Modulating the Bonding Properties of N‐Heterocyclic Carbenes (NHCs): A Systematic Charge‐Displacement Analysis

Abstract

In view of their intensive use as ligands in many reactions catalyzed by transition-metal complexes, modulation of the bonding properties of N-heterocyclic carbenes (NHCs) on a rational basis is highly desirable, which should enable optimization of current applications or even promote new functions. In this paper, we provide a quantitative analysis of the chemical bond between a metal fragment AuCl and a series of 29 different NHCs in [(NHC)AuCl] complexes. NHCs electronic properties are modified through: i) variation of the groups attached to the NHC nitrogen atoms or backbone; ii) change of unsaturation/size of the NHC ring; iii) inclusion of paracyclophane moieties; or iv) heteroatom substitution on the NHC ring. For evaluating the donation and back-donation components of the Dewar-Chatt-Duncanson (DCD) model in the NHC-AuCl bond, we apply the charge-displacement (CD) analysis within the NOCV (natural orbitals for chemical valence) framework, a methodology that avoids the constraint of using symmetrized structures. We show that modulation of the NHC bonding properties requires substantial modification of their structure, such as, for instance, insertion of two ketone groups into the NHC backbone (which enhances the π back-donation bond component and introduces an effective electronic communication within the NHC ring) or replacement of a nitrogen atom in the ring with an sp or sp carbon atom (which increases and decreases the π back-donation bond component, respectively). We extend our investigation by quantitatively comparing the NHC electronic structures for a subset of 13 NHCs in [(NHC)PPh] adducts, the P NMR chemical shift values of which are experimentally available. The latter have been considered as a suitable tool for measuring the NHCs π acceptor properties [Bertrand et al., Angew. Chem. Int. Ed. 2013, 52, 2939-2943]. We show that information obtained using the metal fragment can be transferred to the PPh moiety and vice versa. However, the P NMR chemical shift values only qualitatively correlate with the π acceptor properties of the NHCs, with the stronger π acidic carbenes as the most outliners.

Charge Displacement Analysis—A Tool to Theoretically Characterize the Charge Transfer Contribution of Halogen Bonds

“…On the other hand, the Au-Se bond remains practically unchanged, probably for the compensation of different effects. The decrease of the Au → C back-donation may appear small but it should be considered that the donor/acceptor properties of the carbene are only slightly influenced by chemical modifications on the carbene structure [23,69,70], and, on the other side, activation barriers can be quite sensible to back-donation [71]. The same gold complexes, in the presence of a stronger and more structured XB donor as NIS, gave different results.…”

“…Above all, it provided to be extremely versatile in coordination chemistry. Indeed, thanks to the possibility of decomposing the charge rearrangement upon the bond formation between two species into contributions (see Methodological Aspects), it has been extensively used to characterize metal-ligand bonds [21][22][23][24][25] providing, in some cases, an elegant theoretical framework to better rationalize experimental results [26][27][28][29]. Very recently, CD analysis was the key tool for revealing unexpected acceptor properties of helium atoms [30,31].…”

Cationic Gold(I) Diarylallenylidene Complexes: Bonding Features and Ligand Effects

“…However, only a small subset of these NOCV pairs actually contributes to the overall charge rearrangement Δ ρ ’ because a large part of them presents eigenvalues close to zero . The CD‐NOCV approach has been successfully applied for the characterization of transition metal compounds and for disentangling donation and backdonation in the CD function of non‐symmetric systems containing NHC−Au(I) bond, Au(I)−H bond or Au(III)−CO bond with different ancillary ligands …”

“…However, only a small subset of these NOCV pairs actually contributes to the overall charge rearrangement Δ1' because a large part of them presents eigenvalues close to zero. [18] The CD-NOCV approach has been successfully applied for the characterization of transition metal compounds [19] and for disentangling donation and backdonation in the CD function of non-symmetric systems containing NHCÀ Au(I) bond, [20] Au(I)À H bond [21] or Au (III)À CO bond with different ancillary ligands. [22] Well-defined measurements of the total charge transfer (denoted as CT net ) and of its σ donation and perpendicular and parallel π backdonation contributions (denoted as CT σÀ d°n , CT πÀ back ⊥ and CT πÀ back k , respectively) are obtained by evaluating the corresponding CD-NOCV function at the "isodensity boundary".…”

Orbital Decomposition of the Carbon Chemical Shielding Tensor in Gold(I) N‐Heterocyclic Carbene Complexes

“…A charge displacement study of different [(NHC)AuCl] complexes showed that the NHC bonding properties are quite robust against variation of the structure. This study was extended with [(NHC)PPh] adducts and it was shown that the 31 P NMR chemical shift only qualitatively correlates with the π acceptor properties of the NHCs . A recent computational study of various [(NHC)AuCl] complexes focused on Au NMR found a correlation between the π‐accepting ability of these complexes and the 197 Au chemical shift .…”