Reversible bond formation and cleavage of the oxo bridge of [Ru2(μ-O)-(dioxolene)2(btpyxa)3+ [btpyxa=2,7-Di-tert-butyl-9,9-dimethyl-4,5bis(2,2′:6′,2-terpyrid-4′-yl)xanthene] driven by a three-electron redox reaction

The bis(chlororuthenium) complex [RU2Cl2(3,6-tBu(2)sq)(2-)(btpyxa)](PF6)(2) [1](PF6)(2) [3,6-tBu(2)sq = 3,6-di-tert-butyl-1,2-benzosemiquinone; btpyxa = 2,7-di-tert-butyl-9,9-dimethyl-4,5-bis(2,2':6',2-terpyrid-4'-yl)xanthene] and the oxobridged diruthenium complex [Ru-2(mu-O)(3,6-tBu(2)sq)(2)-(btpyxa)](PF6)(3) [2](PF6)(3) were synthesized, and the redox behavior of these complexes, which contain a non-innocent dioxolene ligand, was investigated by electrochemical and electrospectrochemical methods. Dicationic [1](2+) undergoes two successive metal-centered one-electron and a simultaneous two-electron ligand-based redox reaction at E-1/2 = +0.13 and +0.09 and E-1/2 = -0.75 V (vs. SCE), respectively, in CH2Cl2. The UV/Vis/NIR spectrum of tricationic [2](3+) shows an intervalence-transition (IT) band at 1333 nm (epsilon = 1.52 x 10(4) M-1 cm(-1)) in a near-IR region together with two CT bands at 766 (epsilon = 2.21 x 10(4) M(-1)cm(-1)) and 586nm (c = 1.13 x 104 M-1 cm(-1)) in CH2Cl2. The mixed-valence complex of [2 ](3+) with an Ru-IV-O-Ru-III core is reversibly oxidized and reduced to the Ru-IV-Ru-IV and Ru-ll-Ru-ll oxidation states at E-1/2 = +0.63 and -0.01 V, respectively, in CH2Cl2. On the other hand, three-electron reduction of [2](PF6)(3) is accompanied by the cleavage of the Ru-O-Ru bond at E-p = +0.02 V to give [(Ru(OMe)(3,5-tBU(2)sq))(Ru(OH2)(3,5-tBU(2)sq))(btpyxa)](+) in MeOH. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)