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(NHC^C)(bpyR2)2](PF6), where NHC^C is a bidentate ligand coordinating with the carbene and cyclometalation; bpyR2 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 1H, 13C, 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.