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.