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Quantification and variation of characteristic properties of different ligand classes is an exciting and rewarding research field. N-Heterocyclic carbenes (NHCs) are of special interest since their electron richness and structure provide a unique class of ligands and organocatalysts. Consequently, they have found widespread application as ligands in transition-metal catalysis and organometallic chemistry, and as organocatalysts in their own right. Herein we provide an overview on physicochemical data (electronics, sterics, bond strength) of NHCs that are essential for the design, application, and mechanistic understanding of NHCs in catalysis.

Topographical analysis of the molecular electrostatic potential (MESP) has been carried out for a variety of N-heterocyclic carbenes at the B3LYP, BP86, and M05 levels of density functional theory (DFT) using a 6-311++G** basis set. The electron rich character of the carbene carbon is assessed in terms of the absolute minimum of the MESP at the carbene lone pair region (V(min)), as well as the MESP at the carbene nucleus (V(C)). A linear relationship is established between the V(C) and Tolman electronic parameter (TEP) which suggested the use of the former as a simple and efficient descriptor for the electron donating power of N-heterocyclic carbene ligands toward metal coordination. The V(min) of the carbene also showed good correlation with TEP. However, the deviation from linearity was higher than V(C)-TEP correlation, and the reason for this was attributed to the steric effect of N-substituents at the lone pair region. The greater coordinating power of N-heterocyclic carbenes over phosphines is explained on the basis of deeper V(min) values obtained for the former, and in fact even the V(min) of the least electron rich N-heterocyclic carbene is comparable to the highly electron rich phosphine ligands. Thus the MESP topographical approach presented herein offers quantification of the inherent electron donating power of a free N-heterocyclic carbene ligand.

Reaction of bromanil with N,N'-dimesitylformamidine followed by deprotonation with NaN(SiMe(3))(2) afforded 1,1',3,3'-tetramesitylquinobis(imidazolylidene) (1), a bis(N-heterocyclic carbene) (NHC) with two NHC moieties connected by a redox active p-quinone residue, in 72 % yield of isolated compound. Bimetallic complexes of 1 were prepared by coupling to FcN(3) (2) or FcNCS (3; Fc=ferrocenyl) or coordination to [M(cod)Cl] (4 a or 4 b, where M=Rh or Ir, respectively; cod=1,5-cyclooctadiene). Treatment of 4 a and 4 b with excess CO(g) afforded the corresponding [M(CO)(2)Cl] complexes 5 a and 5 b, respectively. Analysis of 2-5 by NMR spectroscopy and X-ray diffraction indicated that the electron-deficient quinone did not significantly affect the inherent spectral properties or coordination chemistry of the flanking imidazolylidene units, as compared to analogous NHCs. Infrared spectroscopy and cyclic voltammetry revealed that decreasing the electron density at ML(n) afforded an increase in the stretching energy and a decrease in the reduction potential of the quinone, indicative of metal-quinone electronic interaction. Differential pulse voltammetry and chronoamperometry of the metal-centered oxidations in 2-4 revealed two single, one-electron peaks. Thus, the metal atoms bound to 1 are oxidized at indistinguishable potentials and do not appear electronically coupled. However, the metal-quinone interaction was used to increase the electron density at coordinated metal atoms. Infrared spectroelectrochemistry revealed that the average nu(CO) values for 5 a and 5 b decreased by 14 and 15 cm(-1), respectively, upon reduction of the quinone embedded within 1. These shifts correspond to 10 and 12 cm(-1) decreases in the Tolman electronic parameter of this ditopic ligand.

We present a free web application for the calculation of the buried volume (% VBur) of NHC ligands. The web application provides a graphic and user-friendly interface to the SambVca program, developed for the calculation of % VBur values not only of NHC ligands but also of other classic organometallic ligands such as, for example, phosphanes and cyclopentadienyl-based ligands. To provide a reliable procedure for the calculation of % VBur values we tested our approach in the interpretation of the binding energies of NHC ligands in Cp*Ru(NHC)Cl complexes in terms of steric and electronic parameters.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The [3 + 2] cycloaddition of azides and acetylenes followed by nitrogen quaternization was applied for the generation of novel and highly modular triazolium salts. The selective substitution of the 1,3,4-substitution pattern presets such salts as precursors for a new class of abnormal carbene ligands, thus expanding the family of these high-impact ligands. Metalation of the triazolium salts is highly versatile and is illustrated by direct C−H bond activation as well as by applying a transmetalation protocol, thus providing access to Pd(II), Ru(II), Rh(I), and Ir(I) abnormal carbene complexes. The donor properties of these carbenes were analyzed by using Tolman electronic parameters and were found to be slightly stronger than those the most basic normal carbenes.

The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of beta-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

The rate constant, k3, for the direct insertion of carbon monoxide induced by tertiary phosphines into [(η5-C5H5)(CO)3MoCH 2C6H5] in toluene solution decreases steadily with increasing cone angle of the phosphine. In contrast, for [(CO)5MnCH2C6H5], k3 increases with decrease in the Tolman electronic parameter, v, of the phosphine (i.e., with increasing electron donation) and does not correlate with the cone angle. However, k3 reflects the size of the phosphine to a greater degree as the size of the benzyl substituent on manganese increases. An analysis of k1 values in the solvent-assisted insertion pathway has been made for both the molybdenum and manganese systems in THF, 2-MeTHF and 2,5-Me2THF solutions. The general decrease in k1 with increasing size of the solvent for the cyclopentadienyl-molybdenum system becomes less marked as the size of the benzyl substituent increases. The manganese complexes are relatively less sensitive to solvent size. A comparison has been made between the nucleophilic role of a solvent molecule in the k1 step and that of a tertiary phosphine in the k3 step.

A ligand electrochemical parameter, El (L), is described to generate a series which may be used to predict M(n)/M(n-1( redox potentials by assuming that all ligand contributions are additive. In this fashion it performs a similar purpose to the Dq parameter in electronic spectroscopy. The parameter is defined as 1/6 that of the Ru(III)/Ru(II) potential for species RuL6 in acetontrile. The El(L) values for over 200 ligands are presented and the model is tested over a wide range of coordination complexes and organometallic species. The redox potential of a M(n)/M(n-1) couple is defined to be equal to:- E(calc) = f Sigma EL (L) + c. The values of f and C, which are tabulated, depend upon the metal and redox couple, and upon spin state and stereochemistry, but, in organic solvents, are generally insensitive to the net charge of the species. Consideration is given to synergism, the potentials of isomeric species, and the situations where the ligand additivity model is expected to fail. In this initial study, the redox couples are restricted almost exclusively to those involving the loss or addition of an electron to the tzg (in Oh) sub-level.

The electron-donating properties of N-heterocyclic carbenes ([N,N'-bis(2,6-dimethylphenyl)imidazol]-2-ylidene and the respective dihydro ligands) with 4,4'-R-substituted aryl rings (4,4'-R=NEt2, OC(12)H(25), Me, H, Br, S(4-tolyl), SO(4-tolyl), SO2(4-tolyl)) were studied. Twelve new N-heterocyclic carbene (NHC) ligands were synthesized as well as the respective iridium complexes [IrCl(cod)(NHC)] and [IrCl(CO)2(NHC)]. Cyclic voltammetry (DeltaE1/2) and IR (nu (CO)) can be used to measure the electron-donating properties of the carbene ligands. Modifying the 4-positions with electron-withdrawing substituents (4-R=-SO(2)Ar, DeltaE1/2=+0.92 V) results in NHC ligands with virtually the same electron-donating capacity as a trialkylphosphine in [IrCl(cod)(PCy3)] (DeltaE1/2 =+0.95 V), while [IrCl(cod)(NHC)] complexes with 4-R=NEt2 (DeltaE1/2= +0.59 V) show drastically more cathodic redox potentials and significantly enhanced donating properties.

A new N-heterocyclic carbene architecture comprising a 1,4-naphthoquinone annulated to 1,3-dimesitylimidazolylidene (NpQ-NHC) was synthesized in two high yielding steps from commercially available starting materials. The free NpQ-NHC was characterized (solution and solid-state) and was used to synthesize various Rh and Ag complexes that ranged in pi-electron density. Enabled by the quinone moiety, the pi-systems of these complexes were analyzed using infrared spectroscopy and cyclic voltammetry. In contrast to previous reports, pi-backbonding was found to be non-negligible and was directly influenced by the metal's electronic character.

In the last decade the use of diamino N-heterocyclic carbenes (NHCs) A as ancillary ligands for transition-metal catalysts and as organic catalysts on their own has proven very fruitful.[1] The efficiency of NHCs is attributed to their strong σ-donor properties and sterically demanding structure.[1,2] Not only have NHCs yielded improved transition metal catalysts, but they have also lead to the isolation of unusual low-coordinate metal complexes,[2] which often play a key role in catalytic processes. Recently, we reported the synthesis of stable cyclic (alkyl)(amino)carbenes (CAACs) B[3] and demonstrated that CAACs can be even stronger σ donors than NHCs, and lead to highly efficient palladium catalysts for the α-arylation of ketones and aldehydes. Here we report that the peculiar steric and electronic properties of rigid CAAC ligands allow the preparation of low-coordinate metal complexes, hitherto not isolable with any other ligands. To further evaluate the electronic properties of rigid CAAC B1, we attempted to prepare the [RhCl(CO)2(B1)] complex, since the CO stretching frequencies of this type of rhodium complexes are recognized as an excellent measure of the σ-donor ability of the ligand L, and a large amount of data is available for comparison.[4] Reaction of carbene B1 with half an equivalent of [{RhCl(cod)2}2] (cod = 1,5-cyclooctadiene) afforded complex 1, which after purification by column chromatography on silica gel was obtained as orange crystals in 79% yield (Scheme 1). The structure of 1 was unambiguously determined by single-crystal X-ray diffraction.[5] Following the classical procedure, a solution of 1 in chloroform was treated with an excess of CO. Surprisingly, the 13 C NMR spectrum of the product (95% yield) showed only one CO signal, with a large coupling constant (JRh,C = 134 Hz), and not two signals with smaller J values (JRh,C = 33–82 Hz), as observed for [RhCl(CO)2(L)] complexes.[4] It is known that when bulky ligands L are present [RhCl(CO)2L] complexes can readily lose one CO ligand to give a chloro-bridged dimer.[6] However, single-crystal X-ray diffraction[5] revealed that even in the solid state complex 2 is formally a 14-electron [RhCl(CO)L] monomer (Figure 1). Figure 1 Molecular structure of 2 in the solid state. Selected bond lengths [A] and angles [°]: N–C1 1.3174(14), C1–C2 1.5368(15), C1–Rh 1.9399(10) Rh–Cl 2.3740 (3), Rh–C3 1.7955(11), C3–O 1.1433(14), ... Scheme 1 Synthesis of [RhCl(CO)(B1)] complex 2. Interestingly, 2 can also be synthesized in 80% yield by reaction of one equivalent of carbene B1 with half an equivalent of [{RhCl(CO)2}2]. The extreme hindrance provided by the menthyl ring of CAAC B1, which is locked in the most sterically demanding conformation with respect to the metal center, explains in part why dimerization does not occur. Moreover, the short Rh–Ha (2.183 A) and Rh–Hb (2.231 A) distances suggest the existence of agostic interactions, which bring additional stabilization to the metal center. In solution, an agostic interaction is also apparent in the 1H NMR spectrum, which shows a broad multiplet (1H) at 0.08 ppm. Complex 2 is indefinitely stable at room temperature in air. Related [RhClL2] complexes, exemplified by the active species of Wilkinson3s catalyst [RhCl(PPh3)2], are known only as transient species.[7] They have only been generated in situ by ligand dissociation[8] or by hapticity changes;[9] otherwise they readily form chloro-bridged dimers, even when two very bulky ligands L are present.[6] Although other neutral T-shaped formally 14-electron RhI complexes have been isolated, they are still very rare, and none of them have a bridging-capable halide ligand like complex 2.[10,11] The isolation of this low-coordinate RhI complex 2 made us confident that other low-ligated transition-metal complexes could be stabilized by rigid CAAC B1. After close scrutiny of the literature, it came to our attention that other ligands, particularly NHCs, have consistently failed to allow the isolation of cationic 14-electron [Pd(All)(L)]+ complexes (Scheme 2, All =allyl).[12] In 2003, Nolan et al.[12a] were able to abstract chloride from complex 3, but the cationic complex generated was either unstable and decomposed upon workup or isolated and structurally characterized as an acetonitrile adduct. In 2005, Porschke et al.[12b] reported that chloride abstraction with thallium salts from NHC complexes 3 led to ionic binuclear compounds, which result from combination of the transiently formed desired cationic species with the starting neutral complex. Most recently, Glorius et al.[12c] generated cationic complex 4, which, despite intramolecular stabilization by double-bond complexation, appeared to be rather unstable and was only characterized by 1H NMR spectroscopy. Scheme 2 Previous attempts to prepare cationic 14-electron [Pd(All)(L)]+ complexes. In marked contrast, chloride abstraction from [PdCl-(All)(B1)] (5)[3] with a stoichiometric amount of AgBF4 produced the elusive cationic palladium complex 6 as yellow crystals (88% yield, Scheme 3). The structure of 6 was unambiguously established by single-crystal X-ray diffraction (Figure 2).[5] As expected, 6 has a T-shaped geometry with no interaction between the metal atom and tetrafluoroborate anion (shortest Pd–F distance 3.88 A). Similar to the above-mentioned RhI complex 2, at least one of the axial H atoms of the menthyl ring provides a stabilizing agostic interaction (Pd–Ha 2.048, Pd–Hb 2.509 A). This interaction is also apparent from the 1H NMR spectrum, which shows a broad multiplet (1H) at −0.17 ppm. This is the first example of a stable, formally 14-electron, PdII cation.[13] Figure 2 Structure of the cation of 6 in the solid state. Selected bond lengths [A] and angles [°]: N–C1 1.293(3), C1–C2 1.519(3), C1–Pd 2.038(3), Pd–Ha 2.048, Pd–Hb 2.509, Pd–C3 2.272(13), Pd–C4 ... Scheme 3 Synthesis of [Pd(All)(B1)]+ complex 6. To further demonstrate the increased stabilization of metal centers provided by the rigid menthyl moiety of carbene B1, we attempted to prepare complexes analogous to 2 and 6, using the CAAC B2, featuring the unlocked and therefore flexible cyclohexyl ring.[3,14] Treatment of [{RhCl(CO)2}2] with two equivalents of carbene B2 afforded exclusively the classical dicarbonyl complex 7, while treatment of 8[3] with AgBF4 led to rapid formation of palladium black (Scheme 4). Clearly, in contrast to B1, the cyclohexane ring of CAAC B2 can flip to the less sterically demanding conformation, in which no H atoms are available for strong agostic interactions with the metal center. Scheme 4 Synthesis of [RhCl(CO)2(B2)] complex 7, and attempted chloride abstraction from [PdCl(All)(B2)] complex 8. The isolation of complexes 2 and 6 clearly demonstrates the advantages of rigid CAACs over classical ligands. By manipulating the quaternary carbon atom adjacent to the carbene center it is possible to design ligands with an electronically active wall of protection for the metal center. We are currently exploring the possibility that CAAC ligands will stabilize even lower coordinate metal complexes.

The four arsines, As{C6H3(o-CH3)(p-Z)}3 {Z = H (2a) or OMe (2b)} and As{C6H3(o-CHMe2)(p-Z)}3 {Z = H (2c) or OMe (2d)} react with [PdCl2(NCPh)2] or [PtCl2(NCBut)2] to give trans-[MCl2L2] or trans-[M2Cl2(µ-Cl)2L2]. The crystal structures of trans-[PdCl2(2a)2] and [PtCl2(2d)2] have been determined, the latter as its dichloromethane solvate. The structures show that in these complexes, the ligands adopt gga type conformations as do all analogous tri-o-tolyl- and tri-o-isopropylphenylphosphines in square-planar and octahedral complexes. The variable-temperature NMR behaviour of the complexes shows that they are fluxional due to restricted As–C bond rotation. The rate of the fluxionality is more rapid than in the analogous phosphine complexes and this is associated with longer As–C and As–M bonds allowing more free movement. The catalytic activity of the palladium complexes of the arsines and their phosphine analogues for the reaction of 4-bromoacetophenone and n-butyl acrylate has been screened. The results show that the arsines are generally superior to the phosphines as ligands for this catalysis. Tri(o-isopropylphenyl)phosphine and tri(o-isopropylphenyl)arsine are superior to tri-o-tolylphosphine as ligands for this Heck reaction and a p-methoxy substituent improves the arsine catalyst but not the phosphine catalyst. The phosphine catalysts are superior to the arsine catalysts for the reaction of 4-chloroacetophenone and n-butyl acrylate. These observations are discussed in the context of ligand stereoelectronic effects, as measured by the Tolman electronic parameter, νCO of the [NiL(CO)3] {L = AsAr3 or PAr3}.

No example of a simple uncatalyzed dimerization of a diaminocarbene has been clearly established, so it is timely to ask what factors control the thermodynamics of this reaction, and what mechanisms are responsible for the observed dimerizations? In agreement with qualitative experimental observations, the dimerizations of simple five- and six-membered-ring diaminocarbenes are calculated to be 100 kJ mol(-1) less favorable than those of acyclic counterparts. This large difference is semiquantitatively accounted for by bond and torsional angle changes around the carbene centers. Carbenes such as (Et(2)N)(2)C are kinetically stable in THF at 25 degrees C in agreement with calculated energy barriers, but they rapidly dimerize in the presence of the corresponding formamidinium ion. This proton-catalyzed process is probably the most common mechanism for dimer formation, and involves formation of C-protonated dimers, which can be observed in suitable cases. The possibility of alkali-metal-promoted dimerization is raised, and circumstantial evidence for this is presented.

International audience; The calculated (DFT, B3PW91) A(1) v(CO) frequency in LNi(CO)(3) defines an electronic parameter that reliably predicts the relative donor powers of a wide variety of cationic, neutral, and negatively charged ligands. These calculated parameters correlate Very well with the available Tolman and Lever parameters, and also with Hammett's sigma (m), where available. The method avoids any experimental limitations and, in particular, can be used for proposed ligands not yet experimentally available.

Synthese, structure et caracterisation du (1,3-bis [1-adamantyl]-2,3-dihydro)-2-carbenoimidazole prepare par deprotonation du chlorure de (1,3-bis [1-adamantyl]) imidazolium

A high yielding approach to planar chiral carbene ligands is described, in which an imidazo[1,5-a]pyridine-3-ylidene unit is embedded into a cyclophane scaffold. As evident from the IR data of the corresponding rhodium complex 18 as the parent member of this family, these new ligands turned out to be exceptionally strong electron donors, rivaling or even outperforming the other diamino-stabilized five-membered N-heterocyclic carbenes (NHC) known to date. If the remote ring of the cyclophane is substituted by four fluorine atoms, however, the donor capacity is significantly reduced by the through-space interaction of the tetrafluorobenzene moiety with the underneath carbene entity. Since X-ray data suggest that the steric demand of the fluorinated and the nonfluorinated cyclophane-2-ylidenes are virtually identical, these results illustrate that the electronic properties of an NHC ligand can be tuned over a wide range without the need to change its constitution or steric attributes. The synthesis route lea...

Whereas the electronic communication between metals in dimetallic organometallic compounds is often assessed through cyclic voltammetric measurements, we have found that the variations in the Tolman electronic parameter (TEP) can also be an alternative and effective way of determining this type of interaction. The TEP values of several (CO)3Ni–NHC–X systems with five different ditopic NHC ligand systems [triazolyldiylidene (A), bis(imidazolylidene) (B), benzobis(imidazolylidene) (C), cyclopenta[f,g]acenaphthylenebis( imidazolylidene) (D) and bis(imidazolidinylidene) (F)] were determined by means of DFT calculations. Based on these values, the electron-withdrawing character of the X metal fragments employed in this study was found to increase in the order IrCp(CO) → RhCl(COD) → Ni(CO)3 → Cr(CO)5 → RhCl(CO)2. We found that the degree of electronic interaction through the ditopic NHC ligands is the strongest in A, followed by B and F, while being weak in B and C. The TEP values and the quantitative analysis of the upper molecular orbitals of A and F and their (CO)3Ni–NHC–Ni(CO)3 complexes strongly suggest that the principal electronic interaction between the metal centres of the M–NHC–M’ complexes is of σ-type, via the delocalized HOMO and HOMO − 1 orbitals of the NHC ligands.

The synthesis and characterization of three- and four-coordinate Ni(CO)n(NHC) (n = 2, 3; NHC = N-heterocyclic carbene) complexes are reported. Reactions with CO of the Ni(CO)2(NHC) complexes lead to the quantitative formation of Ni(CO)4. Investigation of this reaction under equilibrium conditions allows for the determination of Ni-NHC bond dissociation energies.

Electronic and steric ligand effects both play major roles in organometallic chemistry and consequently in metal-mediated catalysis. Quantifying such parameters is of interest to better understand not only the parameters governing catalyst performance but also reaction mechanisms. Nowadays, ligand molecular architectures are becoming significantly more elaborate and existing models describing ligand sterics prove lacking. This review presents the development of a more general method to determine the steric parameter of organometallic ligands. Two case studies are presented: the tertiary phosphines and the N-heterocyclic carbenes.

This laboratory experiment for use in inorganic or organometallic chemistry allows students to examine the property of ligand electron-donor strength and its impact on a transition-metal center. Students synthesize a series of four cis-Mo(CO)4(PR3)2 complexes with phosphorus(III) ligands having a wide range of electron-donating strengths (PR3 = P(n-butyl)3, PPh3, P(OMe)3, and P(OPh)3). These complexes are characterized by FT-IR spectroscopy, with an A1 carbonyl stretch (νCOMo) used as a handle for quantifying the electron density of the molybdenum center. Using this data, students qualitatively rank the electron-donor strength of this series of ligands. In addition, students apply the Tolman electronic parameter (TEP), the quantitative analysis of ligand effects (QALE) model, and the molecular electrostatic potential (MESP) as quantitative measures of phosphine-donor ability. By developing linear correlations between these parameters and their experimental νCOMo data, students estimate the νCOMo values for additional cis-Mo(CO)4(PR3)2 complexes not synthesized in this laboratory. This combined qualitative and quantitative investigation provides students with practical insight into ligand-donor ability and its application in "tuning" the properties of a metal complex.

Four new stable nucleophilic carbenes have been synthesized and structurally characterized. The remarkable ability of the imidazole nucleus to stabilize a carbene center at the C-2 position is demonstrated by the isolation of 1,3,4,5-tetramethylimidazol-2-ylidene. The isolation of three imidazol-2-ylidenes that bear aryl substituents is counter to speculations based on previous reports