Changes in ligating abilities of the singlet and triplet states of normal, abnormal and remote N-heterocyclic carbenes depending on their aromaticities

Abstract

Quantum chemical calculations at B3LYP/aug-cc-pVTZ level about singlet N-heterocyclic carbene (NHC) ligands, imidazol-2-ylidene, imidazol-4-ylidene, pyrazol-3-ylidene and pyrazol-4-ylidene, and their protonated analogues show that they are considerably aromatic except for pyrazol-3-ylidene. This result is experimentally verified by approximately five thousand NHC transition metal complexes retrieved from the Cambridge Structural Database (CSD). CSD search discloses that NHCs can participate in π-stacking interactions, albeit scarce. Geometry-based HOMA and electronic aromaticity index FLU rather than NICS provide a satisfactory description of the bonding situations in NHC ligands. Singlet state of the normal NHC has electron-deficient aromaticity as compared to those of the abnormal and remote NHCs. Depending on the transition from the singlet to triplet state, NHCs become electron-deficient ligands except for remote NHC. Computational studies regarding electronic nature of free NHC ligands show that the π-electronic population of the formally vacant pπ orbital on the carbene atoms in abnormal and remote NHC is occurred as a result of the aromaticity of NHCs, not as a result of the direct electron donation from LP-orbitals of N atoms to carbene atom according to putative push-pull effect used in understanding the electronic stabilization of normal NHC. Increase in the aromaticity raises σ-donating ability of both imidazol- and pyrazol-based NHC ligands. Free abnormal and remote NHC ligands have higher σ-donation ability than normal NHC ligands. The lack of σ-donating ability of normal NHC is compensated by its relatively high π-accepting ability, whereas π-back donation abilities of abnormal and remote NHCs are prohibited by their almost fully occupied π-orbitals. Aromaticities of the triplet NHC ligands are higher than that of the lowest-lying triplet state of benzene. Increase in the aromaticity of NHC ligands decreases van der Waals shortening in TM-NHC bonds mainly due to diminishing dative character of these bonds.