Pushing ElectronsâWhich Carbene Ligand for Which Application

Tutorial Cite This: Organometallics 2018, 37, 275−289

Pushing ElectronsWhich Carbene Ligand for Which Application? Dominik Munz* Friedrich-Alexander Universität Erlangen-Nürnberg, Egerlandstraße 1, 91058 Erlangen, Germany ABSTRACT: This tutorial explores the diversity and characteristics of C-donor ligands. Particular emphasis is put on the conceptual design of the electronic properties for applications in coordination chemistry. More specifically, the implications of both σ and π effects are discussed. Cyclic (alkyl)(amino)carbenes as well as methanediides and carbones are brought in perspective to “conventional” N-heterocylic carbenes, mesoionic carbenes, and Fischer as well as Schrock carbene ligands.

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singlet state. Concomitant σ-donation from the carbene lone pair to a low-valent metal center and π-back-donation into the formally vacant carbene pπ orbital indicate comparably strong double-bond character. Fischer carbene ligands are electrophilic due to insufficient π-donation from the metal and the adjacent alkoxy substituent (Figure 2).

INTRODUCTION Transition metal carbene complexes have been known for more than a century.1 Nevertheless, it took until the seminal isolation of Bertrand’s2 acyclic push−pull carbene in 1988 and Arduengo’s3 crystalline N-heterocyclic carbene (NHC) in 1991 that ancillary carbene ligands were established as ubiquitous key players in organometallic chemistry (Figure 1). The isolation of these octet-defying molecules has then inspired scientists to develop commercially important applications in catalysis, drug design, and photochemistry.

Figure 2. Stabilization of the Fischer carbene complex through πdonation from an adjacent heteroatom. Figure 1. Bertrand’s acyclic push−pull carbene and Arduengo’s crystalline N-heterocyclic carbene (Ad = adamantyl).

In contrast, alkylidene complexes (i.e., Schrock carbene ligands)7 without heteroatom substituents feature typically a high-valent, early transition metal. A negative partial charge resides on Schrock carbenes, which are consequently nucleophilic. Their electronic structure can be best understood on the basis of five resonance structures, which can serve as prototypes for any carbene−metal bond (Figure 3).8 The most important resonance structure (I) corresponds to the interaction between a metal and a carbene in both their

A huge variety of carbene ligands with distinct electronic properties and steric profiles are now available for the synthetic chemist. Although plentiful reviews have been published on different facets of free carbenes and carbene ligands,4 the choice of the right carbene ligand for a desired property of a metal complex is not a straightforward task. Indeed, spotting the differences between coordination compounds of Fischer carbenes, cyclic (alkyl)(amino)carbenes (CAACs), NHCs, mesoionic carbenes (MICs), bent allenes, or methanediides can be challenging and confusing.5 This tutorial aims therefore at giving the reader a concise and comprehensible overview of the design principles for carbene coordination compounds and their applications. Particular attention is placed on cyclic and acyclic singlet carbene ligands and their relation to other carbon-based donor ligands.

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FROM FISCHER’S CARBENE LIGAND TO HETEROCYCLIC SINGLET CARBENES Fischer’s carbene complex was discovered in 1964.6 The carbene−metal bond was rationalized by a donor−acceptor interaction between the metal and the carbene in its 1A1 σ2 © 2018 American Chemical Society

Figure 3. Resonance structures of tungsten methylidene model complex H3W(CH2) with weight of resonance structures. Received: September 20, 2017 Published: February 12, 2018 275

DOI: 10.1021/acs.organomet.7b00720 Organometallics 2018, 37, 275−289

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Organometallics triplet states. This “textbook” resonance structure is covalent in character. Resonance structures II and III, which mainly account for the polarization of the carbene−metal bond, involve a dative interaction from the ligand to the metal (L→ M). The resonance structure IV describes the bond as the reaction between the dianionic methanediide H2C2− and a biscationic metal center, whereas V refers to a Fischer-type σdonation−π-back-donation interaction. It has been suggested that this resonance structure is more significant for Grubbs’ ruthenium alkylidene complexes,9 which were therefore described as “moderately electrophilic Schrock carbenes”.10 Thus, the carbene−metal bond features mixed nondative covalent and dative character. Classic Fischer and Schrock carbene ligands are usually represented with a double bond due to a historically rooted convention. The description of related carbene−metal or carbene−main-group-element bonds11 with a double bond, an arrow emphasizing a dative bond, or a dash representing a covalent single bond is however a matter of continuous debate.12 In fact, unambiguously deciphering the electronic structure of a particular carbene−metal bond is a challenge. A recent comparative assessment concluded that the dative character of the (NHC):→Rh interaction is on the same order of magnitude as that found for phosphine ligands.13 It was likewise suggested that the dative character of a rhodium(I) phosphonium ylide is still quite strong. In this Tutorial, an arrow represents all bonds of carbene-type ligands with metals for the sake of clarity and consistency. The simplified molecular orbital diagram of a Fischer carbene complex reveals that the σ-bond is polarized toward the carbene atom (Figure 4, left), whereas the π-bond is rather localized on

Figure 5. Acceptor-substituted carbene complexes of rhodium and copper.

the destabilizing π-interaction with the carbene π-orbital, which vice versa is mesomerically stabilized by the conjugated and electron-releasing anisolyl substituent.19 Diarylcarbenes are consequently expected to form comparably stable metal complexes. Note that phenyl substituents are considered to be both weak σ-acceptors and π-donors.20 Indeed, diarylcarbene complexes have been reported for almost all late transition metals (eq 2).21

These complexes are typically electrophilic with metal to ligand charge transfer bands in their UV−vis spectra. A particularly large steric shielding (i.e., kinetic protection) of the electrophilic carbene atoms has been noted for mesityl in relation to phenyl substituents. The energies of the HOMO and LUMO of a carbene can be controlled by interactions of σ- and π-type with the (hetero)atoms bonded to the carbene (Figure 4, right). Substitution of the alkoxy group of a classic Fischer carbene by an amino group will therefore elevate the energy of the HOMO through nitrogen’s reduced negative inductive effect. Accordingly, the carbene will be more nucleophilic and the σinteraction between the carbene and transition metal becomes stronger. Additionally, the energy of the LUMO will be higher due to the stronger positive mesomeric effect of the nitrogen atom. Consequently, such an (alkyl)(amino)carbene ligand is expected to show reduced π-acidity. The metal−carbene bond therefore obtains reduced double-bond character with enhanced σ-bonding and reduced π-back-bonding contributions. These acyclic (alkyl)(amino)carbenes are stable enough to be isolable (Figure 6)22 and coordinate to transition metals such as rhodium(I).23 The introduction of a second amino group provides further stabilization (Figure 7, Alder’s carbene).24 Decreasing π-accepting properties are expected for bonds with transition metals. Note that the weight of the two ylidic resonance structures, which can be represented as well with a

Figure 4. Molecular orbital interactions for Fischer carbene complex (left) and amino Fischer carbene complex (right).

the metal. Importantly, both an energetically high lying carbene HOMO (highest occupied molecular orbital) and a low-lying carbene LUMO (lowest unoccupied molecular orbital) will lead to strong bonding interactions with the metal’s d-orbitals. Free Fischer carbenes with electron-withdrawing α-carbonyl substituents have an energetically low lying HOMO, show strong radical character due to their very small singlet/triplet gap, and can be tamed only at very low temperatures.14 In addition, their coordination compounds are typically too reactive to be isolable.15 In fact, acceptor-substituted carbene complexes of copper and silver,16 which form upon reaction with diazo precursors, react even with methane (eq 1).17 Such carbene complexes are much more stable if an electron releasing p-methoxyaryl substituent is present. The electronwithdrawing ester groups are then positioned perpendicular to the carbene’s pπ-orbital (Figure 5).18 Of course this minimizes

Figure 6. An acyclic (alkyl)(amino) carbene and its rhodium complex. 276


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membered cyclic analogues of the O- or N-substituted Fischer carbene ligands are the tetrahydrofuran-2-ylidenes (oxolan-2ylidenes) and pyrrolidin-2-ylidenes. The latter are also known as cyclic (alkyl)(amino)carbenes (CAACs),34 whose metal complexes were introduced by Lavallo and Bertrand as “cyclic Fischer carbene complexes”.35 The diamino-substituted carbene ligands with enhanced πdonation to the carbene π-orbital are the conventional NHC complexes (saturated backbone, imidazolidin-2-ylidenes; unsaturated backbone, imidazolin-2-ylidenes).36 The mesoionic carbenes (MICs) and amino ylidic carbenes (AYCs) are the cyclic derivatives of the zwitterionic carbene ligands, whereas the cyclic bent allenes (CBAs) are derived from the very electron rich carbones.37

Figure 7. Acyclic diaminocarbene (Alder’s carbene).

bow indicating delocalization of the π-electrons, is close to 100%. Nevertheless, this compound reacts like a “true” singlet carbene and coordinates transition metals readily.25 The overall donor properties of a carbene ligand can be further enhanced by increasing the weight of ylidic or zwitterionic resonance structures. Notably, such carbenes feature strong accumulation of negative partial charge adjacent to the carbene atom (Figure 8, left).26 The electronic structure of a gold complex with such a carbene ligand parallels the related vinyl complexes (Figure 8, right).27

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THE CARBENE−METAL BOND NHC ligands were initially believed to be exclusively σ-donor ligands. Meyer established in two seminal contributions from 2003 and 2004 that the NHC−metal bond shows also significant π-character.38 π-effects amount to 19% of the orbital interaction energy for (CO)5W(NHC) according to energy decomposition analysis (Figure 11).39 Previous calculations40

Figure 8. Resonance structures for ylidic carbene (left) and examples of ylidic/vinyl carbene complexes (right).

Two ylidic, strong electron donors were introduced in a search for a ligand with even higher donor strength. Such bisylidic compounds28 obtain partial allenic character, and it has therefore been suggested to call them “bent allenes”.29 From another perspective, these compounds can be understood as divalent carbon(0) compounds (i.e., carbones).30 Examples comprise compounds with two phosphine (carbodiphosphoranes) or two NHC (carbodicarbenes) substituents (Figure 9).

Figure 11. Decomposition of π- and σ-bonding interactions.42

had shown that π-effects are of the same order of magnitude (23%) for (CO)5WC(OH)2. The parent compound (CO)5W(CH2) without heteroatom substituents shows 48% backbonding.41 Obviously, NHCs and Fischer carbenes are electronically quite similar. The orbital interaction energy ΔEorb for the NHC ligand (−54 kcal mol−1) is quite similar to that of the model dihydroxycarbene (−61 kcal mol−1). However, the interaction energy between a ligand and a metal center is not only dependent on the interaction of orbitals. Additionally, electrostatic effects, the Pauli exchange interaction, and the preparation energy (distortion energy) need to be considered (eq 3).43 Typically, the preparation energy is small for cyclic carbenes (∼1 kcal mol−1).44

Figure 9. Resonance structures of carbodicarbenes.

ΔEint = ΔE orb + ΔEPauli + ΔE elstat + ΔEprep

It has earlier been recognized that the metal complexes of acyclic carbenes are much less stable than their cyclic analogues.31 Metal complexes of acyclic carbenes show a smaller carbene angle than the corresponding free carbenes. Calculations suggested that the coordination involves an energy penalty due to this distortion (preparation energy). 32 Consequently, although ancillary acyclic carbene ligands lead to unique catalytic properties,33 research efforts have focused predominantly on their cyclic cousins (Figure 10). The five-

(3)

The modeling of NHC titanium and copper complexes allowed for a quantification of these contributions (Figure 12).39 The electrostatic attraction term is for all complexes quite large but on the same order of magnitude as for the repulsive Pauli energy. It is stronger for the MIC (ΔEelstat = −153 kcal mol−1)

Figure 12. Energy decomposition analysis of π- and σ-bonding interactions of titanium and copper NHC and MIC complexes.

Figure 10. Classes of cyclic carbenes. 277


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Figure 13. Molecular orbital energies for the carbene lone pairs (σ) and the unoccupied π* (for the bent allenes occupied π) orbitals relevant for πinteraction with a metal.45

than for the NHC (ΔEelstat = −148 kcal mol−1), which is perfectly in line with the mesoionic structure of the MIC. Orbital effects were found to be stronger for the high-valent TiCl4 (ΔEorb = −59 kcal mol−1) than for the Cu(I) (ΔEorb = −45 kcal mol−1) complex. The MIC ligand was predicted to show overall the same orbital contribution as the conventional NHC (ΔEorb = −45 kcal mol−1) but weaker π-effects (24% vs 27%). We conclude therefore that the strength of the carbene− metal bond depends both on electrostatics and on orbital interactions, which are both determined by the electronic properties of the carbene as well as the metal. The σ-donor and π-acceptor capabilities of different carbenes correlate with the energy of the σ-symmetric lone pair orbital (typically the HOMO) and the relevant π*-orbital (often the LUMO) for overlap with the metal’s d-orbitals (Figure 13).46 The saturated NHC is predicted to be both a stronger σ-donor and π-acceptor than the unsaturated NHC. The benzimidazolinylidene is a stronger π-acceptor, but especially the diamidocarbene (DAC) should show outstanding π-acceptor properties (albeit at the expense of weak donor strength).47 The CAAC ligand, on the contrary, is expected to be both a strong donor and acceptor. The mesoionic carbene, ylidic carbene, and bent allene are expected to be very strong donor ligands. Note that the bent allenes do not have a π* orbital with appropriate geometry for overlap with a metal center. Instead, the occupied HOMO-1 orbital shows the equivalent symmetry. The energies of the σ-symmetric lone pair orbitals are of course related to the proton affinity and consequently also the basicity of the carbene (Figure 14).48

properties of ligands is the Tolman electronic parameter (TEP), which relies on the infrared stretching frequency of the CO bonds in tetrahedral Ni(CO)3L complexes. Scales based on (Cl)Ir(CO)2L and (Cl)Rh(CO)2L, which can be converted into one another, have been developed. Note that the TEP includes to some extent steric contributions and that TEP values for N, C, and P donor ligands are therefore not necessarily comparable with each other. Most significant for the determination of the π-accepting properties are Bertrand’s 31P and Ganter’s 77Se NMR scales.52 Those methods rely on the 31 P (respectively 77Se) NMR shift of phosphinidene (selenium) adducts of the free carbenes (Figure 15). The NMR shifts are highly dependent on the amount of π-back-bonding to the former carbene center, which leads to a deshielding (shift to lower field) of the NMR signals.

Figure 15. Resonance structures related to the shifts of carbene adducts.

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P and

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Se NMR

The TEP values shown in Figure 16 confirm that mesoionic carbenes, ylidic carbenes, and carbodicarbenes are exceptionally strong donor ligands. Conventional NHCs are still much more electron releasing than phosphines (e.g., PPh3: TEP = 2068.9). The results obtained by both NMR methods are very similar,54 but steric effects can have some influence.55 It is interesting to see that CAACs (31P 68.9 ppm) and acyclic diaminocarbenes (31P 69.5 ppm) are both strong π-acceptors as well as overall strong donors (TEP = 2047.4 cm−1; TEP = 2036 cm−1). The DAC in contrast is a very strong π-acceptor (31P 78.6 ppm), whereas its donor strength (TEP = 2069 cm−1) is very similar to that of PPh3. Additionally, note that substituents can have a quite strong influence, as evidenced by the mesitylsubstituted (31P −23 ppm) and isopropyl-substituted NHCs (31P −61.2 ppm). The 31P shifts of carbene−phosphinidene adducts are in fairly good agreement with the 13C shifts of the free carbenes as well as the protonated carbenes (Figure 17).52a It has been argued that the NMR shifts of the free carbenes and carbones represent a continuum and are proportional to the population of the carbene’s pπ orbital (sp3 lone pair, respectively).56 A few other methods have been introduced for the comparison of electronic properties of carbene ligands. Huynh’s

Figure 14. Calculated proton affinities in kcal mol−1.49

NHCs are accordingly much less basic than the cyclic bent allenes or mesoionic carbenes. Indeed, 2-imidazolium salts (pKa ≈ 22−24 in dimethyl sulfoxide) can be deprotonated with potassium tert-butoxide, whereas the generation of free imidazolin-4-ylidenes or cyclic bent allenes requires KHMDS (potassium bis(trimethylsilyl)amide).50,51 Several methods have been applied to derive and/or deconvolute σ- and π-capabilities of carbene ligands experimentally. A well-known measure of the overall donor 278


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Figure 16. TEP values and 31P (77Se) NMR shifts in ppm of carbenes and derivatives.53 Legend: (*) 77Se value for CAACMe instead of CAACCy.

propenylidenes (48 kcal mol−1).62 An increase in the carbene angle by an increase in the ring size from a five- to a sixmembered ring typically reduces the HOMO−LUMO gap moderately (Figure 19). This can be mainly attributed to the destabilization of the carbene lone pair (i.e., higher HOMO energy).

Figure 17. Calculated 13C NMR shifts of C-donor ligands.56 13

C NMR scale of palladium(II) carbene complexes allows e.g. for a direct comparison of “primarily σ-donor properties”.57 The effect of NHCs on other ligands in the trans position as well as the ligand field was also investigated by magnetic circular dichroism spectroscopy.58 A very convenient and less explored approach relies on the 15N NMR shift of the Nheterocycles, which can also be used for the deconvolution of σand π-effects (Figure 18).59 The authors claim short acquisition

Figure 19. Relation of HOMO−LUMO gap and carbene angle.39

The three-membered cyclopropenylidene with its very acute carbene angle shows therefore a surprisingly large HOMO− LUMO gap (HOMO, −4.9 eV; LUMO, −1.3 eV) even without heteroatom stabilization. Vinylogous stabilizing amino groups lead to an even larger HOMO−LUMO gap (HOMO, −4.5 eV; LUMO, + 0.1 eV). In perfect agreement with the relative energies of the HOMOs, six-membered CAACs are also found experimentally to be more nucleophilic than their fivemembered analogues. A six-membered CAAC substitutes therefore a five-membered CAAC from a gold(I) complex (eq 4). A similar observation was made for saturated NHCs, which show higher bond dissociation energies in comparison to unsaturated NHCs and especially phosphines.63

Figure 18. 15N-coupled HMBC spectroscopy of CAAC ligands.

times (30 min) through 3 J( 1 H− 15 N) coupled HMBC (heteronuclear multiple bond correlation) NMR spectroscopy with moderately concentrated samples. There have been considerable efforts to predict TEP values and 13C NMR shifts of carbenes computationally.55 Gusev’s work, which was later extended to bent allene ligands, gives a comprehensive overview of calculated TEP values.60

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STERICS, AROMATICITY, AND RING STRAIN The effect of aromaticity on the stability of N-heterocyclic carbenes and derived complexes appears to be moderate. Aromaticity stabilizes mainly the singlet state of carbenes due to enhanced orbital overlap with the pπ orbital of the carbene. The stabilization energy has been suggested to amount to about 20 kcal mol−1 for the imidazolin-2-ylidenes and less for other heteroatom substituents.61 Aromaticity was suggested to be more important for the ring-strained carbocyclic cyclo-

The steric demand (“buried volume”) of ligands can be estimated by Cavallo’s SambVca tool.64 Evidently, steric considerations are significant for the bond dissociation energy of NHCs from metals, and a correlation with the buried volume has been suggested.63 Contradicting simplistic notions of steric repulsion, noncovalent interactions with the N-substituents can however become key for the stability of main-group and transition-metal compounds.65 Low-coordinate and electrondeficient transition-metal centers are accordingly stabilized 279


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Organometallics through interaction with the N-heterocycle aryl substituents (Figure 20).66 Substituents on the aryl ring have therefore often

Scheme 1. Reductive and Oxidative Generation of Carbene Radicals

Figure 20. Examples for NHC complexes showing intramolecular πinteraction with an N-aryl substituent.66,68

accordingly to give a ligand based radical through removal of one electron from their mainly ligand based HOMO (Scheme 1, right). The oxidation of a chelating nucleophilic carbene complex leads therefore as expected to a ligand-centered radical (eq 6).76

a remarkable influence on the overall electronic properties of the NHC ligand.67 A π-face interaction, which persists even in solution, was also reported for the Grubbs II catalysts.68 It is also well-known that sterically demanding ancillary ligands can lead to a dramatic acceleration of the reaction rate in cross-coupling catalysis.69 It has been suggested that “flexible” steric bulk, which enhances both the oxidative addition of substrates and the reductive elimination of products, could be responsible for efficient cross-coupling.70 In fact, also bulky, albeit nonflexible, NHC ligands have allowed for the development of exceptionally active palladium NHC catalysts.71 Calculations72 suggested that noncovalent interactions between the aryl-substituted NHC ligands and the aryl halide aid the oxidative addition, which is the rate-determining state for the Suzuki−Miyaura coupling with aryl chlorides (Figure 21).73

Carbene ligands generated through reaction of a diazo precursor with an open-shell metal precursor can also show high spin density on the carbene atom (eq 7). Such radicals are intermediates for the cyclopropanation of olefins.77

Sterically unprotected carbene radicals are likely to homocouple or abstract H•. CAAC ligands are well shielded by the bulky Dipp substituent. Hence, CAAC ligands have allowed for the spectacular isolation of low-coordinate metal complexes through reduction. Impressive examples include the isolation of the bis-coordinate CAAC complexes of zinc, copper, manganese, and aluminum, where predominant delocalization of spin density onto the carbenes was observed (Figure 23).34c,78 Of particular interest, CAACs stabilize not only coordination compounds but also organic radicals.79

Figure 21. Noncovalent interactions with the isopropyl group and aromatic ring of one 2,6-diisopropylphenyl substituent aid the oxidative addition step of 4-chlorobenzotrifluoride.

Bulky substituents on the N-heteroatoms not only shield the carbene but also increase the π-orbital overlap in the heterocycle due to reduced pyramidalization of the heteroatom. Another very elegant way to modulate the electronic properties of carbene ligands is thus the targeted pyramidalization of the heteroatoms adjacent to the carbene, which will prevent efficient π overlap between the amine lone pair and the carbene. NHCs with one pyramidalized nitrogen atom show therefore higher π-acceptor properties and consequently a higher TEP than the related classical NHC (Figure 22).74

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Figure 23. CAAC-stabilized low-valent zinc complex.

REDOX-NONINNOCENT CARBENE LIGANDS Fischer carbene ligands are well-known to be redox-active and hence to give ligand-based radicals upon reduction (eq 5).75 Note that the electron-accepting LUMO of Fischer carbene complexes is mainly located on the electrophilic carbene atom (Scheme 1, left). Nucleophilic carbene ligands can be oxidized

DACs appear to be even better suited for the stabilization of radicals, as was exemplified for reduced borenium−carbene adducts.80 NHCs delocalize electron density less efficiently,81 and reduced NHC metal complexes with considerable spin density on the ligand are therefore typically highly reactive species. Note, however, that redox-active moieties, which communicate with the coordinated metal, have been incorporated in the ligand design of “conventional” NHCs.82

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PERIODIC TRENDS NHC ligands have been predominantly usedin analogy to the soft phosphineswith the mid and late transition metals.83

Figure 22. π-acidic NHC with pyramidalized nitrogen atom. 280


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chloride substituents and the pyramidalization of the GaCl3 moiety (Bent’s rule) were investigated. Of course, π-acceptor properties of the ligands L are not expected to be important for tetracoordinate gallium. The authors found nevertheless a relationship with TEP values as obtained from LNi(CO)3 but only a fair correlation with the proton affinities of the ligands. Carbene complexes of the f-block elements have been known for a long timeCramer reported the coordination of an ylidic carbene to uranium in 1981.93 Carbenes with a strong dipolar character are arguably expected to form comparably strong bonds with the hard f-block elements. Synthetic attempts toward the synthesis of uranium (or calcium) N-heterocyclic olefins, which are known to be soft σ-donors, led instead to the isolation of the mesoionic carbene complexes.94 The reaction of an NHC with U(Cp*)2I was probed in the presence of Ce(Cp*)2I, bearing in mind that the 5f elements are softer than the 4f metals. The NHC reacted indeed preferably (ratio 9:1) with the uranium compound, and an analysis of the crystal structures corroborated stronger bonding interaction for the NHC−uranium adduct (eq 8).95

The overall bond strength increases with increasing d-electron count (i.e., lower d-orbital energies).84 Complexes of the 5d elements are usually more stable than complexes of the 3d and 4d metals.39 The diffuse d-orbitals of the late 5d transition metals lead to stronger π-back-bonding than their 4d congeners, which is at least partially also due to relativistic effects.21j Copper complexes are more stable than silver complexes and in addition show stronger double-bond character.21j NHC complexes of silver transmetalate consequently to copper and gold, whereas copper transfers the NHC usually only to gold.85 Bonds with the cationic s-block elements are very ionic and are especially weak for the heavier elements.86 Lithium is an exception here, and the formation of lithium carbene adducts is not uncommon. In fact, potassium bases are typically used for the generation of the free carbenes in order to avoid the formation of the lithium adducts.87 Metals in a d0 electronic configuration cannot π-back-donate from their π orbitals. NHC ligands then become strong π-acceptors. Vanadium(V) and titanium(IV) dichlorido complexes therefore show interaction between the chlorido ligands and the carbene (Figure 24).88

Figure 24. NHC ligands in d0 metal complexes are very π-acidic.

Anionic carbenes form strong bonds with the electropositive metals.96 Especially NHC ligands with pendant amido and alkoxy functionalities have proven very successful and allowed even for the isolation of neutral NHC potassium adducts without strong steric protection.97

There is a striking analogy between the π-acidity of the NHC ligands, as evidenced by the solid-state structure of the vanadium(V) complex with the reactivity of high-valent (NHC)AuCl3 and (CAAC)AuCl3 complexes. The NHC trihalogenido complex is perfectly stable, and one of the halides can even be replaced by the very weakly donating triflimidate anion.89 The corresponding complex with the much more π-acidic CAAC ligand has been described in contrast as very reactive.90 Oxidation of the Au(I) precursor at room temperature triggered the reductive elimination of the cyclic 2chloroiminium salt (Figure 25). Note that steric effects should be feeble, because the halides of the gold(III) complexes are positioned in the apical positions relative to the N-heterocycles.

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CONVENTIONAL NHC LIGANDS Conventional N-heterocyclic carbene ligands and their derivatives are the “all-rounders” of carbenes.4 They stabilize both low and high oxidation states well and bond to basically any element of the periodic table. The free NHCs as well as their corresponding acids tend to be comparably stable and are therefore often straightforward to prepare and purify. NHC complexes of the mid and late transition metals are frequently tolerant of basic or acidic aqueous conditions as well as of oxidative or reductive reaction media. Palladium complexes with a chelating bis-NHC ligand are even stable in neat trifluoroacetic acid in the presence of bromine.98 Nevertheless, decomposition through reductive elimination (C−C or C−X coupling) of the NHC or migratory insertion from high-valent metal centers has been observed.99 Ring insertion processes are less common but appear to be more facile for saturated NHCs and earlier transition metals. Numerous variations of the imidazolinylidene scaffold tailored for applications in catalysis and supramolecular chemistry or as switches have been reported.4n,100 The purposeful application of NHC ligands for surface chemistry and nanoparticles is a comparably new albeit dynamicarea of research.101 The variation of substituents of the backbone or N-substituents fine-tunes solubility and the electronic/steric properties and allows for the introduction of chiral information.102 The effect of pendant redox-active or photoactive as well as pH-sensitive groups tends to be in particular large. The synthetic versatility of NHCs allows for the facile tuning of catalysts. For example, cyclometalation of the adamantyl

Figure 25. Stability of gold(III) complexes with NHC or CAAC ligand.

It is not well understood which carbenes form strong bonds with electropositive metals.91 A comprehensive study on the donor strength of C-donor ligands L in LGaCl3 adducts was reported (Figure 26).92 Infrared stretching frequencies of the

Figure 26. Pyramidalization of GaCl3 as an indicator for donor properties of donor ligands L. 281


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substituent led to a spectacular switch from E- to Z-selective olefin cross-metathesis with ruthenium catalysts (Figure 27).103

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CAAC LIGANDS

CAACs34 are an excellent choice for the stabilization of lowvalent and open-shell metal complexes (vide supra) due to delocalization of π-electron density. Only a few variations of the CAAC ligand framework have been reported so far, and their synthetic variability therefore does not yet rival that of classical NHCs.54,112 The even more π-acidic DACs and related Nheterocyclic carbenes have however not yet found comparable application in these areas (Figure 30).113

Figure 27. A ruthenium complex with a cyclometalated NHC ligand affords the Z product in the olefin cross-metathesis reaction.

The selectivity was rationalized by a combination of electronic (π-acceptor properties of the NHC) and steric effects (unfavorable interactions of the substituted olefin with the Nmesityl substituent).104 Transition-metal complexes with functionalized NHC ligands have found numerous applications in catalysis.100a The combination of the soft NHC ligand with a hard O donor moiety has been applied very successfully in both early- and late-transition-metal chemistry.105 The pendant hard donor group facilitates catalysis in the case of the late transition metals typically through either their hemilability or the formation of noncovalent interactions with substrates.106 The hydrogenation of imines by such an iridium NHC complex proceeds therefore through a cooperative transition state (Figure 28).107

Figure 30. Isolated π-acidic carbenes (top) and carbene ligands (bottom) with supposedly strong π-acidic character.

It is interesting to note that pyrazolin-3-ylidene ligands as well as pyridinylidenes are very strong σ-donors and have also been suggested to be strong π-acceptors.12,114 Nevertheless, reports on these ligands are likewise comparably scarce, which might be due to the fact that they have not been isolated in the form of the free carbenes. CAAC complexes of the late transition metals are exceedingly stable compounds and are usually tolerant of moisture, air, or basic/acidic reaction media. They are therefore a good choice for catalytic applications under demanding reaction conditions.112a Examples include the hydrohydrazination of olefins, the borylation of alkynes, and the hydrogenation of aromatics.112,115 Catalysis with CAAC ligands is still underdeveloped in comparison to conventional NHCs or mesoionic carbene ligands. However, note that nucleophiles such as e.g. hydrides appear to migrate more readily to the carbene atom than is the case for the less electrophilic NHC ligands.116 Free CAACs117 and DACs118 dimerize much more readily than NHCs, and considerable steric protection is therefore necessary for their isolation. The cyclic iminium salts, which are the precursors to CAACs, are air stable but react with nucleophiles. Deprotonation to generate the nucleophilic CAAC requires therefore also very bulky bases. Coinage-metal complexes with CAAC ligands can be synthesized without isolation of the free carbenes.119 An early example of the isolation of low-coordinate transition-metal complexes was provided by the report that very bulky CAAC ligands are capable of stabilizing formal 14electron rhodium and palladium complexes (Figure 31).120 Low-valent bis-CAAC metal complexes of iron, cobalt, nickel, palladium, and platinum have been reported subsequently (Figure 32).34c It could be shown that most of the relevant spin (electron, respectively) density resides on the

Figure 28. Cooperative catalysis for iridium NHC complexes with a pendant hydroxyl functionality.

Functionalized NHCs and poly-NHCs108 build supramolecular architectures.109 A supramolecular system based on a mixed NHC/pyrazolyl ligand forms e.g. a tubular metallocavitand structure with gold(I) or silver(I) precursors (Figure 29). Of particular interest, long-chained guest molecules can be sensed by a switch of the photoluminescent properties of the ensemble upon complexation with the host molecule.110 The same cavitand allows for the formation of a pH-switchable rotaxane through reversible release of the metal cations.111

Figure 29. Switchable and photoluminescent supramolecular host.

Figure 31. 14-electron complexes of rhodium and palladium. 282


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bis-MIC palladium(II) complex by chlorine leads to either the formation of the dichlorinated diimidazolium salt or the reductive elimination of the bis-carbene ligand (Scheme 2).128 Scheme 2. Oxidation of bis-MIC Palladium(II) Complex

Figure 32. CAAC-stabilized low-valent metal complexes.

metal atom of the slightly bent bis-CAAC complexes. The electronic structure of an isolated linear (CAAC)2Be complex has been equally interpreted as beryllium in its zero oxidation state.78,121 Copper amido complexes with CAAC ligands show outstanding quantum efficiency as light-emitting molecules for organic light-emitting diodes (OLEDs).122 The π-acidic character of the CAAC allows for efficient charge transfer in the donor−acceptor ensemble with the amido ligand (Figure 33).

Similar results were obtained when the reactivity of a platinum(II) hydride complex with one NHC and one MIC ligand was investigated. Notably, the hydride underwent reductive elimination at 60 °C, forming a bond preferably with the MIC ligand.129 Nevertheless, MIC complexes are typically very robust, show a strong trans influence, and have found numerous applications in electro- and photocatalysis. They appear to be rather redoxinnocent,130 stabilize metals in different oxidation states, and are consequently a good choice for redox catalysis such as the oxidation of water (eq 9).131

Figure 33. Light-emitting (CAAC)Cu(carbazolido) complex.

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MESOIONIC CARBENE LIGANDS Mesoionic carbene (MIC) complexes123 were first reported by Crabtree and Albrecht in 2001.12,124 MICs, which do not have a neutral resonance structure, are very strong donor ligands. Prominent classes of MICs include imidazolin-4-ylidenes, 1,2,3triazolin-4-ylidenes, and alkyl substituted pyrazolin-4-ylidenes (Figure 34).12,114a

Mesoionic carbene ligands are likewise highly promising building blocks for phosphorescent transition-metal complexes. It has been claimed that this is due to their strong σ-donor properties, which stabilizes excited states at the transition metal.132 Imidazolium and pyrazolium salts can be deprotonated twice to afford ditopic, anionic carbene adducts. This allowed for the synthesis of exciting bimetallic complexes (Figure 35).96a,d

Figure 34. Structures of mesoionic imidazolin-4-ylidene, triazolin-4ylidene and pyrazolin-4-ylidene.

Kinetic and not thermodynamic control is usually responsible for the formation of the imidazolin-4-ylidene instead of the 2ylidene complexes. Interestingly, anions in the reaction mixture can have an effect on the regioselectivity. “Acidic” protons need to be avoided, because free MICs are very basic. Free mesoionic carbenes do not dimerize but activate intramolecular CH bonds.51,125 Their bonds with metals have more dipolar character in comparison to those of conventional NHC ligands and are very strong. Importantly, the synthetic access to 1,2,3triazolinylidenes126 is very convenient using azide−alkyne click chemistry. In addition, the synthesis of the imidazole-derived carbenes is short and efficient. Consequently, a remarkable variety of these ligands has been reported. MIC complexes and precursors are often synthesized without rigorous exclusion of air. The bonding situation of these neutral carbene ligands parallels that of vinyl or heteroaryl complexes.127 The reactivity of a bis-MIC palladium(II) complex with chlorine illustrates the heteroaryl character of MICs. Whereas the related bis-NHC palladium(IV) compound is perfectly stable, oxidation of the

Figure 35. Bimetallic MIC complexes.

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BENT ALLENES AND METHANEDIIDES Complexes derived from these C-donor ligands are nucleophilic and electron rich and are potentially redox-active through oneelectron oxidation. The synthesis of the ligands as well as the complexes typically requires inert gas techniques. Bent allenes and the related, charged diide ligands are inherently bifunctional through their two lone pairs. They form potentially bimetallic complexes, whereas the carbene−metal bond of monometallic complexes can undergo addition reactions.133 It is very interesting to recall that NHC ligands have also been suggested very early to have π-donor properties in complexes with coordinatively unsaturated metals.134 Carbodiphosphoranes coordinate two gold atoms (Figure 36). The acyclic carbodicarbene, however, twists out of plane 283


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Figure 38. Zwitterionic resonance structures of diides and yldiide. Figure 36. Acyclic carbodiphosphorane (1) and carbodicarbene (2) complexes of gold(I) and comparison to NHO complex (3).

heteroatoms. Monoanionic yldiides (6) are of course electronically very related.142 The coordination of a nucleophilic carbene to a transition metal leads formally to an increase of the oxidation state by 2. Metal complexes can consequently not only be synthesized through redox-neutral ligand exchange by the bis-deprotonated methanediide but also be synthesized oxidatively from anionic carbenoid surrogates (Scheme 4).143

and accommodates only one metal.135 Fürstner and Alcarazo demonstrated furthermore that the coordination chemistry of carbodicarbenes bears analogies with N-heterocyclic olefins (NHOs),136 which are soft σ-donor ligands. The isolation of a cyclic bent allene (CBA)50 stimulated the synthesis of iron, copper, and ruthenium complexes and their application in catalysis (Figure 37).137 However, only moderate catalytic activity was obtained for the hydroamination of olefins, which was attributed to the catalyst’s low stability.138

Scheme 4. Generation of Palladium Carbene Complex from either Methanediide (Left) or Carbenoid (Right)

Nucleophilic palladium complexes such as that shown in Scheme 4 do not considerably π-bond with the metal and are methylated by methyl iodide.144 Their electronic structure and the chemistry of the related high-valent gold complexes145 parallel accordingly those of the rhodium carbodicarbene complex shown in eq 10. Dianionic methanediide ligands were used for the isolation of carbene complexes of the electropositive metals. Examples include calcium,146 scandium,147 neodymium,148 and uranium.149 The degree of double-bond character appears to vary from case to case. Calculations suggested strong and fairly covalent σ-donor and π-donor interactions for the scandium complex. The bond with the uranium atom was described similarly as a nucleophilic carbene with quite covalent interaction with the metal’s 5f orbitals.149 The bonding situation of the calcium complex was suggested to be highly ionic with strong electrostatic Ca2+−C2− and Ca2+−N− bonds (Figure 39).

Figure 37. Bent allene complexes applied in catalysis.

Chelating ligand frameworks of bent allenes and carbodicarbenes provide enhanced stability.139 For example, a rhodium complex with a chelating carbodicarbene ligand efficiently catalyzes the hydroarylation and hydroamination of olefins (Scheme 3).139c,d Scheme 3. Lewis Acid Promoted Hydroarylation by Carbodicarbene Rhodium Complex

The authors hypothesized on the basis of previous reports that Lewis acidic additives should enhance the catalytic activity.140 Indeed, adding AgCl or LiBF4 to the reaction mixture led to a huge increase in catalytic activity. The treatment of the carbonyl complex with HBF4 led in perfect agreement to the protonation of the ligand (eq 10). Figure 39. Methanediide complexes of electropositive metals.

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CONCLUSION The electronic structure of carbene ligands displays a continuum from strong to weak σ-donors and from π-donors to strong π-acceptors (Table 1). Note that the donor properties of the least electron releasing heterocyclic carbenes are still in the same order of magnitude like phosphines. Likewise, the character of the carbene−metal bond ranges from highly ionic to strongly covalent. Steric and noncovalent interactions can become as well very important. The combination of all these factors defines the properties of carbene complexes with a particular metal and consequently their applications in coordination chemistry, catalysis, or for functional materials.

Bis(thiophosphinoyl)methanediides (4) and bis(iminophosphoranyl)methanediides (5) are chelating as well and parallel with their two available lone pairs the neutral carbones (Figure 38).141 Their electronic structure shows a strong contribution of their zwitterionic resonance structures (cf. Figure 3, resonance structure IV) with strong negative partial charge on the central carbon atom and the N/S 284


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(2) (a) Baceiredo, A.; Bertrand, G.; Sicard, G. J. Am. Chem. Soc. 1985, 107, 4781−4783. (b) Igau, A.; Grützmacher, H.; Baceiredo, A.; Bertrand, G. J. Am. Chem. Soc. 1988, 110, 6463−6466. (3) Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361−363. (4) For thematic issues and books on NHCs, see: (a) Rovis, T.; Nolan, S. P. Synlett 2013, 24, 1188−1189. (b) Diez Gonzalez, S. NHeterocyclic Carbenes: From Laboratory Curiosities to Efficient Synthetic Tools; Royal Society of Chemistry: Cambridge, U.K., 2010. (c) Arduengo, A. J.; Bertrand, G. Chem. Rev. 2009, 109, 3209−3210. (d) Nolan, S. P. N-Heterocyclic Carbenes: Effective Tools for Organometallic Synthesis; Wiley-VCH: Weinheim, Germany, 2014. . For reviews with a focus on cyclic singlet carbenes, see: (e) Herrmann, W. A.; Köcher, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 2162−2187. (f) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122− 3172. (g) Melaimi, M.; Soleilhavoup, M.; Bertrand, G. Angew. Chem., Int. Ed. 2010, 49, 8810−8849. (h) Dröge, T.; Glorius, F. Angew. Chem., Int. Ed. 2010, 49, 6940−6952. For reviews relating to acyclic singlet carbenes, see: (i) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39−91. (j) Vignolle, J.; Cattoën, X.; Bourissou, D. Chem. Rev. 2009, 109, 3333−3384. For a comprehensive review on the electronic properties of NHCs, see: (k) Nelson, D. J.; Nolan, S. P. Chem. Soc. Rev. 2013, 42, 6723−6753. For reviews on applications of NHC complexes, see: (l) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290−1309. (m) Visbal, R.; Gimeno, M. C. Chem. Soc. Rev. 2014, 43, 3551−3574. (n) Mercs, L.; Albrecht, M. Chem. Soc. Rev. 2010, 39, 1903−1912. (o) Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. Nature 2014, 510, 485−496. For a review on nonheteroatom stabilized carbene ligands, see: (p) Santamaría, J.; Aguilar, E. Org. Chem. Front. 2016, 3, 1561−1588. For a review on the synthesis of NHC precursors, see: (q) Benhamou, L.; Chardon, E.; Lavigne, G.; Bellemin-Laponnaz, S.; Cesar, V. Chem. Rev. 2011, 111, 2705−2733. (5) Mindiola, D. J.; Scott, J. Nat. Chem. 2011, 3, 15−17. (6) Fischer, E. O.; Maasböl, A. Angew. Chem. 1964, 76, 645−645. (7) (a) Schrock, R. R. Chem. Rev. 2002, 102, 145−180. (b) Schrock, R. R.; Copéret, C. Organometallics 2017, 36, 1884−1892. “Classic” Schrock carbenes are recently experiencing renewed attention related to the stereoselective metathesis of olefins: (c) Koh, M. J.; Nguyen, T. T.; Lam, J. K.; Torker, S.; Hyvl, J.; Schrock, R. R.; Hoveyda, A. H. Nature 2017, 542, 80−85. (d) Nguyen, T. T.; Koh, M. J.; Shen, X.; Romiti, F.; Schrock, R. R.; Hoveyda, A. H. Science 2016, 352, 569− 575. (8) Cundari, T. R.; Gordon, M. S. J. Am. Chem. Soc. 1991, 113, 5231−5243. (9) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18−29. (10) Occhipinti, G.; Jensen, V. R. Organometallics 2011, 30, 3522− 3529. (11) (a) Haaland, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 992− 1007. (b) Himmel, D.; Krossing, I.; Schnepf, A. Angew. Chem., Int. Ed. 2014, 53, 370−374. (c) Frenking, G. Angew. Chem., Int. Ed. 2014, 53, 6040−6046. (d) Himmel, D.; Krossing, I.; Schnepf, A. Angew. Chem., Int. Ed. 2014, 53, 6047−6048. (12) Schuster, O.; Yang, L. R.; Raubenheimer, H. G.; Albrecht, M. Chem. Rev. 2009, 109, 3445−3478. (13) Lepetit, C.; Maraval, V.; Canac, Y.; Chauvin, R. Coord. Chem. Rev. 2016, 308, 59−75. (14) Toscano, J. P.; Platz, M. S.; Nikolaev, V.; Popic, V. J. Am. Chem. Soc. 1994, 116, 8146−8151. (15) For a review on transition-metal-catalyzed carbene migratory insertions, see: (a) Xia, Y.; Qiu, D.; Wang, J. Chem. Rev. 2017, 117, 13810−13889. For a review on CH activation with carbenes, see: (b) Doyle, M. P.; Duffy, R.; Ratnikov, M.; Zhou, L. Chem. Rev. 2010, 110, 704−724. (16) Caballero, A.; Despagnet-Ayoub, E.; Mar Díaz-Requejo, M.; Díaz-Rodríguez, A.; González-Núñez, M. E.; Mello, R.; Muñoz, B. K.; Ojo, W.-S.; Asensio, G.; Etienne, M.; Pérez, P. J. Science 2011, 332, 835−838.

Table 1. Generalized Characteristic Properties of a Selection of C-Donor Ligands Ligand

Donor Properties

Acceptor Properties

Characteristic Properties

Bent allene, diide MIC

+++

π-donor

++

weak

NHC CAAC

+ ++

+ ++

DAC

weak

+++

Acyclic Fischer Carbene Weak resonance stabilization

+/++

+++

Electron rich, bifunctional, nucleophilic Electron rich, stable complexes, redox-stable “Allrounder”, tunability Low-valent complexes, stable complexes, open-shell compounds Low-valent complexes, openshell compounds Electrophilic, redox active

+

++++

Facile carbene transfer

Bent allenes and diides are nucleophilic, inherently bifunctional, and the latter bond well to the electropositive elements. Mesoionic carbenes excel through their strong overall donor properties, stabilize metals in different oxidation states, and are arguably a good choice for the electropositive elements as well. CAACs and DACs are much stronger π-acceptors than conventional NHCs and tame low-valent complexes and radicals through delocalization of electron/spin density. Acyclic diamino carbenes and Fischer carbenes are very electrophilic, whereas the σ-donor properties span a quite large range. Carbenes with weak heteroatom stabilization or even more so π-acceptor substituted carbenes form highly reactive metal complexes suitable for C atom transfer. Carbene ligands with their outstanding versatility are now drivers of ground-breaking and exciting research for more than 50 years. Certainly, carbenes have still a lot to offer. This tutorial serves hopefully as a guide on the choice of the right carbene ligand for the desired properties of a metal complex or synthetic endeavors toward their main group element analogs.150

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AUTHOR INFORMATION

Corresponding Author

*E-mail for D.M.: [email protected]. ORCID

Dominik Munz: 0000-0003-3412-651X Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS Support by the Fonds der Chemischen Industrie FCI im Verband der Chemischen Industrie e.V. is gratefully acknowledged. I thank G. Bertrand and K. Meyer for their generous support, P. Pinter for helpful discussions, and the reviewers for helpful comments.

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REFERENCES

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(31) Slaughter, L. M. Catalysis with Acyclic Aminocarbene Ligands: Alternatives to NHCs with Distinct Steric and Electronic Properties. In N-Heterocyclic Carbenes: Effective Tools for Organometallic Synthesis, Nolan, S. P., Ed.; Wiley-VCH: Weinheim, Germany, 2014. (32) Schoeller, W. W.; Eisner, D.; Grigoleit, S.; Rozhenko, A. B.; Alijah, A. J. Am. Chem. Soc. 2000, 122, 10115−10120. (33) Slaughter, L. M. ACS Catal. 2012, 2, 1802−1816. (34) (a) Melaimi, M.; Jazzar, R.; Soleilhavoup, M.; Bertrand, G. Angew. Chem., Int. Ed. 2017, 56, 10046−10068. (b) Soleilhavoup, M.; Bertrand, G. Acc. Chem. Res. 2015, 48, 256−266. (c) Roy, S.; Mondal, K. C.; Roesky, H. W. Acc. Chem. Res. 2016, 49, 357−369. (d) Paul, U. S. D.; Radius, U. Eur. J. Inorg. Chem. 2017, 2017, 3362−3375. (35) Lavallo, V.; Canac, Y.; Prasang, C.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2005, 44, 5705−5709. (36) The commonly used trivial name “imidazol-2-ylidene” is discouraged. For a detailed discussion on the correct nomenclature of heterocyclic carbenes according to the Hantzsch−Widman system, see ref 4e and: Huynh, H. V., General Properties of Classical NHCs and Their Complexes. In The Organometallic Chemistry of Nheterocyclic Carbenes; Wiley: Chichester, U.K., 2017; pp 17−51. (37) The nature of cyclic bent allenes has been a matter of controversy. (a) Christl, M.; Engels, B. Angew. Chem., Int. Ed. 2009, 48, 1538−1539. (b) Lavallo, V.; Dyker, C. A.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2009, 48, 1540−1542. (c) Hanninen, M. M.; Peuronen, A.; Tuononen, A. M. Chem. - Eur. J. 2009, 15, 7287−7291. (38) (a) Hu, X.; Castro-Rodriguez, I.; Olsen, K.; Meyer, K. Organometallics 2004, 23, 755−764. (b) Hu, X.; Tang, Y.; Gantzel, P.; Meyer, K. Organometallics 2003, 22, 612−614. (39) Tonner, R.; Heydenrych, G.; Frenking, G. Chem. - Asian J. 2007, 2, 1555−1567. (40) Lein, M.; Szabo, A.; Kovacs, A.; Frenking, G. Faraday Discuss. 2003, 124, 365−378. (41) It has been suggested that intramolecular π-polarization within the NHC fragment is important for the computed π-interaction with the metal. Rezabal, E.; Frison, G. J. Comput. Chem. 2015, 36, 564−572. (42) The carbene−metal bond involves also some carbene→metal πdonation (see also ref 134) as well as an in plane π-interaction. These contributions are condensed into π-back-donation as well as σdonation in Figures 11 and 12 for the sake of simplicity. For further details, see ref 39. (43) Dispersion effects can become very important for sterically crowded molecules but are usually small for sterically relaxed molecules. Furthermore note that the inclusion of solvation effects is necessary for the balance of dispersion effects in solution. For a thorough discussion, see: (a) Hansen, A.; Bannwarth, C.; Grimme, S.; Petrović, P.; Werlé, C.; Djukic, J.-P. ChemistryOpen 2014, 3, 177−189. (b) Grimme, S. ChemPhysChem 2012, 13, 1407−1409. (c) Jacobsen, H.; Cavallo, L. ChemPhysChem 2012, 13, 562−569. (d) Fey, N.; Ridgway, B. M.; Jover, J.; McMullin, C. L.; Harvey, J. N. Dalton Trans. 2011, 40, 11184−11191. (44) Frenking, G.; Solà, M.; Vyboishchikov, S. F. J. Organomet. Chem. 2005, 690, 6178−6204. (45) Andrada, D. M.; Holzmann, N.; Hamadi, T.; Frenking, G. Beilstein J. Org. Chem. 2015, 11, 2727−2736. (46) Values were obtained from ref 45 or were calculated with the same level of theory. (47) The HOMO eigenvalues might not be a good measure for the σ-donor capabilities of DACs. For a discussion, see ref 52b. (48) (a) Tonner, R.; Heydenrych, G.; Frenking, G. ChemPhysChem 2008, 9, 1474−1481. (b) Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am. Chem. Soc. 2004, 126, 8717−8724. (49) Huynh, H. V.; Frison, G. J. Org. Chem. 2013, 78, 328−338. (50) Lavallo, V.; Dyker, C. A.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 5411−5414. (51) Aldeco-Perez, E.; Rosenthal, A. J.; Donnadieu, B.; Parameswaran, P.; Frenking, G.; Bertrand, G. Science 2009, 326, 556−559. (52) (a) Back, O.; Henry-Ellinger, M.; Martin, C. D.; Martin, D.; Bertrand, G. Angew. Chem., Int. Ed. 2013, 52, 2939−2943. 286


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Organometallics (b) Verlinden, K.; Buhl, H.; Frank, W.; Ganter, C. Eur. J. Inorg. Chem. 2015, 2015, 2416−2425. (c) Vummaleti, S. V. C.; Nelson, D. J.; Poater, A.; Gomez-Suarez, A.; Cordes, D. B.; Slawin, A. M. Z.; Nolan, S. P.; Cavallo, L. Chem. Sci. 2015, 6, 1895−1904. (53) The TEP values were obtained from ref 4k. The TEP values for the pyrazolinylidene were obtained from Herrmann and obtained via the regression as described in ref 4k. Herrmann, W. A.; Schütz, J.; Frey, G. D.; Herdtweck, E. Organometallics 2006, 25, 2437−2448. The TEP value for the ylidic carbene was taken from ref 26b and obtained through regression as well. The 31P and 77Se NMR shifts were obtained from refs 52a and 52b. (54) Tomás-Mendivil, E.; Hansmann, M. M.; Weinstein, C. M.; Jazzar, R.; Melaimi, M.; Bertrand, G. J. Am. Chem. Soc. 2017, 139, 7753−7756. (55) Falivene, L.; Cavallo, L. Coord. Chem. Rev. 2017, 344, 101−114. (56) Guha, A. K.; Borthakur, B.; Phukan, A. K. J. Org. Chem. 2015, 80, 7301−7304. (57) Teng, Q.; Huynh, H. V. Dalton Trans. 2017, 46, 614−627. (58) (a) Fillman, K. L.; Przyojski, J. A.; Al-Afyouni, M. H.; Tonzetich, Z. J.; Neidig, M. L. Chem. Sci. 2015, 6, 1178−1188. (b) Iannuzzi, T. E.; Gao, Y.; Baker, T. M.; Deng, L.; Neidig, M. L. Dalton Trans. 2017, 46, 13290−13299. (59) Mondal, K. C.; Roy, S.; Maity, B.; Koley, D.; Roesky, H. W. Inorg. Chem. 2016, 55, 163−169. (60) (a) Gusev, D. G. Organometallics 2009, 28, 6458−6461. (b) Gusev, D. G. Organometallics 2009, 28, 763−770. (c) Tonner, R.; Frenking, G. Organometallics 2009, 28, 3901−3905. (61) Gronert, S.; Keeffe, J. R.; More O’Ferrall, R. A. J. Am. Chem. Soc. 2011, 133, 3381−3389. (62) Lavallo, V.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Science 2006, 312, 722−724. (63) Hillier, A. C.; Sommer, W. J.; Yong, B. S.; Petersen, J. L.; Cavallo, L.; Nolan, S. P. Organometallics 2003, 22, 4322−4326. (64) Falivene, L.; Credendino, R.; Poater, A.; Petta, A.; Serra, L.; Oliva, R.; Scarano, V.; Cavallo, L. Organometallics 2016, 35, 2286− 2293. (65) (a) Liptrot, D. J.; Power, P. P. Nature Rev. Chem. 2017, 1, 0004. (b) Wagner, J. P.; Schreiner, P. R. Angew. Chem., Int. Ed. 2015, 54, 12274−12296. (c) Grimme, S.; Hansen, A.; Brandenburg, J. G.; Bannwarth, C. Chem. Rev. 2016, 116, 5105−5154. (66) (a) Dioumaev, V. K.; Szalda, D. J.; Hanson, J.; Franz, J. A.; Morris Bullock, R. Chem. Commun. 2003, 1670−1671. (b) Kolychev, E. L.; Kronig, S.; Brandhorst, K.; Freytag, M.; Jones, P. G.; Tamm, M. J. Am. Chem. Soc. 2013, 135, 12448−12459. (67) (a) Leuthausser, S.; Schwarz, D.; Plenio, H. Chem. - Eur. J. 2007, 13, 7195−7203. (b) Credendino, R.; Falivene, L.; Cavallo, L. J. Am. Chem. Soc. 2012, 134, 8127−8135. (c) Munz, D.; Allolio, C.; Doering, K.; Poethig, A.; Doert, T.; Lang, H.; Strassner, T. Inorg. Chim. Acta 2012, 392, 204−210. (68) (a) Leuthaeusser, S.; Schmidts, V.; Thiele, C. M.; Plenio, H. Chem. - Eur. J. 2008, 14, 5465−5481. (b) Love, J. A.; Sanford, M. S.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 10103−10109. (c) Fürstner, A.; Ackermann, L.; Gabor, B.; Goddard, R.; Lehmann, C. W.; Mynott, R.; Stelzer, F.; Thiel, O. R. Chem. - Eur. J. 2001, 7, 3236− 3253. (69) Surry, D. S.; Buchwald, S. L. Angew. Chem., Int. Ed. 2008, 47, 6338−6361. (70) (a) Altenhoff, G.; Goddard, R.; Lehmann, C. W.; Glorius, F. Angew. Chem., Int. Ed. 2003, 42, 3690−3693. (b) Würtz, S.; Glorius, F. Acc. Chem. Res. 2008, 41, 1523−1533. (71) For leading reviews on bulky NHC palladium catalysts for cross coupling reactions, see: (a) Froese, R. D. J.; Lombardi, C.; Pompeo, M.; Rucker, R. P.; Organ, M. G. Acc. Chem. Res. 2017, 50, 2244−2253. (b) Valente, C.; Calimsiz, S.; Hoi, K. H.; Mallik, D.; Sayah, M.; Organ, M. G. Angew. Chem., Int. Ed. 2012, 51, 3314−3332. For seminal contributions, see: (c) Calimsiz, S.; Sayah, M.; Mallik, D.; Organ, M. G. Angew. Chem., Int. Ed. 2010, 49, 2014−2017. (d) Wu, L.; Drinkel, E.; Gaggia, F.; Capolicchio, S.; Linden, A.; Falivene, L.; Cavallo, L.; Dorta, R. Chem. - Eur. J. 2011, 17, 12886−12890. (e) Chartoire, A.;

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