Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes.

ABSTRACT: Rh(CO)₂Cl(NHC) complexes of dipyrido-annelated N-heterocyclic carbenes were prepared. From the C-H coupling constant of the respective imidazolium salts and the N-C-N angle of the N-heterocyclic carbene (NHC), a weaker ?-donor character than that of typical unsaturated NHCs is expected. However, the IR stretching frequencies of their Rh(CO)₂Cl complexes suggest an electron-donor character even stronger than that of saturated NHCs. We ascribe this to the extremely weak ?-acceptor character of the dipyrido-annelated NHCs caused by the conjugated 14 ?e^- system that thus allows for an enhanced Rh-CO backbonding. This extremely low ?-acceptor ability is also corroborated by the ⁷⁷Se NMR chemical shift of -55.8 ppm for the respective selenourea, the lowest value ever measured for imidazole derived selenoureas. DFT-calculations of the free carbene confirm the low ?-donor character by the fact that the ?-orbital of the carbene is the HOMO-1 that lies 0.58 eV below the HOMO which is located at the ?-system. Natural population analysis reveals the lowest occupation of the p_?-orbital for the saturated carbene carbon atom and the highest for the pyrido-annelated carbene. Going from the free carbene to the Rh(CO)₂Cl(NHC) complexes, the increase in occupancy of the complete ?-system of the carbene ligand upon coordination is lowest for the pyrido-annelated carbene and highest for the saturated carbene.

SUBMITTER: Nonnenmacher M 

PROVIDER: S-EPMC5082490 | biostudies-literature | 2016

REPOSITORIES: biostudies-literature