Electronic Properties of NâHeterocyclic Carbenes ... - ACS Publications

Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

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Electronic Properties of N‑Heterocyclic Carbenes and Their Experimental Determination Han Vinh Huynh* Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543, Republic of Singapore ABSTRACT: N-Heterocyclic carbenes (NHCs) have become without doubt one of the most exciting and popular species in chemical science due to the ease of their preparation and modularity in stereoelectronic properties. Numerous types of NHCs have been prepared, and various experimental methodologies have been proposed for the study of their electronic properties in order to rationalize reactivities observed. The objective of this article is to provide a comprehensive overview of the most common and popular ones among them. In particular, these include the nickel(0)-based TEP, its rhodium(I) and iridium(I) variants, LEP and related electrochemical methods, the palladium(II)-based HEP, phosphinidene- and selenourea-based methods, as well as the use of direct 1J(C−H) coupling constants of the precarbene carbon in azolium salts. Each individual method and the underlying principle of detection it utilizes will be critically discussed in terms of strength and weakness. In addition, comprehensive amounts of data from various NHCs are compiled for the purpose of comparison. These are also meant to help the scientist in better understanding their own research data and possibly providing directions for their future research, which rely on the unique electronic properties of NHCs.

CONTENTS 1. Introduction 2. Tolman Electronic Parameter (TEP) 3. Rhodium(I) Carbonyl-Based System 3.1. Preparation of Rhodium(I) Complex Probes 3.1.1. Cleavage of Enetetramines 3.1.2. Cleavage of Rhodium Dimers with Free NHCs 3.1.3. Cleavage of Rhodium Dimers with in Situ Generated NHCs 3.1.4. Silver-Carbene Transfer Route 3.1.5. Other Less Common Methods 3.2. Stability of cis-[RhCl(CO)2(NHC)] Probes 3.3. Electronic Properties of NHCs on the Rhodium(I) Scale 3.3.1. Influence of the Halido Ligands 3.3.2. Influence of the Solvent/Media 3.3.3. Other Factors of Influence 3.3.4. Detection of Substituent Effects 3.3.5. Detection of Backbone Effects 3.3.6. Interconversion of RhI Values into IrI Values and TEPs 4. Iridium(I) Carbonyl-Based System 4.1. Preparation and Stability of Iridium(I) Complex Probes 4.2. Electronic Properties of NHCs on the Iridium(I) Scale 5. Redox Potentials and the Ligand Electrochemical Parameter (LEP) 6. Huynh’s Electronic Parameter (HEP) 6.1. Preparation and Stability of Complex Probes 6.2. Understanding HEP 6.3. Electronic Properties of NHCs on the HEP Scale © XXXX American Chemical Society

7. NMR Spectroscopy of Carbene-Phosphinidene Adducts 8. NMR Spectroscopy of Selenoureas 9. 1J(C−H) Heteronuclear Coupling Constants of Azolium Salts 10. Conclusions and Outlook Author Information Corresponding Author ORCID Notes Biography Acknowledgments References

A C D F F F F G G G G H H H H I

Y AA AB AC AD AD AD AD AE AE AE

1. INTRODUCTION N-Heterocyclic carbenes (NHCs) have evolved from laboratory curiosities to state-of-the-art compounds, and nowadays there is no doubt that they have made significant impact in chemical science in general.1−8 In particular, their ability to act as strong donor ligands has been extensively explored in organometallic chemistry and catalysis. Their tremendous popularity can be attributed to several factors. (i) NHC ligands are highly modular leading to their large structural and stereoelectronic diversity. (ii) Transition metal−NHC complexes are easily prepared, and they usually contain strong metal−NHC bonds. Variations of NHCs are essentially limitless, and alterations in their structures can be made by many ways including changes

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Special Issue: Carbene Chemistry Received: February 1, 2018

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DOI: 10.1021/acs.chemrev.8b00067 Chem. Rev. XXXX, XXX, XXX−XXX

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increased back-donation can only occur in electron-rich or low valent complex fragments as opposed to electron-poorer systems. As such a weaker acceptor when bound to an electron-dense complex fragment could experience overall more back-donation than a stronger acceptor bound to a highly Lewis acidic metal center. The second important parameter is the steric bulk that a ligand imposes on the metal center. Compared to the electronic property, the sterics can often be easily estimated from the Lewis structure drawing of a ligand, especially with consideration of the arrangement of substituents in space. Notably, the steric bulk of a ligand can always overrule otherwise favorable electronic interactions. For example, binding of a bulky ligand could be less efficient than predicted solely on electronic grounds due to interligand repulsion, which prevents optimal orbital overlap. On the other hand, one can exploit the electronic dilution by steric interferences to achieve unusual reactivity and regio- or stereoselectivity. These two important factors characteristic for all ligands are together referred to as a ligand’s stereoelectronic property. Ultimately, the bonding picture of a complex is a result of a complicated interplay between several factors including oxidation state, overall charge, and stereoelectronic interactions between the metal center and the various ligands present. The stereoelectronic properties of NHCs, which all contain the carbene donor and at least one nitrogen atom within a ring system, can be fine-tuned by variation of the ring substituents R and the heterocyclic backbone. For the classical NHCs A−D (Chart 1), backbone modifications primarily change the electronics, although the NCN angles may also be slightly affected. Nevertheless, these changes are anticipated to be small and the steric impact through these changes is not very great. In order to understand how these variations affect the electronic property of NHCs, one must first know inductive (I) and mesomeric (M) effects of heteroatoms and functional groups. Electron-withdrawing atoms or groups with respect to the carbon atom exert a negative inductive effect, i.e. −I effect, while electron-releasing functions possess a positive inductive effect, i.e. +I effect. The relative inductive effects of some common functional groups are ranked as follows in terms of decreasing −I or increasing +I effects.

of the heterocyclic backbone and at least two ring-substituents. The most common types of NHCs are derived from imidazole, benzimidazole, imidazoline, and 1,2,4-triazole, but there are many other nonclassical types that are becoming increasingly popular (Chart 1). Many nonclassical NHCs even surpass their classical counterparts in terms of electron donation. Chart 1. Selected Classical (A−D) and Nonclassical NHCs (E−H)

In general, all ligands are characterized by two important parameters that have profound influences on the properties and reactivities of their respective complexes, and NHCs are by no means an exception. The first is the electronic parameter, which contains major contributions from σ- or π-donation as well as π-accepting properties. These are inherently linked to the ligand’s frontier orbitals. For example, the donating ability is associated with the energy level of the highest occupied molecular orbital (HOMO). The higher the HOMO energy of a ligand, the stronger is its electron donating ability. The electron acceptor capability of a ligand is conversely characterized by the energy level of its lowest unoccupied molecular orbital (LUMO). The lower the LUMO energy, the better is the accepting property of a ligand. Figure 1 illustrates a qualitative comparison of the donating and accepting properties among four generic ligands I−IV on

NH3+ > NO2 > SO2 R > CN > SO3H > CHO > COR > COOH > COCl > CONH 2 > F > Cl > Br > I > OR > OH > NH 2 > C6H5 > CHCH 2 > H

Figure 1. Qualitative comparison frontier orbitals of ligands I−IV.

> CH3 > CH 2CH3 < CH(CH3)2

grounds of their frontier orbitals, with species I being the “baseline ligand”. Ligand II is the best donor and at the same time the poorest acceptor, while III is the weakest donor but also the strongest acceptor. The electronic changes going from I to II or III reflect the electronic properties of stereotypical tertiary phosphines and phosphites, where donation and acceptor capabilities are inversely correlated. Species IV, on the other hand, behaves differently. Compared to I, it is a better donor, but also a better acceptor. Indeed, a change from I to IV is characteristic for some NHCs. Although the origins of these donor and acceptor contributions are different, it is apparent that they are not entirely independent from each other within a chosen ligand, and therefore it is very difficult to quantify them individually (vide inf ra). Moreover, it is also true that the overall electron density at the metal center as a result of its oxidation state and the presence of other coligands has to be critically considered when discussing electronic ligand contributions. For instance, it is clear that

The mesomeric effect is reflected in an atom’s or group’s ability to provide a lone-pair for the formation of a multiple bond, i.e. +M effect, or to withdraw a lone pair from a multiple bond, i.e. −M effect, under formation of resonance structures. Thus, it is also referred to as a resonance effect, and +R/−R symbols are sometimes used instead.9 The +M and −M effects of some common substituents decrease in the following orders. Decreasing +M effect: O− > NH 2 > NHR > OR > NHCOR > OCOR > Ph > F > Cl > Br > I

Decreasing −M effect: NO2 > CN > CHO > COR > CO2 COR > CO2 R > CO2 H > CONH 2 > CO2− B


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there is consensus in the community that some NHCs do act as significant π-acceptors. Due to the historical perception, there were comparatively fewer attempts to evaluate the π-accepting properties of NHCs. Nolan and co-workers proposed the use of 1J(Pt−C) heteronuclear coupling constants in trans[PtCl2(DMSO)(NHC)].22 However, only 5 NHCs were compared. More recently, Bertrand suggested the use of 31P NMR spectroscopy on NHC-phosphinidene adducts.23 Along the same line, Ganter proposed the use of 77Se NMR spectroscopy on NHC-selenium adducts (selenoureas).24 Due to the easier access to these selenoureas a larger scope of NHCs has been evaluated. Most recently, Ganter also proposed the used of 1 J(C−H) coupling constants in azolium salts as a measure for the σ-donating ability of the respective NHC.25 All in all, these are currently the most popular experimental methodologies to determine and quantify stereoelectronic properties of NHCs. A few recent articles already provide a detailed account on the steric %Vbur parameter,13,14 which will therefore not be covered in this work. The aim of this article is to provide a detailed, critical, and up-to-date overview on the aforementioned electronic parameters with regards to NHCs. Comprehensive data libraries for monodentate NHCs studied will be given for each methodology to facilitate comparison including the recent literature up to the early/mid 2017. Bridged dicarbenes, Janus-type NHCs, and other related nonNHC species, such as acyclic diaminocarbenes, carbodicarbenes, etc., are not explicitly covered in this article. However, similar considerations with respect to their electronic properties do apply as well. Finally, computational predictions, which at times can be very useful, are also beyond the scope of this review.26

It is easy to understand that components with +I and +M effects increase the electron density of a NHC making it a better donor, while components with −I and −M effects show the opposite behavior. However, some substituents have opposing electronic effects and complicate the situation. For example, halo groups (i.e., F, Cl, Br, I) bound to a carbon atom exhibit a −I effect due to their increased electronegativity, but they also have a +M effect as a result of three lone-pairs available for donation that need to be considered as well. Throughout the years, various methodologies have been developed to evaluate stereoelectronic properties of ligands (Chart 2). The Tolman electronic parameter (TEP)10 and cone Chart 2. Experimental Methods Used for the Donor− Acceptor Evaluation of NHCs

angle (ϕ)11 introduced in the 1970s are named after their developer and have become very useful tools in organometallic chemistry particularly for phosphine ligands. TEP, which detects the net electronic influence comprising both donor and acceptor properties by IR spectroscopy, has also been applied for the evaluation of NHCs, but due to the intrinsic toxicity of nickel−carbonyl compounds, it has since been extended to more popular and less toxic iridium(I)- and rhodium(I)-based systems. Nevertheless, the use of toxic carbon monoxide gas is often still required in the preparation of the respective complex probes. In contrast to TEP, the Tolman cone-angle cannot be used for NHCs, since these do not coordinate in a cone-shaped manner. Different from the substituents in phosphines, which point away from the metal center, those of NHCs point toward the metal center thus creating a greater steric impact. For the evaluation of the latter, the so-called “percent volume buried” (%Vbur) was developed by Nolan and Cavallo.12−16 The ligand electrochemical parameter (EL) developed by Lever and also referred to as Lever’s electronic parameter (LEP) is more often used in Werner-typical coordination chemistry, where they can be used to predict redox-potentials of complexes.17,18 On the other hand, LEP values for NHCs are uncommon. Instead, redox-potentials of NHC complexes have been directly used to compare the electronic contributions of individual NHC ligands by electrochemical means. More recent contributions to electronic parameters make use of multinuclear NMR spectroscopy. Huynh’s electronic parameter (HEP) utilizes the 13Ccarbene NMR chemical shift of the iPr2-bimy ligand in trans-[PdBr2(iPr2-bimy)L] complexes to draw conclusions about the donor ability of the trans-ligand L.19,20 It primarily detects the σ donor strength and can be used for both organometallic (e.g., NHCs, phosphines, isocyanides) and Werner-typical ligands (e.g., amines, pyridines, etc.). This method has also been extended to bidentate ligands.21 Historically, NHCs have been considered to act primarily as σ donors with negligible π-acceptor properties. Nowadays,

2. TOLMAN ELECTRONIC PARAMETER (TEP) The carbonyl ligand is a very popular ligand in organometallic chemistry that forms stable complexes with low-valent metal centers, which often fulfill the 18-electron rule. As one of the strongest field ligands, it is an exceptionally strong π-acceptor that easily engages in metal-to-carbonyl backdonation. The latter leads to a weakening of the initial CO triple bond, which can be quantified by IR spectroscopy. An additional advantage is that the carbonyl band is intense and appears in a unique frequency range, which can be regarded as independent from other vibrations. The Tolman electronic parameter exploits this fact and uses the A1 CO stretching frequency in tetrahedral complexes of the type [Ni(CO)3L] to draw a conclusion of the donor−acceptor ability of a ligand L.10 A stronger net donor would make the complex more electronrich, increasing the amount of backdonation to CO and further weakening the CO bond, and a smaller wavenumber is obtained. Conversely, a nickel complex with a weaker net donor or a better acceptor would be less electron rich, and a relatively larger wavenumber is obtained. The TEP was originally developed for tertiary phosphines and phosphites, for which σ-donation and π-acceptor properties are approximately inversely proportional. The [Ni(CO)3(PR3)] complex probes were synthesized by exposure of phosphine to a solution of [Ni(CO)4] in dichloromethane (Scheme 1, A) and then subjected to measurement in a 0.1 mm NaCl cell. After each run, the spectrum was calibrated using CO gas at 0.1 atm in a 10 cm cell, and many phosphines were ranked this way with a very optimistic standard deviation of σ(68%) = ± 0.5 cm−1 for the A1 band {σ(68%) = ± 2 cm−1 for the broader E band}.10 C


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(R,R)-1,3-(1-phenylethyl)imidazolin-2-ylidene ligand.29,30 Notably, two different values for the A1 band, i.e. 2050 and 2052 cm−1, were reported for this complex in THF by the same authors in different publications. In any case, a direct comparison with the previous values cannot be done due to the different solvents used. The first systematic study on the TEPs of NHCs was reported by Nolan and co-workers, who reacted seven common NHCs with a slight excess of [Ni(CO)4] in hexane or tetrahydrofuran.31,32 In all cases, evolution of CO gas was observed. However, for the most bulky NHCs IAd and ItBu displacement of two carbonyl ligands was observed, which led to the formation of interesting three-coordinate, orange/red [Ni(CO)2(IAd)] and [Ni(CO)2(ItBu)] complexes unsuitable for TEP determination. The presence of only two strong π-acceptors renders these complexes much more electron-rich, and backbonding to the two remaining carbonyl ligands is much more enhanced, giving rise to low wavenumbers of ν̃ = 2007 cm−1 and ν̃ = 2010 cm−1, respectively. On the other hand, off-white or beige [Ni(CO)3(NHC)] complex probes were obtained for the other five NHCs, and their TEP values were determined in dichloromethane and hexane. Later, the same group added data for sterically bulky, but flexible NHCs and methoxysubstituted NHCs.33−37 Comparison of their data revealed that the wavenumbers measured in hexane are usually larger than those in dichloromethane, revealing a significant solvent effect. More recently, Radius and co-workers also reported actual TEPs for three CAACs.38,39 Table 1 summarizes the data, and Chart 3 depicts the structures of NHCs for which actual TEP values have been obtained. In contrast to the data of phosphines, those obtained for these NHCs do not reveal any clear overall trend in terms of backbone and wing-tip variations. For example, unsaturated NHCs (e.g., IMe, IMes, IPr) appear overall to be equal or slightly better net donors than their direct saturated analogues (SIMe, SIMes, SIPr), which is in stark contrast to expectations and their predicted pKa values.40 Similarly, NHCs with N-alkyl do not always appear to be more basic than their N-aryl counterparts (e.g., IMe < IMes or IPr). Finally, the strongest net donor in the classical NHC series appears to be the bulky unsaturated NHCs IPent, IHept, and INon, where additional substituents on the aromatic wing tip cannot be further discerned. Surprisingly, the saturated SIMe is seemingly the weakest in this comparison. Overall, A1 bands of all classical NHCs evaluated only differ by 4 cm−1. All in all and critically viewed, these possibly surprising results already indicate some limitations of TEP and related carbonyl-based methods as highly accurate electronic parameters for NHCs. Notably, the conceptual weakness of the TEP has also been recently discussed by computational means.41 If one would apply Tolman’s very optimiztic standard deviation of σ(68%) = ± 0.5 cm−1 for the A1 band in phosphine complexes also for the mentioned [Ni(CO)3(NHC)] complexes, then the conclusion must be made that TEP cannot differentiate these NHCs within 3σ = ±1.5 cm−1 (>99%). For the structurally very different CAACs, however, lower wavenumbers were obtained, and a reasonable conclusion can be made that they are much stronger net donors than typical NHCs.

Scheme 1. Preparation of [Ni(CO)3(PR3)] and [Ni(CO)3(NHC)] Complex Probes for TEP Measurements

For the evaluation of NHCs by this method, complexes of the type [Ni(CO)3(NHC)] are required, which can be prepared in an analogous way by reaction of free NHCs or their dimers with [Ni(CO)4] (Scheme 1, B). However, the notorious toxicity and low boiling point of [Ni(CO)4] pose a severe limitation to this method. Therefore, only a limited number of NHCs have actually been experimentally evaluated using the nickel−carbonyl system. Early examples of such complexes were reported by Lappert and co-workers, who studied the reactivity of electron-rich enetetramines, i.e. dimers of saturated NHCs. Their complexes contained the saturated NHCs 1,3-dimethylimidazolidin-2-ylidene (SIMe) and 1,3-diethylimidazolidin-2-ylidene (SIEt), and the important stretching frequencies found in hexane are 2058 and 2055 cm−1, respectively (Table 1).27 Comparison between Table 1. A1 Carbonyl Bands [cm−1] in [Ni(CO)3L] Complexes Ligand

DCM

P(tBu)3 SIMe SIEt IMe (R,R)-IEt(Ph) IMes IPr SIMes SIPr ICy IPr* IPent IHept INon IPrOMe SIPrOMe IPr*OMe CAACMe CAACCy CAACMenthyl

2056

Hexane

THF

2058 2055 2055 2050/2052 2051 2052 2052 2052 2050 2053 2049 2049 2049 2050 2050 2051 2046 2046 2042

2054 2055 2055 2056 2052 2053 2053 2052

ref 10 27 27 28 29, 31, 31, 31, 31, 31, 33 34, 35 35 36 36 37 38 39 39

30 32 32 32 32 32 35

these two values could indicate that SIEt is a slightly better net donor, which is in line with the slightly better +I effect of the ethyl groups. Tolman reported a wavenumber of 2056 cm−1 in dichloromethane for P(tBu)3, which is the strongest donor in his original series. Although a quantitative comparison cannot be made due to the different solvents used, it is apparent that these NHCs are exceptionally strong donors. Ö fele and Herrmann were the first to report the reaction of a true free NHC, i.e. 1,3-dimethylimidazolin-2-ylidene (IMe) with [Ni(CO)4] more than a decade later.28 The respective complex [Ni(CO)3(IMe)] shows a stretching A1 frequency of 2055 cm−1 in hexane, which is surprisingly equal to SIEt and lower than its direct saturated analogue SIMe. Shortly after, Herrmann and co-workers described the preparation of another analogue bearing the chiral

3. RHODIUM(I) CARBONYL-BASED SYSTEM Due to the extreme toxicity of [Ni(CO)4], which makes handling difficult, an alternative and safer complex system for the donor strength evaluation by IR spectroscopy can be D


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Chart 3. Lewis Structures of NHCs and the A1 Carbonyl Wavenumbers Observed for Their [Ni(CO)3(NHC)] Complexes in Dichloromethane (Hexane)

considered. Mixed NHC/carbonyl complexes of rhodium(I) of the type cis-[RhX(CO)2(NHC)] (X = halide) appear to be a good choice as they are often stable to air and moisture. Indeed, the RhI-based system has become the most popular method for the electronic evaluation of NHCs. Almost every new NHC synthesized is routinely subjected to this method, and a large amount of data is available for comparison. In spite of its heavy use, the Rh-based system has seldom been analyzed in detail for its strengths and limitations. One shortcoming common to all (FT)IR spectroscopic methods is the relatively limited resolution (typically ∼4 cm−1) obtainable for liquid and solid samples, which is reflected in the broadness of the signals observed in IR spectra. Therefore, IR signals are commonly referred to as “bands” rather than “peaks”. cis-[RhX(CO)2(NHC)] complexes are unsymmetrical and generally exhibit two strong CO vibrational frequencies in the approximate range of 1950−2100 cm−1. The lower energy stretch typically from ∼1950 to 2000 cm−1 is often referred to as an “asymmetric” or “trans” band, while the higher energy stretch from ∼2050−2100 has been called a “symmetric” or “cis” band. The terms cis/trans are used with reference to the carbonyl ligand relative to the NHC mostly responsible for the vibrational stretch despite the fact that these vibrations are inherently coupled.

Among the A1 and E bands observed for the tetrahedral [Ni(CO)3(PR3)] complexes, Tolman chose to compare the sharper A1 stretch. Which of the two bands should be chosen for comparison in the square planar cis-[RhX(CO)2(NHC)] complexes? Some authors have preferred to use the “asymmetric” (trans-CO) band for comparison, since it is supposedly more affected by the NHC. Nevertheless, it has become the norm nowadays to compare the averaged value of both stretches (ν̃av) instead. In analogy to TEP, it is expected that a stronger net donor would induce more electron-density at the metal center, which in turn enhances π-backdonation to the carbonyl ligands leading to a weakening of the CO bond. Thus, a smaller averaged wavenumber (ν̃av) is therefore indicative of a stronger net donor. Another few issues which cannot be neglected in such comparisons are (i) the identity of the halido ligand X and (ii) the mode or media of the IR spectroscopic measurement. In some publications it has been suggested that the vibrational data is independent of the halido ligand. However, there is also ample evidence that such a statement cannot be generalized. Carbonyl-based methods essentially measure the amount of backdonation to the carbonyl ligand, which in turn is affected by the overall electron density of the complex. To assume that different halido ligands, which differ in their donating ability, E


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complexes from carbene dimers is more of historical value and is seldom practiced nowadays. 3.1.2. Cleavage of Rhodium Dimers with Free NHCs. Closely related to Lappert’s method is the reaction of 2 equiv of free NHC with [RhCl(COD)]2 to give [RhCl(COD)(NHC)] by cleavage of the rhodium−chlorido bridges. This can only be done with carbenes that do not dimerize due to electronic reasons (e.g., unsaturated imidazolin-2-ylidenes and 1,2,4-triazolin-5ylidenes) or an enhanced steric bulk. The use of a free NHC for this purpose was only reported a decade after Lappert’s report by Herrmann and co-workers, who employed the free 1,3-dimethylimidazolin-2-ylidene (IMe).43 The introduction of the carbonyl ligands is routinely done by subsequent reaction with CO gas. It is also possible to directly cleave the [RhCl(CO)2]2 dimer with free NHCs (or their enetetramines), which would give the desired complex probes in a single step (Scheme 3).43

would not alter the overall electron density of a complex is rather misleading (vide inf ra). Nevertheless, it has now become the norm to prepare chlorido complexes of the type cis-[RhCl(CO)2(NHC)] as electronic probes. The mode or media of IR data collection will also affect the wavenumbers, which should not be ignored. This is particularly important since IR spectra are commonly recorded as KBr pellets, as Nujol suspensions, in solution, and neat using ATR technology, etc. For data obtained from solutions, only those obtained using the same solvent should be compared. Although these statements are obvious, a survey revealed that not much emphasis was placed on them in early studies. Looking back at the historical development of this RhI-based system until now, the lack of systematicness in earlier studies becomes obvious. Different groups reported and compared data that were obtained by different means. On the other hand, such comparisons could be excused due to the general lack of data in the early days, and still more and more IR data recorded differently continued to be reported. In retrospect, some of the earlier conclusions made in older publications with regard to the donor strength of certain NHCs have to be reconsidered as carbonyl stretches are indeed and as expected sensitive to the media they are recorded in. Currently, the most common solvent for the IR spectroscopic evaluation of cis-[RhCl(CO)2(NHC)] is dichloromethane, and the most extensive data for more than 150 NHCs have been collected in this solvent, which are listed in Table 5 in increasing order of their ν̃av values. The structures of these ligands are shown in Chart 6 in order of increasing averaged carbonyl wavenumbers (ν̃av). In the following sections, methods for the preparation of the complex probes are summarized and their stability is discussed. This is followed by a critical discussion of the data obtained by this methodology.

Scheme 3. Preparation of cis-[RhCl(CO)2(NHC)] Using Free NHCs

3.1.3. Cleavage of Rhodium Dimers with in Situ Generated NHCs. Instead of using free NHCs, it is often more convenient to generate the NHC ligand in situ in the presence of the rhodium-COD dimer. Usually, this can be achieved via (i) α-elimination of the alcohol adducts or (ii) deprotonation of an azolium salt with a base. Both variants were pioneered by Herrmann and co-workers. For the first variant, they heated the tert-butanol adduct of SIMes with the rhodium precursor.44 Elimination of the alcohol from the neutral imidazolidine generates the [RhCl(COD)(SIMes)] complex in a one-pot procedure, which can be easily converted into cis-[RhCl(CO)2(SIMes)] (Scheme 4).

3.1. Preparation of Rhodium(I) Complex Probes

There are various methods for the preparation of the cis-[RhCl(CO)2(NHC)] complex probes that mainly differ in the way the NHC ligand is generated. The two common Rh precursors for all methods are bis[μ-chlorido(cyclooctadiene)rhodium(I)], i.e. [Rh(Cl(COD)] 2 and bis[dicarbonyl(μ-chlorido)rhodium(I)], i.e. [RhCl(CO)2]2. These complexes are commercially available, which has certainly contributed to the popularity of the Rh-based method. 3.1.1. Cleavage of Enetetramines. The first examples of such complexes were reported by Lappert and co-workers, who reacted electron-rich enetetramines with [RhCl(COD)]2 to yield yellow to orange complexes of the type [RhCl(COD)(NHC)] (Scheme 2).42 Subsequent COD displacement is

Scheme 4. Preparation of cis-[RhCl(CO)2(SIMes)] Complexes from an NHC-Alcohol Adduct

Scheme 2. Lappert’s Preparation of cis-[RhCl(CO)2(NHC)] Complexes from Enetetramines The second variant involves the use of [RhCl(COD)]2, an azolium salt, and an additional base. If the base, e.g. alkoxide, is added prior to the addition of the azolium salt, then formation of a new rhodium dimer [Rh(OR)(COD)]2 is proposed that acts as a basic metal precursor for the in situ deprotonation of the azolium salt (Scheme 5).45 If the base is added after the addition of the azolium salt then a direct deprotonation of the latter can be expected that leads to in situ formation of a NHC. The range of bases have been extended to include various alkali alkoxides,46 carbonates,47 and bis(trimethylsilyl)amides (e.g., KHMDS).48 Addition of the

accomplished by passing carbon monoxide gas through the solution of the latter, which afforded the cis-[RhCl(CO)2(NHC)] targets containing saturated imidazolidin-2ylidene ligands (NHC = SIMe, SIEt, SIPh, and SIPhoOMe). The IR data for the complexes were obtained in chloroform or as a Nujol mull, respectively. The preparation of such F


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complex probes have also been prepared using more exotic ways. For example, Raubenheimer and co-workers reported the preparation of the remote NHC complex cis-[RhCl(CO)(rNHC2)] by ligand transfer from a chromium rNHC complex [Cr(CO)5(rNHC2)] to [RhCl(CO)2]2 (Scheme 7).51 James and co-workers used a “reversed” protocol to prepare complex probes of IPr and IMes. In the first step the free carbenes were reacted the rhodium−cyclooctene dimer [RhCl(COE)2]2 to form the respective dimeric NHC complexes [RhCl(COE)(NHC)]2 by COE displacement. In a second step a combined COE displacement and bridge-cleavage reaction is initiated by CO gas giving the desired cis-[RhCl(CO)2(NHC)] probes as end products (Scheme 8).52

Scheme 5. Preparation of [RhCl(COD)(NHC)] Complexes via an Intermediate Rhodium-Alkoxido Species

latter to [RhCl(COD)]2 could give [Rh(N(SiMe3)2)(COD)]2 as a basic metal precursor.49 3.1.4. Silver-Carbene Transfer Route. In 2003, Crabtree and co-workers reported the first preparation of rhodium(I) and iridium(I) (vide inf ra) mixed NHC/carbonyl complexes via the silver-carbene transfer route, and they proposed their use as alternative complex probes for IR spectroscopic evaluation.50 The general preparative route for classical NHCs is outlined in Scheme 6 and commences with treatment of an azolium salt

3.2. Stability of cis-[RhCl(CO)2(NHC)] Probes

Many complexes of the type cis-[RhCl(CO)2(NHC)] have been reported to be stable to air and moisture. However, in some cases, decomposition has been observed, which hampered their purification. Decomposition has been reported to occur via entropically favorable loss of a carbonyl ligand and concurrent dimerization of the remaining complex fragment to give [RhCl(CO)(NHC)]2 (Scheme 9). This can already occur in

Scheme 6. Preparation of cis-[MCl(CO)2(NHC)] Complexes (M = Ir, Rh) Using the Silver Carbene Transfer Route

Scheme 9. Decomposition of cis-[RhCl(CO)2(NHC)] by CO Dissociation

solution at room temperature during crystallization attempts53,54 and is accelerated at higher temperatures.55 A detailed kinetic study of this reaction was conducted by Herrmann and co-workers using the three electronically different NHCs IBn, IBnCl2, and IBnCN2. IR spectroscopic analysis revealed that the CO dissociation is the rate-determining step. The rates of this reaction increase in the order IBn < IBnCl2 < IBnCN2, which suggests that cis-[RhCl(CO)2(NHC)] complexes with weaker donors or better π-acceptors are less stable compared to those with stronger donors. This notion is in line with experimental observations49,53−56 and highly plausible, since stronger donors would increase backdonation to CO and therefore strengthen the M−CO bond.

Scheme 7. Preparation of [RhCl(CO)(rNHC2)] by Ligand Transfer from a Chromium(0) Species

with silver(I) oxide, which leads to formation of the silver− NHC complex with elimination of water. Subsequent addition of [RhCl(COD)]2 or its iridium(I) analogue leads to carbene transfer from silver(I) to rhodium(I) or iridium(I) with concurrent cleavage of the chlorido bridges. Solutions of the resulting mononuclear COD complexes are then purged with carbon monoxide gas to furnish the desired cis-[MCl(CO)2(NHC)] complex probes that are then subjected to IR spectroscopy. This very popular method has also been extended to preparation of complex probes bearing nonclassical NHCs. 3.1.5. Other Less Common Methods. In addition to the common methods mentioned earlier, some rhodium−carbonyl

3.3. Electronic Properties of NHCs on the Rhodium(I) Scale

The electronic properties of numerous NHC ligands (>150 in CH 2 Cl 2 ) have been evaluated using their cis-[RhCl(CO)2(NHC)] complexes. In addition, there are also several NHC ligands that were studied using analogous complexes bearing other halido ligands, i.e. bromide and iodido. In some earlier studies, comparisons of IR data were done without paying strict attention to the halido ligands present in the complex probe or the media of IR spectroscopic measurement due to the general lack of systematic data. There have also been reports that stated that the nature of the halido does not affect

Scheme 8. Preparation of cis-[RhCl(CO)2(NHC)] Complexes from a Rhodium-Cyclooctene Complex

G


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the IR data significantly. The following sections attempt to critically address these issues. 3.3.1. Influence of the Halido Ligands. Table 2 shows the IR data collected for representative NHCs using the

Table 3. IR Data of Selected Identical cis-[RhCl(CO)2(NHC)] Complexes Collected in Different Media Given as ν̃av (ν̃COs) Media CH2Cl2

Table 2. IR Data of Selected cis-[RhX(CO)2(NHC)] (X = Cl, Br, I) Complexes Given as ν̃av (ν̃COs) Obtained in CH2Cl2

“ATR” KBr CDCl3 Media

NHC

cis-[RhCl(CO)2(NHC)]

cis-[RhI(CO)2(NHC)]

IMe Me2-pery Indy1 NHC

2041 (2000, 2082)57 2046 (2007, 2085)59 2030 (1991, 2069)60 cis-[RhCl(CO)2(NHC)]

2037 (2000, 2073)58 2028 (1994, 2061)58 2026 (1991, 2061)60 cis-[RhBr(CO)2(NHC)]

CH2Cl2

2045 (2005, 2085)61 2052 (2011, 2092)61

2030 (1989, 2071)61 2036 (1995, 2077)61

KBr Media

i

Pr2-bimy Cp*RuBI

IMes

IMe 54

2039 2038 2025 2031

(1996, (1996, (1984, (1990,

2081) 2079)63 2066)64 2073)52

2041 2038 2039 2016 2039

(1997, 2084)54 (1995, 2080)67 (1996, 2081)68 (1958, 2074)a,69 (1997, 2081)44 IBuMe

SIMes

CH2Cl2

chlorido and iodido or bromido containing complex probes, respectively. All measurements were conducted in dichloromethane. Comparison of the averaged values indicates significant differences in all three chlorido versus iodido cases with Δν̃av of 4 and 18 cm−1, respectively. The change from chlorido to iodido ligand leads to smaller CO wavenumbers, which is within expectations. Iodido ligands are better donors than chlorido analogues and thus should enhance backdonation from rhodium(I) to the carbonyl ligands. A closer inspection reveals that changes of the “symmetric” or “cis” stretches are more pronounced in these examples. This is also reasonable, since the impact is most severe for the carbonyl ligand trans to the halido ligand. The substitution of chlorido with bromido ligand shows the same trend, but here the differences are somewhat even greater possibly due to better rhodium−bromido interactions. Overall, the conclusion can be made that the nature of the halido ligands does indeed affect the amount of backdonation to the carbonyl ligands. Thus, a proper comparison can only be made using complexes bearing the same halido ligand. 3.3.2. Influence of the Solvent/Media. A non-negligible solvent effect was already noted for the nickel-based TEPs, where larger values were found in hexane compared to dichloromethane. To investigate solvent effects in the rhodium-based system, we have to compare IR values of a given NHC, which were measured in different solvents/media. 1,3-Dimesitylimidazolin-2-ylidene (IMes), 1,3-dimesitylimidazolidin-2-ylidene (SIMes), 1,3-dimetylimidazolin-2-ylidene (IMe), 1,3-(2,6diisopropylphenyl)imidazolin-2-ylidene (IPr), and 1-butyl-3methylimidazolin-2-ylidene are among the more popular NHCs, and a number of reports on the properties of their cis-[RhCl(CO)2(NHC)] complexes have appeared. In addition, the dimesityl-substituted six-membered ring NHC 6-Mes, the bimacrocyclic imidazolidin-2-ylidene SIBm, and the pyridoannulated carbene dipiy were also included in this comparison. Analysis of the data summarized in Table 3 shows the expected solvent influence on the position of the carbonyl stretches, although there is no general trend common to all NHCs. Instead, the extent of solvent-induced differences varies from ligand to ligand. The largest difference of Δν̃av = 19 cm−1 (CH2Cl2 vs DMSO) is noted for the pyrido-annulated NHC ligand dipiy.62 All in all, it appears that measurements using ATR (neat) result in the smallest wavenumbers followed by those in KBr, dichloromethane, and chloroform, respectively. As for TEP, an oftentimes very significant solvent/media effect on the carbonyl stretches is observed. This highlights

“ATR” Nujol Media CH2Cl2 KBr DMSO a

2028 (1989, 2066)71 2037 (1995, 2079)72 SIBm 73

2032 (1995, 2069) 2023 (1984, 2062)73

2041 (2000, 2082)57

2041 (2006, 2076)43 2046 (2004, 2087)65 IPr 2038 (1996, 2079)66

2030 (1990, 2070)52 6-Mes 2029 (1987, 2071)67 2032 (1988, 2076)70 2019 (1976, 2062)46 dipiy 2043 (2003, 2082)62 2033 (1993, 2073)62 2024 (1984, 2064)62

Values in italics are far off and possibly erroneously reported.

once again that comparisons of such IR data cannot be properly done without taking the media and mode of measurement into consideration. Thus, proper reporting of such data must include information about the media and mode of its collection. 3.3.3. Other Factors of Influence. Table 3 also reveals that even analysis in the same solvent can give different results for the same NHC, i.e. IMes, SIMes, and 6-Mes. Another three examples for which different values were reported in the same solvent (CH2Cl2) by different authors are shown in Table 4. Table 4. IR Data of Selected Identical cis-[RhCl(CO)2(NHC)] Complexes Collected in CH2Cl2 Given as ν̃av (ν̃COs) SIoXyl

SIoXylBr

SIMesMe2

2037 (1996, 2078)69 2040 (1996, 2084)68 2041 (1998, 2083)54

2043 (1996, 2092)69 2044 (2000, 2087)54

2013 (1953, 2072)a,69 2036 (NS)68

a

Values in italics are far off and possibly erroneously reported. NS = not stated.

These include 1,3-di(ortho-xylyl)imidazolidin-2-ylidene (SIoXyl), 1,3-di(4-bromo-ortho-xylyl)imidazolidin-2-ylidene (SIoXyl), and 1,3-dimesityl-4,5-dimethyl-imidazolidin-2-ylidene. The differences observed for each individual NHC are in the range of Δν̃av = 1−3 cm−1, which reflects the error of measurement in solution IR spectroscopy. It is also uncommon to state the concentrations of the samples, although these may have some influence on the carbonyl bands as well. Given the error margin of the method and uncertainties in concentrations, reporting IR wavenumbers as decimals, e.g. 2000.4 cm−1, is certainly discouraged. Nevertheless, the difference of Δν̃av = 23 cm−1 for SIMesMe2 is quite unusual and possibly due to some systematic errors. 3.3.4. Detection of Substituent Effects. One way of modulating the stereoelectronic properties of a given NHC is by changing its substituents. Thus, the question arises if and H


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electron-releasing than an isopropyl group, which is again in contrast to expectation. In contrast, the differences observed within the N-mesityl series, which are induced by variation of the 4,5-substituents, are again reasonable. With consideration of the limited resolution mentioned earlier, most of the NHCs in Chart 4 are essentially identical with respect to their carbonyl wavenumbers. Overall, it can be seen that the RhI-based system is in many cases not suitable for the resolution of substituent influences in accordance with the known inductive and mesomeric effects. This notion is not limited to the classical NHCs mentioned but has also been reported for mesoionic 1,2,3-triazolin-4-ylidenes, where identical ν̃av values were obtained for differently substituted derivatives. Resolution of substituent effects is only possible with very significant changes, e.g. by heteroatom substitution. 3.3.5. Detection of Backbone Effects. Another way to tune the stereoelectronic properties of NHCs is to vary the backbone or the heterocycle itself. In order to isolate such effects from those brought about by N-substituents, it is crucial to look at NHCs with identical wing tip groups. Chart 5 depicts various NHCs with the popular tert-butyl and mesityl N-substituents. Comparison of the averaged wavenumbers reveals that the RhI-based method also cannot reveal differences between classical 5-membered NHCs with saturated, unsaturated, and benzannulated heterocycles, which show essentially the same ν̃av values. Even the ferrocenyl-backbone does not seem to induce any significant effect that can be resolved. Only a ring-expansion and the introduction of electronegative heteroatoms or charges lead to detectable changes in line with expectation. In analogy to the substituent effects, one can see that the RhI-based method can generally only reveal very large differences due to the limited resolution of IR spectroscopy in general. Moreover, it is crucial to understand that all IR based methods can only measure an overall effect, which includes both donor and acceptor properties. Despite these limitations, the method is very useful for an estimation of the net electronic properties. However, the ν̃av values obtained must be critically discussed,

how accurately the method can differentiate such changes. To demonstrate this we can evaluate representatives of unsaturated imidazolin-2-ylidenes as the most extensively studied type of NHCs. Relevant N-alkyl and N-aryl substituted imidazolin-2-ylidenes and their ν̃av values detected in CH2Cl2 are depicted in Chart 4. Chart 4. Selected Imidazolin-2-ylidenes and Their ν̃av(Rh) Values Recorded in CH2Cl2 for the Evaluation of Substituent Effects

Surprisingly, the first three NHCs have exactly the same ν̃av values, although they differ in their alkyl substituents. The +I effect of the tert-butyl group is certainly stronger than that of the tolylmethyl groups, and therefore ItBu is expected to be a stronger donor compared to ITm. Yet, it is apparent that this difference cannot be resolved by this method, which could be attributed to steric interferences and/or insufficient resolution. Based on the same argument, one would also expect the less bulky IiPr to be stronger donating than ITm. However, the reported ν̃av values suggest the opposite. On the other hand, the differences between IiPrMe2, IiPr and IMe are in line with chemical intuition. Extension of this comparison to the N-mesityl-substituted carbenes also reveals unexpected results. For example, IMesMe2 and IMesNiPr exhibit the smallest ν̃av values. Comparison, between IMesMe2/IiPrMe2 and IMes/IiPr, respectively, could suggest that the mesityl group is more

Chart 5. Selected NHC with N-tert-Butyl and N-Mesityl Substituents and Their ν̃av(Rh) Values Obtained in CH2Cl2 for the Evaluation of Backbone Effects

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Table 5. continued

and conclusions should only be drawn with consideration of the resolution in IR spectroscopy. 3.3.6. Interconversion of RhI Values into IrI Values and TEPs. The RhI and IrI based methods were initially introduced as less hazardous alternatives to TEP, which has been traditionally used to evaluate the donor/acceptor properties of phosphines. Historically, NHCs were considered phosphine mimics, and thus, it was of interest to compare their electronic properties. This could be done using correlation studies initiated by Crabtree and co-workers, who first reported an equation for the interconversion of IrI based values into TEP (vide inf ra).50 Inspired by this work, Wolf and Plenio proposed an interconversion of RhI into IrI values and vice versa,54 which was modified by Glorius by inclusion of more data points into eqs 1 and 2 with very good regression coefficient R2 = 0.98.74 Using these relationships and the previously established equation for the conversion of IrI values into TEPs (vide inf ra), a correlation of RhI values into TEP is also made possible (eq 3). Rh to Ir: νaṽ (Ir) = 0.9441[νaṽ (Rh)] + 98.9 cm−1

NHC 7-Xyl BNHC1 Indy6 IMesOLi(thf)n Indy1 Indy7 SIPyrMe2 6-Mes mImPy SIBm 7-Cy ANapy1 IMes(NMe2)2 BNHC2 FNHC1 IMesNiPr2 FNHC2 IMesMe2 IMesNiPr SIMesMeEt SIMesMePr 6-Bn*-Me2 aoxyNiPr CAACCy FNHC4 FNHC5 ITm IiPrMe2 IMesOSiMe2Bu IMesNMe2 IoXylNEt2 Iqui6OMe ItBu SIMesMe2

(1)

Ir to Rh: νaṽ (Rh) = 1.035[νaṽ (Ir)] − 56.9 cm−1

(2)

Rh to TEP: TEP = 0.8001[νaṽ (Rh)] + 420 cm−1

(3)

These equations have been and remain very useful for comparisons of NHCs exclusively measured in different systems. However, any interconversion comes with additional uncertainties. As more and more suitable data for the RhI-based system becomes available, it is actually no longer required to convert them e.g. into TEP. A direct comparison of the actually measured values listed in Table 5 is more convenient and Table 5. ν̃av, ν̃(COs), and TEPcalcd Values [cm−1] for cis-[RhCl(CO)2(NHC)] Complexes Obtained in CH2Cl2 NHC rNHC2 rNHC1 athiaPh athia4Ani athia4PhCl Trz2 Trz3 Trz4 Trz5 8-NHC aoxy4Ani aoxy4PhCl Indy2 Indy5 DAC2 7-Mes aoxyPh DAC3 Indy3 Indy4 IPrBCF Trz6

ν̃av 2015 2019 2021 2022 2022 2024 2024 2024 2024 2025 2026 2026 2026 2026 2027 2028 2028 2028 2028 2028 2028 2028

ν̃(COs) 1983, 1978, 1981, 1982, 1982, 1983, 1983, 1983, 1983, 1981, 1986, 1986, 1987, 1985, 1986, 1987, 1989, 1987, 1987, 1987, 1989, 1988,

2046 2059 2061 2061 2062 2065 2065 2065 2065 2068 2065 2066 2064 2066 2068 2069 2067 2069 2068 2068 2067 2068

TEPcalcd 2032 2035 2037 2038 2038 2039 2039 2039 2039 2040 2041 2041 2041 2041 2042 2043 2043 2043 2043 2043 2043 2043

ref. 51 75 76 76 76 77 77 77 77 78 76 76 79 79 70 67 76 70 79 79 80 77

Trz1 IMesOMe SIMesMe SIoXylNEt2 SItBu t Bu2-bimy IiPr F7-NHC IPr IMesNMe2,Cl Iqui IMes SIoXyl

BNHC3 DQ6 DQ7 FNHC6 IMesOP(O)Ph2 J

ν̃av

ν̃(COs)

2029 2029 2029 2030 2030 2031 2031 2029 2032 2032 2032 2031 2035 2034 2034 2034 2034 2035 2035 2035 2035 2035 2035 2036 2036 2036 2036 2036 2036 2036 2036 2036 2036 2036 2036 2036 2013 2036 2037 2037 2037 2037 2037

1986, 1989, 1991, 1988, 1991, 1996, 1990, 1987, 1988, 1992, 1995, 1990, 1994, 1991, 1992, 1994, 1994, 1994, 1995, 1993, 1993, 1993, 1993, 1995, 1996, 1994, 1997, 1997, 1995, 1996, 1994, 1994, 1995, 1996, 1996,

2038 2038 2038 2038 2038 2038 2039 2037 2040 2041 2039 2039 2039 2039 2039

TEPcalcd

ref.

1953, 1996, 1995, 1995, 1996, 1996, 1997,

2071 2069 2066 2071 2069 2066 2071 2071 2076 2072 2069 2071 2075 2076 2075 2073 2074 2076 2075 2077 2076 2077 2076 2077 2075 2077 2075 2075 2076 2076 2077 2077 2076 2076 2076 NS 2072 2075 2079 2079 2079 2077 2076

2043 2043 2043 2044 2044 2045 2045 2043 2046 2046 2046 2045 2048 2047 2047 2047 2047 2048 2048 2048 2048 2048 2048 2049 2049 2049 2049 2049 2049 2049 2049 2049 2049 2049 2049 2049 2031 2049 2050 2050 2050 2050 2050

67 81 79 49 60 79 48 67 70 82 73 83 54 84 85 81 86 87 86 63 88 68 68 89 90 91 92 92 50 93 49 85 54 94 95 68 69 96 49 68 54 95 95

1997, 1997, 1996, 1996, 1998, 1996, 1996, 1996, 1996, 1998, 2000, 2003, 2002, 1998, 1997,

2078 2078 2079 2080 2077 2079 2081 2078 2084 2083 2077 2075 2075 2079 2080

2051 2051 2051 2051 2051 2051 2051 2050 2052 2053 2051 2051 2051 2051 2051

97 98 66 87 94 63 54 69 68 54 81 99 99 92 49


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Table 5. continued NHC IMesOPiv SIMes

SImXyl Trazy BMes DAC5 ImPy1 IoXylOC12 SIoXylOC12 DAC4 IMe ImPy2 IoXyl PorHC SIMes SIMesF2 SIMesF3 SIPrF2 t Bu2-btd ZnPorHC FcCAIm IMesBr2 ImPy11 ImPy3 ImPy4 IoXylBr Iqui5NO2 NiPorHC AlMePorHC IMesCl2 SIoXylBr dipiy Trt DAC6 IMesNMe3 DQ8 IMePPh2 ImPy5 ImPy7 IpFMe MnClPorHC PyI SIMesF5 AlClPorHC FcCABz FcSABz FcSAIm ImPy6 ImPy8 ImPy9 i Pr2-bimy SIArF2 DAC1a ImPy10 Me2-pery SIoXylTs

ν̃av

Table 5. continued TEPcalcd

ref.

NHC

ν̃av

2039 2016 2038 2039 2039 2039 2040 2040 2040 2040 2040 2041 2041 2041 2041 2041 2041 2041 2041 2041 2041

1997, 1958, 1995, 1996, 1997, 1999, 1998, 1998, 2000, 1998, 1998, 2000, 2000, 2001, 1998, 2002, 1997, 2000, 2000, 2000, 2002,

ν̃(COs) 2080 2074 2080 2081 2080 2079 2082 2081 2079 2082 2082 2082 2082 2081 2083 2080 2084 2081 2081 2081 2080

2051 2033 2051 2051 2051 2051 2052 2052 2052 2052 2052 2053 2053 2053 2053 2053 2053 2053 2053 2053 2053

49 69 67 68, 100 69 101 102 70 82 54 54 70 57 57 54 103 54 100 100 100 95

2041 2042 2042 2042 2042 2042 2042 2042 2042 2043 2043 2043 2044 2043 2044 2044 2044 2044 2044 2044 2044 2044 2044 2044 2044 2045 2045 2045 2045 2045 2045 2045 2045 2045 2046 2046 2046 2046

2001, 2002, 2000, 2003, 2002, 2002, 2000, 2003, 2002, 2003, 2000, 1994, 2000, 2003, 2004, 2003, 2002, 2005, 2003, 2003, 2004, 2003, 2004, 2003, 2005, 2005, 2006, 2006, 2007, 2005, 2005, 2005, 2005, 2005, 2005, 2006, 2007, 2002,

2080 2081 2083 2081 2082 2082 2084 2081 2081 2083 2085 2092 2087 2082 2083 2085 2086 2082 2084 2084 2084 2084 2084 2084 2083 2084 2083 2083 2083 2085 2084 2085 2085 2085 2086 2086 2085 2090

2053 2054 2054 2054 2054 2054 2054 2054 2054 2055 2055 2055 2055 2055 2055 2055 2055 2055 2055 2055 2055 2055 2055 2055 2055 2056 2056 2056 2056 2056 2056 2056 2056 2056 2057 2057 2057 2057

104 105 63 57 57 57 54 94 103 104 63 68, 69 54 62 106 70 107 99 108 57 57 109 104 110 100 104 105 105 105 57 57 57 61 100 111 57 59 54

AAC1 DAC7 Nitron TolSAIm IMeDNP SIMesO QY2 SIMesO,CH2 H2PorHC2+ QY1 TsMetazy Triazo1 MeSAIm QY3 SIMesONiPr SIoXylCl Triazo3 Cp*RuBI Cp*RuPeryMe2 IDNP DNPtazy Triazo2 PyIMe DNPtazyCN OAC DNPtazyNO2

2047 2047 2047 2047 2048 2048 2049 2049 2050 2050 2050 2050 2051 2051 2051 2051 2052 2052 2053 2053 2054 2054 2059 2060 2060 2061

ν̃(COs) 2008, 2006, 2007, 2009, 2008, 2005, 2010, 2007, 2010, 2010, 2012, 2011, 2013, 2012, 2009, 1999, 2014, 2011, 2013, 2014, 2015, 2015, 2022, 2021, 2017, 2022,

2086 2087 2086 2084 2087 2090 2088 2090 2089 2089 2088 2089 2088 2089 2092 2103 2089 2092 2092 2091 2093 2092 2095 2099 2103 2100

TEPcalcd

ref.

2058 2058 2058 2058 2059 2059 2059 2059 2060 2060 2060 2060 2061 2061 2061 2061 2062 2062 2063 2063 2063 2063 2067 2068 2068 2069

112 113 114 105, 115 66 49 99 49 103 99 115 116 105 99 88 68, 69 116 117 59 66 118 116 110 118 119 118

also more advisible, since error propagation can be avoided. Nevertheless, the calculated TEP values have been included for completeness. The structures of all these NHCs are depicted in Chart 6 in order of increasing averaged wavenumbers.

4. IRIDIUM(I) CARBONYL-BASED SYSTEM The carbene chemistry of rhodium(I) and iridium(I) closely mirror each other, and analogous complexes can be obtained by essentially the same pathways. Although cis-[RhCl(CO)2(NHC)] complexes have been known long before those of their iridium(I) counterparts, it is the latter that have first been proposed as alternatives for TEP. Due to the similarity of both systems, the same considerations discussed for the rhodium-based methodology also apply for the iridium counterpart and do not have to be dealt with explicitly again. The first systematic study on iridium(I) carbonyl complexes as alternative complex probes for IR spectroscopic evaluation of donor abilities was conducted by Crabtree and co-workers.50 Using the synthetic approach outlined in Scheme 6, the complexes cis-[IrCl(CO)2(ITm)] and cis-[Ir(CO)2(IBu)] (ITm = 1,3-di(4-tolylmethyl)imidazolin-2-ylidene, IBu = 1,3-dibutylimidazolin-2-ylidene) were obtained and their IR spectra measured in CH2Cl2 showing as expected two carbonyl bands. Notably, the two NHCs give rise to the same Ir-based averaged stretching frequencies (ν̃av), which once again indicates that IR spectroscopic methods cannot resolve (smaller) differences, in this case between the tolylmethyl and n-butyl wing tip groups. In order to extrapolate TEP values for these NHCs, the authors plotted the TEPs of selected phosphines and phosphites against the averaged carbonyl stretching frequencies obtained for their known cis-[IrCl(CO)2(PR3)] analogues (Table 6, Figure 2). A good initial K


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Chart 6. Various NHC Ligands in Order of Increasing ν̃av (CO) Values for Their cis-[RhCl(CO)2(NHC)] Complexes Obtained in CH2Cl2

L


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Chart 6. continued

The resulting first linear regression eq 4 obtained allows for the extrapolation of TEP values from cis-[IrCl(CO)2(NHC)] complexes. Its application gives an extrapolated TEP of ν̃ = 2051 cm−1 for both ITm and IBu.

linear correlation (R2 = 0.91) was only obtained if the phosphites were excluded as outliers. Moreover, it must be noted that the data used were obtained in different solvents, while the TEPs were measured in dichloromethane. M


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Table 6. cis-[IrCl(CO)2(PR3)] Carbonyl Stretching Frequencies used for Correlation

Scheme 10. General Routes to trans-[IrCl(CO)2(NHC)] Complex Probes

L

ν̃av (CO)

Solvent

TEP

PCy3 PiPr3 PEt3 P(p-CH3C6H4)3 PMe2Ph PPh3 P(OBu)3 PMePh2 P(OPh)3

2028 2031.5 2037.5 2039 2041.5 2043.5 2044 2044 2049

CH2Cl2 CH2Cl2 CHCl3 CHCl3 CHCl3 CHCl3 CS2 CHCl3 CHCl3

2056.4 2059.2 2061.7 2066.7 2065.3 2068.9 2077 2067 2085.3

Similar to their rhodium analogues, cis-[IrCl(CO)2(NHC)] complexes are usually isolated as solids that are stable to air and moisture. In some cases, slow decomposition was observed in solution under air within hours.97 The decomposition is likely due to irreversible dissociation of a carbonyl ligand, which has also been observed for the analogous rhodium complexes. 4.2. Electronic Properties of NHCs on the Iridium(I) Scale

As for the rhodium-dicarbonyl and TEP system, it is paramount that only IR data collected using the same means and same solvent are used for comparison. In most cases, this has so far been fulfilled for the iridium system, and the large majority of complex probes were measured in dichloromethane indicating overall better systematicness. Table 7 lists the iridium-based Table 7. ν̃av, ν̃(COs) and TEPcalcd. Values [cm−1] for cis-[IrCl(CO)2(NHC)] Complexes Obtained in CH2Cl2

Figure 2. First correlation graph of TEP versus ν̃av(CO) of Selected cis-[IrCl(CO)2(PR3)] Complexes.

NHC

Ir to TEP: TEP = 0.722[νaṽ (Ir)] + 593 cm

−1

aNHC1 aNHC2 aNHC4 aNHC5 aNHC3 aNHC6 aNHC7 CAACMenthyl aNHC8 aNHC9 aNHC11 aNHC12 aNHC13 6-MesMe2 aNHC10 7-Cy IMesOLi,Ph IiPrPh2 IiPrMePh2 nTrz Trz8 6-P2N2 IPhiPr IPrC4 IMesBu2 IMesC5 Trz7 Trz10 Trz11 Trz12 Trz13 ITm IBu

(4)

Further improvement to the currently used eq 5 was later done by Nolan and co-workers by extension of the data set to several common NHCs. In this study, the correlation coefficient could be improved to R2 = 0.971.120 Ir to TEP: TEP = 0.8475[νaṽ (Ir)] + 336.2 cm−1

(5)

The correlation of IrI and RhI values can be done using eqs 1 and 2 (vide supra). Compared to the rhodium(I) system, fewer NHCs have been evaluated using the iridium(I) complexes. Nevertheless, the amount of data covering more than 130 NHCs measured in dichloromethane is still impressive. 4.1. Preparation and Stability of Iridium(I) Complex Probes

The preparation of the cis-[IrCl(CO)2(NHC)] complex probes follows the same strategies as that for their rhodium(I) analogues (vide supra). In most cases, the chlorido-bridged Ir-COD complex [IrCl(COD)]2 is reacted with a suitable NHC source under bridge cleavage. The NHC is either used in its free base form121 or more conveniently generated in situ from the azolium salt and a suitable base (e.g., KHMDS, KOtBu, K2CO3).47,83,122 These reactions give rise to complexes of the type [IrCl(COD)(NHC)] as intermediates, which on exposure to carbon monoxide usually affords the desired dicarbonyl complex probes. As demonstrated by Crabtree, the silver-NHC transfer route can also be used.50 In addition, the direct use of the iridium dicarbonyl dimer [IrCl(CO)2]2 is also feasible. These methodologies are summarized in Scheme 10. N

ν̃av 2003 2008 2010 2010 2011 2012 2012 2013 2013 2013 2014 2014 2014 2014 2015 2016 2016 2017 2017 2017 2018 2018 2019 2019 2019 2019 2019 2019 2019 2019 2019 2020 2020

ν̃(COs) 1961, 1966, 1968, 1968, 1969, 1970, 1970, 1971, 1971, 1971, 1972, 1972, 1972, 1967, 1973, 1973, 1974, 1972, 1974, 1975, 1975, 1976, 1976, 1978, 1976, 1976, 1977, 1976, 1976, 1977, 1977, 1976, 1978,

2045 2049 2051 2052 2053 2053 2053 2055 2055 2054 2055 2055 2055 2061 2057 2058 2058 2061 2059 2058 2060 2060 2061 2065 2062 2062 2061 2062 2062 2060 2060 2063 2062

TEPcalcd.

ref.

2034 2038 2040 2040 2041 2041 2041 2042 2042 2042 2043 2043 2043 2043 2044 2045 2045 2046 2046 2046 2046 2046 2047 2047 2047 2047 2047 2047 2047 2047 2047 2048 2048

123 124 124 124 124 124 124 121 124 124 124 124 124 125 124 83 126 123 123 127 128 129 123 122 122 122 130 131 131 132 132 50 50


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Table 7. continued NHC IMesMe2 IMesC3 IMesC4 IFc 6-NHCpyr Trz14 IoXylNEt2 Trz1 IMesC2Ph IMesMe Trz9 SIoXylNEt2 IAd FNHC2 IPrMe2 IPent I(2-Ad) ICy IMes ItBu ICy8 ICy12 SHInd IMesC5 IMesCy IMesCy7 IMesCy8 IMesCy12 IPrOMe IMesthie2 IiPr IoXyl Ithia1 IPr Fc2-bimy IBn MAAC AAC2 DQ6 InBu SICy2Nap IET SIMes SIoXyl SIPr SIBn BniPr32-bimy SIPrOMe IPr*OMe SICyNap IoXylBr Trazy i Pr2-bimyMe2 IPr* IMesBr2 IPaul Fc,Me-bimy Ph2-bimyFc MesThiaMe DippThiaMe MesThiaC4 MesThiaC5

ν̃av 2020 2020 2020 2020 2020 2020 2021 2021 2021 2021 2021 2022 2022 2022 2022 2022 2022 2023 2023 2023 2023 2023 2023 2023 2023 2023 2023 2023 2023 2023 2023 2024 2024 2024 2024 2024 2024 2024 2024 2024 2024 2024 2024 2025 2025 2025 2025 2025 2025 2025 2025 2026 2026 2026 2026 2026 2026 2027 2027 2027 2027 2027 2027

Table 7. continued ν̃(COs) 1977, 1977, 1977, 1982, 1977, 1978, 1979, 1978, 1978, 1979, 1979, 1980, 1982, 1978, 1979, 1981, 1982, 1981, 1980, 1980, 1981, 1981, 1979, 1981, 1981, 1981, 1981, 1981, 1980, 1980, 1982, 1981, 1982, 1981, 1984, 1983, 1981, 1978, 1986, 1982, 1981, 1981, 1981, 1981, 1982, 1983, 1983, 1982, 1983, 1983, 1982, 1984, 1985, 1984, 1983, 1984, 1986, 1985, 1986, 1986, 1986, 1986,

2063 2063 2063 2058 2063 NS 2064 2062 2064 2064 2062 2065 2063 2062 2065 2065 2063 2064 2065 2066 2065 2065 2064 2066 2065 2065 2065 2065 2065 2066 2065 2066 2067 2066 2067 2064 2065 2066 2069 2062 2065 2067 2066 2068 2069 2068 2067 2067 2067 2067 2067 2069 2067 2067 2068 2069 2067 2068 2068 2067 2067 2067 2067

TEPcalcd.

ref.

NHC

ν̃av

2048 2048 2048 2048 2048 2048 2049 2049 2049 2049 2049 2050 2050 2050 2050 2050 2050 2051 2051 2051 2051 2051 2051 2051 2051 2051 2051 2051 2051 2051 2051 2052 2052 2052 2052 2052 2052 2052 2052 2052 2052 2052 2052 2052 2052 2052 2052 2052 2052 2052 2052 2053 2053 2053 2053 2053 2053 2054 2054 2054 2054 2054 2054

133 122 122 134 135 136 137 96 122 122 128 137 120 134 138 138 139 97 120 137 120 140 140 141 142 142 142 142 142 36 143 97 137 144 120 134 145 125 146 99 147 148 149 137 137 120 145 150 36 36 148 137 101 151 36 152 153 134 134 154 154 154 154

IBniPr3 PyrI n Bu2-bimy 6-CAAC SIoXylBr Ithia2 IPrCl2 i Pr2-bimy IPrBr2 IMesthie2* IoXylSOTol SIoXylSOTol IMeSPh2 IoXylTs QIMes SIEtOMe IPentCl Bn2-bimyMe2 DQ7 DQ8 FcSAIm FcSABz PPI1 AQI QIPr SIoXylTs TPT PPI2 DAC1a SIMesO,Bu PyrIRuCp* SIMesO,Ph DAC1b QY2 CF3Me-bimy CF3iPr-bimy CF3Ph-bimy QY1 QY3 Triazo1 CF3TPh-bimy PyIMe CF3NPh-bimy OAc

2027 2027 2027 2027 2028 2028 2028 2028 2028 2028 2029 2029 2029 2030 2030 2030 2030 2030 2030 2030 2030 2030 2030 2030 2031 2031 2031 2031 2031 2032 2032 2033 2033 2034 2034 2034 2034 2035 2035 2035 2036 2037 2037 2046

ν̃(COs) 1985, 1985, 1985, 1986, 1984, 1986, 1985, 1986, 1985, 1985, 1984, 1985, 1987, 1985, 1988, 1987, 1982, 1988, 1989, 1989, 1990, 1991, 1989, 1989, 1987, 1986, 1989, 1989, 1989, 1989, 1990, 1990, 1991, 1993, 1993, 1993, 1992, 1994, 1995, 1994, 1994, 2003, 1995, 2003,

2068 2068 2068 2068 2071 2069 2071 2069 2071 2070 2073 2073 2070 2074 2072 2072 2069 2071 2071 2070 2070 2068 2071 2071 2074 2075 2072 2072 2072 2075 2073 2076 2074 2075 2075 2075 2074 2075 2075 2076 2077 2084 2078 2089

TEPcalcd.

ref.

2054 2054 2054 2054 2055 2055 2055 2055 2055 2055 2056 2056 2056 2057 2057 2057 2057 2057 2057 2057 2057 2057 2057 2057 2057 2057 2057 2057 2057 2058 2058 2059 2059 2060 2060 2060 2060 2061 2061 2061 2062 2063 2063 2070

150 155 147 156 137 144 120 151 152 143 137 137 157 137 134 158 138 150 99 99 105 105 147 147 138 137 120 147 159 126 155 126 160 99 161 161 161 99 99 116 161 110 161 119

wavenumbers determined for a large number of NHCs and their calculated TEPs. The global Chart 9 displays structures of these NHCs in increasing order of their averaged carbonyl wavenumbers. Only a few ligands were evaluated in another medium. Since all carbonyl-based methodologies are rooted on the same concept, it is not a surprise that the same advantages and weaknesses observed for the previous two carbonyl-based methods are also encountered here. Therefore, there is no need to discuss halido and solvent/media effects, which obviously do play a role here as well, again. Measuring the degree of backdonation to the carbonyl ligand does not allow for separation of σ and π contributions, and only a net contribution of the NHC in question can be observed. In some cases, the oftentimes complicated interplay between them can give results that are difficult to reason. Overall, one can conclude that nonclassical NHCs with a reduced heteroatom stabilization are stronger net donors compared to classical NHCs. This notion is reasonable as removal of an electron-withdrawing nitrogen neighbor is O


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Chart 7. Selected Dimesityl-Substituted NHCs and Their ν̃av(Ir) Values in CH2Cl2

imidazolin-2-ylidenes as the most common type of NHC along with their ν̃av(CO) values. The ligands all differ in their N-alkyl or N-aryl substituents. On first sight it becomes apparent that differences between tert-butyl, isopropyl, cyclohexyl, cyclododecane, and even mesityl cannot be differentiated using this method. The ν̃av(CO) values for NHCs with variations of these substituents are identical. Despite this observation, one should not jump to the conclusion that these NHCs are electronically identical. On the contrary, it is well-known that the electronic influences of aryl and secondary or tertiary alkyl groups are indeed different (e.g., Hammett σ constants). Identical ν̃av(CO) values for NHCs with different substituents are not limited to imidazole-derived carbenes but have also been observed for other types of NHCs, e.g. mesoionic 1,2,3-triazolin-5-ylidenes (Chart 9). Another observation worthy of comment is that the formal replacement of one isopropyl with a phenyl group leads to a decrease of the averaged wavenumber, i.e. IiPr vs IPhiPr. Could the latter be regarded as the better net donor? If this is indeed so, then what are the contributing factors? Clearly, more research is required to address these issues. Overall, the conclusion can be made that all carbonyl-based methodologies have some limitations in resolving the effects induced by different N-substituents. This is clearly a disadvantage as a large library of NHCs are generally prepared by modular variations of the wing tip groups. Despite their wide acceptance, interpretations of the results obtained should always be done cautiously. Nevertheless, these methods remain attractive and are more reliable in particular when more dramatic variations in the ligand structure are carried out.

expected to increase the electron density around the carbene atom. The position of CAACs (i.e., CAACMenthyl) with only one ring-nitrogen atom and a fully saturated heterocycle compared to some mesoionic imidazolin-4-ylidenes with two ring-nitrogen atoms and an essentially unsaturated heterocycle is somewhat surprising. The same trend was also observed for the rhodium system (Chart 6). Maybe their good π-accepting properties reduce the amount of backdonation to the carbonyl ligands, thus “masking” their anticipated very strong donating ability. Structural variations in the N-substituents and backbones are, on the other hand, more difficult to resolve using this method. Chart 7 shows ten selected NHCs which all contain dimesitylsubstitution. Their ν̃av(CO) values obtained in CH2Cl2 suggest that the six-membered saturated 6-MesMe2 is the strongest net donor followed by the mesoionic Trz11 ligand, which is in turn stronger than the famous IMes ligand. However, subsequent attachment of two thienyl moieties leading to IMesthie2 does not lead to any measurable differences, which is surprising. Moreover, expanding the ring and attaching one or two oxygen substituents, e.g. MAAC and AAC2, seemingly result in a marginal change, which cannot be considered significant considering the error of IR spectroscopy. The next ligand in this series is SIMes, which compared to IMes appears to be an overall weaker donor. The problematic differentiation between imidazolin-2-ylidenes and their saturated imidazolidin-2-ylidene counterparts is a weakness common to all carbonyl-based methodologies. The ranking of the last three NHCs, i.e. IMesthie2*, QIMes, and OAc, all bearing electron-withdrawing groups as the weakest donors in this series, is reasonable again. It is interesting to note that annulation by ring closure of IMesthie2* leads to a measurable decrease of the net donor strength. Also as anticipated, the effect of endocyclic keto-groups in OAC is markedly larger than that of exocyclic ones in QIMes, which are more remote. Next, the capability of the iridium system to discern wing tip variations is evaluated. Chart 8 depicts eight selected

5. REDOX POTENTIALS AND THE LIGAND ELECTROCHEMICAL PARAMETER (LEP) While the TEP and related carbonyl-based methods are common in organometallic chemistry, electronic properties of classical Werner type ligands (mainly N, O, S, halido, and pseudohalido donors) are commonly evaluated using their ligand electrochemical parameter EL. The methodology has often been termed Lever electronic parameter (LEP), after Alfred Beverley Philip Lever, who introduced it in 1990.17 EL values of various ligands have been established using redox potentials of most commonly RuII/III metal complexes, but other redox couples have also been investigated.18 The purpose was to provide an easy way to predict redox potentials of metal complexes using a general formula (e.g., eq 6), which is based on the assumption that ligand contributions to the overall redox potential of a complex are additive.17 Summation of experimentally determined ligand electrochemical parameters EL with consideration of parameters specific to the spin state

Chart 8. Selected Imidazolin-2-ylidenes with Varying Wing-tip Groups and Their ν̃av(Ir) Values in CH2Cl2

P


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Chart 9. Various NHC Ligands in Order of Increasing ν̃av Values for Their cis-[IrCl(CO)2(NHC)] Complexes

Q


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Chart 9. continued

(e.g., Sm = 0.97 for RuII/III) and redox couple of the respective metal (e.g., Im = 0.04 for RuII/III) gives the predicted redox potential of a given complex of interest. In most cases, realistic results that are close to the experimentally determined redox potentials can be obtained. Eredox = Sm(∑ E L(L)) + Im

The EL values are not a direct measure of ligands’ donor strengths, but they reflect their relative capacity to stabilize a metal in a certain oxidation state. For example, a ligand with a smaller EL value is expected to better stabilize the RuIII state in the RuII/III couple than those with larger EL values. The sum of ligands with small EL values would obviously result in a lower redox potential reflecting a more facile oxidation process.

(6) R


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In some way, EL values are therefore related to ligand donor strength.162 Since this measure relies on an electrochemical process, only complexes that exhibit reversible or quasireversible redox chemistry can be studied to determine an EL value of a new species. Moreover, noninnocent ligands that interfere in the metal-centered redox processes are difficult to study. Due to these drawbacks and the requirement for relatively less common electrochemical setups, EL determination for NHCs is very rare. Albrecht and co-workers determined the first general EL value for generic unsaturated imidazolin-2-ylidenes with the assumption of insignificant N-substituent effects.163 The FeII/III redox couples of piano-stool carbene complexes of the type [Fe(Cp)(CO)2(NHC)]+ and [Fe(Cp)(CO)(diNHC)]+ were used for this purpose, and an EL = 0.29 was determined for generic imidazolin-2-ylidenes, which is surprisingly similar to that of pyridine (EL = 0.25, Table 8). This observation was

Table 9. Redox Potentials of [RhCl(COD)(NHC)] Complexes in CH2Cl2 with Bu4NPF6 as Electrolyte NHC

E1/2 [V]

ref.

Bu2-bimy 7-Cy SIoXylNEt2 t Bu2-btd IoXylNEt2 i Pr2-bimy ANapy1 ANapy2 SIoXylOC12 SIMes SIoXyl IMes IoXylOC12 7-DAC IoXyl CF3Ph-bimy Et2-bimy i Pr2-bimy SIoXylBr IoXylSOTol IoXylBr dbpy SIoXylTs FNHC2 FNHC1 CF3iPr-bimy CF3TPh-bimy Cp*RuBI CF3NPh-bimy

+0.58 +0.608 +0.651 +0.683 +0.718 +0.773 +0.78 +0.78 +0.785 +0.791 +0.817 +0.833 +0.836 +0.84 +0.855 +0.858 +0.86 +0.875 +0.903 +0.923 +0.926 +0.948 +0.961 +0.98 +1.00 +1.012 +1.025 +1.042 +1.053

167 54 54 95 54 161 84 84 54 54 54 54 54 168 54 161 167 61 54 54 54 169 54 86 86 161 161 61 161

t

Table 8. EL Values for Selected Common Ligands

explained by a considerable amount of π-backbonding from electron rich FeII centers to the NHC ligands.163 To the best of our knowledge, EL values for specific NHCs are unknown, and they are insignificant in NHC chemistry. More commonly, the donating abilities of NHCs have been ranked by directly comparing the electrochemical redoxpotential of analogous NHC complexes, whereby a stronger donor would give rise to a smaller redox-potential. Of course, the metal and all other coligands must not be varied in such studies. Redox potentials of a few ruthenium-NHC complexes have been determined.141,159,164 However, the amount of available data is insufficient for a detailed comparison of various NHC ligands. On the other hand, significantly more electrochemical data is available for rhodium and iridium complexes. cis-[MCl(CO)2(NHC)] complexes that are commonly used for IR spectroscopic evaluation often show an irreversible electrochemistry, probably due to loss of CO during the redox process.137 It has also been reported that the metal-centered redox process of interest could not be observed within the solvent window.134 However, their [MCl(COD)(NHC)] complex precursors have been identified as the currently best suitable redox probes. As with any methodology, data collections should be done under the same conditions to allow for a proper comparison. Commonly, cyclic voltammetric studies of the [MCl(COD)(NHC)] complexes are carried out in dichloromethane with the use of tetrabutylammoniumhexafluorophosphate (Bu4NPF6) as electrolyte. Tables 9 and 10 list redox potentials in increasing order for the MI/II couple of rhodium and iridium complexes that solely differ in the NHC ligand. Chart 10 depicts the Lewis structures of the NHCs and the redox potentials they induce. The comparison of such redox potentials has allowed for the determination of remote substituent effects on the donating ability of N,N′-diaryl substituted imidazolin- and imidazolidin2-ylidenes. In addition to the detection of substituent effects, these electrochemical studies could also corroborate that saturated imidazolidin-2-ylidenes are stronger donors than unsaturated imidazolin-2-ylidenes, which is within expectation.

Moreover, decoration of the backbone with electron-withdrawing substituents, e.g. in diamidocarbenes (DAC), also leads to an increase of the redox potential. All in all, the data highlights that the electrochemical approach is often superior to the more popular carbonyl-based methodologies in terms of sensitivity and accuracy. Nevertheless, there are also limitations to this methodology. As already mentioned, only complexes with reversible or quasireversible redox processes can be investigated. Moreover, the presence of noninnocent ligands must be avoided. In this respect, NHCs with a ferrocenyl backbone or substituent (e.g., FNHC1, FNHC2, etc.) and other redox-active groups are considered noninnocent, which cannot be evaluated properly using this method.165 The processes involving the FeII/III couple of ferrocenyl substituents, e.g. oxidation of FeII, will alter the properties of the initial NHC, thus affecting the RhI/II or IrI/II redox potential.166

6. HUYNH’S ELECTRONIC PARAMETER (HEP) To circumvent disadvantages related to carbonyl-based and electrochemical methodologies, Huynh and co-workers introduced a new methodology for the evaluation of various types of organometallic and classical Werner-type ligands in 2009.19,20 The method is based on the 13C NMR spectroscopic analyses of generally stable palladium(II) NHC complexes of the type trans-[PdBr2(iPr2-bimy)L] (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene). In essence, the influence of a transstanding ligand L on the chemical shift of 13Ccarbene NMR signal of the iPr2-bimy reporter ligand (i.e., HEP value) is measured. S


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only contain two ligands, and steric interferences in the determination of electronic properties, which cannot be ruled out, are expected to be less severe. A very good linear correlation (R2 = 0.98) between HEPs of NHCs and their respective gold based 13Ccarbene chemical shifts for the iPr2-bimy reporter ligand has been established (Figure 4, Table 11), and conversion of gold(I)-based values into HEP can be done using eq 7.173

Table 10. Redox Potentials of [IrCl(COD)(NHC)] Complexes in CH2Cl2 with Bu4NPF6 as Electrolyte NHC

E1/2 [V]

ref.

IQu2 IQuMe SIoXylNEt2 IQuBu 6-MesMe2 IQuBn IoXylNEt2 t Bu2-btd SIoXylOC12 SIMes SIoXyl IoXylOC12 IMes MAAC IoXyl IPPCMe2 SIoXylBr 7-DAC SIoXylSOTol SIoXylFc IoXylBr ANaphy3 IoXylSOTol SIoXylTs IoXylTs DAC1a QIMes dbpy FNHC2 SIDNMes

+0.572 +0.580 +0.591 +0.594 +0.61 +0.616 +0.648 +0.662 +0.730 +0.735 +0.759 +0.761 +0.765 +0.77 +0.786 +0.804 +0.838 +0.84 +0.846 +0.858 +0.862 +0.862 +0.870 +0.910 +0.920 +0.94 +0.95 +0.959/0.962 +1.02 +1.041

170 170 137 170 125 170 137 95 137 137 137 137 137 125 137 171 137 168 137 166 137 171 137 137 137 125 134 169 134 172

HEP = 1.19[Au] − 45.0

(7)

Carbonyl based methodologies use low valent metal centers and measure the amount of π-backdonation to the carbonyl ligands. However, any ligand of interest can also compete with the carbonyl ligands for electron density, and thus such methods can only detect the net donating ability of a ligand primarily comprising both σ-donor and π-acceptor contributions. HEP, on the other hand, utilizes a more Lewis acidic palladium(II) metal center, where metal-to-ligand backdonation is insignificant. This is so because palladium is a rather electronegative transition metal and tends to retain its valence electrons. This tendency is even more pronounced in its +II oxidation. The observation that [PdBr2(iPr2-bimy)]2 does not react with carbonyl ligands to form [PdBr2(iPr2-bimy)(CO)] also provides evidence that the [PdBr2(iPr2-bimy)] complex fragment is a poor π-donor. As such, HEP primarily detects the σ-donating ability of a ligand.20 The 13C NMR based methodology has since also been extended to the evaluation of Wernertype and organometallic bidentate ligands including diNHCs using complexes of the type [PdBr(iPr2-bimy)L2]PF6, i.e. HEP2.21 6.1. Preparation and Stability of Complex Probes

HEP utilizes the 13Ccarbene NMR signal of an NHC to measure the electronic impact of ligands including that of other NHCs. Access to the reporter ligand iPr2-bimy and the dimeric [PdBr2(iPr2-bimy)]2 parent complex is crucial. The respective benzimidazolium salt iPr2-bimy·HBr can be prepared by conventional N-alkylation of benzimidazole. Notably, 13C2 labeled benzimidazole can be easily prepared by condensation of 13 C formic acid with 1,2-diaminobenzene as well. Due to the reduced electrophilicity of isopropyl bromide and the tendency of secondary alkyl halides to undergo base-catalyzed elimination reactions, the use of excess isopropyl bromide with K2CO3 as a relatively mild base and prolonged reaction time is required for good yields.174 Subsequent palladation of the i Pr2-bimy·HBr salt with Pd(OAc)2 and a bromide source such as NaBr affords the [PdBr2(iPr2-bimy)]2 parent complex using standard protocols.174 In general, HEP complex probes are easily prepared by bridged cleavage reactions of the dimeric [PdBr2(iPr2-bimy)]2 complex with 2 equiv of free ligands. Heterobis(NHC) complexes of the type trans-[PdBr2(iPr2-bimy)(NHC)] can also be prepared via this route, where the free NHC is available.152 More convenient is the bridge cleavage reaction with NHCs generated in situ from the respective azolium salt and an external base. Such reactions do not require the exclusion of air and moisture and can be conducted in standard glassware. A special variation involves the use of silver(I) oxide as a base, where silver−NHC complexes are proposed as intermediates transferring the NHC to palladium (Scheme 11, route A). For azolium salts with weakly coordinating anions, e.g. BF4− and PF6−, a halide source should be added to provide the driving force by silver halide precipitation (route B). Alternatively, a dimeric palladium complex containing the NHC

To ensure valid comparisons, all complex probes are measured in CDCl3 as concentrated solutions, which also makes data collection easier. Importantly, the CDCl3 solvent signal must be referenced at 77.7 ppm for proper comparison. In contrast to IR spectroscopy (bands) or cyclic voltammetry (waves), 13 C NMR signals generally show very little line broadening and are detected with greater accuracy as sharp lines. A standard deviation of σ = 0.01 ppm can be estimated by the full-width-athalf-maximum (fwhm ∼0.02 ppm) for the iPr2-bimy 13Ccarbene NMR signal. Since 13C NMR chemical shifts are commonly reported rounded to the nearest tenth place, one could argue that HEP differences of 0.1 ppm are very significant, i.e. 10σ. Empirically, it was found that a stronger donating ligand induces a downfield shift, while a weaker donor results in an upfield shifted HEP signal. For comparisons among NHC ligands, heterobis(NHC) complexes of the type trans[PdBr2(iPr2-bimy)(NHC)] are required, which contain the NHC of interest as well as the iPr2-bimy reporter NHC. In some cases, the two carbene signals are closely spaced. However, the HEP signal can be easily identified by HMBC NMR spectroscopy (Figure 3) or by using 13C labeled complex probes (vide inf ra). In cases where the desired palladium complexes cannot be prepared for whatever reasons, alternative gold(I) probes [Au(iPr2-bimy)(NHC)]BF4 can be targeted as well. For example, cis-configured palladium complexes are not suitable for HEP determination. On the other hand, linear gold(I) probes are not subjected to such isomerization. Moreover, they T


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Chart 10. Redox Potentials of cis-[MCl(COD)(NHC)] Complexes (M = Rh, Ir) Determined in CH2Cl2 with Bu4NPF6 as Electrolyte

in question can also be cleaved by the iPr2-bimy reporter NHC using a similar approach (route C). The preparation of the alternative gold probes can also be achieved by routine gold-NHC chemistry as depicted in

Scheme 12. Route A involves introduction of the iPr2-bimy reporter to a given gold-NHC complex already bearing the NHC to be evaluated. This can be achieved by reaction of [AuCl(NHC)] with the iPr2-bimy·HBr benzimidazolium salt U


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Scheme 11. Preparation of trans-[PdBr2(iPr2-bimy)(NHC)] Complexes

Scheme 12. Preparation of [Au(iPr2-bimy)(NHC)]BF4 Complexes as Alternative Probes

Figure 3. Portion of the HMBC NMR spectrum of trans-[PdBr2(iPr2bimy)(IMes)].

CDCl3. Ligand disproportionation reactions have not been observed for palladium, and no decomposition has been observed upon prolonged standing in solution. However, slow ligand redistribution has been observed for the gold analogues of some NHCs, which led to the formation of bis(NHC) complexes of the type [Au(NHC)2]+.173 Nevertheless, this process is slow and does not interfere with the detection of the desired iPr2-bimy carbene signal.

Figure 4. Correlation of the 13Ccarbene(iPr2-bimy) NMR values for [Au(iPr2-bimy)(NHC)]BF4 complexes and HEP.

6.2. Understanding HEP

Table 11. Summary of iPr2-bimy Carbene Resonances in [Au(iPr2-bimy)(NHC)]BF4 and [Au2(iPr2-bimy)2(μditz)](BF4)2 Complexes and Their Respective Ligand HEPs NHC b

ditz Bn-btzy Bn2-tazy IPr Bn2-bimy Indy8 Pyry3

13

Ccarbene (iPr2-bimy)a 185.6 186.2 186.6 186.7 187.6 190.8 192.5

Why do stronger donors induce a downfield shift of the transstanding iPr2-bimy 13Ccarbene NMR signal, and how can we understand this fact? The carbene atom of free NHCs generally exhibits very downfield signals of >200 ppm. The magnitude of this downfield shift is approximately inversely proportional to the singlet−triplet (S-T) energy gap and the likeliness for an S-T transition by promoting an electron from the σ (NHC lone pair) to the initially vacant pπ orbital of the NHC.6 This process has the strongest contribution to the paramagnetic shielding term, which in turn leads to a downfield shift. Thus, NHCs with a larger S-T separation, i.e. unsaturated imidazolin-2-ylidenes (∼211−221 ppm), would exhibit smaller chemical shifts compared to those with a smaller S-T gap, i.e. saturated imidazolidin-2-ylidenes (∼238−245 ppm). Benzimidazolin-2ylidenes have intermediate S-T gaps and chemical shifts in between those of the former two (∼223−232 ppm). Metal coordination of NHCs occurs via donation of their carbene lone pair into empty metal orbitals, and thus the probability for the aforementioned S-T transition is essentially removed. This is why a significant upfield shift of the carbene atom is observed upon complexation. HEP exploits the interplay between availability of the carbene lone pair and its chemical shift in trans-[PdBr2(iPr2-bimy)L] complexes. A stronger trans donor L would therefore weaken the Pd−iPr2-bimy

HEPa 175.2 176.4 176.6 177.5 178.3 181.4 184.0

a

Measured in CDCl3 and referenced to the solvent signal at 77.7 ppm. ditz denotes the triazole-derived Janus-type diNHC ligand 1,2,4triazolidin-3,5-diylidene.

b

and an external base such as K2CO3.175,176 In route B, a gold complex already containing the iPr2-bimy ligand can be treated with the NHC precursor of interest. A suitable complex is [Au(OAc)(iPr2-bimy)], which already contains a “built-in” base. The acetato ligand in this complex can deprotonate the azolium salt to generate the NHC in situ for coordination to gold.173 The heterobis(carbene) complexes of palladium and gold are both generally air and moisture stable and highly soluble in V


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Figure 5. Relationship of carbene chemical shifts and ligand donor strength in HEP.

bond more so than a weaker one (trans influence), leading to an enhanced “free iPr2-bimy” character, which in turn leads to a downfield shift (Figure 5). In general, such a bond elongation has also been observed in solid-state molecular structures of relevant “HEP” complexes, e.g. from 1.936(2) Å for L = CH3CN to 2.019(6) Å for 1,2-diethylindazolin-3-ylidene (Indy9).

Table 12. HEP Values of NHCs in Increasing Order NHC Br2

IPr IMesBr2 Bn-btzy H-bozy Bn2-tazy IAd IMes IPr SIMes SIPr ItBu SIAd SIPrtBu IMesBn Bn2-bimy SItBu bimy-A SIPrPh bimy-B IPrMe IBn bimy-C bimy-D bimy-E Trz11 Trz15 bimy-F Trz23 SIBn Trz16 Trz17 Trz18 i Pr2-bimy IiPr Trz19 Trz20 Trz21 ICy Trz22 Indy8 Indy9 aNHC14 Pyry1 Pyry2 Pyry3

6.3. Electronic Properties of NHCs on the HEP Scale

The HEPs of 45 NHCs have currently been determined, and these are listed in Table 12. Figures 7 and 8 show a plot of their structures on a 13C NMR scale according to their HEP values. Notably, the methodology has allowed for the detection of both backbone and substituent-effects of classical and nonclassical NHCs. Among the four types of classical NHCs with the same N-benzyl substituents (Chart 11), HEP could reveal that the donor ability decreases in the order imidazolidin-2-ylidene (saturated) > imidazolin-2-ylidene (unsaturated) > benzimidazolin-2-ylidene (benzannulated) > 1,2,4-triazolin-5-ylidene (additional nitrogen atom). This order is fully consistent with chemical intuition and can be easily understood by comparing the different electronegativities/inductive effects of the backbones. For example, the sp3-hybridized C4/5 carbon atoms in SIBn are expected to be more electron releasing than the sp2-hybridized counterparts in IBn. Benzannulation in Bn2-bimy further reduces the +I effect, and exchange of a carbon atom with an electron-withdrawing nitrogen atom results in the weakest donor in Bn2-tazy. The influence of substituents is exemplified in a series of imidazolin-2-ylidenes, which constitutes the most common type of NHCs (Chart 12, upper). Stepwise replacement of the aromatic mesityl groups in the famous IMes ligand with a more electron releasing benzyl group leads to a stepwise increase of the donating ability that is reflected in the stepwise increase of HEPs of the IMesBn and IBn ligands. On the other hand, installation of two electronegative bromo-substituents at C4 and C5 leads to a lowering of the HEP value in line with a significantly lower donor strength of IMesBr2. Similar substituent effects can also be detected in the increasingly popular mesoionic 1,2,3-triazolin-5-ylidenes. As depicted in Chart 12 (lower), the HEPs increase from Trz17 (Ph) via Trz20 (Bn) to Trz22 (iPr), which is consistent with the increasing +I effect of their N1-substituents in the order Ph < Bn < iPr. On the other hand, the electronics of Trz19 and Trz20 cannot be further differentiated. These two NHCs differ only at the more remote N3-position with respect to the carbene donor atom. Moreover, the methyl and the benzyl group are electronically quite similar. It is worth noting that a wide range of such mesoionic NHC could not been differentiated using the IrI or RhI dicarbonyl systems, which gave identical averaged wavenumbers.

HEPa b

175.3 175.6b 176.4 176.5 176.6 177.1 177.2 177.5 177.6 177.6 177.6 177.8 178.2 178.3 178.3 178.6 178.6 178.7 178.8 178.9 179.0 179.2 179.3 179.3 179.5b 179.5b 179.6 180.0 180.1 180.2b 180.3 180.4b 180.6 180.6 180.8 180.8 181.0b 181.2 181.2 181.4 181.6 181.9 182.4 182.8 184.0

ref. 152 152 173 178 19, 173 177 19 19, 173 19 19 177 177 177 19 19, 173 177 179 177 179 177 19 179 179 179 180 180 179 181 19 180 182 180 19, 183 177 182 182 180 177 182 173 19 19 19 184 173, 176

a

Measured in CDCl3 and referenced to the solvent signal at 77.7 ppm. Obtained by addition of 0.54 ppm to the original value referenced to 77.16 ppm.

b

W


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will always interfere with the electronics of any system. As such, the potential donating abilities of the very bulky ligands ItBu, SItBu, IAd, and SIAd appear to be underestimated also on HEP scale. A first stereoelectronic NHC map that plots HEP versus %Vbur determined using the same complex probes (Figure 6) has recently been employed to rationalize the structure−activity relationship (SAR) of NHC ligands in the iron catalyzed Kumada coupling.177 The plot clearly shows that the best performers are very bulky NHCs located in the upper left area and that electronic effects are secondary in nature for this reaction. Using the same plot, one can also see that very bulky NHCs with %Vbur > 34 have relatively small HEPs, which in some cases could be underestimated. The increased steric bulk probably leads to a less efficient metal coordination and thus dilutes electronic contributions. Overall, HEP is a viable electronic parameter that detects primarily the σ-donating ability of ligands. It is particularly useful for the detection of more subtle variations in the ligand structure that other methodologies are incapable of resolving.

Figure 6. Plot of HEP versus %Vbur. Yields obtained for coupling product are given in parentheses.

As with any methodology, HEP also has its limitations that remain to be explored. It is obvious that increasing steric bulk

Figure 7. Donor abilities of N-heterocyclic carbene ligands on the 13C NMR spectroscopic scale.

Figure 8. Donor abilities of nonclassical N-heterocyclic carbene ligands on the 13C NMR spectroscopic scale. X


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Chart 11. Evaluation of Backbone Effects of Classical NHCs using HEP

Scheme 13. Preparative Routes to NHC-Phosphinidene Adducts

Chart 12. Evaluation of Substituent Effects in Imidazolin-2ylidenes and Mesoionic 1,2,3-Triazolin-5-ylidene Using HEP as byproducts. Hudnall reported that addition of TMSOTf improves the reactivity.185 The third pathway is a further improvement of route two, whereby the NHC is reacted with one equiv of dichlorophenylphosphine to furnish an intermediate salt. In a second step, the isolated salt is then reduced to the desired product by potassium graphite or elemental magnesium.23 So far, a small library of structurally diverse NHCs have been studied using this method mostly in C6D6 as the solvent of choice and covering a wide chemical shift range from −61.2 ppm to +83.0 ppm (Table 13). The structures of the respective Table 13. 31P NMR Data of NHC-Phosphinidene Adducts NHC

7. NMR SPECTROSCOPY OF CARBENE-PHOSPHINIDENE ADDUCTS In 2013, Bertrand and co-workers proposed the use of carbenephosphinidene adducts as probes for the evaluation of π-accepting properties of carbenes.23 The basis for this concept is the fact that such adducts can be represented by either the phosphinidene form A or the phosphaalkene form B (Figure 9).

DAC1a OAC CAACCy DippThiaC4 CAACMenthyl 6-NHCpyr 6-Dipp MAAC SIPr SIMes IPr IMes i Pr2-bimy IMeMe2 IiPr a

Figure 9. Relationship between the two extreme resonance forms and 31 P NMR spectroscopy.

31

P NMR [ppm] 83.0 78.6 68.9 57.0 56.2 34.9 14.8 39.7/37.7 -10.2a −10.4 −18.9 −23.0 −34.6 −53.5 -61.2b

ref. 185 185 23 23 23 23 23 23, 185 23 23 23 23 23 23 23

Measured in CDCl3. bMeasured in THF-d8.

NHCs are depicted in Chart 13 in order of decreasing chemical shifts. Using this methodology, it was found that saturated imidazolidin-2-ylidenes (SIPr, SIMes) induce a more downfield 31 P NMR shift compared to their unsaturated counterparts (IPr, IMes). It was therefore concluded that the former are better π-acceptors. Pyramidalization (e.g., 6-NHCpyr) or substitution of an adjacent nitrogen atom (e.g., CAACs) reduces the competing donation from nitrogen into the pπ acceptor orbital of the carbene, making such carbenes even better acceptors. Finally, the acceptor capability of an NHC can also be increased by introduction of electron-withdrawing carbonyl-functions, which lower the energy of the pπ acceptor orbital. The results obtained via this methodology enhance our knowledge on the electronic properties of NHCs and related species. Moreover, some “unexpected” observations made using the carbonyl-based methods can be better rationalized, e.g. IMes vs SIMes.

Weak π-acceptors would adopt an electronic situation closer to form A, in which the carbene acts primarily as a σ-donor leading to an electron rich phosphorus atom with formally two lone pairs. This in turn would result in an upfield 31P NMR signal. Form B is dominant with good π-acceptors leading to a deshielded phosphorus nuclei. The enhanced backdonation contributes to the paramagnetic shielding term, and a significant downfield 31P NMR signal is obtained. The preparation of the phosphinidene adducts has been accomplished from the free carbenes by three routes as shown in Scheme 13. The first route involves direct reaction of the free NHC with pentaphenylcyclopentaphosphane. Alternatively, the free NHC can be reacted with dichlorophenylphosphine. Here, one equiv of NHC acts as a sacrificial reagent to trap the released chlorine atoms to give 2-chloroazolium chloride salts Y


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Chart 13. Lewis Structures of NHCs and the 31P NMR Chemical Shifts of their Phosphinidene Adducts in C6D6

Table 14. 77Se NMR Data of NHC-Derived Selenoureas Obtained in Acetone-d6 NHC 6-CAAC OAC DAC1a MAAC AAC1 DippThiaC4 i Pr2-pery 6-Mes SIMes ItBu Trazy SIMesO PyI SIPr SIPhF SIPrOMe IPrCl2 Triazo1 Triazo3 CMB IMesNMe3 INon IPent IHept Triazo4 IPr SICy12 Tr3 IPrOMe Tr2 i Pr2-bimy Tr4 IMesNMe2 Tr1 IMes ICy12 IMeMe2 IiPr ICy

Figure 10. Simplified HOMO−LUMO diagram of three generic ligands I−III.

Figure 11. Relationship between the two extreme resonance forms and 77Se NMR spectroscopy.

Scheme 14. General Synthetic Route to Selenoureas

Nevertheless, it must be noted that the method does not necessarily determine the pure π-acceptor property of a ligand. From a more critical point of view, it is actually the magnitude of backdonation to the carbene that is being measured, which is not entirely influenced by (i) the π-acceptor property of the carbene alone but also by (ii) the overall electron-density at phosphorus. The latter is in turn affected by the donating ability of the carbene. In classical synergic bonding, the lone pair donation of an L- or X-type ligand to a Lewis acid is the first interaction that leads to a single bond. Backdonation is “only” the second interaction that then leads to an enhanced doublebond character. As such, it cannot be truly independent from the σ-donating ability of the ligand. Z

77

Se NMR [ppm] 1180 856 847 472 437 396 364 271 116, 113 197 196 184 183 181 178 177 174 137 119 103 102 97 96 96 91 87 86 86 83 74 67 49 43 40 35 4 3 −3 −4

ref. 156 24 25 24 25 25 25 24 188, 25 187 25 25 25 24 25 36 188 116 116 189 107 187 187 187 116 24 187 187 36 187 24 187 107 187 188 187 25 25 187


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Table 15. 77Se NMR Data of NHC-Derived Selenoureas Obtained in CDCl3 NHC CAArC4 CAArC3 FNHC8 FNHC7 CF3NPh-bimy CF3FPy-bimy CF3TPh-bimy CF3Ph-bimy CF3Me-bimy CF3Bu-bimy IAd CF3iPr-bimy SIMesO2BPh SIPr ItBu IPrCl2 SIMes(OH)2 IPtBu2 SIMesOH,OMe Trt SIMesF5 Tr5 IPrPtBu2 SIMes TPT IPr* Nitron IPr*OMe INon IPent

77

Se NMR [ppm]

ref.

616 601 307 303 276 275 264 249 215 204 197 196 194 190 183 174 169 167 167 163 155 131 131 110 110 106 106 104 102 101

190 190 191 191 161 161 161 161 161 192 187 161 193 36 187 188 193 194 193 106 192 187 194 188 195 36 195 36 187 187

NHC IHept IPr CMB IPrOMe SIMesCy Tr2 SICy12 Tr3 IMesNMe2,Cl Me2-bimy Bu2-bimy Tr4 IMesNiPr2 Tr1 IMesNMe2 IoXyl IMes IMes(NMe2)2 Me2-tazy ImPy9 ImPy8 ImPy7 IMesBu ImPy5 ImPy11 ImPy3 ICy12 IiPrMe2 ICy dipiytBu

77

Se NMR [ppm] 100 90 89 87 86 77 75 74 68 65 55 47 36 34 32 30 27 27 20 19 14 9 9 5 −3 −10 −11 −18 −22 −56

ref. 187 36 189 36 192 187 187 187 87 57 192 187 87 187 107 187 188 87 57 57 57 57 192 57 57 57 187 188 187 62

Chart 14. Comparison of 77Se NMR Shifts of Selected NHC-Derived Selenoureasin CDCl3

Along the same line, 77Se NMR spectroscopy of these compounds should allow for the detection of π-backbonding to carbenes since very similar extreme resonance forms A and B can be proposed for selenoureas as well (Figure 11).186 The preparation of the required selenoureas can be straightforwardly achieved by deprotonation of the respective azolium salts in the presence of elemental selenium in THF at low temperatures (Scheme 14). KHMDS, NaHMDS, and KtOBu were found to be suitable bases. The ease of access to selenoureas has resulted in a much larger number of NHCs being evaluated for their acceptor abilities. Notably, the 77Se NMR chemical shift is quite sensitive to solvent, concentration, temperature, and pH value. This is particularly important since all studies so far have employed either acetone-d6 (KSeCN in D2O as the external standard)24 or CDCl3 (relative to SeMe2 and externally referenced to PhSe-SePh)187

Figure 10 is a simplified illustration for this notion. It also depicts the HOMOs and LUMOs of three generic ligands. All three LUMOs are of equal energy, which would mean their π-accepting properties are equal. However, their donating abilities differ as indicated by their HOMO levels. When bound to the same fragment one could conclude that the amount of backdonation decreases in the order II > I > III due to decreasing overall electron density of their adducts. Therefore, the chemical shifts of these adducts will also differ despite containing “equal” π-acceptors. In reality, the situation must be even more complicated, since donation of the lone pair in the HOMO will also alter other orbital energies.

8. NMR SPECTROSCOPY OF SELENOUREAS Inspired by Bertrand’s work, Ganter proposed the use of synthetically less challenging selenourea probes in the same year.24 AA


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Chart 15. Lewis Structures of NHCs and the 77Se NMR Chemical Shifts of their Selenoureas in Acetone-d6

identical resonance as that measured for IMes (27 ppm). Also surprising is that the more donating diisopropylamino substituent leads to a marginally more downfield shift compared to the dimethylamino substituent. Clearly, more in-depth studies need to be done to understand these results, which should be treated, as all other data, with some caution before conclusions are made. In particular, any NMR spectroscopic evaluation of NHCs with aromatic substituents can be influenced by anisotropy effects, which may lead to unexpected results.

as the solvent of choice, and it must be emphasized once more that comparisons of NHCs can only be made in the same solvent system. Tables 14 and 15 summarize the data thus available, and Charts 15 and 16 depict the NHC structures in order of decreasing 77Se NMR chemical shifts. Overall, the NMR data covers an even larger range compared to the phosphinidene system, i.e. −4 ppm to 1180 ppm for acetone-d6 and −56 ppm to 616 ppm for the CDCl3. A reasonable linear correlation (R2 = 0.91) has been established between the 31P and 77Se NMR shifts of phosphinidene adducts and selenoureas, respectively.25 Thus, the same general trends with respect to π-acceptor properties were observed for most cases (vide supra). However, analysis of the available data also gives some surprises that trigger new questions (Chart 14). For example, the shift for the selenourea of Me2-bimy (65 ppm) is significantly more downfield that that for Me2-tazy (20 ppm). Can one conclude that benzimidazolin-2-ylidenes are indeed better π-acceptors than 1,2,4-triazolin-5-ylidenes? Moreover, for a given type of NHC, certain substituent influences are difficult to understand. The NHCs IAd (197 ppm) and ItBu (183 ppm) with tertiary alkyl substituents exhibit unexpectedly downfield signals. What makes them such good π-acceptors compared to their analogues with secondary alkyl substituents, i.e. IiPrMe2 (−18 ppm) and ICy (−22 ppm)? Maybe these results are due to steric interferences or due to their expected strong σ-donating ability, which in turn promotes backdonation (Figure 10). Another peculiar observation is made when dialkylamino-substituents are introduced at the C4/5 positions of the parent IMes ligand. Monosubstitution leads to a downfield chemical shift (IMesNMe2, 32 ppm), while disubstitution (IMes(NMe2)2, 27 ppm) gives an

9. 1J(C−H) HETERONUCLEAR COUPLING CONSTANTS OF AZOLIUM SALTS In addition to HEP, the arguably most recent method for evaluating electronic properties of NHCs and related species also makes use of 13C NMR spectroscopy. Azolium salts are the most common NHC precursors, and they could arguably be considered as “complexes”, where the carbene is coordinated to the proton as the Lewis acid. Ganter proposed that the respective heteronuclear 1J(C−H) coupling constant of an azolium salt could be potentially used to gauge the σ-donating ability of the respective NHC. They hypothesized that the magnitude of the 1J(C−H) coupling constants, which correlates to the s-character of the C−H bond, also inversely correlates with the σ-donor strength of the parent NHC. Thus, a weaker donor would have a larger 1J(C−H) value indicating larger s-character of the carbene lone pair (σ-orbital).25 So far, a small number of azolium salts have been tested for this notion, and the relevant data is summarized in Table 16, while the structures of these salts are depicted in Chart 17. The coupling constants cover a range from 200 to 233 Hz. At first sight, one can notice that the AB


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Chart 16. Lewis Structures of NHCs and the 77Se NMR Chemical Shifts of their Selenoureas in CDCl3

the latter should intuitively be the weakest donor. Nevertheless, the method holds promise for the evaluation of NHCs due to the easy access to azolium salts. At this early stage, however, clear conclusions on the suitability methodology cannot be made. More detailed studies are required, but all future comparisons should only be systematically made among salts bearing the same anion, which are measured under identical conditions.

various salts investigated differ in the counteranions. In some cases, the latter have not been indicated in the original literature, and these are represented with “X”. Moreover, analyses were done in different deuterated solvents. It must be noted that both counteranions and solvents influence the chemical shift of the C−H bond in an azolium salt and possibly also its coupling constant.196 Thus, the data presented should be treated with caution to avoid misinterpretations. For example, the same coupling constant has been determined for salts of the three structurally very different NHCs iPr2-bimy, DippThiaC4, and AAC1, although

10. CONCLUSIONS AND OUTLOOK This review article has brought the most common experimental methods for the study of electronic properties of NHCs AC


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Table 16. 1J(C−H) Coupling Constants of Azolium Salts Azolium salt Trazy·HBr 6-Mes·HX SIMes·HCl MAAC·HCl i Pr2-bimy·HBr DippThiaC4·HX AAC1·HNO3 CMB·HBF4 IPr·HBF4 IMesNMe2·HOTf IMes·HCl TPT·HClO4 Nitron·HClO4 IPrCl2·HX Trt·HBF4 IMesNMe3·HOTf dipiy·HBr IPrPMePPh2·HOTf

1

J(C−H) [Hz]

Solvent

ref.

200 200 206 206 218 218 218 224 224 225 225 229 229 229 230 231 233 233

DMSO-d6 DMSO-d6 DMSO-d6 CDCl3 DMSO-d6 DMSO-d6 DMSO-d6 acetone-d6 DMSO-d6 Not stated CDCl3 CD3CN CD3CN DMSO-d6 DMSO-d6 CD3CN DMSO-d6 CD2Cl2

25 25 25 25 25 25 25 189 25 107 25 195 195 25 106 107 197 198

in many cases. On the other hand, redox potentials of complexes are more reflective of the overall electron density of a complex and are more useful in terms of ranking NHCs’ donicities. Nevertheless, electrochemical methods are limited to noninnocent ligands, and only complex probes with a reversible redox chemistry can be truly evaluated. The HEP is a more recent nondestructive methodology that uses well-resolved 13 C NMR spectroscopy of stable palladium(II)-NHC complexes for evaluation. Due to the more Lewis acidic metal center, backdonation becomes insignificant, and this method detects primarily the σ-donating ability of NHCs. The acceptor properties of NHCs can be studied by NMR spectroscopy of their phosphinidene adducts or selenoureas, where the 31P or 77 Se NMR chemical shifts are sensitive to the double-bond character of the NHC-adduct. However, the amount of elementto-NHC backdonation cannot be completely independent from the donicity of the NHC itself due to the nature of synergic bonding. Overall, various methodologies are available as a tool box to the carbene chemist. Even so, more detailed research and further developments can be anticipated in this important area for a more in depth understanding of the more recent parameters. However, it is important that data analysis and conclusions are done in a proper and circumspect manner. These contributions will pave the way to a better understanding of structure−activity relationships observed in NHC chemistry, which in turn will aid in the design of more efficient systems for applications in various areas.

together in an attempt to reveal and compare their strengths and weaknesses. The different methods make use of different spectroscopic signatures in the complex probes and do not necessarily measure the same electronic property. Thus, no methodology can be regarded as “the best”. As highlighted throughout the article the overall electronic properties consist of donor and acceptor contributions. The most popular carbonyl-based methodologies measure the amount of backdonation to the carbonyl ligands by IR spectroscopy and utilize electron-rich metal centers (Ni0, RhI, and IrI). As such, they can only detect an overall effect of a NHCs on these metal centers. In contrast to common phosphines, the donor−acceptor abilities of NHCs are not inversely proportional. Thus, backdonation to the NHC competing with that to the carbonyl ligands complicates proper interpretation of the data, clouding any clear and rational trends

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Han Vinh Huynh: 0000-0003-4460-6066 Notes

The author declares no competing financial interest.

Chart 17. Structures of Azolium Salts and the 1J(C−H) Coupling Constants of their Precarbene Carbon Atoms

AD


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Biography

(16) Falivene, L.; Credendino, R.; Poater, A.; Petta, A.; Serra, L.; Oliva, R.; Scarano, V.; Cavallo, L. SambVca 2. A Web Tool for Analyzing Catalytic Pockets with Topographic Steric Maps. Organometallics 2016, 35, 2286−2293. (17) Lever, A. B. P. Electrochemical Parametrization of Metal Complex Redox Potentials, Using the ruthenium(III)/ruthenium(II) Couple to Generate a Ligand Electrochemical Series. Inorg. Chem. 1990, 29, 1271−1285. (18) Lever, A. B. P. Electrochemical Parametrization of Rhenium Redox Couples. Inorg. Chem. 1991, 30, 1980−1985. (19) Huynh, H. V.; Han, Y.; Jothibasu, R.; Yang, J. A. 13C NMR Spectroscopic Determination of Ligand Donor Strengths Using NHeterocyclic Carbene Complexes of Palladium(II). Organometallics 2009, 28, 5395−5404. (20) Teng, Q.; Huynh, H. V. A Unified Ligand Electronic Parameter Based on 13C NMR Spectroscopy of N-Heterocyclic Carbene Complexes. Dalton Trans. 2017, 46, 614−627. (21) Teng, Q.; Huynh, H. V. Determining the Electron-Donating Properties of Bidentate Ligands by 13C NMR Spectroscopy. Inorg. Chem. 2014, 53, 10964−10973. (22) Fantasia, S.; Petersen, J. L.; Jacobsen, H.; Cavallo, L.; Nolan, S. P. Electronic Properties of N-Heterocyclic Carbene (NHC) Ligands: Synthetic, Structural, and Spectroscopic Studies of (NHC)platinum(II) Complexes. Organometallics 2007, 26, 5880−5889. (23) Back, O.; Henry-Ellinger, M.; Martin, C. D.; Martin, D.; Bertrand, G. 31P NMR Chemical Shifts of Carbene-Phosphinidene Adducts as an Indicator of the π-Accepting Properties of Carbenes. Angew. Chem., Int. Ed. 2013, 52, 2939−2943. (24) Liske, A.; Verlinden, K.; Buhl, H.; Schaper, K.; Ganter, C. Determining the π-Acceptor Properties of N-Heterocyclic Carbenes by Measuring the 77Se NMR Chemical Shifts of Their Selenium Adducts. Organometallics 2013, 32, 5269−5272. (25) Verlinden, K.; Buhl, H.; Frank, W.; Ganter, C. Determining the Ligand Properties of N-Heterocyclic Carbenes from 77Se NMR Parameters. Eur. J. Inorg. Chem. 2015, 2015, 2416−2425. (26) Nelson, D. J.; Nolan, S. P. Quantifying and Understanding the Electronic Properties of N-Heterocyclic Carbenes. Chem. Soc. Rev. 2013, 42, 6723−6753. (27) Lappert, M. F.; Pye, P. L. Carbene Complexes. Part 12. Electron-Rich Olefin-Derived Neutral Mono- and Bis-(Carbene) Complexes of Low-Oxidation-State Manganese, Iron, Cobalt, Nickel, and Ruthenium. J. Chem. Soc., Dalton Trans. 1977, 2172−2180. (28) Ö fele, K.; Herrmann, W. A.; Mihalios, D.; Elison, M.; Herdtweck, E.; Scherer, W.; Mink, J. Mehrfachbindungen Zwischen Hauptgruppenelementen Und Ü bergangsmetallen. J. Organomet. Chem. 1993, 459, 177−184. (29) Herrmann, W. A.; Goossen, L. J.; Köcher, C.; Artus, G. R. J. Chiral Heterocylic Carbenes. Angew. Chem., Int. Ed. Engl. 1996, 35, 2805−2807. (30) Herrmann, W. A.; Goossen, L. J.; Artus, G. R. J.; Köcher, C. Metal Complexes of Chiral Imidazolin-2-Ylidene Ligands. Organometallics 1997, 16, 2472−2477. (31) Dorta, R.; Stevens, E. D.; Hoff, C. D.; Nolan, S. P. Stable, ThreeCoordinate Ni(CO)2(NHC) (NHC = N-Heterocyclic Carbene) Complexes Enabling the Determination of Ni-NHC Bond Energies. J. Am. Chem. Soc. 2003, 125, 10490−10491. (32) Dorta, R.; Stevens, E. D.; Scott, N. M.; Costabile, C.; Cavallo, L.; Hoff, C. D.; Nolan, S. P. Steric and Electronic Properties of NHeterocyclic Carbenes (NHC): A Detailed Study on Their Interaction with Ni(CO)4. J. Am. Chem. Soc. 2005, 127, 2485. (33) Balogh, J.; Slawin, A. M. Z.; Nolan, S. P. Bulky N -Heterocyclic Carbene IPr* in Selected Organo- and Transition Metal-Mediated Catalytic Applications. Organometallics 2012, 31, 3259−3263. (34) Collado, A.; Balogh, J.; Meiries, S.; Slawin, A. M. Z.; Falivene, L.; Cavallo, L.; Nolan, S. P. Steric and Electronic Parameters of a Bulky yet Flexible N-Heterocyclic Carbene: 1,3-Bis(2,6-bis(1-Ethylpropyl)phenyl)imidazol-2-Ylidene (IPent). Organometallics 2013, 32, 3249− 3252.

Han Vinh Huynh completed his doctoral degree (Dr. rer. nat.) in 2002 from the University of Münster, Germany, under the supervision of Professor F. E. Hahn, where he worked on complexes of polythiolato ligands. He then moved to the Department of Chemistry at the National University of Singapore (NUS) and started his independent research career as a Feodor-Lynen Research Fellow (Alexander von Humboldt-Foundation) embarking on the study of N-heterocyclic carbenes and related species. In 2007, he became Assistant Professor at NUS and was promoted to Associate Professor in 2011. His research areas include organic and organometallic chemistry with particular focus on N-heterocycles and catalysis.

ACKNOWLEDGMENTS I thank all my current and former co-workers for their trust and dedication to our research. Financial support from the National University of Singapore and the Singapore Ministry of Education (WBS R-143-000-669-112) is gratefully acknowledged. REFERENCES (1) Herrmann, W. A.; Köcher, C. N-Heterocyclic Carbenes. Angew. Chem., Int. Ed. Engl. 1997, 36, 2162−2187. (2) Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. Stable Carbenes. Chem. Rev. 2000, 100, 39−92. (3) Herrmann, W. A. N-Heterocyclic Carbenes: A New Concept in Organometallic Catalysis. Angew. Chem., Int. Ed. 2002, 41, 1290−1309. (4) Hahn, F. E.; Jahnke, M. C. Heterocyclic Carbenes: Synthesis and Coordination Chemistry. Angew. Chem., Int. Ed. 2008, 47, 3122−3172. (5) Schuster, O.; Yang, L.; Raubenheimer, H. G.; Albrecht, M. Beyond Conventional N-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization. Chem. Rev. 2009, 109, 3445−3478. (6) Tapu, D.; Dixon, D. A.; Roe, C. 13C NMR Spectroscopy of “Arduengo-Type” carbenes and Their Derivatives. Chem. Rev. 2009, 109, 3385−3407. (7) de Frémont, P.; Marion, N.; Nolan, S. P. Carbenes: Synthesis, Properties, and Organometallic Chemistry. Coord. Chem. Rev. 2009, 253, 862−892. (8) Huynh, H. V. The Organometallic Chemistry of N-Heterocyclic Carbenes; John Wiley & Sons, Ltd: Chichester, UK, 2017. (9) Hansch, C.; Leo, A.; Taft, R. W. A Survey of Hammett Substituent Constants and Resonance and Field Parameters. Chem. Rev. 1991, 91, 165−195. (10) Tolman, C. A. Electron Donor-Acceptor Properties of Phosphorus Ligands. Substituent Additivity. J. Am. Chem. Soc. 1970, 92, 2953−2956. (11) Tolman, C. A. Steric Effects of Phosphorus Ligands in Organometallic Chemistry and Homogeneous Catalysis. Chem. Rev. 1977, 77, 313−348. (12) Hillier, A. C.; Sommer, W. J.; Yong, B. S.; Petersen, J. L.; Cavallo, L.; Nolan, S. P. A Combined Experimental and Theoretical Study Examining the Binding of N-Heterocyclic Carbenes (NHC) to the Cp*RuCl (Cp* = η5-C5Me5) Moiety: Insight into Stereoelectronic Differences between Unsaturated and Saturated NHC Ligands. Organometallics 2003, 22, 4322−4326. (13) Clavier, H.; Nolan, S. P. Percent Buried Volume for Phosphine and N-Heterocyclic Carbene Ligands: Steric Properties in Organometallic Chemistry. Chem. Commun. 2010, 46, 841. (14) Gómez-Suárez, A.; Nelson, D. J.; Nolan, S. P. Quantifying and Understanding the Steric Properties of N-Heterocyclic Carbenes. Chem. Commun. 2017, 53, 2650−2660. (15) Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.; Cavallo, L. SambVca: A Web Application for the Calculation of the Buried Volume of N-Heterocyclic Carbene Ligands. Eur. J. Inorg. Chem. 2009, 2009, 1759−1766. AE


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