Singlet diradical complexes of ruthenium and osmium: geometrical and electronic structures and their unexpected changes on oxidation

Abstract

Reaction of HL, HL^a (2-[(2-N-phenylamino)phenylazo]pyridine), HL^b (2-[{2-N-(4-methylphenyl)amino}phenylazo]pyridine), or HL^c (2-[{2-N-(4-chlorophenyl)amino}phenylazo]pyridine), with KRuO₄ or OsO₄ and PPh₃ under exhaustive deoxygenation (PPh₃ → OPPh₃) yields diamagnetic compounds ML_2. Crystal structure determination for M(L^a)₂ indicates the radical dianion state, L*^2-, for the ligands as evident from the typical N-N bond length of about 1.33 Å for a one-electron reduced azo function. The resulting spin-coupled complexes, M^IV(L*^2-)₂, can be oxidized in two reversible one-electron steps, as probed by cyclic voltammetry and UV-vis-NIR spectroelectrochemistry. The paramagnetic intermediates, [M(L^a)₂]⁺, are distinguished by intense NIR absorption, largely metal-centered spin as revealed by EPR, and, in the case of [Os(L^a)₂]I₃, by crystallographically determined shortening of the N=N bond to about 1.30 Å, corresponding to a coordinated unreduced azo function. Thus, oxidation of the complex M^IV(L*^2-)₂ involves partial reduction of the metal in [M^III(L^-)₂]⁺ because intramolecular double electron transfer is offsetting the external charge removal. Density-functional theory calculations were employed to confirm the structural features and to support the spectroscopic assignments.