Effect of substituents at the heteroatom on the structure and ligating properties of carbodicarbenes and its silicon analogs: a theoretical study

Abstract

Density functional calculations have been carried out to investigate the effect of substituents attached to the heteroatoms of N-heterocyclic carbenes (NHCs) on the structure and ligating properties of carbon(0) [C(NHC)2] and silicon(0) [Si(NHC)2] compounds. The substituents were found to have a profound role on the structure and ligating properties of these classes of compounds. Fluoro- and chloro-substituted carbon(0) compounds were found to have quasi-linear geometries in which their C(0) characteristics are “masked.” However, their C(0) characteristics become prominent in their protonated species. Large negative charges and shallow bending potential of the central Cc–C0–Cc angle provide evidence for the “hidden C(0) characteristics” of these two compounds. Electron withdrawing substituents at N-atoms of the two NHCs dramatically decreases the basicity of these compounds. Both natural bonding and atoms in molecules analysis suggest that the most favorable Lewis structure of C(NHC)2 and Si(NHC)2 in their equilibrium geometries should be described (portrayed) as L=C=L and L → Si ← L, respectively, where L = NHCs.