Osmium Complexes Containing N-Heterocyclic Carbene-Based C,N,C-Pincer Ligands

Osmium(II) complexes bearing C,N,C-pincer ligand 2,6-bis(alkylimidazolin-2-ylidene)pyridine (CNC-Me and CNC-Bu for alkyl = methyl and n-butyl, respectively) or 2,6-bis(3-butylbenzimidazolin-2-ylidene)pyridine (CNC′-Bu) and aromatic diimine (2,2′-bipyridine (bpy)/1,10-phenanthroline (phen)) have been prepared. The X-ray crystal structure of [Os(CNC-Me)(bpy)Cl](PF6) shows that the Os−C bonds are essentially single (Os−C distances = 2.042(4) and 2.059(4) Å). Spectroscopic comparisons of [Os(C,N,C)(diimine)Cl]+ and [Os(tpy)(bpy)Cl]+ (tpy = 2,2′;6′,2′′-terpyridine) suggest that the lowest-energy absorption bands for [Os(C,N,C)(diimine)Cl]+ originate from a dπ(OsII) → π*(diimine) metal-to-ligand charge-transfer (MLCT) transition. Density functional theory calculations reveal that the π*(diimine) levels in [Os(C,N,C)(diimine)Cl]+ are lower lying than the π*(C,N,C). The Os(II/III) oxidation waves for [Os(C,N,C)(diimine)Cl]+ are reversible, with E1/2 = −0.03 to 0.16 V vs Cp2Fe+/0. The absorption spectra for the Os(III) species have been obtained by spectroelectrochemical methods.