Electron Transfer. 135. Pendant Carbonyl Groups in the Mediation of the Reactions of Indium(I) with Bound Ruthenium(III)1

Abstract

Reductions, using In(I)(aq) and Ti(III)(aq), of (NH(3))(5)Ru(III) derivatives of pyridines having carbonyl-bearing substituents (-CONH(2), -COOCH(3), and -COC(6)H(5)) yield the corresponding (NH(3))(5)Ru(II) complexes. Reactions with Ti(III) are kinetically straightforward, exhibit only slight responses to structural alteration, and give no indication of inner-sphere mediation involving the carbonyl group. Kinetic profiles for In(I) reductions of the 3-CONH(2), 3-COOCH(3), and 3-COC(6)H(5)-substituted complexes (in the range [H(+)] = 0.030-0.15 M) begin with a nearly linear section, with rates independent of [In(I)], but show curvature during the later stages of reaction. These profiles are consistent with a reaction sequence in which the predominant carbonyl form of the oxidant is hydrated to a more reactive gem-diol form (>C=O + H(2)O right harpoon over left harpoon >C(OH)(2)), which undergoes reduction by In(I), yielding the observed Ru(II) product and In(II). The latter is then rapidly consumed by a second unit of Ru(III). Rate constants for the hydration step giving optimal fit to the experimental curves are 6 x 10(-4) s(-1) (for the 3-CONH(2) complex) and 1.3 x 10(-3) s(-1) (for the 3-COOCH(3) oxidant). Lower limits for the rate of attack by In(I) on the active forms of the oxidants are estimated as 10(3.3)-10(4.6) M(-1) s(-1), about 10(2)-10(3) times are rapid as the reduction of the unsubstituted pyridine complex. Our results suggest the utilization, by In(I), of a hydroxyl-bridged path featuring the sequence In(I)-OH-C(OH)-py in these reductions, a path which is overshadowed, in the case Ti(III) reactions, by a more facile outer-sphere process.