Photochemical generation of alkyl radicals and their reactions with methyl viologen radical cation and with transition-metal complexes in aqueous solution

Abstract

The irradiation of aqueous solutions of RCo((14)aneN{sub 4})(H{sub 2}O){sup 2+} (R = CH{sub 3}, C{sub 2}H{sub 5}, 1-C{sub 3}H{sub 7}, CH{sub 2}Cl, CH{sub 2}Br, CH{sub 2}OCH{sub 3}) with visible light, {lambda} {le} 400 nm, results in the homolysis of the cobalt-carbon bond. These complexes are quite unreactive toward a variety of dealkylating and redox reagents and thus represent a convenient source of small unstabilized carbon-centered radicals in the presence of potential substrates. The radicals react rapidly with the methyl viologen radical cation, k{sub MB} = (1-2) {times} 10{sup 9} M{sup {minus}1} s{sup {minus}1}. The kinetics of the formation of RCr{sup 2+} and RCo((14)aneN{sub 4})(H{sub 2}O){sup 2+} in the reactions of radicals with Cr{sup 2+} and Co((14)aneN{sub 4})(H{sub 2}O){sub 2}{sup 2+}, respectively, were studied by using MV{sup +} as a probe. The respective rate constants have values k{sub Cr} = (1.9-2.4) {times} 10{sup 8} M{sup {minus}1} s{sup {minus}1} and k{sub Co} = (1-2) {times} 10{sup 7} M{sup {minus}1} s{sup {minus}1} for all the radicals examined. 36 refs., 2 figs., 1 tab.