Electron transfer. 44. Decreases in the effectiveness of redox catalysts with use

Article abstract of DOI:10.1021/ic50204a024

Pyridine derivatives, which, when uncoordinated, catalyze outer-sphere electron-transfer reactions between metal centers, often undergo loss of catalytic activity with use. The active intermediates in such catalyzed reactions are radicals (cat?) formed by initial reaction of the catalysts with the primary reducing centers. This study examines the deterioration of a number of such catalysts in reactions of Eu²⁺ and V²⁺ with (NH₃)₅Co(py)³⁺. This deterioration, when it occurs, arises from interaction of the catalyst and the reducing center; it does not require Co(III). Degrees of deterioration vary widely. The very powerful catalysts derived from 2,4-pyridinedicarboxylic acid (III) lose much or all of their catalytic activity during the course of a single 1-min run with Eu²⁺ in excess, whereas catalytic erosion of isonicotinic acid (I), its esters, and its nitrile is negligible after treatment with excess reductant for 10-20 min. Erosion is much more marked with Eu²⁺ than with the less strongly reducing V²⁺ and is much less severe when the oxidant, rather than the reductant, is taken in excess. Deterioration in Eu²⁺ systems may be decreased strikingly by addition of excess Eu³⁺ and that in V²⁺ systems by addition of V³⁺. The spectra of the products formed when the more fragile catalysts react rapidly with Eu²⁺ (in the absence of Co(III)) correspond to those formed by reduction of the catalysts with zinc amalgam, which is presumed to be a two-electron reductant. The more robust isonicotinate catalysts are not affected by Eu²⁺ under similar conditions. Evidence is presented in support of two attrition mechanisms. The catalytic deterioration of 2,4-pyridinedicarboxylic acid in Eu²⁺ systems appears to involve disproportionation of catalyst-radical pairs (2cat? + 2H⁺ → cat + catH₂), converting one member of each pair to an inactive dihydro species and returning the other to the catalyst pool. Measurements of the rate of deterioration of this catalyst allow us to estimate the specific rate for the disproportionation as 5.8 × 10⁸ M^-1 s^-1. In systems featuring less fragile catalysts (e.g., isonicotinamide) the steady-state concentration of the radical, cat?, is so low that attrition, if it takes place at all, occurs mainly by reductive deterioration (cat? + Eu²⁺ →^2H+ Eu³⁺ + catH₂). In two such instances, comparison of kinetic runs using catalyst preparations that have undergone partial attrition permits an estimate of the specific rates of such deteriorative processes. Although both modes of attrition are presumed to occur in each catalytic system, bimolecular disproportionation appears to compete most favorably with reductive deterioration when the extent of reduction of the catalyst to its radical is greatest.