Synthesis and Photophysical Characterization of Cyclometalated Ruthenium Complexes with N-Heterocyclic Carbene Ligands

In the search of new ligand environment for the ruthenium(II) complexes with pyridine-type ligands, we developed seven new cyclometalated ruthenium complexes with N-heterocyclic carbenes (NHCs). In a two-step procedure, we synthesized complexes with the general formula Ru(NHCC)(bpyR(2))(2), where NHCC is a bidentate ligand coordinating with the carbene and cyclometalation; bpyR(2) is either 2,2'-bipyridine, when R = H, or 4,4'-dimethoxycarbonyl-2,2'-bipyridine, when R = CO2Me. To investigate the photophysical properties of these complexes, various NHC and cyclometalated ligands of different electron donating potency were used. The new compounds were characterized by H-1, C-13, and COSY NMR methods, cyclic voltammetry, absorption spectroscopy, and single crystal X-ray crystallography. These complexes exhibit perfect redox reversibility, and in contrast to the reported analogues without cyclometalation, they do not have photoemission in solution.