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.