Electron transfer 122. Reductions of an N 4-coordinated complex of silver(III)

Abstract

The silver(III) complex of ethylenebis(biguanide), (ebbg)Ag III, which is stable in aqueous acid, reacts readily with 2 equiv. each of V 2+, Cr 2+, Eu 2+, Fe 2+ and Ru(NH 3) 6 2+ yielding Ag +. Slower reactions with Ti(III) and Fe(Me 2phen) 3 2+ consume only 1 equiv, of these reductants but again yield Ag +. The 2 e − reductants H 3AsO 3 and HSO 3 − do not react. No Ag(II) species is detected in these reactions, either as a product or as a transient. At low reductant concentrations, reactions are first order in both redox partners, but at high [Fe 2+], there is evidence for partial formation of a 1:1 (ebbg)Ag IIIFe II complex ( Q assn = 10 M −1). Rates appear to be determined by the initial 1 e − reduction of (ebbg)Ag III to its Ag II analog, after which the latter is rapidly converted to Ag(I), either by a second reductant molecule or, in the case of the slower reductants, internal electron transfer with oxidation of the ligand. The latter process provides substantial driving force for it allows complete conversion to Ag(I) by several reductants having much more positive E° values than that reported for (ebbg)Ag III/II (0.32 V). Kinetic selectivity patterns are consistent with the predominance of an outer-sphere path for all metal-center reductants examined. Specific rates for these reductions, in conjunction with the treatment of Marcus, led to a self-exchange rate 10 5.8+0.8 M −1 s −1 for this Ag III/II couple. The latter value is comparable to rates recorded for a number of 4d- and 5d-based systems, is considerably greater than many of those for 3d-based couples, and lies close to that for Ag III/II when both states are porphyrin-bound. The present results support the view that reductions of Ag(III) to Ag(I) in this system take place most easily by successive 1 e − steps, rather than by single 2 e − transactions. These 2 e − reductants that react most readily (e.g. ascorbic acid and H 2O 2) are those which can be oxidized in single electron steps. Reductions by formic acid and by primary alcohols, which may instead involve hydride transfer to Ag(III) are found to proceed much more sluggishly.