Methyltrioxorhenium-catalyzed oxidation of a (thiolato)cobalt(III) complex by hydrogen peroxide

Abstract

The oxidation of (en)[sub 2]Co(SCH[sub 2]CH[sub 2]NH[sub 2])[sup 2+] by H[sub 2]O[sub 2] is catalyzed by CH[sub 3]ReO[sub 3]. Studies of the kinetics and mechanism were carried out in aqueous solutions of dilute perchloric acid. The (thiolato)cobalt(III) complex is oxidized first to a sulfenato complex, (en)[sub 2]Co(S(O)CH[sub 2]CH[sub 2]NH[sub 2])[sup 2+], which is in turn more slowly oxidized to the sulfinato complex, (en)[sub 2]Co(S(O)[sub 2]CH[sub 2]CH[sub 2]NH[sub 2])[sup 2+]. The two steps are well resolved in time, the second being some 1500 times slower than the first. Both steps fit the same kinetic pattern, which is consistent with a Michaelis-Menten scheme in which there are two substrates. This scheme involves the reversible formation of a 1:1 H[sub 2]O[sub 2]/CH[sub 3]ReO[sub 3] adduct. The reversible formation of a 2:1 H[sub 2]O[sub 2]/CH[sub 3]ReO[sub 3] adduct also occurs, but it appears to be a dead-end process, in that B, if involved at all, is much less reactive than A. Rate constants were determined at 25[degrees]C, [mu] = 0.10 M (HClO[sub 4]), for the formation of the 1:1 H[sub 2]O[sub 2]/CH[sub 3]ReO[sub 3] adduct A (k[sub 1] = 77 [+-] 1 L mol[sup [minus]1] s[sup [minus]1]) and its dissociation (k[sub [minus]1] = 9.0more » [+-] 0.5 s[sup [minus]1]) and for the oxidation by A of the thiolato complex, (4.2 [+-] 0.3) [times] 10[sup 5] L mol[sup [minus]1] s[sup [minus]1], and of the sulfenato complex, 265 [+-] 7 L mol[sup [minus]1] s[sup [minus]1].« less