Carbones as Ligands in Novel Main-Group Compounds E[C(NHC)_2]_2 (E=Be, B~+, C~(2+), N~(3+), Mg, Al~+, Si~(2+), P~(3+)): A Theoretical Study

摘要

Quantum chemical calculations of the main-group compounds E[C(NHC~(Me))_2]_2 (E=Be, B~+, C~(2+), N~(3+), Mg, Al~+, Si~(2+), P~(3+)) have been carried out using density functional theory at the BP86/def2-TZVPP and BP86-D3(BJ)/def2-TZVPP levels of theory. The geometry optimization at BP86/def2- TZVPP gives equilibrium structures with two-coordinated species E and bending angles C-E-C between 152.5° (E=Be) and 110.58 (E=Al). Inclusion of dispersion forces at BP86- D3(BJ)/def2-TZVPP yields a three-coordinated beryllium compound Be[C(NHC~(Me))_2]_2 as the only energy minimum form. Three-coordinated isomers are found besides the two-coordinated energy minima for the boron and carbon cations B[C(NHC~(Me))_2]_2~+ and C[C(NHC~(Me))_2]_2~(2+). The three-coordinated form of the boron compound is energetically lower lying than the two-coordinated form, while the opposite trend is calculated for the carbon species. The theoretically predicted bond dissociation energies suggest that all compounds are viable species for experimental studies. The X-ray structure of the benzoannelated homologue of P[C(NHC~(Me))_2]_2~(3+) that was recently reported by Dordevic et al. agrees quite well with the calculated geometry of the molecule. A detailed bonding analysis using charge and energy decomposition methods shows that the two-coordinated neutral compounds Be[C(NHC~(Me))_2]_2 and Mg[C(NHC~(Me))_2]_2 possess strongly positively charged atoms Be and Mg. The carbodicarbene groups C(NHC~(Me))_2 serve as acceptor ligands in the compounds and may be sketched with dative bonds (NHC~(Me))_2C ←E→C(NHC~(Me))_2 (E=Be, Mg). Dative bonds in which the carbones C(NHC~(Me))_2 are donor ligands are suggested for the cations (NHC~(Me))_2C→E ←C(NHC~(Me))_2 (E=B~+, Al~+). The dications and trications possess electron-sharing bonds in which the bonding situation is best described with the formula [(NHC~(Me))_2C]~+-E-[C(NHC~(Me))_2]~+ (E=C, Si, N~+, P~+).