Electron Delocalization in Acyclic andN-Heterocyclic Carbenes and Their Complexes:  A Combined Experimental and Theoretical Charge-Density Study

Abstract

Combined experimental and theoretical charge-density studies on free and metal-coordinated N-heterocyclic carbenes have been performed to investigate the extent of electron delocalization in these remarkable species. Tracing the orientation of the major axis of the bond ellipticity (the least negative curvature in the electron density distribution) along the complete bond paths distinguishes unambiguously between fully delocalized systems and those with interrupted cyclic electron delocalization. Evaluation of charge-density-based properties such as atomic quadrupole moments serves as a direct and quantitative measure of the extent of pi-electron delocalization and reveals consistency between theory and experiment. A detailed topological analysis of theoretical charge densities for two benchmark carbene systems, viz., 1,2-dimethylpyrazol-3-ylidene 1a and 1,3-dimethylimidazol-2-ylidene 2a, and their corresponding stable chromium pentacarbonyl complexes 1 and 2, highlights the advantages of charge-density-based criteria to analyze such complex electronic situations. Thus, 1a and 2a display a different extent of electron delocalization; yet nearly identical p(pi) occupations at the carbene center are computed for 1a and 2a. However, atomic quadrupoles Q(zz) - the charge-density analogues of p(pi) occupation - reveal faithfully the electronic differences in 1a and 2a and demonstrate the sensitivity of charge-density-based properties to the bonding situation. The acyclic aminocarbene (iPr(2)N)(2)CCr(CO)(4) has also been studied, and the high barrier to rotation about the C-N bond is shown not to arise solely from p(pi)-p(pi) bonding.

Beyond ConventionalN-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

“…However, the trans influence is similar, i.e., the crystallographically determined metal-carbonyl bonds in the Cr complex 64b and (IMe)Cr(CO) 5 are of equal length (Cr-CO trans ) 1.86 and 1.87 Å, respectively). 55 Quantum mechanical investigations suggest that the nitrogen lone pairs of these heterocyclic carbenes are largely localized, thus allowing (some) mesomeric interaction solely via the carbon framework. 55 This result is in marked contrast to the general assumption that the stability of imidazol-2-ylidene systems can be predominantly attributed to π-donation by the N atoms.…”

“…55 Quantum mechanical investigations suggest that the nitrogen lone pairs of these heterocyclic carbenes are largely localized, thus allowing (some) mesomeric interaction solely via the carbon framework. 55 This result is in marked contrast to the general assumption that the stability of imidazol-2-ylidene systems can be predominantly attributed to π-donation by the N atoms. http://doc.rero.ch 9 In an attempt to compare the relative donor strengths of different carbenes, Herrmann et al have synthesized a range of NHC complexes of the type cis-RhI(NHC)(CO) 2 .…”

N-Heterocyclic carbene bonding to cobalt porphyrin complexes

“…In contrast, the bonding between a NHC and a metal center comprises a much higher covalent contribution [5]. Metal bonding is thus, generally, under kinetic control, and the NHC-metal bond is kinetically inert, i.e.…”

Theoretical Study of Rhodium(I) Carbene Complexes: The Structural Versatility of Phosphino‐ Compared with Aminocarbenes

“…[25] This is apparent in the NBO population analyses that reveal the presence of p NC interactions for all aminocarbene h 1 -complexes 7±9, while p PC bonding is only observed for the phosphinocarbene h These results demonstrate the propensity of phosphinocarbenes to act as two-as well as four-electron donors, while only h 1 -coordination is strongly favored for aminocarbenes, in agreement with experimental data. Indeed, the h 2 -coordination mode has only been observed in the case of acyclic FULL PAPER di(amino)carbenes, [11] in which one amino group is engaged in p-donation to the carbene center, which leaves the other available for coordination to the transition metal center. Table 5.…”

“…Indeed, the h 2 -coordination mode observed in most of the PC complexes [10] remains unknown for NHCs and is very rare for acyclic aminocarbenes. [11] To obtain more insight into ligand properties of PCs versus NHCs, here we report a theoretical investigation of model complexes with the general formula [(carbene)RhClL 2 ]. The influence of the L ligand (L = C 2 H 4 , PH 3 , CO) on the struc- Abstract: Density functional calculations are reported for complexes of general formula [(carbene)RhClL 2 ] featuring model phosphino-and aminocarbenes.…”