SLOW INTRAMOLECULAR ELECTRON-TRANSFER IN A RU(III)-TI(III) REDOX INTERMEDIATE HAVING A DIANION DERIVED FROM A DIHYDROXYQUINONE AS BRIDGING LIGAND

Slow intramolecular electron transfer (ET) has been observed in the binuclear intermediate of a Ti(III)-Ru(III) redox reaction that involves a dianion derived from a dihydroxyquinone as bridging ligand; this indicates that this ligand provides small but significant cross-bridge Ru(III)-Ti(III) electronic interaction. A Ti(III)-Co(III) reaction with a similar bridging ligand follows a different intimate mechanism (formation of a quinone radical ion), but intramolecular Co(III)-Ti(III) ET through squarate as bridging ligand is so slow that much of the overall redox change is accomplished by a competing outer-sphere reaction of that intermediate with TiOH2+. A two-phase reaction was observed for the Ti(III) reduction (in M LiCl at 25-degrees-C) of Co(en)2SQ+ (en is ethylenediamine; SQ2- is the dianion of squaric acid). Relatively rapid formation of a binuclear intermediate is followed by competition between outer-sphere attack of TiOH2+ on the deprotonated intermediate and intramolecular ET within that deprotonated species: Ti3+ half arrow right over half arrow left TiOH2+ + H+ (K(a)); Co(en)2SQ+ + Ti3+ half arrow right over half arrow left Co(en)2(SQ)Ti4+ (K1); Co(en)2(SQ)Ti4+ half arrow right over half arrow left Co(en)2(SQ)TiOH3+ + H+ (K2); Co(en)2(SQ)TiOH3+ --> products (k3); Co(en)2(SQ)TiOH3+ + TiOH2+ --> products (k4). Here, k4K1K2K(a) = 14 s-1 and k3K1K2 = 0.8 s-1. Reduction of Co(trien)Q+(trien is triethylenetetramine, and Q2- is the dianion of 2,5-dihydroxy-1,4-benzoquinone) by Ti(III) (in M LiCl at 25-degrees-C) also involves formation of a dinuclear complex between Ti3+ and the oxidant (K1' = 4.5 X 10(2) M-1) followed by further reaction of that complex by two competing paths. One path is intramolecular ET within the intermediate (k3' < 0.5 s-1); the other path is reaction between a second TiOH2+ and the preassembled (not deprotonated) intermediate (k4' = 1.3 x 10(3) s-1 M-1). Reduction of Ru(trien)CQ+ (CQ2- is the dianion of 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone) by Ti3+ also involves formation of an intermediate (K1'' = 6 x 10 M-1). Intramolecular ET within that intermediate has a rate constant of 1.1 x 10(2) s-1, and DELTAH* = -16 kJ/mol and DELTAS* = -150 J/(deg mol). Intramolecular ET in this kinetic intermediate is many orders of magnitude slower than that in analogous pyrazine-bridged mixed-valence diruthenium systems; electronic coupling between donor and acceptor orbitals is smaller in the dihydroxyquinonate-bridged species than in the pyrazine-bridged dimers due to orbital symmetry mismatch in the former case.