Titanium(III)-ruthenium(III) electron transfer through 3,6-dihydroxy-1,4-benzoquinonate ligands: an ultra-short insulated wire

Substituents in positions 2 and 5 of a 3,6-dihydroxy-1,4-quinonate ligand ( BQ(2-)) bring about gross changes in the kinetics and mechanism by which ru thenium(III) complexes containing that ligand (with triethylenetetramine, t rien, as spectator ligand) are reduced by aqueous titanium(III). Ti-III-red uction of Ru(trien)(BQ)(+) requires two mols of Ti-III per mol of Ru-III; t he product is a Ru-III hydroquinonate complex. An intermediate is involved in this reaction. Both formation and decay of the intermediate follow the k inetic pattern characteristic of outer-sphere electron-transfer (ET). In co ntrast, reduction of the chloro-substituted analogue, Ru(trien)(Cl(2)BQ)(+) , by Ti-III has 1/1 stoichiometry, no detectable intermediate, and displays the rate behavior characteristic of inner-sphere reactions. A third patter n obtains for reduction of the methoxy-substituted analogue, Ru(trien)((OMe )(2)BQ)(+) by Ti-III. This reaction has 1/1 stoichiometry, does not involve an intermediate, and follows outer-sphere kinetics. The difference in stoi chiometry caused by change of substituents is understood to arise from stab ilization of quinonoid radicals by both chloro- and methoxy-substituents. T he inner-sphere mechanism in the case of Ru(trien)(Cl(2)BQ)(+) is ascribed to higher basicity and complexing ability of oxygen in this species. When T i-III reduces Ru(trien)(BQ)(+) that is encapsulated by alpha-cyclodextrin, 2/1 stoichiometry, with an intermediate, is observed, and the kinetic patte rn, changes from that characteristic of outer-sphere ET to that of inner-sp here electron transfer. Cyclodextrin changes the reaction mechanism by bloc king attack of Ti-III on the face of the quinonate ring. The alpha-cyclodex trin-encapsulated quinonate ligand functions as an insulated conductor of c harge between titanium and ruthenium centers.