title: Singlet diradical complexes of ruthenium and osmium: geometrical and electronic structures and their unexpected changes on oxidation.
abstract: 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, L2(.-), 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 NN bond to about 1.30 A, corresponding to a coordinated unreduced azo function. Thus, oxidation of the complex MIV(L(2.-))2 involves partial reduction of the metal in [MIII(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.
authors.affiliation: Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India.
authors.forename: Subhas
authors.initials: S
authors.lastname: Samanta
authors.forename: Priti
authors.initials: P
authors.lastname: Singh
authors.forename: Jan
authors.initials: J
authors.lastname: Fiedler
authors.forename: Stanislav
authors.initials: S
authors.lastname: Zális
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Kaim
authors.forename: Sreebrata
authors.initials: S
authors.lastname: Goswami