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

Rh(CO)Cl(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 σ-donor character than that of typical unsaturated NHCs is expected. However, the IR stretching frequencies of their Rh(CO)Cl complexes suggest an electron-donor character even stronger than that of saturated NHCs.