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

Reaction of HL, HLa (2-[(2-N-phenylamino)phenylazo]pyridine), HLb (2-[{2-N-(4-methylphenyl)amino}phenylazo]pyridine), or HLc (2-[{2-N-(4-chlorophenyl)amino}phenylazo]pyridine), with KRuO4 or OsO4 and PPh3 under exhaustive deoxygenation (PPh3 → OPPh3) yields diamagnetic compounds ML2. Crystal structure determination for M(La)2 indicates the radical dianion state, L•2−, for the ligands as evident from the typical N−N bond length of about 1.33 A for a one-electron reduced azo function. The resulting spin-coupled complexes, MIV(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(La)2]+, are distinguished by intense NIR absorption, largely metal-centered spin as revealed by EPR, and, in the case of [Os(La)2]I3, by crystallographically determined shortening of the N═N bond to about 1.30 A, corresponding to a coordinated unreduced azo function. Thus, oxidation of the complex MIV(L•2−)2 involves pa...