Organoosmium complexes of imidazole-containing chelate acceptor ligands

The complexes [(L)Os(η6-Cym)Cl](PF6), Cym = p-cymene and L = bis(1-methylimidazol-2-yl)ketone (bik) or bis(1-methylimidazol-2-yl)glyoxal (big), were obtained and characterized with respect to spectroscopy, crystal structure (big complex) and (spectro)electrochemical behaviour at variable temperatures. DFT calculations confirm the structure of [(big)Os(η6-Cym)Cl]+ with imidazolyl-N-bonded OsII in a boat-shaped seven-membered chelate ring with small N–Os–N angles (<84°). Reduction of this compound proceeds reversibly to a neutral complex of the α-semidione radical anion ligand big−; EPR and IR spectroelectrochemistry indicate very little participation from the heavy metal in the spin distribution. The analogous [(bik)Os(η6-Cym)Cl]+ could not be reduced reversibly to the ketyl radical complex but displayed a more reversible oxidation at high potential.