NHCs in Main Group Chemistry - Chemical Reviews (ACS Publications)

Review Cite This: Chem. Rev. 2018, 118, 9678−9842

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NHCs in Main Group Chemistry Vitaly Nesterov, Dominik Reiter, Prasenjit Bag, Philipp Frisch, Richard Holzner, Amelie Porzelt, and Shigeyoshi Inoue*

Chem. Rev. 2018.118:9678-9842. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/09/18. For personal use only.

Department of Chemistry, WACKER-Institute of Silicon Chemistry and Catalysis Research Center, Technische Universität München, Lichtenbergstrasse 4, Garching bei München 85748, Germany ABSTRACT: Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC−main group element adducts, and might be useful for the broad research community.

CONTENTS 1. Introduction 1.1. Electronic Effects and Bonding in Main Group Element−NHC Complexes 1.2. Synthetic Methodology 2. NHC Complexes of Main Group Elements 2.1. NHC Complexes of Group 1 Metals 2.1.1. Lithium 2.1.2. Sodium 2.1.3. Potassium 2.2. NHC Complexes of Group 2 Elements 2.2.1. Beryllium 2.2.2. Magnesium 2.2.3. Calcium, Strontium, and Barium 2.3. NHC Complexes of Group 13 Elements 2.3.1. Boron 2.3.2. Aluminum 2.3.3. Gallium and Indium 2.3.4. Thallium 2.4. NHC Complexes of Group 14 Elements 2.4.1. Carbon 2.4.2. Silicon 2.4.3. Germanium 2.4.4. Tin 2.4.4.2. Tin(II) NHC Complexes 2.4.4.3. Tin(0) NHC Complex 2.4.5. Lead 2.5. NHC Complexes of Group 15 Elements 2.5.1. Nitrogen 2.5.2. Phosphorus 2.5.3. Arsenic, Antimony, and Bismuth 2.6. NHC Complexes of Group 16 Elements

© 2018 American Chemical Society

2.6.1. Adducts with Monocoordinated Chalcogen Atoms 2.6.2. Adducts with Two-Coordinated Chalcogen Atoms 2.6.3. Adducts with Three-Coordinate Chalcogen Atoms 2.6.4. Adducts with Three- and Higher-Coordinate Chalcogen Atoms Containing Ch−O Bonds 2.7. NHC Complexes of Group 17 Elements 3. Conclusion and Perspectives Author Information Corresponding Author ORCID Notes Biographies Acknowledgments Abbreviations References

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1. INTRODUCTION Nowadays it is widely recognized that some classes of lowvalent main group element compounds possess properties similar to those of transition metals regarding some aspects of their bonding nature and chemical reactivity.1−4 In fact, representatives of singlet carbenes5 and related main group element compounds can activate small molecules under mild Special Issue: Carbene Chemistry Received: February 6, 2018 Published: July 3, 2018 9678

DOI: 10.1021/acs.chemrev.8b00079 Chem. Rev. 2018, 118, 9678−9842

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ground states. This determines the high stability of NHCs, their nucleophilic (basic) nature, and results in characteristically strong σ-donating properties, sharply different from other classes of carbenes. On the other hand, due to the presence of the nominally vacant pπ orbital, NHCs still possess not negligible π-accepting abilities, which may vary significantly depending on the carbene structure. It can be exemplified by the comparison of electronic effects in imidazol(id)in-2-ylidenes (classical NHCs) with those in pyrrolydin-2-ylidenes [cyclic (alkyl)(amino)carbenes, CAACs], to date the most explored NHC ligands in main group chemistry (Figure 1). Presence of σdonating quaternary carbons adjacent to the carbene atoms instead of one nitrogen in CAACs decreases the HOMO− LUMO as well as the singlet−triplet gap with the HOMO slightly higher compared to classical NHCs. Consequently, CAACs exhibit stronger σ-donating and enhanced π-accepting properties (i.e., stronger nucleophiles and electrophiles). This directly reflects in their chemical properties (e.g., ability of CAACs to activate H2, CO, and NH3)5 and the differences in electronic structures of the corresponding main group element adducts. Supposing π-back-donation from the main group element is possible, the use of CAACs increases the double bond character of the CCarbene−E bond along with bond strengthening. Thus, dependent on the element E and its coordination number, NHC adducts with main group elements possess either single dative covalent CNHC−E bonds45 (π-backdonation from E is negligible or absent) or, in extreme case, double bonds. The varying nature of these bonds can generally be presented by the ylide resonance structure with the negative formal charge at E and positive charge delocalized over the heterocycle, and by the ylene structure with a CNHC−E double bond (Lewis structures I and II, respectively, Figure 2). The π-

conditions, catalyze organic reactions, and act as ligands in transition metal catalysis.6−12 N-Heterocyclic carbenes (NHCs) are a class of stable singlet carbenes with a neutral divalent carbon atom directly attached to at least one nitrogen within a heterocyclic scaffold.13 Since the outstanding discoveries of the first stable acyclic (phosphino)(silyl)carbene by Bertrand (1988)14 and the first “bottle-able” imidazolin-2-ylidene by Arduengo (1991),15 a plethora of stable carbenes have been reported. Among them, the NHCs represent the most important and investigated class of compounds, widely used as ligands in transition metal chemistry and catalysis,16,17 in f-block element chemistry,18 as organocatalysts,19 and beyond. In main group element chemistry, NHCs proved to be an effective tool for the stabilization of low-coordinate elements in different oxidation states, which are difficult to access using alternative approaches. This provided wide opportunities to not only characterize and get insights into the bonding in these reactive species, but also to explore their unique reactivity and potential in synthesis and catalysis. A number of general reviews on NHC complexes of main group elements have been published to date,20−25 and also a number of more specific reviews covering selected aspects of the topic.26−33 The intention of this Review is to present a comprehensive and concise overview of the most important advances in this rapidly developing field with an emphasis on the major features of main classes of compounds to demonstrate their potential for further applications in different branches of chemistry. NHC adducts with elements of group 12, considered here as transition metals, and the chemistry of frustrated Lewis pairs (FLPs)34 are not included. 1.1. Electronic Effects and Bonding in Main Group Element−NHC Complexes

The structural variety of NHCs includes carbenes, which differ in the number of nitrogen atoms adjacent to the carbene carbon atom, in the ring size, ring constitution and functionalization. It includes also cyclic diaminocarbenes with 1,1′-ferrocenediyl backbone.35 Together with the possibility to employ mesoionic (“abnormal”) NHCs,36,37 they deliver a broad set of strong Lewis bases38 with different electronic and steric environments. It is obvious that the resulting bonding and stability of adducts with main group element Lewis acids may vary significantly with respect to the NHC nature.39 Electronic and structural characteristics of stable singlet carbenes have been discussed elsewhere.40−44 The σ-electronwithdrawing and π-electron-donating effects of nitrogen atom(s) within a cyclic framework of NHCs (Figure 1) decrease, respectively, the HOMO (sp2-hybridized lone pair orbital, σ-orbital) and increase the LUMO (unoccupied pπ orbital) energies, thus increasing the energy separation between these orbitals and between the singlet and triplet

Figure 2. Ylide (I) and ylene (II) limiting resonance structures of imidazol(id)in-2-ylidene adducts with main group elements E, and the formula (III) showing a dative bond between NHC and E.

back-donation ability of E and the π-accepting properties of the used NHC determine the relative contributions of the possible resonance Lewis structures. “Genuine” or Classical CE double bonds (electron-sharing bonds, see below) are often formed by lighter elements such as C, N and O. For heavier elements, a decreased tendency toward hybridization leads to increased contribution of the ylide resonance structure when descending the group in the periodic table. NHC ligands are very efficient in the stabilization of electron-deficient main group element species due to their strong σ-donating properties, while stronger π-accepting abilities provide an additional stabilization for ambiphilic and paramagnetic species.27,46 The rational choice of an appropriate NHC ligand39 enables effective tuning of electronic properties and effective stabilization of different elusive main group element species. Several theoretical47 and experimental approaches40,41 are used to evaluate electronic properties of NHCs and CNHC−E bonding. A latest review on the electronic

Figure 1. Schematic representation of electronic effects in imidazol(id)in-2-ylidenes and CAACs. 9679


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Figure 3. Relationship between structure and electronic properties of NHCs.

C−N angle, replacement of one nitrogen with a less electronegative atom, ring annulation, and introduction of electron-withdrawing groups (Figure 3). Comparing the trends obtained by the two different methods allows for a rough evaluation of σ-donor properties of NHCs: the stronger the π-acidity and the overall donation effect, the higher is the σ-donating property of the NHC ligand. As can be deduced from Figure 3, the strongest σ donor ligands, also possessing high π-acidity and therefore exhibit stronger binding properties, are CAACs and novel BICAACs.57 The 13C NMR spectral data can also be used for the analysis of CNHC−E bonds, although in this case the δ(13C) signals of the carbene C2 nuclei appear in a significantly narrower region (Δδ(13C) = ca. 40 ppm), and the measurements require longer acquisition times. The δ(13C) NMR signals generally correlate with the δ(E) data (E = P, Se), while the 1JC,Se coupling constants of selenium adducts and the 1JC,H coupling constants for the C2 atoms of cationic carbene precursors are associated with the σ-donating properties of NHCs.56 15 N NMR spectroscopy is especially useful for bonding analysis of CCAAC−E bonds as, in contrast to classical NHCs, correlation between the δ(15N) chemical shifts and the σdonation strength/π-back-donation could be demonstrated.58 The modern concept of dative (donor−acceptor) covalent bonding in main group element complexes, actively developed by G. Frenking59−61 and others,62 proved its usefulness for the description of electronic structures of main group element− NHC adducts. It is in line with the covalent bond classification method provided by Green and Parkin. 62 Supported theoretically by the quantum chemistry methods of charge and energy decomposition analysis,63 this concept is based on the transfer of the Dewar−Chatt−Duncanson bonding model for transition metal complexes to the complexes of main group elements and on the interpretation of covalent bonding in terms of dative and electron-sharing interactions.45 Dative bonds A→B are considered as arising from the orbital mixing of the doubly occupied valence orbitals of A and the vacant valence orbital of B, while the electron-sharing bonds A−B originate from the mixing of the singly occupied valence orbitals of A and B. Important characteristic features of dative bonds are the presence of charge transfer and their heteropolar dissociation. Accordingly, the nature of the CNHC−E bonds varies from the structures with a single dative bond (depicted with an arrow as CNHC→E, structure III, Figure 2) to the structures with double dative bonds comprised of σ-donation

properties of NHCs and their experimental determination was published very recently by Huynh.48 The most common method to estimate the overall donation capability of NHC ligands is the use of Tolman electronic parameters (TEPs),49,50 that is, A1 IR-stretching frequencies of CO ligands in model transition metal complexes, such as [Ni(CO)3(L)] or cis-[MCl(CO)2(L)] (M = Ir, Rh). TEP values of NHCs change in the relatively narrow region of ca. 2030−2068 cm−1 (Figure 3), decreasing upon enhanced donation from the d-orbital of the transition metal into the π*(CO)-orbital. Being originally developed for phosphine ligands and further widely employed in the investigation of NHCs, this method is not free from some intrinsic limitations in the description of M−L bonding in transition metal complexes,51 and does not characterize σ-donor and πaccepting effects of the ligand. The latter issue is especially important when the assumed strength of the M−L bonding, based on the evaluation of TEP values, is compared to that of the ligands possessing distinctly different π-accepting properties. Analysis of NMR signals of the nuclei involved in the CNHC−E bonds of main group element−NHC complexes provides a valuable experimental tool for primary appreciation of the bonding and evaluation of the π-accepting properties of NHCs. The two main methods, generally giving comparable results (linear correlation), are based on the comparison of 31P and 77Se NMR chemical shifts for sets of NHC phenylphosphinidene52 and selenium adducts,53,54 respectively. Theoretical calculations showed good correlation between chemical shielding and π-contribution in the CNHC−E (E = Se, P) bonds of investigated complexes,55 as well as good correlation with the LUMO energies of NHCs,56 thus strongly supporting these techniques for characterization and quantification of the π-accepting abilities of NHC ligands. The resonance signals δE of these complexes are changing in relatively broad ranges (Δδ(31P) = ca. 144 ppm, Δδ(77Se) = ca. 850 ppm), which allows one to differentiate even slight structural changes in carbene structures. Increased contribution of the ylide resonance structure I (Figure 2, E = P, Se) with a single bond and negative charge at E affords more shielded nuclei, with E resonating upfield with respect to the downfield signals of the complexes with a higher degree of πback-donation (increased contribution of the ylene resonance structure II, Figure 2). The 31P and 77Se NMR data clearly support enhanced π-acidity of NHCs upon increase of the N− 9680


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Figure 4. Frequently used NHCs in main group chemistry and the acronyms used in this Review.

first carbodicarbenes L2C (G. Bertrand et al., 2008)68 and NHC-supported diatomic allotropes L2E2 (E = B, Si, P in formal oxidation state zero, G. Robinson et al., 2008).32,69,70 This was accompanied by the active development of the modern theoretical model of dative (donor−acceptor) bonding for main group element compounds (G. Frenking et al.)61 and parallel development of novel NHCs (e.g., discovery of CAACs in 2005 by G. Bertrand et al.71). Those achievements evoked an extensive development of this field, especially concerning the use of NHCs for stabilization of elusive main group element species. Synthetic methods used for the preparation of main group element−NHC adducts can be classified into two main categories. The first includes reactions of free NHCs with main group element compounds, while the second includes reactions of NHC precursors or synthetic equivalents (e.g., bis(imidazolidin-2-ylidenes), 2-halogeno-, and 2-carboxyl-substituted imidazolium salts). The first group of reactions is best exemplified by the reactivity of well-investigated imidazol(id)in-2-ylidenes (Scheme 1). It includes addition reactions (a and b), leading to the formation of Lewis Base/Lewis acid adducts III and IV; redox (X = Hal) and deprotonation (X = H) reactions to imidazolium derevatives V; formation of cationic adducts VI; and 1,1- and 1,2-elimination reactions yielding the complexes with low-coordinate main group elements VII and VIII. The reaction outcome can differ from the outline in Scheme 1. It depends on the stability/reactivity of the obtained adducts and the amount of NHC used in the reaction. Noteworthy, complexes of types III and VI often serve as starting materials for the preparation of NHC-stabilized low-coordinate main group elements, for example, by their further reduction. Replacement of imidazol(id)in-2-ylidenes with NHCs of other types in reactions with main group element compounds or in complexes by ligand exchange reactions can lead to the formation of different adducts. It can be illustrated by the

and π-back-donation (i.e., CNHC ⇆ E), characterized by different partial charge distribution. In these terms, electronsharing double bonds CE can formally be considered as those formed between an element and triplet carbene (homolytic dissociation), while singlet carbenes generally build dative double bonds (heterolytic dissociation). For lighter elements with higher electronegativity (e.g., O, N), and for singlet carbenes displaying high π-acidic properties (e.g., CAACs and DACs), these double bonds are often electronsharing. It should be noted that the structures III and I (Figure 2) are two alternative representations of a dative CNHC−E bond. Toward each other, these are not resonance structures.62 To avoid misunderstanding of electronic structures schematically presented by Lewis formulas and formulas showing the dative bonds, it is also important to differentiate between formal and partial charges.64,65 In this Review, the single CNHC−E bonds with negligible double bond character are mainly denoted with arrows, provided it is not misleading, taking into account the debates about graphical representation of such bonds.65−67 To keep consistency, some formulas presented in this Review are different from those from the original reports. For the reader’s convenience, Figure 4 provides an overview of the structures and corresponding acronyms of the most commonly used carbenes in this Review. In addition, the two opposing charges in the representation of abnormal NHCs are omitted throughout this Review for the sake of clarity. 1.2. Synthetic Methodology

Shortly after the discovery of stable NHCs, their application has been smoothly extended to the chemistry of main group elements. The earlier achievements, summarized in 2005,24 include various examples of NHC complexes with group 1, 2, and 13−17 elements, which are mainly limited to simple Lewis base/acid adducts. A significant breakthrough in the field has been achieved after seminal experimental discoveries of the 9681


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corresponding imidazolium salt precursor. Overall, the number of crystallographically characterized complexes remains rather small. No NHC complexes of Rb or Cs have been reported. This is unsurprising, given the trend in stability for the alkali metal cations: in line with the decreasing Lewis acidity, the strength of the metal−NHC bond for group 1 metals decreases with increasing atomic number of the alkali metal. This can be easily illustrated by looking at the 13C NMR shift of the carbene carbon atom in a variety of lithium, sodium, and potassium complexes and then comparing the shifts to the free NHC (Table 1).76,77 The larger is the atomic number of the alkali metal, the further downfield shifted is the carbene carbon atom, that is, the closer to the shift of the free NHC.

Scheme 1. Typical Reactivity of Imidazol(id)in-2-ylidenes toward Main Group Element Compounds

Table 1. Comparison of the 13C NMR Data [ppm] of NHC Alkali Metal Complexes (M = Li, Na, K) in Toluene-d8

reactivity of CAACs, which can afford open-shell main group element complexes as intermediates or final products instead.26,27 Cleavage of the CNHC−E bonds in main group element− NHC complexes generally occurs heterolytically with the release of main group element Lewis acids and NHCs. This feature is used synthetically to obtain the former in its base-free form, for example, via bonding of NHCs as corresponding triarylborane−NHC complexes. Under certain reaction conditions, NHC ligands can be noninnocent and undergo transformations, such as ringopening or ring-expansion reactions, etc., leading to unexpected reaction products.29

NHC

free NHC

Li−NHC

Na−NHC

K−NHC

176 276 377

236.1 206.8 242.7

216.8 195.7 219.4

221.3 196.4 224.9

226.7 201.1 241.0

In the following section, for the sake of clarity, charges on the alkali metal cations and ligands are omitted unless they are needed. 2.1.1. Lithium. Unsurprisingly, the majority of reported group 1 NHC complexes contain lithium as the metal center. One of their most important applications is their ability to act as affordable transmetalation agents like silver−NHC complexes (vide infra). The first group 1 NHC complex was reported in 1995 by Boche et al. with a lithiated 4-tertbutylthiazole 4 (Figure 5). 78 It was synthesized via deprotonation of 4-tert-butylthiazole using MeLi and shows a dimeric structure with long C−Li distances (2.531(5) Å). A few years later, the Li borane-carbene species 5a was reported by Siebert et al.79 via deprotonation of the trimethylimidazolborane precursor with n-BuLi. The 13C NMR chemical shift of the carbene carbon in compound 5a is observed at 191.3 ppm, whereas free IMe4 can be observed at 213.7 ppm. Similar to compound 4, the structure in the solid state is also dimeric with one short (2.169(5) Å) and one long (2.339(5) Å) carbene−lithium distance. In 2002, the same group further reported different substitution patterns (5b−d) on the imidazole moiety and transmetalation reactions with various transition metal complexes (Fe, Mn, V, Ti).80 While in complexes 4 and 5 the lithium is also coordinated to a heteroatom in addition to the carbene carbon, the first example of a group 1 carbene complex with only coordinated carbon centers was reported by Arduengo et al. in 1999 via reaction of free NHCs (ItBu, IAd, and IMes) with 1,2,4tris(trimethylsilyl)cyclopentadienide.81 The lithium center in 6a (NHC = ItBu) shows a η5-coordination to the Cp ring as well as a short carbene lithium (2.155(4) Å) interaction (Figure 5). Similar to other lithium NHC complexes, the 13C NMR signal of the carbene carbon appears upfield-shifted by around 20 ppm (190.7 ppm) as compared to that of free ItBu.82 In the same year, Alder and co-workers reported the

2. NHC COMPLEXES OF MAIN GROUP ELEMENTS 2.1. NHC Complexes of Group 1 Metals

This section details the advances made in the field of group 1 metal NHC complexes, ranging from generally simple Lewis adducts to dimeric or even polymeric complexes with interesting ligand systems. The diverse reactivity of the Nheterocyclic dicarbene (NHDC) lithium complex72 first reported by Robinson et al. will be discussed, as well as group 1 aNHC and CAAC complexes will be detailed. As opposed to groups 13, 14, and 15, which show a staggering amount of publications in recent years for a variety of NHCstabilized compounds and their reactivity, significantly less papers have been published about NHC complexes of group 1 and 2 metals. Some of the compounds covered in sections 2.1 and 2.2 have been already discussed in earlier reviews.16,17,22,23,33 Discussion of the chemistry of NHC− hydrogen adducts, such as imidazolium salts,73,74 bis(NHC)−proton adducts,75 and dihydrogen addition to NHCs,5 is not included. Many of the alkali metal NHC complexes are simple Lewis adducts, typically generated by the addition of an alkali metal base (e.g., LiHMDS) either directly to the free NHC or to the 9682


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Figure 5. Various lithium NHC complexes 6−13 (THP = tetrahydropyran; L = [N(SiMe3)2]).

alkali metals sodium and potassium, isolated via the same route (vide infra).85 Additionally, the same group also reported the lithium gallate complex 13 in 2015 by reaction of LiCH2SiMe3 with the gallium NHC complex [(IDipp)Ga(CH2SiMe3)3] (cf., section 2.3.3).86 The compound is structurally similar to that of the lithium zincate complex 9. Reversing the order of addition (i.e., LiCH2SiMe3 followed by Ga(CH2 SiMe 3 ) 3 ) leads to the expected abnormal heteroleptic gallium NHC complex (vide infra). They also reported a sodium zincate NHC complex via direct zincation, which will be discussed during the sodium section of this Review (vide infra).84 The lithium IEt2Me2 adduct 11 was synthesized from a NHC carbodiimide adduct (vide infra) with LiN(SiMe3)2.87 The complex shows a 13C NMR carbene resonance at 194.2 ppm (vs free IEt2Me2 at 211.1 ppm88). The complex is a dimeric form of LiN(SiMe3)2 with one lithium center being coordinated by the NHC with a Li−C bond length of 2.189(4) Å. An NHC-stabilized lithium alkynyl species 12 has also been reported by Roesky and Stalke et al.89 Simple reaction of t BuCCLi with equimolar amounts of IMe4 leads to the NHC adduct [tBuCCLi·IMe4]4. SC-XRD analysis revealed a tetrameric structure with a slightly distorted tetrahedral Li4core where each of the lithium corners is terminally coordinated by one IMe4 and each of the four Li3-planes is μ-capped by one alkynyl ligand. The CNHC−Li distances are in the range of 2.240 Å. A related magnesium alkynyl NHC complex was also reported, which will be discussed during the earth alkaline section of this Review (vide infra).89 Lithium complexes 14−16 with a chelating neutral bis(NHC) ligand have been reported by Hofmann et al. (Figure 6).90 They were synthesized by addition of LiHMDS to the corresponding methylene bridged bis(imidazolium) salt, and all structures were determined by single-crystal XRD: complex 14 shows a dimeric structure with two bridging bromide ligands between two lithium centers, each coordinated by two NHC moieties from the same chelating ligand with Li−C distances of 2.212(8) and 2.255(8) Å. The monomeric complex (15a) can be obtained when the crystallization is carried out at low temperature. Instead of bridging bromo

lithium NHC complex 7b synthesized by reaction of the free six-membered NHC 1,3-diisopropyl-3,4,5,6-tetrahdyropyrimid2-ylidene (1) and LiHMDS.76 The 13C NMR of 7b in toluened8 shows the carbene carbon resonance at 216.8 ppm, while the free NHC resonates at 236.1 ppm.76 The related complex 7a with IiPr2Me2 as the NHC was also isolated in the same fashion. A similar trend for the chemical shifts in the 13C NMR of the free NHC (206.8 ppm) and the lithium complex 7a (195.7 ppm) can be observed. Additionally, they also described the heavier alkali metal complexes (M = Na, K, vide infra), synthesized via the same route; however, only the potassium complex 72 was structurally characterized (cf., section 2.1.3). Very recently, Rivard et al. reported an extremely bulky NHC with trityl wingtip substituents and its lithium complex 8 (Figure 5).83 In addition to the lithium complex, they also reported a thallium(I) complex and a chlorogermyliumylidene complex, which will be discussed in their respective sections (vide infra). Complex 8 was obtained by addition of MeLi to the chlorogermyliumylidene complex in Et2O. The structure was elucidated using SC-XRD analysis, which revealed one lithium center coordinated by one carbene moiety as well as one Et2O solvent molecule with BArF4 as the anion. The complex shows a short Li−C bond distance of 2.076(6) Å as well as significant arene−lithium interaction with Carene−Li contacts in the range of 2.57−3.43 Å. Reaction of the lithium complex with GeCl2·dioxane returns the chlorogermyliumylidene complex in quantitative yield.83 Hevia and co-workers reported the reaction of the IDipp complex of Zn(tBu)2 with 1 equiv of tBuLi, which leads to the homoleptic lithium zincate complex 9 with a lithium bound NHC.84 The two metals are connected by two tBu bridges, while the third tBu moiety is connected to Zn in a terminal fashion. In 2011, Hill et al. reported the charge separated lithium bis(NHC) complexes [Li(IDipp)2][M[N(SiMe3)2]3] (M = Mg, Ca) (10).85 They were synthesized by reaction of an equimolar amount of LiHMDS and M(HMDS)2 (M = Mg, Ca) in the presence of 2 equiv of free IDipp. Single-crystal structure determination revealed a CNHC−Li bond distance range of 2.128(6)−2.179(6) Å for the complexes. Hill and coworkers further described related complexes with the heavier 9683


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bite angle of the ligand being too small for the larger alkali metals (vide infra).92 Very recently, Kunz and co-workers prepared novel complexes with lithium, sodium, and potassium coordinated to chelating bis(NHC) 2,7-dihydro-2,7-dialkyldiimidazo[1,5b:5′,1′-f ]pyridazine-1,8-diylidene (vegi) ligand. Strong chelating properties of the ligand precluded the isolation of the free carbene from the reaction of corresponding bis(imidazolium) salts with alkali-metal bases.93 A variety of lithium NHC complexes with (anionic) functional groups as tethers on one wingtip substituent have been reported by several groups. Arnold et al. have been the first to report the amine-functionalized lithium NHC complex 18 (Figure 7) in 2003.94 The complex was synthesized by two subsequent deprotonation steps of the alkylammoniumimidazolium dibromide salt precursor. The first deprotonation with tBuLi leads to the amine-imidazolium salt, while the second deprotonation with n-BuLi affords the NHC lithium complex. X-ray analysis revealed a dimeric structure bridged by two LiBr units with dative bonds from both the amine and the carbene directed toward the lithium cation. The Li−C bond in complex 18 (2.196 Å) is slightly longer than that in 6a. The adduct was successfully used as a transmetallating agent to synthesize anionic amido-NHC samarium and yttrium complexes from Ln(HMDS)3.94 The mesityl derivative 20 was reported in 2009 by Ong et al. and was utilized to synthesize (NHC)AlR3 (R = Me, Et) adducts95 (vide infra), boronium and borane adducts96 (vide infra), and transition metal complexes.97 Interestingly, the SC-XRD analysis showed a monomeric structure with one THF molecule coordinated to the lithium (d(Li−C) = 2.125(7) Å). The 13C NMR shift of complex 20 can be observed at 206.5 ppm, shifted upfield from the free NHC (216.3 ppm). The corresponding amido−NHC lithium complexes 19 can be obtained when 2 equiv of n-BuLi is used in the final deprotonation step.98 Again, the lithium complex shows a dimeric structure via bridging amido-groups with a distorted carbene lithium bond (2.124(4) Å). The complex was utilized to synthesize a UO2-complex with UO2Cl2(THF)2 as the metal precursor. A magnesium NHC complex was also synthesized with the same ligand framework (vide infra).98 Danopoulos

Figure 6. Bis(NHC) lithium complexes 14−17 with a neutral chelating bis(NHC) ligand.

ligands, only one terminal bromide is observed, with the empty coordination site being saturated by a THF solvent molecule. The Li−C distances are slightly longer than in the dimeric compound (2.291(6) and 2.253(6) Å). Similar structures are observed for different anions (PF6 instead of Br) and different wingtip substituents (Dipp instead of tBu). Prolonged crystallization time of the Dipp-substituted bis(NHC) lithium adduct led to the polymeric complex 16.90 The complexes have been used to synthesize Ni(0) and Pt(0) complexes with chelating bis(NHC) ligands.91 Westerhausen et al. reported that addition of the free methylene bridged bis(NHC) to LiHMDS leads to the chelating bis(NHC) adduct 17.92 Crystal structure analysis shows a monomeric structure with both NHC moieties connected to the lithium center (d(Li−C) = 2.271(4) and 2.234(4) Å), which is also coordinated by one [N(SiMe3)2] ligand. Therefore, the complex can essentially be regarded as a NHC-coordinated monomeric form of LiN(SiMe3)2. The same group also reported multiple sodium and potassium complexes with the same ligand; however, no similar complexes with a chelating ligand were obtained due to the

Figure 7. Various lithium NHC complexes 18−30 (L = [N(SiMe3)2]). 9684


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2.145(8) Å. The complex was utilized in the synthesis of lanthanoid NHC complexes.110 Fehlhammer et al. synthesized the dimeric hydrotris(NHC)borate complex 28 (Figure 7) by deprotonation of the corresponding imidazolium tetrafluoroborate salt with nBuLi.111 Each lithium cation in the NHC adduct is coordinated by four carbenes (two terminal (d(Li−C) = 2.138(4), 2.147(4) Å) and two bridging (d(Li−C) = 2.201(4), 2.311(4) Å), while no additional solvent molecules are coordinated to the lithium cations. The 13C NMR signal for the C2 carbon atom is observed at 191.8 ppm. Related borate bis(NHC) complexes 29−30 have also been isolated in both dimeric as well as monomeric forms.112,113 The BH2-bridged bis(NHC) complex 29a was reported by Smith et al. as the transmetallating agent to a related nickel complex.113 Crystal structure analysis showed a dimeric structure with two central lithium atoms, each coordinated by two NHCs, one from each ligand, with one of them being further coordinated by one Et2O solvent molecule. Li−C distances are in the range of 2.111(4)− 2.186(4) Å. The Ph2B- (30) and Me2B-bridged (29b) bis(NHC) complexes were published by Hofmann and coworkers in 2009.112 While the phenyl-substituted complex 30 shows a monomeric structure with a lithium-bound Et2O solvent molecule in the solid state, the methyl-substituted complex 29b has a dimeric structure comparable to that of the BH2-bridged complex 29a. Li−C distances are 2.089(10) Å (30) and 2.056(2) Å (29b).112 Recently, Hevia et al. reported the alkali metal-mediated ring opening of saturated NHCs.114 Addition of LiCH2SiMe3 to SIMes leads to the saturated NHC complex 31 (Scheme 2). X-

and co-workers reported the lithium complexes 21 of a monodeprotonated, dearomatized PNCNHC pincer-type ligand.99 Structural parameters of the complexes were elucidated using X-ray analysis. Depending on the solvent used during the synthesis, either no coordination (THF or Et2 O) or coordination (benzene) of the PR2 moiety to the lithium cation could be observed. The complexes show a Li−C distance in the range of 2.139(4)−2.156(5) Å. They also reported related potassium complexes (vide infra).99 Derivatives with two anionic tethers (22) or one anionic tether linking two NHC moieties (23, 24) have also been reported (Figure 7).100−103 Hall et al. reported the anionic bis(amido) complexes 22 in 2006, which were synthesized via deprotonation from the corresponding bis(amine)-substituted free NHC with 2 equiv of n-BuLi.100 No crystal structure data were obtained due to the poor solubility; however, the dilithium structure shown in Figure 7 was proposed because of the dilithium structure reported for similar complexes (Li2[NPN]).104,105 The 13C NMR shows a resonance at 189.9 ppm, shifted upfield from the free NHC (215 ppm).106 Transmetalation from the lithium complexes with different tantalum halides as metal precursors resulted in a variety of tantalum NHC complexes.100 The anionic amido-bridged lithium bis(NHC) complexes 23 (synthesized via n-BuLi deprotonation) were reported by Arnold et al. in 2007 (Figure 7).101 It was not possible to unambiguously confirm the structure of the complex by X-ray diffraction, but 13C NMR data show a carbene signal at 203.4 ppm. The series of carbazole bridged complexes was reported by Kunz et al.102,103 Chemical shifts of the carbene carbons in the 13C NMR range from 203.7 ppm (R = iPr) to 206.1 ppm (R = Me). Single-crystal analysis of complex 24d (R = iPr) revealed the structure to be the lithium iodide adduct of a monomeric lithium NHC complex. Each NHC moiety is coordinated to one lithium center, with one of the ions being coordinated to the iodide anion and the other one being coordinated to a DME solvent molecule.103 The two different Li−C distances are quite similar (2.142(11) and 2.131(11) Å). The complexes were employed in transmetalation reactions.107 Arnold and co-workers also reported the asymmetric alkoxyfunctionalized NHC lithium complexes 25 (Figure 7) by deprotonation of the imidazolium salt with Li(SiMe3) or nBuLi.108 SC-XRD analysis of 25b revealed a dimeric structure with stoichiometric amounts of lithium iodide incorporated into the structure. The lithium carbene distance of 2.135 Å is quite short. The complexes were used to synthesize mono- or bis-substituted copper NHC complexes.108 The group of Arnold also did work on Sc and Y NHC complexes with the same ligand framework where they were able to isolate the heterobimetallic “ate” complex 26, which was formed during the synthesis of the actually desired monometallic complexes.109 Combining both the anionic alcoholate moiety as well as the amino functional group, Shen et al. reported the aminophenoxo-functionalized lithium complex 27 in 2010 (Figure 7).110 The complex was synthesized by deprotonation of a phenol-functionalized, imine-substituted imidazolium salt with 2 equiv of n-BuLi. The 13C NMR spectrum shows a resonance at 200.5 ppm. The structure in the solid state is a dimer with a central (OLi)2-unit, with each lithium atom being further coordinated by a carbene moiety and one nitrogen atom to form a distorted tetrahedral geometry. The Li−C distance is

Scheme 2. Ring Opening of SIMes to Complex 31

ray analysis revealed one coordinating NHC moiety, one coordinating THF solvent molecule, and one amido-functionalized indole-based amido N-bound ligand resulting from the activation of SIMes. A related potassium complex resulting from the activation with KCH2SiMe3 was also reported (vide infra). The SIMes activation presumably happens after coordination via metalation of the ortho methyl group of the mesityl substituent followed by C−C coupling/C−N cleavage. They further investigated the NHC activation pathway by performing different trapping reactions, which led to two NHC lithium complexes 32 and 33 of different steps of the activation intermediates (Scheme 2).114 The coordinaton adduct 32 was trapped using 0.5 equiv of Mg(CH2SiMe3)2. Addition of LiTMP/Al(TMP)(iBu)2 (TMP = 2,2,6,6-tetramethylpiperidine) allows the trapping of the lithium NHC aluminate 9685


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species 33, confirming the metalation of a mesityl ortho methyl group. In 2010, Robinson and co-workers described the synthesis and characterization of a synthetically viable anionic Nheterocyclic dicarbene (NHDC) 34a (and 34a·THF) by direct lithiation of IDipp at the C4 position with n-BuLi or lithium metal (Scheme 3).72

34a and led to the AlMe3 (35a) and BEt3 (35b) adducts (Scheme 4).72 In both cases, the lewis acids bind to the abnormal C4 position, while the lithium cation is coordinated to the C2 position in addition to two THF molecules. The C− Li bonds are rather short in both complexes (2.094(6) and 2.096(4) Å, respectively). Related MMe3 (M = Al, Ga, In) lithium NHDC complexes of ItBu have also been reported, synthesized by deprotonation of the aNHC group 13 complexes with n-BuLi.115 On the other hand, reaction of 34a with TMSCl led to the TMS-functionalized (C4 position) free NHC 39b (vide infra).72 Since then, 34a and its derivatives have been successfully used to synthesize various normal, abnormal, and anionic (bis)NHC complexes of main group elements and transition metals. Scheme 4 gives an overview over a variety of different compounds reported in the last years. Nonlithium main group NHC complexes will be discussed during their respective sections. Since their initial report of the NHDC lithium compound 34a and its Lewis acid adducts 35,72 Robinson et al. reported a variety of heterobimetallic anionic bis(NHC) complexes and backbone-functionalized NHCs.116−118 Reaction of 34a with diethylzinc in hexane leads to the lithium zincate 38.116 The compound was characterized as the TMEDA adduct and shows the expected structure, with lithium being coordinated to the regular C2 position of the NHDC (d(Li−C) = 2.093(4) Å) and one TMEDA molecule, while the zinc is coordinated to the abnormal C4 position. When the reaction is carried out in THF with an excess amount of Et2Zn, a triorganozincate compound is obtained, where one zinc bridges two NHDC moieties at their C4 positions and two additional Zn are coordinated to each normal carbene position. Addition of THF to the lithium zincate 38 leads to decomposition, yielding a

Scheme 3. Synthesis of the Anionic Dicarbene 34a and 34a· THF

Because of the anionic character of 34a, the 1H NMR resonance of the C5 proton is shifted upfield to 6.16 ppm as compared to the 7.19 ppm in the precursor IDipp. SC-XRD analysis of the THF adduct revealed a polymeric chain structure with lithium cations bridging two NHC moieties. The metal centers are connected to the C2 (d(Li−C) = 2.216(6) Å) carbon of one carbene and the C4 carbon (d(Li−C) = 2.125(6) Å) of another. The lithium center is also coordinated by one THF solvent molecule. Reaction with group 13 Lewis acids unambiguously demonstrated the dicarbene nature of Scheme 4. Diverse Reactivity of Anionic NHDCs 34

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Pb[N(SiMe3)2]2 with NHDC 34a. Further reaction of 48 with one additional equivalent of Pb[N(SiMe3)2]2 yields complex 49 by deprotonation of one amide ligand (cf., section 2.4.5). In addition to the already discussed cases where the lithium cation is only coordinated to the normal C2 position of the carbene, aNHC complexes of lithium have also been reported. Bertrand et al. reported the synthesis of the aNHC lithium complex 50 (Scheme 5) by blocking the C2 position of the NHC followed by the deprotonation with LDA or n-BuLi; however, no crystal structure was reported.131

mixture of the triorganozincate and a diorganozincate lithium complex.116 Robinson and co-workers further described the isolation of the SiCl3-functionalized NHC 39a by reaction of 34a72 with SiCl4118 as well as an anionic dithiolene radical by addition of elemental sulfur to 34a.119 Furthermore, they showed that reaction of 34a with iodomethane conveniently produces the N-heterocyclic olefin 44 in high yield.120 Tamm et al. isolated a variety of NHDC lithium complexes with borate backbone substituents (46)121−123 by addition of BAr3 (Ar = C6F5, 3,4-(CF3)2C6H3, p-Tol) to 34a−c. As expected, 13 C NMR signals are slightly shifted upfield from the free NHC (e.g., 46c = 218.9 ppm vs 220.6 ppm). All compounds share similar structural motifs with comparable Li−C distances (2.092(2)−2.198(2) Å). The complexes have been employed in transmetalation reactions,124 which in turn have been used as hydrogenation catalysts of alkenes in nonpolar solvents.123 The group also synthesized the Mes2B and 9-BBN backbonesubstituted free NHCs 42 and 43 by addition of substituted haloboranes to 34c.122,125 Goicoechea et al. isolated the potassium analogue 45 of 34a by addition of KOtBu (vide infra). However, isolation of the free carbanionic species, for example, by addition of the potassium sequestering agent 2,2,2-crypt, failed.126 Ghadwal et al. reported a convenient route to R2Si-bridged (R = Me, Ph) di-NHCs 41 as well as the backbone silylated NHC 40 by addition of dihalosilanes to 34a. Addition of 1 equiv of the halosilane leads to the monosubstituted product, while addition of substoichiometric amounts results in the diNHC compounds.127 Ghadwal and co-workers further reported the lithium tungsten NHDC complex 37 by reaction of W(CO)5THF with 34a.128 As expected, the W(CO)5-fragment coordinated to the C4 position, while the lithium cation remains at the C2 position (d(Li−C) = 2.106(4) Å). Similar to Robinsons AlMe3 and BEt3 adducts 35a,b, Hevia et al. isolated the related Ga(CH2SiMe3)3 adduct 36.86 The compound was used to synthesize various gallium aNHC complexes (vide infra). Complex 34a has also been utilized for the fixation of N2O, leading to a bis N2O adduct of IDipp (vide infra).129 Very recently, Goicoechea and co-workers also reported the synthesis of the bis(aNHC)-ditin complex 47, where one of the NHC moieties is also coordinated to a lithium cation in the “regular” C2 position (Figure 8).130 The complex was obtained by reaction of Sn[N(SiMe3)2]2 with NHDC 34a. They also reported two related aNHC−lead complexes 48 and 49. Compound 48 was isolated by reaction of 0.5 equiv of

Scheme 5. Synthesis of the aNHC Lithium Complex 50 and Complexes 51−55

Roesky and Stalke et al. reported an NHDC complex where lithium is coordinated in the abnormal position of the NHC (51a).132 The complex was synthesized by addition of BH3 to free IDipp followed by deprotonation of the backbone with nBuLi.132 The analogous complex 51b with BEt3 instead of BH3 was also isolated.117 Interestingly, heating a THF solution of 51b results in the formation of the C2 lithiated compound with BEt3 in the backbone. Like previously mentioned, this complex can also be synthesized by deprotonation with n-BuLi followed by addition of the Lewis acid (cf., 35b, Scheme 4).72 As expected, structural paramaters of 51a and 51b (d(Li−C) = 2.123(3) and 2.151(9) Å, respectively) and NMR data are quite similar. The BEt3 adduct was used to synthesize the first aNHC gallium complex (vide infra).117 Rivard et al. reported the NHDC lithium complex 52, synthesized by deprotonation of the NHCs backbone of the (IDipp)B(Cl)O·B(C6F5)3 substrate with MeLi or PhLi.133 The bond length in this complex is slightly shorter (2.082(6) Å) than that in the other C4 lithiated NHDC complexes. Robinson et al. were able to isolate the lithiated NHC phosphinidene adduct 53 from the reaction of an NHC diphosphorus adduct (vide infra, Scheme 191) and lithium metal.134 The lithium−C4 bond length (2.116(5) Å) is in a range similar to that of the BR3 adducts 51. Studies about regioselective backbone deprotonation of methylene-bridged bis(imidazole-2-thiones) leading to aNHC lithium complexes were also reported.135 Lithium aNHC complexes have been utilized in the postcoordination backbone functionalization of NHCs.136

Figure 8. Tin/lead NHDC lithium complexes 47−49. 9687


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structure via X-ray diffraction studies, the proposed structure 57 was supported experimentally by deuteration studies. While the abnormal carbene complex is stable as a solid, a solution of 57 slowly (t1/2 = 2 weeks) isomerizes to the normal NHC lithium complex. Utilization of 3 equiv of n-BuLi as the base allows the isolation of the NHDC dilithium complex 58. The complex shows two downfield shifted resonances in the 13C NMR that correspond to the C2 (195.6 ppm) and C4 (169.5 ppm) positions of the NHC. SC-XRD studies revealed a dimeric structure where the C2 centers are coordinated to one lithium cation each, whereas the C4 centers are bridged by two lithium cations. The C2−lithium bonds (2.103(4) Å) are slightly shorter than the C4−lithium bonds (2.157(5) Å). Agostic interactions between the lithium cation and several B− H bonds are suggested due to their close contacts.141 Lavallo and co-workers further described asymmetric carborane-substituted NHC complexes of lithium. Substitution of one carborane moiety with a mesityl allowed the isolation of the regular NHC lithium complex 59 as well as the NHDC dilithium complex 60 (Scheme 6).142 Deprotonation of the imidazolium salt with LiHMDS oder KHMDS results in the formation of the regular NHC alkali metal complex, while addition of LDA leads to decomposition. The 13C NMR spectrum of the regular NHC complex shows a carbene carbon resonance at 199.9 ppm. The solid-state structure revealed three lithium-coordinated THF molecules in addition to the NHC with a lithium carbene distance of 2.214(3) Å, which is slightly longer than the symmetrical carborane-substituted NHC complex 56. The potassium analogue was also isolated but not structurally characterized. Isolation of an abnormal NHC complex (like for the symmetrical abnormal NHC complex 57) was unsuccessful; however, addition of 2 equiv of n-BuLi allowed the isolation of the NHDC dilithium complex 60. The second deprotonation occurs selectively at the position adjacent to the mesityl moiety. The 13C NMR spectrum shows carbene carbon resonances at 193.0 ppm (normal carbene) and 166.8 ppm (abnormal carbene). Structural analysis revealed a monomeric structure, which contrasts with the dimeric structure observed for the symmetrical complex 58. Again, the C2−Li bond distance (2.079(4) Å) is shorter than the C4−Li bond distance (2.133(4) Å). Gold NHC complexes were synthesized from both the symmetrical as well as the asymmetric regular NHC complexes.143 2.1.2. Sodium. Sodium NHC complexes are quite rare; only a handful of them have been reported so far with almost all of them during the past decade. The first sodium NHC complexes 61a,b were reported in 1999 by Alder et al. from the reaction of free NHCs 1,3-diisopropyl-3,4,5,6-tetrahdyropyrimid-2-ylidene (1) and IiPr2Me2 with NaHMDS (Figure 9).76 In both cases, as compared to the related lithium complexes 7a (216.8 ppm) and 7b (195.7 ppm), a downfield shift of the 13C NMR signal of the carbene carbon atom in toluene-d8 to 221.3 ppm (61a) and 196.4 ppm (61b) can be observed. While these complexes were only characterized by multinuclear NMR spectroscopy, the related potassium homologue was also characterized crystallographically (vide infra).76 The first fully characterized sodium NHC complexes were reported in 2011 by Hill et al.85 Similar to their reported lithium complexes 10a,b (vide supra) and potassium complexes (vide infra), they prepared a series of charge separated sodium bis(NHC) complexes 62a−c with [M(HMDS)3]− (M = Mg, Ca, Sr) as counteranions (Figure 9).

Lithium NHC complexes with an (anionic) backbonesubstituent have also been reported.137−140 Danopoulos and co-workers synthesized the lithium NHC complex 54 (Scheme 5) with an anionic amido functional group connected to the NHC in the backbone. It was synthesized from the corresponding imidazolium zwitterion by addition of LiCH2SiMe3 followed by TMEDA.137 Single-crystal analysis showed a lithium NHC complex (d(Li−C) = 2.093(3) Å) with the TMEDA ligand being coordinated to the lithium cation. The complex was used to prepare an iron complex via transmetalation. They further reported direct lateral lithiation of the iPr substituents on the same ligand framework.138 They also did work on related potassium complexes (vide infra).139 The lithium adduct 55 and the related potassium adduct of a P-anionic phosphanide NHC derivative (cf., section 2.5.2) were reported by Streubel et al.; however, only the potassium adduct was structurally characterized (cf., section 2.1.3).140 In 2014, Lavallo et al. introduced N,N-carborane anion bissubstituted NHCs and characterized their normal (56) and abnormal lithium (57) complexes as well as the related NHDC dilithium complex 58 (Scheme 6).141 Addition of LiHMDS as Scheme 6. Synthesis of Complexes 56−60 from Carborane Anion-Substituted Imidazolium Saltsa

a

Unsubstituted vertices equal B−H bonds.

the base to the corresponding carborane-substituted imidazolium salt resulted in the smooth deprotonation at the regular C2 position leading to the normal carborane-substituted NHC lithium complex 56. The 13C NMR shows a carbene carbon resonance at 196.9 ppm. The complex shows a carbene lithium bond lengh of 2.110(2) Å, which is in the regular range for lithium NHC complexes. Two close B−H contacts with the lithium cation are also observed. Carrying out the deprotonation at low temperatures using the bulkier base LDA resulted in the proton abstraction at the C4 position, yielding the abnormal NHC lithium complex 57. The complex shows a characteristic C4 carbon resonance of 174.7 ppm. Although they were unable to determine the 9688


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coordinated to the aNHC position (isolated from [(TMEDA)NaMg(TMP)2(n-Bu)]) was also reported (vide infra).144 In 2016, Hevia et al. further described the synthesis of a sodium gallate NHC complex 66 as a precursor for functionalized abnormal NHC gallium complexes (Scheme 7).146 Addition of NaCH2SiMe3 to free IDipp led to the Scheme 7. Synthesis of Sodium Gallate NHC Adduct 66 and Reactivity with Electrophiles

Figure 9. Sodium NHC complexes 61−65 (L = [N(SiMe3)2]).

They were isolated via reaction of an equimolar amount of the group 1 and group 2 bis(TMS)amides in the presence of 2 equiv of IDipp in toluene.85 All three sodium NHC adducts show a narrow CNHC−Na bond distance range of 2.439(6)− 2.452(2) Å. In 2013, Hevia and co-workers isolated the sodium NHC complex 63 by addition of the sodium−zincate metalating agent [(TMEDA)NaZn(TMP)(tBu)2] to equimolar amounts of free IDipp.84 The normal C2-position of the NHC coordinates to sodium, while deprotonation in the abnormal C4 position and subsequent coordination to the zinc occurs. In addition to the coordination of the NHC, the sodium center is also coordinated by three THF molecules. The resonance of the C2 carbene carbon in the 13C NMR is shifted upfield to 201.4 ppm (vs 220.5 ppm in the free NHC), whereas the C4 carbene carbon is significantly shifted downfield to 159.4 ppm (vs 122.3 ppm in the free NHC). The CNHC−Na distance with 2.510(3) Å in 65 is slightly longer than that in the complexes 64a−c. The complex was used as a transmetallating agent to synthesize a dinuclear gold(I) NHC complex with gold being coordinated in the normal as well as the abnormal NHC position. The group also reported several other sodium and potassium NHC complexes from various metalating agents (e.g., [(TMEDA)NaMg(TMP)2(n-Bu)].144 In 2015, four years after the isolation of Hill’s bis(NHC) adducts 62a−c (Figure 9), Hevia et al. synthesized an analogous sodium bis(NHC) complex 62d with [Fe(HMDS)3]− as the counteranion instead of the earth alkaline metal-based anions.145 The CNHC−Na distances are essentially identical to those of the complexes 62a−c. Furthermore, the mono(NHC) adduct 64 was synthesized by addition of the sodium ferrate [NaFe(HMDS)2(CH2SiMe3)]n to free IDipp, and the heteroleptic ferrate 65 was obtained by addition of NaCH2SiMe3 to the iron NHC complex [(IDipp)Fe(HMDS)2] (Figure 9).145 The sodium center in both 64 and 65 exhibits a distorted-tetrahedral geometry via coordination to three THF molecules similar to 63. The CNHC−Na bond in 64 and 65 with 2.551(3) and 2.510(4) Å, respectively, is in the general range for sodium NHC complexes. The structure of 65 is quite similar to 63 and shows essentially the same Na−NHC distance.84 A similar complex with a magnesium cation

formation of completely insoluble solids. However, addition of Ga(CH2SiMe3)3 to the insoluble products resulted in the formation of the heteroleptic complex 66 similar to 65 and 63. The CNHC−Na bond distance (2.530(3) Å) is in good agreement with the related complexes. Similar to the previously discussed sodium complexes, the carbene carbon atom resonance in the 13C NMR is observed at 202.8 ppm. The same complex was also isolated with a coordinated potassium ion instead of the sodium cation (cf., section 2.1.3).146 Reaction of these types of bimetallic complexes with electrophiles leads selectively to the neutral abnormal gallium NHC complexes 67 with the electrophile adding exclusively to the C2 position of the NHC while the gallium−C4 bond is preserved. Recently, Westerhausen et al. reported two different sodium NHC adducts 68 and 69 (Figure 10) with a methylene-

Figure 10. Sodium NHC adducts 68 and 69 (L = [N(SiMe3)2]).

bridged bis(NHC) as the ligand starting from the corresponding bis(imidazolium) diiodide (or the free carbene) by addition of NaHMDS.92 The sterically not very demanding iso-propyl wingtip substituents result in the polymeric bis(NHC) adduct 68, where the sodium cation is coordinated by a THF molecule and connected to another sodium cation via two iodide bridges. Utilization of the significantly bulkier Dipp group instead of the iso-propyl substituent results in the dinuclear mono(NHC) adduct 69. While the reaction with LiHMDS leads to the chelating mononuclear complex 17 (Figure 6), the bite angle of the bis(NHC) ligand is too small 9689


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for the heavier alkali metals. Additionally, the bulky HMDS and Dipp groups hinder aggregation, resulting in the dinuclear complex. The CNHC−Na bond distances for 68 and 69 (2.500(2) and 2.577(2) Å, respectively) and NMR chemical shifts are in the regular range for sodium NHC complexes. Related polymeric and dinuclear complexes were also isolated with KHMDS in a similar fashion (vide infra).92 2.1.3. Potassium. Similar to sodium, only a handful of potassium NHC complexes have been reported so far. As mentioned earlier, Alder et al. reported the first potassium NHC complexes 70a and 70b (Figure 11) in 1999 by reaction

Figure 12. Various potassium NHC complexes 73−76 (L = [N(SiMe3)2]).

equiv of KH leads to the corresponding imidazolium alcoholate. Single-crystal X-ray analysis showed a polymeric structure comprised of a network of [K-NHC]4-tetramers where each potassium cation is coordinated to three alcoholate moieties and one carbene. Potassium−carbon bond distances are in the range of 2.984(5)−3.157(4) Å, which is similar to other potassium NHC complexes and, as expected, significantly longer than the related lithium complexes 25.108 The carbene carbon shows a resonance at 208.4 ppm in the 13C NMR spectra. Arnold et al. further reported the potassium NHC adducts of the amido-tethered abnormal carbene yttrium and samarium complexes 74 by reduction of the corresponding normal NHC complexes with potassium naphthalenide.148 The 13C NMR resonance for the potassium bound carbon in complex 74a can be observed at 199.2 ppm. SC-XRD analysis revealed a dimeric structure and a potassium−carbon bond distance of 2.954(2) Å. The potassium cation is further coordinated by one DME molecule, and an additional K−NHC interaction with the C4 carbon of the second NHC moiety (3.182(2) Å) is also present. The samarium complex 77b is isostructural to the yttrium complex. Shortly after, Danopoulos and co-workers reported the synthesis of indenyl- and fluorenyl-functionalized potassium NHC complexes 75a and 76a (Figure 12) via addition of 2 equiv of KHMDS to the corresponding imidazolium salt.149 13 C NMR data (206.9 ppm for 75a and 211.0 ppm for 76a) are consistent with the previously discussed potassium complexes. The structure of the fluorenyl-substituted complex 76a was determined crystallographically, which revealed a polymeric structure consisting of potassium ions bridged via fluorenyl units. The metal atom is sandwiched by two fluorenyl moieties as well as coordinated by one NHC. The CNHC−K bond length is slightly shorter (2.896(5) Å) than previously reported bond distances. After these initial results, they also varied the length of the linker between the indenyl substituent and the NHC. Furthermore, they introduced different substitution patterns on the NHC wingtip (R2 and R4, Figure 12) and the aromatic moieties (R1 and R3, Figure 12) of 75 and 76.150 These

Figure 11. Potassium NHC complexes 70−72 (L = [N(SiMe3)2]).

of equimolar amounts of the corresponding free NHC and KHMDS.76 While the IiPr2Me2 complex 70b was only characterized via NMR spectroscopy, complex 70a was characterized crystallographically. Single-crystal XRD studies revealed a potassium carbon bond distance of 3.00 Å. As expected, as compared to the related lithium complexes 7 (216.8 and 195.7 ppm, respectively) and sodium complexes 61a and 61b (221.3 and 196.4 ppm), a further downfield shift for the carbene carbon resonance in the 13C NMR in toluened8 is observed for 70a and 70b (226.7 and 201.1 ppm).76 As mentioned earlier, in addition to their work on lithium and sodium bis(NHC) complexes, Hill et al. also synthesized and characterized several analogous potassium bis(NHC) complexes [K(IDipp)2][M[N(SiMe3)2]3] (M = Mg, Ca, Sr, Ba) (71) (Figure 11).85 It was only possible to isolate the barium derivative 71d with potassium and not with lithium or sodium, possibly due to the more labile coordination chemistry associated with the larger alkali metal and alkaline earth metal centers. As expected, CNHC−K bond lengths (2.8210(2)− 2.8720(17) Å) are significantly longer than their lithium (2.150(4) Å) and sodium (2.441(2) Å) counterparts. Additionally, only in the potassium complexes was a significant η3interaction with the π-system of the ipso- and ortho-carbon centers of one Dipp-group observed. The dimeric potassium NHC complex 72 is the IDipp analouge of complexes 70 (Figure 11). It was formed during the synthesis of complex 71d (M = Ba) as a side product, further reinforcing the observation that these kinds of NHC complexes become increasingly labile with increasing atomic number of the metals. Compound 72 shows a potassium−carbon bond distance of 3.0291(17) Å, which is quite similar to that of complex 70a. In 2005, Arnold and co-workers reported the synthesis of potassium NHC complexes 73 with an alkoxide wingtip substituent (Figure 12).147 These complexes can be prepared in high yield by adding excess KH to the corresponding alcohol-functionalized imidazolium salt. Addition of only 1 9690


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215.1 ppm, and the carbanionic site can be observed at 176.3 ppm (vs 126.8 ppm for the protonated alkenic center). X-ray analysis revealed one-dimensional zigzag chains in which the ditopic anionic carbenes are bridged via potassium ions and hence two distinct carbene potassium bond distances are observed (2.812(2) and 2.905(2) Å). Unsurprisingly, the carbanionic contact is slightly shorter due to a greater electrostatic interaction between the formally anionic carbon and the potassium cation. The complex has been successfully employed in transmetalation reactions to yield abnormal NHC germanium, tin, and lead complexes (cf., section 2.4).126 Sadow et al. reported mixed NHC-bis(oxazolinyl)-borato potassium complexes 80 by deprotonation of the corresponding imidazolium salt with benzyl potassium (Scheme 8).153

complexes have been utilized as transmetallating agents for various transition metals, including titanium, vanadium, zirconium, chromium, rhodium, and iridium.149−152 Danopoulos et al. further reported potassium complexes of amino-substituted NHCs 77 and 78 (Figure 13) and their

Scheme 8. Structure of Complex 80 and Solvent-Dependent Isomerization of 81a

Figure 13. Potassium NHC complexes 77−79 with anionic backbone substituents.

tautomeric equilibrium with mesoionic imidazolium aminides.139 Addition of 1.2 equiv of benzyl potassium in THF to an imidazolium aminides precursor with relatively small groups on the amine backbone substituent (R1, Figure 13) leads to the potassium NHC complexes 77 where the potassium ion is coordinated to an NHC as well as to the anionic amine backbone substituent to form polymeric chains. 13 C NMR carbene carbon signals can be observed between 205.8 ppm (77c) and 208.6 ppm (77a), and the potassium− carbene bond distance is in the range from 2.924(2) Å (77b) to 3.021(2) Å (77a). If the bulkier Dipp substituent is used with 1.7 equiv of KBz, the copolymer complex 78 can be observed. The repetition-unit is comprised of two anionic NHCs with the amide backbone substituents being connected via a potassium ion as well as the NHC coordinated to another potassium ion. The latter potassium ion is also bound in a bridging η6-fashion to a benzylic group with two THF molecules completing the potassium coordination sphere. The carbene carbon signal in the 13C NMR can be observed at 206.1 ppm. The potassium carbene bond length (2.934(3) Å) is on the short end of the reported CNHC−K bond lengths, while the distance to the aromatic moiety is rather long (up to 3.284(5) Å).139 In addition to the previously mentioned lithium complex 55 of a backbone-substituted P-anionic NHC derivative, Streubel et al. also synthesized the analogous potassium complex 82 (Figure 13).140 The compound exists as a monomer in the solid state where the potassium ion is coordinated by two crown ether molecules (12-crown-4) as well as the NHC. The complex shows a potassium−NHC bond length of 3.066(6) Å.140 Goicoechea et al. isolated the potassium salt of a ditopic carbanionic carbene 45 via reaction of the lithiated precursor 34a with KOtBu at low temperature (Scheme 4).126 The carbene carbon resonance in the 13C NMR is observed at

The structure in the solid state is a polymeric chain of potassium cations bridged by anionic ligands, with each potassium ion coordinated to the ipso-carbon of the boronbound phenyl moiety, the NHC, and two oxazoline moieties (of different ligands). SC-XRD revealed a carbene potassium bond distance of 2.991(2) Å. The complexes have been utilized in transmetalation reactions to a variety of rhodium and iridium complexes.153 Related borate complexes have also been reported by Driess et al. They synthesized the potassium bis(NHC)borate complex 81a and its isomerization to the (NHC-imidazolyl)borate complex 81b (Scheme 8).154 When the corresponding imidazolium salt is deprotonated in THF with benzyl potassium, only complex 81a could be observed. However, deprotonation in toluene leads to the isomerization product 81b. It was shown that the isomerization between the two complexes is solvent-dependent: dissolution of 81b in THF slowly leads to complex 81a and vice versa for toluene. Singlecrystal XRD analysis revealed a dimeric structure for complex 81b where each potassium cation is coordinated by an NHC moiety, an imidazolyl nitrogen, an imidazolyl ring in η5-fashion, and one of the boron bound phenyl rings in η6-fashion. The CNHC−K distance for 81b is 2.877(2) Å. Kunz et al. reported the synthesis of a palladium(0) bis(NHC) complex 82 (Figure 14) with incorporated potassium ions via reduction of the corresponding Pd(II) complex with KC8.155 Complex 82 shows a dimeric structure in the solid state. The potassium cation is coordinated by the anionic nitrogen in the carbazolide moiety as well as to the 9691


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signal for the carbene carbon is shifted downfield to 210.7 ppm for 84 from 202.8 ppm for 66.146 Goicoechea et al. reported two potassium adducts 85 of the abnormal transition metal complexes of ditopic carbanionic carbenes.159,160 They were obtained by reduction of [M(IDipp)(Mes)2] (M = Fe, Mn) with 1 equiv of KC8. Both complexes show a very similar structure, except that in the case of M = Mn, an additional THF molecule is coordinated to the metal center, which leads to a distorted tetrahedral geometry. Potassium−carbon bond distances are 2.811(3) Å (M = Fe) and 2.844(3) Å (M = Mn). For both complexes, reactivity toward Lewis acids (AlEt3) in the presence of 2,2,2-crypt (to sequester the potassium ion) was investigated, which led to the AlEt3 complexes (cf., section 2.3.2). The magnesium analogue 85c was reported by Hevia and Mulvey et al. with an n-Bu substituent on the magnesium instead of a mesityl group. It was synthesized by addition of the potassium-magnesiate [KMg(TMP)2(n-Bu)]6 to IDipp.144 Already mentioned earlier, Westerhausen et al. recently reported a polymeric (86) as well as a dinuclear (87) potassium NHC complex (Figure 16) related to the sodium

Figure 14. Structure of the potassium−palladium(0) complex 82 (arrows and lone pairs are omitted for clarity).

carbene carbon atom (3.132(2) Å). Contacts between the Pd and K are also present. In 2015, Danopoulos and co-workers described the synthesis of potassium complexes 83 with a monodeprotonated dearomatized PNCNHC pincer-type ligand.99,156 Three different routes to the complexes with different deprotonation methods and sequences are outlined in Scheme 9. In the solid state, Scheme 9. Synthetic Routes to Complexes 83 with a Monodeprotonated PNCNHC Pincer-type Ligand

Figure 16. Polymeric methylene-bridged potassium bis(NHC) adduct 86 and dinuclear mono(NHC) adduct 87 (L = [N(SiMe3)2]).

complexes 68 and 69 (vide supra).92 Reaction of a methylenebridged bis-imidazolium iodide precursor with iso-propyl wingtips with 3 equiv of KN(SiMe3)2 leads to the polymeric complex 86, due to the low steric demand of the iPr group. While the sodium complex 68 has two bridging iodide anions, 86 is bridged via two N(TMS)2 groups. Again, introduction of the bulky Dipp group hinders aggregation and leads to the dinuclear complex 87. Carbene carbon 13C NMR shifts (211.6 and 217.3 ppm, respectively) as well as carbon−potassium bond lengths (3.121(3) and 2.986(2) Å) for the complexes are in the expected range. Mandal et al. recently reported a rare example of an aNHC potassium complex 88 (Figure 17), obtained by reaction of equimolar amounts of KHMDS and the corresponding free aNHC.161 The complex exhibits a 13C NMR resonance of the abnormal carbene at 197.2 ppm (upfield shifted from 201.9 ppm for the free aNHC) and potassium NHC bond length of 2.973 Å. It was successfully utilized in the ring-opening polymerization of ε-caprolactone. The first example of a potassium CAAC complex (89) was reported in 2015 by Turner et al. (Figure 17).162 It was obtained by reaction of the in situ generated CAAC ligand (from the iminium salt with equimolar amounts of KHMDS) with KHMDS and 0.5 equiv of [Sr(HMDS) 2] 2. The homoleptic complex contains a central potassium cation coordinated by three CAAC ligands with [Sr(HMDS)3]− as the anion. All three CAAC ligands are equivalent in the 1H NMR and the 13C NMR spectra and show a high frequency resonance at 270.6 ppm for the carbene carbon. SC-XRD

complex 83a shows a dimeric structure, connected via a η2coordination of the potassium ion to the 3- and 4-positions of the six-membered heterocycle. The potassium carbene−carbon bond distance is 2.901(4) Å. The carbene carbon in the 13C NMR spectrum can be observed at 208 ppm. The complexes 83 have been successfully utilized in transmetalation reactions to form chromium and cobalt complexes.157,158 The synthesis of alkali metal gallate NHC complexes by Hevia et al. has already been mentioned for the sodium complex 66; they also described the potassium equivalent 84 via the same route (Figure 15; for reactivity with electrophiles, see Scheme 7).146 Like in 66, the potassium ion is also coordinated by three THF molecules. The potassium−carbon bond length in 84 (2.902(3) Å) is in good agreement with similar complexes and, as expected, significantly longer than that of the sodium analogue 66 (2.530(3) Å). The 13C NMR

Figure 15. NHDC complexes 84−85. 9692


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increase in average bond length as well as a further downfield shift in the 13C NMR spectrum. 2.2. NHC Complexes of Group 2 Elements

In general, crystallographically characterized NHC complexes of the alkaline earth metals remain rather scarce, with the bulk of the reported complexes being concentrated on magnesium. In most cases, only the synthesis and characterization are reported, with applications only in a select few cases. In recent years, the Mg(I)Nacnac complex from Jones et al.163 found numerous applications in a variety of fields as a mild reducing agent. Because NHCs are well suited for the stabilization of low-valent main group element compounds, similar applications are conceivable.164,165 2.2.1. Beryllium. Because of the high toxicity of beryllium and its compounds,166 beryllium chemistry has unsurprisingly only received sporadic scientific interest. Nevertheless, some NHC-stabilized compounds have been isolated, characterized, and investigated in regard to reactivity in the last 20 years. The first beryllium NHC compound was reported in 1995 by Herrmann et al.167 Treatment of BeCl2 with 3 equiv of IMe2 resulted in the formation of the four-coordinate ionic Lewis adduct 91 (Figure 18). CNHC−Be bonds in 91 (1.807(3) and

Figure 17. aNHC potassium complex 88 and potassium CAAC complexes 89−90 (L = [N(SiMe3)2]).

revealed a carbene potassium bond distance of 2.998(2) Å, which is longer than the homoleptic bis(NHC) complexes 71 (2.8210(2)−2.8720(17) Å, Figure 11) from Hill et al.85 The only other potassium CAAC complex was reported recently by Mandal et al. Reaction of KHMDS with free CAAC yielded the dimeric complex 90.161 The two potassium centers are bridged by two hehamethyldisilazide ligands as well as coordinated to one CAAC ligand each. The CCAAC−K bond distance (2.9769(18) Å) is quite similar to Turner’s compound 89, whereas the 13C NMR spectrum shows a significantly downfield shifted carbene signal at 307.5 ppm. Interestingly, this resonance signal is shifted downfield from that of the free CAAC ligand (304.2 ppm). Like the related complex 88, 90 was also successfully used in the ring-opening polymerization of ε-caprolactone with a similar activity.161 No NHC complexes of rubidium or cesium have been reported so far. This is unsurprising, considering the already apparent instability of potassium complexes. This trend in decreasing stability of NHC adducts with increasing atomic number of the alkali metal has already been illustrated in the introduction of this section; however, it remains yet to be shown that, for example, CAACs cannot stabilize such complexes. As a concluding summary to this section, Table 2 lists the ranges and averages of both the alkali metal NHC bond

Figure 18. Various beryllium NHC complexes 91−95.

1.822(3) Å) are in the regular Be−C single bond range (∼1.71−1.85 Å),167 while the Be−Cl bonds are significantly elongated (2.091(7) Å). After this initial report, it took over 10 years until the first NHC complex of a diorganoberyllium compound was reported in 2006 by Gottfriedsen.168 Reaction of diphenylberyllium with 1 equiv of IiPr2Me2 yielded the corresponding complex 92. The compound exhibits a CNHC− Be bond length (1.807(4) Å) similar to that of 91. Two related similar compounds 93, obtained from dimethylberyllium and IDipp or IMes, were also reported recently by Hill et al.169,170 Here, it was observed that, upon addition of the silane PhSiH3 to 93, reduction to the dimeric organoberyllium hydrides 96 takes place (Scheme 10). Upon further reduction, unexpected insertion of beryllium into the C−N bond of the NHC is observed, leading to compounds 100 containing a six-membered ring. Similarly, at higher temperature, they observed the activation of both C−N bonds in the carbene unit of 93a leading to compound 94 (Figure 18).170 Theoretical calculations regarding the reaction mechanism of such a ring expansion comparing boranes, silanes, and beryllium hydride have also been carried out.171 The mechanism consists of (i) hydrogen atom migration to the carbene carbon, (ii) insertion of the beryllium-hydride into the

Table 2. CNHC−M Bond Length Ranges and Average and 13 C NMR Chemical Shift Ranges and Average of the Carbene Carbon Atom of Alkali Metal NHC Complexes CNHC−M [Å]

δ(13CNHC) [ppm]

M

range

average

range

average

Li Na K

2.056−2.531 2.439−2.577 2.810−3.157

2.163 2.482 2.945

181.3−221.1 196.4−220.7 199.2−217.3

200.5 207.6 209.6

lengths as well the 13C NMR shifts of the coordinating carbene carbon atoms. All reported compounds are included except for CAAC and aNHC complexes, as they generally give significantly different values, especially in regards to NMR data. From the compiled data, while there are outliers (especially for lithium complexes), a clear trend can be observed: an increase in atomic number is accompanied by an 9693


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Scheme 10. Reaction of 93 with PhSiH3 (Top) and “Dual Reduction” of an NHC (Bottom)

NHC ligands. The compounds were isolated via reduction of the corresponding (CAAC)BeCl2 adducts 99 with KC8 in the presence of one additional equivalent CAAC ligand. Without additional carbene, no defined product could be isolated. The beryllium atom in 100 exhibits linear coordination, while the CCAAC−Be bond lengths in 100 (1.664(2) and 1.659(4) Å, respectively) are significantly shorter than those of 99 (1.779(3) and 1.791(2) Å) and fall within the range of C− Be single bonds (∼1.71−1.85 Å)167 and CBe double bonds (1.58 Å).179 The C−Be−C bond is best described as a 3center-2-electron bond with partial double bond character, which explains the shortened bond length and linear coordination. 2.2.2. Magnesium. The first magnesium NHC adduct was reported 25 years ago, in 1993 by Arduengo et al. They were able to isolate the simple NHC diethylmagnesium adducts 101 via addition of IAd or IMes to diethylmagnesium (Figure 19).180 While the utilization of IMes afforded a dimeric

C−N bond, and (iii) a second hydrogen migration to the carbene carbon. Very recently, Braunschweig and co-workers reported the isolation of the beryllium bis(diazaborolyl) NHC adduct 95, which was synthesized from the corresponding bis(diazaborolyl) adduct via simple addition of 1 equiv of IMe2 to demonstrate the electrophilic nature of the beryllium atom.172 It exhibits a CNHC−Be bond length of 1.781(3) Å, which is slightly shorter than those of previously reported adducts. In 2012, Robinson et al. reported the NHC-stabilized beryllium borohydride 98,173 which was synthesized from the (IDipp)BeCl2 adduct via addition of 2 equiv of LiBH4. The NHC adduct shows a CNHC−Be bond length of 1.765(2) Å. Remarkably, the compound is stable for days in air, while other adducts and unstabilized beryllium borohydride are highly pyrophoric.174 It also shows an interesting reducing ability, being able to perform a “dual reduction” of the imidazole ring of an NHC via hydroboration of the CC bond as well as hydrogenation of the C2 carbon atom (Scheme 10).173 Low-valent beryllium species (i.e., Be(I) and Be(0)) have been theoretically investigated in several reports and are generally assumed to exhibit interesting reactivities.175−177 For example, it was suggested that three-coordinate Be(0) compounds could exist as free molecules utilizing NHCs as ligands176 or that they could be stabilized as ligands in transition-metal complexes.175 Indeed, shortly after these theoretical predictions were published, Braunschweig et al. reported the CAAC-stabilized Be(0) compounds 100 (Scheme 11).178 However, as opposed to the theoretical predictions, the beryllium center is only coordinated by two and not three

Figure 19. Various magnesium NHC complexes 101−104.

compound (with bridging Mg−CEt−Mg contacts) with a very high melting point (360−363 °C), application of IAd leads to a monomeric complex with a significantly lower melting point of 145−148 °C. Complex 101b exhibits a CNHC−Mg bond length of 2.279(3) Å. The related (IMes)MgCl2 adduct 102 was reported in 2009 as a THF adduct with a bond length of 2.200(2) Å.181 It was used as a latent precatalyst in the polyurethane synthesis.181 Five years after their initial result, Arduengo and co-workers also described the synthesis of decamethylmagnesocene IMe4 complex 103 along with related complexes with the heavier group 2 elements Ca, Sr, and Ba (cf., section 2.2.3).182 Interestingly, only one of the coordinated Cp* ligands shows a η5 coordination, while the other one exhibits a η3 coordination, most likely due to the steric crowding around the magnesium center. The CNHC−Mg bond length is 2.194(2) Å. In 2001, Schumann et al. also synthesized similar group 2 complexes 104 with varying magnesocene substituents and the bulkier carbene IiPr2Me2 (Figure 19).183 The CNHC−Mg bond length in 104a (2.2260(13) Å) is slightly longer than that in 103 due to the increased steric bulk of the carbene wingtip substituents. In 2004, Arnold and co-workers synthesized the first amidofunctionalized magnesium NHC complex 105 (Figure 20) via addition of MgMe2 to an amino-carbene ligand.98 Similar complexes utilizing alkoxy (107, 108)184 and aryloxy (109)185

Scheme 11. Synthesis of CAAC-Stabilized Be(0) Compounds 100

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Figure 20. Various magnesium NHC complexes (TMP = 2,2,6,6-tetramethylpiperidine; L = [N(SiMe3)2]).

substituents on the NHC wingtips as well as related tridentate binuclear complexes (106)186 have also been reported. Additionally, lithium NHC complexes with the same ligand frameworks were isolated (vide supra). The CNHC−Mg bond length in 105 is 2.263(2) Å, while the carbene−magnesium bond in the binuclear complex 106a is slightly shorter (2.180(2) Å). Complexes 107 and 108 have been employed as catalysts in the polymerization of lactides; however, the dimeric compound 107a appears to be much more active with 98% monomer conversion as compared to 27% for the monomeric compound 108.184 Complex 110a, reported by Hill et al., was obtained by reaction of the IDipp carbene with Mg(HMDS)2.187 Similar compounds have also been reported shortly after by Mulvey et al.188 Addition of free carbene to alkyl magnesium amides results in the formation of complexes 111. In addition, Mulvey and co-workers were also able to isolate an NHC-stabilized dimeric complex connected by chloride bridges (112a) from Grignard reagents and a tetranuclear magnesium complex supported by NHC ligands with bridging butyl chains (113) from n-Bu2Mg (Figure 20). They also reported NHC-induced monomerization of polymeric dialkylmagnesium compounds leading to complexes 114.189,190 Different derivatives of complex 110a with IEt2Me287 and ItBu191 instead of IDipp were also reported (110b,c). The ItBu derivative 110c was successfully employed in a catalytic C−N bond formation reaction.191 Additionally, derivatives of compound 112a with different bridging halides and halides as terminal Mg ligands have been synthesized (112b,c).192 The NHC-stabilized organomagnesium amide complexes 115 were isolated by a salt elimination reaction of the corresponding organomagnesium bromide NHC complex with LiHMDS.193 The complexes were utilized as catalysts in the dehydrocoupling of organosilanes with amines. The tris-NHC-borate complex 116 has been synthesized via addition of 3 equiv of MeMgBr to the imidazolium bromide salt precursor and was subsequently employed as a transmetalation agent to synthesize the corresponding four coordinate Fe(II) complex.194 The magnesium−hydride cluster 117 has been reported in 2009 by Hill et al. (Scheme 12).187 It was synthesized by conversion of the Mg-bis(N(TMS)2) IDipp complex 110a with excess PhSiH3. It consists of an adamantane-like Mg4H6 core with two coordinating NHCs as well as two N(SiMe3)2

Scheme 12. Synthesis of the Mg−Hydride Cluster 117

ligands. The compound shows a higher hydrogen/magnesium ratio (1:1.5) than any other complex reported so far and is resistant to further Si−H/Mg−N metathesis reactions. In 2009, Roesky and Stalke et al. could isolate the first magnesium−alkynyl NHC complex 118 (Figure 21).89 The

Figure 21. Mg−alkynyl NHC complex 118 and N-methylbenzimidazole complex 119.

Mg centers are coordinated by an NHC moiety as well as two alkynyl ligands in an end-on fashion. Additionally, one of the alkynyl ligands is coordinated side-on to the other magnesium atom. The heavier Ca and Sr analogues were also isolated. A dimeric benzimidazole magnesium complex has been described in 2014 by direct magnesiation of N-methylbenzimidazole with a butylmagnesiate reagent supported by a chelating silylbisamido ligand (119).195 The CNHC−Mg bond length is in the range of regular Mg−NHC bonds (2.233(3) Å). NHCs have also been employed in Grignard reactions to increase the nucleophilicity of the Grignard reagent.196 While the increase in nucleophilicity was ascribed to the formation of a Mg−NHC complex, only a tentative catalytic cycle without structure confirmation of the complex could be given. In addition to the regular magnesium−NHC compounds, abnormal NHC magnesium complexes have been reported 9695


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more recently, with the first example being published in 2015 by Hevia and Mulvey et al.144 Addition of different K/Mg or Na/Mg heterobimetallic (e.g., [KMg(TMP)2(n-Bu)]6 or [(TMEDA)NaMg(TMP)2(n-Bu)]) bases to IDipp led to the formation of two or three coordinate aNHC complexes 120 and 121 (Figure 22). The magnesiation in the C4 position of

reduction with KC8.197 The CAAC activation pathway most likely proceeds via the formation of (CAAC)2Mg(0), followed by ligand dissociation/rearrangement to a cyclobutane with an exocyclic imine group. Subsequent C−C oxidative addition of the magnesium into the cyclobutane ring forms 125. The complex shows a 13C NMR resonance at 314.3 ppm and a long CNHC−Mg bond distance of 2.3158(13) Å. 2.2.3. Calcium, Strontium, and Barium. Similar to the first magnesium adducts, the first NHC complexes of the heavier group 2 elements calcium, strontium, and barium were reported by Arduengo et al. in 1998.182 Addition of free IMe4 to the heavier alkaline earth decamethylmetallocenes Cp*2M (M = Ca, Sr, Ba) leads to the same adducts 126 (Figure 24) as

Figure 22. Abnormal NHC magnesium complexes 124−127.

the imidazole moiety was only possible through alkali-metal mediation. In the 13C NMR, the C4 carbon resonance is shifted downfield to 156.4 ppm (120) and 161.0 ppm (121) from 122.5 ppm in free IDipp. Potassium complex 120 shows an infinite zigzag chain structure with bridging potassium ions between regular NHC moieties and bridging magnesium ions between abnormal NHC positions. Shortly after, Ghadwal and co-workers also reported the synthesis of an abnormal magnesium NHC complex.192 By protecting the C2 position of the imidazolium moiety with a phenyl ring, they could isolate the aNHC complex 122 via utilization of MeMgI as the base. Iodine ligand exchange to [N(SiMe3)2]− using KHMDS was also achieved. Compound 123 shows a magnesium carbene bond length of 2.165(3) Å. CAAC magnesium complexes 124 have been reported in 2015 (Figure 23).162 They were obtained by reaction of the

Figure 24. Decamethylmetallocene IMe4 (126) and bis(IMe4) (127) complexes as well as IiPr2Me2 derivatives (128).

previously discussed for magnesium (103, Figure 19), with the key difference being that both Cp* rings coordinate in a η5fashion for the heavier group 2 metals. As expected, with increasing size of the alkaline earth metal center, the CNHC−M bond increases in length. The same trend can also be seen in the 13C NMR spectra of these compounds: with increasing atomic number of the metal center, a downfield shift of the carbene carbon (C2) resonance is observed (Table 3). With that, the NHC−M bond can be described as quite covalent for magnesium, while the NHC−Ba bond is rather ionic.182 Table 3. CNHC−M Bond Lengths of the Alkaline Earth Metallocene NHC Complexes and Their Carbene Carbon Atom 13C NMR Chemical Shift182

Figure 23. CAAC magnesium complexes 128−129.

free CAAC ligand (generated in situ by deprotonation of the corresponding cyclic iminium salt with KHMDS) with 0.5 equiv of [Mg(HMDS)2]2. CCAAC−Mg bond lengths are quite long (2.2931(12) Å for 124a and 2.2989(12) Å for 124b). Complex 124a shows a carbene carbon resonance in the 13C NMR spectrum at 266.6 ppm. In addition to the potassium CAAC complex 89,162 Turner and co-workers reported ligand rearrangement of CAACs to CAAC-stabilized metallocyclopentane 125 (Figure 23), realized by addition of MgCl2 to free CAAC followed by

compound

d(C−M) [Å]

δ(13C) [ppm]

MgCp*2(IMe4) (103) CaCp*2(IMe4) (126a) SrCp*2(IMe4) (126b) BaCp*2(IMe4) (126c) free IMe4

2.194(2) 2.562(2) 2.861(5) 2.951(3)

185.7 196.2 198.2 203.5 213.7

Because of the increased size, Sr and Ba are also able to form bis(IMe4) adducts 127 (Figure 24).182 These types of complexes are isostructural to bent metallocene complexes. Similar to the previously discussed magnesium complexes 104 (Figure 19), Schumann et al. also synthesized calcium, strontium, and barium analogues 128 with different metallocene substituents and the bulkier carbene IiPr2Me2.183 Hill and co-workers reported the synthesis of NHC adducts of heavier group 2 bis(TMS)amides, either through addition of 9696


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of the corresponding imidazolium salt with KHMDS in the presence of MI2 (M = Ca, Sr). In addition, the dinuclear dimeric complex 134 and the mononuclear tetracoordinated complexes 133 were successfully isolated. The calcium complex 132a shows a 13C NMR carbene resonance at 195 ppm and a C−Ca bond length of 2.583(3) Å. In comparison, the Sr complex 132c is downfield shifted to 201.5 ppm and shows a bond length of 2.739(3) Å. The coordination sphere of the metal is completed by coordinated THF.200 The tris(NHC)borate complexes 135 were also synthesized by deprotonation of the imidazolium salt in the presence of MI2 (M = Ca, Sr, Ba).201 In contrast to the bis(NHC) complexes 132, where the coordination sphere is completed by solvent molecules, the coordination sphere for the heavier tris(NHC) complexes 135 is completed by tBu-imidazole, which stems from the fragmentation of the ligand precursor during the reaction. The complexes show metal carbene distances and 13C NMR shifts similar to those of the related complexes.201 The group also isolated and characterized the dimeric 136 and the hexacoordinate complex 137. Very recently, Westerhausen et al. isolated calcium complexes with a chelating CCC202 (138) and CNC203 (139) ligand system as well as the bis-coordinated complexes 140 and 141 (Figure 25). Complexes 139c,d with the heavier metals strontium and barium have also been isolated. Directed ortho-metalation of 1,3-bis(NHC)-benzene with the heavy Grignard reagent Me3SiCH2CaX (X = Br, I) resulted in the calcium complexes 138a,b with the chelating CCC ligand.202 The bis-coordinated complex 140 was obtained by calciation of the ligand with Ca(CH2SiMe3)2. The 13C NMR signals are upfield shifted from the free NHC (213.2 ppm) to 197.3 ppm (138a), 197.1 ppm (138b), and 199.9 ppm (139a). CNHC−Ca bond lengths (2.591(7)−2.661(4) Å) are in the typical range for calcium carbene complexes. Complexes 139 with the CNC ligand system were synthesized by addition of [(THF)5CaI][BPh4] or MI2 (M = Sr, Ba) to the free ligand, while addition of CaI2 resulted in the formation of the bis-coordinated complex 141.203 Because of the insolubility of these complexes, the soluble complex 139b was also prepared, either by exchange of the iodide anions with KNPh2 or by direct

the group 2 amides to an imidazolium salt or through addition of free carbene to the amides (Scheme 13).198 Addition of 2 Scheme 13. Ca, Sr, and Ba NHC Adducts 129−131

equiv of [M(HMDS)2] (M = Ca, Sr, Ba) to the imidazolium salt (R = Mes) leads to the NHC bis(amide) complexes 129a−c. On the other hand, addition of 1 equiv of Ca(HMDS)2 to free IDipp leads to the related complex 129d. Additionally, the (IMes)CaCl(N(SiMe3)2) complex 130 by addition of 1 equiv of Ca(HMDS)2 to the corresponding imidazolium salt was reported. As expected, CNHC−M bond lengths elongate with increasing atomic number of the group 2 metal (2.598(2) Å (129a), 2.731(3) Å (129b), 2.915(4) Å (129c)). The Dipp-substituted complex 129d has a bond length similar to that of the related mesityl-substituted complex. NMR data show the same trend observed for the complexes 126. Herrmann et al. described the related bis(NHC) bis(HMDS) adducts 131; however, no structural parameters were given.199 The authors described decreasing stability of the complexes with increasing atomic number of the metal, with the barium complex only being stable in solution. Hill et al. reported the bis(NHC)borate and tris(NHC)borate complexes 132200 and 135,201 respectively (Figure 25). The bis(NHC) complexes were synthesized by deprotonation

Figure 25. Various Ca, Sr, and Ba complexes 132−141 (lone pairs and arrows for compounds 135−137 are omitted for clarity). 9697


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compounds in all of their possible oxidation states. Among the NHC-stabilized triel-complexes, boron clearly dominates the field, to a lesser extent followed by aluminum and gallium. There are only a few reports concerning indium and thallium complexes. In this section, we provide a comprehensive coverage of the development and application aspects of various NHC-supported stable group 13 complexes isolated so far. 2.3.1. Boron. 2.3.1.1. NHC-Supported Boron(III) Compounds. There are plenty of reported remarkably stable, neutral NHC−borane adducts, containing boron in the +3 oxidation state. Because of their rich chemistry, they found various applications as reagents and catalysts in organic synthesis. The complementary nature of the species, where the NHC acts as a strong Lewis base and borane as a Lewis acceptor, facilitates the formation of stable adducts, analogues to other Lewis acid−base complexes of boranes, such as ether, amine, sulfide, or phosphine adducts. However, the exceptionally strong σ-donating nature of NHCs confers their adducts a way higher stability as compared to other ligands. Moreover, most of the NHC−borane complexes can be easily synthesized in multigram scale and have profound solubility in common organic solvents. Some of those complexes found diverse applications, such as co-initiators for radical polymerization and stoichiometric reducing agents for various aryl, alkyl, or alkenyl halides, triflates, and carbonyl compounds. In 2011, the group of Curran published a comprehensive review regarding the synthetic application and the prospects of NHC−borane chemistry.204 Herein, we present a brief, yet concise, account, regarding the general synthetic approach to NHC-stabilized borane complexes and their role as catalytic and stoichiometric reagents in various organic transformations, up to this date. Oxazolidin-2-ylidene triphenylborane 146 is the first isolated carbene-borane complex, synthesized in 1968 by Bittner and co-workers (Scheme 14).205 This complex was obtained from the reaction of the in situ generated isonitrile-triphenylborane complex 144 with the lithium base and acetone, to produce the borate anion 145.

reaction with Ca(NPh2)2. Carbene calcium bond lengths are similar to those of the related CCC complexes. As expected, bond lengths for the heavier group 2 complexes 139c,d are significantly longer (2.779(3) Å (M = Sr) and 2.925(4) Å (M = Ba)).203 As was already discussed in the magnesium section, Turner et al. also reported strontium and barium CAAC complexes 142,143 in 2015 (Figure 26).162 They were synthesized by

Figure 26. CAAC complexes 142 and 143 of strontium and barium.

reaction of the free CAAC ligand (generated in situ by deprotonation of the corresponding cyclic iminium salt with KHMDS) with 0.5 equiv of [Sr(HMDS) 2 ] 2 or Ba(HMDS)2(THF). The attempted isolation of the related calcium complex resulted in decomposition of the ligand. As expected, the CNHC−Ba (3.1209(17) Å) bond length is quite long, and with increasing atomic number of the alkaline earth metal a further downfield shift of the carbene resonance in the 13C NMR is observed (266.6 ppm (M = Mg), 283.8 ppm (M = Sr), 303.0 ppm (M = Ba)). The complexes have been investigated as catalysts in ring-opening polymerization reactions.162 As a final note in this section, a compilation of bond lengths and 13C NMR data of the coordinating carbene carbon and the corresponding averages in alkaline earth metal NHC complexes is given in Table 4. Because of their in general Table 4. CNHC−M Bond Length Ranges and Average and 13 C NMR Chemical Shift Ranges and Average of the Carbene Carbon Atom of Alkaline Earth Metal NHC Complexes CNHC−M [Å]

δ(13CNHC) [ppm]

M

range

average

range

average

Li Na K

2.056−2.531 2.439−2.577 2.810−3.157

2.163 2.482 2.945

181.3−221.1 196.4−220.7 199.2−217.3

200.5 207.6 209.6

Scheme 14. Synthesis of the First Carbene−Borane Complex 146

significant deviation in analytical data, CAAC and aNHC complexes are not included in this summary. Similar to the trend observed for group 1 complexes, an increase in atomic number is also reflected in an increase in bond length and a downfield shift for the carbene carbon atom in the 13C NMR. 2.3. NHC Complexes of Group 13 Elements

The strong Lewis acidic nature of group 13 element compounds emanates from the unfilled octet, and makes them amenable to form strong coordination complexes with σdonating ligands, such as amines, phosphines, and carbenes. Otherwise they exist in their dimeric or polymeric forms, to mitigate their electron deficiency. NHCs feature the strongest σ-donor properties among the aforementioned electrondonating ligands, as well as π-accepting character, depending on the nature of carbenes. They play a pivotal role in the stabilization of highly reactive and unstable group 13 molecular

Upon aqueous workup, 145 gets protonated to form the final triphenyl borane complex 146 (Scheme 14). In addition, benzo[d]oxazol-2-ylidene adducts with triphenyl borane (147) were also isolated by several groups using direct reaction of the corresponding lithiated oxazole with BPh3.206 Among the trivalent NHC−borane adducts, the parent borane complexes (NHC)BH3 represent the largest family. It 9698


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slightly downfield shifted (−27 to −30 ppm) as compared to the corresponding NHC complexes. Theoretical calculations by Rablen et al. showed a dramatic reduction of the B−H bond dissociation energy (74−80 kcal/ mol) for the parent borane−NHC adducts, as compared to the free BH3 (106.6 kcal/mol) and corresponding phosphine or amine congeners (94−102 kcal/mol).210 This qualifies (NHC)BH3 complexes as potentially better hydride donors and reducing agents. Consequently, NHC−borane complexes are proven to be fascinating reagents and catalysts in several organic transformations over the years. The starting point was the use of NHC−borane reagents in the radical chain reduction of xanthates and related compounds. The easy accessibility and air and moisture stability of (IDipp)BH3 (150b) and the triazolylidene complex 154 (Figure 27) make them viable reagents. Reactions proceed in the presence of an excess of radical initiators to produce deoxygenated products through the signature radical rearrangement reactions.211,212 The mechanism of these transformations follows the initial addition of an in situ generated boryl radical (NHC)BH2• to xanthates ROC(S)SR′ and subsequent fragmentation of the resulting radical to an alkyl radical R•, which eventually abstracts hydrogen furnishing the reduced product RH. The second generation NHC−borane reagents, such as 148a and 153 (Figure 27), are better hydrogen donors than 150b because their radicals are less persistent. Consequently, more efficient reductions of sugar-derived xanthates213 and alkyl halides containing nearby electronwithdrawing groups214 can be realized with lower loadings of initiator additives, such as AIBN, Et3B, or di-tert-butylperoxide. The additional advantage of these reduction reactions is that the boron-containing byproducts can be easily removed using flash chromatography. NHC−borane complexes do not reduce alkyl, aryl, or alkenyl halides and triflates. However, in the presence of transition metal catalyst, such reduction reactions can be achieved. For instance, heating of substituted aryl iodides or triflates in the presence NHC−borane 154 and the Pd(II) catalyst directly led to the isolation of the corresponding reduction products in excellent yields (Scheme 15a).215 Selective production of phenyl triflate as sole component, achieved by reduction of 4-iodophenyl triflate, clearly reflects the chemoselective nature of this reduction reaction for iodoover triflyl substituents.

all started with the successful isolation of the (IMe4)BH3 (148b) adduct by Kuhn and co-workers in 1993 from the reaction of the NHC with BH3·SMe2 (Figure 27).207 Analogously, they also obtained the stable adduct (IMe4)BF3, using BF3·OEt2 as precursor.

Figure 27. Examples of parent NHC−borane complexes.

Following this seminal work, several other stable NHC− borane complexes were documented. They are mostly synthesized in a multigram scale directly from reactions of free or in situ generated NHCs with various borane sources, such as LiBH4, NaBH4, or Lewis acid/base adduct of boranes (amines, phosphines, or sulfides) through MH (M = Li, Na) elimination or by Lewis-base exchange reactions, respectively. Some of these notable complexes (148a,c, 149−155) are depicted in Figure 27.204 Interestingly, they are thermally quite stable and do not release free borane (BH3) even under moderately elevated temperature (120 °C). Furthermore, they are easy to handle and also do not show tendencies to hydroborate the unsaturated carbene backbone, despite the strong hydridic nature of the B−H bond and the delocalized positive charge that resides on the carbene fragment. Shielded by relatively sterically demanding carbenes, the parent borane complexes are even air and moisture stable. Formation of the (Me2CAAC)BH3 complex (156a) has only been demonstrated through the in situ spectroscopic measurements, but it has not been isolated so far, presumably due to the side carbene insertion reaction into B−H bond upon complex formation. In contrast, the corresponding (Me2CAAC)BF3 complex (157) is quite stable and was structurally characterized.208 The use of the bulkier CyCAAC allowed one to synthesize the parent borane complex (CyCAAC)BH3 (156b), without observing the formation of even a trace amount of B−H bond activation product.209 The adduct 156b was isolated in 84% yield and structurally characterized. The 11B NMR resonance signals of all currently known (NHC)BH3 complexes are observable in the range from −32 to −40 ppm.204 The (CAAC)BH3 signal has been found

Scheme 15. Reduction of Aryl Halides or Triflates and Arylation Using NHC−Borane Adducts

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Curran and co-workers also revealed the ability of the triphenyl borane complex (IDipp)BPh3 (158) as an effective phenyl group transfer reagent in palladium-catalyzed Suzuki− Miyaura cross-coupling reactions with aryl halides or triflates (Scheme 15b).216 Expanding the scope of NHC−borane compounds as reducing agents, the groups of Lindsay and McArthur were able to carry out the asymmetric reduction of acetophenone to (S)-1-phenylethanol using chiral complexes 155 (Figure 27) in the presence of Sc(OTf)3 as Lewis acid catalyst.217 The enantioselectivity in this case was moderate (42−50% ee, yields 92−95%). Employing a similar but relatively bulkier chiral 9-BBN-derived complex, coupled with a less sterically hindered Lewis acid additive BF3·OEt2, they were able to improve the enantioselectivity of acetophenone reduction up to 85% ee. In addition to the application as reducing agents, NHC− boranes were also utilized as suitable reagents for hydroboration of arynes,218,219 alkenes, and alkynes.220 Kawamoto et al. have shown that (IMe2)BH3 acts as a hydrohen donor in hydroxymethylation reactions of alkyl and aryl iodides, using carbon monoxide under UV irradiation or catalytic amounts of AIBN, respectively.221 These reactions are highly chemoselective and only take place at aryl/alkyl−iodine bonds without effecting the bromide or chloride substituents. B−H bonds of electron-rich NHC-boranes were found to act as two electron donors forming metal complexes. Braunschweig and co-workers have reported transition metal complexes [CpMn(CO)2(η1-H3B(IMe2))] (159) and [M(CO)5(η1-H3B(IMe2)] (M = Cr, Mo, W) (160a−c), where (IMe2)BH3 (148a) behaves as an η1-ligand (Figure 28).222

Scheme 16. Synthesis and Reactivity of Hydrido(hydroborylene) Complexes

be best described by the combination of two bonding interactions, a W−B σ-bond and a W−H−B three-center two-electron bond. Furthermore, hydrido(hydroborylene) tungsten complexes act as hydroboration reagents with terminal alkynes, affording the NHC-stabilized η3-boraallyl complexes, for example, Cp*(CO) 2 W[η 3-PhHCCHBH(NHC)] 163 (δ(11B) = −9.5 and −8.9 ppm). Most of the NHC−borane complexes possess pKa values less than two and the B−H bonds with a highly hydridic nature. They show a natural tendency to undergo acid/base reactions or nucleophilic substitution at the boron center. Exploiting the hydridic nature of such B−H bonds, Curran et al. have successfully synthesized a series of mono- and disubstituted boranes of the general formula (IDipp)BH2X 164a−f (δ(11B) = −8 to −23 ppm), and (IDipp)BH(Nu′)2 167a−c, employing 150b as borane source using three complementary routes: (i) reactions with various alkyl electrophiles either via ionic substitution reactions or via radical chain reactions, (ii) Brønsted acid/base reactions with HX (Scheme 17), and (iii) direct substitution at the tetracoordinate boron center in the presence of various nucleophiles at elevated temperature (Scheme 18b).224

Figure 28. Examples of (NHC)BH3 transition metal carbonyl complexes.

These products were obtained upon irradiation of the metal carbonyls in the presence of (IMe2)BH3. For each complex, the 11 B NMR spectra show a quartet, which appears in the range from −42 to −47 ppm. Solid-state structures of all four compounds revealed the boranes coordinating to the metal center in a B−H−M fashion via three-center two-electron bonds. Interestingly, due to the negligible π-back-bonding from the metal to BH3, borane in 159 can be exchanged by stronger binding ligands. Recently, the group of Tobita showed that the reaction of boranes (NHC)BH3 (NHC = IMe4, IiPr2Me2) with the methyl(pyridine)tungsten complex Cp*(CO)2W(py)Me furnishes the NHC-stabilized hydrido(hydroborylene)tungsten complexes Cp*(CO)2W(H)[BH(NHC)] 162a,b (δ(11B) = 74.0 and 74.4 ppm). The complexes were obtained as dark brown crystals in high yields after the liberation of methane and the coordinated pyridine ligand (Scheme 16).223 SC-XRD analysis revealed that both complexes contain a hydride-bridge between tungsten and boron with the W−B bond distances (2.174(4) Å) shorter than W−B single bond lengths (2.47 Å). The bonding in the W−B−H three-membered ring of 162 can

Scheme 17. Syntheses of Monosubstituted Borane Complexes

Adduct 150b also reacts with several halogenating reagents, such as N-bromosuccinimide (NBS), N-iodosuccinimide (NIS), bromine, and iodine, to form NHC-supported halogen-substituted boranes.224 The reaction with equimolar amounts of bromine is not selective and produces a mixture of mono-, di-, and tribromo-substituted derivatives, while the reaction with 0.5 equiv of iodine in benzene proceeds selectively and smoothly at room temperature to produce only the monoiodo-substituted product (IDipp)BH2I 164c. Despite the formal negative charge being located on the boron 9700


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fluoride (δ(11B) = 1.8 ppm), azide, and cyanide (δ(11B) = −36.7 ppm) substituents were obtained by one-pot synthesis from the in situ generated ditriflate 166 and corresponding nucleophiles (Scheme 18b).224 NHC aryldifluoroboranes (NHC)BF2Ar (168) can be obtained by fluorination of NHC−arylboranes (NHC)BH2Ar with 2 equiv of 1-chloromethyl-4-fluoro-1,4diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (Selectfluor). These NHC aryldifluoroboranes found applications in Suzuki coupling reactions where they act as an aryl group transfer agent toward various aryl halides.229 Apart from the role as stoichiometric reagents, (NHC)BH3 complexes can act as catalysts in various organic transformations. In the organocatalysis area, Hoveyda et al. have shown that reactions of a variety of cyclic and acyclic α,βunsaturated ketones and esters with bis(pinacolato)diboron B2(pin)2 in the presence of an in situ generated NHC (2.5−10 mol %) provide respective 1,4-adducts as single diastereomers. They have described the plausible involvement of the NHC− diborane adduct as the reaction intermediate in the catalytic cycle.230 Recently, the goup of Curran has reported that in situformed monosubstituted NHC−boryl iodide (IDipp)BH2I (164c) efficiently catalyzes the reaction of NHC−borane and diazoesters to produce a number of stable α-NHC−boryl esters.231 As far as NHC−alkylborane complexes are considered, in 2004 Yamaguchi et al. led the way to the first serendipitous synthesis of NHC-coordinated BEt3 as yellow-colored complexes (IiPr2Me2)BEt3 169a and (IMes)BEt3 169b, by treating the corresponding imidazolium salts with superhydride LiBEt3H.232 The elongated C−B bond distances in these adducts (169a, 1.683(5) Å and 169b, 1.678(6) Å), as compared to those of the parent borane (1.596(4), 1.603(3) Å) and trifluoroborane (1.635(5), 1.6669(6) Å) adduct of the same NHCs, suggest weaker coordination of the trialkyl borane. These adducts act as carbene transfer agent toward both BH3 and BF3. Additionally, they undergo facile reaction with molybdenum hexacarbonyl Mo(CO)6 in refluxing toluene to furnish corresponding (NHC)Mo(CO)5 complexes as pale yellow solids in a moderate yield (67%). Later, various other stable alkyl and aryl complexes 170−178 were isolated by

Scheme 18. Nucleophilic Substitution Reactions at the Boron Center

center of monohalo-, triflyl-, or sulfonyl-substituted boranes, they have proven to be suitable precursors for further nucleophilic substitution reactions. Therefore, a diverse range of monosubstituted NHC−borane complexes of the general formula (IDipp)BH2Nu 165a−j (δ(11B) = −6 to −37 ppm), including fluorides, chlorides, cyanides, azides, isonitriles, isocyanates, nitro compounds, nitrous esters, and other derivatives, was synthesized by employing reactions of monosubstituted derivatives 164c−e (Scheme 18a).224 Among the monosubstituted complexes, NHC−boryl sulfides show the tendency to cleave homolytically, furnishing NHC−boryl or NHC−thioboryl radicals, which makes them good type I photopolymerization initiators for the polymerization of acrylates under air.225,226 Additionally, boryl sulfides and N-borylthioamides were found to serve as neutral sources of sulfur nucleophiles and react with several alkyl or acyl halides to form thioethers or thioesters in high yields.227 Thermal 1,3-dipolar cycloaddition reactions of the boryl azide (IDipp)BH2N3 (165d) with alkynes, nitriles, and alkenes led to respective NHC-boryl-substituted triazoles, tetrazoles, and triazolidines in excellent yields.228 Most of the monosubstituted NHC−boranes are resistant to further acid−base reactions. However, an exception is the reaction of (IDipp)BH3 (150b) with 2 equiv of triflic acid, in which the borane undergoes rapid and clean conversion to the bis-triflyl product (IDipp)BH(OTf)2 (166 (δ(11B) = −2.5 ppm).224 Labile triflate groups in 166 can be easily substituted by a nucleophile providing an access to various doublesubstituted boranes. In fact, such complexes 167a−c with

Figure 29. Examples of NHC-stabilized alkyl- and arylboranes. 9701


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employing both mono and chelating bis(carbenes) (Figure 29).117,233−236 The neutral, doubly base-stabilized, aryl-substituted diborane complex 175 (Figure 29)233 was obtained upon Cp*2Comediated reduction of the corresponding borenium cation. Complex 175 showed some interesting reactivities and undergoes facile reactions with TEMPO (PhC(O)O)2, PhNO, (THT)AuCl (THT = tetrahydrothiophene)) and S8 leading to the formation of corresponding adducts through the homolytic cleavage of the B−B bond. Borenium cations (see the next section) generated in situ from the complexes 176−178 by hydride abstraction using [Ph3C][B(C6F5)3] were found to catalyze asymmetric hydrogenation of imines, following a FLP-type reaction mechanism. However, afforded enantioselectivities were low.234 Very recently, Xie et al. reported the adduct 179, obtained by the treatment of NHC-stabilized cyclic amino(carboranyl) silylene 697 (cf., section 2.4.2.3) with benzophenone. The cycloaddition reaction affords the NHC-coordinated cageopening product 179 in 59% yield.237 The 11B NMR spectrum of 179 displays a broad resonance at 29.3 ppm for the unique bridging boron nucleus. In 2006, Stephan introduced the concept of “frustrated Lewis pairs” (FLPs), in which the two Lewis acid/base components cannot form a covalent dative bond due to large steric encumbrance. FLPs were found to reversibly activate dihydrogen molecules. This seminal discovery has prompted a great deal of interest in the FLP chemistry.238 In line with this concept, various FLP combinations were explored for stoichiometric and catalytic activation of otherwise inert chemical bonds.239,240 With NHCs being strong Lewis base donors, a variety of combinations with several bulky Lewis acidic boranes and alanes is extensively used. Among them, the combination of ItBu as Lewis base and B(C6F5)3 as Lewis acid leads to cooperative activation of dihydrogen,241,242 ring opening of THF,242 N−H bond activation of ammonia, aniline, diphenyl amine,241 and dehydrogenation of saturated alkane backbone parts of carbenes243 and germanes to generate germylenes in a single step.244,245 Similar to the discussed (NHC)BH3 complexes, the (NHC)BX3 and (NHC)BRX2 (X = Cl, Br; R = Ar, Alk, etc.) adducts can also be obtained from the one-pot 1:1 stoichiometric reactions of the two Lewis acid/base components in appropriate solvents. For convenience, all of those complexes are discussed later in the relevant sections. Apart from the parent borane, alkyl-, aryl-, or haloborane complexes, NHCs also have been found to stabilize some unusual boron compounds. Herberich et al. have described the first borabenzene adduct 180 (11B = 23.9 ppm) as a colorless compound, which was readily accessible through the IMe4 triggered 1,2-elimination of Me3SiCl from the 1-chloro-3,5dimethyl-2-(trimethylsilyl)-1,2-dihydroborinine precursor (Figure 30).246 The solid-state structure of 180 (CNHC−B bond distance 1.596(2) Å) revealed considerable deviation from interplanarity (34.75(6)°) for the planar borabenzene and NHC rings, assumably enforced by a repulsive interaction between the N-Me groups and the α-CH groups of the borabenzene. Piers et al. characterized SIMes-stabilized 9-boraanthracenes 181. Notably, 181 shows a considerably smaller HOMO− LUMO energy gap (2.6 eV), as compared to the parent anthracene molecule.247 Moreover, the boraanthracene readily

Figure 30. Examples of NHC-stabilized substituted borabenzene and boraanthracenes.

reacts with dioxygen to form air- and moisture-stable 9,10endoperoxide 182 (Figure 30). Later, the group of Rivard isolated IDipp-stabilized 4coordinate borafluorene-based luminogen 9-bromo-9-borafluorene 183, which shows a bright blue luminescence upon UV-light irradiation (λexcit = 320 nm) with a high quantum efficiency (Figure 30).248 Compound 183 can be reduced to 9hydro-9-borafluorene by proton abstraction from the solvent molecules upon reduction with KC8, and it produces the monocationic borafluorenium ion when treated with AgOTf via elimination of AgBr. Very recently, Harman et al. have synthesized 9,10diboraanthracene 185 (δ(11B) = 20.1 ppm) stabilized by IDipp (Scheme 19).249 It was obtained upon magnesium Scheme 19. Synthesis of 9,10-Diboraanthracene

reduction of the corresponding dibromo bis(NHC) adduct 184 (δ(11B) = −3.6 ppm). This 9,10-diboraanthracene reversibly undergoes a one-electron oxidation at a mild potential (E1/2 = −1.4 V vs Fc/Fc+). The corresponding radical-cation [(IDipp)2(BA)]•+ (BA = boranthrene), generated in solution, can also be synthesized from the comproportionation reaction of 184 and 185. Boranthrene 185 also undergoes a facile formal [2+4] cycloaddition across the central diborabutadiene moiety with several unsaturated small molecules, such as CO2, O2, and ethylene. Recently, the groups of Marder and Radius together reported reversible formation of the thermally stable adducts 186 (Scheme 20a)250 in reactions of aryl boronates with IiPr2Me2. Interestingly, similar reactions with Me2CAAC proceed differently, leading to the unprecedented reversible oxidative addition (insertion) of the CAAC to the B−C bond of cyclic boronate esters (Scheme 20b).250 Multiple cooling/ heating cycles, performed to demonstrate the reversible nature 9702


Chemical Reviews Scheme 20. Reactions of the Classical NHC and with Boronate Esters

Review Me2

borenium cation led them to find extensive applications in many stoichiometric or catalytic transformations.255,256 In 2009, Matsumoto and Gabbai ̈ successfully isolated the first borenium cation 188 from the reaction of Mes2BF with trimethylsilyl triflate and [Ag(IMe2)2][Ag2I3] in refluxing chlorobenzene (Scheme 21a).257 The molecular structure of

CAAC

Scheme 21. Synthesis of Diaryl and Dialkyl Borenium Cations

188 (δ(11B) = 64 ppm) shows a CNHC−B bond length (1.579(7) Å), comparable with that of a covalent B−CAr bond (1.562(7) and 1.560(7) Å), implying strong coordination of the NHC ligand to the highly electrophilic boron center. Cyclic voltammetry showed reversible one-electron reduction of 188 (E1/2Red = −1.81 V vs Fc/Fc+) to the neutral radical, which was only detected and characterized in solution by EPR spectroscopy. Two years later, the first dialkyl borenium cation with a triflate counteranion 189 was synthesized from the reaction of (IMes)(9-BBN) with triflic acid (Scheme 21b).258 However, this compound was not structurally characterized. The ratio of the cation to anion diffusion coefficient value 0.85, calculated from the 1H and 19F DOSY NMR spectra, strongly reflects the separated ion pair nature of compound 189. Shortly afterward, Curran and co-workers isolated the first dihydroxyborenium cation 191 by sequential treatment of (IDipp)BH3 (150b) with an excess of triflic acid (Scheme 22).259 During the course

of the oxidative addition reaction, were monitored by 1H NMR spectroscopy. At high temperature (80 °C), the equilibrium shifts left, that is, the product to reactant ratio decreases, while taking back the solution to room temperature shifts the equilibrium back toward the oxidative addition products. From the 1H NMR data, calculated energy parameters at equilibrium are ΔH = −56.1 kJ mol−1, ΔS = −0.18 kJ mol−1. Similarly, Me2CAAC is reported to activate the B−B bond of B2pin2, B2cat2, B2neop2, and B2eg through the oxidative insertion of carbene carbon into the boron−boron bond.251 In this case, the reaction is irreversible in nature. In contrast, classical imidazol(id)in-2-ylidene-based NHCs form in such reactions 1:1 or 1:2 Lewic acid/base adducts depending on the reagent ratio.252 2.3.1.2. NHC-Stabilized Cationic Boron Compounds. As compared to neutral three-coordinate boron(III) compounds, corresponding cationic boron compounds, being more electron deficient, exhibit higher Lewis acidity and more reactivity features. Depending on the coordination number of the boron center, monocationic boron compounds are divided into three distinct subclasses.253 Two-coordinate borinium cations [BR2]+, formally four-electron species, are the most reactive species among them. The other two classes are threecoordinate borenium cations [(L)BR2]+ and four-coordinate boronium cations [(L)2BR2]+,254 possessing external Lewis bases. The strong donor ability of NHCs is often crucial for isolation of such monocationic species by partial mitigation of their electron deficiency. Until now, there is no report of NHC-stabilized borinium cations, presumably because of the lack of suitable precursors, as it needs a dianionic ligand counterpart along with a carbene as donor. On the other hand, boron in the three-coordinate cationic state is more stable and comparatively more reactive than the four-coordinate boronium cation. Consequently, there is a considerable number of borenium cations documented, featuring NHCs in its first coordination sphere. The delicate balance between the steric and electrophilic nature of

Scheme 22. Synthesis of Dihydroxyborenium Cation

of the reaction, the intermediary complexes (IDipp)BH(OTf)X (X = OTf, Cl) 190 were isolated. The crystal structure analysis revealed that the triflate anion forms hydrogen bonds with the hydroxyl proton of the borenium cation, thereby imparting a great deal of stability to the complex 191 (δ(11B) = 22.5 ppm). Subsequently, many other borenium cations were isolated (Figure 31).32,260−267 Both steric and electronic effects were 9703


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Figure 31. Examples of various borenium cations.

crucial for isolating Weber’s diamino borenium ions 192,261obinson’s aminochloroborenium 193,32 and Aldridge’s dibromoborenium 194260 ions. Complex 196 has been obtained from the reaction of the (IBn)BH3 borane with [Ph3C][B(C6F5)4], and proceeds through the sequential hydride abstraction and intramolecular dehydrogenative cationic borylation.264 The boron center in the complex 196 can add donor molecules such as PPh3, Et3PO, or DABCO to form four-coordinate boronium cations. Other compounds 197−200 (Figure 31) were obtained by adapting a common synthetic protocol, using [Ph3C][B(C6F5)4] or B(C6F5)3 as hydride ion scavenger from the corresponding NHC-supported 9-BBN.262,263,265,267,268 Interestingly, some of these complexes emerged as efficient catalysts for the activation of dihydrogen, concomitant hydrogenation of imines and enamines through a FLP mechanism,262,263,267 and trans-hydroboration of alkynes with (NHC)(9-BBN).265 Very recently, the groups of Wang and Li reported the first dihydrogen activation by a single boron center of boryl borenium cation 203 (Scheme 23).11 This cation was obtained starting from the carbene adduct (IMe4)B(H)2Mes (201), which furnishes (IMe4)B(H)(Bpin)Mes (202) after monoiodination and subsequent borylation of the (IMe4)B(H)(I)Mes intermediate with (IDipp)CuBpin. Subsequent hydrogen abstraction by [Ph3C][B(C6F5)4] afforded 203, isolated in 96% yield. The cationic boron center appears as a broad singlet at δ = 78.0 ppm in the 11B NMR spectra. Reactivity studies showed that the cation 203 undergoes facile cleavage of dihydrogen within a minute, even at very low temperatures (−40 °C) to form dihydroboronium complex [(IMe4)B(H)2(MesBpin)]+ 204, accompanied by elimination of the Mes and Bpin substituents as MesBpin. Complex 204 showed a broad signal at δ = 2.87 ppm for the BH2 moiety in the 1H NMR spectrum. The mechanistic studies revealed a concurrent addition/elimination process in which polarization of the B−B bond upon coordination of H2 plays a crucial role. The cationic complex 203 could also be converted to the fourcoordinated boronium cation 205 upon reaction with the Lewis base DMAP. Complex 205 in the 11B NMR spectrum

Scheme 23. Synthesis and Dihydrogen Activation by a Boryl Borenium Cation and Its Conversion to Four-Coordinate Boronium Cation

shows a broad signal at δ = 7.8 ppm characteristic of a fourcoordinate monocationic boron center, which lies upfield of the signal of the three-coordinate borenium cation of 203 (δ = 78.0 ppm). The NHC−borenium cation [(IMe2)BH2···H···BH2(IMe2)] [NTf2] (206) was postulated as an intermediate in the hydroboration of such alkenes as 3-hexene, 3-octene, and 1cyclohexyl-1-butene with the (IMe2)BH3 (148a) complex in the presence of a catalytic amount of triflamide (HNTf2).268 An analogous synthetic approach allowed one to obtain a polycyclic diborenium ion 207, containing two threecoordinate monocationic boron centers (Scheme 24).269 Reaction of the in situ formed [(IiPr2)BH2]+ cation with 9,10-distyrylanthracene proceeds through the tandem hydro9704


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Scheme 24. Synthesis of the Polycyclic Diborenium Dication

boration of the alkene functions followed by electrophilic borylations of aromatic rings. Dication 207 was obtained as a 1:1 mixture of pseudo cis- and trans-isomers and isolated as dark-red crystals in 62% yield. The pseudo trans-isomer 207 possesses a CNHC−B bond distance of 1.596(3) Å, which is elongated as compared to other NHC-supported (di)arylborenium ions (ca. 1.54−1.58 Å). DFT calculations of 207 highlighted that the LUMO and NBO charges are principally located at the two boron atoms. Finally, compound 207 was converted to the highly fluorescent 3,9-diboraperylene (Φ = 0.63) upon dehydrogenation of the diborenium ion with TEMPO radical, followed by an acidic workup. In 2013, Vidović and co-workers reported a series of dichloroborenium cations [(NHC)BCl2]+ (208) (NHC = IMe4, IiPr2Me2, ItBu, IDipp) with various counteranions, such as AlCl4−, OTf−, NTf2−, and B(ArCl)4−, ArCl = 3,5-Cl2− C6H3.270 All of them were synthesized by treating the neutral (NHC)BCl3 adducts with various chloride abstracting reagents, such as AlCl3, AgOTf, or AgNTf2, at room temperature. The compounds containing AlCl4− counteranions were structurally characterized, and it turned out that there exists a different degree of cation−anion interaction, depending on the steric encumbrance around the NHC involved. Furthermore, this magnitude of cation−anion interaction was corroborated through the 11B NMR spectra, displaying downfield shifted resonance signals for compounds with only the least of such interactions. The most downfield signals (38.6−51.9 ppm) provided the [(NHC)BCl2]+ cations bearing AlCl4− anion, while the corresponding (NHC)BCl3 precursors showed significantly upfield-shifted signals lying in the range of 2.0−2.9 ppm. Moving from borenium to boronium cations with an increased coordination number of boron center, only a few examples have been reported so far (Figure 32). The cation 209 was stabilized employing a chelating amido-pendant NHC ligand.96 It can be considered as a parent boronium cation. An unusual reaction leading to a bis(borane) complex containing two chemically different boron centers takes place when 209 is treated with silver triflate. Boronium cation 210 was synthesized through the interception of the freshly generated, hydride-bridged dimer 195 (Figure 31), containing [B(C6F5)4] as counteranion, with DMAP as coordinating Lewis base.266 Braunschweig and co-workers isolated two NHC-coordinated boronium ions 211, obtained by heterolytic Fe−B bond cleavage of ferroborirene complex [Cp*Fe(CO) 2 ][BC2(SiMe3)2] upon reactions with NHCs (Figure 32).271

Figure 32. Examples of NHC-stabilized boronium cations.

Kinjo and co-workers have structurally authenticated the first diboron(II) dication, which can formally be considered as bis(boronium) dication [L2PhB−BPhL2][2X] (X = OTf, BF4, AlCl4) 213, using oxazol-2-ylidenes as Lewis bases without the aid of any chelating ligand. The synthesis of 213 was achieved through one-electron oxidation of borylene L2PhB (L = oxazol2-ylidene) 212 using AgOTf, AgBF4, or Mes*AlCl2 (Mes* = 2,4,6-tri-tert-butylphenyl) (Scheme 25).272 The bis(boronium) Scheme 25. Synthesis of Oxazol-2-ylidene-Stabilized Bisboronium Dications

dication shows a sharp singlet at −16.7 ppm in the 11B NMR spectra, conferring to a four-coordinate boron center. DFT calculations revealed that the HOMO mainly consists of a σbonding orbital. Additionally, the CV experiment produced irreversible oxidation and reduction waves at 1.190 and −2.401 V, respectively, suggesting an unstable nature of the oneelectron oxidized or reduced species. Complex 213 can further react with isocyanides at elevated temperatures to form the cyano-substituted boronium cation [L2PhBCN]BF4 214 through the oxidative B−B bond cleavage (Scheme 25). In contrast to the monocations, the dicationic boron species contain one more formal positive charge and two vacant p orbitals at the boron center. This electronic structure makes them extremely reactive. Therefore, the synthesis and isolation of the dicationic species remains a daunting task. 9705


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Scheme 26. Synthesis of IMes-Supported Dicationic Boron Complex 217 and Its Conversion to the Borabenzene Complex 218

With the aid of NHCs, Chiu et al. were able to stabilize and structurally characterize the dicationic boron species 217 (Scheme 26).273 It was synthesized upon stepwise treatment of [Cp*BCl2(IMes)] with AlCl3 as chloride ion scavenger in chlorobenzene. Compound 217 (CNHC−B bond distance 1.558(4) Å) features a sharp signal at −49 ppm in the 11B NMR spectra. This hypercoordinated boron dication with a pentagonal pyramidal geometry can be viewed as nidocarborane having six vertices with 16 cluster electrons, one from the B2+ center and 15 from the C5 fragment. It has been postulated that the stability of this highly electrophilic dicationic species arises from the strong σ-donating ability of the NHC in conjunction with steric and electronic stabilization imparted by the η5-Cp* ligand. Interestingly, the dication upon reaction with superhydride LiHBEt3 produces the NHCstabilized borabenzene 218 through the ring expansion of the Cp* ligand. Similarly, the CAAC-stabilized boron dication [(η5-Cp*)BCy ( CAAC)][2AlCl4] (220) was also isolated, using the reaction of [Cp*BCl2(CyCAAC)] (219) [δ(11B) = −48.8 ppm, CNHC−B bond distance 1.560(7) Å] with 2 equiv of AlCl3 in chlorobenzene.274 This compound shows a reversible one-electron reduction wave (E1/2Red = −0.85 V vs Fc/Fc+) in the cyclic voltammetry experiment. Chemical one-electron oxidation of 220 by tetrakis(dimethylamino)ethylene produced the corresponding radical cation, which was detected only by EPR spectroscopy. Interestingly, the halide abstraction from both 215 or 219 using Krossing salt Ag[Al(OC(CF3)3)4]275 allowed the isolation of corresponding chloroborenium cations [(η1-Cp*)BCl(NHC)][Al(OC(CF3)3)4] (221) (NHC = IMes, CyCAAC). On the other hand, this reagent, as well as TMSOTf, were not efficient to produce the desired dicationic compounds. 2.3.1.3. NHC-Stabilized Anionic Boron Compounds. As compared to the plethora of cationic boron species, it is imperative to mention that some sporadic examples of NHCstabilized anionic boron species are reported. Humongous efforts were dedicated to isolate six electron anionic boron species BR2−, employing strong σ-donor ligands. The breakthrough in this field was achieved in 2006 by Nozaki and Yamashita with the isolation of the cyclic diamino-substituted boryllithium (222),276 an isoelectronic analogue of imidazolin2-ylidenes (classical NHCs). This molecule is the first representative of compounds exhibiting boron centered nucleophilicity. Since then, only four compounds containing NHC-stabilized anionic boron species were reported. In 2010, Braunschweig and co-workers isolated the first, deep reddish purple colored NHC-stabilized monoanionic borole compound 224 [δ(11B) = 12 ppm]. It was obtained through facile two-electron reduction of the SIMes−chloroborole adduct 223 with potassium graphite (Scheme 27).277 Theoretical calculations suggested that an aromaticity triggers the stabilization of this anionic compound. The HOMO of 224 mainly represents a π-like bonding orbital between the boron and the

Scheme 27. Synthesis of Borole Monoanion NHC Complex and Its Reactions with Various Electrophiles

carbene carbon of IMes with ample contribution from the boron atom (15%). This result clearly implies the significant nucleophilic character of the π-boron atom, which was demonstrated by reaction with such electrophiles like MeI or using a weak Brønsted acid Et3NHCl. These reactions led to the isolation of the colorless NHC-stabilized 1-methyl-2,3,4,5tetraphenylborole 225 [δ(11B) = −11.2 ppm] and the parent 1-hydroborole 226 [δ(11B) = −18.4 ppm]. Compound 226 was produced via two successive nondegenerate [1,5]sigmatropic hydrogen migrations (Scheme 27).278 In the same year, the group of Curran attempted to obtain the parent boryl anion 227 (IDipp)BH2Li (Figure 33) by reduction of iodoborane adduct (IDipp)BH2I (164c) with an excess of lithium di-tert-butylbiphenylide (LDBB) in the presence of TMEDA.279 While it was not possible to isolate

Figure 33. Examples of NHC- and CAAC-stabilized boryl anions. 9706


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227 due to its highly instability at ambient temperature, it was trapped in situ using various electrophiles to furnish a range of monoalkyl- or arylborane complexes. In 2013, Bertrand and co-workers isolated the CAACstabilized dicyano boryl anion [(CyCAAC)B(CN)2] (228) (Figure 33) obtained by the deprotonation of borohydride (CyCAAC)BH(CN)2 (229) with KHDMS.280 The π-accepting ability of the CAAC plays a pivotal role in the stabilization of 228 (δ(11B) = −17.9 ppm, CNHC−B bond distance: 1.473(2) Å). Furthermore, the anionic character of 228 was demonstrated using reactions with Me3PAuCl, and isopropyl iodide, affording the corresponding coordination complex with gold, and the isopropyl-substituted borane adduct (CyCAAC)B(iPr)(CN)2 (230), respectively. Recently, a series of NHC-stabilized boryl anions of the type (NHC)B(CN)2− 231 (δ(11B) = −24 to −28 ppm) were isolated, exploiting the combination of steric and electronic effects of classical NHCs and the π-accepting properties of the cyano group (Figure 33).281 They were obtained in several sequential reaction steps. First, the cyano group was introduced via an thioalkyl/cyano exchange reaction at boron between (NHC)B(SEt)3 (232) and Me3SiCN, affording (NHC)B(CN)2SEt (233). Compounds 233 reacted further with MeI to form monoiodoboranes (NHC)B(CN)2I (234). Finally, the reductive dehalogenation with an excess of KC8 or K/NH3 provided boryl anions 231a,b in quantitative yields. It is interesting to note that reductions of the iodide (IMe4)B(CN)2I, thioalkyl derivatives 233, and the bromide (IMes)B(CN)2Br (235) did not yield corresponding boryl anions. The structural analysis of 231 revealed a trigonal planar geometry around the boron center and shortened CNHC−B bond distances (e.g., 1.495(2) Å in 231a vs 1.607(3) Å in 234a), while the C−N bonds in both the NHCs and the nitrile groups were elongated as compared to the starting iodo precursors 234. This result unequivocally indicates a strong resonance stabilization of the boron-centered lone pair with the pz-orbital at the carbene carbon atom and the π*-orbital of the nitrile groups. The decreased π-acidity of NHCs as compared to CAACs leads to decreased CNHC−B bond order and increased negative Mulliken charge density on the boron. Facile reactivity of boryl anions 231 leading to the formation of new B−E bonds was demonstrated toward C-, Si-, Sn-, P-, and Au-electrophiles. 2.3.1.4. NHC-Stabilized Neutral Radical Boron Compounds. Synthesis of stable species containing boron centered radical has fascinated chemists for a long time.282 The anionic radical species Ar3B•−, which are isoelectronic to the trityl radicals Ar3C•, represent the prototypical and earliest class of boron-centered radicals, which were generally obtained by reduction of the corresponding triarylboranes with alkali metals.283 Recently, numerous neutral radical compounds of the common formula [(NHC)BR2]•, presenting an interest in application as reagents in organic synthesis and efficient coinitiators in various radical polymerization reactions,284 have been synthesized. In the late 2000s, Gabbai ̈ and co-workers reported the first example of the NHC-stabilized neutral persistent boryl radical 236, obtained by one-electron reduction of the corresponding borenium cation (188) with magnesium metal; however, they failed to crystallographically characterize it (Figure 34).257 Shortly afterward, Lacote and Curran independently reported several transient parent boryl radical species, for example, 237 and 238, prepared in situ by hydrogen atom abstraction from

Figure 34. Examples of NHC-stabilized neutral boryl radicals.

the corresponding boranes employing AIBN or hydrogen peroxide as hydrogen atom scavenger.211,285,286 Because of their considerably low lifetime, none of those complexes were structurally characterized. However, they were persistent enough to be detected by EPR spectroscopy, which has suggested substantial spin density delocalized in a CNHC−B π bonding orbital. All of those short-lived boryl radicals were found to be excellent initiators for radical polymerization reactions. In 2014, the group of Braunschweig isolated the first NHCcoordinated neutral borole-based radical 239 (Figure 34).287 It was synthesized through a single electron transfer (SET) from the corresponding borolyl anion 224 (cf., Scheme 27) upon reaction with bulky triorganotetrel halides Ph3ECl (E = Sn, Pb)). The solid-state structure of 239 confirmed considerable deviation from coplanarity of the two heterocyclic rings (torsion angle ca. 40°), thereby suppressing the possibility of the unpaired electron to delocalize over the carbene fragment. This was further corroborated through EPR spectroscopy, which revealed the entrapment of the unpaired electron mainly on the five-membered borolyl ring, representing an open-shell 5π-electron system. When the borolyl anion 224 was reacted with the less bulky triorganotetrel halides Me3ECl (E = Sn, Pb), compounds with rare B−Sn and B−Pb bonds were generated in contrast to the radical formation in the previous case. Braunschweig and co-workers recently reported radical 240 (Figure 34)288 featuring predominant boron-centered radical character as compared to 236, achieved by introduction of electron-withdrawing CF3 substituent on the aryl counterpart. It was obtained by the facile decamethylcobaltocene-mediated reduction of the corresponding monocationic boron species. The large hyperfine coupling constant [A(11B) = 8.5 G and A(10B) = 2.0−2.4 G)] values in the EPR spectrum confirmed that the spin is mainly localized on the boron with a negligible contribution from the NHC moiety. Further DFT calculations revealed a spin density ratio (BR2/NHC) of ca. 9:1, affirming the boron-based radical nature of 240. Alternatively, by employing carbenes with enhanced electrophilic and π-acidic character, such as CAACs and diamidocarbenes (DACs), neutral boryl radicals 241−243 (Figure 35) with more delocalized spin density have been reported.12,289−291 In all three cases, the unpaired electron mainly resides on the carbene part; however, in compounds 241290 and 242a,289 because of the coplanar arrangement of the boryl moiety and the CAAC, a significant part of the spin 9707


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Braunschweig et al. reported a series of symmetrically substituted diborenes 248−250, 252, 254−256 using more selective reactions (Scheme 29),297−301 as compared to Scheme 29. Synthesis of NHC-Stabilized Diborene Compounds Figure 35. Examples of neutral boryl radicals stabilized by carbenes with enhanced electrophilic and π-acidic properties.

density is delocalized over the boron atom. On the other hand, due to the perpendicular orientation of the boryl and carbene moieties in 243, only a minor fraction of spin density resides at the boron center and 90% is located on the DAC fragment.291 Interestingly, the B-(2-thienyl)-substituted analogue of the radical 242a appeared to be unstable. 2.3.1.5. NHC-Stabilized Multiply Bonded Boron Compounds. In this section, our discussion will focus solely on the successful isolation of NHC-stabilized multiple-bonded lowcoordinate boron compounds. Without efficient stabilization, these species are highly reactive and their existence can only be perceived through the low temperature matrix isolation techniques.292 Initial attempts to synthesize kinetically stabilized diborenes RBBR, using only single bulky substituents, led to unwanted side reactions, such as radical hydrogen abstraction or borylene insertion into C−H or C−C bonds.293,294 Therefore, multiple bonded boron compounds remained elusive until the pioneering work by Robinson and co-workers in 2007, who isolated parent NHC-stabilized diborene compounds 245a,b using IDipp and IMes. Both compounds were synthesized by reduction of the corresponding (NHC)BBr3 complexes in the presence of an excess of KC8 in diethyl ether at room temperature (Scheme 28).295,296 Both

Robinson’s parent diborenes. Either lithium metal or KC8 was utilized for the reductive coupling of the corresponding haloboranes. The 11B NMR resonance signal varies from 18− 25 ppm, while the B−B bond distances lie in the range of 1.578(3)−1.593(5) Å. The major structural difference of the aryl-substituted diborenes in comparison to their heteroaryl-substituted counter parts is that the duryl or mesityl groups are twisted out of the >BB< plane, whereas thienyl or furanyl rings lie within the plane, resulting in an enhanced π-conjugation between the heteroaromatic substituents and the BB bond unit. DFT studies confirmed that the HOMO−1 and the HOMO represent a boron−boron σ and π bond, respectively, in each diborene molecule. Therefore, the substitution pattern has a significant impact on the HOMO−LUMO (π−π*) energy gap of the BB double bond and thus determines the reactivity of those molecules, discussed in the next section. Recently, an unsymmetric polar diborene 258 was reported by Kinjo et al. (Scheme 30a).302 It was obtained by reductive dehalogenation of the asymmetrically substituted tetrabromodiborane 257. The B−B bond length [1.602(4) Å] is slightly longer than that in the previously reported neutral symmetric diborenes. This polar diborene 258 [δ(11B) = 27.8, 51.7 ppm] is highly reactive and undergoes complete B−B bond cleavage upon reaction with aryl isonitrile, unlike in the case of the symmetrical diborenes for which such reactions are not documented. In a similar approach, they also prepared the first structurally authenticated, neutral, allenic-type diborene 260 by employing PMe3 as another external donor (Scheme 30b).303 Compound 260 was obtained via KC8 reduction of the zwitterionic boraalkenyl boronium complex 259. In the 11B

Scheme 28. Synthesis of Neutral Parent Diborenes and Diboranes

compounds 245a and 245b were isolated as red colored crystals in 12% and 15.8% yields, respectively. Their formation is believed to proceed via an in situ hydrogen atom abstraction from solvent diethyl ether. Notably, the ratio between 245 and the NHC-stabilized parent diboranes (NHC)2B2H4 246 depends on the amount of KC8 employed. The BB double bond lengths were found to be 1.561(18) and 1.582(4) Å for 245a and 245b, respectively, which are considerably shorter than the B−B single bond lengths (1.682(16)−1.762(11) Å) in three-coordinate diborane compounds. Interestingly, XRD analysis revealed a planar structure of the IDipp-coordinated diborene 245a, while the IMes-stabilized congener (245b) exhibits a higher degree of conformational flexibility showing planar, trans-, and cis-bent solid-state structures for three different fractions of single crystals. DFT calculations showed that the HOMO−1 represents a B−B σ-bond, while the HOMO is essentially a B−B π-bonding orbital and mainly constitutes from overlap of the boron 2p orbitals. The WBIs of the B−B bonds in 245a and 245b are 1.408 and 1.445, respectively. Later, 9708


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Scheme 30. Synthesis of the Unsymmetric and Allenic Diborenes

Scheme 32. Synthesis of Dibora[2]ferrocenophane

strained ansa-metallocene with a multiply bonded bridge apart from carbon. The synthesis was achieved via direct KC8 reduction of the carbene-stabilized 1,1′-bis(dibromoboryl)ferrocene 262. DFT calculations revealed that compound 263 possesses a substantially low HOMO−LUMO energy gap (ΔE = 2.11 eV) as compared to other trans-diborenes, probably due to considerable ring strain of 263. This diborene was found to be highly reactive and readily undergoes complete cleavage of the B−B double bond upon reaction with 1,2-diphenyldiselenide to form the base-stabilized 1,1′-bis(boryl)ferrocene. Comparatively, compounds containing boron in the formal oxidation state zero are fewer in numbers. The group of Braunschweig demonstrated that reduction of IDipp, SIDipp, IDep, SIDep, or Me2CAAC-stabilized tetrabromodiboranes 264 with sodium napthlenide furnishes boron allotropes 266−268 with a linear structure of the CNHC−B−B−CNHC units (Scheme 33).306−310 The B−B triple bond character in 266 was elucidated by X-ray crystallography, spectroscopic analysis, and detailed computational analysis. Interestingly, the BB stretching frequencies of 266a,b, observed in Raman spectra between 1600 and 1750 cm−1, are lower than those observed for the CC bond in alkynes (∼2100−2260 cm−1) and the NN bond of dinitrogen (∼2300 cm−1), which also reflects the relative strength of these bonds.308 Because of the large π-accepting character of CAACs versus NHCs, the B−B bond length in 268 (1.489(2) Å)307 is longer than those in 266 (266a, 1.449(3) Å;306 266b, 1.446(3) Å308). The Me2CAAC-stabilized diboron compound 268 can be best described as diboracumulene having an intermediate bonding situation between the electron precise diborene (>BBSiE− (E = N, silaimines; E = P, phosphasilenes; E = As, arsasilenes; E = Sb, stibasilenes; E = Bi, bismasilenes) is a challenging target, because these compounds are considered as highly reactive species, due to the relatively small HOMO−LUMO gap. Therefore, only a rather small amount of NHC complexes is known. In addition to the stabilization of these coordinatively 9737


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unsaturated molecules, NHC coordination can also enhance the reactivity of >SiE− complexes, allowing for unusual abilities, such as the activation of small molecules. The first NHC-stabilized dichlorosilaimines 533a−c were synthesized by Roesky et al. in 2010, and recently the pool of reported compounds was extended with the synthesis of the similar complex 534 by Cui and co-workers (Figure 63).569−571 The implementation of the sterically demanding

Scheme 87. Synthesis of NHC-Stabilized Silaaziridines 540 from trans-1,2-Disilylimines 539

were synthesized in moderate to high yield, tolerating a variety of aryl-substituted nitriles. The nature of the aryl substituents has a negligible influence on the 29Si NMR chemical shifts; thus the detected signals fluctuate around −100 ppm. Complexes 539 undergo a nucleophilic attack by phenyllithium reagents to afford the first silaaziridines 540 with an NHC-stabilized exocyclic silaimine moiety. The corresponding 29 Si NMR chemical shifts (δ ≈ −109.5 ppm) are shifted to higher field as compared to the starting material. The aziridine derivatives are thermally unstable and form five-membered rings via electrophilic aromatic substitution reactions upon heating. The NHC adducts of the silaimine derivatives 541 and 542 are accessible through controlled oxidation of the respective phosphinoaminosilylene NHC complex (IiPr2Me2)[Dipp(TMS)N]SiPPh2 (543) (vide infra) with dioxygen (Figure 64).577 The NHC-stabilized 1-siloxysilaimine 541 contains a

Figure 63. NHC-stabilized silaimine species 533−538.

triphenylsilyl group led to the isolation of the further silaimine NHC adduct 535.572 The overall detected chemical shifts for the central silicon atoms range from δ = −75.3 to δ = −107.1 ppm. The experimentally measured SiN double bond lengths vary from 1.563(4) to 1.594(2) Å and are in the range of similar reported compounds (1.53−1.68 Å570). The earliest mentioned donor-stabilized 1-hydrosilaimine 536 was isolated one year later.573 NHC adduct 536 bears a relatively long SiN double bond (1.6209(13) Å). The Si−H bond is clearly observed in the corresponding proton (δ = 6.21 ppm) and silicon (δ = −72.0 ppm) NMR spectra with a coupling constant 1JSi−H of 218 Hz. A rather similar NHC-stabilized 1-hydrosilaimine 537 was also described by Jana et al. not long ago (Figure 63).574 Instead of the silylamino motif of compound 536, they introduced a meta-terphenyl-based ligand framework. The silicon−nitrogen double bond length of 1.583(5) Å in the tetra-coordinated 537 is significantly shorter as compared to that of complex 536. However, the chemical shifts of the Si−H moiety in the 1H NMR (δ = 6.44 ppm, 1JSi−H = 199 Hz) and 29 Si NMR (δ = −76.5 ppm) spectra are in good agreement with the respective signals of 536. NHC adduct 537 reacts with water under the formation of an imidazolium silanolate. A series of NHC adducts of cis-1,2-bis-silylated alkenes 538, with trimethylsilyl groups and silaimine moieties, were synthesized in overall high yield.575 As starting materials the respective internal or terminal alkynes with mainly electronwithdrawing groups (EWG) were used. The chemical shifts in the 29Si NMR spectra occur between δ = −87.2 and δ = −105.5 ppm. XRD analysis of several of the synthesized compounds revealed an average SiN double bond length of 1.590 Å. Akin to the Z-configured alkenes 538, the same group reported a set of trans-1,2-disilylimines 539 containing a silaimine component (Scheme 87).576 The complexes 539

Figure 64. NHC complexes of 1-siloxysilaimine 541 and dioxysilaimine 542.

SiN double bond with a length of 1.6021(12) Å, which exhibits a doublet signal (δ = −84.9 ppm, 1JP−Si = 38.3 Hz) in the 29Si NMR. The silaimine bond (1.596(2) Å) in the rare NHC-stabilized 1-siloxy-1-phosphinoxysilaimine 542, with the tetra-coordinate silicon atom attached to two oxygen atoms, is a bit shorter as compared to that of complex 541. The corresponding 29Si NMR chemical shift at δ = −104.0 ppm with a coupling constant 2JP−Si of 14.5 Hz is shifted to higher field indicating a more shielded silicon nucleus, due to the additional oxygen donor. Filippou et al. recently synthesized the three additional NHC-supported silaimine compounds 544−546 (Figure 65).578 These complexes symbolize the first NHC-stabilized silaimine derivatives including Si−I and Si−N3 moieties. SCXRD analysis revealed distorted tetrahedrally coordinated silicon nuclei with SiN double bonds varying from 1.568(4) to 1.608(1) Å. 29Si NMR spectroscopy exposed sharp singlets for the central Si atoms (544, δ = −223.9 ppm; 545, δ = −88.9 ppm; and 546, δ = −67.4 ppm). The unprecedented NHC-stabilized mononuclear silazine complexes 547 and 548 are the first described examples of their kind. The solid-state structure of 547a shows an Econfigured silazine unit with a rather long SiN double bond length of 1.655(2) Å. The respective Si(IV) nuclei exhibit 9738


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Figure 65. Further NHC-stabilized silaimine and novel silazine complexes.

Figure 67. NHC-stabilized “half-parent” phosphasilene 551 and NHC adducts of phosphasilenes bearing Si−H moieties 552−555.

resonances in the 29Si NMR spectra (547b, δ = −51.9 ppm; and 548, δ = −149.6 ppm), which are shifted to lower fields in comparison to the corresponding silaimine derivatives 544 and 545. The group of So described the isolation of the unique NHCstabilized siladiimide complex 549, which represents the first room-temperature stable example of its kind (Figure 66).579

compared to the corresponding donor-free phosphasilene (δ = 101.5 ppm, 1JP−Si = 186.4 Hz), indicating a more electronrich silicon center. The SiP double bond is with a length of 2.1431(10) Å about 3.5% elongated in comparison to the “naked” phosphasilene and is in good agreement with other described phosphasilenes (2.053−2.165 Å582). A high contribution of the zwitterionic character was revealed by DFT studies. The first NHC-stabilized phosphasilene containing a Si−H fragment and a bulky meta-terphenyl group was reported in 2013.583 The 2-aminophosphasilene complex 552 is synthesized in high yield and can easily be transformed to the analogous NHC-free phosphasilene by reaction with the Lewis acid BPh3. The tetra-coordinate silicon exhibits a doublet signal (δ = −21.1 ppm, 1JP−Si = 144.4 Hz) in the 29Si NMR and contains a very long SiP double bond (2.1652(8) Å), thus indicating a significant contribution of the zwitterionic resonance structure. The Driess group succeeded in the isolation of the unprecedented 1,2-dihydrophosphasilene NHC adduct 553a and the novel NHC-stabilized 1-silyl-2-hydrophosphasilene 553b (Figure 67).582 The appropriate 29Si NMR spectra show doublets at δ = −25.6 ppm (1JP−Si = 120.7 Hz) and δ = −27.5 ppm (1JP−Si = 130.8 Hz) for the central silicon nuclei. The increased 1JP−Si coupling constant in 553b reflects the increased shielding of the SiP bond by hyperconjugation of the P-TMS group, therefore enhancing the double bond character. However, compound 553a is labile in solution and undergoes a head-to-tail dimerization under liberation of NHC. Further stabilization of 553a is achieved by complexation with [W(CO)5·THF] to afford the donor−acceptorstabilized phosphasilene 554.582 NHC adduct 553b shows with a SiP bond length of 2.1459(7) Å an increased SiP character as compared to that of complex 554 (2.2105(4) Å), which is in the range of typical Si−P single bonds (average 2.25 Å583). In addition, the calculated WBI of 554 is only 1.09, thus elucidating the reduced SiP double bond character and the high zwitterionic structure of the transition metal complex. Recently, a series of NHC-stabilized 2-hydrophosphasilenes bearing two aryl substituents 555a−d was described by Jana and colleagues.584 NHC (IiPr2Me2)-mediated dehydrochlorination of the respective chlorophosphasilane precursors furnishes the complexes 555a,b in good yield. Treating solutions of these NHC adducts with an ImMe4 solution results in the formation of the NHC-stabilized phosphasilenes

Figure 66. Unprecedented NHC-coordinated siladiimide 549 and the first CAAC-stabilized silaimine complex 550.

The detected 29Si NMR resonance (δ = −65.0 ppm) is in the same range as those of the above-mentioned siliaimine complexes. XRD analysis exposed a distorted trigonal planar geometry around the allylic-like N−Si−N moiety with a bond angle of 147.7(2)° in the solid state. The relatively short SiN double bonds (1.591(5) and 1.593(5) Å) and the calculated WBIs (1.14 and 1.17) elucidate the significant double bond character. Lately, the same group accomplished the isolation of the first CAAC-supported silaimine complex 550.576 The silicon nucleus of the CAAC-iminosilane adduct displays a distinct highfield chemical shift at δ = −180.7 ppm in the 29Si NMR spectrum, which correlates well with the iodine-substituted silaimine and silazine derivatives 544 and 548. The SiN double bond with a length of 1.585(4) Å is comparable to those of the NHC-stabilized silaimines. Compounds containing a SiP double bond are referred to as phosphasilenes; thus they are formal heavier analogues of imines. Coordinating NHCs enhance the stability of phosphasilenes, which is concomitant with an elongation of the SiP double bond. Recent achievements in this section of main group chemistry were summarized in a comprehensive review by our group;580 therefore, these findings are not discussed in detail here. Driess et al. were able to increase the stability of a “halfparent” phosphasilene with a β-diketiminate skeleton by the coordination of DMAP or IMe4, resulting in the isolation of NHC adduct 551 (Figure 67).581 The respective 29Si signal (δ = −7.0 ppm, 1JP−Si = 116.4 Hz) is highfield-shifted as 9739


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555c,d under NHC exchange. The 29Si NMR chemical shifts vary from δ = −26.7 to δ = −32.9 ppm with 1JP−Si coupling constants ranging from 148.3 to 157.8 Hz. XRD analysis of complex 555d revealed a SiP double bond length of 2.1585(9) Å, which agrees well with the above-mentioned NHC phosphasilene adducts. The collaboration between the Tokitoh and Streubel groups resulted in the isolation of the stable NHC complex of 2,2chlorosilylphosphasilene 556 with the very bulky Tbt group (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl) (Figure 68).585 The 29Si NMR spectrum shows a doublet signal (δ =

in main group chemistry research. Especially silanones are thermodynamically and kinetically unstable species, due to the weak π-bond of the strongly polarized, ylide-like SiO double bond. The high instability generally leads to a facile head-totail oligomerization affording oligosiloxanes. Nevertheless, Driess et al. succeeded in the remarkable isolation of the first NHC-stabilized silanone (cyclic sila-urea complex) 559a in 2009 (Figure 69).589 Complex 559a is obtained in almost

Figure 69. NHC adducts of silanones 559 and dioxasiliranes 561 and cyclic urea-stabilized sila-urea 562 reported by the Driess group.

quantitative yield by the oxidation of the respective NHCsupported silylene 560 (vide infra) with nitrous oxide. The observed chemical shift in the 29Si NMR spectrum appears at δ = −74.2 ppm, and the experimentally measured SiO distance was 1.541(2) Å. A high contribution of the zwitterionic NHC+−Si−O− resonance structure was suggested, due to the relatively short Si−N bonds (1.745(2) and 1.750(2) Å) and a significant pyramidalization at the silicon center. The similar compound 559b with the more sterically encumbered NHC IiPr2Me2 was described one year later by the same group.590 The silicon nucleus exhibits a signal in the 29Si NMR at δ = −72.9 ppm, which is close to that of NHC adduct 559a. XRD analysis revealed two independent molecules of 559b in the asymmetric unit with even shorter SiO double bonds (1.527(2) and 1.534(2) Å). Utilization of dioxygen instead of N2O leads to the formation of the unprecedented dioxasilirane NHC adducts 561.591 Interestingly, the penta-coordinate silicon nuclei adopt rare square-pyramidal geometries with a side-on coordination of the peroxo ligand moiety. 29Si NMR spectroscopy shows singlets in the highfield region at δ = −131.9 (561a) and δ = −133.3 ppm (561b), respectively, due to the five-coordinate silicon atoms. Strikingly, complex 561b undergoes an oxygen transfer reaction at ambient temperature furnishing the first urea-stabilized silanone 562.591 The silicon nucleus resonates at δ = −77.1 ppm in the 29Si NMR, which is in good comparison to the signals of the NHC-stabilized silanones 559. The SiO double bond is quite short with a length of 1.532(2) Å. The groups of Cui and Roesky successfully synthesized donor−acceptor-stabilized silanone species (Figure 70). The first heavier enone analogues 563 and 564, featuring α,βunsaturated SiO double bonds, were synthesized in 2011.592

Figure 68. NHC- and CAAC-stabilized mono- and dichlorophosphasilenes.

−22.4 ppm, 1JP−Si = 225.5 Hz). The SiP double bond length determined by X-ray crystallographic analysis (2.1319(11) Å) is shorter as compared to the prior described NHC-stabilized phosphasilenes, thus implying a more pronounced silicon− phosphorus double bond. The first 2,2-dichlorophosphasilene complexes were reported in 2015.586 Dark red NHC-stabilized phosphasilene 557, bearing a sterically demanding 2,4,6-triisopropylphenyl group, is synthesized from the reaction of Tipp-P(SiCl3)2 with KC8 in the presence of NHC in 90% yield. The silicon nucleus resonates at δ = −19.1 ppm (1JP−Si = 197.7 Hz) in the 29Si NMR. Performing the same reaction with different CAACs instead of the NHC results in the formation of similar compounds 558a−c in moderate yield. However, the silicon atoms of the CAAC adducts exhibit signals in the 29Si NMR shifted to slightly lower field (δ = 6.6 to −7.89 ppm, 1JP−Si = 195.5 to 203.6 Hz) as compared to the NHC analogue. The most captivating distinction of the NHC/CAAC substitution is the accompanied color change from dark red to blue-black, which is consistent with the lower lying LUMOs of CAACs by 0.78 eV. Complexes 557 and 558 show similar structural characteristics and possess relatively short SiP double bond lengths (2.1129(10)−2.1225(9) Å). Each HOMO is located on the πSiP double bond, whereas the LUMOs are located on the carbenes. In the case of the heavier phosphasilenes, only a few isolable derivatives of arsa- and stibasilenes are known, but bismasilenes are still unknown.587,588 However, there is no example of a NHC-stabilized heavier phosphasilene complex reported until today. Compounds bearing a double bond between silicon and group 16 elements, silanechalcogenones >SiCh (Ch = O, silanones; Ch = S, silanethiones; Ch = Se, silaneselones; Ch = Te, silanetellones), are the heavier homologues of the ubiquitous ketones and currently an object of great interest

Figure 70. Lewis acid/base-stabilized silanone derivatives 563−565. 9740


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demanding m-terphenyl group and a diphenylphosphorus ligand, shows a doublet in the 29Si NMR at δ = −42.1 (1JP−Si = 11.7 Hz) and a SiO absorption band at νSiO = 1080 cm−1. Thus, the presence of a Si+−O− single bond was suggested, which is supported by the relatively long SiO double bond (1.5544(10) Å). Oxidation of Robinson’s famous NHC-stabilized diatomic Si(0) complex (IDipp)SiSi(IDipp) (572)69 (vide infra) with different oxidizing agents delivered a series of tetra-coordinate NHC adducts of silicon oxides 569−571. Utilization of the frequently used oxygen atom donor N2O furnishes NHCstabilized Si2O3 compound 569 in 50% yield.598 Despite the fact that both silicon atoms are in the formal oxidation-state +3, the complex is shown in this section. On the other hand, oxidation with O2 provides the carbene-stabilized Si2O4 complex 570 under complete cleavage of the SiSi double bond. NHC-supported (SiO2)2CO2 571, representing the first NHC adduct of a silicon−carbon mixed oxide, is obtained via either direct oxidation of 572 or consecutive oxidation of intermediate 569 with CO2.599 The silicon nuclei show increasing highfield chemical shifts in the 29Si NMR spectra from δ = −49.1 (569), to δ = −76.3 (570), and finally to δ = −91.5 ppm (571). The SiO stretching vibrations range from νSiO = 1092 to νSiO = 1165 cm−1. SC-XRD analysis revealed SiO double bond lengths (1.521(4)−1.5347(18) Å) featuring the shortest reported among the described NHCstabilized silanone complexes. Baceiredo and Kato observed a rare thermal retro-[2+2]cycloaddition reaction of a donor-stabilized silacyclobutanone, which affords in the presence of IiPr2Me2 cis-stilbene and the first NHC-stabilized 1-silaketene 573 in high yield (Scheme 88).600 Complex 573 exhibits a doublet signal in the 29Si NMR

In contrast to the unstable formyl chloride congener, silaformyl chloride adduct 565 was reported one year later.593 Complex 565 sets a pioneering example of an acyclic silanone species bearing a halide substituent. The silicon atoms of the Lewis acid/base-stabilized complexes 563−565 exhibit signals in the 29Si NMR ranging from δ = −15.1 to −49.8 ppm. The SiO double bonds (563, 1.5894(17) Å; 565, 1.568(15) Å) determined by XRD are elongated as compared to the previously mentioned donor-stabilized silanones. Filippou and co-workers reported the fascinating isolation of the NHC-supported cationic chromiosilanone 566 bearing a three-coordinate silicon atom (Figure 71).594 The thermally

Figure 71. NHC-stabilized cationic silanone species 566 and 567.

stable, trigonal planar (sum of bonding angles at Si: 359.9°) complex features a very short SiO double bond (1.526(3) Å), which is significantly shorter than those observed in the complexes described before. In addition, the 29Si NMR signal is detected with a notable downfield shift at δ = 169.6 ppm, due to the lowered coordination number at the silicon nucleus. Nevertheless, the cationic silanone complex 566 can be considered as NHC adduct of silicon monoxide stabilized by the coordination sphere of the chromium fragment. Two related compounds, the unprecedented NHC-stabilized sila-acylium ions 567a and 567b, were synthesized in our group.595 These aryl-substituted complexes represent the first experimental realization of the elusive heavier acylium ions. The 29Si NMR spectra display singlets at δ = −62.1 (567a) and δ = −60.4 ppm (567b), respectively. The determined Si O double bond length of 567a (1.548(2) Å) and the bathochromically shifted SiO absorption band (νSiO = 1098 cm−1; typical SiO stretching vibrations: νSiO = 1150− 1300 cm−1)596 illustrate the intricate inherent electronic structure. Theoretical calculations uncovered a significant contribution of the zwitterionic character to the bonding situation of the sila-acylium ions. Another NHC adduct of a silanone 568 was reported in late 2015 (Figure 72).597 Complex 568, containing a sterically

Scheme 88. Retro-[2+2]-cycloaddition Reaction Yielding NHC-Stabilized 1-Silaketene 573 under Liberation of cisStilbene

spectrum at δ = −49.6 ppm with a coupling constant 2JP−Si of 30.3 Hz and contains a relatively long SiO (1.549(4) Å) and a very long SiC double bond (1.835(6) Å). Moreover, the isolation of the donor−acceptor-stabilized monomeric silicon dioxide complexes 574 has successfully been achieved (Scheme 89).601 Similar to the retro-[2+2]cycloaddition reaction of the donor-stabilized silacyclobutanone, cycloreversion of the corresponding base-stabilized silaβ-lactone602 furnishes NHC adducts 574 in a stereoselective manner (97:3 ratios of diastereoisomers). Whereas 574b with the labile NHC is transient in solution at room temperature, complex 574a with the less sterically encumbered NHC is completely stable. Doublet signals in the 29Si NMR spectra are observed at δ = −76.2.1 (574a, 2JP−Si = 8.5 Hz) and δ = −74.9 ppm (574b, 2JP−Si = 9.2 Hz). Reaction of 574b with phenylsilane furnishes the respective silanoic ester complex under liberation of IiPr2 Me2.

Figure 72. NHC-supported complexes 568−571 bearing a SiO moiety. 9741


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determined SiCh double bond lengths (579, d(SiS) = 2.006(2)/2.106(2) Å; 580, d(SiSe) = 2.129(2)/2.241 Å; and 581, d(SiTe) = 2.389(4)/2.436(2) Å) are in good agreement with those of the above-described heavier silanechalcogenone adducts. Driess and colleagues also synthesized the NHC-supported silanethione 583a and silaneselone 583b, which are the heavier congeners of the NHC-stabilized silanone 568 (Figure 74).597

Scheme 89. Synthesis of the Donor−Acceptor-Stabilized Complexes 574a,b via a Cycloreversion Reaction of a Sila-βlactone

By going from oxygen to the heavier analogues, the difference in electronegativity between silicon and the chalcogen atom decreases, resulting in a less polarized Si Ch double bond. Accordingly, dimerization is less favorable, and thus a series of the higher silanechalcogenone homologues could be successfully isolated in their monomeric form. Driess and co-workers enlarged their study on the NHCsupported β-diketiminato silanones 559 by the preparation of the NHC-stabilized silanechalcogenones 575−577 (Figure 73).590 The corresponding less-shielded silicon atoms resonate Figure 74. NHC-supported heavier silanechalcogenone complexes.

The complexes exhibit downfield-shifted doublet signals (583, δ = −13.2 ppm, 1JP−Si = 29.4 Hz; and 583b, δ = −18.5 ppm, 1 JP−Si = 37.5 Hz) in the 29Si NMR in comparison to silanone 568. XRD analysis of NHC-stabilized silaneselone 583b revealed a SiSe double bond length of 2.156(2) Å. Oxidation of the respective NHC-stabilized phosphinosilylene 584 (vide infra) with an excess of elemental chalcogens results in chalcogen insertion into the Si−P bond and affords the NHC adducts of silanethione-thiophosphansulfide 585a and silaneselone-selenophosphanselenide 585b.597 The corresponding doublets in the 29Si NMR (585a, δ = −22.6 ppm, 2 JP−Si = 4.7 Hz; and 585b, δ = −33.6 ppm, 2JP−Si = 6.4 Hz) occur at a slightly higher field. The determined SiS (1.999(6) Å) and SiSe (2.142(11) Å) double bond lengths correspond well to those mentioned above and are significantly shorter than the measured Si−S (2.231(6) Å) and Si−Se (2.380(11) Å) single bonds. A series of related compounds was reported by Müller et al. in 2016 with the isolation of NHC-supported heavier silaaldehydes 586.608 The NHC-stabilized complexes display significant chemical shifts for the Si−H moieties in the 1H NMR spectra ranging from δ = 5.74 to δ = 5.86 ppm with 1 JSi−H coupling constants of approximately 209 Hz. The tetracoordinate silicon nuclei resonate in the 29Si NMR spectra from δ = −37.3 (586a), to δ = −44.1 (586b), and finally to δ = −77.7 ppm (586c). SC-XRD analysis exposed relatively short SiCh double bond lengths of d(SiS) = 2.018(8) Å (586a), d(SiSe) = 2.160(3) Å (586b), and d(SiTe) = 2.397(5) Å (586c), respectively. Experimental analysis and supportive DFT calculations revealed a close relationship between the NHC adducts of the heavier silaaldehydes 586 and the isoelectronic phosphine chalcogenides R3PCh. Heavier homologues of the NHC-supported sila-acylium ions 567 were described only recently. The NHC-stabilized complexes 587 were isolated by facile oxidation of the corresponding meta-terphenyl substituted chlorosilyliumyli-

Figure 73. Isolated NHC adducts of heavier silanechalcogenone complexes.

at a significantly lower field (δ = −32.9 to −49.6 ppm) than those of complexes 559. Because of the presence of the SiCh moieties, a depiction of various zwitterionic resonance structures is conceivable. However, a significant contribution of the SiCh double bond representation is justified by their characteristic SiCh stretching vibrations and the relatively short SiCh bond lengths (575a, d(SiS) = 2.006(1) Å; 576a, d(SiSe) = 2.1457(9) Å; 576b, d(SiSe) = 2.1399(9) Å; and 577b, d(SiTe) = 2.383(2) Å), which are slightly elongated in comparison to the series of reported silanoic ester derivatives (d(SiCh) = 1.980(2)−2.346(1) Å).603 The same group used their bis-NHC-stabilized monatomic zerovalent silicon complex (silylone) 582604 (vide infra) as a suitable precursor for the synthesis of the unprecedented silicon dichalcogenides 578−581.605,606 This chemistry has recently been reviewed in a comprehensive report and is therefore not discussed in detail here.607 Oxidation of 582 with elemental sulfur yields the first-ever isolated monomeric SiS2 complex 578. Treatment of compound 578 with GaCl3 gives the donor−acceptor-stabilized SiS2·GaCl3 complex 579. The selenium SiSe2·GaCl3 580 and tellurium SiTe2 581 analogues were also reported. The observed chemical shifts for the central silicon atoms in the 29Si NMR spectra show an increasing upfield shift (δ = −32.5 to −143.9 ppm) along with the increase in atomic numbers. The experimentally 9742


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dene 588a609 (vide infra) with elemental chalcogens in overall high yield (Figure 74).610 Interestingly, chalcogen exchange reactions from the heavier congeners 587c and 587b to the lighter ones were observed by addition of an equimolar amount of the respective chalcogen. However, the analogue reverse conversions are not possible. In addition, reduction of the heavier sila-acylium ions with coinage metal halides (AgI and AuI) regenerates the Si(II) silyliumylidene complex 588a. The corresponding signals of the complexes 587 in the 29Si NMR spectra, varying from δ = −36.0 to δ = −72.2 ppm, and the short SiCh double bond lengths (2.013(1)−2.3941(6) Å) coincide well with the analytical data of the compounds mentioned before. Recently, So and co-workers introduced a series of NHCsupported, three- and four-coordinate silicon chalcogenides 589−593, which are strongly related to the NHC adducts of silicon oxides 569−571598,599 reported by Robinson et al.611 Although the silicon atoms show all formal oxidation states from +1 to +4, the complexes are shown in this section. Reaction of (IDipp)SiSi(IDipp) 57269 (vide infra) with two molar equivalents of elemental tellurium furnishes a mixture of the first dimeric NHC-stabilized silicon monotelluride complex 589 and its structural isomer 590 (Scheme 90). Prolonged

Roesky et al. extended the list of carbene-stabilized disilicon tetrachalcogenides Si2Ch4 (Si2O4, 570;598 Si2Te4, 591;611 vide supra) with the successful isolation of the dimeric CAACstabilized silicon disulfide and semiconducting silicon diselenide complexes 594−596 (Figure 75).613,614 Treatment of

Figure 75. CAAC-stabilized silicon sulfides and silicon diselenides.

the corresponding diatomic silicon(0) compounds (CAAC)2Si2 598 (vide infra) with elemental sulfur and black selenium results in the formation of the (CAAC)2Si2Ch4 complexes 594−596. These compounds feature a symmetric Si2Ch4 core with an inversion center and thus resemble geometrically the respective Si2O4 complex 570 described by Robinson and colleagues. While neither solution nor solid-state 29 Si NMR signals were observed for the CAAC adducts 595 and 596, complex 594 resonates at δ = −25.0 ppm. X-ray analysis revealed short SiCh double bond lengths (d(SiS) = 2.011(5) Å (594) and d(SiSe) = 2.151(10) Å (595)), which are in good agreement with the earlier mentioned ones. The same group synthesized the CAAC-stabilized siliconthiodichloride complex 597, the first heavier silicon analogue of thiophosgene isolated in its monomeric form.615 The chemical shift of the silicon nucleus of (CAAC)SiSCl2 in the 29 Si NMR spectrum occurs at δ = 3.8 ppm, which is close to the calculated value of δ = 5.7 ppm. SC-XRD analysis unambiguously confirmed the monomeric molecular structure of CAAC adduct 597 with a SiS double bond length of 1.990(10) Å. On the contrary, the corresponding NHC analogue of 597 is not isolable, due to the beneficial superior σ-donor and π-acceptor properties of CAACs as compared to those of NHCs. 2.4.2.3. Low-Valent Silicon Complexes: Silylenes. As compared to silicon(IV) complexes, silicon compounds in low oxidation states are much more reactive and therefore quite challenging to isolate. For example, divalent silylenes, the heavier carbene congeners, feature a central silicon atom with a lone pair of electrons and an unoccupied 3pz orbital.616−619 The high reactivity of silylenes originates from this unoccupied orbital, while the lone pair is relatively chemically inert due to its high s-character. As a result of their intrinsically high singlet−triplet gap, silylenes are usually observed in the singlet ground state. NHC coordination reduces the electrophilicity of the Si(II) center, thereby limiting the reactivity of NHCstabilized silylenes. On the other hand, NHC silylene adducts often display a more pronounced nucleophilic character, which is responsible for the observation of intriguing reaction patterns. One of the first examples of a three-coordinate NHCsupported divalent silicon complex was reported by Lappert et al. in 1999.620 Reaction of the free carbene with the corresponding N-heterocyclic silylene (NHSi) affords the dark red-brown NHC silylene adduct 599 in high yield (Scheme 91). Silylene 599 resonates at 77.1 ppm in the 29Si NMR and features a long Si−C distance of 2.162(5) Å.

Scheme 90. Isomerization Pathway and Further Reactivity of the First Dimeric NHC-Stabilized Silicon Monotelluride Complex 589

reaction times lead to exclusive formation of the mixed-valent species 590. Further oxidation of 590 by elemental Te affords the NHC adduct of the dimeric silicon ditelluride complex 591 via the intermediary formed NHC-supported disilicon tritelluride complex 593. Additionally, 590 reacts with elemental sulfur under formation of the unprecedented NHC-stabilized disilicon sulfide ditelluride 592. The first molecular ternary silicon heterodichalcogenide 592 is formed concurrently with 591, 593, and Te by a disproportionation reaction of the former. The reaction pathways were elucidated by theoretical studies that support the experimental findings. The silicon nuclei resonate at lower frequencies (δ = −107.0 to δ = −166.4 ppm) as compared to the related NHC-stabilized silicon oxides 569− 571. The experimentally determined SiTe double bond lengths (2.246(7)−2.390(2) Å) comprise the shortest reported among the described NHC-supported silanetellone derivatives, which is even shortened in comparison to that of the parent H2SiTe (2.288 Å612). They are significantly shorter than the measured Si−Te single bond lengths (2.517(2)−2.641(8) Å) and thus display the inherent highly polarized double bond character, supported by DFT calculations. 9743


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[IDippH]Cl, can easily be removed by filtration. Very recently, the research group around Beckmann identified [(IDippH)2Cl][Si(SiCl3)3] as another byproduct generated by following the initial reaction procedure.625 The verified imidazolium salt features the interesting weakly coordinating anion Si(SiCl3)3−. On the other hand, reaction of (IDipp)SiCl4 (501) with the strong reducing agent KC8 in toluene leads to the same product 602, but with a much lower yield. It is noteworthy that the reduction is strongly solvent-dependent, thus furnishing a mixture of [(IDipp)Si]2 (572) and [(IDipp)SiCl]2 (603) in THF or n-hexane.69 The complexes 572 and 603 will be discussed in detail later. Similarly, reduction of the sterically less demanding Si(IV) halide complex (IMes)SiCl4 (502) affords NHC-stabilized dichlorosilylene 604. Later, it was shown that the reaction of Si2Cl6 with IDipp or IMes results in a Lewis base-induced disproportionation affording 602/604 and 501/502, respectively.626 Filippou and colleagues used the ionic NHC adduct [(IDipp)SiBr3]Br (506) as precursor for the synthesis of the unprecedented dibromosilylene analogue. Reduction of the former in THF gives the silicon(II) compound 605 in moderate yield.546 They also characterized the first Si(II) diiodide complex (IDipp)SiI2 (606), starting from the triiodosilylimidazolium salt 507 and using the same synthetic approach. Reaction of 507 with a molar excess of potassium graphite affords the corresponding NHC-stabilized diiodosilylene 606 as yellow crystals in high yield.547 The related NHCstabilized SiX2 complexes 607 and 608 with the saturated carbene SIDipp were also reported.548 The overall similar solid-state structures of the Si(II) dihalide complexes 602, 605, 606, and 608 feature trigonal pyramidal-coordinated silicon nuclei, due to the presence of stereochemically active lone pairs of electrons. The relatively short CNHC−Si distances (1.984(7)−2.007(5) Å) indicate a comparatively strong donor−acceptor interaction of the NHCsilylene adducts. Halide substitution (Cl → Br → I) is accompanied by the observation of more shielded Si(II) nuclei in the 29Si NMR spectra (602, δ = 19.1 ppm; 604, δ = 17.8 ppm; 605, δ = 10.9 ppm; 608, δ = 10.8 ppm; and 606, δ = −9.7 ppm). The isolation of the NHC-supported silicon(II) dihalides heralded the start of a very fruitful era in organosilicon chemistry. Above all, Roesky’s dichlorosilylene (IDipp)SiCl2 (602) paved the way for the synthesis of a plethora of novel silicon compounds. Adduct 602 displays a nucleophilic and redox-active lone pair, thus acting as a source of molecular SiCl2. Because most of the results have already been comprehensively summarized elsewhere, only a brief recapitulation is given here.627,628 Because the reaction of silylenes with unsaturated compounds is often observed, the reactivity of 602 toward diphenylacetylene was preliminarily investigated.543 The resulting cyclization reaction with an excess of PhCCPh furnishes the perchlorinated trisilacyclopentene complex 609 in 68% yield (Scheme 94). In a similar manner, 602 undergoes [1+2] and [1+4] cycloaddition reactions with benzophenone derivatives and diphenylethanedione.629 The NHC-stabilized silaoxiranes 610, illustrating monosilicon variants of heavier epoxides, are the first examples of their kind. In addition, compound 611 represents the dioxa analogue of NHCsupported trisilacyclopentene 609. The observed sharp signals in the 29Si NMR spectra, varying from δ = −99.5 to δ = −142.5

Scheme 91. Examples of Early Isolated NHC-Stabilized Silylene Complexes Including the Thermal Isomerization of 560a to 600

In a similar fashion, Driess and co-workers succeeded in isolating the NHC-stabilized β-diketiminato silylenes 560 as yellow crystals.589,590 The experimentally measured Si−C distances (560a, 2.016(3) Å and 560b, 2.065(2) Å) are significantly shortened in comparison to that of NHSi adduct 599, but much longer than the typical Si−C single bond (1.87 Å), thus indicating a stronger donor−acceptor correlation. The observed 29Si NMR shifts for the complexes 560 (560a, δ = −12.0 ppm and 560b, δ = −7.6 ppm) are upfield shifted as compared to the donor-free silylene 560′ (δ = 88.4 ppm621), illustrating stronger shielded Si(II) nuclei with a pronounced nucleophilic character. As a result, 560′ is relatively inert against chalcogen sources, whereas the NHC adducts 560 react with N2O, O2, S, Se, and Te to afford the NHC-stabilized complexes 559, 561, and 575−577 mentioned above.589−591 Additionally, silylene 560a with the less sterically hindered carbene IMe4 thermally tautomerizes via C−H activation to give the asymmetric N-heterocyclic silylcarbene 600.622 Novel triaminosilanes are formed via the reaction of 560a/600 and 560b with cyclohexyl isocyanide. Interestingly, compound 600 was used as an unprecedented bidentate carbene-silylene ligand for the synthesis of several nickel complexes.623 Percival, Driess, West, and colleagues used muon spin rotation spectroscopy to study the reaction of the NHCstabilized Si(II) complex 560b with muonium (Mu; singleelectron atom hydrogen equivalent with only one-ninth of its mass) (Scheme 92).624 Thereby, the formation of the radical species 601a and 601b was observed via two distinct muonium addition pathways. Scheme 92. Reactivity Investigations of 560b by Muon Spin Spectroscopy

A major breakthrough in low-valent silicon chemistry was achieved in 2009, when the groups of Roesky and Filippou simultaneously reported the preparation of NHC-supported SiX2 complexes (Scheme 93). Roesky et al. described two different methods for the synthesis of (IDipp)SiCl2 (602).543 On the one hand, treating trichlorosilane with 2 equiv of IDipp provides 602 under carbene-induced dehydrochlorination in 78% yield. The side product, the ionic imidazolium chloride 9744


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Scheme 93. Synthesis of NHC-Stabilized Si(II) Dihalides via Carbene-Induced Dehydrochlorination Reaction of HSiCl3, Reduction of Si(IV) Halide Precursors, or Lewis Base-Mediated Disproportionation Reaction of Hexachlorodisilane Si2Cl6

Scheme 94. Reaction of 602 with Alkynes, Ketones, Muonium, and Lewis Acids

Scheme 95. Preparation of a Series of “Push−Pull” Stabilized Silylene Species

ppm, are consistent with those of other five-coordinate silicon atoms. Percival, Roesky, West et al. irradiated (IDipp)SiCl2 (602) with muons to afford the radical 612 visualized by muon spin resonance spectroscopy and supported by DFT calculations.630 Comparable to the radical species 601b, the unpaired electron density is mainly localized on the carbene moiety. Treating the NHC-stabilized Si(II) dichloride 602 with the strong Lewis acid tris(pentafluorophenyl)borane leads to the donor−acceptor-stabilized complex 613.631 The silylene−borane adduct 613 highlights the intriguing ambiphilic nature of SiCl2, which simultaneously acts as both σacceptor and σ-donor. The associated stable dichlorosilylene− BH3 adduct 614 is obtained by the reaction of 602 with an equimolar amount of either lithium borohydride or BH3· THF.632 However, the intended hydride replacement with LiBH4 to form the unprecedented NHC-stabilized dihydrosilylene (IDipp)SiH2 was not observed. Whereas 614 resonates at lower field in the 29Si NMR spectrum (δ = 30.7 ppm) as compared to 602 (δ = 19.1 ppm), the silicon nucleus of complex 613 displays a large shift to higher field (δ = −53.2 ppm). XRD analysis revealed slightly shortened Si−C distances (613, 1.965(5) Å; and 614, 1.937(2) Å). The Rivard group used Roesky’s dichlorosilylene (IDipp)SiCl2 (602) and the dichlorosilylene−borane adduct (IDipp)SiCl2(BH3) (614) as precursors for the synthesis of various “push−pull” stabilized silylene derivatives (Scheme 95).633 Reaction of the hydride source lithium aluminum hydride with the dichlorosilylene−BH3 adduct 614 provides the donor−

acceptor-stabilized parent silylene complex 615 via chloride replacement.634 Because SiH2 displays the highest ambiphilicity of the parent tetrylenes, NHC adduct 615 shows a remarkable thermal stability. Treatment of 615 with the strong Lewis acidic transition metal carbonyl fragment [W(CO)5· THF] leads to the formation of the tungsten complex [(IDipp)SiH2]W(CO)5 (616) and the byproduct BH3·THF by a silylene group transfer reaction. The synthesis of the novel NHC-stabilized aminochlorosilylene 617 was described in 2012.635 Complex 602 reacts with the corresponding lithium amide to furnish 617 in 45% yield. LiBH4 converts the NHC adduct 617 into the Lewis acid/base-stabilized Si(II) amidohydride [(IDipp)Si(H)NHDipp(BH3)] (618). 29Si NMR spectroscopy shows signals in the highfield region at δ = −6.0 (617), δ = −55.6 (615), and δ = −71.6 ppm (616), respectively. The strong nucleophilic character of (IDipp)SiCl2 (602) enabled the reaction with coordinatively labile tungsten complexes, providing the perhalogenated “push−pull” stabilized germasilene and stannasilene complexes 619.636 Exposure of 619a to LiAlH4 and 619b to LiBH4 furnishes the first reported heavier ethylene homologues 620. Reacting 619a with LiBH4 instead results in the formation of the intermediate [(IDipp)SiCl2(GeH2)W(CO)5] by partial chloride exchange. The silicon nuclei exhibit signals in the 29Si NMR spectra varying from δ = −6.1 to δ = −91.1 ppm. Complex 620a undergoes a clean hydrosilylation reaction with acetylacetone to form an imidazolium salt. 9745


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628 and 629, respectively.128,634,638,639 The complexes 625− 629 show molecular geometries around the divalent silicon nuclei similar to those of the former ones. 29Si NMR spectroscopy revealed an analogous deshielding of the silicon atoms (δ = 64.6 to δ = 24.8 ppm), due to the coordination to the metal atoms. Additionally, conversion of the NHC adduct of dichlorosilylene 602 with LiP(Mes*)TMS results under TMSCl elimination in the formation of the first NHC-stabilized phosphasilenylidene (IDipp)SiPMes* (630), which will be shown in detail below.640 Filippou et al. utilized the saturated NHC-stabilized silicon(II) dihalides 607 and 608 as an entry into the chemistry of mononuclear chromium complexes. Treatment of 607 or 608 with Li[CrCp(CO)3] affords the first NHCstabilized halosilylidyne chromium complexes 631 in moderate yield (Scheme 96).548 The isotypic three-legged piano-stool

(IDipp)SiCl2 (602) was further used to gain access to some multiply bonded Si(IV) complexes. Accordingly, the NHCstabilized dichlorosilaimines 533a−c and 535 were synthesized by treating 602 with the carbodiimide DippNCNDipp, metaterphenyl azides, or Ph3SiN3 as previously mentioned (vide supra). 569,572 However, the reaction of 602 with 1azidoadamantane AdN3 did not furnish a silaimine complex but rather provided backbone-functionalized NHC and triazene derivatives.560 The NHC-supported silaformyl chloride 565 was synthesized via the reaction of 602 with H2O· B(C6F5)3 in the presence of IDipp, acting as a HCl scavanger.593 In addition, the capability of NHC-stabilized dichlorosilylene 602 to serve as a neutral σ-donoating ligand for transition metal complexes was investigated. Indeed, the research groups of Roesky, Rivard, and Ghadwal described a series of Lewis base-supported Si(II) dichloride transition metal complexes (Figure 76). Reaction of (IDipp)SiCl2 (602) with the metal

Scheme 96. NHC-Supported Halosilylidyne Chromium Complexes

complexes consist of trigonal planar-coordinated silicon nuclei and feature SiCr double bonds (631a, 2.160(7) Å and 631b, 2.162(9) Å). The NHC adducts are characterized by considerably downfield-shifted 29Si NMR resonances at δ = 113.6 (631a) and δ = 95.1 ppm (631b). A ligand substitution reaction is observed when 631b is exposed to the less bulky NHC IiPr2Me2, thus furnishing NHC adduct 632. Complex 632 shows a prolonged Si−Cr bond length of 2.252(7) Å and a more shielded Si nucleus at δ = 17.3 ppm, due to the additional coordination of NHC. Bromide abstraction via the reaction of 632 with Li[B(C6F5)4] results in the isolation of the dinuclear siloxycarbyne complex 633. The unsoluble dicationic NHC adduct 633 represents the formal dimer of the unprecedented chromium silylidene complex salt [Cp(CO)2CrSi(IiPr2Me2)2][B(C6F5)4]. Employing the same procedure as described for the synthesis of complex 631b, the NHC-stabilized zwitterionic bromosilylidene 634 was isolated from the reaction of (SIDipp)SiBr2 (608) with Li[CrCp*(CO)3] (Scheme 97).594 Complex 634 features similar structural characteristics and a comparable 29Si NMR chemical shift at δ = 74.8 ppm. Treatment of 634 with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (Na[B(ArF)4]) gives the chromium silylidyne salt 635 in 89% yield. The thermally stable complex represents the first example of a compound containing a CrSi triple bond (2.122(9) Å) and resonates at even lower field than 634 in the 29Si NMR spectrum (δ = 127.8 ppm). Because of the high electrophilic nature, NHC-stabilized silylidyne 635 readily reacts with carbon monoxide under

Figure 76. NHC-stabilized dichlorosilylene transition metal complexes.

carbonyls VCp(CO)4, Fe2(CO)9, and CoCp(CO)2 gives the corresponding donor−acceptor-stabilized complexes 621−623 including the first structurally characterized vanadium−silylene complex.637 SC-XRD analysis uncovered that the tetracoordinate Si(II) nuclei adopt distorted tetrahedral geometries in the solid state. Only recently was a transition metal SiCl2 complex with the group 6 metal tungsten isolated.128 NHC adduct 624 is obtained by converting 602 with W(CO)5·THF under liberation of THF and represents the first example of the hitherto unknown group 6 metal dichlorosilylene complexes. The examined resonances for the relatively distinct Si(II) centers of the complexes 621−624 in the 29Si NMR spectra (δ = 88.7 to δ = 26.9 ppm) are downfield-shifted in comparison to that detected for 602. Complex 624 reacts with the strong Brønsted base CsOH in the presence of IDipp furnishing the aNHC-complex (aIDippH)W(CO)5 and thus provides a new method for the transformation of a NHC to an aNHC. The pool of dichlorosilylene metal complexes also contains a variety of disubstituted 2:1 adducts of the general formula (IDipp)(SiCl2)2MLn. Reaction of the metal carbonyl derivatives Ni(CO)4, Cr(CO)5·THF, W(CO)5·THF, Co2(CO)8, and [Rh(CO)2Cl]2 with (IDipp)SiCl2 (602) leads to the formation of the neutral complexes 625−627 and the ionic complexes 9746


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are observed strongly upfield shifted (δ = −16.7 ppm) as compared to the donor-free complexes (R = Me: 275.3 ppm). The first NHC-supported hydrosilylyne complexes 638 are obtained upon treating the NHC adducts 637with another equivalent of IMe4 under alkane elimination of TMS3CH. The observed 29Si NMR resonances (δ = −25.4 to −16.7 ppm) are shifted to higher field in comparison to that of the related chromium complex 632 (δ = 17.3 ppm). SC-XRD analysis revealed three-legged piano-stool solid-state structures and a significantly shorter W−Si bond length (638b: 2.363(4) Å) than that of complex 637a (2.521(8) Å) and the respective pyridine compound (2.481(7) Å). Furthermore, Filippou and colleagues isolated the unprecedented NHC-stabilized bromo(silyl)silylene 639 bearing the bulky 2,6-bis[bis(trimethylsilyl)methyl]-4-tert-butylphenyl (Tbb) group, which was synthesized by two different methods (Scheme 99).642 Reaction of 2 equiv of the dibromosilylene

Scheme 97. Synthesis and Reactivity of the Novel Chromium Silylidyne and Chromiosilylene Complex Salts 635 and 636

Scheme 99. Synthetic Approach to NHC-Stabilized Bromo(silyl)silylene 639

formation of the two-coordinate cationic chromiosilylene 636. The experimentally determined Si−Cr bond length (2.393(2) Å) agrees well with the typical Si−Cr single bond length of 2.399 Å. NBO analysis revealed the presence of a lone pair of electrons with a high s-character localized at the silicon nucleus. The divalent silicon atom of the NHC adduct 636 exhibits a remarkable high isotropic 29Si NMR resonance at δ = 828.6 ppm, which reflects the electron deficiency. UV/vis spectroscopy depicted a strong bathochromic shift at 724 nm corresponding to the Sin → Si3p transition. Because of the small HOMO−LUMO gap (2.87 eV), NHC-stabilized chromiosilylene 636 reacts rapidly with small molecules such as H2, HCl, H2O, NH3, and N2O to provide novel silylchromium complexes of the general formula [Cp*(CO)3CrSi(H)X(SIDipp)][B(ArF)4] (X = H, NH2, OH, Cl) and the striking cationic chromiosilanone 566 (vide supra). Metallosilanone 566 is converted into the respective dihydroxysilyl complex [Cp*(CO)3CrSi(OH)2(SIDipp)][B(ArF)4] by contact with water. The Tobita group succeeded in the isolation of some organometallic compounds containing silicon−tungsten bonds (Scheme 98). Exposure of the pyridine adducts of silylene

(SIDipp)SiBr2 (608) with either the in situ generated transient silylene TbbBrSi: (thermolysis of the disilene TbbBrSi SiBrTbb643) or LiTbb furnishes 639 in moderate yield. SCXRD analysis showed a trigonal pyramidal geometry around the divalent silicon atom and a Si−C distance of 1.978(3) Å, which is in good agreement with other base-stabilized silylenes. Complex 639 exhibits a singlet for the Si(II) center in the 29Si NMR spectrum at δ = −1.9 ppm. Selective reduction of 639 with KC8 affords the NHC-stabilized disilavinylidene 640 (vide supra). Treating the NHC-supported Si(II) diiodide complex (IDipp)SiI2 (606) with additional σ-donating NHC ligands results in the formation of ionic products via iodide separation. In this manner, the iodosilyliumylidene cation NHC adduct 641 and the NHC-stabilized silicon(II) dication 642 were isolated and will be discussed thoroughly later.547 IDipp-stabilized diiodosilylene 606 further reacts with bis(dialkylamino)acetylenes in a [2+2+1] cycloaddition to provide the silicon(IV) 1,1-diiodo-2,3,4,5-tetraamino-1H-siloles 643 (Scheme 100).644 This conversion is rather intriguing, because (IDipp)SiX2 (X = Br, 605; X = I, 606) does not react with various alkynes and (IDipp)SiCl2 (602) forms the perchlorinated trisilacyclopentene complex 609 (vide supra) with diphenylacetylene.543 Treating the more stable 1H-silole derivative 643b with 2 equiv of IMe4 results in the formation of the unusual NHCstabilized dicationic Si(IV) salt 644. The short CNHC−Si distance (1.898(4) Å), which is only slightly longer than the Si−Cα(ring) bonds (1.862(3) Å), expresses the significant contribution of the respective bis(imidazolium) salt resonance structure. Two-electron reduction of 644 with potassium graphite gives the IMe4-stabilized divalent silacyclopentadienylidene 645 in 49% yield. Despite diverse attempts, no suitable single

Scheme 98. Synthesis of the NHC-Stabilized Hydrosilylyne Tungsten Complexes 638

complexes (C5Me4R)(CO)2(H)W SiH(Tsi)·py (R = Me, Et; Tsi = tris(trimethylsilyl)methyl) to the strong σ-donor IMe4 affords the NHC-stabilized complexes 637 via a Lewis base exchange reaction in high yield.641 Compound 637 shows 1 H NMR resonances for the W−H bonds at around −6.40 ppm with 1JW−H coupling constants of about 27 Hz, which implies the presence of weak Si···H−W bonding interactions. The corresponding 29Si NMR signals for the central Si nuclei 9747


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Scheme 100. Synthesis of 1H-Siloles 643 and Their Reactivity toward NHCs

Scheme 102. Ligand Substitution Reaction between 602 and an aNHC

dichlorosilylene complex but the stable dark blue unpaired biradical species 648 along with the functionalized IDipp derivative 649 (Scheme 103).648 SC-XRD analysis showed the Scheme 103. Conversion of the NHC-Supported Singlet Silylene 602 with Me2CAAC to the Biradical Derivative 648 and 649 and Further Examples crystals for SC-XRD analysis could be grown, and thus 645 was only characterized by multinuclear NMR spectroscopy and elemental analysis. The observed resonances in the 29Si NMR spectra (644, δ = −29.2 ppm; 645, δ = −69.6 ppm; 643b, δ = −83.5 ppm; and 643a, δ = −87.8 ppm) illustrate more shielded silicon nuclei as compared to that of (IDipp)SiI2 (606) (δ = −9.7 ppm). Recently, the reactivity of the NHC-stabilized Si(II) diiodide 606 toward an azide and a diazoalkane was reported.578 Conversion of 606 with MesN3 and (p-Tol)2CN2 affords the NHC-supported silaimine 544 and silazine 548 as already shown above. Another astonishing example of the versatile chemistry of the NHC-stabilized silicon(II) dihalides (NHC)SiX2 602−608 has been shown lately (Scheme 101). The selective reaction of existence of two almost identical polymorphs with a tetrahedral coordination sphere around the Si atoms and very short CCAAC−Si bond lengths of about 1.847 Å (cf., 602: d(Si−C) = 1.985(4) Å), which are in the range of typical Si−C single bonds. The major polymorph features a diamagnetic closedshell electronic configuration and exhibits an upfield shifted 29 Si NMR resonance at δ = 4.1 ppm, whereas the minor paramagnetic one is EPR active. The inherent bonding situation is therefore best described with two electron-sharing bonds instead of classical donor−acceptor bonds. The application of the carbene derivative CyCAAC and/or (IDipp)SiBr2 (605) resulted in the isolation of the similar 1,3biradicals 650 and 651, including the heavier homologues of complex 648.649 The analogous CAAC coupled NHC 652 was identified as a byproduct. However, due to the better stabilization of the biradical state with more electronegative halide substituents, the bromo derivatives are significantly less stable and thus only isolated in low yield. The unpaired biradicals 650 and 651 feature isostructural geometries in the solid state with similar short Si−C distances varying from 1.843(2) to 1.859(18) Å, but remain silent in the 29Si NMR measurements. Strikingly, the reduction of the unpaired biradical species 648 and 651 with potassium graphite or organolithium reagents results in the formation of the biradicaloid dicarbene-coordinated silylones 653 (vide infra).649,650 In addition, the laborious approach to biradical 648 was significantly shortened by conversion of the Si(IV) adduct (Me2CAAC)SiCl4 (522)555 with lithium diisopropylamide (LDA).649 The non-nucleophilic base acts in a peculiar fashion

Scheme 101. Novel Synthetic Approach to Decamethylsilicocene 646

602 or 608 with 2 equiv of potassium pentamethylcyclopentadienide (KCp*) furnishes Jutzi’s decamethylsilicocene SiCp*2 (646)645 in good yield.646 This rather simple synthetic approach displays a clear simplification as compared to the original multistep process. However, using 605 or 606 as Si(II) source leads to the formation of several byproducts, such as Cp*H and Cp*2. The stronger σ-donor character of aNHCs as compared to that of NHCs enabled a selective Lewis base exchange reaction (Scheme 102). Exposing complex 602 to an aNHC gives the aNHC-stabilized Si(II) dichloride 647 in high yield.647 The revealed 29Si NMR resonance (δ = 24.2 ppm) does not differ significantly from that observed for NHC adduct 602 (δ = 19.1 ppm). Contradictorily, performing the same reaction with Me2CAAC does not provide the expected CAAC-stabilized 9748


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as a reducing agent and 648 is obtained in 48% yield. Nonetheless, adopting this novel method to the related (CyCAAC)SiCl4 (523) was unsuccessful. To still gain access to the CAAC-supported SiCl2 complex, the reduction of the CAAC-stabilized silicon tetrachlorides was further investigated in depth. However, due to the different nature of CAACs in comparison to NHCs, the desired compound is not available so far. Treating the CAAC adducts of SiCl4 522 and 523 with an equimolar quantity of KC8 results in the formation of the unprecedented trichlorosilylcarbene radicals 654 in moderate yields (Scheme 104).555 The

Scheme 106. Preparation of the Radicals (CAAC)SiCl2PPh2

quasi-reversible process (E1/2 = −0.96 V vs ferrocenium/ ferrocene (Fc+/Fc)), indicating the formation of the respective anion. In addition to the unpaired biradical species (CAAC)2SiX2 mentioned above, Roesky and co-workers were also able to stabilize the parent SiH2 moiety in an acceptor-free manner for the first time (Scheme 107).656 The paramagnetic compound

Scheme 104. Reduction of CAAC-Stabilized SiCl4 with One Equivalent of KC8

Scheme 107. Preparation of the CAAC-Stabilized Biradical (CAAC)2SiH2 paramagnetic carbon-centered radicals display a fluorescent yellow color and do not resonate in the 29Si NMR, due to the radical character. The experimentally determined Si−C bond lengths (654a, 1.815(12) Å and 654b, 1.819(8) Å) are even shorter than those of the biradicals 648, 650, 651, and the precursor complex 522 (1.944(2) Å). Thus, the bonding situation changed from a dative bond to a covalent one as a result of the reduction. The utilization of excessive amounts of KC8 for the conversion of the complexes 522 and 523 delivered a variety of novel compounds such as (Me2CAAC)2Si2Cl2 (655) and (CAAC)2Si2 598, which is strongly related to the reduction of (IDipp)SiCl4 (501) reported by Robinson et al.69 (see below).614,651−654 The reaction of the trichlorosilylcarbene radical 654b with phenyllithium furnishes the red carbon-centered radical 656 via chloride substitution in 90% yield (Scheme 105).525 The

658 is synthesized by the reduction of diiodosilane with KC8 in the presence of Me2CAAC in moderate yield. SC-XRD analysis, EPR spectroscopy, and theoretical calculations revealed a similar bonding situation as inherent in the biradicals 648, 650, and 651, with the triplet state being lower in energy than the corresponding singlet state. Only very recently was the CAAC-stabilized silicon tetrafluoride 525 isolated and its subsequent reduction investigated (Scheme 108).557 Treatment of (Me2CAAC)SiF4 Scheme 108. Reactivity of Me2CAAC·SiF4 (525) toward KC8 and Me2CAAC

Scheme 105. Conversion of (CyCAAC)SiCl3 with PhLi to (CyCAAC)SiPh3

(525) with two molar equivalents of KC8 in the presence of an additional equivalent of Me2CAAC affords the dark purplecolored CAAC-coordinated silicon difluoride 659 in 45% yield. The solid-state structure is in good accordance with the related (CAAC)2SiR2 compounds; however, biradical 659 is EPR silent and thus displays no paramagnetic nature. On the other hand, 29Si NMR spectroscopy exposed a triplet signal at δ = −29.7 ppm with a coupling constant 1JF−Si of 275 Hz. Thorough DFT studies revealed a biradicaloid character with the electronic singlet state being lower in energy than the respective triplet state, which is contradictory to the similar reported species. Moreover, performing the same reaction with an equimolar amount of KC8 in the absence of Me2CAAC provides the first silicon trifluoride monoradical 660 in low yield. The experimental and theoretical data of (Me2CAAC)SiF3 (660) are in line with those obtained for the related radicals 654 and 656. Remarkably, 660 features the shortest CCAAC−Si bond

tetra-coordinate silicon nucleus adopts a distorted tetrahedral geometry similar to that observed in the precursor. The triphenylsilylcarbene radical 656 exhibits three hyperfine lines, in contrast to the 15 for 654, in the X-band EPR spectrum at g = 2.0019, due to the coupling with one 14N nucleus. Supportive DFT calculations confirm the experimental data. Additionally, a similar pair of CAAC-supported monoradicals 657 is synthesized via one-electron reduction of Cl3SiPPh2 with KC8 in the presence of 1 equiv of RCAAC (R = Me2, Et2) in 22−25% yield (Scheme 106).655 Complexes 657 represent the first experimentally realized examples of stable radicals containing phosphinochlorosilane moieties. Analytical and theoretical data uncovered similar structural motifs in comparison to the radicals 654 and 656, with the unpaired electron mainly localized on the carbene carbon atom. Cyclic voltammetric (CV) inverstigations showed a one-electron 9749


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length (1.795(1) Å) in the series, slowly approaching the typical range for SiC double bonds (1.70−1.76 Å).566 It is noteworthy that initial fluorination attempts of the respective chloro derivatives with several reagents such as Me3SnF, CsF, and C5F5N to produce 659 and 660 were unsuccessful. Finally, So and colleagues synthesized the first CAACstabilized dihalosilylene (Me2CAAC)SiI2 (661) by adopting the same method as Filippou et al. used for the preparation of the corresponding NHC derivative (IDipp)SiI2 (606) (Scheme 109).547,576

Scheme 110. Synthesis of Arylchlorosilylene NHC Adducts and Further Reaction of 664b To Afford the First NHCStabilized Phosphinosilylene 584

higher field in the 29Si NMR spectra (664a, δ = 1.3 ppm; 664b, δ = 0.8 ppm) than do the later ones. Reaction of the more sterically demanding derivative 664b with lithium diphenylphosphide results in the formation of the unprecedented NHC-coordinated acyclic phosphinosilylene 584.597 The silicon nucleus of 584 exhibits a doublet resonance signal at δ = −39.2 ppm in the 29Si NMR spectrum, therefore indicating a higher electron density at the Si(II) center. A considerable contribution to the electronic structure of 584 by the zwitterionic resonance structure was suggested and supported by the short CNHC−Si bond length (d(Si−C) = 1.958(3) Å). The NHC−silylene adduct 584 was used as a versatile precursor for the reaction with various chalcogen sources. Thus, the series of novel NHC-stabilized silanechalcogenones 568, 583, 585 with SiE moieties (E = O, S, Se) (vide supra) was reported. Conversion of 664b with the alkali metal phosphide derivatives LiPH2·dme and LiPTMS2·dme gives the aforementioned NHC-stabilized phosphasilene complexes 553a and 553b.582 Treatment of 553a with the Lewis acidic tungsten complex [W(CO)5·THF] provides the “push−pull” stabilized phosphasilene 554. The NHC-supported silaimines 545 and 546 are obtained from the reaction of 664b with the azides MesN3 and TMSN3 in high yield.578 Similarly, the diazoalkane (2,6-Mes2-C6H3)HCN2 reacts with the arylchlorosilylene NHC adducts 664 under formation of the NHC-stabilized silazines 547 (vide supra). The arylchlorosilylene adduct 664b was further used to gain access to the first complex ever described with a metal−silicon triple bond (Scheme 111).658 The NHC-stabilized arylsilyli-

Scheme 109. Synthesis of the First CAAC-Stabilized Diiodosilylene 661 and the Synthesis of Silaimine 550

Conversion of the CAAC-coordinated SiI4 524 with 2 equiv of potassium graphite in toluene furnishes the divalent diiodosilylene 661 in 58% yield. Remarkably, the formation of radical species as in the case of the lighter congeners was not observed during the two-electron reduction; however, the utilization of excessive amounts of KC8 forms 2,3-disiladiiodobutadiene (Me2CAAC)2Si2I2 (662) as byproduct (vide infra). The 29Si NMR resonance at δ = −2.1 ppm is observed at a slightly lower field as compared to that of (IDipp)SiI2 (606) (δ = 9.7 ppm). XRD analysis revealed the presence of a stereochemically active lone pair of electrons, due to the distorted trigonal pyramidal-coordinated Si(II) nucleus. Suprisingly, the Si−CCAAC distance (Me2CAAC)SiI2 (661) (2.013(5) Å) is significantly elongated in comparison to those of the mono- and biradical derivatives and even to those of the NHC-stabilized SiX2 complexes 602, 604−608 (1.984(7)− 2.007(5) Å). Thus, the bonding situation is best described as a coordinative bond with a negligible π-backbonding. DFT calculations underpin these observations and unveiled the HOMO showing a high s-character on the low-valent silicon atom. The reaction of complex 661 with azido(trimethyl)silane, affording the first CAAC-stabilized silaimine 550 under liberation of N2, demonstrates the nucleophilic nature of (Me2CAAC)SiI2 (vide supra). In addition, treating CAAC adduct 661 with 1 equiv of NHC results in the formation of the NHC- and CAACstabilized iodosilyliumylidene iodide 663, which is strongly related to the conversion of (IDipp)SiI2 (606) to the NHC adduct 641 observed by the Filippou group. These results will be discussed in detail below.547,576 Filippou et al. used the dehydrochlorination approach to isolate the first NHC-stabilized arylchlorosilylenes 664.657 Reaction of the respective aryl-substituted chlorosilanes ArSiHCl2 (Ar = 2,6-Mes2C6H3, 2,6-Tipp2C6H3) with IMe4 yields the air-sensitive complexes 664 in high yield (Scheme 110). SC-XRD analysis of 664b revealed a structural motif that is very similar to that of the dihalosilylenes. The C−Si distance with a length of 1.963(2) Å and the high degree of pyramidalization compare well with those observed for the silicon(II) dihalides. The tetrel atoms resonate at a slightly

Scheme 111. Synthesis of the First Transition Metal Silylidyne Complex 666

dene complex 665 was isolated from the reaction of (IMe4)SiCl(2,6-Tipp2-C6H3) (664b) with Li[MoCp(CO)3] in moderate yield. Subsequent NHC abstraction via conversion of 665 with B(p-Tol)3 gives the Lewis base-free molybdenum silylidyne complex [Cp(CO) 2 MoSi(2,6-Tipp 2 -C 6 H 3 )] (666). Complex 666 shows a contracted MoSi triple bond of 2.224(7) Å (665: 2.345(6) Å) and a considerable downfield resonance in the 29Si NMR at δ = 320.1 ppm (665: δ = 201.8 ppm). 9750


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aminochlorosilylene 617 and the arylchlorosilylenes 664 (δ = 1.34 to δ = −6.0 ppm). The NHC-stabilized aminochlorosilylene 670 proved to be a valuable starting material for the preparation of a series of novel compounds, as already mentioned above. Reaction of 670 with an equimolar amount of SiCl4 in diethyl ether provides the NHC-supported silaimine 534 in low yield.570 All attempts to increase the yield by changing the reaction conditions only led to the formation of other products such as (IiPr2Me2)SiCl4 (491c), Dipp(TMS)NSiCl3, the silaimine dimer, etc. Recently, a novel dehydrosilylation approach for the synthesis of complex 534 has been reported.571 Treatment of the aminodichlorosilane Dipp(TMS)NSiHCl2 with less than 2 equiv of IiPr2Me2 in THF yields the NHC-stabilized silaimine 534 in addition to the aminochlorosilylene 670. The corresponding equimolar reaction in Et2O instead gives the disilane Dipp(TMS)NCl2Si−SiHClN(TMS)Dipp in good yield. Conversion of NHC adduct 670 with electron-deficient internal or terminal alkynes affords the variety of NHCcoordinated cis-1,2-bis-silylated alkenes 538 in an efficient and regiospecific manner.575 In a similar fashion, several NHC-stabilized trans-1,2disilylimines 539 are accessible in a highly regioselective process by treating 670 with different aryl-substituted nitriles.576 The complexes 539 react with phenyllithium reagents in a subsequent reaction to form the first NHCsupported silaaziridines 540. Furthermore, the reactivity of NHC adduct 670 toward ketones was investigated thoroughly.660 Whereas the treatment of 670 with enolizable ketones results in the synthesis of silicon bis-enolate derivatives, the reaction of 670 with benzophenone leads to a bis-silylation of the carbonyl moiety and affords a four-membered ring product. The NHC-coordinated phosphinoaminosilylene 543 is obtained through a salt metathesis reaction of complex 670 with lithium diphenylphosphide in moderate yield (Scheme 113).577 The solid-state structure of NHC adduct 543 revealed

Tokitoh, Sasamori, Matsuo, and co-workers used the stable 1,2-dibromodisilenes (E)-ArBrSiSiBrAr (Ar = 2,6-bis-[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl (Bbt) and 1,1,7,7-tetraethyl-3,3,5,5,-tetramethyl-s-hydrindacen4-yl (EMind)) to synthesize the NHC-stabilized arylbromosilylene derivatives 667 and 668a by treating the former ones with 2 equiv of NHCs (Figure 77).643

Figure 77. NHC-stabilized arylbromosilylenes 667 and 668.

The complexes 667 and 668a have not been isolated in pure form, due to their extreme instability. Because the 1,2dibromodisilenes can react with NHCs to furnish the NHCstabilized silyliumylidene ions 669 (vide infra), the isolation of the intermediary arylbromosilylene NHC adducts is hindered. However, the observed signals in the 29Si NMR spectra (667, δ = 10.9 ppm; 668, δ = 13.1 ppm) agree well with the related arylchlorosilylenes 664 (664a, δ = 1.3 ppm; 664b, δ = 0.8 ppm) and the theoretical ones (calculated by GIAO-DFT). Most recently, they extended the pool of NHC-stabilized arylbromosilylenes by isolating complex 668b, bearing the more sterically demanding Eind (1,1,3,3,5,5,7,7-octaethyl-shydrindacen-4-yl) group.659 The NHC adduct 668b is accessible via treatment of either (Eind)BrSiSiBr(Eind) or (Eind)SiHBr2 with 2 equiv of IiPr2Me2. The divalent silicom atom resonates at an even lower field (18.0 ppm) in the 29Si NMR spectrum. The first NHC-supported aminochlorosilylene 670 was isolated by Cui et al. in 2011 (Scheme 112).559 Because the

Scheme 113. Synthesis of the NHC-Stabilized Phosphinoaminosilylene 543

Scheme 112. NHC-Mediated Dehydrochlorination Furnishes the First NHC-Stabilized Aminochlorosilylene 670

the characteristic trigonal pyramidal-coordinated silicon nucleus, exhibiting the presence of a stereochemically active lone pair of electrons at the divalent silicon center. The observed doublet in the 29Si NMR spectrum (δ = 4.2 ppm, 1 JP−Si = 78.4 Hz) is further shifted to a lower field as compared to the precursor 670 (δ = 3.1 ppm). Controlled oxidation of complex 543 with O2 furnishes the NHC-stabilized 1siloxysilaimine 541 and the 1-siloxy-1-phosphinoxysilaimine 542 derivatives (vide supra). Reaction of 670 with LiPH(2,6-Mes2-C6H3)·THF instead provides the NHC-supported 2-aminophosphasilene 552 in high yield, because the respective NHC-stabilized phosphinoaminosilylene is thermally labile and undergoes a 1,2 H shift.583 Successive NHC abstraction by reacting complex 552

disproportionation reaction of the 1,2-diaminotrichlorodisilane Dipp(TMS)NCl2Si−SiHClN(TMS)Dipp with 3 equiv of IiPr2Me2 in refluxing deuterated benzene already slowly decomposes the NHC adduct 670, a low temperature approach was also developed. NHC-induced dehydrochlorination of the corresponding aminodichlorosilane affords the NHC-coordinated aminochlorosilylene 670 in 75% yield. Complex 670 features similar structural characteristics as compared to other NHC-stabilized silylenes and resonates slightly downfield-shifted in the 29Si NMR spectrum (δ = 3.1 ppm) in comparison to the related NHC adducts of 9751


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with the Lewis acid BPh3 yields the corresponding base-free phosphasilene, as was already mentioned above. In addition, reactivity studies of the NHC-stabilized aminochlorosilylene 670 with respect to Lewis acids and Lewis bases were conducted (Scheme 114).661 Treatment of

Scheme 115. NHC-Stabilized 1-Silacyclopentadienylidenes 675 and the Reactivity toward Phenylacetylene

Scheme 114. Reactivity of Complex 670 toward Lewis Acids and Lewis Bases

675b with 2 equiv of phenylacetylene to give the 1-alkenyl-1alkynylsilole 676 exclusively as the E-isomer, which is contradictory to the usually observed [2+1] cycloaddition reaction of silylenes affording silacyclopropene derivatives. Treating 675b with various aldehydes RCHO (R = Et, iPr, t Bu, CH2tBu) in the presence of AlCl3 results in the formation of the donor−acceptor-stabilized silanone species 563 and 564 via proposed cyclopropanation of the CC bond and subsequent CO bond cleavage (vide supra).592 Recently, the reactivity investigations of the NHC-stabilized carbocyclic silylene 675b toward alkynes were extended (Figure 78).568 Exposure of complex 675b to an equimolar

(IiPr2Me2)[Dipp(TMS)N]SiCl (670) with an equimolar amount of the sterically less encumbered IMe4 results in the formation of the NHC-coordinated silylene 671 via a selective NHC substitution in high yield. Complex 671 displays almost the identical 29Si NMR resonance at δ = 2.7 ppm than its precursor 670 (δ = 3.1 ppm). Notably, NHC adduct 671 is not directly accessible through IMe4-induced dehydrochlorination of the respective aminodichlorosilane because the reaction furnishes the NHC-stabilized silaimine 536 (vide supra).573 On the other hand, the utilization of the bulkier NHC ItBu provides the functionalized NHC 672. Thus, steric effects play a crucial role in the chemistry of the NHC-supported silylene 670 and its precursors. Reaction of complex 670 with the Lewis acid complex BH3· THF furnishes the donor−acceptor-stabilized silylene−borane adduct 673 in 82% yield. The remarkable air-stable compound 673 adopts a distorted tetrahedral geometry around the divalent silicon nucleus in the solid state and exhibits a lowfield-shifted quartet in the 29Si NMR spectrum at δ = 12.6 ppm, due to the coordination to the boron moiety. In contrast, conversion of the NHC-stabilized aminochlorosilylene 670 with the stronger Lewis acid B(C6F5)3 in THF is rather unselective. However, some colorless crystals of the THF ringopening product 674 suitable for SC-XRD analysis were isolated. This observation points to the formation of an unprecedented frustrated Lewis pair, which consists of a silylene and a boron-based Lewis acid, capable of activating THF and presumably other small molecules. Cui and colleagues described the isolation of the first NHCstabilized 1-silacyclopentadienylidenes 675 synthesized via the NHC-mediated dehydrochlorination reaction of the corresponding chlorotetraphenylsilole Ph4 C4 SiHCl (Scheme 115). 662 The divalent silicon nuclei exhibit highfield resonances in the 29Si NMR spectra (675a, δ = −48.6 ppm and 675b, δ = −43.6 ppm), which are comparable to that of the tetraamino-substituted complex 645 (δ = −69.6 ppm). SCXRD analysis of 675b showed a three-coordinate silicon atom with a short CNHC−Si bond length of 1.926(3) Å, indicating a strong donor−acceptor interaction and thus suggesting a highly nucleophilic nature of the complexes 675. This character was confirmed experimentally by the conversion of

Figure 78. Reaction products of the conversion of 675b with internal alkynes.

amount of aryl-substituted internal alkynes affords the NHCsupported fused 1-silabicyclo[3.2.0]hepta-1,3,6-trienes 532 bearing silene moieties in moderate yield (see above). Treatment of the NHC adduct 532a with another equivalent of diphenylacetylene leads to the formation of the bicyclic 1,1′spirobisilole 677a. The direct conversion of 675b to 677a is also accessible without the isolation of the intermediary formed NHC-stabilized complex 532a. This reaction tolerates a variety of functionalized alkyne derivatives providing compounds 677 and thus underlines the potential of the NHC-stabilized 1silacyclopentadienylidenes to serve as building blocks for unprecedented silabicycles. Cui et al. used the silafluorene derivative 678 as a starting material for a NHC-mediated dehydrochlorination reaction with the intention to synthesize a novel disilene.663 Nevertheless, the reaction of 678 with IiPr2Me2 results in the cleavage of the Si−Si single bond under formation of the NHC-stabilized 1-silacyclopentadienylidene 679 (Scheme 116). The observed resonance for the Si(II) nucleus in the 29 Si NMR spectrum at δ = −33.0 ppm is close to those of the related complexes 645 and 675 (δ = −43.6 to −69.6 ppm), nonetheless considering the differences in the ligand framework. Sekiguchi and co-workers performed reductive debromination reactions of dibromosilanes with potassium graphite in the 9752


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Scheme 116. Preparation of the NHC-Coordinated 1Silacyclopentadienylidene 679

Scheme 118. Formation of the NHC-Coordinated Si3Cyclopropylidene 683

disilenide 682. The low-valent silicon nucleus exhibits a downfield-shifted singlet in the 29Si NMR spectrum at δ = 110.5 ppm as compared to the bis(silyl)silylene complexes 680. The IiPr2Me2-stabilized Si(II) center adopts a trigonal pyramidal geometry, a characteristic feature of NHCsupported silylenes. The first example of an acceptor-free NHC-stabilized silylene monohydride complex was reported by our group. Dehydrochlorination of the supersilyl-substituted chlorosilane t Bu3SiSiH2Cl with 2 equiv of IMe4 affords the NHC adduct of hydrosilylene 684 in 41% yield (Scheme 119).667

presence of NHCs to obtain the first examples of NHCsupported bis(silyl)silylenes 680 in moderate yield (Scheme 117).664 The solid-state structure of 680b revealed the typical Scheme 117. Synthesis of the NHC-Stabilized Bis(silyl)silylenes 680 and the Synthesis of Silylene Radical Cation 681

Scheme 119. Synthesis of the NHC-Stabilized Hydrosilylene 684

The divalent Si−H moiety is clearly observed in the corresponding proton (δ = 3.17 ppm) and silicon (δ = −137.8 ppm) NMR spectra with a coupling constant 1JSi−H of 101 Hz. XRD analysis confirmed the monomeric structure of 684 containing a highly pyramidalized Si(II) nucleus and a CNHC−Si distance of 1.942(3) Å, which is in good agreement with the silylene species mentioned above. Additional DFT calculations revealed that the HOMO represents a lone pair orbital at the silicon center. To gain insight into the nucleophilicity of the NHCstabilized hydrosilylene 684, the complex was treated with bis(1,5-cyclooctadiene)nickel(0) (Ni(COD) 2 ) (Scheme 120).667 The highly selective reaction provides the intriguing 1,2-dihydrosilene nickel(0) complex 684 in 86% yield. The reaction mechanism involves the stepwise NHC migration from the divalent Si(II) nucleus to the transition metal. On the basis of the experimentally determined data and supportive DFT calculations, the bonding situation is best described as metallacyclopropane rather than a typical disilene transition metal π-complex. Because silylenes bearing Si−H moieties may be capable of undergoing hydrosilylation reactions, the reactivity of NHC adduct 684 toward internal and terminal alkynes was thoroughly investigated.668 Conversion of 684 with 2 equiv of diphenylacetylene leads to the formation of the silole derivative 686 in moderate yield via a [2+2+1] cycloaddition reaction. On the other hand, the reaction of 684 with the C−H acidic phenylacetylene furnishes the 1-alkenyl-1-alkynylsilane 687, which is strongly related to the formation of silole 676 observed by Cui et al.662 The postulated mechanisms, supported by detailed theoretical calculations, include the nucleophilic attack of the lone pair on the individual alkyne

pyramidal geometry around the tricoordinate low-valent silicon center (sum of bonding angles at Si: 344.3°) and a short CNHC−Si distance of 1.933(4) Å. The central silicon atoms exhibit highfield-shifted resonances in the 29Si NMR spectra at δ = −132.3 ppm (680a) and δ = −128.9 ppm (680b) as compared to other NHC-stabilized silylenes, due to the adjacent σ-donating silyl groups. One-electron oxidation of complex 680b with a trityl cation derivative provides the unprecedented NHC-coordinated silylene radical cation 681 in high yield. SC-XRD analysis showed a planar geometry with a widened Si−Si−Si bond angle from 134.81(6)° (680b) to 144.96(4)°. NHC adduct 681 shows a quintet signal in the EPR spectrum at g = 2.00466 with a hyperfine coupling constant (hfcc) of a(14N) = 0.26 mT. According to the experimental and supportive theoretical data, complex 681 can best be described as a π-type radical cation. The NHC-stabilized silylene 680b can be regenerated by the reversible reduction of 681 with KC8. Scheschkewitz et al. isolated a NHC-stabilized cyclic bis(silyl)silylene with a Si3 scaffold. Reaction of the Si(IV) tetrahalide NHC adduct (IiPr2Me2)SiBr4 (494) with the lithium disilenide Tipp2SiSiTippLi (682)665,666 forms the NHC-supported cyclopropylidene 683 in 62% yield along with the 1,1,2-tribromosilane TippBr2Si−SiBrTipp2 as the main byproduct (Scheme 118).539 The proposed mechanism involves the initial formation of a NHC-stabilized disilenylbromosilylene and subsequent isomerization to complex 683, due to the nucleophilic and reductive nature of the lithium 9753


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Scheme 120. Reactivity of Hydrosilylene Complex 684 toward a Nickel Complex, Alkynes, Benzophenone, and Carbon Dioxide

Scheme 121. Donor−Acceptor-Stabilized Silylene Transition Metal Complexes

ones observed for silylene transition metal complexes (d(Si−Fe) = 2.154−2.363 Å and d(Si−W) = 2.337−2.636).541 A few related complexes with different substitution patterns were also synthesized to gain a deeper understanding of the influence of the substituents on the bonding situations.541 Treating the NHC-stabilized Si(IV) compounds 491a, 495, and 496 with the Collman’s reagent Na2[Fe(CO)4] forms the donor−acceptor-stabilized silylene transition metal complexes 693 in overall good yield. Increasing the electronegativity of the substituents leads to considerably lower field resonances observed in the 29Si NMR spectra (692a, δ = −48.3 ppm → 693b, δ = −25.5 ppm → 693c, δ = 29.4 ppm → 693a, δ = 63.0 ppm). The Si−Fe bond lengths of the complexes 693 (693a, 2.242(3) Å; 693b, 2.295(3) Å; and 693c, 2.327(5) Å) are shorter than that of NHC adduct 692a, thus illustrating that electron-withdrawing substituents shorten the Si−Fe bond by decreasing the electron density at the divalent silicon atom. In depth DFT calculations support these observations. Müller et al. recently reported a novel synthetic approach to NHC-stabilized hydrosilylenes, which complements the NHCassisted dehydrohalogenation of halosilanes and the reduction of silicon dihalides in the presence of NHC. Conversion of the respective meta-terphenyl-substituted dibenzosilanorbornadienes with an equimolar amount of IMe4 affords the NHCstabilized arylhydrosilylenes 694 under liberation of anthracene in low yield (Scheme 122).671 The low yield evolves from

carbon atoms, resulting in the formation of NHC-supported zwitterionic intermediates and transition states. Moreover, the reaction of the NHC-stabilized hydrosilylene 684 with 1-phenyl-2-trimethylsilylacetylene in the presence of an additional equivalent of IMe4 gives the NHC derivative 688.669 Remarkably, the silyl-substituted alkyne exhibits the slowest reactivity of the investigated alkynes. The reactivity studies of the NHC-stabilized silylene monohydride 684 were extended to compounds with CO double bond functionalities (Scheme 120).670 The outcome of the conversion of 684 with benzophenone strongly depends on the applied reaction conditions. Treating NHC adduct 684 with one molar equivalent of benzophenone at low temperatures furnishes the bicyclic product 689 as a racemic mixture. The instability of compound 689 (mainly decomposition to t Bu3SiH) prevented the clean isolation. However, the same reaction with 2 equiv at room temperature exclusively forms the stable bicycle 690 in high yield. Furthermore, exposure of the NHC-stabilized hydrosilylene 684 to an excess of carbon dioxide stereoselectively affords the cis/trans-cyclotrisiloxane 691 in 82% yield. Compound 691 symbolizes the formal trimer of the respective silaaldehyde obtained by a facile headto-tail cyclotrimerization. Recently, the coordination ability of the NHC-coordinated silylene monohydride complex 684 toward metal carbonyls was studied.541 Reaction of 684 with Fe(CO)5 or W(CO)5· THF gives the Lewis acid/base-stabilized hydrosilylene complexes 692 (Scheme 121). The observed downfield-shifted signals of the Si−H moieties in the 1H NMR (692a, δ = 4.97 ppm and 692b, δ = 4.50 ppm) and 29Si NMR spectra (692a, δ = −48.3 ppm and 692b, δ = −94.2 ppm) prove the successful coordination to the transition metal centers. SC-XRD analysis revealed tetrahedral coordination spheres around the divalent silicon nuclei and typical Si−C distances varying from 1.934(6) to 1.952(7) Å. However, abnormally long Si−M bond lengths (d(Si−Fe) = 2.372(16) Å (692a) and d(Si−W) = 2.640(19) and 2.668(2) Å (692b)) were found, which deviate significantly from the usual

Scheme 122. NHC-Mediated Fragmentation of 7Silanorbornadienes To Furnish the NHC-Stabilized Arylhydrosilylenes 694

the tedious purification procedure, involving sublimation at a rather high vacuum (110 °C at 2.0 × 10−6 mbar) and subsequent recrystallization. The divalent silicon nuclei exhibit downfield resonances in the 29Si NMR spectra at δ = −87.6 ppm (694a) and δ = −80.5 ppm (694b) as compared to the silylhydrosilylene 684 (δ = −137.8 ppm), but they resonate at a higher field in comparison to the related arylchlorosilylenes 664 (δ = 0.8/1.3 ppm). 9754


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induced dehydrochlorination of the respective chlorosilane precursor furnishes complex 697 in high yield. The observed signal in the 29Si NMR spectrum at δ = −4.3 ppm is in the range of other NHC-supported silylenes. The solid-state structure revealed a trigonal pyramidal coordination sphere around the low-valent silicon nucleus and a dihedral angle of the silylene SiNC3 and the NHC N2C3 plane of 67.8°, which underpins the strong σ-donating character of the NHC and excludes considerable π-acceptor properties. Reaction of complex 697 with the Lewis acid complex BH3·THF affords the donor−acceptor-stabilized silylene−borane adduct 698 in 37% yield, demonstrating the nucleophilc nature of 697. The coupling of the Si(II) atom with the boron nucleus leads to a broad resonance at lower field (δ = 15.2 ppm) in the 29Si NMR spectrum. Conversion of NHC adduct 698 with the unsaturated compounds diphenylacetylene and benzophenone results in the formation of the NHC-coordinated silacyclopropene derivative 699 and the cage-opening product 179 (cf., section 2.3.1) via cycloaddition reactions. The 29Si NMR spectrum of 699 displays a significant highfield-shifted singlet at δ = −149.8 ppm. XRD analysis revealed slightly shortened C−Si distances (698, 1.974 Å and 699, 1.970 Å) as compared to that observed for complex 697 (2.018(2) Å). 2.4.2.4. Low-Valent Silicon Complexes: Silyliumylidene Ions. Another important class of low-valent silicon species is represented by silicon(II) cations, the so-called silyliumylidene ions. These in modern main group chemistry highly soughtafter compounds illustrate the formal fusion of silylene and silylium ion moieties. Thus, positively charged silyliumylidene ions bear only four valence electrons, including a lone pair at the silicon center and two vacant orbitals, resulting in a more distinct electrophilicity as compared to silylenes. Accompanied by their high Lewis acidity, they possess strong σ-donor as well as π-acceptor abilities and are considered as highly reactive species. Despite their difficult isolation, more than a handful of silyliumylidene ions were described over the past decade, especially due to the effective utilization of NHC-stabilization. The first example of a NHC-stabilized silyliumylidene ion was reported by Filippou et al. in 2013 with the synthesis of the iodosilyliumylidene cation NHC adduct 641 (Figure 80).547 Reaction of the NHC-stabilized diiodosilylene 606 (vide supra) with IiPr2Me2 affords compound 641 in 84% yield. According to the results of the crystal structure determination, the trigonal pyramidal silicon(II) center is in close proximity to one methine proton of the IiPr2Me2 carbene. This interesting finding together with the observation of a doublet signal in the 29Si NMR spectrum (δ = −55.3 ppm, JSi,H = 10.4 Hz) and supporting DFT calculations revealed the first anagostic interaction between a C−H bond and a divalent silicon nucleus. Moreover, reaction of (IDipp)SiI2 (606) with an excess of the less sterically demanding NHC IMe4 results in the formation of the rare example of a Si(II) dication 642 via a carbene substitution reaction in high yield.547 The novel dicationic silicon NHC complex 642 shows an even stronger highfield chemical shift (δ = −89.9 ppm) in the 29Si NMR spectrum as compared to the NHC-supported iodosilyliumylidene 641, due to the highly shielded silicon nucleus by the three coordinating NHC ligands. The solid-state structure revealed an almost C3 symmetric, propeller-like coordination sphere around the Si(II) atom with a high degree of pyramidalization.

The NHC adducts 694 display structural and bonding features in the solid state similar to those of other NHCstabilized silylenes. Although the conversion of the 7-chloro-7silanorbornadiene derivative with IMe4 provides the arylchlorosilylene 664a, the necessary harsher reaction conditions already lead to decomposition. Accordingly, this new method also reaches its limits, because further replacement of the Si−H by an Si−CH3 moiety or utilization of a sterically more demanding NHC already prevents the fragmentation reaction. However, extensive theoretical calculations suggest significant improvements for other leaving groups and central elements. The high reactivity of the NHC-supported arylhydrosilylene 694b has already been exploited for the synthesis of several novel compounds. For instance, the treatment of complex 694b with an excess of water results in the selective formation of the tetrahydridodisiloxane 695 (Figure 79).671 The

Figure 79. Reaction products of the conversion of NHC adduct 694b with H2O, Fe2(CO)9, and elemental chalcogenes.

proposed strong σ-donor ability was proven with the successful isolation of the Lewis acid/base-stabilized silylene iron complex 696. The experimentally determined analytical data correspond well with those of the previously mentioned complexes (δ29Si = −11.1 ppm; d(Si−Fe) = 2.327(6) Å). In addition, the reaction of the NHC adduct 694b toward elemental chalcogens provides the first examples of the NHCstabilized heavier silaaldehydes 586 in 61−67% yield (vide supra).608 Only recently did the group of Xie report the synthesis, structure, and reactivity of the novel NHC-stabilized cyclic amino(carboranyl) silylene 697 (Scheme 123).237 IiPr2Me2Scheme 123. NHC-Stabilized Cyclic Amino(carboranyl) Silylene 697 and Its Reactivity toward a Borane and Unsaturated Substrates

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furnishes 588a and 588b in 54% and 45% yield, respectively. The experimentally found chemical shifts in the 29Si NMR spectra (δ = −68.9 ppm and δ = −69.5 ppm) are in good agreement with the calculated values (δ = −67.3 ppm and δ = −68.6 ppm) and are within the range of the NHC-supported arylbromosilyliumylidene ions 669. The crystal structure of 588a uncovered a pyramidalized Si(II) center, whereas the sum of the bond angles around this atom is roughly 310°. Theoretical calculations revealed a diffusion over the carbene moieties for the HOMO (lone pair orbital at Si) as well as the LUMO. Because the reactivity of both NHC-stabilized arylchlorosilyliumylidene ions 588a and 588b was recently summarized in a mini-review by us,672 only a brief recap is given here. Treatment of 588a with an excess of phenylacetylene produces a m-terphenyl-substituted 1-alkenyl-1,1-dialkynylsilane via a C−H insertion reaction.609 In contrast to the Econfigured 1-alkenyl-1-alkynylsilole 676 and -silane 687 (vide supra), the alkylene substituent adopts solely Z-configuration. Most interestingly, exposure of 588a and 588b to carbon dioxide gas provided the first experimental realization and isolation of the unprecedented sila-acylium ion NHC adducts 567.595 The corresponding heavier congeners of the NHCstabilized sila-acylium ions 567 were reported very recently. The silanechalcogenone adducts 587 are obtained via conversion of the meta-terphenyl-substituted chlorosilyliumylidene 588a with the respective elemental chalcogens in overall high yield.610 So and colleagues expanded the versatile utilization of CAACs in main group element chemistry with the isolation of the first mixed NHC-/CAAC-stabilized iodosilyliumylidene ion 663 (Figure 81).556 Reaction of the particular CAAC-

Figure 80. Examples of NHC-stabilized silyliumylidene ions including the unprecedented Si(II) dication 642.

In the same year, Driess and co-workers reported the unprecedented, three-coordinate chlorosilyliumylidene complex 700 stabilized by a neutral chelating bis-NHC ligand.604 Compound 700 is formed by the reaction of (IDipp)SiCl2 (602) with the corresponding bis(carbene) in moderate yield, which displays another striking reactivity of Roesky’s NHCstabilized dichlorosilylene.543 The chelate coordination leads to a boat conformation of the puckered six-membered C3N2Si ring in the crystal structure. The strong electron-donating capability of the bis(carbene) moiety resonates with the upfield 29 Si NMR shift at δ = −58.4 ppm. Computational studies uncovered the nature of the HOMO (σ lone pair of electrons) and LUMO (π-type contribution at the central Si nucleus) being mainly localized on the low-valent silicon atom, thus confirming the silyliumylidene ion character. Reductive dechlorination of complex 700 with sodium naphthalenide yields the NHC-coordinated silylone 582, which will be discussed in detail below. The groups of Tokitoh, Sasamori, and Matsuo used sterically crowded diaryldibromodisilenes as precursors to obtain the novel NHC-stabilized silyliumylidene ions 669 (Figure 80).643,659 Reaction of the respective 1,2-dibromodisilenes or dibromosilanes with the strong σ-donor carbenes IMe4 and IiPr2Me2 provides NHC adducts of the arylsilyliumylidene cation bromide salts. The observed 29Si NMR shifts for the divalent silicon nuclei (varying from δ = −60.8 to δ = −75.9 ppm) are upfield shifted as compared to the above-mentioned silyliumylidene ions, indicating stronger shielded silicon(II) atoms. With proper choice of the aryl moiety and carbene substituents, the reaction affords the NHC−arylbromosilylene adducts 667 and 668 apart from the silyliumylidene complexes 669 as described earlier. Another pair of mono-organyl-substituted, NHC-stabilized chlorosilyliumylidene ions was found in our group.609 An uncomplicated one-pot synthesis starting with the corresponding bulky aryldichlorosilanes RSiHCl2 and 3 equiv of IMe4

Figure 81. Iodosilyliumylidene cation NHC-/CAAC-adduct 663 and NHC-stabilized parent silyliumylidene 701 reported by So et al.

diiodosilylene 661 with 1 equiv of IiPr2Me2 provides compound 663 in 88% yield in a manner analogous to the preparation of complex 641 from (IDipp)SiI2 (606).14 The 29 Si chemical shift (δ = −51.5 ppm) is observed at a slightly lower field as compared to the related iodosilyliumylidene cation NHC adduct 641 (δ = −55.3 ppm). SC-XRD analysis revealed a trigonal pyramidal geometry around the divalent silicon nucleus with two distinct Si−C bond distances. The CCAAC−Si bond length (1.878(5) Å) is significantly shorter than the CNHC−Si bond length (1.946(5) Å), due to the stronger σ-donor and π-acceptor properties of the CAAC carbene. Additional MO analysis showed the nature of the HOMO being the lone pair orbital at the Si(II) atom with a slight contribution of the π-backbonding to the C−N π*-MO of the Me2CAAC moiety. 9756


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The two-coordinate disilene 572 and the three-coordinate bis(silylene) 603 exhibit 29Si NMR resonances in the lowfield region at δ = 224.5 (572) and δ = 38.4 ppm (603), respectively. Crystal structure determinations showed a rather short Si Si double bond of 2.229(11) Å for 572, which is in the range of reported disilenes (2.118(1)−2.289(14) Å536,566,676) and a trigonal pyramidal geometry around the Si nuclei of 603 (sum of bonding angles at Si: 308.0°) with a Si−Si single bond of 2.393(3) Å. Noteworthy, these results played a pivotal role for the successful isolation of the silicon(II) dihalides 602, 604− 608, because reduction of the respective silicon(IV) tetrahalides with 2 equiv of KC8 provides the dihalosilylenes (vide supra). An initial reactivity investigation of the NHC-coordinated diatomic Si(0) complex 572 was performed with a Lewis acidic boron compound (Scheme 125).677 Exposure of 572 toward

Treating complex 663 with KC8 results in a two-electron reduction and formation of the NHC-/CAAC-stabilized monatomic silicon(0) species 702 (vide infra).556 The same group recently described the isolation of the first NHC-stabilized parent silyliumylidene ion 701.673 Complex 701 is obtained by the reaction of the NHC-supported iodosilicon(I) dimer [(IDipp)SiI]2 (703)674 (vide infra) with 4 equiv of IMe4 in 72% yield. Regarding the reaction mechanism, the homoleptic cleavage of the Si−Si bond with a concomitant replacement of IDipp and iodide was proposed. The intermediary formed cationic NHC-coordinated silicon(I) radical finally forms silyliumylidene ion 701 via abstraction of a H• radical from the solvent toluene. The low-valent Si−H moiety is clearly observed in the corresponding proton and silicon NMR spectra at δ = 9.73 ppm and δ = −77.9 ppm, respectively. The 1H NMR resonance is considerably shifted to lower field in comparison to those of the hydrosilylene complexes 684 and 694 (δ = 3.17−4.00 ppm), whereas the 29 Si NMR signal and the X-ray crystal structure are consistent with the NHC-stabilized silyliumylidene ions mentioned above. NHC adduct 701 is generally highly unstable in solution and even slowly activates fluorobenzene under formation of the imidazolium salts [IMe4H]I and [1-F-2IMe4-C6H4]I. Furthermore, methanolysis of 701 provides Si(OMe)4, H2, and unidentified NHC-containing compounds. Despite the fact that the chemistry of silyliumylidene ions is still in its infancy, it might produce innovative reagents with respect to small molecule activation or even complexes for the application in catalysis in the future. Furthermore, those compounds could be used as promising building blocks for novel low-valent organosilicon species. 2.4.2.5. Low-Valent Silicon Complexes: Multiply Bonded Compounds. Similarly to multiply bonded silicon(IV) complexes, the isolation of compounds bearing a SiE double bond (E = main group element) with a low-valent silicon atom is a challenging target, due to the unfavorable overlap of the pπorbitals. Therefore, these transient species need a tailor-made ligand framework for a sufficient kinetic and thermodynamic stabilization of the electron-deficient weak π-bonds. However, the application of carbenes with their strong electron-donating nature has enabled the preparation of a variety of otherwise elusive complexes. A milestone in low-valent organosilicon chemistry has already been set in 2008 with the isolation of a NHCstabilized silicon “allotrope” by Robinson et al.69 This chemistry has already been subject to several review articles and thus will not be discussed in full detail here.30,32,70,675 Reduction of the silicon(IV) tetrahalide (IDipp)SiCl4 (501) with 6 equiv of potassium graphite in n-hexane furnishes the NHC-stabilized diatomic Si(0) complex 572 accompanied by the NHC-supported silicon(I) chloride 603 only in very low yields (Scheme 124). The former is obtained with an increased yield of 23% when using THF as solvent and 4 equiv of KC8.

Scheme 125. Reactivity of [(IDipp)Si]2 (572) toward Borane-tetrahydrofuran

BH3·THF provides two quite distinct donor−acceptorstabilized silylene units via facile cleavage of the SiSi double bond and a borane insertion reaction, which is strongly dependent on the purity of 572. The unique boron-substituted “push−pull”-stabilized complex 704 is obtained in 72% yield when pure crystals of 572 are used and represents the first isolated example containing the parent silylene moiety (H2Si:). In contrast, utilization of an IDipp-contaminated sample results in the formation of both complex 704 and the Lewis acid/base-stabilized cyclic silylene 705 in low yield. Compound 705 incoporates an unprecedented three-membered silylene ring. SC-XRD analysis unambiguously confirmed the molecular structures and revealed C−Si bond distances varying from 1.934(4) to 1.944(4) Å, which are consistent with other coordinative NHC−Si bonds. Because of the strong linebroadening originating from the quadrupolar boron nuclei, no 29 Si NMR resonances are detectable. The nucleophilicity of the two-coordinate disilene 572 was studied by examining its coordination ability toward transition metal complexes. Treating 572 with CuCl leads to the formation of the dark purple red-colored NHC-stabilized disilicon−copper(I) chloride complex 706a in 51% yield (Figure 82).227 On the other hand, the reaction with iron pentacarbonyl Fe(CO)5 affords the related donor−acceptorstabilized dark purple iron complex 706b in high yield.678 The solid-state structures exposed trigonal planar coordination spheres around the tricoordinate Si(0) atoms with slightly shortened SiSi double bond lengths of 2.206(12) Å (706a) and 2.195(12) Å (706b) as compared to complex 572. The Si−Fe distance (2.327(10) Å) in 706b matches those of the NHC-stabilized silylene-iron complexes 622, 692a, 693, and 696 (2.229(11)−2.372(16) Å). Whereas the iron complex

Scheme 124. Synthesis of [(IDipp)Si]2 (572) and [(IDipp)SiCl]2 (603) from (IDipp)SiCl4

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Scheme 126. Preparation of the Donor−Acceptor-Stabilized Parent Monochlorosilylene Complex 708

δ29Si = 44.0 and 45.3 ppm) are consistent with those of the aforementioned carbene−silylene−iron complexes and support the presence of diastereoisomers. The group of Filippou reported a novel synthetic route to NHC-supported Si(I) halide complexes [(IDipp)SiX] 2 (Scheme 127).674 Facile oxidation of [(IDipp)Si]2 (572)

Figure 82. Reaction products of complex 572 with transition metal fragments.

706b displays two upfield shifted signals in the 29Si NMR at δ = 201.3 and δ = 142.5 ppm, only one resonance at δ = 226.7 ppm is observed for the copper complex 706a. This observation suggests the existence of a dynamic σ−π interconversion behavior of compound 706a in solution via a π-complex intermediate (side-on coordination). Variabletemperature (VT) NMR studies and DFT calculations support this assumption. The “push−pull”-stabilized iron complex 706b reacts with another equivalent of Fe(CO)5 at elevated temperatures to furnish almost quantitatively the NHCstabilized complex 707, thus illustrating the first experimental realization of a SiSi double bond cleavage induced by a transition metal carbonyl fragment. The observed signal in the 29 Si NMR spectrum (δ = 109.4 ppm) indicates more shielded silicon nuclei. In addition, the NHC-stabilized diatomic Si(0) complex [(IDipp)Si]2 (572) proved to be a valuable precursor for oxidation and chalcogenation reactions. Conversion of complex 572 with N2O, O2, and CO2 affords the NHCcoordinated Si2O3 569, Si2O4 570, and the silicon−carbon mixed oxide (SiO2)2CO2 571 derivatives in moderate yield (vide supra).598,599 So et al. used [(IDipp)Si]2 (572) for the preparation of a series of NHC-coordinated silicon sulfide and telluride compounds 589−593, including the NHC-stabilized [(IDipp)SiTe]2 (589) and the heavier congeners of 570 (Si2Te4, 591) and 569 (Si2Te3, 593).611 Interestingly, the reaction of the two-coordinate disilene derivative 572 with the organic azide DippN3 provides the first NHC-stabilized siladiimide complex 59 in 35% yield as already mentioned above.579 In the course of the past decade, the parent silylene (H2Si:) and dichlorosilylene (Cl2Si:) have been successfully trapped with the aid of donor(−acceptor)-stabilization and isolated as shown above (cf., section 2.4.2.3). However, the synthesis of a stable complex of the related monochlorosilylene (HClSi:), which has been observed concomitantly with the former silylenes in the chemical vapor deposition of silicon from chlorosilanes and SiH4, still has to be achieved. Thus, to gain access to a stabilized HClSi: moiety, Robinson and co-workers treated complex 572 with pyridine hydrochloride py·HCl.679 Nevertheless, the reaction does not furnish a monochlorosilylene species, but a rather complicated mixture containing among others the NHC-stabilized bis(silylene) 603, dihydroaminal IDippH2, and free IDipp. However, performing the same reaction with the donor−acceptor-stabilized iron complex 706b results in the formation of the “push−pull”stabilized parent monochlorosilylene adduct 708 as two diastereoisomers in 58% yield (Scheme 126). The experimentally and theoretically determined structural and bonding features (average d(Si−Fe) = 2.213 Å; δ1H = 6.20 and 6.22 ppm;

Scheme 127. Synthesis of Halides [(IDipp)SiX]2 via Oxidation of [(IDipp)Si]2 (572)

with 1,2-dihaloethanes results in an improved synthesis of [(IDipp)SiCl]2 (603) and the first molecular Si(I) bromide 709 and iodide 703 NHC adducts in a diastereoselective manner in 49−98% yield. The use of an equimolar amount of 1,2-C2H4X2 is very crucial, because further oxidation provides the Si(II) dihalides (IDipp)SiX2 602, 605, 606 as inseparable byproducts. The heavier congeners [(IDipp)SiBr]2 (709) and [(IDipp)SiI]2 (703) show an overall similar bonding pattern as compared to the NHC-stabilized bis(silylene) 603 and exhibit an increasing upfield shift in the 29Si NMR spectra (603, δ = 38.4; 709, δ = 34.9; and 703, δ = 18.7 ppm) concomitant with the increase in atomic numbers. With this better synthetic access to the NHC-supported bis(halosilylenes) in hand, the high reactivity of Si(I) iodide 703 has been exploited for the synthesis of several unprecedented compounds (Scheme 128). For instance, iodide abstraction from complex 703 with [Li(Et2O)2.5][B(C6F5)4] leads to the formation of the disilicon(I) salt [(IDipp)2Si2I][B(C6F5)4] (710) in 62% yield.674 SC-XRD analysis revealed the presence of a short SiSi double bond (2.174(9) Å; cf., 572: d(SiSi) = 2.229(11) Å) between the trigonal planarcoordinated Si(II)−I and the dicoordinate Si(0) nuclei bearing a lone pair of electrons in a V-shaped geometry. Interestingly, NHC adduct 710 undergoes a dynamic topomerization process in solution leading to an exchange of the heterotopic silicon centers. Variable-temperature (VT) NMR studies in combination with theoretical calculations proposed a NHC-stabilized π-bonded disilaiodonium ion intermediate. Thus, the two 29Si NMR resonances at δ = 75.3 ppm (Si(0)) and δ = −26.4 ppm (Si(II)) are observable only at lower temperatures. Robinson and colleagues recently investigated the reactivity of the NHC-stabilized diiodobis(silylene) 703 toward the imidazole-based thiolate 711.680 The reaction of [(IDipp)SiI]2 (703) with 2 equiv of 711 in toluene furnishes the NHCsupported complex 712 in moderate yield. Utilization of THF 9758


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Scheme 128. Reactivity of NHC-Stabilized [(IDipp)SiI]2 (703)

Scheme 129. Further Reactivity Studies of the NHCStabilized Diatomic Si(0) Complex [(IDipp)Si]2 (572)

as the solvent instead provides complex 712 (43%) together with the isomeric NHC adduct 713 as a minor side product (6%). The assumed mechanism involves in both cases a double thiolate coordination under liberation of one IDipp carbene and subsequent C−H or C−N insertion of the SiI2 unit. The existence of silylene moieties was shown by X-ray analysis, because these divalent nuclei adopt distorted trigonal pyramidal geometries with Si−Si single bonds (712, 2.349(2) and 713, 2.416(11) Å). The 29Si NMR resonances of complex 713 (δ = 2.21 and δ = −56.7 ppm (Si(II)) are lowfield-shifted as compared to those of its isomer 712 (δ = −27.0 and δ = −73.0 ppm (Si(II)). Conversion of [(IDipp)SiI]2 (703) with 4 equiv of IMe4 affords the first NHC-coordinated parent silyliumylidene iodie complex 701 as already mentioned above.673 In the course of their studies, So et al. also found a new method for the preparation of complex 703 via reduction of the triiodosilylimidazolium salt 507 with 3 equiv of potassium graphite. The group of Filippou extended the reactivity investigations of the NHC-supported diatomic Si(0) complex 572 (Scheme 129). Reversible one-electron oxidation of NHC adduct 572 with [FeCp*2][B(ArF)4] (ArF = 3,5-(CF3)2-C6H3) provides the NHC-stabilized disilicon radical cation 714 in 71% yield under liberation of decamethylferrocene.681 Conversion of 714 with an equimolar amount of KC8 in THF recovers complex 572. NHC adduct 714 is particularly thermolabile in solution, but stable as a solid at −30 °C. The solid-state structure of the radical cation 714 exposed a lower symmetry with nonequivalent Si nuclei and a shortened SiSi double bond length of 2.178(3) Å as compared to [(IDipp)Si]2 (572) (2.229(11) Å). Consistent with these observations, the calculated WBI increased from 1.703 to 2.046. Continuouswave (cw) EPR spectroscopy and DFT calculations indicated a

topomerization process with a low activation barrier leading to an interconversion of the two Si sites of radical cation 714. Treating [(IDipp)Si]2 (572) with Brookhart’s acid [H(Et2O)2] [B(ArF)4]682 results in the formation of the NHCstabilized hydridodisilicon(I)−borate complex 715 in high yield via protonation of the former.683 The similar mixedvalent alkyldisilicon(I) borates [IDipp(R)Si(II)Si(0)IDipp][B(ArF)4] (R = Me, 716a and R = Et, 716b) are obtained upon treatment of complex 572 with the respective iodoalkanes RI in the presence of Na[B(ArF)4] in moderate yields. SC-XRD analysis revealed isotopic structural motifs for the complexes 715,716, which are strongly related to the analogous disilicon(I) salt [(IDipp)2Si2I][B(C6F5)4] (710) with similar short SiSi double bonds varying from 2.173(9) to 2.191(8) Å. However, a degenerate isomerization (topomerization) accompanied by the observation of 29Si NMR resonances at low temperatures only was exclusively found for NHC adduct 715 (π-bonded disilahydronium ion intermediate), as the more sterically demanding alkyl groups lead to less flexible structures in solution. The corresponding silicon nuclei exhibit highfield-shifted signals in the 29Si NMR spectra in comparison to [(IDipp)Si]2 (572) (δ = 224.5 ppm) and display a progressive deshielding for the Si(II) centers (715, δ = 69.4 ppm → 716a, δ = 102.8 ppm → 716b, δ = 111.6 ppm). The opposite trend is observed for the respective zerovalent silicon atoms (715, δ = 125.4 ppm → 716a, δ = 115.2 ppm → 716b, δ = 87.2 ppm). Additionally, the reaction of the NHC-stabilized diatomic Si(0) complex 572 with the alkali metal borates [Li(Et2O)2.5][B(C6F5)4] and Na[B(ArF)4] furnishes the alkali metal disilicon(0) complexes 717 in 43% and 70% yield.683 Remarkably, with the isolation of the NHC adduct [Na(IDippSi)2][B(ArF)4] (717b), the intermediate of the electrophilic alkylation reactions affording the complexes 716 could be successfully trapped. The solid-state structures of the NHC adducts 717 elucidate the electrostatic encapsulation of the alkali metal cations in the cavity of [(IDipp)Si]2 (572) via η2coordination to the π-bonded SiSi moiety and η6coordination to two wing tip aryl substituents (Dipp groups). 9759


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located at 608 and 704 nm being responsible for the intense green color, which was further analyzed by time-dependent density functional theory (TDDFT) investigations. X-ray crystallography exposed a pseudo-Ci symmetry of 718 contradictory to the C1-symmetric molecular structure of complex 715 with a considerably elongated Si−Si bond length (2.281(3) Å; cf., 715: d(SiSi) = 2.187(8) Å), which is still in the range of reported SiSi double bonds (2.118(1)− 2.289(14) Å536,566,676). The calculated lowered WBI of 1.17 versus 1.70 (715) corroborates these results. In addition, the experimentally determined νSi−H stretching absorption band at 2089 cm−1 lies between that of NHC adduct 715 (νSi−H = 2142 cm−1) and that of the NHC-stabilized hydrosilylene (IMe4)SiHSitBu3 (684) (νSi−H = 1984 cm−1), indicating a decrease in the stretching vibrations with an increasing degree of pyramidalization of the respective Si nuclei. Moreover, EPR studies and theoretical calculations revealed that NHCstabilized [(IDipp)Si]2H (718) exists as a π-type radical with the singly occupied molecular orbital (SOMO) being the Si Si π* orbital. Roesky et al. studied the reductive dechlorination of the CAAC-stabilized silicon(IV) tetrahalides (Me2CAAC)SiCl4 (522) and (CyCAAC)SiCl4 (523) in depth. Because these investigations have already been summarized in several review articles, these results will not be discussed thoroughly here.26,29,46,58,60,554 The reaction conditions, such as the steric hindrance of the carbene substituents (dimethyl vs cyclohexyl), the amount of the reducing agent, and the control of the reaction temperature, are critical to the outcome of the reaction, and thus a series of CAAC adducts has been isolated

The SiSi double bonds are only slightly elongated (717a, 2.234(1) and 717b, 2.248(2) Å) as compared to the starting material 572, indicating a rather weak Si···M interaction. The zerovalent silicon nuclei of the NHC-stabilized complexes 717 resonate at higher frequencies in the corresponding 29Si NMR spectra (δ = 288.8−301.1 ppm) with respect to the NHC adduct 572. Because cyclic voltammetric (CV) studies of the NHCstabilized hydridodisilicon(I)-borate complex 715 exposed a reversible one-electron reduction at a low redox potential (E1/2 = −2.15 V vs ferrocenium/ferrocene (Fc+/Fc)), the former NHC adduct was treated with an equimolar amount of KC8 in THF.684 In fact, the reaction provides the NHC-supported mixedvalent disilicon(0,I) hydride complex 718 as a dark green crystalline solid in 55% yield (Scheme 130). The strong Scheme 130. Reversible One-Electron Oxidation of Disilicon(I) Complex 715

reducing nature of the [(IDipp)Si]2H radical 718 facilitates the reoxidation under recovery of the NHC adduct 715. UV−vis− NIR spectroscopy revealed the significant absorption maxima

Scheme 131. One-, Two-, Three-, and Four-Electron Reduction of the CAAC-Stabilized Silicon(IV) Tetrachlorides 522 and 523

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furnishes the unprecedented CAAC-coordinated triatomic silicon(0) cluster 720 instead of [(Me2CAAC)Si]2 (598a) in 25% yield (Scheme 131).654 29Si NMR spectroscopy displays a considerably lowfield-shifted singlet at 7.2 ppm as compared to those of the CAAC-stabilized diatomic silicon(0) complexes 598. Synchrotron single-crystal X-ray diffraction analysis of [(Me2CAAC)Si]3 (720) revealed each of the three-coordinate Si(0) nuclei adopting trigonal pyramidal geometries, which indicates the presence of stereochemically active lone pairs of electrons. Therefore, complex 720 bearing a triangular Si3 moiety can also be considered as a CAAC-supported cyclic trisilylene. The CAAC-stabilized diatomic silicon(0) complexes [(Me2CAAC)Si]2 (598a) and [(CyCAAC)Si]2 (598b) have already been subjected to several initial reactivity studies. CV measurements of complex 598 showed a quasi-reversible reduction at E1/2 = −1.40 V vs Cp*2Fe/Cp*2Fe+ indicating the formation of the corresponding radical anion.653 According to these results, CAAC adduct 598b was treated with an equimolar amount of potassium metal. However, the intermediary formed radical anion [(CyCAAC)Si]2•− proved to be too reactive, and only the follow-up reaction products isomer 721 (60% yield) and the dimeric potassium salt 722 (minor byproduct) were isolated (Figure 83). SC-XRD

(Scheme 131). Reduction of the sterically less encumbered derivative 522 with an equimolar amount of LDA affords the CAAC-supported biradical (CAAC)2SiCl2 (648) in moderate yield under elimination of SiCl4 as already mentioned above.649 On the other hand, the analogous utilization of 1 equiv of the strong reducing agent KC8 results in the formation of the trichlorosilylcarbene monoradicals 654 in 48−52% yield (vide supra).555 The first compound containing the unprecedented disilicontetrachloride moiety [(CyCAAC)SiCl2]2 (719) is obtained by the two-electron reduction of (CyCAAC)SiCl4 (523) with either KC8 at −105 °C or by LDA at 0 °C in 60% and 63% yield, respectively.651 Alternatively, the CAAC-stabilized complex 719 is also accessible via conversion of the monoradical 654b with 1 equiv of KC8 in high yield. The silicon nuclei exhibit one signal at δ = 3.3 ppm in the 29Si NMR spectrum and adopt distorted tetrahedral geometries with a trans-olefin-like configuration of the Si2Cl4 unit in the solid state. The CCAAC−Si distance of 1.846(5) Å is consistent with those of the complexes 648 and 654 (1.815(12)− 1.854(2) Å). According to the results of DFT calculations, 719 should be considered as a singlet 1,4-biradical. Furthermore, reductive dechlorination of (Me2CAAC)SiCl4 (522) with 3 equiv of KC8 in THF at −78 °C provides [(Me2CAAC)SiCl]2 (655) in 48% yield (Scheme 131).652 In comparison to the analogous NHC-stabilized derivative [(IDipp)SiCl]2 (603) reported by Robinson and co-workers,69 a highfield-shifted resonance at δ = 25.6 ppm (cf., 603: δ = 38.4 ppm) is observed in the 29Si NMR spectrum for complex 655. SC-XRD analysis revealed distorted trigonal pyramidal coordination spheres around both Si nuclei with increased sums of bond angles (325.34° and 327.96°) and shortened Si− Si (2.306(13) Å) and CCarbene−Si (1.823(3)−1.826(3) Å) bond lengths as compared to NHC adduct 603 (sum of the bond angles at Si: 308.0°, d(Si−Si) = 2.393(3) Å, and d(Si−C) = 1.929(7)−1.939(6) Å). Thus, the experimental data together with auxiliary computational studies suggest a change in the bonding situation from dative bonds to electron-sharing ones via carbene substitution from NHCs to CAACs. Compound 655 is therefore best described as a CAAC-supported 1,4diamino-2,3-disila-1,3-butadiene complex bearing a CSi− SiC moiety, rather than as a CAAC-stabilized threecoordinate bis(silylene) like [(IDipp)SiCl]2 (603). Treatment of the CAAC adducts of SiCl4 522 and 523 with KC8 in a molar ratio of 1:4 at −78 °C results in complete dechlorination reactions under formation of the CAACstabilized diatomic silicon(0) complexes 598 in moderate yields.614,653 The corresponding silicon nuclei exhibit resonances at higher frequencies in the 29Si NMR spectra (δ = 249.1−252.3 ppm) as compared to the related NHC complex [(IDipp)Si]2 (572) (δ = 224.5 ppm). X-ray analysis exposed slightly elongated SiSi double bond lengths of 2.232(9)− 2.254(3) Å (572: 2.229(11) Å) but shortened CCAAC−Si distances varying from 1.849(4) to 1.887(4) Å (572: 1.921(15) Å). These observations corroborate with the partial double bond character of the CCAAC−Si bonds, due to the superior σ-donor and π-acceptor properties of CAACs in comparison to NHCs. In addition, Raman spectroscopy (stretching vibration νSiSi at 482 cm−1) and DFT calculations (HOMO−1: SiSi π-bond) clearly showed the presence of SiSi double bonds in the [(CAAC)2Si]2 complexes 598. The similar four-electron reduction of (Me2CAAC)SiCl4 (522) with KC8 only at even lower temperatures (−107 °C)

Figure 83. Products of the one-electron reduction of [(CyCAAC)Si]2 (598b).

analysis, DFT calculations, and the observed significantly highfield-shifted 29Si NMR resonances at δ = −5.0 to −58.2 ppm (cf., 598a: δ = 249.1 ppm) support this outcome of the reaction. In addition, oxidation of the two-coordinate disilene derivatives 598 with elemental sulfur and black selenium affords the CAAC-stabilized disilicon tetrachalcogenides (RCAAC)2Si2Ch4 complexes 594−596 in 30−84% yield (vide supra).613,614 Roesky and colleagues also synthesized the first CAACsupported siliconthiodichloride complex (Me2CAAC)SiSCl2 (597) via conversion of either the undescribed Me2CAAC analogue of [(CyCAAC)SiCl2]2 (719) or [(Me2CAAC)SiCl]2 (655) with elemental sulfur in moderate yields as already mentioned above.615 Recently, various CAAC-stabilized Si2R4 and Si2R2 complexes have been reported, which are strongly related to the CAAC adducts [(CyCAAC)SiCl2]2 (719) and [(Me2CAAC)SiCl]2 (655) (Figure 84). For instance, the reductive dechlorination of Me2SiCl2 and MeSiCl3 with 2 equiv of KC8 in THF in the presence of an equimolar amount of Me2CAAC provides the CAAC-stabilized paramagnetic biradicals 723a and 723b in 65% and 48% yield, respectively. 656 Both [( Me2 CAAC)SiMe 2 ] 2 (723a) and [(Me2CAAC)SiMeCl]2 (723b) display similar bonding patterns in the solid state as compared to complex 719 with consistent Si−Si (2.472(6) and 2.437(6) Å) and CCAAC−Si (1.863(11) and 1.847(12) Å) bond lengths. In contrast to CAAC adduct 9761


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Si]2 (572) (δ = 224.5 ppm) and [(IDipp)SiCl]2 (603) (δ = 38.4 ppm).69 An elongation of the SiSi triple bond (2.062(9) Å) leading to a SiSi double bond (2.199(6) Å) upon Lewis base coordination was shown by SC-XRD analysis. UV−vis spectroscopy revealed the π−π* (HOMO → LUMO) transition at 409 nm. The experimental and calculated data suggest the representation of 727 as a NHC-stabilized zwitterionic disilene-like structure with a lone pair of electrons at one Si center. Conversion of complex 727 with the Lewis acid ZnCl2 affords the “push−pull”-stabilized derivative 728 in moderate yield, demonstrating its nucleophilic character. The complexation results in significantly shifted 29Si NMR resonances (δ = 190.8 ppm and δ = 66.9 ppm), but maintains the SiSi double bond length (2.201(13) Å). Surprisingly, the SiSi double shows a Z-configuration despite the sterically demanding silyl substituents. The nucleophilic nature of complex 727 was further demonstrated by the reaction of 727 with MeOTf, which furnishes the striking NHC-stabilized disilenyl cation 729 in 66% yield.691 The first example of a heavier group 14 vinyl cation 729 resonates at δ = 168.8 ppm and δ = 54.0 ppm in the 29 Si NMR spectrum and shows nearly planar coordination spheres around the silicon nuclei of the almost unchanged Si Si double bond (2.192(2) Å). The groups of Scheschkewitz and Jutzi synthesized the cyclotrisilene−NHC adduct 731 via the reaction of the corresponding cyclic disilene 730 with IiPr2Me2 in moderate yield (Scheme 133).692

Figure 84. Further [(CAAC)SiR]2 and [(CAAC)SiR]2 complexes.

719, the paramagnetic biradicals 723a and 723b are silent in the 29Si NMR spectroscopy, but EPR active. Moreover, the three-electron reductions of the trichlorosilanes HSiCl3 and MeSiCl3 with KC8 accompanied by one CAAC equivalent furnish the complexes [(RCAAC)SiH]2 (R = Cy, 724 and R = Me2, 725) and [(Me2CAAC)SiMe]2 (726) in moderate yields.685,686 Noteworthy, the conversion of HSiCl3 and other hydrochlorosilanes with CAACs in the absence of KC8 leads to the formation of Si−H bond insertion products, such as Cl3Si(CAACH) and Dipp(TMS)NCl2Si(CAACH).687 Similar to the reduction of (Me2CAAC)SiCl4 (522), the treatment of (Me2CAAC)SiI4 (524) with 3 equiv of KC8 yields the analogous complex of 655 [(Me2CAAC)SiI]2 (662).576 The [(RCAAC)SiR]2 complexes 662, 724−726 feature isotopic structural motifs (trigonal pyramidal geometries around the Si nuclei) with similar Si−CCAAC distances (1.798(12)−1.846(3) Å) but slightly elongated Si−Si single bonds (2.314(11)− 2.337(11) Å) as compared to CAAC adduct 655. The corresponding 29Si NMR resonances vary from δ = 0.7 to −45.5 ppm, due to the different nature of the silicon substituents. Overall, detailed theoretical bonding situation analyses suggested a similar contribution of both the 2,3-disila1,3-butadiene and the CAAC-stabilized interconnected bis(silylene) resonance forms. The first disilynes, compounds containing a SiSi triple bond, were independently synthesized by the groups of Sekiguchi and Wiberg in 2004, due to the efficient stabilization provided by sterically crowded silyl groups.688,689 Later, Driess and Sekiguchi et al. obtained the unprecedented NHC− disilyne adduct 727 from the reaction of the respective disilyne i PrDsi2Si−SiSi−SiDsi2iPr (Dsi = CH(TMS)2) with an equimolar amount of IMe4 in 81% yield (Scheme 132).690 The two- and three-coordinate silicon nuclei exhibit signals in the 29Si NMR spectrum at δ = 276.3 ppm and at δ = 28.7 ppm, respectively, which are rather close to the complexes[(IDipp)-

Scheme 133. Reversible Coordination of IiPr2Me2 to Cyclic Disilene 730

XRD analysis of suitable red crystals uncovered a pyramidalized three-coordinate Si atom (sum of bonding angles at Si: 340°) and a relatively long CNHC−Si distance of 1.984(14) Å, indicating the presence of a stereochemically active lone pair of electrons and a rather labile NHC coordination. The former SiSi double bond is with a length of 2.270(5) Å still in the range of reported disilene species (2.118(1)−2.289(14) Å536,566,676). Interestingly, VT NMR studies revealed an equilibrium between the NHC-coordinated 731 and donor-free cyclotrisilene 730, and thus the highfieldshifted 29Si NMR resonances of 731 (δ = −61.5 to −85.6 ppm) are solely observable at 223 K (cf., 730: δ = 57 to −15.3 ppm). A related coordination equilibrium was observed by dissolving the stunning NHC-stabilized disilenyl silylene 733 (Scheme 134).693 Complex 733, representing the first example of a heavier analogue of the elusive vinyl carbenes, is obtained in 23% yield by treating the aryl-substituted cyclotrisilene 732692 with IiPr2Me2. 29 Si NMR spectroscopy clearly revealed the signals for the disilene (δ = 54.1 and 102.0 ppm) and silylene (δ = −60.3 ppm) moieties. At low temperatures, however, the corresponding NHC-coordinated cyclotrisilene intermediate is formed as evidenced by upfield shifted 29Si NMR resonances (δ = −45.6

Scheme 132. NHC−Disilyne Adduct 727 and Its Reactivity toward Electrophiles

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Å (cf., 710, 715, 716: d(SiSi) = 2.173(9)−2.191(8) Å) and the high WBI of 1.79. The nature of the HOMO (SiSi π-bond orbital) and the observed signals in the 29Si NMR spectrum (Si(0), δ = 34.6 ppm and Si(II), δ = 86.0 ppm) further elucidate the strong relationship between these isolobal complexes. The same group obtained the unprecedented NHCstabilized phosphasilenylidene 630 via salt metathesis reaction of the versatile dichlorosilylene (IDipp)SiCl2 (602)543 with LiP(Mes*)TMS under liberation of LiCl and TMSCl in 39% yield, as was already mentioned above.640 By monitoring the reaction kinetics at low temperatures with 29Si and 31P NMR spectroscopy, the assumed intermediate phosphinosilylene (IDipp)ClSiP(TMS)Mes* was identified. The solid-state structure of (IDipp)SiPMes* (630) is similar to those of its isolobal congeners P2Mes*2695 and [(IDipp)Si]2 (572)69 and bears a SiP double bond (2.119(7) Å), which is consistent with other reported phosphasilenes (2.053−2.165 Å582). The 29Si NMR doublet signal of the salient Si nucleus (δ = 267.3 ppm, 1JP−Si = 170 Hz) appears at an even lower field than that of [(IDipp)Si]2 (572) (δ = 224.5 ppm). To gain access to the analogous CAAC-stabilized phosphasilenylidene 739, Roesky and colleagues investigated the two-electron reduction of the CAAC-stabilized dichlorophosphasilene 558c586 (vide supra) with sodium naphthalenide (Scheme 135).696 However, the intermediary formed

Scheme 134. Equilibrium between the NHC-Stabilized Disilenyl Silylene 733 and Cyclotrisilene 732 in the Presence of IiPr2Me2

to −126.8 ppm) similar to those of complex 731. The experimentally determined molecular structure of NHC adduct 733 displays a trigonal pyramidal geometry around the divalent Si center with an adjacent short SiSi double bond (2.180 Å). Exposure of a mixture of the coexisting NHC-supported disilenyl silylene 733, cyclotrisilene 732, and free IiPr2Me2 to CO results in the formation of the NHC-stabilized 1,3-disila-2oxyallyl zwitterion 734 in good yield (Figure 85).694 SC-XRD analysis and 29Si NMR spectroscopy (δ = 15.6 to −47.8 ppm) unambiguously confirmed the structure of the silenolate species 734.

Scheme 135. Dimerization of the Transient Stabilized Phosphasilenylidene 739

Cy

CAAC-

Figure 85. NHC-coordinated three- and four-membered ring derivatives.

Scheschkewitz et al. isolated the complexes 735 and 736 during reactivity investigations of the NHC-stabilized 2chlorosilylsilagermenylidene 737 (vide infra).539 Conversion of 737 with 2 equiv of IMe4 provides 735 in 63% yield. The reaction of 737 with MesLi affords the cyclic bis(silyl)germylene 738 and the NHC-coordinated cyclosilagermene 736. Both complexes 735 and 736 are structurally similar to the NHC-coordinated cyclotrisilene 731 with consistent 29Si NMR resonances varying from −56.1 to −82.0 ppm. Filippou et al. successfully isolated the first NHC-stabilized disilavinylidene (Z)-(SIDipp)SiSi(Br)Tbb (640) as a bright-red solid via reductive debromination of the NHCsupported bromo(silyl)silylene 639 (vide supra) with KC8 in 60% yield (Figure 86).642 Thorough theoretical analyses uncovered similar electronic structures and thus the isolobal analogy between NHC adduct 640 and the disilicon(I)−borate complexes 710, 715, and 716.674,683 Accordingly, complex 640 features comparable structural features like a quite covalent SiSi double bond, due to the short bond length of 2.167(2)

complex 739 (δ29Si = 288.3 ppm, 1JP−Si = 163 Hz) is thermally labile and undergoes a dimerization process affording CAACsupported (CAAC)2(SiPTipp)2 (740) in 61% yield. SC-XRD analysis uncovered an elongation of the former SiP double bond to Si−P single bonds (2.266(7)−2.295(8) Å) and short CCAAC−Si bond distances of 1.812(2) Å, indicating electronsharing rather than coordinative CCAAC−Si bonds. 29Si NMR spectroscopy revealed a highfield shift for the three-coordinate silicon atoms (δ = 37.1 ppm, 1JP−Si = 44 Hz) in comparison to the precursor complex 558c (δ = −6.6 ppm, 1JP−Si = 198 Hz). CV measurements of dimer 740 showed a quasi-reversible reduction at E1/2 = −0.87 V vs Cp*2Fe/Cp*2Fe+, indicating the formation of the corresponding radical anion, which was further supported by EPR studies. Driess and co-workers synthesized the first bis-NHCcoordinated silicon(II) monoselenide complex 741 by oxidation of the respective donor−acceptor-stabilized silylone 742605 (vide infra) with red selenium in good yield (Figure 87).606 The Lewis acid/base adduct 741 symbolizes a stunning example of a heavier CO homologue and is closely related to the silicon dichalcogenides 578−581.605,606 The Si(II) nucleus adopts a tetrahedral geometry with a SiSe double bond (2.135(1) Å) and exhibits a singlet signal in the 29Si NMR spectrum at −56.6 ppm.

Figure 86. Unique examples of NHC-coordinated disilavinylidene 640 and phosphasilenylidene 630. 9763


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particularly with respect to the bent silaallene resonance form.58,554,698,699 According to these studies, carbene-stabilized silylones are best described with donor−acceptor bonds. Because most of the results in subvalent NHC-supported silylone chemistry have already been summarized elsewhere, only a brief recapitulation is given in the following section.26,46,607,700 Roesky and co-workers successfully isolated the first example of carbene-stabilized silylones in 2013 (Scheme 137).649,650

Figure 87. Further NHC-stabilized complexes with SiE double bonds.

Scheme 137. Synthesis of the First Examples of CarbeneStabilized Silylones

The Cui group described the aforementioned IiPr2Me2supported silicon−carbon mixed cumulene complex 530, representing another example of an unprecedented molecular silicon compound in a low oxidation state stabilized by NHCs.565 Very recently, the striking isolation of the first acyclic, neutral, three-coordinate, and donor-free silanones 743 was reported by Inoue and Rieger et al. (Scheme 136).697 The Scheme 136. Isomerization of a Unique Silanone via a 1,3Silyl Shift under Formation of the Novel NHC−Disilene Adduct 744

Reductive dechlorination of the unpaired biradical species 648 and 651a with 2 equiv of KC8 affords the dark blue CAACstabilized silylones 653a and 653b in 95% and 85% yield, respectively. Alternatively, (CyCAAC)2Si (653b) is synthesized via the reaction of (CyCAAC)2SiCl2 (651a) with the organolithium reagents tBuLi, MeLi, and PhLi, however in significantly lower yields (35−72%). The zerovalent silicon nuclei exhibit downfield-shifted resonances in the 29Si NMR spectra (δ = 66.7−71.2 ppm) in comparison to that of the CAAC-supported biradical 651a (δ = 4.1 ppm). Additionally, the experimentally determined C CAAC −Si distances (1.841(13)−1.853(14) Å) are in a range similar to those of the precursors 648 and 651a (1.843(2)−1.859(18) Å),648,649 illustrating the considerable π-backbonding character. DFT calculations support the presence of Si−C π-back-donations and revealed three-center C−Si−C π-bonds. The formation of a silylone radical anion was indicated by CV measurements of the CAAC-stabilized silylone (CyCAAC)2Si (653b) (quasi-reversible reduction at E1/2 = −1.55 V vs Cp*2Fe/Cp*2Fe+) and supportive EPR studies.701 In fact, reduction of the CAAC-coordinated silylones 653 with an equimolar amount of metallic potassium results in the formation of the six-membered cyclic silylenes 745 in 40−80% yield (Scheme 138). A detailed analysis of the reaction mechanism indicated the radical anion being the key intermediate for the activation of the strong Dipp C−H bond. The three-coordinate silylone isomers 745 possess a closed-shell singlet ground state and exhibit upfield shifted signals in the 29Si NMR spectra at δ = 54.6−56.0 ppm, as

tailor-made ligand framework, containing a σ-donating silyl and a π-donating N-heterocyclic imino (NHI) group, provides efficient kinetic and thermodynamic stabilization and thus enabled the successful synthesis of the silanones 743. Conversion of the persilyl-substituted derivative 743a with an equimolar amount of IMe4 affords the NHC-stabilized zwitterionic disilene adduct 744 in 58% yield. SC-XRD analysis uncovered an extremely elongated Si−Si bond (2.330(9) Å) in comparison to the range of reported disilenes (2.118(1)− 2.289(14) Å536,566,676). The calculated WBI of 1.04 is consistent with the rather negligible double bond character. The observed 29Si NMR resonances for the central silicon nuclei at δ = −35.2 ppm (SiOTMS) and at δ = −174.6 (SiTMS2) indicate a significant contribution of the zwitterionic character, because the highfield-shifted Si illustrates silyl anion character. Therefore, the bonding situation of complex 744 is best described with the depicted zwitterionic resonance structure. 2.4.2.6. Low-Valent Silicon Complexes: Silylones. Zerovalent monatomic silicon complexes (silylones), featuring a central silicon nucleus in the formal oxidation state of zero and bearing two stereochemically active lone pairs of electrons, represent the silicon analogues of the so-called carbones. The group of these heavier congeners has been emerging only recently, as the need for an efficient stabilization by strong Lewis base donors for the isolation of these highly reactive compounds is apparent. Because of the lack of adequate stabilization methods, the development of NHCs has once again played a pivotal role in modern main group chemistry, since their utilization enabled the synthesis of elusive silylone derivatives. In depth theoretical calculations have investigated the nature of the inherent bonding situation of silylones,

Scheme 138. Reactivity of CAAC-Stabilized Silylones 653

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CAACs. Thorough DFT investigations confirmed the more electron-rich and thus more nucleophilic Si nucleus of the bisNHC-supported silylone 582. Moreover, the nature of the HOMO (silicon π-type orbital) and the HOMO−1 (silicon σlone pair orbital) is consistent with the bonding situation in carbene-stabilized silylones. Contradictory to the rather unreactive CAAC-coordinated silylones 653, the bis-NHC-stabilized complex 582 exhibits a high and versatile reactivity. The strong σ-donor ability of silylone 582 is shown by its coordination to the Lewis acid GaCl3 under formation of the “push−pull”-stabilized complex (bis-NHC)Si(GaCl3) (742) in moderate yield (Figure 88).605

compared to the corresponding silylones 653. The geometry around the silicon nuclei changed from a bent structure (Si(0)) to a distorted trigonal pyramidal coordination sphere (Si(II)). Exposure of (Me2CAAC)2Si (653a) and (CyCAAC)2Si (653b) to N2O selectively furnishes the lactam derivatives 746 and SiO2 under oxidation of the former in high yield.649 Aerial oxidation of the silylones leads to the same result. So and colleagues recently reported an alternate method for the synthesis of the CAAC-stabilized silylone (Me2CAAC)2Si (653a).673 Treatment of H2SiI2 with 3 equiv of Me2CAAC in toluene leads to the formation of the silylone complex 653a in 32% yield via a dehydroiodination pathway. However, the reaction is strongly solvent dependent and thus provides the unprecedented cationic CAAC-silicon(I) radical complex 747 in THF or DME in 5% and 62% yield, respectively (Scheme 139). On the other hand, using diethyl ether as solvent affords, Scheme 139. Preparation of the Novel Cationic CAACSupported Silicon(I) Radical 747 and Its Subsequent Reduction to Silylone 653a

Figure 88. Donor−acceptor-stabilized silylone derivatives.

Complex 742, representing the first example of a Lewis acid/ base adduct of a silylone, displays a upfield shifted broad 29Si NMR signal at δ = −119.0 ppm, due to the additional coordination. The three-coordinate silicon nucleus adopts a pseudotetrahedral coordination sphere with an elongated CNHC−Si distance of 1.942(2) Å in the solid state. Recently, the 2-fold coordination capability of both lone pairs of electrons was evidenced by the isolation of the germinal silylone−ZnCl2 complex 748.607 Nevertheless, only the molecular structure has been reported to date, which revealed a tetrahedral geometry around the central Si(0) atom. Remarkably, it has been briefly stated that (bis-NHC)Si (582) can also serve as a reducing agent for GeCl2(dioxane) and (IDipp)SiCl2 (602)543 providing elemental Ge, Si, the respective tetryliumylidenes 749 (vide infra) and 700, and the NHC-stabilized diatomic Si(0) complex [(IDipp)Si] 2 (57269).607 On the other hand, the oxidation of the bis-NHCcoordinated silylone 582 with various chalcogen sources was intensively studied. For instance, the reaction of the complexes 582 and 742 with elemental sulfur, selenium, and tellurium furnishes the NHC-stabilized silicon dichalcogenides 578−581 and the silicon(II) monoselenide complex 741 (vide supra).605,606 The silanechalcogenone NHC adducts display unique examples of heavier CO and CO2 congeners. The reactivity of (bis-NHC)Si (582) toward carbon dioxide was reported only recently.702 Exposure of 582 to four molar equivalents of CO2 forms the first silicon dicarbonate complex 750 in 75% yield via the reduction of CO2 to CO (Scheme 141). The extremely shielded hexa-coordinate silicon nucleus resonates at −195.0 ppm in the 29Si NMR spectrum and exhibits a distorted octahedral geometry in the molecular

besides trace amounts of radical 747, the 2,3-disiladiiodobutadiene derivative [(Me2CAAC)SiI]2 (672)576 in low yield. The CAAC-supported silicon(I) radical 747 adopts a bent geometry with a widened C−Si−C bond angle of 123.0(15)° with respect to silylone 653a (117.7(8)°). The radical nature of complex 747 was elucidated by EPR and DFT investigations, indicating the unpaired electron being localized in a p-orbital of the Si center. Notably, the one-electron reduction of radical 747 with LDA in toluene furnishes once again the CAAC-stabilized silylone 653a in high yield. Driess et al. synthesized the first NHC analogue of a silylone 582 by the reductive dechlorination of the three-coordinate chlorosilyliumylidene chloride 700 with sodium naphthalenide (NaNaph) in 68% yield (Scheme 140), as was already mentioned above.604 Scheme 140. Formation of the Bis-NHC-Stabilized Silylone 582 via Dechlorination of the Chlorosilyliumylidene Ion 700

SC-XRD analysis of dark red crystals revealed a narrowed C−Si−C angle of 89.1(1)° and considerably longer CNHC−Si distances (1.864(1)−1.874(1) Å) in comparison to the CAAC derivatives 653 (C−Si−C angles: 117.2(8)−118.2(5)°). Additionally, the zerovalent silicon atom resonates at a significant higher field (δ = −80.1 ppm) in the 29Si NMR spectrum, demonstrating the stronger σ-donor but weaker π-acceptor capabilities of the bidentate bis-NHC moiety as compared to

Scheme 141. Reaction of Silylone 582 with Carbon Dioxide

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2.4.3.1. Germanium(IV) NHC Complexes. As compared to silicon, only a few examples of halogenated, NHC-coordinated germanium compounds in the formal oxidation state +4 are reported. Because GeCl2·dioxane is a convenient, readily available Ge(II) source, Ge(IV) complexes are less common as precursors for low-valent germanium chemistry. For the most part, Ge(IV)−NHC complexes derive from reactions with low-valent germanium species. In 2012, Rö schenthaler et al. published the NHCcoordinated GeF4 and GeCl4 complexes 752 and 753 (Scheme 143).707 The reaction of the 2,2-difluoroimidazolidine 751 with equimolar amounts of GeCl2·dioxane affords a mixture of NHC-coordinated Ge(IV) halide complexes 752 and 753. Because the fluoro derivative is insoluble in THF, these compounds can be easily separated from each other by fractional precipitation. Both complexes are isostructural and display trigonal bipyramidal germanium centers. However, due to the expected stronger Lewis acid−base interaction between the NHC and GeF4, the CNHC−Ge bond length in 752 (1.981 Å) is slightly shorter than that in 753 (2.002 Å). Upon treatment of 752 with an excess of 751, the imidazolium salt 754 with hypercoordinated germanium is formed. Not surprising, this reaction is accompanied by a structural change at the Ge atom to an octahedral geometry and increase of the CNHC−Ge bond distance (2.057 Å).707 One year later, the group of Rivard described the IDipp adduct of GeCl4 (755) obtained in a straightforward reaction, by mixing the two components in diethyl ether (Figure 89).708 This compound can be reduced to the donor−acceptorstabilized germylene 756, which will be discussed later.708 In the course of their investigations on germylene reactivity, Baines and co-workers reported Ge(IV) NHC complexes 757 and 758a,b depicted in Figure 89.709 Adduct 757, bearing a hypercoordinated germanium center, is formed by the reaction of free carbene with respective GeCl2 precursor and decomposes to the germylene (IiPr2Me2)GeCl2 (759b) and 2,3-dimethylbutadiene upon heating (80 °C, 18 h). The CNHC−Ge distance in this compound (1.965 Å) compares well to the examples of Röschenthaler (vide supra).707 Complexes 758 were obtained by reacting the respective germylene NHC adducts (IiPr2Me2)GeX2 (759b,g, vide infra) with 3,5-di-tertbutylorthoquinone, a known trapping reagent for divalent germanium compounds. Besides these Ge(IV) halogen complexes, several germanium(IV) chalcogen compounds, stabilized by NHC ligands, are reported. The first examples of these heavier ketone analogues, the germanones 760, stabilized either by IMe4 or by IiPr2Me2 (Figure 90), were isolated by the group of Driess.710 These elusive species were obtained by oxidation of the respective NHC-germylenes 761 (vide infra) with N2O at room temperature in toluene, from which germanones 760 precipitate. Interestingly, the free germylene shows no reactivity toward N2O. The larger sensitivity toward oxygenating reagents can be explained by the higher nucleophilicity of the NHC-coordinated Ge(II) center. This example underlines the importance of NHCs in synthetic main group chemistry. In contrast to ketones, the EO double bond (E = Si, Ge, Sn, Pb) in the heavier analogues is highly polarized due to the difference in electronegativity. However, the revealed Ge−O bond lengths in 760 (1.664 Å for 760a, 1.670 Å for 760b) are distinctly shorter than typical Ge−O single bonds in germoxanes (1.75−1.85 Å), indicating a significant GeO double bond character.710

structure. Profound quantum chemical calculations identified the NHC-stabilized silicon monoxide (bis-NHC)SiO and dioxide (bis-NHC)SiO2 complexes as key intermediates in the reaction pathway. The group of So described a striking example of a mixed NHC-/CAAC-stabilized monatomic silicon(0) complex.556 Reductive deiodination of the corresponding iodosilyliumylidene iodide 663 with 2 equiv of KC8 affords complex 702 in 70% yield (Scheme 142). The corresponding signal in the 29Si Scheme 142. Two-Electron Reduction of the Iodosilyliumylidene Cation NHC-/CAAC-Adduct 663

NMR spectrum at δ = 2.0 ppm occurs between those of the silylone complexes 653 (δ = 66.7−71.2 ppm) and (bis-NHC) Si (582) (δ = −119.0 ppm). Furthermore, SC-XRD analysis revealed an intermediary C−Si−C angle of 102.8(2)° (cf., 653, 117.2(8)−118.2(5)° and 582, 89.1(1)°) and a very short CCAAC−Si bond (1.792(4) Å) and a rather long CNHC−Si distance (1.957(5) Å). DFT calculations uncovered the nature of the HOMO (CCAAC−Si π-MO) and the HOMO−1 (silicon σ-lone pair orbital) and a WBI for the CCAAC−Si bond of 1.42, indicating the SiC double bond character. Thus, according to the experimental and theoretical data, complex 702 can also be considered as a NHC-stabilized silene (bent 2-silaallene) instead of a NHC-/CAAC-supported silylone. To conclude this section, Table 6 provides a compilation of important analytical data for the carbene−silicon bonds of selected examples of NHC-coordinated silicon complexes. 2.4.3. Germanium. This section gives an overview on NHC- and CAAC-stabilized germanium complexes, beginning with the pioneering work of Arduengo and co-workers on the preparation of dihalogermylenes in 1993,703 to this date. Like in the silicon section, the germanium compounds are shown and discussed in an order of decreasing oxidation state. Partially, the presented compounds are already covered by Prabusankar’s review on NHC-supported compounds of heavier group 14 elements.28 However, there have been interesting developments in this field of chemistry in recent years. In contrast to silicon, Ge(IV) compounds do not play an equally important role. This can be explained by the easy accessibility of low-valent Ge-precursors (e.g., GeCl2·dioxane). Hence, only a few examples are reported, and therefore this section will be focused on low-valent Ge−NHC complexes. Heavier carbene and silyliumylidene congeners are presented in the Ge(II) section. Besides being precursors for elusive Ge(0) species,607,704 they have already been shown to undergo complex formation with transition metals.705,706 Consequently, these compounds might become interesting and important ligands and reagents in the field of coordination chemistry. At the end of this section, germanium complexes in the formal oxidation state zero and their reactivity are presented. 9766


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Table 6. Key Features of Selected Examples of NHC-Stabilized Silicon Complexesa NHC adduct

CNHC−Si [Å]

(IMe4)SiCl4 (491a) (IDipp)SiF4 (500) (IDipp)SiCl4 (501) (IDipp)SiBr4 (503) [(IDipp)SiBr3][Br] (506) [(IDipp)SiI3][I] (507) (IDipp)2SiF4 (510) (Me2CAAC)SiCl4 (522) (Me2CAAC)SiI4 (524) (Me2CAAC)SiF4 (525) (IiPr2Me2)SiPPh2(N(TMS)Dipp) (543) [(IDipp)Si]2 (572) (bisNHCDipp)Si (582) (IMe4)SiPPh2(2,6-Tipp2-C6H3) (584) [(IMe4)2Si(2,6-Mes2-C6H3)][Cl] (588a) [(IMe4)2Si(2,4,6-iPr3-C6H2)][Cl] (588b) [(Me2CAAC)Si]2 (598a) [(CyCAAC)Si]2 (598b) (IDipp)SiCl2 (602) [(IDipp)SiCl]2 (603) (IMes)SiCl2 (604) (IDipp)SiBr2 (605) (IDipp)SiI2 (606) (SIDipp)SiBr2 (608) [(IDipp)(IiPr2Me2)SiI][I](641) [(IMe4)3Si][I]2 (642) (Me2CAAC)2Si (653a) (CyCAAC)2Si (653b) (Me2CAAC)SiI2 (661) (IMe4)SiCl(2,6-Mes2-C6H3) (664a) (IMe4)SiCl(2,6-Tipp2-C6H3) (664b) (IiPr2Me2)SiCl(N(TMS)Dipp) (670) (IiPr2Me2)Si(SiMetBu2)2 (680a) (IMe4)Si(SitBu3)2 (680b) [(IMe4)Si(SitBu3)2][B(4-SiMe2tBu-C6F4)4] (681) (IMe4)SiH(SitBu3) (684) (IMe4)SiH(2,6-Mes2-C6H3) (694a) (IMe4)SiH(2,6-Tipp2-C6H3) (694b) [(bisNHCDipp)SiCl][Cl] (700) [(IMe4)2SiH][I] (701) (IiPr2Me2)(Me2CAAC)Si (702) [(Me2CAAC)Si]3 (720)

− 2.004(2) 1.928(2) 1.935(2) 1.880(9) 1.911(3) 2.0088(14)/2.0109(15) 1.944(2) − − 1.9909(18) 1.9271(15) 1.864(1)/1.874(1) 1.958(3) 1.9481(19)/1.9665(19) − 1.887(4) 1.876(4) 1.985(4) 1.929(7)/1.939(6) − 1.989(3) 1.997(4) 2.012(2) 1.947(2) 1.909(3)/1.917(3)/1.918(3) 1.8411(18)/1.8417(17) 1.8407(13)/1.8531(14) 2.013(5) − 1.963(2) 2.0023(19) − 1.933(4) 1.915(3) 1.942(3) 1.9506(21) − 1.960(4)/1.963(4) 1.931(3)/1.932(3) 1.792(4)/1.957(5) 1.834(7)/1.854(7)/1.878(7)

δ(13CNHC) [ppm] a

153.2 − − − 136.3b 125.5c − 206.1d − 221.0a 166.3d 196.3a 188.7d 166.7f 160.3g 159.7g 236.7a 234.7a 168.5a 180.0a − 164.5a 158.4a 188.7a 151.5/158.3b 150.7b 210.9a 210.8a 230.1a 165.2a 166.7a 164.1a 172.1a 172.6a − 176.9a 169.1a 168.5a 161.6g − 177.2/200.7a 207.1a

δ(29Si) [ppm]

ref

−105.9a −141.0a −108.9b −89.6a −63.9b −225.8c −184.0a −103.5d −203.1e −133.5d 4.2a 224.5a −83.8a/−80.1d −39.2 a −68.9g −69.5g 252.3a 249.1a 19.1a 38.4a 17.8d 10.9a −9.7a 10.8a −55.3b −89.9b 66.7a 71.2a −2.1a 1.3a 0.8a 3.1a −132.3a −128.9a − −137.8a −87.6a −80.5a −58.4g −77.5e 2.0a 7.2a

535 544 69 544 546 547 544 555 556 557 577 69 604 597 609 609 614 653 543 69 543 546 547 548 547 547 650 649 556 657 657 559 664 664 664 667 671 671 604 673 556 654

NMR solvents: a = C6D6, b = CD2Cl2, c = CDCl3, d = THF-d8, e = pyridine-d5, f = toluene-d8, g = CD3CN. − = unknown/not observed.

a

Scheme 143. Synthesis of Ge(IV)−Halogen NHC Complexes 752−754

Figure 89. Ge(IV) NHC adducts.

NHC-stabilized germylene 762 (vide infra) readily reacts with sulfur and selenium, furnishing respective compounds 763. The germathione 763a turned out to be extremely unstable. It decomposes in solution within several hours, and its formation was only verified by 1H and 31P NMR spectroscopy. In contrast, the analogues selenium compound 763b is stable enough to be isolated and fully characterized. In the 13C NMR

Two other examples of heavier ketone analogues were reported by Escudié and co-workers.706 The corresponding 9767


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Figure 90. NHC-stabilized heavier ketone analogues 760 and 763 and heavier CO2 congeners 764.

spectrum, the chemical shift of carbene C-nucleus (149.9 ppm) clearly indicates a coordination of the NHC. Furthermore, the coupling with the phosphorus atoms can be observed (3JC−P = 10.0 Hz). The 77Se NMR signal (−173.7 ppm) is in the range of reported germasilenones (−28.7 to −348 ppm). The Ge Se double bond length of 2.2426 Å is also in good agreement with previous examples. The CNHC−Ge bond length (2.048 Å) is about the same as that in the starting material 762. It turned out that, upon heating to 80 °C in the presence of elemental selenium, the germaselenone 763b decomposes forming a dimeric molecule bridged by three selenium atoms without NHC coordination.711 Later, Driess and co-workers isolated two heavier congeners of CO2, 764 (Figure 90), by reacting the germylone GaCl3 adduct 765 (vide infra) with elemental sulfur or selenium.712 The push−pull effect provided by Lewis base (NHC) and Lewis acid (GaCl3) was effective to stabilize these products. Treatment of germylenes 759a,c,g with alkyl iodides furnishes the germanium cations 766 (Scheme 144)713 The

Figure 91. Isolated NHC-stabilized acceptor-free germylenes.

767a.703 It exhibits a CNHC−Ge bond length of 2.102 Å,703 which is longer than the range of typical Ge−C single bonds (1.90 Å−2.05 Å)714 and can therefore be described as a dative bond. Adduct 767a shows a remarkable thermal stability and melts between 210 and 214 °C.703 Baines et al. reported the related (IiPr2Me2)GeCl2 adduct (759b).714 They obtained it by reacting GeCl2·dioxane complex with the carbene and used it as starting material for a variety of further germylenes. The reaction with the corresponding halotrimethylsilane yields the diiodogermylene 759a and dibromogermylene 759e. In the case of triflate, they were not able to afford the ditriflylgermylene. Although using an excess of trimethylsilyltriflate in a similar reaction, only the monotriflyl-substituted germylene 759f was formed. Treatment of 759f with varying amounts of 2,2,2-cryptand affords a mixture of a naked encapsulated germanium dication, the dichlorogermylene NHC adduct 759b, and the first chlorogermyliumylidene 768 (vide infra).715 The difluoro analogue 759d was prepared by treating 759b with KF in the presence of crown ether 18-C-6. Furthermore, (IiPr2Me2)GeCl2 (759b) can undergo substitution reactions with potassium tert-butoxide and potassium thiocyanide, which leads to compounds 759g and 759h.716 The dimesitylgermylene 759c can be prepared in two different fashions: either by substitution of 759b with dimesityl magnesium716 or by cleavage of tetramesityldigermene with 2 equiv of IiPr2Me2. Besides these N,N′-diisopropyl NHC adducts, the group of Baines synthesized similar germylene complexes 769a−e with sterically less demanding carbene IMe4 and reacted dichlorogermylene 759b with (IMe4)GeMes2 (752c) to obtain the germanium-substituted germylene 759i in a poor yield of 25%.713 Later, Tobita et al. were able to increase the yield of 759i to 87% by using GeCl 2·dioxane complex instead of the diclorogermylene NHC adduct 759b.717 Furthermore, Baines and co-workers were able to abstract the two iodine atoms from (IiPr2Me2)GeI2 (759a) with an excess of NHC to generate the germanium dication 770, which is coordinated in a pyramidal geometry by three identical carbenes (Scheme 145).714 Moreover, Schulz et al. isolated the NHC-stabilized diazidogermylene 767c from the reaction at the dichlor-

Scheme 144. Synthesis of Ge(IV) Cations 766

observed CNHC−Ge bond length in 766a (1.994 Å) is shorter than that in the precursor 759a due to the involvement of the germylene electron lone pair in a bonding interaction and the appeared cationic charge. In chloroform, the mesitylsubstituted germanium cations 766c,d (X = Mes) undergo chlorination to the respective Mes2Ge(Cl)R.713 2.4.3.2. Germanium(II) NHC Complexes. Going down the ladder of oxidation states, Ge(II) NHC complexes represent the major part of this section. These compounds comprise germylenes, germyliumylidenes, as well as the heavier congeners of carbon monoxide. Germylenes are isovalent with singlet carbenes; that is, their frontier orbitals consist of an electron lone pair and an empty p-orbital at the germanium center, and therefore show amphiphilic reactivity. This high reactivity can be regulated by coordination with strong Lewis bases, such as NHCs, to isolate the low-valent germanium compounds (Figure 91). As mentioned above, Arduengo and co-workers established this field of chemistry in 1993 by reacting IMes with GeI2 and isolating the first germylene 9768


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complex of parent digermene H2GeGeH2. Complex 773 (Figure 92) was isolated by fractional crystallization and fully characterized. The 1H NMR spectrum of 773 contains distinctive multiplet resonances at 3.05 and 4.08 ppm, which indicates the presence of four magnetically inequivalent germanium-bound hydrides. The Ge−Ge bond distance in 773 is 2.4212(7) Å and lies in the range of single bonds. In contrast to the stable dihydrogermylene 772b, complex 767 decomposes in benzene solution at ambient temperature, yielding 772b and presumably elemental germanium.721 Besides exploring the low-valent germanium hydride chemistry, Rivard et al. used the dichlorogermylene 767b as starting material to access carbene-stabilized GeCl2 oligomers. It turned out that the nucleophilic lone pair of germylene 767b can react with 1 equiv of GeCl2·dioxane complex to form the NHC-stabilized formal tetrachlorodigermene 768a in a good yield of 75%. With 2.6304 Å, the Ge−Ge bond in 768a is significantly longer than most typical Ge−Ge single bonds (2.40−2.50 Å) and indicates a dative bonding.722 Moreover, the sum of bond angles at the germanium atom within the terminal GeCl2 group (276.36°) shows that there is a lone pair of electrons and therefore another possible coordination site. In THF, the lengthened Ge−Ge bond cleaves releasing the starting material 767b.722 Encouraged by the successful isolation of the carbenesupported Cl2Ge−GeCl2 array, Rivard et al. added either 1 or 2 equiv of GeCl2·dioxane complex to 774a and isolated in both cases the branched Ge4Cl8 adduct 774b in low yields. This compound is the first polygermane, which was synthesized by sequential addition of germanium halides. Further attempts to grow the chain by addition of GeCl2·dioxane failed.722 The synthesis of similar Ge(II) compounds is depicted in Scheme 146. Rivard and co-workers substituted one chloride

Scheme 145. Synthesis of Germanium Dication 770

ogermylene 767b with sodium azide.This compound is soluble and stable in aprotic, polar solvents, shows no shock-sensitivity, and decomposes only at 205 °C under nitrogen elimination.718 Filippou et al. prepared the IMe4 adducts of dichlorogermylene 769b and aryl-substituted chlorogermylenes 769f,g either by chlorine substitution with the respective aryllithium reagents or by coordination of aryl(chloro)germylenes ArGeCl with the NHC.657 Because of the presence of free lone pair of electrons, germylenes can act as Lewis bases furnishing various NHCstabilized germylene−Lewis acid adducts (Figure 92). By

Figure 92. Lewis acid adducts of NHC-stabilized germylenes.

Scheme 146. Synthesis of Amidogermylene 775 and Germylene Borane Adduct 776

treating dimesitylgermylene complex 759c with either BH3 or Ph3PBH3, Baines and co-workers obtained the germylene borane adduct 771. It shows metrics similar to those of the starting material 759c and is described as an “in-series” carbene−germylene−borane complex.719 Dichlorogermylene (IDipp)GeCl2 (767b) was published by the groups of Rivard and Jones independently at the same time.704,720 Rivard et al. treated it with an excess of lithium borohydride to obtain the donor−acceptor-stabilized parent germylene 756 (Figure 92).720 This compound is unstable upon heating in refluxing toluene, releasing the NHC borane adduct (IDipp)BH3.720 They assumed that the replacement of the Lewis acid BH3 with the stronger electron acceptor W(CO)5 would lead to more stable adducts and were able to prepare the dihydrogermylene complex 772b by two distinct routes. Treatment of the dichlorogermylene tungsten pentacarbonyl adduct 772a with 2 equiv of Li[BH4] or reaction of the borane adduct 756 with the tungsten pentacarbonyl THF complex provides the product, which shows no signs of decomposition even after 16 h in refluxing toluene. It is to note that the BH3/W(CO)5 metathesis provides the higher yield of 772b (43%). In the 1H NMR spectrum, the GeH2 group of 772b is detected at 4.23 ppm and thereby within the expected range for low oxidation state Ge(II) hydrides.705 A rather unorthodox three-component reaction between (IDipp)GeCl2 (767b), THF·GeCl2·W(CO)5 complex, and the hydride source Li[BH4] yielded a mixture of 756 (40%), 772b (40%), and complex 773 (20%), regarded as donor/acceptor

of the dichlorogermylene 767b with an amino group to obtain the amidogermylene 775, which was subsequently converted to the amidogermylene hydride borane adduct 776.635 In 2000, Lappert et al. synthesized a number of low-valent group 14 NHC complexes and among those the germylene 778 (Scheme 147). They treated the N-heterocyclic germylene 777 with the NHC to isolate 772, which displays a pyramidal geometry at the germanium center due to the stereochemically active lone pair.723 Scheme 147. Synthesis of NHC−N-Heterocyclic Germylene Adduct 778

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In a similar fashion, the group of Driess prepared the germylenes 761 (Figure 93). The free six-membered cyclic

Figure 94. Structures of cyclic germylenes 781 and 782.

60 °C was a sharp signal at −9.3 ppm observed. This is most likely due to the lack of configurational stability at the germanium atom at room temperature.726 In contrast to the result of Escudié et al., the group of Marschner was able to synthesize hypersilyl-substituted germylenes using potassium hypersilanide. Instead of starting from the NHC-stabilized dichlorogermylene 759b, they reacted GeX2·dioxane (X = Cl, Br) complexes with the silanide in the presence of the respective NHCs (IMe4, IiPr2Me2) to obtain the dihypersilylgermylenes 783a,b (Scheme 149).727,728

Figure 93. NHC-stabilized N-heterocyclic germylenes 761.

germylene reacts at −30 °C straightforward with an equimolar amount of NHCs to give the respective adducts 761 in yields of 85−86%. In both cases, the C3N2Ge rings remain almost planar, and the CNHC−Ge bond lengths (761a, 2.147 Å; 761b, 2.192 Å) are in the range of other NHC−germylene complexes. Both complexes were successfully oxidized with N2O to the corresponding NHC-coordinated germanones 760 (vide supra).710 Escudié and co-workers presented three germylene compounds 779−780 with bulky E(TMS)3 (E = Si, Ge, Sn) substituents (Scheme 148).724 After attempts to displace one

Scheme 149. Synthesis of Dihypersilylgermylenes 783 and Germylene Coinage Metal Complexes 784 and 785

Scheme 148. Synthesis of Hypermetallyl Germylenes 780 and 779

chloride in compound 759b using lithium- and potassium hypersilanide failed, the substitution was achieved by addition of 0.5 equiv of dihypersilylmagnesium. Compound 779a was isolated in a good yield of 71%, and in an analogous reaction, compound 779b was prepared using a digermylmagnesium reagent. However, the chlorohyperstannyl germylene could not be synthesized by a similar experimental procedure. Reaction of 759b with 0.5 equiv of distannylmagnesium led to the formation of 780 in a poor yield of 30%, which was later improved to almost quantitative yield by using 1 equiv of the stannylmagnesium reagent. It was shown that the replacement of a chlorine atom by an E(SiMe3)3 group (E = Si, Ge, Sn) has almost no influence on the CNHC−Ge bond length.724 The group of Marschner also published a number of NHCstabilized, silyl-substituted germylenes, including two cyclic ones that are depicted in Figure 94.725,726 Both compounds 781 and 782 (Figure 94) were prepared in a similar fashion: the respective bis(potassium silanide) was reacted with GeBr2· dioxane in the presence of IMe4.725,726 For the bicyclo[2.2.1] backbone of compound 782, two resonances for the SiMe2 groups were observed in the 29Si NMR spectrum, which indicates configurational stability of the germylene atom. Interestingly, no 29Si NMR signal for the SiMe3 groups was detectable at ambient temperature. Only at

Furthermore, the reactivity of 783a toward coinage metal cyanides was investigated. Even though the reaction with CuCN and AgCN led to ill-defined product mixtures, the subsequent addition of B(C6F5)3 gave the germylene silver complex 784. Without the Lewis acid, the reaction afforded gold complex 785.728 Compounds 784 and 785 both feature the germylene unit as neutral ligands and display almost linear geometries around the transition metals (177.2° (Au), 176.1° (Ag)). These complexes are the first reported examples of compounds bearing the structural motives Ge−Au−Si and Ge−Ag−Si. For both compounds, the Ge−M (d(Ge−Au) = 2.448 Å; d(Ge−Ag) = 2.502 Å) bonds are in the range of reported Ge−Au and Ge− Ag single bonds, respectively.728 Further examples of germylene−transition metal complexes were discovered by Escudié and colleagues exploring the reactivity testing of their NHC−germylene adduct 762 (Scheme 150). There are two ways to synthesize 762, either by reaction of (IiPr2Me2)GeCl2 (759b) with the carbenoid Mes*PC(Li)Cl or by coordination of the NHC to the metastable free germylene. Germylene 762 exhibits a geometry 9770


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Since 2013, Scheschkewitz et al. contributed to the lowvalent germanium NHC chemistry with a variety of unprecedented compounds. They prepared a NHC-stabilized silagermylidene 789, the heavier analogue of vinylidene, and tested its reactivity as depicted in Scheme 151.730 Reduction of

Scheme 150. Synthesis of 763 and Germylene−Transition Metal Complexes 786 and 787

Scheme 151. Synthesis and Reactivity of Silagermenylidene Complex 789

the NHC−germylene adduct 759b together with Tipp2SiCl2 with 4 equiv of lithium naphthalenide yields a 2:3 mixture of compound 789 and the disilene (Tipp)2SiSi(Tipp)2. Silagermylidene 789 was isolated by fractional crystallization from hexane in a poor yield of 8%, however in a reproducible manner. The red crystals turned out to be thermally stable and melted at 128−130 °C. In the 29Si NMR, 789 gives one signal at 158.9 ppm, which is significantly downfield shifted as compared to aryl-substituted disilenes and silagermenes. They state the dicoordinate Ge(II) moiety appears to be more electron-rich than the silylene fragment Si(Tipp)2, and therefore the 29Si NMR chemical shift is more comparable to that of silyl-substituted disilenes ((iPr3Si)2SiSi(iPr3Si)2: δ = 154.5 ppm). The Si−Ge bond length of 2.252 Å is an intermediate value between the bond length in (Tipp)2Si Si(Tipp)2 (Si−Si bond length: 2.144 Å) and that in the bisNHC Ge(0) complex IDipp2Ge2 832 (Ge−Ge bond length: 2.349 Å, vide infra). Noteworthy, the NHC coordinates to the Si−Ge bond almost orthogonal (98.90°). Because of the double bond between the silicon and germanium atom, complex 789 can undergo a [2+2] cycloaddition with phenylacetylene to form the cyclic NHC-coordinated germylene 790. The upfield-shifted 29Si NMR signal (−39.1 ppm) and the elongated Si−Ge bond length (2.498 Å) as compared to the starting material clearly confirm the absence of the Si−Ge double bond.730 Only one year later, Scheschkewitz and co-workers described another NHC-stabilized silagermenylidene 737, similar to 789 (Scheme 152).731 Reaction of the dichlor-

that compares well with other NHC−germylene adducts (distorted pyramidal structure around the Ge center, d(Ge−C) = 2.087 Å). In the presence of elemental sulfur or selenium, it forms the heavier ketone analogues 763, which have been previously discussed. Furthermore, it can react with different gold sources (e.g., AuCl·SMe 2), cleanly affording the corresponding gold halide complexes 786. The gold atoms in these compounds display an almost linear geometry (bond angles: Ge−Au−Cl = 177.07°; Ge−Au−I = 176.75°), and the Ge−Au bond lengths are similar to those of previously investigated Ge(II) gold complexes.706 Besides that, upon reaction with tungsten and molybdenum carbonyls, germylene 762 affords the respective germylene− transition metal complexes 787 in good yields. In both cases, the 31P NMR spectrum contains two signals indicating that only one phosphorus atom interacts with the metal center. In 787a, the tungsten center is octahedrally coordinated with two carbonyl ligands occupying the apical positions, and the germanium atom displays a distorted tetrahedral geometry. With 2.6508 Å, the Ge−W bond distance is one of the longest to be reported for germylene complexes.706 Very recently, Marschner et al. published a study dealing with a variety of metal complexes containing a modified silatrane ligand, including the disilylgermylene 788 (Figure 95). They introduced the ligand to GeCl2·dioxane as

Scheme 152. Synthesis of Silagermenylidene 737 and Its Iron Complexes 791 and 792

Figure 95. Disilylgermylene 788 bearing silatrane ligands.

potassium silanide in the presence of PMe3 as electron donor and subsequently replaced PMe3 with the stronger Lewis base IMe4. In the 29Si NMR spectrum of 788, the silatrane ligand resonates at −41.9 ppm, which is upfield shifted in comparison to other silatrane units and indicates diminished degree of hypercoordination.729 9771


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contradict the formation of the NHC exchange product, but indicate the formation of the three-membered ring 793. When compound 737 is reacted with an excess of IMe4, it consumes 2 equiv and forms the imidazolium salt 735 via 793. Because the CNHC−Si bond (1.934 Å) is not only significantly shorter than the CNHC−Ge bond (2.035 Å) but also close to a regular C−Si single bond, compound 735 could also be depicted as an imidazolium salt. Furthermore, silagermylidene 737 was reacted with mesityllithium, yielding the threemembered cyclic germylene NHC adduct 738. As a byproduct of this reaction, a three-membered cyclic molecule similar to 738 but with the mesityl group at the germanium atom and the NHC bound to the silicon was isolated. One year later, Scheschkewitz and co-workers published the reactivity study of 737 toward isonitriles.732 This kind of reactivity of isonitriles has been observed previously for Ge(II)-centers, SiSi double bonds, as well as insertion reactions into Si−Si bonds of strained ring systems.732 In this case, the purple compound 794 was isolated in a moderate yield of 58% (Scheme 154).

ogermylene 759b with lithium disilenide 682 leads to the compound 737 in a moderate yield of 62%. The presumable intermediate of this reaction is the NHC-stabilized chloro(disilenyl)germylene, which undergoes a 1,3-migration of chloride from germanium to the β-silicon atom. The lowfield 29Si NMR shift of the Sisp2 (162.5 ppm) as well as the Si− Ge bond length (2.276 Å) are comparable with the previously reported similar silagermylidene 789. In benzene solution, compound 737 undergoes stereoisomerization (regarding the position of the chlorosilyl group to the NHC), which reaches equilibrium after 4 h, essentially unaffected by temperature, with an E/Z ratio of 0.85:0.15 (calculated from the 29Si NMR spectrum). An E/Z-mixture of 737 initially affords the iron carbonyl complex 791-Z (position of chlorosilyl group in regard to iron position) upon treatment with diiron nonacarbonyl. At ambient temperature, compound 791-Z undergoes a slow but in this case irreversible E/Z-isomerization to 791-E, which was confirmed by SC-XRD: the Si−Ge double bond adopts a trans-bent as well as twisted geometry and is shortened (2.248 Å) in comparison to the starting material 737. With 2.378 Å, the Ge−Fe bond is slightly longer than that in reported germylene-coordinated iron(0)tetracarbonyl complexes.731 When the isomerization of 791-Z takes place at 65 °C, along with the major product 791-E (56%), the cyclic germylene iron complex 792 (14%) is generated. Because 792 cannot be afforded by heating of 791-E, spatial proximity of the chlorosilyl group and the iron moiety seems to be necessary for the isomerization, which proceeds under release of CO. The structure of 792 is described as a bicyclo[1.1.0]butane-like butterfly structure with a distance between the bridging germanium and iron atoms of 2.688 Å.731 Besides the preparation of the iron complex 791, Scheschkewitz et al. further investigated the reactivity of their silagermylidene 731 toward mesityllithium and IMe4. These reactions are depicted in Scheme 153.539

Scheme 154. Synthesis of Cyclic Germylene 794

Compound 794 shows the characteristics of a NHCcoordinated germylene, such as 13C NMR CNHC shift (169.8 ppm), CNHC−Ge bond length (1.986 Å), as well as a pyramidal geometry around the germanium center (sum of bond angles = 313.07°). Notably, complex 794 is completely stable in the solid state and in solution, even at an elevated temperature of 65 °C, but upon removal of the coordinated NHC by a Lewis acid (BPh3), it forms the dimeric digermene.732 In 2016, the group of Scheschkewitz also reported another reactivity of their silagermylidene 737 (Scheme 155).733,734

Scheme 153. Reactivity of 737 toward Mesityl Lithium and IMe4

Scheme 155. Synthesis of NHC-Stabilized Germylenes 795 and 796

When 737 is treated with methyllithium, it generates the methylgermylene 795a, which is not stable in solution and rearranges to a three-membered Si2Ge cycle that reacts further under release of NHC.734 Reaction with phenyllithium however leads to the isolable phenyl-substituted germylene 795b, bearing a disilene moiety. At 40 °C in toluene solution, it rearranges to the cyclic three-membered NHC-coordinated germylene 796, which displays a structure and characteristics similar to those of compound 738 (13C NMR shift CNHC, δ = 172.9 ppm; CNHC−Ge bond distance, 2.059 Å). Both germylene isomers, the acyclic 795b and the cyclic 796, were tested in cycloaddition reactions with phenyl-

Upon treatment of 737 with the smaller NHC IMe4, an immediate color change from red to yellow occurred, and the release of free IiPr2Me2 was confirmed by 1H NMR spectroscopy. Interestingly, the 13C NMR shift of 170.8 ppm of the carbene C-nucleus indicated a coordination by NHC. However, 29Si NMR shifts (δ = −18.2 ppm, −71.6 ppm) 9772


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acetylene and isonitriles (Scheme 156).733 Compound 795b readily reacts with phenylacetylene already at low temperatures

Very recently, Hahn and co-workers published a number of intramolecular NHC germylene complexes 802 (Figure 96).736 The germanium atom was introduced by treating the corresponding chloro imidazolium salt with germanium hexymethyl disilazide to afford the chlorogermylene 802 in a good yield. In the 13C NMR spectrum of 802a, the CNHC signal can be observed at 171.4 ppm, which clearly indicates the coordination of the NHC to the germylene center and also the CNHC−Ge bond length (2.062 Å) is within the expected range.736 In substitution reactions with potassium nucleophiles, the chloride can be replaced by tert-butoxide (802b) and hexamethyldisilazide (802c), respectively,736 which is in accordance with the results published by Baines et al.716 Reacting the (IiPr2Me2)GeCl2 adduct 759b with the boryl lithium reagent (THF)2Li[B(NDippCH)2], Aldridge and coworkers synthesized the NHC-stabilized germylene 803. Subsequent reduction of this compound with 0.5 equiv of Jones’ magnesium(I) Nacnac reagent furnishes the unsymmetrical mono-NHC-coordinated formal digermyne 804 (Scheme 157). Compound 804 displays a GeGe bond length of 2.279 Å, which is in the range of the terphenylsubstituted Ge2 systems.737

Scheme 156. Reactivity of 795b (Left) and 796 (Right) toward Phenylacetylene (Top) and Xylylisonitrile (Bottom)

Scheme 157. Synthesis of NHC-Stabilized Digermyne 804 from Boryl Germylene 803

to yield the five-membered germylene cycle 797, which is stable for several weeks in solution and does not undergo further reactions with an excess of the reagent. Surprisingly, the phenyl group migrated from the germanium to the silicon atom. However, harsher conditions (70 °C, 22 h) are necessary to react 796 with phenylacetylene to generate 798. Because arylacetylenes polymerize at these conditions, an excess of phenylacetylene is needed for the complete conversion of 796. In contrast to the regioisomeric 797, the CC double bond in the compound 798 is directly attached to the NHCcoordinated germanium center. Complex 796 also shows lower reactivity toward xylylisonitrile cycloaddition and reacts only at elevated temperatures. Yet, both reactions yield fourmembered cyclic germylenes with NHC-coordinated germanium centers. Scheschkewitz and co-workers showed that West’s Nheterocyclic silylene readily inserts in the Ge−Cl bond of the dichlorogermylene (IiPr2Me2)GeCl2 (759b), furnishing the silyl-substituted germylene 801 (Figure 96).735 In comparison to the starting material, the 29Si NMR signal is strongly downfield-shifted from −3.4 to 78.3 ppm. The CNHC−Ge bond length (2.081 Å) is within the expected range.

Metallagermylenes represent the subclass of germylenes, bearing a transition metal substituent on the Ge(II) center. In 2012, Tobita and co-workers reported the first metallagermylene 805 (Scheme 158).717 They reacted the literature Scheme 158. Synthesis of the First Metallagermylene 805 and Its Conversion to Complex 806

known chloro(germanyl)germylene−NHC adduct 752b with the anionic tungsten Cp tricarbonyl complex to obtain the [Cp(CO)3W]-functionalized germylene 805. Notably, the substitution reaction occurred only at the sterically less hindered GeCl unit to form exclusively the compound 805, which turned out to be unstable at ambient temperature. Nevertheless, a crystal structure of 805 was obtained. Tungsten is coordinated in a four-legged piano stool configuration, and the germanium center displays a pyramidal geometry due to the stereochemically active lone pair. In

Figure 96. Structure of silyl-germylene NHC adduct 801 and cyclic germylenes 802. 9773


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at room temperature in fluorobenzene, compound 808 was formed in a moderate yield (59%) and described as cationic metallogermylene. Compound 808 displays a two-coordinate Ge center with a bent geometry (W−Ge−C angle: 112.4°). Because of an increase of the π back-donation from the filled d orbital at the tungsten to the empty p orbital of the germanium, the revealed W−Ge bond length is significantly shortened in comparison to 807 (2.5787 Å vs 2.7413−2.7894 Å). This result indicates the delocalization of the positive charge over the W−Ge−C bond. Therefore, 808 is presented as imidazolium salt and not as germyliumylidene. In contrast, the chloride abstraction from the germylenes 807b,c coordinated by less sterically demanding NHCs led to the formation of the dicationic dimetalladigermenes 809 and 810, which are almost insoluble in organic solvents.738 Complex 809 shows a twisted (50.0°) Z-configuration with trans-bent angles of 34.6° and 35.3°. With 2.4286 Å, the Ge− Ge bond is within the range of previously reported double bond lengths in digermenes.738 In contrast to that, digermene 810 adopts an E-configuration with trans-bent geometry (trans-bent angle: 30.5°), which is not twisted (twist angle: 0°). The GeGe distance in 810 (2.345 Å) is shorter than that in 809. These differences in geometry are attributed to the different steric demand of the attached NHC ligands. For instance, the iso-propyl groups as wingtips in 809 lead to an elongated and distorted Ge−Ge bond. The CNHC−Ge bonds in 809 and 810 are shorter than those in 807b and 807c, respectively, indicating distribution of the positive charge over the NHC rings and germanium motif. Theoretical calculations for compound 810 revealed a considerable double bond character of the GeGe bond (Wiberg bond index: 1.432). Furthermore, natural population analysis (NPA) revealed that the Ge-atom as well as the NHC both are highly positively charged (+0.65 and +0.44 respectively). Thus, the two dominant resonance structures can be drawn for 810: the one, depicted in the Figure 97, and the dimetallodigermene-1,2-diylium ion structure, where the NHC is bound to a vacant p orbital of the germanium cations.738 Later, Tobita et al. investigated the reactivity of the cationic germylene 808. Notably, they found that it activates dihydrogen and reacts reversibly with hydrosilane and hydroborane (Scheme 160).739 At elevated temperature (60 °C), the reaction of 808 with hydrogen selectively affords the insertion product 811 isolated in a very good yield. In the 1H NMR spectrum, the GeH2 signals resonate at 3.78 ppm. Furthermore, complex 810 was found to insert into the Si−H bond of dimethylethylsilane to form 812. Interestingly, this reaction turned out to be reversible: at 60 °C, the hydrosilane was eliminated from the germanium center and a temperature-dependent equilibrium was achieved. Above 80 °C, decomposition of both 808 and 812 occurred. This is the first example of a reversible insertion of an isolable germylene into a Si−H bond. The 1H NMR chemical shift of the GeH unit was observed at 3.56 ppm, and the 29Si NMR spectrum contains only one signal at −5.3 ppm (SiMe2Et). In a similar reversible reaction, compound 808 inserts into the B−H bond of HBpin to yield the addition product 813. In 2012, Filippou and co-workers reported two transition metal germylene complexes (Scheme 161).740 They treated their germylidyne complexes 814 with IMe4 and obtained the compounds 815. The CNHC−Ge bond lengths are comparable

comparison to the starting material, the Ge−Ge bond length has not changed significantly. With 2.8127 Å, the W−Ge bond is longer than in base-free tungsten metallagermylenes,717 while the CNHC−Ge bond (2.082 Å) is shorter in regard to the starting material. The reason for the elongation of the W−Ge bond is probably the steric repulsion between the CO ligands and the NHC, as well as the reduced π-back-donation by the competitive electron donation from the NHC to the p orbital at the α-Ge. At the same time, this effect makes the tungsten center more electron-rich, which shifts the CO frequencies to lower wavenumbers in the IR spectrum. One carbon monoxide ligand of 805 can be cleaved off by irradiation with a mercury lamp, to form the germylyne complex 806 (Scheme 158).717 Although no publishable crystal structure could be obtained, the preliminary one, in combination with multinuclear NMR spectra, supports the proposed structure. The 13C NMR shift of the CNHC (175.3 ppm) of 806 is within the range reported for NHCcoordinated germylenes. Two years later, the group of Tobita described the metallagermylenes 807738 with structures similar to that of 805. The synthesis affords isostructural 807a−c in moderate (IMe4: 46%) to excellent (IDipp: 95%) yields (Scheme 159). With 2.7413 Å, the W−Ge bond in 807c is slightly shorter than that in 805; however, the other metrics are within the expected range. Scheme 159. Synthesis of Metallagermylenes 807

Furthermore, the reactivity of 807a−c toward chloride abstraction reagent NaBArF4 (ArF = 3,5-(CF3)2C6H3) was investigated. Depending on the seric demand of the NHC ligand, three different products 808−810 were isolated (Figure 97).738 Upon reaction of 807a (NHC = IDipp) with NaBArF4

Figure 97. Cationic derivatives of metallagermylenes. 9774


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By the treatment of 816 with NaBArF4 (ArF = 3,5(CF3)2C6H3), similar to the procedure published by Tobito et al., the chloride was abstracted affording the cationic germylidyne complex 817. The W−Ge bond length (2.2813 Å) is shorter than that in the starting material 816, laying within the range of reported W−Ge triple bonds. The linear geometry of the W−Ge−CNHC fragment indicates the presence of a W−Ge triple bond. The CNHC−Ge bond in 817 is significantly shortened as compared to the precursor 816 (1.952 vs 2.025 Å).741 Whereas the stabilization by the NHC coordination is a widespread method in modern main group chemistry to make elusive low-valent compounds accessible, the application of abnormal carbenes (aNHCs) is still in its infancy (for a general overview on aNHCs, cf., section 1). In 2012, the group of Roesky reported the abnormal carbene-supported dichlorogermylene 818 (Figure 98),550 a heavier congener of the

Scheme 160. Reactions of 808 with Dihydrogen, Hydrosilane, and Hydroboranea

a

ArF = 3,5-(CF3)2C6H3.

Scheme 161. Synthesis of Transition Metal Germylene Complexes 815 Figure 98. Germylenes stabilized by aNHC (818) and CAACs (821, 822).

previously discussed (cf., section 2.4.2) aNHC-stabilized dichlorosilylene 647. The synthesis proceeds straightforwardly: the free aNHC reacts with GeCl2·dioxane to give the complex 818, which displays a distorted trigonal pyramidal structure at the germanium center. The distance between the carbene Catom and the germanium was determined to be 2.071 Å.550 Two years later, Goicoechea and Waters described two Ge(II) compounds 819 and 820, which are coordinated by abnormal carbenes (Scheme 163).126 The unstable diamido-

with the Ge−Ctrisyl distances, which indicates a strong CNHC− Ge donor−acceptor interaction. Starting from Tobita’s metallagermylene 807a, Filippou, Lebedev, and co-workers reported the tungsten germylidyne complex 817 (Scheme 162).741 First, they converted 807a to

Scheme 163. Conversion of Abnormal NHC Complex 819 to Germyliumylidene 820

Scheme 162. Synthesis of Tungsten Germylidyne Complex 817 via 816a

a

ArF = 3,5-(CF3)2C6H3.

germylene 819 was obtained from reaction of KIDipp (vide supra) with 1 equiv of Ge[N(SiMe3)2]2. Elimination of K[Ge[N(SiMe3)2]3] from 819 in THF solution at room temperature results in the formation of the germyliumylidene 820. No single crystals of 819 were obtained, but because it cocrystallizes in the presence of 18-crown-6 and 820, its structure was clarified. Akin to other NHC-stabilized germylenes, compound 819 displays a trigonal pyramidal geometry at the germanium center, but is coordinated to the anionic carbene via the backbone (C4/C5 position). The bond distance between this carbon atom and the germanium center (2.059 Å) is slightly longer than an average Ge−C single bond

cation 808 and attempted to abstract one carbonyl ligand from the tungsten center with the aim of obtaining a W−Ge triple bond, which turned out not to work. Instead, the germylidyne 817 was prepared via 816. Upon UV-irradiation, chlorogermylene 807a releases carbon monoxide and complex 816 is formed. Although the reaction does not proceed completely, 816 can be precipitated from the aromatic solvents and was isolated in a moderate yield of 56%. Compound 816 features a three-legged piano stool configuration of the tungsten atom and a trigonal planar-coordinated germanium center. With 2.3496 Å, the W−Ge bond is shorter than in 815b and the shortest W−Ge double bond reported so far.740 9775


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Figure 99. Structures of various germyliumylidenes (ArF = 3,5-(CF3)2C6H3).

display a positive charge as well as a lone pair of electrons at the germanium center, which can be stabilized by one or two Lewis base molecules. The first NHC-stabilized germyliumylidene 768 was described by the group of Baines in 2008 (Figure 99).715 In a rather unorthodox synthesis, they treated the triflate-substituted germylene 759f with cryptand[2.2.2] and obtained an encapsulated naked Ge(II) dication. The side products of this reaction, which is strongly dependent on the cryptand amount, were the dichlorogermylene 759b and the germyliumylidene 768. Besides the already discussed germyliumylidene 820 (vide supra), several other examples are described in the literature (Figure 99).715,736,743−745 In 2013, Driess and co-workers reported the chlorogermyliumylidene chloride 749a, stabilized by a bidentate NHC ligand.743 They treated the chelating biscarbene with an equimolar amount of GeCl2·dioxane and obtained 749a in a very good yield of 95%. Because of its high iconicity, it is only soluble in acetonitrile. The 13C NMR signal of the coordinated CNHC atoms is observable at 166 ppm, being strongly upfield-shifted in comparison to the free carbene (220 ppm). The crystal structure of 749a reveals a 3-fold-coordinated germanium center with two almost equidistant carbenes (CNHC−Ge bond lengths: 2.057, 2.058 Å) and one chlorine atom (Ge−Cl bond length: 2.310 Å) as well as a far dislocated chloride counterion (d(Ge−Cl) = 6.53 Å). In comparison to (IDipp)GeCl2 (767b), the CNHC−Ge bond lengths in 749a are shorter. This indicates a stronger interaction of the chelating carbene with the positively charged germanium center.743 In a follow-up publication, the group of Driess investigated the reactivity of 749a toward sodium azide.746 Interestingly, the reaction of 749a with an equimolar amount of NaN3 results always in a simultaneous substitution of both chlorides yielding azidogermyliumylidene 749c. However, it was possible to exchange the chloride counterion by treating 749a with NaBPh4 to obtain 749b. In this compound, the germanium− chlorine bond can be cleaved with sodium azide, affording the azidogermyliumylidene 749d. The subsequent exchange of the counter aniom with another equivalent of sodium azide turned out not to work. The covalent Ge−N bond lengths in 749c (2.037 Å) and 749d (2.003 Å) compare well to that in Schulz’s diazidogermylene (IDipp)Ge(N2)2 (767c, 1.969 Å).746 One year later, Driess et al. described another two germyliumylidenes 823 with relatively similar structures.745 They used again a bidentate NHC, this time linked by an anionic boron center to complexate the germanium atom. The

(1.90−2.05 Å), which indicates a dative interaction. It compares well to the Roesky’s (aNHC)GeCl2 adduct 818.550 The 13C NMR shift of the coordinating C atom can be observed at 157.1 ppm, which is upfield shifted in comparison to the normal NHC germylene complexes. Compound 820 can be drawn as germyliumylidene coordinated by two negatively charged abnormal NHCs bearing one negative net charge for the complex. This mesomeric structure can be supported by the observation of similar bond distances between the carbon atoms of the coordinating carbenes and the germanium center (2.067 and 2.038 Å), which are in the range of coordinative C−Ge bond lengths and comparable to that found in 819. Because the 13C NMR shifts could not be assigned, no comparison to the starting material 819 is possible. On the other hand, the 1H NMR spectrum indicates the presence of two equivalent carbenes in 820. Alternatively, the germyliumylidene 820 can be directly synthesized by treating Ge[N(SiMe3)2]2 with 2 equiv of the potassium species KIDipp. In comparison to imidazole-based NHCs, cyclic alkyl amino carbenes (CAACs) play only a minor role in the stabilization of germanium(II) compounds (for a general overview on CAACs, cf., section 1). The group of Roesky reported the CAAC-dichlorogermylene adduct 821 (Figure 98).742 It was synthesized by treating GeCl2·dioxane with free Me2CAAC in the presence of catalytic amounts of LDA (3 mol %). In the 13C NMR spectrum, the carbene C atom resonates at 245.2 ppm, which is significantly upfield shifted in comparison to the free carbene (304.2 ppm), however strongly downfield shifted when compared to the NHC-coordinated germylenes. With 2.1321 Å, the CNHC−Ge bond is longer than in comparable chlorogermylene NHC adducts. Roesky et al. prepared the germanium CAAC adduct 822 (Figure 98) by reduction of MeGeCl3 with 3 equiv of KC8 in the presence of the free carbene. With 2.4483 Å, the Ge−Ge bond in 822 is intermediate between the reported Ge−Ge double bonds and single bonds. The CNHC−Ge bonds (1.9240 and 1.9069 Å) are shorter than those in in the CAAC dichlorogermylene adduct 821, but longer than the reported GeC double bonds. Quantum-chemical calculations revealed that, in contrast to the analogues silicon compound 726 (vide supra), the depicted structure (interconnected bisgermylene with an electron lone pair at each germanium center) is the predominant mesomeric form.686 Germyliumylidenes are another class of low-valent germanium compounds. These heavier analogues of silyliumylidenes 9776


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(2.112 Å) is longer as compared to examples with bidentate carbenes, which indicates that ITr is a weaker coordinating ligand than the chelating NHCs. Recently, the group of So reported the bis(germyliumylidene)silver(I) complex [{L(IMe4)Ge}2 Ag(OTf)][OTf]2 (L = C6H3=2,6-(HCNtBu)2), obtained by the reaction of the corresponding bis(germylene)silver(I) complex [{L(TfO)Ge}2Ag(OTf)] with 2 equiv of the NHC.748 Only a few reactivities of germyliumylidenes are published. Driess et al. investigated the hydrogermyliumylidene 823b toward hydride abstraction with a tritylium borate salt (Scheme 164).745 They attempted to remove the Ge-bound

chlorogermyliumylidene 823a was synthesized in a moderate yield (61%) starting from the corresponding potassium bis(NHC) complex 81a (vide supra) and GeCl2·dioxane as germanium source. The average CNHC−Ge distance of 823a (2.037 Å) as well as the Ge−Cl bond length (2.304 Å) are comparable to those in 749. Upon treatment of 823a with K(HBsecBu3), a Cl/H exchange occurred yielding almost quantitatively the hydrogermyliumylidene 823b. This compound turned out to be stable in solution and exhibits CNHC− Ge distances (2.037 Å) and a distorted trigonal pyramidal geometry around the germanium center similar to those of its precursor. Notably, the GeH signal can be observed at 5.69 ppm in the 1H NMR spectrum. As discussed previously, the bis(NHC) ligand in the potassium NHC adduct 81a can undergo solvent dependent isomerization into the complex 81b bearing the bidentate C,Nligand, where one of the carbene centers is coordinated to boron. When 81b was treated with GeCl2·dioxane, they obtained compound 824a, which displays metrics similar to those of 823a. In analogy, 824a can be converted to the corresponding hydrogermyliumylidene 824b by reacting it with K(HBsecBu3).154 The 1H NMR shift of the GeH proton (5.69 ppm) indicates a hydridic character. This observation was backed up by theoretical calculations. For example, the NBO analysis revealed a positive charge on the germanium center (+0.43) and a negative charge on the hydrogen (−0.21).745 Another chlorogermyliumylidene 825 was published by Kinjo et al. Compound 825 is stabilized by a chelating iminoNHC ligand and can be prepared by reacting the free ligand with GeCl2·dioxane in a moderate yield of 67%.744 In the 13C NMR spectrum, the carbene signal can be observed at 198.4 ppm, which is upfield-shifted in comparison to the free carbene (242.3 ppm). Both the CNHC−Ge bond length (2.0588 Å) and the Ge−Cl distance (2.2646 Å) are comparable to those of the other reported chlorogermyliumylidenes.744 Very recently, Kinjo and co-workers presented an interesting example of the chlorogermyliumylidene 826, featuring two Ge−Cl unuts.747 This compound displays a puckered boat conformation of the Ge2N2C2 ring. In this ring, the C and N atoms are nearly coplanar, whereas the two Ge atoms deviate from this plane. The long distance between the two Ge centers (3.957 Å) rules out any possible Ge−Ge interaction. However, theoretical calculations revealed a contribution from other resonance forms of 826 to the actual electronic structure, as compared to the one depicted herein. The positive charge resides mainly on the N-coordinated germanium atom, and the Wiberg bond index of the Ge−N bond (0.41) is lower than that of the Ge−C bond (0.66). Recently, Hahn, Glorius, and co-workers prepared the twocoordinated germyliumylidene 827, which is stabilized by a chelating amino NHC ligand. The germanium center is linked to the NHC wingtip via an ethyleneamino bridge, thus creating a C3N2Ge heterocycle.736 It was synthesized by removing the chlorine from the germylene 802a with NaBArF4 (ArF = 3,5(CF3)2C6H3). The most recent example of a chlorogermyliumylidene (828), bearing a two-coordinate germanium center, was reported by Rivard and co-workers.83 They successfully isolated the extremely bulky NHC ITrit and utilized it to isolate the tallium 441 and the lithium complex 8 (vide supra), both of which turned out to react with GeCl2·dioxane yielding the chlorogermyliumylidene 828. The CNHC−Ge bond length

Scheme 164. Synthesis of Germanium Dication 830 via Germyliumylidene Adduct 829

hydrogen in 823b, which is hydridic in nature, by treating it with 1 equiv of the tritylium salt to obtain the bis(NHC)stabilized germanium dication. However, this reaction yielded a mixture of compounds 829 and 830, which were separated by fractional crystallization. Treated with an equimolar amount of 823b, 829 selectively affords the compound 830. This result indicates that the desired two-coordinate germanium dication is too reactive and readily undergoes a donor−acceptor stabilization with unreacted 823b. Because of the coordination to the trityl cation, the 1H NMR signal of the GeH unit in 829 (5.20 ppm) is significantly downfield shifted in comparison with the starting material and compound 830. The long distance between the Ge center and the CPh3 atom in 829 (2.063 Å) implies a weak interaction, which likely makes the hydride abstraction possible. The Ge−Ge bond length in 830 (2.556 Å) is comparable to that in the Rivard’s (NHC)GeCl2(GeCl2) adduct 774a (2.630 Å),722 but much longer than Ge−Ge single bonds in digermanes (around 2.41 Å).745 2.4.3.3. Germanium(0) NHC Complexes. To complete this section, the NHC-stabilized germanium compounds in the formal oxidation state zero are discussed. A few of the so-called germylones, the heavier analogues of carbones and silylones, have been published in recent literature (Figure 100). These compounds can only be isolated due to the stabilization with such Lewis bases as NHCs and CAACs. Driess and co-workers reduced germyliumylidene 749a with 2 equiv of sodium naphthalenide to the corresponding Ge(0) species 831, isolated in a moderate yield of 45%.743 Interestingly, the CNHC signal in the 13C NMR spectrum was downfield shifted (196 ppm) in comparison to the germyliumylidene 749a (166 ppm) and compares better to 9777


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carbon signals of 832 are upfield shifted (832a, 232.6 ppm; 832b, 232.8 ppm) and the CNHC−Ge bonds are elongated. Interestingly, the CNHC−Ge bond lengths in 832b differ significantly, a phenomenon that has also been observed for the silicon analogue 653.742 The third NHC-stabilized germylone 833 to this date was reported by the group of Kinjo.744 Again, the synthesis proceeds via the corresponding germyliumylidene 825 that is reduced with an excess of KC8 to furnish the Ge(0) compound 833. The 13C NMR signal of the coordinated carbene (194.3 ppm) is comparable to the starting material; however, the CNHC−Ge bond length is significantly shorter (1.8870 Å), now ranging between typical C−Ge single and double bonds. Also, the calculated molecular orbitals and the WBI value for the CNHC−Ge bond (1.21) support some double bond character of this bond and thereby a delocalization of the π system throughout the five-membered heterocycle. As the electron lone pairs at the germanium atom anticipate, the germylone 833 readily reacts with electrophiles. Treatement with 2 equiv of methyl triflate yields the dication 826 (Figure 101).744

Figure 100. NHC-stabilized germylones 831−833.

the CNHC signal observed in the similar silylone 582 (210.9 ppm). As expected, compound 831 is extremely air- and moisturesensitive and shows signs of decomposition in THF solution after 2 days. Nevertheless, a crystal structure was obtained. The CNHC−Ge bonds (1.967 and 1.962 Å) are significantly shorter than those in the precursor. To get an insight into the electronic structure of 831, DFT calculations were employed. The HOMO consists of the π-type orbital at the germanium center, including Ge−C π back-bonding. In addition, the HOMO−1 with the contribution of the σ lone-pair orbital at Ge and the calculated proton affinities underline the germylone character of 831.743 Further discussion of this and the following Ge(0) compounds can be found in the Driess’ review published recently.607 Compound 831 can easily be converted to the Lewis acid− base adduct 765 by treatment with GaCl3. Reactions with elemental chalcogenes furnish the donor−acceptor-stabilized heavier congeners of carbon monoxide 834 (Scheme 165) and

Figure 101. NHC-stabilized Ge dication 835.

Scheme 165. Reactivity of Germylone GaCl3 Adduct 765

The dimeric Ge(0) complex 836 as described by Jones and co-workers is discussed (Figure 102).704 They reduced the

Figure 102. NHC-stabilized Ge(0) dimer 836.

carbon dioxide 764 (vide supra). SC XRD analysis of 834a reveals a pyramidal coordination of the Ge center (sum of angles: 289.5°) with an average CNHC−Ge bond distance of 2.050 Å, which is very close to that in 765 (2.038 Å). The Ge− Se bond (2.438 Å) turned out to be significantly longer than in the diselenide complex 764b, contradicting a GeSe double bond character. This result is backed up by theoretical calculations: Natural Population Analysis shows a positive charge at the bis-carbene moiety donating electrons and a negative charge at the GaCl3 fragment, which indicates a strong chalcogen−GaCl3 interaction. The tellurium analogue 834b shows similar characteristics.712 In addition, Roesky and co-workers reported the first acyclic, CAAC-stabilized germylones 832 in 2013.742 Upon treatment of GeCl2·dioxane with KC8 in the presence of 2 equiv of the carbene, respective two-coordinate germylones are generated. Notably, the direct reduction of the (Me2CAAC)GeCl2 adduct (821) does not lead to the formation of 832a. It turned out that the synthesis delivered higher yields (75% for 832a) than was the case of Driess’ cyclic germylone 831, apparently due to the higher nucleophilicity and electrophilicity of CAACs as compared to NHCs, making them superior for germylone stabilization. In comparison to 831, the 13C NMR carbene

dichlorogermylene 767b with the Mg(I) Nacnac reagent to obtain 836 as red crystals in a poor yield of 20%. The 13C NMR signal of the carbene carbon atom can be observed at 203.3 ppm, which is close to the signal of Robinson’s silicon analogue 572 (vide supra)69 but significantly downfield shifted in comparison to the starting material 767b. The X-ray crystal structure turned out to be isomorphous to the silicon congener 572: a double-bonded ditetrele unit, coordinated almost orthogonal by two NHCs. With 2.3490 Å, the Ge−Ge bond length lies within the range of reported digermenes; however, it is much longer than in digermynes (2.206−2.285 Å), which indicates a double bond character. The distance between the carbene C atoms and the respective coordinated Ge center (2.030 Å) is slightly shorter than that in the dichlorogermylene complex 767b. These bonding descriptions are backed up by DFT calculations in the example of a sterically reduced model compound.704 To conclude this section, analytical data of some NHC− germanium adducts are presented (Table 7). 2.4.4. Tin. As compared to silicon and germanium, significantly less advances have been made in the field of NHC-stabilized tin complexes. A possible reason for that might 9778


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Table 7. Key Features of Selected Examples of NHC-Stabilized Germanium Complexesa NHC adduct

CCarbene−Ge [Å]

δ(13CCarbene) [ppm]

solvent

ref

(SIMe2)GeF4 (754) [(SIMe2)GeF5][SIMe2F] (754) (IDipp)GeCl4 (755) Me2(C2CH)2GeCl2 (757) Dipp NacnacGe(IiPr2Me2) (761b) i (I Pr2Me2)GeSe(ClCPMes*)2 (763b) (IMes)GeI2 (759a) (IDipp)Ge(N3)2 (767c) (IMe4)GeCl2 (759b) (IDipp)GeCl2(GeCl2) (774a) (NHC)(biL)Ge (778) [Me2SiSi(TMS)2]Ge(IMe4) (781) [(IiPr2Me2)Ge(NC)(SiTMS3)][(AuSiTMS3)] (785) [XylNCSi(Tipp)2SiTipp(Cl)]Ge(IiPr2Me2) (794) (biL)GeCl (802a) (IDipp)GeCl[WCp*(CO)3] (807a) [LGe(H)CPh3][BArF4] [(829) [(ITr)GeCl][BArF4] (828) [biLGeCl][GeCl3] (825) [biLGeH] (824b) (bis-NHCDipp)Ge (831) (Me2CAAC)2Ge (832a) (biL)Ge (833) (IDipp)GeGe(IDipp) (836) LGeCl(IiPr2Me2) (803) (L)GeGe(L)(IiPr2Me2) (804)

1.981 2.057 1.9921 1.965 2.192 2.048 2.102 − 2.098 2.032 2.339 − 1.980 1.986 2.062 2.119 1.968; 1.974 2.110 2.0588 2.022 1.961; 1.965 1.9386; 1.9417 1.8870 2.030 2.084 2.003

175.5 − 156.5 − 151.2 149.9 158.8 141.7 166.1 146.2 224.9 178.5 162.4 169.8 171.4 178.0 145.1 173.7 198.4 168.8 not assigned 232.6 194.3 203.3 172.7 171.8

CD3CN − CD2Cl2 − C6D6 C6D6 THF-d8 THF-d8 C6D6 CD2Cl2 C6D6 C6D6 C6D6 C6D6 THF-d8 C6D6 CD3CN C6D6 CDCl3 THF-d8 THF-d8 C6D6 C6D6 C6D6 C6D6 C6D6

707 707 708 709 710 711 703 718 657,713 722 723 725 728 732 736 738 745 83 744 154 743 742 744 704 737 737

− = unknown/not observed. L = various donor ligands. biL = various bidentate ligands. ArF = 3,5-(CF3)2C6H3.

a

Treatment of Ph2SnCl2 with the respective NHC results in the formation of the compounds 837 (Figure 103).535 These NHC

be the high toxicity of organotin compounds that discourages people to engage with this element.749 On the other hand, NHC−tin adducts are often less stable and therefore more difficult to prepare and isolate than the lighter congeners. For heavier tetrel atoms, the tendency to form sp-hybrid orbitals decreases, which hampers their ability to form stable complexes.750 It is also reported that the Lewis acidity and basicity of the parent tetrylenes EH2 decrease from carbon to lead. This further complicates the formation of stable NHC− stannylene adducts.751 Nevertheless, a number of NHCsupported tin complexes have been isolated. Some of them have been previously reviewed by Prabusankar et al. in 2014.28 Like in the previous sections (Si, Ge), this section is presented in an order of decreasing formal oxidation states, starting with Sn(IV) adducts, moving to Sn(II) compounds, and finally to the Sn(0) complex. Selected analyticatcal data of some NHC− tin complexes are presented at the end of the section (Table 10). In contrast to the lighter analogues, tin hydrides show different reactivity toward NHCs. For example, Wesemann et al. employed NHCs to abstract dihydrogen from SnR2H2-type Sn(IV) hydrides, yielding unprecedented tin(II) complexes. This synthetic approach for low-valent tin compounds is generally not feasible for the lighter group 14 elements. These compounds will be presented in the Sn(II) section together with other stannylenes, of which some have already been successfully applied as polymerization catalysts.450 Furthermore, stannyliumylidenes, the heavier congeners of silyliumylidenes, as well as the elusive diatomic Sn(0) allotrope of Jones et al. will be discussed.752 2.4.4.1. Tin(IV) NHC Adducts. The first four-valent tin NHC complexes are reported by the group of Kuhn in 1995.

Figure 103. NHC-supported Sn(IV) complexes 837−840.

adducts were obtained as colorless crystals in good yields. The 119 Sn NMR shifts in CDCl3 (837b, −310.9 ppm; 837c, −314.4 ppm) are upfield-shifted about 30 ppm in comparison to relative phosphine complexes. Complex 837c displays a trigonal bipyramidal geometry around the tin center with both Sn−Cl bond lengths in the expected range (2.53 Å). As compared to the Sn−CPh distances (2.122, 2.139 Å), the CNHC−Sn bond (2.179 Å) is slightly longer, indicating a dative bonding situation. In 2002, the group of Kuhn presented related Sn(IV) NHC adduct 838.753 Its CNHC−Sn bond length (2.203 Å) compares well to the first example. It turned out that the oxygen atoms from the ether group are not coordinating to the tin center. 9779


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Recently, Goicoechea and colleagues reported the distannane 48 (cf., Figure 8)130 that is coordinated by two abnormal NHCs. Formation of 48 from the abnormal (aNHC)Li adduct 34a and Sn[N(SiMe3)2]2 proceeds via an intermediary aNHC stannylene adduct 843, observed by 1H NMR spectroscopy, which further undergoes dimerization and rearrangement reaction (Scheme 168). The 119Sn NMR

Rivard et al. reported on the (IDipp)SnCl4 adduct (839), which is isostructural to the germanium congener 755 (vide supra).708 It contains a trigonal bipyramidal-coordinated tin atom with the CNHC−Sn bond length of 2.186 Å, which compares well to 837c. However, the 119Sn NMR signal of 839 (−422.6 ppm, CD2Cl2) is shifted significantly upfield. In 2016, Schneider and co-workers used rather uncommon NHC IiPr2 to coordinate dialkyl(dichloro)stannanes, yielding the adducts 840.754 Compound 840a is structurally very related to Kuhn’s 837c and displays nearly the same 119Sn NMR shift (−317.7 ppm, CDCl3). As compared to the starting material Ph2SnCl2 (−26.4 ppm), this is strongly upfield shifted. Also, the CNHC−Sn distances (840a, 2.1773 Å; 840b, 2.1908 Å) are in perfect agreement with the other examples of Sn(IV) NHC adducts. In analogy to their pentafluorogermanium NHC complex 754, the group of Röschenthaler prepared the tin congener 841 (Scheme 166).707 Addition of the difluoro−NHC adduct 751 to a suspension of tin difluoride in acetonitrile leads to a clear solution of 841.

Scheme 168. Synthesis of Distannane 48

Scheme 166. Synthesis of the Hypervalent Sn(IV) Complex 841

chemical shifts of 48, observed at −12.5 and −126.2 ppm, clearly show the presence of two inequivalent tin atoms and indicate rather a covalent bonding between the tin centers. Both CaNHC−Sn bond lengths (2.186 and 2.256 Å) are within the observed range of abnormal bonded NHC tin complexes. With 2.870 Å, the Sn−Sn bond is slightly longer than the sum of the covalent radii and longer than that in hexaphenylditin; however, it still remains in the range of Sn−Sn single bonds.130 2.4.4.2. Tin(II) NHC Complexes. Besides these tin(IV) NHC adducts, the major part of this section is tin complexes in the oxidation state +2. As mentioned in the introduction to this section, the parent stannylene SnH2 exhibits weaker Lewis acidic character than do its lighter congeners. Therefore, Sn(II)−NHC complexes are generally less stable than germylenes or silylenes. Nevertheless, various research groups were able to synthesize and characterize these species. An overview of NHC−stannylene complexes is shown in Figure 104.450,535,711,720,724,756−761 Pioneering work of the Kuhn’s group also delivered the first dichlorostannylene adduct (IiPr2Me2)SnCl2 (844a).535 In analogy to the preparation of 837c, reaction of the free carbene with tin dichloride afforded 844a as colorless crystals in 38% yield. Interestingly, in contrast to the Sn(IV) analogue, this synthesis works only with IiPr2Me2. The use of sterically less hindered N-alkyl NHCs such as IMe4 and IEt2Me2 presumably led to the formation of insoluble bis-NHC SnCl2 complexes, which could not be isolated or characterized. The tin center in 844a displays a pyramidal geometry with a CNHC− Sn distance of 2.290 Å. This is 0.112 Å shorter than that in the tin(IV) complex 837c. The 119Sn NMR signal of 844a is observed at −59.4 ppm. In the same year, Weidenbruch et al. presented the diarylstannylene (IiPr2Me2)SnTipp2 (844b).756 Because of the presence of sterically demanding aryl ligands, the CNHC− Sn bond in 844b (2.379 Å) is elongated as compared to the Kuhn’s stannylene 844a. The 119Sn NMR signal is extremely downfield shifted to 710.0 ppm.

Crystal structure analysis revealed an octahedral-coordinated hypervalent Sn(IV) center with 2.256 Å distance between the tin and CNHC atoms. In the 119Sn NMR spectrum, a doublet of quintets is observable at −749.8 ppm, which is within the range of neutral hexacoordinated tin(IV)−fluorides. This multiplicity derives from the coupling of the tin nucleus with one trans-standing fluorine atom (1JSnF(trans) = 1530 Hz) and four equivalent 19F nuclei in cis position (1JSnF(cis) = 1980 Hz).707 Jurkschat and co-workers presented the dimeric stannathione 842, which contains two abnormal NHCs (Scheme 167).755 The tin centers in this compound are in a tetrahedral Scheme 167. Synthesis of Stannathione Dimer 842

coordination sphere of the two bridging sulfur atoms, the bulky aryl group and the abnormal NHC (d(Sn−C) = 2.1555 Å). Interestingly, the 119Sn NMR spectrum shows that in solution compound 842 exists in equilibrium with the monomeric stannathione. Corresponding signals were observed as a singlet at −148 ppm (monomer) and a triplet at −228 ppm (3JSn−P = 61 Hz). 9780


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The groups of Rivard and Goicoechea reacted IDipp with SnX2 (X = Cl, OTf) and obtained the respective stannylene adducts (IDipp)SnX2 (X = Cl, 846a;720 X = OTf, 846b757). For both compounds, the analytical parameters are within the expected range. Upon treatment with Li(NHDipp), complex 846a furnishes the amido-substituted stannylene 846c, which displays a structure comparable to the germanium congener 775 (vide supra).635 Compounds 847,759 848,760,761 and 844h762 were synthesized by Wesemann and co-workers by treating the corresponding dihydrostannane with 2 equiv of NHC, removing dihydrogen as the (NHC)H2 adduct. These complexes display similar CNHC−Sn bond lengths (around 2.30 Å); however, they differ in 119Sn NMR shifts (cf., Table 8).759−762 Complexes 847c, 848b,c, and 844h are the first examples of NHC-stabilized tin(II)-monohydrides.759−762 Table 8. 119Sn NMR Chemical Shifts [ppm] of Selected (NHC)SnX2 Complexes complex

Figure 104. NHC-supported stannylenes. δ( Sn) [ppm] 119

Escudié and co-workers reported bis(hypermetallyl)stannylenes (IiPr2Me2)Sn[E(SiMe3)2]2 (E = Si, Ge, Sn) 844c−e, formed in reactions of dichlorostannylene complex 844a with the corresponding magnesium dihypermetallyl reagents in analogous approach to the preparation of the germylene 774 (vide supra).724 Compounds 844c−e turned out to be unstable in the solid state, slowly decomposing at room temperature after 24 h. Going down from the hypersilyl (834c) to the hyperstannyl-substituted compound (834e), the stability is decreasing. The 13C NMR shifts of the carbenic C atoms (170−175 ppm) indicate NHC coordination. In the 119 Sn NMR spectra of 844c−e, singlet signals were observed at −196.8, −115.0, and −138.3 ppm, respectively. The CNHC−Sn bond lengths of 844c and 844e (2.328 and 2.309 Å) are in the same range as those in 844a and 844b.724 Treatment of the dichlorostannylene complex 844a with Mes*PC(Li)Cl afforded the complex 844f. As its germanium analogue 762 (vide supra), the ligand-free stannylene can be synthesized in analogous manner from SnCl2. It decomposes in solution; however, it can be stabilized by addition of the NHC leading to 844f.711 The 119Sn NMR shift (−130.8 ppm) is upfield shifted as compared to the starting material 844a; however, the CNHC−Sn bond length (2.316 Å) is consistent with 844a. Upon reaction with an excess of elemental sulfur, compound 844f forms a sulfurbridged, donor-free stannathione.711 With compound 844g, the group of Escudié published another NHC-stabilized stannylene similar to 844f. Again, it was prepared by reaction of the dichlorostannylene 844a with the respective lithiated phosphaalkenyl ligand. Besides, the 119 Sn NMR shift (52.2 ppm) of 844g displays structural features similar to those of 844a. Buchmeiser and co-workers reported mesityl-NHC-SnCl2 adducts with saturated (845a) and unsaturated (845b) carbene backbones.450 The 119Sn NMR shifts (−61.5 and −63.9 ppm) as well as the metrics of 845 are comparable to those of the dichlorostannylene 844a. Compounds 845 were successfully applied as highly active polymerization catalysts for isocyanates in the polyurethane synthesis.450

δ(119Sn) [ppm]

847a

847b

−150.7

−121.0

847c −339.9 complex

848a −160.7

848b

848c

844h

−329.5

−349.4

−290.6

It turned out that the stoichiometry between the dihydrostannane and the NHC is crucial for the outcome of the reaction: application of only 1.5 equiv of NHC led to the formation of the stannylstannylenes 849 as depicted in Scheme 169.762 Instead of the expected Sn−Sn triple bond formation, a Scheme 169. Synthesis of Stannylstannylene NHC Adducts 849

partial dehydrogenation and the NHC coordination occurred. All three complexes 849 display almost identical CNHC−Sn bond lengths (2.2511−2.2636 Å) as well as similar 119Sn NMR shifts (−192.4 to −246.6 ppm).762 Recently, Wesemann and co-workers investigated the reactivity of trihydrostannane (Me3Si)2CHSnH3 (850) toward varying amounts of IMe4. The results are shown in Scheme 170.763 In benzene, 2 equiv of IMe4 was used to abstract dihydrogen and stabilize the resulting stannylene (Me3Si)2CHSn(IMe4)H (851). Because of its extreme instability (it decomposes in solution within hours), the compound was only characterized by NMR spectroscopy (119Sn NMR signal: − 278 ppm). When the same reaction is conducted in hexane, a different adduct of the 1,4-bis-stannylene 852 is generated. In the 119Sn NMR spectrum of 852, a doublet from the SnH units at −334 ppm (JSn−H = 782 Hz) and a singlet from the stannylene groups at −93 ppm are observable. The distance between the two central 9781


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Scheme 170. Reactivity of Hydrostannane 850 toward IMe4 in Different Stoichiometries and Solvents

NHC. The synthesis is similar to that of 855 with the difference that the corresponding NHC dimer is applied instead of the free NHC. Unexpectedly, compound 856 turned out to be extremely unstable. It decomposes in solution and is light-sensitive. Therefore, no NMR data ere collected. The CNHC−Sn distance (2.399 Å), however, compares very well to that of 855.764 Filippou et al. reported the synthesis of the metallastannylene 857, an analogue of Tobita’s metallagermylenes 807a and 808. Compound 857 was obtained by treating the (IDipp)SnCl2 adduct (846a) with Li[Cp*W(CO)3] (Scheme 171).741 It was further converted to the cationic stannylene

tin atoms is 2.8417 Å, and that between the central and the outer ones is 2.8938 and 2.9022 Å, respectively. These bond lengths are within the range of reported Sn−Sn single bonds in stannylstannylenes and distannanes. It is noteworthy that each tin atom in 852 is a center of chirality, and 10 stereoisomers are possible. By increasing the NHC amount to 2.5 equiv, the reaction furnishes the bis(stannylene) 853 in a poor yield of 24%. The Sn−Sn distance in this molecule is 3.0572 Å. Furthermore, the pyramidalization at the tin centers indicates the presence of stereoactive lone pairs. With a larger IMe4 excess of 20 equiv, stannane 850 reacts to the ionic alkyltin cluster 854. The Sn−Sn bond lengths found in the cluster (around 2.90 Å) are comparable to those of previously reported tin clusters (2.575−3.486 Å).763 Both the cation and the anion contain NHC-coordinate tin units.763 The cation in 854 is a stannyliumylidene, a cationic tin(II) species with one substituent and two coordinated electrondonating NHCs. Stannyliumylidenes are discussed more thoroughly later. In an approach, analogous to the preparation of NHCcoordinated silylene 599 and germylene 778, Lappert and colleagues synthesized stannylene complex 855 by reacting the corresponding free NHC and N-heterocyclic stannylene (Figure 105).723 Compound 855 shows similarities to the

Scheme 171. Synthesis of Metallastannylene 858

858 by reaction with lithium Krossing salt. This conversion is accompanied by a significant shortening of the CNHC−Sn bond from 2.380 to 2.287 Å. Therefore, compound 858 is depicted with a covalent Sn−C single bond between the tin center and the imidazolium unit. Notably, the W−Sn bond was also shortened upon chloride abstraction (from 2.9514 to 2.8029 Å), and the observed 119Sn NMR signal was extremely downfield-shifted (from 455.5 to 3318 ppm in 858). Akin to the germanium congener, the conversion of 858 to a compound containing an E−W triple bond by irradiation with UV light failed. Marschner and co-workers synthesized disilylstannylenes 859 and 860a,b (Figure 106) by reacting the corresponding potassium silanides with SnCl2 in the presence of IMe4.728 All three compounds show characteristics very similar to those of

Figure 105. NHC adducts with N-heterocyclic stannylenes.

lighter congeners with the pyramidal geometry at the metal center due to the sterochemically active electron lone pair. The CNHC−Sn distance of 2.472 Å in 855 is rather long as compared to other NHC-coordinated stannylenes, indicating a relatively weak coordination of the NHC. This result is backed up by variable-temperature NMR spectroscopic investigations, which revealed the presence of a dissociative equilibrium of the NHC and the tetrylene. The 119Sn NMR signal is observable at 10.5 ppm.723 Hahn et al. later discribed the closely related complex 856 depicted in Figure 105.764 It also comprises an N-heterocyclic stannylene moiety coordinated by benzimidazole-derived

Figure 106. Structure of disilylstannylene NHC complexes 859−860. 9782


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other NHC-stabilized stannylenes (119Sn NMR shifts lie between −272.9 and −318.8 ppm, while the CNHC−Sn bond lengths are around 2.29 Å). Upon reaction of 859 with AuCN, the stannylene auride complex 861 is formed. Partial decomposition of the starting material explains the poor yield of 861 (17%), the presence of hypersilyl group at the gold, and side formation of hypersilyl cyanide detected in the reaction mixture. The gold complex 861 displays almost linear geometry around the Au center (Sn−Au−Si = 177.75°) and a relatively long Sn−Au bond length of 2.639 Å, indicating a neutral stannylene adduct character. In comparison with the starting material, the 119Sn signal is downfield-shifted to −136.8 ppm.728 In analogy to the germylenes 802 (vide supra), the group of Hahn synthesized the intramolecular NHC stannylene adducts 862 (Figure 107).736 Stannylenes 862 are isostructural to their

to the coupling with tungsten are observable with a coupling constant of 1JSn−W = 828 Hz. As is the case silicon and germanium adducts, the stabilization of low-valent tin compounds with abnormal bound NHCs plays only a marginal role. Nonetheless, there are two reported examples of aNHC stannylene complexes 865 and 866 (Figure 108).126,647 These compounds have already been discussed in Goicoecheas’ review on ditopic carbanionic NHCs.37

Figure 108. NHC−stannylene complexes stabilized by aNHCs (865 and 866) and CAAC (867).

Roesky et al. synthesized 865 by reacting the free aNHC with tin dichloride. With 2.308 Å, the distance between the distorted trigonal-pyramidal-coordinated tin center and the C4 atom of the abnormal carbene lies within the range of normal NHC stannylene adducts. The 119Sn NMR shift can be observed at −60.0 ppm.647 Reaction of the anionic Nheterocyclic dicarbene KIDipp 45 with Sn(HMDS)2 furnishes the ionic stannylene 866. Both the 119Sn NMR shift of 10.9 ppm as well as the CaNHC−Sn distance (2.277 Å) are comparable to normal NHC-stabilized stannylenes. In contrast to the analogue germanium 819 and lead 883 compounds, 866 does not decompose in THF solution to the corresponding bis-aNHC-tetryliumylidene.126 Moreover, there is an example of a (CAAC)SnCl2 adduct (Figure 108).647 Compound 854 was prepared by the group of Roesky from the reaction of SnCl2 with free Me2CAAC. The complex was obtained as colorless crystals in 72% yield. Both the 119Sn NMR shift (−53.1 ppm) and the Sn−CNHC distance (2.358 Å) are very similar to those in (aNHC)SnCl2 865.647 Several stannyliumylidenes, that is, Sn(II) cations, have also been described in the literature in recent years. Most of these highly sensitive complexes need two electron donors for stabilization. The example of Wesemann and co-workers has already been discussed. Their anionic tin cluster 854 has a stannyliumylidene as the counterion (Scheme 170).763 Further stannyliumylidenes are depicted in Figure 109.736,757,766 The unique adduct 868a, featuring a dicarbene ligand, was obtained using a procedure similar to that in the synthesis (IDipp)Sn(OTf)2 (846b), but applying 2 equiv of NHC.757 Because of extensive steric crowding at the tin center, one carbene in 868a is bound in the normal mode and the other adopts an “abnormal” coordination mode (via the C4 position). Interestingly, during the “abnormal” coordination, the proton of the IDipp ligand migrates from the C4 carbon atom to the C5 position, leaving the C2 carbene center free instead of its protonation. Notably, the CaNHC−Sn distance (2.248 Å) is significantly shorter than the CNHC−Sn distance (2.302 Å), but still longer than the expected value for a Sn−C σ-bond (2.15 Å), indicating a dative bonding of both NHCs. The trigonal pyramidal geometry at the tin atom indicates significant s orbital character of the electron lone pair. Crystals of the isostructural chlorostannyliumylidene 868b were

Figure 107. Intramolecular-stabilized stannylenes 862 and stannylene−transition metal complexes 863 and 864.

Ge congeners. The 119Sn NMR signal of 862a (−149 ppm) clearly confirms the formation of an electron-rich Sn(II) compound. Upon treatment with the respective nucleophiles, 862a can be converted to 862b and 862c.711 Rivard and co-workers prepared a number of NHC− stannylene tungsten705 and chromium765 complexes 863, 864 (Figure 107), closely related to the germanium analogues 772 (vide supra). Their 119Sn NMR shifts are compiled in Table 9. The dichlorostannylene complexes 863 were synthesized by simple treatment of (THF)2·SnCl2·M(CO)5 (M = W, Cr) with IDipp. Table 9. 864a,b

119

Sn NMR Shifts [ppm] of Complexes 863a,b− complex

δ(119Sn) [ppm]

863a

863b

864a

864b

−71.3

165.6

−309

−106.6

Compounds 863 feature a Sn(II) center coordinated to an electron-donating IDipp and an electron-accepting transition metal pentacarbonyl moiety. By the treatment with Li[BH4], the dichlorostannylenes 863 can be converted to the respective, structurally similar dihydrostannylene complexes 864. The CNHC−Sn bond distance in 864a (2.230 Å) is shortened in comparison to (IDipp)SnCl2 (846a), indicating stronger bonding of the NHC. The Sn−W bond length, however, compares well to previously reported tin−tungsten dative bonds.705 In the 1H NMR spectrum, the SnH2 group of 864a resonates at 5.56 ppm, while in the proton coupled 119Sn NMR spectrum a triplet (1JSn−H = 1158 Hz) can be observed at−309 ppm. Additional satellites of the 119Sn NMR signal due 9783


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analogue silicon 572 and germanium 836 compounds (vide supra), having the highly trans-bent geometry. The NHC ligands are almost orthogonal (91.82°) to the Sn−Sn axis. With 2.7225 Å, the Sn−Sn bond length lies within the range of three-coordinate distannenes R2SnSnR2 (2.668−2.851 Å).752 It is comparable to the Sn−Sn distance in elemental tin (2.81 Å).752 2.4.5. Lead. As compared to the lighter group 14 elements, the chemistry of NHC−lead adducts has been explored significantly less. For example, lead is the only group 14 element without an isolated NHC-stabilized diatomic Pb(0) compound.752 As was mentioned before, going down in group 14, the tendency to form hybrid orbitals from atomic s and p orbitals decreases.750 On the basis of this trend and relativistic effects in the lead atom,767 all low-valent lead NHC adducts known to date share the characteristic of a lone pair as well as an empty p orbital, which is reflected by pyramidalization around the lead atoms in all three-coordinated lead NHC adducts. The first NHC adduct of lead 873 was reported in 1999 by the group of Weidenbruch, prepared from diplumbene (872)2, which dissociated into the monomeric plumbylene 872 in solution (Scheme 172).768 Compound 873 features a relatively long CNHC−Pb bond (2.540(5) Å), and thus a rather weak bond. It prevents the NMR characterization of 873 as it is only stable in solution in the presence of an excess of 872. One year later, N-heterocyclic plumbylene 874 was treated with carbene 875, yielding compound 876 featuring an even longer CNHC− Pb bond (2.586(7)Å).723 Variable-temperature NMR studies revealed the dissociative equilibrium between 876 and its educts 874 and 875. Jones, Frenking, Stasch, and co-workers reported compound 877, formed by reacting PbBr2 and IDipp in THF, as the first NHC lead dihalide adduct. The chloride congener could not be obtained (Scheme 173).752 They further reported efforts to reduce 877 with [(MesNacnac)Mg]2, in a fashion similar to (IDipp)SnSn(Idipp) (871),752 were not successful due to immediate deposition of lead metal, even at −80 °C. On the basis of this observation, they concluded IDipp was not the right choice to enable the isolation of a diatomic Pb(0) compound. This is in contrast to previous calculations of Wilson et al. investigating the possibility of thermodynamically stable compounds of L−E−E−L (L = NHC, E = group 14 or 15 element), which should be possible with the exception of nitrogen.769 Frenking and co-workers published further theoretical studies in the last years that focused on ylidone complexes EL2 (E = C−Pb, L = PPh3, NHC, bicyclic NHCs, CAAC), but to the best of our knowledge isolation of novel lead NHC adducts with lead in oxidation state zero has yet to be reported.770−772 Compound 877 was further reacted with Li[NHDipp] by the group of Rivard yielding 878, a Pb(II) amide carbene adduct.708 The CNHC−Pb bonds of 877 and 878 are almost the same (877, 2.443(11) Å; 878, 2.437(2) Å), shorter as in compounds 873 and 876 also reflecting their higher stability. The original purpose of the report by Rivard and co-workers was the preparation of new element hydride adducts by reaction of the corresponding NHC-stabilized element halides with Li[BH4], which was hampered in the case of lead due to the lack of crystal structures of the products. Most recently, the group of Wesemann reported compound 880, the first reported hydride of lead in oxidation state II.773 It is obtained via conversion of 879 with catecholborane (catB−

Figure 109. Structures of various stannyliumylidenes 868−870 (ArF = 3,5-(CF3)2C6H3).

obtained from the same solution as the crystals of 868a. It is highly likely,that residual KCl from the IDipp-preparation (deprotonation of the imidazolium chloride with KOtBu) led to an exchange of weakly coordinating OTf− by chloride at the tin center.757 The group of Jurkschat reported the stannyliumylidene 869 with a bulky aryl substituent possessing two phosphonate groups in ortho-positions.766 It is stabilized by one IDipp molecule and the two neighboring electron-donating oxygen atoms. For its preparation, ArSnCl was treated with NaBArF in the presence of free NHC. The Sn(II) center exhibits a distorted pseudotrigonal-bipyramidal geometry with the two oxygen atoms occupying the axial positions. The equatorial positions are occupied by the lone pair, the C atom of the aryl ligand, and the coordinating carbenic center. With 2.287 Å, the Sn−CNHC bond length is within the expected range for NHCstabilized tin(II) compounds. In the 119Sn NMR spectrum, a triplet at −169 ppm is observable due to the coupling with the two 31P nuclei (JSn−P = 142 Hz). The geometry around the tin center implies high s character of the lone pair. This result is backed up by NBO analysis: the lone pair at the Sn(II) center mainly occupies the 4s type orbital with only 14% p character. Hahn and co-workers obtained compound 870 by chloride abstraction from their stannylene 862a with NaBArF. The stannyliumylidene 870 is stabilized intramolecularly by NHC moiety and displays a structure similar to the Ge congener 827. It is the only example of a two-coordinate stannyliumylidene. In comparison to the starting material, the 119Sn NMR signal is significantly downfield-shifted from −149 to −60 ppm.736 2.4.4.3. Tin(0) NHC Complex. Jones and co-workers synthesized the diatomic Sn(0) NHC adduct 871 (Figure 110),752 by reductive dechlorination of the dichlorostannylene (IDipp)SnCl2 (846a) with Mg(I)Nacnac.163 Adduct 871 was isolated as red crystals in a poor yield of 5%. Compound 871 turned out to decompose in solution and solid state at room temperature. The complex shows structural similarities to the

Figure 110. Structure of diatomic tin(0) compound 871. 9784


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Table 10. Key Features of Selected Examples of NHC-Stabilized Tin Complexesa NHC adduct

CCarbene−Sn [Å]

(IMe4)SnPh2Cl2 (837a) (IiPr2)SnMe2Cl2 (840a) (IiPr2)SnPh2Cl2 (840b) [(SIMe2)SnF5][SIMe2F] (841) (IiPr2Me2)SnTipp2 (844b) (IiPr2Me2)Sn[Sn(SiMe3)3]2 (844e) (IMes)SnCl2 (845a) (IDipp)SnCl2 (846a) (IEt2Me2)SnH(2,4-Tipp2C6H3) (847c) (IMe4)SnH(2,4-Mes2C6H3) (848c) (IMe4)Sn(CHTMS2)-Sn(CHTMS2)(IMe4) (853) (NHC)(biL)Sn (855) (IDipp)SnCl[WCp*(CO)3] (857) {(IDipp)Sn[WCp*(CO)3]}[Al(OC(CF3)4] (858) (IMe4)Sn(SiTMS3)2 (859) (IMe4)Sn(SiTMS3)2[AuSiTMS3] (861) bi LSnOtBu(862b) (IDipp)SnCl2[W(CO)5] (863a) (IDipp)SnH2[W(CO)5] (864a) (IDipp)SnH2[Cr(CO)5] (864b) [K][(aIDipp)Sn(NTMS2)2 (866) (Me2CAAC)SnCl2 (867) (IDipp)SnSn(IDipp) (871)

− 2.1908 2.1773 2.256 2.379 2.309 2.311 2.341 2.295 2.2804 2.2922 2.472 2.481 2.287 2.287 2.258 − − 2.230 2.2358 2.277 2.358 2.280

δ(13CCarbene) [ppm] g

161.4 161.5a 163.7a 191.6f 177.2 − not assignedd 184.2a 172.3a 173.3a 177.4a 200.8a 179.4a 193.1c 169.9a 165.4a 207.5a 166.9a 167.5a 169.5a 163.2e not assignede 210.7b

δ(119Sn) [ppm]

ref

− −227.0a −316.7a −742.0f 710.0 −655.5a −63.9e −68.7 a −337.9a −349.4a −82a 10.5b 455.5a 3318c −297.2a −136.8a −148a −71.3a −309a −104.8a 10.9e −53.1e −

535 754 754 707 756 724 450 720 759 762 763 723 741 741 728 728 736 705 705 765 126 647 752

a NMR solvents: a = C6D6, b = toluene-d8, c = C6D5F, d = CDCl3, e = THF-d8, f = CD3CN, g = CD2Cl2. − = unknown/not observed. biL = a bidentate ligand.

formation of a hydride as intermediate in the synthesis of the heavier alkyne analogue TippTerPbPbTerTipp (882) was already postulated by Power and co-workers.774 This was confirmed by the group of Weseman through warming the reaction solution of 880 to ambient temperature to give 882 under release of dihydrogen. Further conversion with an excess of IMe4 yields the NHC adduct 881 (Pb−CNHC, 2.411(2) Å; 1H NMR, 23.81 ppm (Pb−H)), which was shown to be in equilibrium with 880 by usage of 1 equiv of IMe4. Furthermore, aNHC−lead adducts were prepared from the N-heterocyclic dicarbene (NHDC) precursors 34a (M = Li) and 45 (M = K) via reaction with Pb[N(SiMe3)2]2, giving the intermediate 883 (isolation of a small fraction was only possible for the germanium complex 819), which decomposes to give 48b (Scheme 174).126 Plumbyliumylidene 48a,b, which can also be directly prepared by addition of 0.5 equiv of Pb[N(SiMe3)2]2 to 35a and 45, features two nearly equal CNHC−Pb bonds (2.363(5), 2.339(5) Å). In 2017, Goicoechea and co-workers converted 48a with another equivalent of Pb[N(SiMe3)2]2, giving compound 49 as an adduct of 48b and a carbene-coordinated four-membered cyclic plumbylene, formed upon deprotonation of the N(SiMe3)2 moiety.130 In solution, the plumbylene moiety shows rapid intermolecular exchange resulting in inversion of the configuration, which was investigated by variable-temperature NMR, and further evidenced by two conformations of 48a in the XRD analysis. The structural parameters of both conformers show only a minor deviation, and the previously identical CNHC−Pb bond lengths in the plumbyliumylidene unit (in 48a) are no longer equivalent, one remaining nearly unchanged and one being elongated due to the coordination of the plumbylene (2.393(9) and 2.397(10) Å). The carbene-coordinated plumbylene possesses a longer CNHC−Pb bond length (2.472(9) and 2.522(9) Å in the two conformers), reflecting

Scheme 172. Synthesis of NHC-Stabilized Lead Compounds 873 and 876

Scheme 173. Synthesis of NHC-Stabilized Lead Dihalide and Lead Hydride

H) at low temperature, and features a 1H NMR signal at the lowest field observed ever (35.61 ppm). Interestingly, the 9785


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reported to activate N−H bonds reversibly,776 apparently through a different reaction mechanism as compared to that of more electrophilic congeners. Bis(imidazolidin-2-ylidenes) are known to activate N−H bonds of amines, amides, lactams, and imides yielding corresponding 2-functionalized imidazolidines or products of their further transformations.777 Reaction of free or in situ generated imidazolin-2-ylidenes with nitrous oxide yields air- and moisture-stable NHC−N2O adducts 884a−g (Scheme 176).778,779 Upon heating (65−100 °C), these adducts release N2 yielding corresponding ureas.

Scheme 174. Conversion of Abnormal Carbenes with Pb[N(SiMe3)2]2

Scheme 176. Reaction of NHCs with Nitrous Oxide

Solid-state structures of 884a−g possess bent N−N−O units with the bond angles ranging from 110.19(17)° to 118.7(4)°. The C−N−N−O units are flat (dihedral angles: 172.65(7)°− 118.7(4)°), while N−N bond distances lie in the range 1.270(5)−1.33(2) Å. The C NHC −N bond distances (1.354(2)−1.3783(12) Å) correspond to single bond lengths; the presence of free rotation around this bond is also evidenced from NMR spectra. Fixation of nitrous oxide to anionic centers of mesoionic or anionic NHCs yields respective neutral or anionic adducts 885−887 (Scheme 177).129

the weak bond, which enables configuration inversion in solution. 2.5. NHC Complexes of Group 15 Elements

The chemistry of the NHC adducts with group 15 elements is largely presented by the low-coordinate pnictogen compounds possessing CNHC−E bonds with a double bond character or dative single bonds. Upon descending the group, the stability of the CNHC−E bond decreases, which hampered investigation of heavier pnictogen−NHC adducts (E = As, Sb, Bi). On the other hand, the chemistry of the lighter congeners (E = N, P) has been extensively developed with the exploration of the synthetic potential of free NHCs. While the nitrogen adducts are mostly presented by the compounds with an imino function, traditionally considered as guanidine derivatives or imine bases, the chemistry of the phosphorus adducts is more rich and not limited to heavier imine analogues. 2.5.1. Nitrogen. NHCs with enhanced electrophilic properties, such as CAACs and DACs, activate N−H bonds of ammonia775,498 or amines776 under mild conditions (Scheme 175). The reactivity toward ammonia27 is similar to that of electrophilic heavier group 14 carbene analogues, but different from most transition metal complexes, which form Lewis acid−base adducts LnM−NH3. The SIMes carbene is

Scheme 177. Reactions of Mesoionic and Anionic NHCs with Nitrous Oxide

Scheme 175. Activation of Ammonia by Electrophilic NHCs

Protonation of (IMes)N2O 884a (Scheme 178)779 afforded the respective cation [(IMes)NH2]+ 888 (CNHC−N bond distance: 1.344(4) Å). DFT calculations485 on the electronic structure of the similar [(IMe2)NH2]+ cation (experimental CNHC−N bond distance: 1.332(5) Å)780 confirmed weak double bond character of the CNHC−N bond (WBI = 1.22); the nitrogen and carbene carbon atoms bear negative (−0.842) 9786


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Scheme 178. Protonation of the NHC−N2O Adduct 888a

Scheme 180. Syntheses of Aminyl Radicals 891 and Biradical 892

and positive (0.664) atomic partial charges, respectively. Reaction of 884a with [Ph3C][BF4] led to addition of the elelectrophile to the oxygen atom of the N2O unit. In reactions with metallic Lewis acids (CrCl3, Cu(OTf), CuCl2, Fe(OTf), CuCl2, Fe(OTf)2, and SnCl), NHC−N2O adducts 884 act as N, O, or chelating N,O-donors,779 while reaction with Ni(COD)2 led to metal insertion into the N−N bond.781 In the presence of aluminum chloride, preformed NHC− N2O adducts react with various aromatic compounds (benzenes, polyaromatic compounds, thiophene) affording industrially relevant azoimidazolium dyes 889 and 890 in good yields (Scheme 179).782,783 According to the patent literature,

Scheme 181. Syntheses of Imidazolin-2-imines and Their Derivatives Based on the Reactivity of NHCs

Scheme 179. Syntheses of Azoimidazolium Dyes

similar to 889, azoimidazolium dyes can also be synthesized using reactions of imidazolium salts with diazonium salts [ArN2][X] in the presence of a base.784 Reduction of N-arylsubstituted azoimidazolium salts 889 (Ar = Ph, 2,3,5,6C6Me4H) and 890 with potassium metal or potassium naphthalenide gave stable radicals 891 and biradical 892, respectively (Scheme 180).783 Open-shell species 891 possess elongated (Δca. 0.06 Å) N−N and shortened CNHC−N (Δca. 0.08 Å) bond distances, as compared to those in starting materials (e.g., the CNHC−N bond distances in 889[BF4] and 891[BPh4] (R = Mes, Ar = Ph) are 1.414(2) and 1.316(3) Å, respectively). EPR investigation supported with theoretical DFT calculations revealed 41.5% of the spin density on the N-aryl atom of the N2 unit. The electronic structure of 892 is a biradical with largely independent unpaired electrons. Interestingly, attempts to obtain stable radicals by reduction of N-alkylsubstituted 889 were not successful. Imidazolin-2-ylidenes react with diazoalkanes, producing corresponding azines (e.g., 893),785 while reactions with organic azides give in good yields the corresponding triazenes 894 (Scheme 181),786 which are difficult to access using other synthetic approaches not involving reactions of free NHCs.

Reactions of NHCs with trimethylsilyl azide, reported by Tamm and co-workers, proceed in a different fashion, yielding corresponding N-trimetylsilylimines 895 in good yields via Staudinger-type transformation. Further methanolysis of 895 provides imines 896, which can be deprotonated to imides 897 (Scheme 181).787−790 Exocyclic CNHC−N bond distances in 895 and 896 (e.g., 895, 1.267(2) Å (R = Mes, R′ = H); 896 (R = Me, R′ = H), 1.296(2) Å)788 lie rather in the range of CN double bonds (1.29 Å), while in 893 (1.312(3) Å)785 and 894 (ca. 1.34 Å)786 these bonds are slightly elongated. The NBO analysis of imidazolin-2-imines (nitrogen analogues of 2-alkylidene imidazolines, NHOs) showed dominant contribution of the ylene resonance structure (structure A, Figure 111).485 Nevertheless, the imino CN bond in 1,3-dimethyl-2-iminoimidazoline (896, R = Me, R′ = H) exhibits significant charge polarization (atomic partial charges +0.606 at C, and −0.845 at N), and the bond order of 9787


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1.66 indicates partial double bond character. Notably, the proton affinity of this iminoimidazoline is higher than that of guanidine (253.4 vs 244.2 kcal/mol) calculated at the same level of theory.485 Thus, exocyclic nitrogen atoms of imidazolin-2-imines,485,791 or so-called N-heterocyclic imines (NHIs), possess significantly enhanced basicity and nucleophilicity (contribution of the ylide resonance structure B, Figure 111) as compared to ketimines, where the conjugation of the CN bond with πelectron donor atoms is not present. In terms of the dative bonding concept, these species can be formally considered as NHC-stabilized nitrenes (structure C, Figure 111), although theoretical calculations showed that the CNHC−N bond in a model compound (imidazolin-2-imine) is rather an electron-sharing covalent bond.792 Respective anionic imidazolin-2-iminato derivatives NHCN− (Figure 111)

Figure 111. Mesomeric structures of imidazol(id)in-2-imines and imidazolin-2-iminato ligands.

Table 11. Key Features of Selected Examples of NHC−Phosphorus Adductsa NHC adduct (IMe2)PPh (945) (IMe4)PPh (903) (IMes)PPh (904a) (SIMes)PPh (IDipp)PPh (CyCAAC)PPh (MesDAC6)PPh (CyCAAC)PTMP (926) (IMe4)PH (IMes)PH (SIDipp)PH (IDipp)PH (931) (IDipp)2P2 (922a) (IDipp)PSiMe3 (932) (IMes)PGePh3 (958a) (IMes)PSnPh3 (958b) [(IDipp)PRhCp*Cl] (948) [(IDipp)P]2Hg (957) [(IDipp)PPh(CuCl)] (IMes)PPh(BH3)2 (941) (Me2CAAC)PH (940/940′) (Me2CAAC)PCl (937a) [(Me2CAAC)PLi(THF)2]2 (967) (CyCAAC)2P2 (915) ([(IDipp)2P2]BH2)[B2H7] (942) [(IDipp)P2NiPr][GaCl4] (1008) [(IDipp)2P2][OTf]2 (9222+) [(IDipp)2P3][Cl] (950) [(IiPr2Me2)2P][Cl] (977b) [(IDipp)P(aIDipp)][OTf] (994) [(IAr*)PH2][OTf] (1007a) [(IDipp)PPh2][Cl] (1001) [(Me2CAAC)PPh2][SbF6] (999) [(IMe4)PCl2][OTf] (1011a) [(IiPr2Me2)2PCl][OTf]2 (1012b) [(IMe4)3P][OTf]3 (1013) (IMe4)P(Mes)CPh2 (1021) (IDipp)PCl3 (921) (IDipp)PBr3 (1016) (IDipp)2PO4 (1023) [(IMes)PPh2F][B(C6F5)4]2 (1026)

CNHC−P [Å]

δ(13CNHC) [ppm]

1.7917(14) [1.7911(15)] 1.794(3) 1.763(6) 1.746(4) 1.7658(10) 1.7336(15) 1.726(3) 1.7376(14) 1.772(1) 1.747(2) 1.743(2) 1.752(1) 1.7504(17) 1.7744(13) [1.7800(13)] − 1.778(3) 1.822(6) 2.399(1) 1.8097(19) 1.856(2) − 1.7355(11) 1.7036(12) 1.719(7) 1.830(3) 1.821(2) 1.840(2) 1.810(2) 1.824(2) 1.773(3), 1.818(3) 1.840(4) − 1.865 1.845(4) 1.827(2), 1.850(2) 1.813(2)−1.825(2) 1.7420(19) 1.871(11) 1.872(2) 1.895(3) 1.852(3)

a

170.1 169.1a 170.0d 184.3d 172.9a 208.1a 172.0a 207.3a 174.7d 180.0d 195.1a 180.18a − 175.2a 168.7b 169.4b 190.9a − − 152.3b 212.3/217.2a 210.9a 177.4d 202.2a − − − 161.37b − 166.0b 148.0c 132.7e 215.3b 139.0e 133.3e − − − 149.2d − −

δ(31P) [ppm]

ref

−49.1a −53.5a −23.0d −12.0d −18.9a 68.9a 83.0a 135.4a −148.8d −147.3a −116.7d −136.7a −52.4a −129.5a −155.2b −179.5b 551.8a −56.0d −44.6d 4.0b −38.3/−44.9a 161.9a 202.10d 59.4a −185.9d 158.1, 492.1 d 451.8e 190.6, 591.9b −124.2b −66.1b −166.6c −12.9e 0.0b 107.8e 19.9e −82.9e 200.7a 16.9a 24.8d 5.8a 78.1b

848 805 804 804 52,846 52 53 827 837 837 836 832 824 831 851 851 831 836 846 841 840 840 854 818 842 850 825 832 858 865 835 561 877 886 886 886 893 134 888 896 897

NMR solvents: a = C6D6, b = CD2Cl2, c = CDCl3, d = THF-d8, e = CD3CN; − = unknown/not observed.

a

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The first complexes regarded as NHC-stabilized phosphinidenes (or phosphanylidenes)803 (903−905, Scheme 182),

represent an important class of N-donor ligands. The main feature of these ligands is that they can act as 2σ,4π-electron donors, isolobal to dianionic imido systems (ylidic resonance structure B of imidazolin-2-iminato ligand, Figure 111). Their use in transition metal chemistry and catalysis,790 in rare-earth metal complexes,793 and in main group element chemistry794 has been extensively reviewed. Detailed discussion of their chemistry and application in catalysis here is out of the scope of this Review. 2.5.2. Phosphorus. The chemistry of NHC adducts with phosphorus compounds dates back to 1964, when the first complexes with two-coordinate phosphorus, regarded as phosphacyanines, were synthesized by Dimroth and Hoffmann.795 Recognition of the NHC−phosphinidene adducts and the use of free NHCs for activation of P−P and P− halogen bonds in phosphorus compounds led to active development of the field and allowed an access to various neutral and cationic adducts with low-coordinate phosphorus. In the following, the NHC-adducts are generally discussed in ascending order of coordination number of the phosphorus atoms. Moreover, important analytical data of selected phosphorus−NHC adducts are presented (Table 11). Phosphorus-functionalized NHC ligands796 obtained without using free NHCs or their synthetic equivalents in the key step of synthesis (e.g., backbone or N-functionalization using classical methods of imidazolium chemistry) are generally not included. 2.5.2.1. NHC-Stabilized Phosphorus(I) Compounds. Inversely polarized phosphaalkenes,797 presented by benzimidazolin-2-ylidene phosphine 898798 and other related mono- and diamino derivatives 893−901,798−800 have been known since the beginning of the 1980s (Figure 112). As compared to other

Scheme 182. Syntheses of NHC−Phosphinidene Complexes 903−905

obtained by reactions of NHCs with oligomeric cyclic phosphinidenes (method a) or, alternatively, with P(III) dichlorides (method b), have been reported in 1997 by Arduengo and Cowley.804,805 Structural and spectroscopic data clearly suggest dominant ylidic character of these adducts. The phosphorus nuclei of 903−905 resonated in high field (903, −53.5; 904a, −23.0; 904b, −23.0; 905, −12.0 ppm), while the 1 H and 13C NMR spectroscopic data indicated the presence of free rotation around the CNHC−P bonds. The CNHC−P bond lengths were found to be in the range of 1.76−1.79 Å, being the longest in the (IMe4)PPh complex 903 (an average CP double bond length of the phosphaalkenes with positively polarized P atom is 1.67 Å,806 while a single C−P bond length in PPh3 is ca. 1.83 Å807). Interestingly, the P−CPh bond in 903 of 1.81 Å is only slightly longer than the corresponding CNHC− P bond (Δ = ca. 0.02 Å). Theoretical calculations on the electronic structures of adducts 903, 904,808 and the adducts of parent phosphinidene PH with the acyclic and cyclic diaminocarbenes ([(H2N)2C]PH, and imidazol(id)in-2-ylidene-PH adducts)809 based on the electron localization function analysis, and analysis of bond dissociation energies and aromaticity, supported the donor−acceptor model for the description of the Cylidene−P bonds in these inversely polarized phosphaalkenes. Newertheless, for convenience such bonds in this Review are mainly shown as a CP double bond. It should also be noted that during the submission of this Review, a detailed review on the neutral NHC−phosphinidene adducts from Slootweg and Krachko810 became available. Bertrand and co-workers were the first to recognize the usefulness of 31P NMR spectroscopic data of phenylphosphinidene adducts for evaluation of π-accepting properties of NHCs and other singlet carbenes (for a general discussion, cf., Introduction).52 Syntheses of a series of carbene−PPh adducts (including main types of NHCs) have been performed by reacting carbenes with (PhP)5 (method a, Scheme 182), or by using a two-step approach, based on reactions of equimolar amounts of free or prepared in situ carbenes with dichlorophenyl phosphine and following reduction of the resulting salts with KC8 or Mg. Hudnall et al. extended this list of (NHC)PPh adducts using Mes DAC 6 and Mes DAC 5 carbenes.53

Figure 112. Selected earlier examples of inversely polarized phosphaalkenes 898−901 and typical phosphaalkene 902.

phosphaalkenes displaying the Cδ−Pδ+ polarization (electronsharing CP bonds, formal adducts of triplet carbenes and triplet phosphinidenes), this type of conjugated phosphaalkenes due to the π-donation from neighboring nitrogen atoms possess essential ylidic character, with the negative charge at the phosphorus atom (or dative C→P bonds). This results in significant high-field resonance shift of the low-coordinate phosphorus nuclei and elongation of the C−P distances. Notably, it has also been demonstrated that inversely polarized phosphaalkenes, for example, (Me2N)2CPSiMe3, can undergo phosphinidene transfer reactions accompanied by release of the carbene and phosphinidene (heterolytic C−P bond cleavage),801,802 thus supporting the dative nature of the C− P bonds in these compounds. 9789


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The tetraphosphinediide [Na2(thf)4(PMes)4] reacts with IMe2 as phosphinidene-transfer reagent affording the (IMe2)PMes adduct (the CNHC−P bond length is 1.768(4) Å) in good yield.811 Primary phosphines react with NHCs to afford corresponding (NHC)PR adducts. Direct interaction of phenylphosphine PhPH2 with the IiPr2 carbene required relatively harsh reaction conditions (105 °C, 4h, toluene) yielding quantitatively the 1:1 mixture of the phosphinidene adduct (IiPr2)PPh and 1,3bis(diisopropyl)-2,2-dihydroimidazole (IiPr2)H2.812 The use of catalytic amounts of M[N(SiMe3)2] (M = Fe, Co) in reactions of ArPH2 (Ar = Ph, Mes) and NHCs (IDipp, IMes, IMe4) allows one to obtain the P(I) adducts with moderate to good yields under mild reaction conditions. Formation of the (IMe4)PMes adduct is catalyzed also by the phosphinidene-bridged iron complex [(IMe 4 ) 2 Fe(μPMes)].813 The IMe4 carbene reacts with the phosphaalkyne iPr2NC P to yield the bicyclic P(I) complex 906. In contrast, reaction of IMe4 with another phosphaalkene tBuCP and the reaction of iPr2NCP with another NHC led to N-heterocyclic olefins (e.g., 907, Scheme 183). In contrast, reaction of more

Scheme 184. Reactions of Diphosphenes 909a,b with IMe4

Scheme 185. Formation of Carbene-Supported 2,3,4,5Tetraphosphatrienes and P12 Cluster

Scheme 183. Reactions of Phosphaalkynes with the NHC

The use of a larger excess of sterically less hindered CyCAAC led to the formation of other carbene-supported acyclic P4 species 914 and stabilized P2 fragment 915, isolated in 67% and 12% yields, respectively (Scheme 186).818 Hudnall et al. reported the selective formation of analogous P4 species 916 in the reaction of more electrophilic MesDAC6, Scheme 186. Syntheses of NHC-Stabilized P4, P2, and P8 Species electrophilic MesDAC6 with tBuCP led to the carbene insertion into the CP triple bond and formation of respective strained spirophosphirene 908 (Scheme 183).814 The latter is demonstrated to be a source of respective vinylphosphinidene, transiently formed via phosphirene− phosphinidene rearrangement accompanied by the P−CDAC bond cleavage. Reaction of 2 equiv of IMe4 with diphosphenes 909a,b carrying bulky indacenyl groups led to activatation of the PP double bonds and quantitative formation of the corresponding (IMe4)PRind complexes 910a,b (Scheme 184).815 NHCs readily react with white phosphorus under mild conditions, affording adducts (NHC)nPm of different composition, which depends on the nature of carbenes (electronic and steric properties) and reaction conditions (reagent ratios, solvents). As has been demonstrated by Bertrand and coworkers, reaction of MentCAAC with white phosphorus yields the corresponding CAAC-stabilized 2,3,4,5-tetraphosphatriene 911,816 while the reaction with the less electrophilic SIDipp affords similar but less stable product 912, which was selectively converted to P12 cluster 913817 (Scheme 185). 9790


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decreased double bond character of this bond in 922a. The P− P bond distance of 2.2052(10) Å lies in the range of a single bond length (P−P distance in 915: 2.184(3) Å). DFT calculations on the simplified model of 922 suggested the presence of σ CNHC−P bonds and two lone pairs on each phosphorus atom; the WBIs of the CNHC−P bonds are 1.397. Another DFT study,825 by means of NBO analysis, revealed a negative atomic partial charge of −0.11e at the P2 fragment of 922a, while being positive (0.20e) in the 915, thus reflecting higher π-acidity of the CyCAAC ligand. Bertrand and colleagues explored the reactivity of imino and amino-substituted dichlorophosphines 923 and 925 toward Cy CAAC to get access to the corresponding stable PN adduct 924826 and aminophosphinidene 926827 (Scheme 189).

while analogues monoamidocarbene under the same reaction conditions afforded the P8 cluster 917.819 Interestingly, selective formation of the same type of P8 cluster (918, 919) has been reported for the reactions of Mes DAC6 and CyCAAC with an excess of white phosphorus in benzene (Scheme 186).820 The CNHC−P bond lengths in the complexes 911 and 913−919 lie in the range 1.728(6)− 1.777(4) Å, being longest in 898 and 900. The accepted general mechanism for the formation of these adducts,821−823 dictated by the dominance of nucleophilic properties in NHCs over electrophilic, is different from the activation of white phosphorus with other related low-valent main group element species. Initial nucleophilic attack of NHCs on the LUMO of P4 leads to the ring-opening and further formation of corresponding transient (NHC)P−P3 species containing three-membered cyclotriphosphirene ring. These intermediates can rearrange to very reactive NHCsupported 2,3,4,5-tetraphosphatrienes (NHC)2P4 (e.g., 911, Scheme 185) and undergo further transformations (e.g., cycloaddition reactions) to form final products. The presence of these intermediates has been proven experimentally by using 2,3-dimethyl-1,3-butadiene as a trapping reagent.816,817,819 Reaction of the more electrophilic seven-membered diamidocarbene with white phosphorus proceeds differently, affording the product of carbene insertion into the P−P bond, the cage 920 (Scheme 187).820 This type of reactivity is typical for electrophilic heavier carbene analogues and transition metal complexes.

Scheme 189. Syntheses of Carbene-Stabilized Phosphorus Mononitride 924 and Aminophosphinidene 926

Scheme 187. Insertion of the NHC into the P−P Bond of White Phosphorus Reaction of the only known stable free phosphinidene 927828 with free NHCs afforded the corresponding adducts 928.829 Alternatively, the adduct 928 can be prepared by the ligand exchange at the phosphinidene phosphorus, through the reaction of the phosphinidene−phosphorane 929 with NHCs (Scheme 190).4 Treatment of the diphosphorus adduct 922a with lithium metal led to the reductive P−P bond cleavage and formation of the backbone-lithiated NHC-adduct of parent P−H phosphinidene 54 (Scheme 191).134 The product displayed a signal at −143.0 ppm, 1JP,H = 171 Hz in 31P NMR spectrum. The

The syntheses of imidazolin-2-ylidene-stabilized P2 allotropes 922a,b has been achieved by Robinson and coworkers824 using an alternative synthetic approach based on the reduction of (NHC)PCl3 adducts 921a,b with KC8 (Scheme 188). As compared to the CAAC-analogue 915, the CNHC−P bond distances in 922a are longer (1.7504(17) vs 1.719(7) Å, respectively). The 31 P NMR chemical shift appeared significantly upfield (−52.4 vs 59.4 ppm), suggesting higher polarization of the CNHC−P bonds toward phosphorus and

Scheme 190. NHC Complexes of Stable Diaminophosphanylphosphinidene

Scheme 188. Synthesis of the Imidazolin-2-ylideneStabilized P2 Allotrope

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Scheme 191. Synthesis of the Anionic NHC Adduct of Parent Phosphinidene

Scheme 193. One-Step Syntheses of (NHC)PH Complexes Based on Reactions of Imidazolium or Imidazolinium Salts

CNHC−P bond distance is 1.763(2) Å. The WBI of this bond, calculated for simplified model at the B3LYP/DZP level of theory, is 1.397. Neutral P−H phosphinidene complex (IDipp)PH 931, first described by Driess and co-workers,830 has been obtained by phosphinidene transfer from the phosphasilene 930 (Scheme 192). The same complex could also be prepared in 20% yield differently, yielding (IDipp){cyclo-(CO)P2C(O)} and the (IDipp)P2C(O)(Dipp) adducts.838 The latter contains the P2 (P−P bond distance 2.153(3) Å) and CO units, and can be obtained from the former in the presence of IDipp at elevated temperatures via extrusion of CO. Alternative two-step synthesis of (SIDipp)PH is based on the dehydrogenation of the phosphinoderivative 933 with 9,10-phenantrenoquinone (Scheme 194).836 An attempt to

Scheme 192. Syntheses of Neutral NHC Adducts of Parent and PSiMe3 Phosphinidenes

Scheme 194. Two-Step Synthesis of (SIDipp)PH

by reduction of the (IDipp)PCl3 complex 921 with an excess of KC8, or in better yield from the adduct (IDipp)PSiMe3 932, obtained by reaction of the corresponding 2,2-difluoroimidazoline with tris(trimethylsilyl)phosphine.831 The 31P NMR of spectrum of 931 shows a signal at δ(31P) = −133.8 ppm, which is slightly downfield shifted as compared to the anionic analogue 53 (δ(31P) = −143.0). The CNHC−P bond length (1.7510(2) Å/1.752(1) Å) is slightly shorter than that of 53. The adduct 932 possesses an elongated CNHC−P bond length of 1.7744(13) Å/1.7800(13) Å and exhibits a signal in the 31P NMR spectrum at −129.5 ppm. Theoretical calculations at B3LYP/6-31G(d) confirmed the very weak double bond character of the CNHC−P bond in 931. The WBI of this bond in this compound is 1.36, and the calculated rotation barrier of 14.0 kcal/mol suggests almost free rotation around this bond.830 Practical synthetic approaches toward (NHC)PH adducts are based on reactions of imidazolium salts with phosphorus reagents (Scheme 193). Grützmacher et al. reported reaction of the imidazolium salt [IDippH]Cl with sodium ethynolate Na(OCP) producing (IDipp)PH complex 931 in 71% yield.832 The same approach has also been used to synthesize (IMes)PH,833 (SIMes)PH,834 and (IPr*)PH835 adducts. While the methods utilizing sodium ethynolate Na(OCP) and (Me3Si)3P7832,836 are quite efficient, they are limited to the preparation of adducts with N-aryl substituents. In contrast, the use of sodium heptaphosphanide Na3P7 allows one to synthesize also the complexes with N-alkyl groups.837 Interestingly, reaction of the 2-chloro-substituted imidazolium salt [(IDipp)Cl][Cl] with 2 equiv of Na(OCP) proceeds

synthesize the adduct 933 with an unsaturated backbone using this approach was not successful. The use of the [SIMesH]Cl imidazolinium salt led to the neutral P,P-bis(imidazolidinyl)substituted phosphine (SIMesH)2PH, which could not be dehydrogenated further.839 Three-component reactions of white phosphorus with imidazolium salts and KOtBu (in situ formation of IMe2, IMe4, IMes, IDipp carbenes) in THF allowed one to obtain the corresponding (NHC)PH adducts as the main reaction products (isolated yields up to 54%) along with other cationic or insoluble phosphorus compounds.837 Temperature-dependent 1H NMR spectroscopic measurements have been used to evaluate rotation barriers for CNHC−P bonds in (SIMes)PH and (IMes)PH adducts.834 Under standard conditions, rotation around the P−C bond in (SIMes)PH is hindered (rotation barrier is ca. 21 ± 3.6 kcal/mol, coalescence of the signals of methyl protons of mesityl groups observed at ca. 80 °C), while this was not the case for (IMes)PH (rotation barrier is 13.7 ± 1 kcal/mol, coalescence temperature is −33 °C). This is in agreement with the better π-accepting properties of NHCs with a saturated backbone as compared to those with an unsaturated. Streubel and co-workers reported the first synthesis of the phosphinidene adduct with abnormal NHC (aNHC)PPh 936 (CaNHC−P and P−CPh bond distances: 1.8107(16) and 1.8068(16) Å, respectively; δ(31P) = −73.6 ppm), using reactions of reductive P−C bond cleavage and desulfurization of backbone-functionalized phosphanylthiones 934 and 935, 9792


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with much smaller CCAAC−P bond distances (937a′−c′, 1.688(7), 1.435(12), and 1.615(4) Å, respectively; δ(31P) signals, 129.4−135.0 ppm), but because of the low occupancy and high uncertainty of these data, discussion of their structural parameters in this case was excluded. Spectroscopic data and theoretical calculations supported the assigned structures. It should be noted that reactivity of CAACs toward phosphorus trihalides is different from that of less electrophilic imidazolin-2-ylidenes, and the respective (NHC)PCl complexes have not yet been reported. Because of the essential ylidic character of the CNHC−P bonds in NHC−phosphinidene adducts, the phosphorus atom in these molecules is very nucleophilic. The HOMO in the imidazolin-2-ylidene complexes is mainly located at phosphorus atom,830 and it has been demonstrated that both phosphinidene lone pairs are generally available as Lewis base centers for further transformations. The CNHC−P bonds of (NHC)PR ligands upon complex formation become longer and, respectively, weaker, due to exclusion of the lone pair(s) from the π-backbonding. For the same reason, phosphorus nuclei of the complexes normally resonate upfield in 31P NMR spectra, as compared to free phosphinidene adducts. Already in 1997, Arduengo and Cowley reported the reaction of (IMes)PPh 904a with an excess of BH3·THF leading to the stable bis(borane) adduct 941 (CNHC−P bond distance: 1.856(2) Å, δ(31P) = 4 ppm) (Scheme 197).841

respectively (Scheme 195).140 Further deprotonation of the product with KHMDS afforded the corresponding monomeric Scheme 195. Syntheses of Zwitterionic Compound 936 and Carbene Complex 79

complex 79 (CaNHC−P bond distance: 1.823(6) Å; δ(31P) = −65.7 ppm) with the potassium metal coordinated to the carbene center. Despite the presence of two molecules of crown ether coordinated to the potassium, the CNHC−K distance of 3.066 Å falls into the range of typical CNHC−K interactions. Recently, the syntheses of the CAAC-supported halogenophosphinidenes (CAAC)PHal 937/937′−939/939′ and derived pearent phosphinidene (Me2CAAC)PH 940/940′ have been reported (Scheme 196).840 Each isolated (CAAC)-

Scheme 197. Reactions of the NHC−Phosphinidene with Boranes

Scheme 196. Syntheses of (CAAC)PCl and (CAAC)PH Adducts

Interestingly, no evidence for 1:1 borane adduct formation could be found spectroscopically. On the other hand, reaction with BPh3 led to the cleavage of dative CNHC−P bond and the formation of cyclic oligophosphines. Analogous bis(borane) adducts have also been reported for (IMes)PH834 and (IPr*)PH835 phosphinidenes. Carbene-stabilized diphosphorus 922a reacts with an excess of BH3·THF to give the boronium salt 942, which slowly dissociates to the starting materials in solution (Scheme 198).842 The 31P and 11B NMR chemical shifts of 942 (−185.9 and −31.6 ppm, respectively) lie in the range typical for cyclic bisphosphine−boronium salts. The CNHC−P bond in 942 is about 0.08 Å elongated, as compared to the starting material. PHal complex displayed sets of signals in different ratios of two different compounds in NMR spectra, which remained nearly the same even after heating. Crystal structures of (CAAC)PCl 937a,b/937a′,b′ exhibit distortions of the P−Cl units in each probe. Major components 937a−c (site occupation factors ca. 95% 937a,b and 85% in 937c), were interpreted as expected (CAAC)PCl complexes (CCAAC−P bond lengths (937a−c), 1.7355(11), 1.7404(12), and 1.7513(15) Å, respectively; δ(31P) signals, 160.3−163.3 ppm). Residual densities were explained by the presence of unusual conformational isomers

Scheme 198. Synthesis of the Cationic Borane-Bridged Diphosphorus Adduct

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The syntheses of various monometallic phosphinidene carbonyl complexes [(NHC)PR]M(CO)5 943 (NHC = IDipp, M = Cr, Mo, W, R = H, Ph, Mes;839,843 NHC = IMes, M = W, R = H833) and [(IPr*)PH]Fe(CO)4835 are reported for reactions of (NHC)PR phosphinidenes with the corresponding metal carbonyls. Formation of bimetallic species in these reactions is hindered, apparently due to the steric reasons. Alternatively, tungsten pentacarbonyl complexes can be prepared by reactions of corresponding phosphinidenoid complexes 944 with NHCs (Scheme 199).844 The TEP value of the

Scheme 200. Synthesis and Phosphinidene Transfer Reactions of 946

Scheme 199. Syntheses of Group 6 Transition Metal Complexes of NHC−Phosphinidene Adducts

of insoluble coordination polymer [(IMe2)ZnCl2]n and were performed with an in situ formed or preisolated reagent. In the solid state, 946 exists as mixed mono- and bimetallic species. The CNHC−P bonds in 946 are only slightly longer than those in the free ligand (1.819(4)/1.818(4) Å vs 1.7917(14) Å). On the other hand, in solution it exists as a symmetric molecule with equivalent phosphorus atoms (946, δ(31P) = −88.1 ppm vs 945, δ(31P) = −49.1 ppm). Tamm and co-workers reported syntheses of the first NHCphosphinidenyl transition metal complexes 947−949, and also derived di- and trimetallic complexes [(947)AuCl], [(948)AuCl], and [(949)AuCl], respectively, using the (IDipp)PSiMe3 adduct 932 as starting material (Scheme 201).831 Spectroscopic and especially structural features of 947 (δ(31P) = 531.5 ppm) and 948 (δ(31P) = 551.8) are similar to those of arylphosphinidene complexes (e.g., [(η6-p-cumene)(PCy3)Ru(PMes*)]), suggesting a double bond character of the metal− phosphorus bonds in these complexes and the presence of

(IDipp)PPh ligand (2024 cm−1) is defined using the cis[{(IDipp)PPh}Rh(CO)2Cl] complex.843 It confirms stronger electron-donating properties of the phosphorus(I) ligand as compared to tertiary phosphines (e.g., PPh3 or PtBu3) and even NHCs. NHC−phosphinidene adducts (NHC)PR have been utilized as ligands to form mono- and bimetallic complexes with coinage metals, [{(NHC)P}(MX)] and [{(NHC)P}(MX)2], respectively. Their formation depends on the amount of metal salt used for their preparation. Dias et al. reported the preparation of bimetallic coinage metal complexes [{(IMes)PPh}(MCl)2] (M = Cu, Ag, Au) and [{(IMes)PPh}(CuBr)2],845 while Tamm and co-workers described similar mono- and bimetallic complexes bearing the (IDipp)PPh ligand.846 Derived from the latter, cationic [{(IDipp)PPh}{Au(THT)2}][SbF6]2 (THT = tetrahydrothiophene) and [{(IDipp)PPh}Au2Cl4][BArF]2 complexes showed catalytic activity in enyne cyclization and carbene-transfer reactions. Formation of the gold complex [{(IPr*)PH}(AuCl)2] has also been reported.835 The ruthenium benzylidene complex [{(IMes)PPh}RuCl2(PPh3)(CHPh)] has been reported by Lavoie et al.847 It was inactive as catalyst in ring-closing metathesis of diallyl sulfide. As has been already discussed, NHC phosphinidene adducts can undergo phosphinidene transfer reactions, accompanied by the CNHC−P bond cleavage. Recently, Slootweg, Grützmacher, and co-workers found that sterically less hidered (IMe2)PPh (945) complex can itself act as phosphinidene transfer reagent and also reported the Lewis acid promoted phenylphosphinidene [PPh] transfer to organic substrates under mild reaction conditions (Scheme 200).848 Reaction of 945 with ZnCl2 led to isolation of stable complex 946, while reactions with BPh3, AlCl3, MgCl2, and Zn(OAc)2 led to the P−C bond cleavage and formation of oligophosphines (PPh)n. Phosphinidene transfer reactions of 946 were accompanied by the formation

Scheme 201. Syntheses of Phosphinidyne Complexes

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carbene rings. Localization of the spin density at the central E atoms of P−E−P units in 952 and 953 was supported experimentally and theoretically (952: 64.4% on central P, 3.8% on each terminal P atoms). Notably, the open-shell 952 reacts with acetylenedicarboxylate in a [3+2] cycloaddition manner splitting off the carbene (Scheme 203).832 The assumed mechanism involves the phosphinidene transfer and formation of the transient cycloaddition product 954, followed by a redox process yielding the final product 955.

2σ,2π-electron donation from the NHC−phosphinidenyl (NHCP) ligand. More recently, Tamm et al. reported alternative syntheses of phosphinidenyl complexes 947 and 948, and new osmium and iridium complexes [{(IDipp)P}(LMCl)] (M = Ru/Os, L = η6p-cumene; M = Rh/Ir, L = η5-C5Me5) starting from the (IDipp)PH adduct 931.849 The method is based on the preparation of [{(IDipp)PH}(LMCl2)] complexes by reactions 931 with [LMCl2]2, and their following dehydrochlorination using DBU as a base. Grützmacher and co-workers have employed reactions of (IDipp)PH 931 with PCl3 in the presence of organic base to approach the NHC-supported P3 and P2As cations 950 and 951, respectively (Scheme 202).832 Following reduction of

Scheme 203. Transfer of the P3 Radical Unit

Scheme 202. Syntheses of NHC-Supported Cationic and Radical P3 and P2As Species

The same research group explored the reactivity of (NHC)PH (NHC = IDipp, SIDipp) toward other electrophiles (Scheme 204).836,850 Phosphinidenyl-substituted chlorScheme 204. Reactions of (NHC)PH with Phosphorus Nucleophiles and with HgCl2 in the Presence of Organic Bases

these salts with magnesium metal afforded corresponding stable radicals 952 and 953. Structural analysis revealed 950 and 951 as separated ion pairs. The CNHC−P bonds in these compounds are significantly elongated as compared to the starting material (950, 1.811(2) Å; 951, 1.800(2) Å vs 1.752(1) in 931). The P−P bond distances in [(IDipp)2(μP3)]Cl 950 of 2.090(1) and 2.097(1) Å are relatively short (e.g., P−P bond distance in P4 is 2.1994(3) Å, and that in Mes*PPMes* is 2.034 Å), suggesting substantial double bond character of these bonds. The P−As bond distance in 951 is 2.202(1) Å. Shortening of the carbene−phosphorus bonds (e.g., to 1.763(2) and 1.769(2) Å in 952), and lengthening of the central P−E bonds (P−P bond lengths in 952: 2.145(1) and 2.144(1) Å) have been observed for the P3 radical species. Notably, all systems 950, 951 (green color) and 952, 953 (blue and purple colors, respectively) exhibit long wave absorptions in UV/vis spectra, at λmax = 714−776 nm. According to DFT calculations on simplified models, the electronic ground-state structures of 950 and 951 are best described as a carbene-supported P3 triphosphaallyl unit bonded to an imidazolium cation, and as a P−P•−P radical stabilized with two NHCs, respectively. The P3 unit in 950 is only slightly positive (NPA charge: +0.07), while the main positive charge (+0.45) is equally distributed over both

ophosphines [(IDipp)P]P(R)Cl 956a−c have been obtained using reactions of (IDipp)PH with phosphorus nucleophiles in the presence of DABCO.850 The P−P distances in 956a (2.13 Å) and 956c (2.15 Å) suggest some double bond character of these bonds. In addition, reactions of (NHC)PH adducts with HgCl2 in the presence of DBU yielded the corresponding bis(NHC−phosphinidenyl) complexes 957.836 Grützmacher and co-workers also reported the phosphinedenyl-substituted germanes and stannanes (NHC)PGePh3 (958a) and (NHC)PSnPh3 (958b) obtained in reactions of corresponding phosphaketenes Ph3E−PCO with NHCs (NHC = IDipp, IMes, and SIDipp; E = Ge, Sn).851 Hänisch and colleagues developed an effective method to introduce a phosphinidenyl functionality based on the use of NHC−phosphinidenyl potassium 959 (Scheme 205).834 The reagent was obtained by P−H deprotonation of the weakly acidic (SIMes)PH with benzyl potassium. Because of its low 9795


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well described as dative double bond or as classical electronsharing double bond (difference in energy is only around 4 kcal/mol). Notably, more recently, the same group achieved the synthesis of CAAC-stabilized lithium phosphinidenide [(Me2CAAC)P−Li(THF)2]2 (967) obtained by lithiation of (Me2CAAC)PH (940) with MeLi (Scheme 207).854 Structural

Scheme 205. Synthesis of the NHCP−Potassium and NHCP/Al and NHCP/Ga Complexes

Scheme 207. Synthesis of the CAAC-Supported Lithium Phosphinidenide and Its Reactivity toward Electrophiles

solubility in organic solvents, neither NMR data nor crystal structure have been reported. Anionic phosphinidene 959 and (SIMes)PH have been employed in reactions with dialkylaluminum and gallium chlorides to get an access to new group 13/15 adducts 960−963. More recently, the same group utilized (SIMes)PK (959) in the synthesis of phosphinidenyl-substituted pnictogen compounds (SIMes)P−PntBu2 (Pn = P, As, Sb, and Bi) using corresponding pnictogen halides tBu2PnX (X = Cl, Br) as a second reaction component.852 Roesky and co-workers reported the CAAC-supported silylene−phosphinidene 966 (CCAAC−P and P−Si bond distances: 1.7303(17) and 2.2970(7) Å, respectively). It was successfully obtained from the reaction of chlorophosphinidene adduct (Me2CAAC)PCl 937a with chlorosilylene 964 (Scheme 206).853 Theoretical calculations (NBO analysis, at M06-2X/def2TZVPP level) confirmed the presence of lone electron pairs localized at P and Si atoms, characteristic for phosphinidene and silylene species, and the P−Si single bond (WBI = 0.89). According to results of energy decomposition analysis (EDA), the double CCAAC−P bond (WBI = 1.57) in 966 can be equally

analysis revealed a relatively short CCAAC−P bond distance of 1.7042(12) Å. Interesting to note is the unusual low-field resonance of 967 (δ(31P) = 179.3 ppm), supporting high double bond character of the CCAAC−P bond (WBI = 1.72). The compound decomposes slowly in ethereal solvents yielding the diphosphorus adduct (Me2CAAC)2P2. Reactions of 967 with C-, Si-, Ge-, and P-electrophiles opened an access to CAAC-supported P-functionalized phosphinidenes 968−971. 2.5.2.2. NHC-Stabilized Phosphorus(I) Cations. Discovery of the first compounds of two-coordinate phosphorus possessing PC double bonds as a part of a conjugated system by Dimroth and Hoffmann in 1964795 is an important landmark in phosphorus and main group element chemistry. These phosphaalkenes, regarded as phosphamethine cyanines (973, Scheme 208), comprise a delocalized 2-phosphaallyl cation. The synthesis is based on reactions of 2-chlorobenzothiazolium salts 972 with tris(hydroxymethyl)phosphine. A few years later, Märkl and Lieb reported an alternative synthesis of 973 by reacting 972 with tris(trimethylsilyl)phosphine.855 In 1983, Schmidpeter and co-workers described the synthesis of related P-imidazoliniumyl systems 976 using reactions of bis(imidazolidin-2-ylidenes) 974 with the triphosphenium salt 975,856 and the synthesis of similar systems bearing acyclic mono- and diaminomethylidene moieties. The main structural features of 973 (R = Et, R′ = H, and [ClO4] as counteranion)857 are the identical C−P bond distances of 1.76 Å, near coplanarity of benzothiazole rings (twisting angle is about 3°), and short S−S distance of 2.95 Å, suggesting an interaction between these atoms.

Scheme 206. Synthesis of the Silylene−Phosphinidene Adduct

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The same research group also described the preparation of various P(I) cationic systems, including cyclic bis(NHC)supported P(I) cations 979,859 and P-thiazoliumyl-substituted salts 980[I],860 using the triphosphenium bromide 978[Br] as phosphorus transfer reagent (Scheme 210).

Scheme 208. Syntheses of Phosphamethine Cyanines

Scheme 210. Syntheses of Adducts 979 and 980

Novel methods to approach the systems related to 973 and 976, based on the use of NHCs for their synthesis, have been developed more recently by Macdonald and co-workers (Scheme 209).858 Reactions of 3 equiv of nonbulky carbenes Cyclic cations 979 (C−P−C angles: 91.3°−92.9°) are isoelectronic to the bis(NHC)-supported E(0) tetrylones (E = Si, Ge) described by Driess.604,743 (cf., section 2.4). Despite restricted twisting of NHC fragments in 979, this system retains a description as a P(I) species, as was supported by DFT calculations. The NBO analysis showed that the phosphorus atom in 979 bears two electron pairs, while stabilization of 979 through the π-back-donation from the P atom to NHC is higher, as compared to the acyclic system 977. Structural features of P-thiazoliumyl derivatives 980 are very similar to those of S,N-heterocyclic phosphamethine cyanines 973. The phosphorus atom in 980 is significantly deshielded. The 31P NMR spectrum of 980 (R = Et) displayed a downfield-shifted signal at δ(31P) = 17 ppm, as compared to the resonances observed for chelates 957 (δ(31P) = ca. −82 ppm) and acyclic P(I) cations 977. In contrast to the latter systems, the NBO analysis of 980 (R = Et) showed the presence of the π-bond between P and C atoms and only one lone electron pair remaining at the phosphorus atom, thus supporting the description of 980 and 973 as P(III) species and true phosphamethine cyanines. Schulz and co-workers reported selective formation of the salt 977[(MesTerN)2P] upon reaction of the biradicaloid [P(μNMesTer)]2 981 with IMe4 (Scheme 211).861

Scheme 209. Syntheses of P(I) Cations Using Free NHCs

with phosphorus trichloride proceed differently from the analogous reactions of CAACs or bulkier IDipp, providing the NHC-stabilized P-imidazolium-2-yl salts 977a−c[Cl] selectively. Alternatively, reactions of 2 equiv of NHCs with triphosphenium salt 978 afforded 977a,b in a more atom economic way. The phosphorus nuclei in 977 are shielded, and the respective 31P NMR signals were observed in high field at δ(31P) = −124.2 to −129.2 ppm. Molecular structures of 977 possess elongated and roughly equal single C−P bonds (977b: 1.824(2), 1.823(2) Å). The molecules are bent at the P atom (C−P−C′ angle: 97.35(9)° in 977b); the NHC planes are twisted to ca. 50−60°. These experimental data, supported with theoretical calculations, suggest the presence of P(I) species and dominant contribution of the ylide resonance structure 977A (Figure 113).

Scheme 211. Synthesis of the Cation 977[(MesTerN)2P]

Phosphorus atoms in P-imidazoliumyl−NHC adducts 977, 979 and their thiazolium analogues 980 are nucleophilic and can react with Lewis acids or oxidizing agents. They can be oxidized, for example, with sulfur to form the same type of dithiophosphonium salts (982, Scheme 212).860,862 Triflate salts 977[OTf] react with triflic acid and alkyl triflates ROTf (R = H, Me) yielding the corresponding dicationic bis(imidazoliumyl)phoshanes 983.862

Figure 113. Canonical structures of 977. 9797


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214).865 The 31P NMR spectrum of 994 displayed a downfield signal at −66.1 ppm relative to P-imidazolium-2-yl adducts

Scheme 212. Reactions of 977[X] with Sulfur, Triflic Acid, and Methyl Triflate

Scheme 214. Synthesis of the First NHC-Supported PImidazolium-4-yl Phosphinidene Salts 994[X]

Similar to the neutral P(I)−NHC adducts, cationic Pimidazoliumyl analogues 977 can form mono- and dinuclear transition metal complexes, such as 984−991 (Figure 114).862,863 IR spectroscopic data analysis and theoretical 977, apparently due to a wider C−P−C angle (109°), as compared to the less sterically hindered cations 977 (e.g., 97.35(9)° in 977c). The CNHC−P bond in 994 (1.773(3) Å) is shorter than the CaNHC−P bond (1.818(3) Å). Cation 994 preserves the reactivity characteristic for neutral and cationic NHC−phosphinidene complexes, forming the corresponding mono- and bimetallic complexes with coinage metals,865 and reacting with electrophilic reagents (TfOH, MeOTf) to form the corresponding dicationic bis(imidazoliumyl)phosphines.866 Isolation of the first carbene-stabilized P3 triphosphaallyl cation 996 and P2 neutral adduct 997 upon the NHC-induced fragmentation of the P4-derived cage [RP5Cl]+ 995 has also been accomplished by the group of Weigand (Scheme 215).867 Scheme 215. Fragmentation of the Cationic Cage 995 Figure 114. Transition metal complexes with P(I) cationic ligands.

studies on carbonyl [{(IMe2)2P}2M(CO)5]+ complexes 987[BPh4] uncovered cationic P(I) ligands [(NHC)2P]+ as weaker π-acceptors and σ-donors as phosphines.863 The [(IMe4)2P]+ ligand possesses lower in energy HOMO (−8.11 eV) and LUMO (−3.64 V), as compared to PPh3 (HOMO = −6.21 eV, LUMO = −0.70 eV). According to Schmidpeter,864 NHC-supported cationic P(I) adduct 976 can also act as phosphinidene transfer reagents, substituting stepwise NHC ligands at the phosphorus atom upon reactions with strong nucleophiles (Scheme 213). The group of Weigand reported the NHC-stabilized Pimidazolium-4-yl phosphinidene 994, obtained by reduction of 992 or 993 with potassium graphite in good yields (Scheme Scheme 213. Stepwise Ligand Substitution at P Atom of 976

The obtained P3 cation 996 possesses structural and electronic features similar to those of the discussed 950[Cl], reported later by Grützmacher et al. (cf., Scheme 202).832 In contrast, the reaction of equimolar amounts of 995 and NHC afforded cationic P5 species 998. 2.5.2.3. Mono-, Bis-, and Tris(carbenio)phosphines and Related Species. Nucleophilic substitution of halogen atoms in halogenophosphines HalPR2 by NHCs affords cationic phosphorus compounds [(NHC)PR 2 ] + , which can be 9798


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considered as carbeniophosphines [(NHC+)PR2] (e.g., structure A, Figure 115) or as NHC-stabilized phosphenium cations

Similar synthetic methods have been employed for the preparation of bis(imidazoliumyl)phospines [(NHC+)2PR] through reactions of dichlorophosphines RPCl2 with free NHCs, methylene-bridged bisNHC,870 or imidazolium 2carboxylates,879 and 2-trimethylsilyl imidazolium triflate.540 Analogously, reactions of 2-chloroimidazol(in)ium salts with primary phosphines RPH2 afford the corresponding dicationic species.881 According to structural data analyses,871 central phosphorus atoms in monocationic species [(NHC)PR2]+ (R = Alk, Ar) have a pyramidal environment. The CNHC−P bond distance is ca. 1.84 Å. Bis(imidazolium) derivatives possess similar structural data and slightly shortened CNHC−P bond distances. The CNHC−P bond in monocationic (imidazoliumyl)phoshines [(NHC)PR2]+ can also undergo heterolytic cleavage, suggesting dative character of these bonds. Reaction of imidazolium-2-yl-substituted diphenylphosphine 1001 with KHMDS (Scheme 217) led to lithiation of the backbone and

Figure 115. Bonding models used for the description of [(NHC)PR2]+ adducts.

[(NHC)P+R2] (structure B, Figure 115). Theoretical investigations support both models for the description of bonding situation in these molecules.868−870 Carbeniophosphines [(NHC)PR2]+ belong to a class of αcationic phosphine ligands,871−873 useful in the design of novel transition metal catalysts with increased Lewis acidity of a metal center. As compared to them, bis- and tris(carbenio)phosphines ([(NHC+)2PR] and [(NHC+)3P], respectively) possess a significantly reduced coordination ability to transition metals.871 Because the synthesis of α-cationic phosphine ligands and their use in transition metal catalysis have been extensively reviewed,796,871−873 in this section we covered only selected aspects of the carbeniophosphine chemistry excluding coordination chemistry and transition metal catalysis. The first synthesis of the monocationic imidazoliumyl phosphine [(IMe2)PPh2]+ by reaction of the in situ generated free NHC with chlorodiphenylphosphine Ph2PCl has been described already in 1988 by Zoller.874 Generally, free NHCs readily react with monochlorophosphines to form the corresponding monocations [(NHC)PR2]+ (Scheme 216,

Scheme 217. Examples of Heterolytic Cleavage of the CNHC−P Bond in Monocationic Phosphines

Scheme 216. Synthetic Approaches to Imidazoliumyl Phosphines Based on the Reactivity of Monochlorophanes

following formal transfer of the diphenylphosphanyl moiety to form 4-diphenylphosphanyl-substituted NHC 1002 (apparently, a multistep process including intermediate formation of 4,2-diphosphinylimidazolium salt, followed by reaction with the free NHC).561 Futhermore, reactions of (benzimidazoliumyl)phosphine 1003 with n-BuLi or [Et4N] Cl led to splitting of the diphenylphosphinyl group with concomitant release of the free carbene, which was either trapped by elemental sulfur, hydrolyzed by water, or protonated (Scheme 217).869 Theoretical calculations on the bond dissociation energies confirmed that heterolytic cleavage of the CNHC−P bonds in monocationic imidazoliumyl phosphines869 [(IMe2)PPh2]+ and their oxides882 [(IMe2)P(O)Ph2]+ is energetically more favored (by ca. 15 kcal/mol), as compared to the homolytic dissociation, thus supporting dative character of these bonds (structure B, Figure 115). In contrast, in dicationic bis(imidazoliumyl)phosphines and phosphanoxides ([(IMe2)2PPh]2+ and [(IMe2)2P(O)Ph]2+, respectively),883 these bonds are electron-sharing covalent, as their homolysis is more favorable. Notably, it has been shown

NHC = IiPr2Me2,875 IDipp,561 IMe2PPh2,876 IBox-iPr2;870 R′ = Ph). Notably, CAACs, such as Me2CAAC and CyCAAC, react similarly, affording the corresponding cationic phosphines [(Me2CAAC)PR2]+ 999 (R = Ph, Cy, p-(CF3)Ph)877 and [(CyCAAC)PPh2]+ 1000870 in good yields (61−89%). Carbene precursors, such as imidazolium 2-carboxylates878−880 and 2-(trimethylsilyl)imidazolium triflate,540 can also be employed in these reactions instead of free NHCs (Scheme 216). The synthesis of monocationic imidazolinium phosphines (e.g., [(SIMe2)PR2]+, R = Ph, Cy)881 has also been achieved by reaction of 2-chloroimidazolinium salts with secondary phosphines R2PH. 9799


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experimentally that NHC ligand in the cationic phosphine oxide [(NHC)P(O)Ph2]+ can be exchanged by another carbene ligand with stronger σ-donor properties.882 Reactions of the 4,5-dichloro-substituted NHC 1004 with chlorophosphines R2PCl (R = Ph, Cy) in the presence of TMSOTf afforded the expected imidazolium-2-yl phosphines 1005 (Scheme 218).866 Interestingly, rearrangement of 1005 to imidazolium-4-yl phosphines 1006 has been observed, if the carbene was used in excess (e.g., 20%) or added (0.3 equiv) to the 1005.

Scheme 220. Synthesis of the Dicationic Diphosphorus Adduct 9222+

Scheme 221. Syntheses of the P2 Cation 1008 and P4 Dications 1009

Scheme 218. Rearrangement of the Imidazoliumyl Phosphines 1005

Weigand and co-workers demonstrated the potential of 2(trimethylsilyl)imidazolium salts as imidazolium-transfer reagents.884 While interactions of free NHCs with PCl3 are known to not afford three-coordinate imidazoliumyl phosphines, the use of 2-(trimethylsilyl)imidazolium triflates (1010, Scheme 222) instead of NHCs allowed stepwise substitution of chlorine atoms in PCl3 to obtain the corresponding mono-, bis-, and tris(imidazoliumyl)phosphines 1011−1013, respectively.540,885,886 Dichloro(imidazoliumyl)phoshines 1011 are versatile starting materials for the preparation of various imidazoliumyl-

The group of Bertrand utilized reactions between NHCstabilized P(I) complex (IPr*)PH and electrophilic reagents with the aim to synthesize the NHC-stabilized parent phosphenium cation [(IPr*)PH2]+ 1007a (δ(31P) = −166 ppm, CNHC−P bond distance: 1.840(4) Å) and cations 1007b,c (Scheme 219).835 The calculated NBO charges of CNHC and P atoms are 0.23e and 0.62e, respectively. Scheme 219. Synthesis of the Parent NHC-Supported Cation [(NHC)PH2]+ 1007a and Cations 1007b,c

Scheme 222. Syntheses of Mono-, Di-, and Tricationic Imidazoliumyl Phosphines from 1010

Oxidation of the neutral diphosphorus adduct (IDipp)2P2 922a with 2 equiv of ferrocenyl triflate (Scheme 220)825 afforded the corresponding dicationic adduct 9222+ (CNHC−P and P−P bond lengths: 1.84, 2.08 Å; δ(31P) = 452 ppm), possessing a PP double bond. Chloride abstraction from the P-chlorophosphinyl NHC− P(I) adducts 956a−c led to the cations [(NHC)PPNR]+, which could be isolated as monomeric species 1008 (CNHC−P and P−P bond distances: 1.821(2) and 2.0611(7) Å, respectively; δ(31P) = 492.1, 158.1 ppm, 1JP,P = 525 Hz), or as a dimer 1009 (Scheme 221).850 The PP double bond of the [(NHC)PPNR]+ cations is reactive toward dienes (2,3dimethylbuta-1,3-diene, cyclopentadiene), yielding the corresponding [3+2] cycloaddition products. 9800


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substituted phosphines. Reactions of 1011 and 1012 with two molar equivalents of TMSX (X = N3, CN) allowed one to obtain the corresponding products of chlorine substitution [(NHC)PX2][OTf] and [(NHC)2PX][OTf]2.540,886 Sulfurization of 1011a,b with bis(trimethylsilyl) sulfide (Me3Si)2S led to the substitution of chlorides for sulfur and the formation of the salts featuring eight-membered P,Sheterocyclic scaffold {[(NHC)PS]4}[OTf]4 (NHC = IMe4, IiPr2Me2), which can be considered as the tetramerization products of transient cationic phosphorus monosulfides [LP S]+.887 Reaction of 1011c[X] with the IDipp carbene afforded the dichloro(imidazolium-4-yl)phosphine−NHC adducts 992[X] (CNHC−P and CaNHC−P bond distances: 1.8672(2), 1.8509(2) Å, respectively; δ(31P) = −98.9 ppm) with four-coordinate phosphorus (Scheme 223).865 Subsequent reduction of the

Scheme 224. Reduction of the Dichloro(imidazolium)phosphine 1011c

Scheme 223. Syntheses of the Adduct 992 and Dicationic Chlorophosphine 993 Scheme 225. Synthesis and Reactions of the BromineBridged Dicationic Diphosphine 1017

992[X] with KC8 (2 equiv) led to the formation of the cationic P(I)−NHC adduct 994[X], while chloride abstraction with TMSOTf or GaCl3 afforded the corresponding dicationic chlorophosphines 993[X]2 (δ(31P) = 8.3 ppm). Weigand, Wolf, and co-workers studied reduction of 1011c[OTf] with various reducing agents. Reduction with an excess of KC8 afforded known NHC-supported diphosphorus 922a in good yield. Reactions with Na, K, KC8 (1.1−1.2 equiv), or equimolar amounts of [K(18-crown-6){Cp*Fe(η4naphthalene)}] proceeded differently, yielding dicationic chlorine-bridged diphosphine 1014 (P−P bond distance 2.242(1) Å, δ(31P) = −34.4 ppm) and monocationic diphosphorus compound 1015 (δ(31P) = −22.4, 132.1 ppm, 1 JP,P = 382.2 Hz) in different ratios (Scheme 224).885 Phosphorus atoms in 1014 are bridged symmetrically by chlorine (P···Cl distances: 2.602(1) and 2.594(1) Å). The bond distances between phosphorus and terminal chlorine atoms are in the normal range of single P−Cl bonds (2.138(1) and 2.142(1) Å). Thermal decomposition of the adduct (IDipp)PBr3 1016 (Scheme 225), described by Goicoechea and co-workers,888 afforded dicationic bromide-bridged diphosphine 1017[Br] (P−P bond distance 2.252(1) Å), a bromine analogue of 1014. Halogen abstraction from 1017[Br] led to the formation of dicationic P,P-dibromodiphosphines 1018[X]2 (P−P bond distance 2.232(1) Å). Reductive debromination of 1017[Br] with SnBr2 yielded the diphosphorus compound 1019[SnBr5](THF), structurally authenticated in [1019]2[SnBr6](thf) (CNHC−P bond distances: 1.847(5) and 1.845(5) Å). The P−P bond length of

2.096(2) Å in 1019 indicates some double bond character (contribution of the [(IDipp)(Br)PP(IDipp)]+ resonance structure); the P−Br single bond is elongated as compared to dicationic salts 1017 and 1018. In the reaction solution, 1019[SnBr5(THF)] slowly forms the P2 dication 922[SnBr6]. 2.5.2.4. Miscellaneous NHC Adducts with Three- to SixCoordinate Phosphorus. Low-coordinate phosphorus compounds containing P−E (E = N, P, C) multiple bonds readily react with NHCs to form different types of adducts. Reactions of NHCs with phosphaalkynes (RCP) and diphosphenes (Ar−P = P−Ar) leading to P(I) adducts have been discussed in section 2.5.2.1. In contrast, iminophosphines ArNPX (Ar = Mes*, Dipp, X = halogens or triflate) form stable Lewis base−acid P-adducts [ArNPX](IiPr2Me2).889,890 The less sterically protected adduct with Ar = Mes, X = Cl slowly decomposes to afford a mixture of products,858 which includes the corresponding cationic imidazoliumyl-P(I) adduct. Rivard et al. described the preparation of the NHC-stabilized cyclic iminophosphine−phosphazene [(IDipp){PN(PC2N)2}] by 9801


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POCl2]Cl, which has been further hydrolyzed or partially hydrolyzed to the corresponding acid (NHC)PO2OH or the neutral monomeric phosphenic chloride complex (NHC)PO2Cl (CNHC−P bond distance: 1.843(2) Å),894,895 respectively. In contrast, analogous reaction of the carbene with PSCl3 led to the formation of the corresponding imidazoline-2thione and PCl3. Robinson and co-workers reported the synthesis of the NHC-stabilized diphosphorus tetraoxide 1023 (P−P and CNHC−P bond distances: 2.310 and 1.895(3) Å, respectively) by oxidation of the diphosphorus complex 922a with molecular oxygen (Scheme 228).896

reductive dehalogenation of the hexachlorophosphazene [Cl2PN]3.891 The outcome of reactions of the phosphaalkene MesP CPh2 with NHCs depends on the backbone substitution pattern of the carbene (Scheme 226). NHCs with an Scheme 226. Reactions of the Phosphaalkene with NHCs

Scheme 228. NHC-Supported Diphosphorus Tetraoxide 1023

unsubstituted backbone (IMes, IMe2) form the corresponding 4-phosphinyl-substituted carbenes 1020a,b,892,893 while the formation of an adduct 1021 with three-coordinate phosphorus (CNHC−P bond, 1.8512(18) Å; PC bond, 1.7420(19) Å; δ(31P) = 206.1 ppm in THF) has been observed,893 if the transient formation of the abnormal carbene was hindered. Reactions of equimolecular amounts of IDipp with phosphorus trichloride or tribromide afford stable Lewis base−acid adducts (IDipp)PCl3 921134 and (IDipp)PBr3 1016,888 respectively (Scheme 227). Crystal structures of

Stefan and co-workers described the synthesis of the cationic difluorophosphorane 1025 (CNHC−P bond distance: 1.866(5) Å) via fluorination of the imidazoliumyl-substituted phosphine 1024 with XeF2 (Scheme 229).897 Scheme 229. Syntheses of Cationic Phosphoranes and Dicationic Phosphonium Salt

Scheme 227. Syntheses of NHC Adducts of Phosphorus Trihalides and Dichlorophosphines

Following fluoride abstraction from 1025 allowed one to access the dicationic phosphonium salt 1026 (CNHC−P bond distance: 1.852(3) Å). The same group reported later a number of other difluorophosphoranes 1027−1029 using the same synthetic methodology.870 Dicationic phosphonium salt 1026 exhibits remarkable Lewis acidity and is effective electrophilic phosphonium catalyst. It displayed catalytic activity in the hydrodefluorination of fluoroalkanes with silanes, hydrosilylation of alkenes, alkynes,897 amides,898 and reduction of phosphine oxides.899 NHCs (IMes, IMesCl2) react with PF5 and PhPF4 to form the corresponding adducts (NHC)PF5 and (NHC)PF4Ph 1030a,b possessing a six-coordinate phosphorus atom (Scheme 230).900,901 Analogous complexes (SIMe2)PF4R (R = F, Me, Ph) 1031, 1032 bearing less sterically hindered NHC can be obtained alternatively by reacting 2,2-difluoroimidazolines with PF3 or RPCl2.902 As might be expected, the CNHC−P bond distances in these adducts are significantly elongated

921 and 1016 (CNHC−P bond distances: 1.8711(2) and 1.872(2) Å, respectively) exhibit pseudotrigonal-bipyramidal geometries around the phosphorus; the one P−Cl distance in 921 and two P−Br bond distances in 1004, corresponding to axial positions of the halogen atoms, are significantly elongated as compared to other P−halogen bond(s) in the same molecules. While the use of less sterically demanding carbenes in reactions with PCl3 led to the reduction of phosphorus and formation of the cationic phosphinidene adducts (e.g., 977, Scheme 209), it has been demonstrated551 that reactions of the (SIMe2)SiCl4 adduct (514) with PCl3 or dichlorophosphines provide otherwise difficult to access stable chlorophosphine adducts 1022a−c (Scheme 227). As has been shown by the Kuhn group, reaction of IiPr2Me2 with an excess of POCl3 led to the nucleophilic substitution of chloride and formation of the P-imidazoliumyl salt [(NHC)9802


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1.840(2) Å; 924•+, 1.788(5) Å, CCAAC−P distance, 1.313(5) Å CNHC−N distance). The same trend has been observed for 926•+, where the P−N (1.6805(14) Å) and CCAAC−P bond distances (1.8137(17) Å) were shortened and elongated, respectively. Elongation of the CNHC−P bonds might be attributed to decreased π-back-donation from the phosphorus atoms to NHC ligands. EPR measurements and DFT calculations revealed better delocalization of spin density for CAAC-supported adducts. Spin density is mainly localized at central P2, PN, and P units of 915•+ (0.54e), 922•+ (0.77e), 924•+ (P, 0.40e; N, 0.18e), and 926•+ (P, 0.67e; NTMP, 0.16e). Goicoechea et al. reported an alternative synthesis of the 922•+ by reduction of the bromide-bridged diphosphorus cation 1017 with tetra(diethylamino)ethylene (Scheme 231).888

Scheme 230. Syntheses of NHC Adducts with SixCoordinated Phosphorus Atom

Scheme 231. Syntheses of Radical Dications 922•+ and [994•]2+ (e.g., in (IMes)PF4Ph, it reaches 1.910 Å, while another P−CPh bond distance is 1.842 Å).900 2.5.2.5. Open-Shell Phosphorus−NHC Adducts. Oneelectron redox reactions of NHC-adducts with low-coordinate phosphorus compounds present a convenient and practical synthetic approach to the corresponding stable radical species. Bertrand and co-workers investigated one-electron oxidation reactions of neutral P(I) adducts, such as NHC-supported diphosphorus compounds ( Cy CAAC) 2 P 2 915 825 and (IDipp)2P2 922a,825 phosphorus mononitride (IDipp)N− P(CyCAAC) 924,826 and aminophosphinidene (CyCAAC)PTMP 926. Cyclic voltammetry revealed reversible oxidations of these adducts, for example, at E1/2 = −0.536 V (915); −1.408, −0.872 V (922a), −0.51 V (924) in THF/0.1 M nBu4N[PF6] vs Fc+/Fc. One-electron oxidation with the tritylium salt [Ph3C][B(C6F5)4] allowed one to obtain the corresponding radical-cations, isolated as stable compounds (Figure 116). As compared to the starting materials, single crystals of the NHC-supported P2 and PN radical cations possess shortened P−P and P−N bonds (915•+, 2.094(2) Å; 922•+, 2.0826(12) Å; 924•+, 1.645(4) Å) suggesting some double bond character of these bonds, while the corresponding CNHC−E (E = P, N) are elongated (915•+, 1.777(3) Å; 922•+,

One-electron oxidation of the cationic P(I) adduct 994[OTf] (E1/2 = 0.22 V in THF/0.1 M n-Bu4N[OTf] vs Fc+/Fc) with nitrosyl triflate, described by Weigand and colleagues, yielded the corresponding radical-dication [994•]2+[OTf]2 (Scheme 231).866 EPR and DFT studies confirmed high spin density at the phosphorus atom in [994•]2+ (ca. 80%). Both CNHC−P (1.808(4) Å) and CaNHC−P bonds (1.800(4) Å) in [994•]2+ are elongated as compared to the closed-shell cation 994. Recently, Alcarazo and co-workers succeeded in the isolation of CAAC-stabilized neutral P(III) radicals 1033a−c (Scheme 232), obtained from the corresponding monocationic Scheme 232. Synthesis of α-Radical Phosphines 1033a−c

phosphines [(Me2CAAC)PR2]+ 999a−c after reduction with KC8.877 Spin density in 1033a−c is mainly located at the CAAC ligands (ca. 65−70% on the carbene carbon atoms, 20− 25% on the N atoms), with only ca. 1% residual spin density at phosphorus atoms. The CCAAC−P bonds in 1033a−c are significantly shorter, as compared to their precursors (e.g., 1.7881(14) Å in 1033a vs 1.865 Å in 999a).

Figure 116. Radical-cations obtained by one-electron oxidation of corresponding phosphinidene−NHC adducts. 9803


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Notably, it has been demonstrated that these α-radical phosphines can form Au(I) complexes [{(Me2CAAC)PR2}AuCl]• and [{(Me2CAAC)PR2}AuC6F5]•, and thus serve as spin-labeled ligands. Apeloig, Tumanskii, and co-workers explored the potential of the NHCs as radical trapping reagents.903,904 Reaction of the IMes with the transient phosphonyl radical (iPrO)2(O)P•, generated in situ from [(iPrO)2(O)P]2Hg under irradiation (λ > 300 nm), yielded the new unstable radical adduct [(IMes)P(O)(OiPr)2]• with increased stability (half-lifetime τ1/2 = 7.1 s), which was characterized by EPR spectroscopy. DFT calculations revealed 76% of the spin density on the NHC ligand (39% on the carbene carbon atom) and 7.5% on the phosphorus. Analogous reaction with bis(N-adamantyl)benzimidazolin-2-ylidene afforded the less stable radical (τ1/2 < 1 s) with more spin density (83%) localized on the NHC ligand. 2.5.3. Arsenic, Antimony, and Bismuth. The chemistry of the NHC adducts of heavier pnictogens (E = As, Sb, Bi) is significantly less explored, which can be in part explained by the intrinsic stability decrease of these adducts and the diminishing effectiveness of 2pπ−npπ interactions descending in the group, which are important for NHC adducts with lowcoordinate elements. For syntheses of heavier pnictogen adducts, generally similar synthetic approaches that have been developed for phosphorus congeners are applied. Arsamethine cyanines 1034 (Scheme 233), which can formally be considered as NHC-stabilized cationic As(I)

in these adducts (1035a, 1.977(3) Å; 1035b, 1.976(7) Å) are only ca. 4% greater than the CNHC−As bond distances. Tamm, Scheer, Goicoechea, and co-workers reported the syntheses of NHC adducts with parent arsinidene AsH and AsSiMe3 (1036a−c and 1037a,b, respectively, Scheme 234).907 The CNHC−As bond distances of AsH adducts Scheme 234. NHC Adducts of Parent AsH and AsSiMe3 Arsinidenes

(1.883(2)−1.896 Å) are comparable to those of AsPh 1035a,b and shorter than those in AsSiMe 3 1037a,b (1.899(3)−1.9130(15) Å) adducts. The carbene carbon atoms of 1036a−c adducts resonate at 179.4−184.5 ppm, which are in lower field in comparison to 1037a,b (176.8 and 173.6 ppm). The bonding situation in AsH adducts 1036a−c has been analyzed by means of DFT calculations at PBE1PBE level of theory. Computed molecular structures were in good agreement with experimental data. WBI values for the CNHC−As bonds in these adducts (1.26−1.27) support partial double bond character of these bonds. NRT analysis showed contribution of the ylene (33%) and ylide (38%) resonance structures into the ground state of the 1036 (N−Me model). The computed NBO charges supported dative character of the CNHC−As bond in 1036a−c, which is strongly polarized toward the As atom. It should be noted that, similar to phosphorus analogues, inversely polarized arsaalkenes (Me2N)2CAsR can undergo arsinidene transfer reactions with release of a carbene.802 Reaction of the IMes carbene with diphenyliodoarsine Ph2AsI afforded imidazoluim arsine 1038[I], isolated as a stable solid in good yield (Scheme 235).908 The CNHC−As bond distance in 1038 (1.9858(8) Å) is slightly longer than the other two As−CPh distances (1.9468(9) and 1.9461(9) Å). Salt metathesis reactions allowed isolation of 1038[X] (X = OTf, BF4, SbF6) with different counteranions. Interestingly, addition of trimethylsilyltriflate led to the heterolytic dissociation of the CNHC−As bond in 1038[BF4] and formation of the corresponding 2-(trimethylsilyl)imidazolium salt. Reaction with (Me2S)AuCl afforded Au(I) complex 1039[BF4]. Reactions of arsenic and antimony trichlorides PnCl3 with the onium-transfer reagents [NHC−SiMe3][OTf] 1010b,c afforded stable imidazolium pnictogen dichloride adducts 1040b,c and 1041b (CNHC−As bond distance in 1040b: 1.974(a) Å) and 1041b (CNHC−Sb bond distance: 2.212(3) Å).886 Adducts 1040b,c, obtained in good yields, reacted

Scheme 233. Arsamethine Cyanines 1034 and Arsinidenes 1035a,b

compounds, have been prepared by Märkl and Lieb in 1967 using the same approach as for the preparation of phosphorus congeners 972.855 The first neutral arsinidene−NHC adducts (inversely polarized arsaalkenes) 1035a,b have been synthesized in 1997 by Arduengo, Cowley, and co-workers using reactions of the free NHC with cyclic oligoarsinidenes (Scheme 233).804 Similar to the phosphorus analogues 904a,b, reported in the same publication, spectroscopic and structural data suggested dominance of the ylidic resonance structure in these adducts. The CNHC−As bonds are elongated (1035a, 1.899(3) Å; 1035b, 1.902(7) Å) as compared to the CAs bond length in the (fluorenylidene)As−Mes* arsaalkene (1.807(3) Å, electron-sharing bond)905 and in other acyclic arsaaalkenes without conjugated C-amino functions (C−As bond distances: 1.816−1.827 Å).906 The corresponding As−CPh single bonds 9804


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Scheme 235. Synthesis of the Imidazoliumylarsine 1038 and Au(I) Complex 1039[BF4]

Scheme 237. Syntheses of NHC Adducts with Chloropnictogens 1044−1048

further with TMSCN or TMSN3 to give the corresponding dicyano and diazido derivatives 1042b,c and 1043b,c, respectively (Scheme 236).

(NHC)Sb(CF3)3 adduct with an unusually long CNHC−Sb single bond distance of 2.821(5) Å, suggesting weak interaction.913 The 13C NMR spectrum revealed the low-field signal of the carbene carbon atom at δ(13C) = 207 ppm, indicating carbene-like character (the signal for the free carbene is 219.9 ppm). Recently, Goikoechea and co-workers described the synthesis of the NHC adducts with antimony-and bismuth tribromides (IDipp)PnBr3 (Pn = Sb, Bi), obtained quantitatively from the reaction of equivalent amounts of the NHC with corresponding tribromides.914 Reactions of these adducts with aluminum tribromide gave the corresponding monocationic compounds [(IDipp)PnBr2][AlBr4]. Interestingly, under the thermal treatment (75 °C, THF), the pnictogen tribromide adducts (IDipp)PnBr3 rearrange to the abnormal (aIDipp)PnBr3 complexes. Moreover, reaction of the (IDipp)SbBr3 with an additional equivalent of IDipp yielded the cationic [(aIDipp)2SbBr2][Br] adduct, which strongly suggests involvement of the free NHC in aforementioned rearrangement. Further treatment of this adduct with AlBr3 yielded the corresponding dicationic species [(aIDipp)2SbBr]2+. NHCs (IDipp, IMes2Cl2) react with pnictogen pentafluorides PnF5 (Pn = As, Sb) to yield respective adducts with six-coordinate pnictogen atoms (NHC)PnF6.804 The CNHC−As and CNHC−Sb bond distances in (IMes2Cl2)PnF5 are 2.006(5) and 2.175(5) Å, respectively. NHC adducts of chloropnictogens 1044−1048 have further been explored in the syntheses of corresponding lowcoordinate derivatives and open-shell species. Reduction of the (NHC)AsCl3 adduct 1044 with potassium graphite gave the NHC-supported diarsenic 1049 in low yield as red crystals (Scheme 238).909 Diarsenic 1049 has structural features similar to those of the diphosphorus analogue 922a, thus supporting description of the molecule as a bis(arsinidene). It adopts trans-bent geometry around the As−As single bond (bond distance: 2.442(1) Å), with nearly coplanar imidazole rings (torsion N−C−As−As angle is 2°). The CNHC−As bond length of 1.881(2) Å suggests partial double bond character. The adduct 1049 was further oxidized to radical-cation 1049•+, and dication 10492+ using different amounts of gallium trichloride (Scheme 238).915 As is the case with the

Scheme 236. Syntheses of Cationic Pnictogen Dichloride Adducts and Their Derivatives

Heavier pnictogen chlorides PnCl3 (Pn = As, Sb, Bi), similar to phosphorus congeners, react with free NHCs to form the corresponding stable Lewis acid−base adducts with fourcoordinate pnictogen atoms (Scheme 237). Reactions with IDipp led to the corresponding adducts 1044 (CNHC−As bond distance: 2.018(3) Å),909 1045 (E = Sb),675 and 1046a (CNHC−Bi bond distance: 2.370(1) Å).910 Reaction of BiCl3 with sterically less bulky IiPr2Me2 afforded the (NHC)BiCl3 adduct 1046b (CNHC−Bi distance: 2.370(1) Å). Halogen abstraction from 1046b using TMSOTf in THF yielded the adduct [(NHC)BiCl2(OTf)(thf)] with five-coordinate bismuth atom. Similar to imidazolin-2-ylidenes, reaction of antimony trichloride with CyCAAC yields directly the (CyCAAC)SbCl3 complex 1047 (CCAAC−Sb bond distance: 2.223(3) Å) in good yield.911 Reaction of MesDAC6 with dichlorophenylstibane PhSbCl2 (Scheme 237)912 afforded the adduct 1048 (CDAC− Sb and Sb−CPh bond distances: 2.326(4) and 2.178(4) Å). The 4,5-dichloro-substituted IMes carbene reacts smoothly with tris(trifluoromethyl)antimony yielding the corresponding 9805


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bond distances in 1051 (2.082(5) Å, WBI = 1.282) and in 1052 (2.084(11)/2.088(10) Å, WBI = 1.234) correspond to dative bonds with a weak double-bond character. The Sb−Sb bond distance in 1052 (2.8125(10) Å) corresponds to a single bond, comparable to those observed in distibanes. Reduction of (MesDAC6)Sb(Ph)Cl2 1048 with two molar equivalents of magnesium powder afforded DAC-supported phenylstibinidene (MesDAC6)SbPh 1053 (CDAC−Sb and Sb− CPh bond distances: 2.068(7) and 2.160(7) Å), isolated as a thermally stable solid in 86% yield.912 In contrast, reductive dechlorination of the NHC-supported bismuth trichloride (IDipp)BiCl3 1046a or [(IiPr2Me2)BiCl2(OTf)(THF)] with KC8 (3 equiv) or Mg metal failed to produce the desired low-coordinate adducts, leading to release of the free NHC and presumably metallic bismuth.910 Apaprently, the use of the NHC ligands with better πaccepting properties such as CAACs is necessary for the stabilization of the low-coordinate bismuth.

Scheme 238. Syntheses of the NHC-Supported Diarsenic and Derived Radical-Cation and Dication

2.6. NHC Complexes of Group 16 Elements

NHC adducts of group 16 elements are mainly represented by the well-known classes of organic compounds, which can be synthesized using classical synthetic methods. On the other hand, the use of NHCs for their preparation significantly extended their availability and opened convenient access to some interesting classes of compounds. In the following brief discussion, the NHC complexes are generally presented in order of increasing coordination number of group 16 elements. The main accent is made on the nature of carbene−chalcogen interactions and novel synthetic applications. 2.6.1. Adducts with Monocoordinated Chalcogen Atoms. The most common group of NHC adducts with group 16 elements is represented by the corresponding chalcogenoureas, containing CNHC−Ch (Ch = O, S, Se, Te) double bonds. Generally, thermodynamic stability of the C Ch bonds in chalcogenones (R2C=Ch, R = Alk, Ar)916 for heavier elements is drastically decreased due to reduced tendency toward hybridization, and, respectively, lower efficiency of the 2p(C)−np(Ch) orbital overlap. Therefore, these bonds in heavier homologues (Ch = S, Se, Te) require kinetic or thermodynamic stabilization. In NHC-derived chalcogenones or chalcogenoureas, high stabilization (more effective as, e.g., in acyclic analogues) is achieved by the conjugation with aminogroup(s), leading to substantial contribution of the ylidic resonance structure to the ground state of these molecules. Theoretical NBO analysis of N−H imidazolin-2-ylidene derived chalcogenoureas as model compounds showed an expected decrease of the double bond character of CNHC−Ch bonds descending the group (WBIs: 1.59 (C−O), 1.45 (C−S), 1.33 (C−Se)).485 Description of these bonds for Ch = S, Se, Te as donor−acceptor interactions with substantial π-back bonding were supported at the B3LYP/6-31G* level of theory using NBO, charge decomposition analysis, and electron localization function.792 The CNHC−O double bond is rather electron-sharing covalent. NHCs are remarkably stable toward molecular oxygen,82,917 while reaction of ItBu with singlet oxygen was reported to produce the respective diimine and carbene−CO2 adduct.917 Nitrogen monoxide is known to oxidize free NHCs to the corresponding ureas.82 Oxidation with N2O leads to the formation of stable NHC−N2O adducts, which eliminate N2

diphosphorus analogues, gradual elongation of the CNHC−As bond distances (1049•+, 1.960(4), 1.938(5) Å; 10492+, 1.977(2) Å) and shortening of the As−As dictances (1049•+, 2.332 Å; 10492+, 2.2803(5) Å) has been observed moving from neutral to dicationic species. The computed WBIs for the As−As bonds in simplified models of 1049•+ and 10492+ are 1.218 and 1.78, respectively. EPR spectroscopic investigation of 1049•+ supported the presence of open-shell species. DFT calculations revealed localization of spin density mainly at the As2 unit (0.41e on each As). Reductive dechlorination of the (CyCAAC)SbCl3 adduct 1045 with different amounts of KC8 afforded stable radical 1050, CAAC-stabilized chlorostibinidene 1051, and bis(stibinidene) 1052 (Scheme 239).911 Theoretical and experimental EPR investigations suggested dominant localization of the spin density on Sb atom in 1050. The CNHC−Sb Scheme 239. Syntheses of the NHC-Supported Diarsenic and Derived Radical-Cation and Dication

9806


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and form ureas upon heating (60−100 °C).778,779 While free NHCs are inert to oxidation with CuO, Cu2O, or HgO,82 their copper complexes (NHC)CuCl are prone to form ureas in the presence of oxidation agents.918,919 Analogously, reactions of NHC−AgCl complexes with elemental sulfur led to the formation of thiones.920 Oxidation of [SIMes-H]I with PhI(OAc)2 in the presence of succinimide921 and oxidation of bis(benzimidazolidin-2-ylidenes)922−925 under air yields the corresponding imidazolidin-2-ones. The C−O bond distances in N-substituted imidazolones vary only slightly in the range 1.23−1.24 Å; carbene carbon atoms resonate at δ(13C) = 151−163 ppm. Free or in situ generated NHCs react with elemental sulfur, 926−932 selenium, 54−56,927−929,933,934 and tellurium927−929,935,936 similarly to other main group compounds with unshared electron pairs (e.g., phosphines or heavier carbene analogues) to give the corresponding chalcogenoureas 1054−1056 in high yields (Scheme 240). Reactivity of other NHCs (e.g., CAACs,927 1,2,4-triazolin-5-ylidenes,937,938 and bis(imidazolidin-2-ylidenes)939,940) toward elemental chalcogens is similar.

Some imidazoline-2-thiones and their derivatives possess useful biological activity and have therefore attracted interest in medicinal research.944,945 N-Heterocyclic chalcogenoureas are used as ligands in transition-metal and main group chemistry. While the role of the imidazol(id)ine-2-thiones as S-donor ligands is dominant, heavier analogues attract growing research attention.928,946−951 2.6.2. Adducts with Two-Coordinated Chalcogen Atoms. Protonation of chalcogenoureas (NHC)Ch 1054− 1056 affords the corresponding NHC-supported stable parent sulphenyl H−S+, selenyl H−Se+, and tellurenyl H−Te+ (unstable in solution) cations 1057−1059, respectively (Scheme 241).952 Similarly, reactions with methyl triflate allowed the isolation of stable chalcogen-methylated analogues 1060−1062. Scheme 241. Syntheses of NHC-Supported Chalcogenyl Cations

Scheme 240. Reaction of NHCs with Elemental Chalcogens

Monocoordinated cationic chalcogen atoms in these adducts are isoelectronic to nitrenes and phosphinidenes. Protonation or methylation led to lengthening of the CNHC−Ch bond distances in the reaction products; for example, in [(IDipp)ChMe]+ 1060a−1062a for Ch = S, Se, and Te, the lengthening is about 0.067, 0.036, and 0.062 Å, respectively. In 77Se NMR and 125Te NMR spectra, the signals of Ch−H and Ch−Me adducts appeared significantly downfield as compared to those of the starting chalcogenoureas, thus supporting the increased electron-deficiency of these nuclei. Theoretical NBO analysis revealed an increasing positive charge (0.13−0.54 au) at the chalcogen atoms in the order S < Se < Te for [(SIMes)ChH]+ adducts 1057−1059, and decreasing WBIs (1.17−0.99) for heavier homologues. NHC-supported stable arytellurenyl salts 1065[X] (CNHC− Te and Te−CAr bond distances: 2.097(2) and 2.161(2) Å (X = I), respectively, δ(125Te) = 444.7 ppm) can be obtained by reactions of the free NHC with aryltellurenyl iodide 1063 or aryltellurenyl halides 1064 (Scheme 242).953 NBO analysis

Imidazol(id)ine-2-thiones 1054 and related N−H-substituted adducts941,942 are a well-studied class of compounds, accessible through various alternative synthetic approaches.88,920,943 In carbene chemistry, N-alkylimidazoline-2thiones 1054 serve as convenient starting materials for NHC syntheses.88 Reported CNHC−S bond distances in 1054 (1.66−1.69 Å) do not vary significantly. The CNHC−Se distances in selenoureas 1055 lie in a relatively narrow region of ca. 1.82−1.86 Å and display no correlation to δ(77Se).55 The 77Se NMR chemical shifts of NHC−Se adducts are changing in a broad range of Δδ(77Se) = ca. 850 ppm, which can be used for quantification of πaccepting properties of carbenes (for details, cf., section 1).54−56 As compared to the lighter congeners, NHC-derived telluroureas 1056 are significantly less represented. The reported CNHC−Te bond distances of imidazoline-2-tellones vary from 2.050(3) to 2.087(4) Å, being longer in N-alkyl derivatives; 125Te NMR signals change from −4 to −200 ppm, depending on the π-accepting properties of NHCs.928,935,936 The most low-field resonance of δ(125Te) = 472.04 ppm has been observed for the (Me2CAAC)Te adduct.927 The 13C NMR signals of the carbene atoms of (NHC)S adducts usually appear upfield as compared to resonances of this carbon in free NHCs (e.g., for (IDipp)S,928 167 ppm vs 220.6 ppm (IDipp); for (CyCAAC)S,927 213.6 ppm vs 309.4 ppm (CyCAAC), respectively). These nuclei of (NHC)Se and (NHC)Te adducts resonate upfield to sulfur analogues (e.g., (IDipp)Se, 162.2 ppm; (IDipp)Te, 145.9 ppm).

Scheme 242. Synthesis of the Tellurenyl Cation 1065

9807


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(NHC)2Pn2 (Scheme 245). These compounds are commonly prepared by oxidative coupling of the corresponding

showed nearly equal distribution of the positive charge between the aryltellurenyl cation and the carbene moiety (0.489 and 0.511, respectively) in the less bulky model compound [(IMe4)TePh]. 2-(Methylthio)imidazolium salts, for example, [(IMe2)SMe][X], are known to react with nucleophiles with substitution of the −SMe group at the C2 atom of imidazolium ring.954,955 In contrast, 2-(alkynylchalcogeno)imidazolium salts [(NHC)ChCCR][SbF6] 1066, 1067 can act as S- and Seelectrophiles (umpolung) and serve as useful thioalkylation reagents (Scheme 243).956

Scheme 245. Synthesis of Dicationic Dichalcogenides 1074 and 1075 by Coupling of Corresponding Thiones and Selenones

Scheme 243. Use of Imidazolium Sulfides as Reagents in Organic Synthesis

chalcogenoureas 1054 and 1055 using such reagents as (CF3CO)2O,959,960 Cu(OTf)2,961 Cu(BF4)2·6H2O,962 Cu(ClO4)2·6H2O,963 Br2,964 interhalogens (IBr, ICl),965 and others.966−969 It should be noted that a change of reaction conditions (e.g., reagent ratio) may lead to other reaction products like adducts with a three-coordinate chalcogen atoms (cf., section 2.6.3). The S−S distances in 1074 (ca. 2.1 Å) are slightly longer than those in acyclic urea derivatives and in diaryl sulfides (ca. 2.1−2.05 Å); the CNHC−S distances are slightly elongated, if compared to starting thiones (e.g., CNHC−S bond distances in (IDipp)S and [(IDipp)S2]2+: 1.670(3) and 1.736(3) Å, respectively). The imidazole moieties are arranged in a trans position. The diselenides 1075 are isostructural to 1074. The Se−Se bond distances (2.409(2)−2.440(2) Å)965 are generally greater than the sum of covalent radii of Se atom (2.40 Å).970 Interaction of selenium atoms with counteranions (e.g., halogens) influences the geometry of the dications. The signals of diselenides 1075 in 77Se NMR spectra are shifted significantly downfield as compared to the corresponding selenones 1055 (e.g., δ = 272−426 ppm vs −13.4 to 47 ppm, for 1075 and 1055 (R,R′ = Alk), respectively).968 Dicationic bis(imidazoliumyl)chalcogenides [NHC2Ch]2+ 1076−1079 (structures A and B, Figure 117) can be considered as NHC-supported chalcogen dications, which are isoelectronic to neutral NHC-stabilized tetrylones NHC2E (E = Si, Ge), and to monocationic NHC-supported phosphinidenes [(NHC)2P]+(cf., sections 2.4 and 2.5, respectively).

Reactions of these salts with various Grignard reagents R′MgX led to heterolytic CNHC−Ch bond cleavage and formation of alkynylthio- and alkynylselenoethers 1068, 1069 in good to excellent yields. Cationic imidazolium thiocyanate [(NHC)SCN][SbF6] 1070 can act as CN-transfer reagent in reactions with alkynes, yielding, in the presence of BCl3, 1,2-chlorocyanoalkenes 1071 and the corresponding dicationic disulfide salt [(NHC)2S2][SbCl4]2 as a byproduct (Scheme 243).957 (Dimethylamino)imidazoliumyl sulfide 1072[X] (CNHC−S bond distance: 1.7602(13) Å, X = BPh4] is reported to undergo a [3+2] cycloaddition reaction with acetylenedicarboxylate to give spiroheterocycle 1073 (Scheme 244).958 Dicationic dichalcogenides 1074 and 1075 (Ch = S, Se) are isoelectronic to the neutral NHC-supported bis(pnictinidenes) Scheme 244. Cycloaddition Reaction of 1072[Cl] with Acetylenedicarboxylate

Figure 117. NHC-supported chalcogen dications 1076−1079, and examples of sulfur dications 1077a,b. 9808


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The first representative of this class of compounds, the adduct 1077a (C−S−C bond angle: 103.3(5)°),971,972 has been synthesized by N-methylation of the corresponding sulfide-bridged heterocycle with methyl triflate. Another dicationic sulfur derivative [(IMe4)2S][SbCl6] 1077b has been obtained accidently by refluxing of the corresponding thione (IMe4)S in dimethylformamide.973 The first dicationic ether [(SIMe2)2O][OTf]2 1076a (CNHC−O bond distances, ∼1.30 Å; C−O−C′ bond angle, 120.3(7)°), reported at the beginning of the 1980s, has been synthesized via reaction of the corresponding urea (SIMe2)O with triflic anhydride Tf2O.974,975 It has been demonstrated that 1076a can act as an efficient onium-transfer reagent in reactions with various C- and N-nucleophiles. Reactions of its analogue, ether 1076b, with imidazoline-2-thiones and selenones yielded the corresponding dicationic adducts 1077c and 1078a, respectively. Similarly, reactions of the ether 1081 yielded dications 1082 and 1083 (Scheme 238).960 Exchange of the diazabutadiene ligand in dications 1084 and 1085 to the NHC ligand yielded stable dications 1078b (CNHC−Se bond distances: 1.915(3)/1.920 Å)976 and 1079 (CNHC−Te bond distances: 2.136(4)/ 2.138(3) Å), 977 respectively (Scheme 247). The reaction of 1085 with NHC

Scheme 247. Syntheses of Dicationic Selenium and Tellurium NHC Adducts

allowed one to obtain the fluoride analogue, adduct (IiPr2Me2)SF2 (1090).

Scheme 246. Synthesis of Chalcogen Dications from Chalcogenoureas

Scheme 248. Syntheses of NHC Adducts with Chalcogen Dihalogenides 1087−1089 Based on the Reactivity of the NHC

in a 1:4 molar ratio yielded the four-coordinated tellurim dication 1086 (CNHC−Te bond distances: 2.342(6)−2.519(7) Å). In both molecules 1078b and 1079, the electrophilic chalcogen centers are coordinated to oxygen atoms of the triflate anions (Ch···O distances: 2.755(3)/2.969(2) Å and 2.740(3)/2.921(3) Å, respectively). The C−Te−C′ bond angle in 1079 (91.5(1)°) is more narrow as compared to the C−Se−C′ angle (96.3(1)°) in the lighter homologue 1078b. Theoretical calculations revealed a positive atomic charge (+0.65) and the presence of two lone pairs at the tellurium in 1079. 2.6.3. Adducts with Three-Coordinate Chalcogen Atoms. Reaction of the free NHC (IiPr2Me2) with sulfur dichloride afforded selectively sulfur dichloride adduct (NHC)SCl2 1087 (Scheme 248).978 Subsequent substitution of chlorine atoms in 1087 for fluorine using silver fluoride

Chalcogen tetrahalides SeCl4 and TeCl4 are reported to not form stable NHC adducts with five-coordinate chalcogen atoms upon reaction with the IiPr2Me2 carbene. In this case, formal reduction of the chalcogen centers (Ch(IV) → Ch(II)) and direct formation of NHC adducts with the respective chalcogen dihalides 1088 (X = Cl, δ(77Se) = −266 ppm; X = Br, δ(77Se) = 66 ppm)979 and 1089980 has been observed instead (Scheme 248). An alternative approach to chalcogen dihalide adducts (NHC)ChX2, suitable for large-scale preparations, is based on the reaction of corresponding chalcogenoureas with halogenating reagents. 981 First representatives of this class of compounds, the dihalo(imidazolium)sulfuranes 1091 (Scheme 249, X = Cl, Br; R, R′ = Me, H), have been obtained in the late 1970s by reaction of N,N-dimethylimidazolin-2-thione and molecular chlorine or bromine.982 Selenoureas react with bromine933,964,983,984 and chlorine984 similarly, yielding adducts 1092 (Ch = Se, X = Br, Cl). Both thiones and selenones form respective dicationic dichalcogenides 1074 and 1075 (cf., section 2.6.2, Scheme 245), if 0.5 9809


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(I i Pr 2 Me 2 )Se). 979 The C NHC −Ch bond distances in (IiPr2Me2)SCl2 1087978 and (IiPr2Me2)TeI2 1089980 are 1.732(3) and 2.105(4) Å, respectively. Halogen atoms in chalcogen dihalide adducts (NHC)ChX2 can be substituted for other groups and/or anions, which gives rise to various useful synthetic reagents. The (NHC)S(CN)Br adduct (NHC = IiPr2Me2) obtained from the reaction of the respective sulfur dibromide adduct with TMSCN, and its derivative [(NHC)SCN][SbF6] 1070, are efficient electrophilic cyanation reagents (N-, S-, and C-cyanation of secondary amines, thiols, CH-acidic compounds, and aromatic compounds, respectively). 993 Alkynylthioimidazolium salts [(NHC)SCCCO2Et][SbF6] derived from the same adduct are reagents for electrophilic S-alkynylation of thioles and Calkynylation of activated amides and ketoesters.993 Some other interesting synthetic applications of 1070 as well as alkynylthio- and selenoimidazolium salts were already discussed (cf., Scheme 243). Another interesting example of an NHC adduct with a Tshaped geometry of the chalcogen atom is the dicationic tris(selenourea) complex 1096 (Figure 118), obtained upon

Scheme 249. Halogenation of Chalcogenoureas

equiv of bromine is employed. While reactions of imidazolin-2ylidene-derived seleno-985 and telluroureas986 with I2 can be used for the syntheses of respective (NHC)ChI2 adducts (1092 and 1093), analogous reactions of thiones927,987,988 and CAAC-derived chalcogenoureas927 lead to the formation of corresponding charge-transfer complexes bearing two-coordinate chalcogen atoms (NHC)S−I−I (e.g., 1095a,b, Scheme 249) and (CAAC)Ch−I−I (Ch = S, Se, Te). This difference in the reactivity is explained by a generally more sufficient interaction of HOMOs of the donor molecules with the σ* molecular orbital of iodine to cleave the I−I bond in the case of imidazolin-2-ylidene-derived seleno- and telluroureas possessing higher in energy HOMOs as compared to respective thiones.981,989 Moreover, the HOMO energy level of the latter is lower than the LUMO energy level of I2, which makes the formation of charge-transfer adducts energetically favorable in this case.989 Chlorination of the thione (IiPr2Me2)S with thionyl chloride yielded the respective sulfur dichloride adduct 1094 without the need to employ chlorine gas.958 The following reaction of 1094 with TMSNMe2 resulted in adduct 1072 (cf., section 2.6.2). Selenium dihalide adducts (NHC)SeX2 (X = Cl, Br, I, NHC = N,N ′-di-n-butylbenzimidazolin-2-ylidene) have been used to produce tellurium adducts (NHC)TeX2 from the tellurourea (NHC)Te via a metathesis reaction.990 Halogenation of chalcogenoureas (Ch = S, Se), including Nmethylene- and ethylene-bridged bis(selenoureas), using interhalogens IX (X = Cl, Br) has also been investigated.991,992 The main structural feature of chalcogen dihalide adducts (NHC)ChX2 is the T-shaped geometry of the chalcogen centers, indicative of two lone electron pairs present at these atoms. The X−Ch−X bond angles are almost linear and only slightly depend on X. For Ch = S and Se, they vary from 170° to 175°, while for Ch = Te, these bond angles are slightly more narrow (165−169°). The X−Ch−X plane in these adducts is almost perpendicular to the plane of the heterocycle. The CNHC−Ch bond distances are nearly the same for different X and slightly elongated, as compared to those of the corresponding chalcogenoureas (e.g., for 1088 (X = Cl), 1.884(2) Å; 1088 (X = Br), 1.901(11) Å vs 1.853 Å in

Figure 118. Dicationic tris(selenourea) 1096 and cationic adducts 1097 and 1098.

oxidation of the respective selenone (IMe2)Se with Cu(OTf)2.961 The Se−Se−Se bond angle is 174.11(2)°, and the average Se−Se bond length is 2.6501(8) Å. The CNHC−Se bond distances in 1096 change from 1.865(5) to 1.885(4) Å (shorter in terminal selenoureas) and are similar to that of the starting selenone (IMe2)Se (1.884(10) Å). Cationic adducts 1097 (CNHC−Se bond distance: 1.960(3) Å) and 1098 (Figure 118) have been obtained by reactions of IDipp with the corresponding cationic Se and Te ligands.994 2.6.4. Adducts with Three- and Higher-Coordinate Chalcogen Atoms Containing Ch−O Bonds. (NHC)SO2 adducts 1099 (Figure 119)995,996 have been obtained by reactions of free NHCs (SItBu, ItBu, and IMeEt) with sulfur dioxide. While reaction with SItBu afforded the stable adduct 1099a,995 the analogous product with an unsaturated backbone appeared to be unstable. In contrast, the adduct 1099b is a stable compound.996 Both adducts 1099a and 1099b were structurally characterized. The CNHC−S bond distance in 1099a is relatively long (2.030(2) Å), while in 1099b it is significantly shorter (1.859(1) Å). DFT calculations on 1099 (R = R′ = Me) showed that the sulfur atom accepts only a small additional charge from the carbene moiety, while most of the transferred electron density resides on the more electronegative oxygen atoms.995 Selenium dioxide adducts 1100 (Figure 119) have been obtained by oxidation of the corresponding selenones with aqueous solution of hydrogen peroxide.968 Structural data for these adducts are not reported. 9810


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carbon at 193.6 ppm, shifted around 25 ppm upfield as compared to the free NHC. 1H NMR spectroscopy confirmed that in solution the adduct 1105 exists in equilibrium with starting materials. Prolonged standing of a solution of 1105 at room temperature results in the cleavage of the I−Caryl bond, leading to pentafluorobenzene and the corresponding 2iodoimidazolium cation 1106a. The solid-state structure of 1105 was elucidated with single-crystal XRD analysis. The compound features a nearly linear central C−I−C moiety (178.9(2)°) with Caryl−I and CNHC−I bond distances of 2.159(3) and 2.754(3) Å, respectively. As expected, the NHC−I bond distance in 1105 is significantly longer than that in the 2-iodoimidazolium salt 1106a (2.131(11) Å). Zhang et al. reported the analogous compound 1106b by addition of I2 to the corresponding 2-hydroimiazolium iodide in the presence of KOtBu.999 Arduengo and co-workers further reported the bis(IMes) adducts 1107a,b (Figure 120) of an iodine (+1) cation by Figure 119. Adducts 1099−1104 containing Ch−O bonds.

The adduct 1101 (Figure 119; CNHC−S bond distance: 1.811(3) Å) has been obtained by reaction of the respective free carbene with thionyl chloride SOCl2.978 In contrast, reactions of this carbene with SO2Cl2, SO2ClF, SO2F2, as well as with SF4 led to the reduction of sulfur centers yielding the corresponding 2-haloimidazolium salts (cf., section 2.7). Subsequent treatment of 1101 with methyl iodide afforded the cation 1102. Reactions of 2-haloimidazolium salts [(IiPr2Me2)X][SO2X] (X = Cl, F) with aqueous solutions of KCN afforded the adduct 1103 (Figure 119; CNHC−S bond distance: 1.822(2) Å).997 It should be noted that acyclic urea−SO3 adducts can be obtained alternatively, by oxidation of thioureas with peroxides. Protonation of 1103 with HBF4 and HSbF6 gave salts 1104.894

Figure 120. Bis(NHC) adduct of an iodine (+I) cation.

addition of free IMes to the 2-iodoimidazolium salt in THF.1000 The compounds show a high melting point (255− 258 and 218−221 °C), consistent with their ionic structure, as well as good solubility and stability in acetonitrile. 1107b was characterized by FAB-MS (m/z = 735, corresponding to [(IMes)2I]+) and SC-XRD. Like in 1105, the central C−I−C unit is almost linear (177.5(2)°). A slight asymmetry can be observed regarding the two C−I bond lengths (2.286(4) and 2.363(4) Å). The 13C NMR signal of the carbene carbons in 1107b (155.2 ppm) is almost the average (163.5 ppm) of the free IMes (220 ppm) and the 2-iodoimidazolium salt (107 ppm). A variety of haloimidazolium salts have been reported over the last 20 years. Not surprisingly, they found synthetic applications as useful halogenating reagents or synthetic building blocks. Depending on the halogen nature (source), employed NHC (Lewis basicity/acidity), and solvent, reactions of free carbenes with various halide sources can result in three different NHC adducts A−C (Figure 121).

2.7. NHC Complexes of Group 17 Elements

Halogen-NHC adducts are mainly presented by haloimidazolium salts [NHC−X]+[Y]− with only a few examples of genuine NHC complexes reported so far. This section will be mostly limited to adducts synthesized directly from the free NHC. The first of such complexes was isolated by Arduengo et al. in 1991.998 Reaction of the free NHC IAd with iodopentafluorobenzene in THF results in the 1:1 adduct 1105 (Scheme 250). The complex shows a 13C NMR signal for the carbene Scheme 250. Formation and Decomposition of 1105

Figure 121. Possible structures of halide adducts (A,B) obtained upon halogenation of NHCs (X = F, Cl, Br, I).

Heavier halogens (I, Br, and Cl) generally form the adducts of types A (charge-transfer complexes) and B (2-haloimdazolium salts), while the C2 dihalogen structure C is usually reserved for fluorine.17,23 Halide sources include dihalogens (Cl2, Br2, I2), alkyl halides (e.g., C2Cl6, (CH2Cl)2, (CH2Br)2), and other halogenation reagents (e.g., SO2Cl2). 9811


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(IMes)AsSiMe3 (1037b)907 (vide supra). Interestingly, in contrast to N-aryl 2-chloroimidazolium chlorides (B, Figure 122), their difluoro analogues, like 1110a,b, are highly moisture sensitive, easily forming the corresponding ureas upon hydrolysis. Scheme 251 gives an overview of a variety of halogenation reactions of free NHCs with different halide sources. Reaction of IEt2Me2 or IiPr2Me2 with elemental iodine leads to the charge-transfer complexes 1111a,b.1007,1008 The nearly linear geometry of the C−I−I fragment (176.0(1)° and 178.8(2)°, respectively) and elongated I−I bond lengths (3.348(1) and 3.348(6) Å, respectively) clearly show the hyper-valency of the central iodine atom. While the structure of 1111a is retained upon dissolving in dipolar solvents, protic solvents cause ionic dissociation to the corresponding 2-iodoimidazolium iodide. Complexes 1111a,b can essentially be considered as the isolated transition state of a nucleophilic attack of an NHC on I2.1007 Kuhn et al. further reported the 2-chloroimidazolium salts 1112a (X = SO2Cl−)1009 and 1112b (X = SO2F−),978 which are formed upon addition of SO2Cl2 (or SO2ClF) to free IiPr2 Me 2. They further described the synthesis of 2chloroimidazolium chlorides 1113a−c by addition of 1,2dichloroethane to the free NHCs.1010 The resulting chargetransfer complexes are able to chlorinate benzene as well as perform various chlorination, oxidation, and reduction reactions.1010−1013 Reaction of 1113c with SO2 leads to complex 1112a. Additionally, 1113c can also be synthesized by reaction of C2Cl6 with free NHC.1014 Complex 1113c shows a near linear bond angle of the C−Cl−Cl fragment (166.1(1)°) and an elongated Cl−Cl bond (3.159(3) Å). Interestingly, reaction with CH2Cl2 does not yield a C2-chlorinated

The 2-fluoroimidazolium salt 1108a (Figure 122) was obtained by addition of SF4 to the free NHC IiPr2Me2.978

Figure 122. 2-Fluoroimidazolium salts 1108 and difluoro-NHCs 1109 and 1110.

While Kuhn et al. expected the coordination of the NHC to the sulfur center, only the formation of the imidazolium salt with SF3− as the anion was observed. Similarly, the imidazolium salt 1108b was obtained upon reaction with SO2F2. For the sake of completeness, the difluoro-compound 1109 should also be mentioned, even though the synthesis follows a different pathway (reaction of chloroamidinium chloride and 1,3-dimethyl-2-imidazolidinone with potassium fluoride). The compound was utilized to isolate different Ge and Sn NHC complexes (vide supra).707 The related difluoro derivative 1110a with Dipp wingtip substituents can be isolated by fluorination of the corresponding 2-chloroimidazolium chloride salt with excess CsF.1001 This commercially available flurinating agent (as PhenoFluor) is effective for late-stage (deoxy)fluorination of complex alcohols and heteroaromatics as well as for the formation of alkyl-aryl-ether bonds and nucleophilic aromatic fluorination.1001−1006 Because of its useful fluorinating properties, the compound was also used to obtain (IDipp)PSiMe3 (932)831 and (IDipp)AsSiMe3 (1037a)907 adducts. Similarly, the adduct 1110b was employed in the synthesis of

Scheme 251. Various Haloimidazolium Salts 1111−1118 Derived from Free NHCs by Reaction with Halogen Sourcesa

a 1

R = H, alkyl; R2 = alkyl, aryl. 9812


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compound, but an N-heterocyclic olefin instead (cf., section 2.4.1).999 Jones and Junk et al. also described various reactions of free NHCs with different halide sources. Reaction of IMes with C2Cl6 in acetonitrile results in the 1113a−c analogue 1114. However, unlike the charge-transfer complexes 1113a−c and 1118a,b, no close contacts of the chloride anion to the C2− chlorine can be observed. This is presumably due to the synthesis being carried out in acetonitrile, which favors the ionic dissociation (cf., 1111). Reaction of IMes with Br2 in THF leads to the chargetransfer complex 1115 with a bromine−bromide contact of 3.113(8) Å (which is significantly longer than Br2 (2.28 Å)) and a nearly linear C−Br−Br bond angle of 176.6(6)°.1015 On the other hand, reaction of IMes with 1,2-dibromoethane results in a mixture of 1115 and the corresponding 2hydroimidazolium bromide salt. This is in contrast to the reactivity observed for 1,2-dichloroethane (vide supra, 1113a− c).1010 Furthermore, reaction of IMes with CBr4 leads to the backbone brominated free NHC IMesBr 2 (1116). 1015 Arduengo et al. described the similar reaction with 2 equiv of CCl4, providing the analogous backbone-chlorinated NHC IMesCl2 (1117).935 Interestingly, carbene 1117 is stable in air for several days. Reaction with additional CCl4 finally leads to the backbone chlorinated 2-chloroimidazolium chloride salt 1118a. Again, SC-XRD analysis showed a close contact of the chloride anion and the C2−chlorine substituent with an almost linear C−Cl−Cl bond angle (175°). A saturated analogue of imidazolium salt 1114 was also obtained by the reaction of the free NHC with 0.5 equiv of CCl4.935 Bertrand et al. reported the synthesis of C4functionalized imidazolin-2-ylidenes by addition of KHMDS to 2-halo-substituted imidazolium halides.561 In addition to the previously mentioned 2-bromoimidazolium compound 1115 by Jones and Junk et al., Kuhn and coworkers reported three related 2-bromoimidazolium derivatives 1119−1121 (Figure 123).1008 Reaction of IiPr2Me2 with

TeBr4 lowers its nucleophilicity significantly, leading to the 2bromoimidazolium pentabromotellurate 1121.1008 A significant lengthening of the closest bromine−bromide interaction can be observed (3.745(4) Å). Kuhn et al. also isolated a BrCF2CF2I bridged bis(2iodoimidazolium bromide) 1122 by addition of the free NHC to the alkyl halide in Et2O. The compound exhibits near linear C−I−Br moieties (177.8(8)° and 174.1(7)°) as well as short Br−Br (3.313(7) Å) and Br−I (3.220(6) Å) contacts to the BrCF2CF2I-linker.1016 Apart from classical NHCs, halogen adducts with other carbenes are also reported. Dihaloimidazolidinediones 1123 (Figure 124), which can be considered as diamidocarbene

Figure 124. Halogen adducts of DACs and CAACs.

(DAC) derivatives, are known since 1959.1017 These are mainly presented by chloro derivatives (X = Cl), generally synthesized by condensation of carbodiimides RNCNR with oxalyl chloride.1018 Akin to difluoroimidazolines 1110, dichloroimidazolidinediones 1123 easily undergo hydrolysis or alcoholysis. More recently, the group of Bielawski reported the N,N′-dicyclohexyl derivatives 1123 (X = Cl, Br, I) as versatile (deoxy)halogenating reagents.1018 Notably, structural analysis revealed two halogen atoms covalently bound to the C2 carbon in all three derivatives, even with X = I. The signal of the C2 carbon of the diiodo derivative, observed at −19.9 ppm in 13C NMR spectrum, is significantly upfield-shifted as compared to its bromo (66.6 ppm) and chloro (102.9 ppm) analogues. It should also be noted that reduction of dichloroimidazolidinediones bearing relatively bulky N-substituents with potassium metal is a convenient synthetic method to approach the corresponding five-membered DACs.473 The CAAC derivative 488 (Figure 124),57,520 reported by the Bertrand group, was obtained in high yields (90−93%) from the reaction of Et2CAAC with bromine and found some interesting synthetic applications (e.g., the synthesis of the radical cation 473b•+,520 vide supra). The resonance of the C2 carbon of the CAAC moiety of 488 in the 13C NMR spectrum (CDCl3) was observed at 191.1 ppm. Another CAAC derivative, described by Bochmann and Romanov, was obtained as a mixture of 1124a,b or 1125a,b (Figure 124) upon oxidation of respective (AdCAAC)AuX complexes with either PhICl2 or CsBr3 in dichloromethane at ambient temperature. Assumably, the reaction proceeds through the intermediate formation of (AdCAAC)AuX3 as a primary oxidation product and further photoinduced reductive elimination of X2, finally leading to the CCAAC−Au bond cleavage and formation of 1124 and 1125. Bromoiminium salt 488 was employed in the synthesis of the BICAAC derivative 1126 (Scheme 252).57 Reaction of 488 with the strongly nucleophilic and electrophilic free BICAAC is comparably fast, irreversible, and quantitative, suggesting

Figure 123. 2-Bromoimidazolium bromides 1119−1121 and the I(CF2)2Br-bridged 2-iodoimidazolium bromide dimer 1122.

Br2 results in the charge-transfer complex 1119 with structural parameters (d(Br−Br) = 3.101(7) Å, C−Br−Br = 176.3(3)°) similar to those of the mesityl-substituted complex 1115 (3.113(8) Å, 176.6(6)°). The CBr4 adduct 1120 was isolated by utilization of excess CBr4 instead of Br2. Interestingly, no backbone functionalization (cf., 1116) was observed. The close contact of the C2−bromine with the bromide remains nearly unchanged (d(Br−Br) = 3.190(16) Å, C−Br−Br = 170.6(6)°). However, as expected, coordination of the bromide anion to 9813


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Notes

Scheme 252. Synthesis of the Bromoiminium Bromide 1126 by Ligand Exchange Reaction

The authors declare no competing financial interest. Biographies Vitaly Nesterov received his Ph.D. in organophosphorus chemistry at the Institute of Bioorganic and Petrochemistry, National Academy of Science of Ukraine, under the supervision of Prof. O. I. Kolodiazhnyi (Kyiv, Ukraine). After research stays in Prof. R. Streubel’s group at Bonn University (phosphinidenoid chemistry) and Prof. N. Tokitoh’s group at Kyoto University (phosphasilene chemistry), he joined Prof. S. Inoue at the Technische Universität München. His research interests lie in the field of low-coordinate compounds of group 14 and 15 elements.

stronger binding properties of the BICAAC ligand as compared to Et2CAAC. It present an example of NHC ligand exchange reaction at a main group element center. In the 13C NMR spectrum of 1126, the C2 carbon resonates significantly upfield (187.5 ppm, CDCl3) as compared to that of the free ligand (334 ppm, C6D6). Alternatively, 1126 can be synthesized in good yields using a traditional approach, the reaction of the free carbene with bromine.57

Dominik Reiter received his B.Sc. and M.Sc. degrees from the Department of Chemistry at the Technische Universität München in 2013 and 2015 under the supervision of Prof. K.-O. Hinrichsen and Prof. B. Rieger. He then joined the group of Prof. S. Inoue for his Ph.D. studies in 2016. His current research interest focuses on the synthesis, characterization, and reactivity studies of novel silanones. Prasenjit Bag received his Ph.D. in 2014 from the Indian Institute of Technology Kanpur, India, under the supervision of Prof. V. Chandrasekhar. Afterward, he spent a short time as a postdoctoral fellow at the Saarland University under Prof. David Scheschkewitz, where he worked on the multiple-bonded silicon compounds. Since March of 2016, he joined the group of Prof. Shigeyoshi Inoue at the Technische Universität München (TUM). His current area of research includes the synthesis of multiple-bonded aluminum compounds and their applications in small molecule activation reactions.

3. CONCLUSION AND PERSPECTIVES This Review highlights the progress achieved by exploring the synthetic potential of NHCs in main group element chemistry. Nowadays this area is very broad, ranging from applications of NHCs and their adducts as organocatalysts to their use for stabilization of various elusive molecules. NHCs appeared to be superior to many other Lewis base ligands for the stabilization of low-valent and low-coordinate main group elements, which has been exemplified by dramatic recent achievements in the chemistry of group 13, 14, and 15 elements. Here, numerous novel classes of compounds with interesting properties have been isolated and investigated. Many of them, being chemically less active due to the stabilization effect of NHC ligands, preserved unique reactivity and can, similar to transition metals, activate unreactive bonds under mild conditions. On the other hand, NHC ligands can also be removed, which is already used in laboratories to access some compounds. Investigation of various NHC−main group element adducts also contributed significantly to the development of the modern concept of donor−acceptor interactions in main group chemistry. In the near future, further increased research attention to this field is expected. More rational approaches using highly tunable NHC ligands combined with effective substitution patterns at the elements will allow one to access novel kinds of adducts, including stable open-shell species and novel suitable reagents for activation of such unreactive substrates as dinitrogen. The potential of NHCs and NHC−main group adducts will further be explored in catalysis. Driven by a great interest in novel energy storage devices and organic electronics, increased attention will be paid to the development of novel NHC-based stable redox-active systems and conjugated materials.

Philipp Frisch graduated from the Department of Chemistry at the Technische Universität München with a B.Sc. in 2012 and M.Sc. in 2015 under the guidance of Prof. Fritz E. Kühn. In 2016 he joined the group of Prof. Inoue as a Ph.D. student. He is currently working on NHC-stabilized low-valent silicon compounds and their transition metal complexes, in particular silyliumylidene ions. Richard Holzner obtained his B.Sc. and M.Sc. degrees from the Technische Universität München in 2012 and 2016 in the group of Prof. Rieger. He joined Prof. Inoue’s group in 2016 for his doctoral studies. His research interests focus on the synthesis of novel silicon radicals and their application in organic radical batteries. Amelie Porzelt received both her B.Sc. and M.Sc. degrees in chemistry from the Technische Universität München. In 2016 she joined the group of Prof. Shigeyoshi Inoue in Munich as a Ph.D. student. Her research focuses on combined experimental and theoretical studies on the reactivity of silyliumylidenes. Shigeyoshi Inoue studied at the University of Tsukuba and carried out his doctoral studies under the supervision of Prof. Akira Sekiguchi, obtaining his Ph.D. in 2008. As a Humboldt grantee as well as a JSPS grantee, he spent the academic years 2008−2010 at the Technische Universität Berlin in the group of Prof. Matthias Drieß. In 2010, he established an independent research group within the framework of the Sofja Kovalevskaja program at the Technische Universität Berlin. Since 2015 he has been on the faculty at the Technische Universität München (TUM). His current research interests focus on the synthesis, characterization, and reactivity investigation of compounds containing low-valent main group elements (groups 13, 14, and 15) with unusual structures and unique electronic properties, with the goal of finding novel applications in synthesis and catalysis. A particular emphasis is placed on low-coordinate silicon compounds.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Shigeyoshi Inoue: 0000-0001-6685-6352 9814


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ACKNOWLEDGMENTS

m MAC Me MeCN Mes Mes* MMA Mu Nacnac NaNaph NBO NBS NHC NHCD NHI NICS NIR NIS NMR Np o OTf p p-Tol PDMS Ph Pr py rt SC-XRD SOMO t Tbb

meta (heterocyclic) N-monoamidocarbene methyl acetonitrile mesityl, 2,4,6-trimethylphenyl supermesityl, 2,4,6-tri-tert-butylphenyl methyl methacrylate muonium a β-diketiminate ligand sodium naphthalenide natural bond order N-bromosuccinimide N-heterocyclic carbene N-heterocyclic dicarbene (anionic) N-heterocyclic imine nucleus-independent chemical shifts near-infrared N-iodosuccinimide nuclear magnetic resonance neopentyl ortho trifluoromethylsulfonyl, OSO2CF3 para para-tolyl, 4-methylphenyl polydimethylsiloxane phenyl propyl pyridine room temperature single-crystal X-ray diffractometry singly occupied molecular orbital tert 2,6-bis[bis(trimethylsilyl)methyl]-4-tert-butylphenyl [2,6-[CH(SiMe3)2]2-4-tBu-C6H2] Tbt 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, 2,4,6[CH(SiMe3)2]3-C6H2 TDDFT time-dependent density functional theory Ter terphenyl, diphenylbenzene TEMPO 2,2,6,6-tetramethylpiperidinyloxyl THF tetrahydrofuran THP tetrahydropyran THT tetrahydrothiophene Tipp 2,4,6-triisopropylphenyl TMP 2,2,6,6-tetramethylpiperidinyl TMS trimethylsilyl Tsi tris(trimethylsilyl)methyl UV−vis ultraviolet−visible vs against (Latin) VT variable temperature WBI Wiberg bond index vide infra see below (Latin) vide supra see above (Latin) XRD X-ray diffraction

Financial support from WACKER Chemie AG and the European Research Council (SILION 637394) is gratefully acknowledged.

ABBREVIATIONS AIBN 2,2′-azobis(2-methylpropionitrile) aIDippH 1,3-bis(2,6-diisopropylphenyl)imidazolin-4-ylidene aNHC abnormal (mesoionic) N-heterocyclic carbene Ar aryl Bbp 2,6-bis[bis(trimethylsilyl)methyl]phenyl, 2,6-[CH(SiMe3)2]2C6H3 Bbt 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl, 2,6-[ CH(SiMe3)2]2-4-C(SiMe3)3C6H2 BCF tris(pentafluorophenyl)borane, B(C6F5)3 BICAAC bicyclic (alkyl)(amino)carbene Bn benzyl Bu butyl CAAC cyclic (alkyl)(amino)carbene Ch chalcogen (group 16 element) COD 1,5-cyclooctadiene Cp cyclopentadienyl Cp* pentamethylcyclopentadienyl CV cyclic voltammetry Cw continuous-wave Cy cyclohexyl d day(s) d (bond) distance DAC (heterocyclic) N,N′-diamidocarbenes DFT density functional theory Dipp 2,6-diisopropylphenyl DMAP 4-dimethylaminopyridine DME 1,2-dimethoxyethane Dsi bis(trimethylsilyl)methyl, CH(SiMe3)2 Dur duryl [2,3,5,6-tetramethylphenyl] EMind 1,1,7,7-tetraethyl-3,3,5,5-tetramethyl-s-hydrindacen-4-yl Eind 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl equiv equivalent et al. and others (Latin) EWG electron-withdrawing group Fc+/Fc ferrocenium/ferrocene FLP frustrated Lewis pair GIAO gauge-including atomic orbitals h hour(s) hfcc hyperfine coupling constant HMDS hexamethyldisilazide HOMO highest occupied molecular orbital i iso IDipp 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene IMe4 1,3,4,5-tetramethylimidazolin-2-ylidene IiPr2Me2 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene IMes 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene IPr* 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazolin-2-ylidene IR infrared ItBu 1,3-di-tert-butylimidazolin-2-ylidene KCp* potassium pentamethylcyclopentadienide LDA lithium diisopropylamide LiNaph lithium naphthalenide LUMO lowest unoccupied molecular orbital

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