Electron transfer reactions of heteroaromatic diols

Abstract

The kinetics and mechanism of oxidation of four heteroaromatic diols, namely 2-methyl-4,7-dihydroxybenzothiazole, 4,7-dihydroxy-1,2,3-benzothiadiazole, 2-methyl-5,6-dihydroxybenzothiazole, and 5,6-dihydroxy-1,2,3-benzothiadiazole, by means of a series of monoelectron acceptor metal complexes [iron(III) complexes of 1,10-phenanthroline and related compounds (Fe^IIIL₃), hexachloro-, aquapentachloro-, and diaquatetrachloro-iridate(IV)(Ir^IV), and octacyanomolybdate(V)] were investigated by means of a stopped-flow spectrophotometric technique. The rates were found to be first order in diol and in metal complex in 1M-HClO₄. A linear correlation between the logarithms of the specific rate constants and standard reduction potentials were found for both Fe^IIIL₃ and Ir^IV systems. The slopes of these linear free energy plots were similar to those observed for other benzenediols with the same oxidants, and are those predicted by the Marcus theory for outer-sphere electron transfer. This allows an estimate of the reduction potentials and the intrinsic parameters of the cation radicals which are formed in the rate-determining step.