Electron transfer. 33. Rate enhancement by external pyridinedicarboxylato derivatives

Article abstract of DOI:10.1021/ic50183a010

The reductions, using Eu²⁺ and Cr²⁺, of such outer-sphere oxidants as Co(NH₃)₅py³⁺ and Co(en)₃³⁺ are strongly accelerated by derivatives of pyridinedicarboxylic acids. In the Cr²⁺ systems, the carboxylate species is consumed during the course of the reaction and thus does not function as a true catalyst. The Eu²⁺ reductions are straightforward and, as in earlier work, 4 are interpreted in terms of a sequence in which the catalyst is reduced (k₁) to a radical intermediate, after which this intermediate may participate in reversal of the initial step (k_-1) or may react with Co(III) (k₂). Differences in catalytic effectiveness exhibited by the various dicarboxylato derivatives reflect principally differences in k₁, which is greatest when both carboxyl groups are conjugated with ring nitrogen and least when neither group is conjugated. In the Cr²⁺ sequence, the active intermediate appears to be a Cr(III)-bound radical which, because of slow substitution at the Cr(III) center, is not in rapid equilibrium with Cr(H₂O)₆³⁺. Kinetic data for Cr²⁺ reductions as accelerated by the methyl ester of 2,4-pyridinedicarboxylic acid (Est) allow calculation of: k₁, pertaining to formation of the radical-ion intermediate Cr^IIIEst·; the ratio k_-1/k₂, which governs the competition between reversal of the initial step and reaction of Cr^IIIEst· with Co(III); K_A, the acidity constant of the catalytically inactive conjugate acid of the diester. The selectivity exhibited by Cr^IIIEst· toward the various Co(III) oxidants in this study corresponds closely to that displayed by unbound pyridyl radicals and also by reducing metal ion centers in uncatalyzed reactions. Evidence is presented that the reaction of Cr²⁺ with 2,4-pyridinedicarboxylic acid can yield two different radical ions, one of which is much more reactive toward Co(III) centers than is the other.