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 (FeIIIL3), hexachloro-, aquapentachloro-, and diaquatetrachloro-iridate(IV)(IrIV), 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 FeIIIL3 and IrIV 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.