METHYLTRIOXORHENIUM-CATALYZED OXIDATION OF A (THIOLATO)COBALT(III) COMPLEX BY HYDROGEN-PEROXIDE

The oxidation of (en)2Co(SCH2CH2NH2)2+ by H2O2 is catalyzed by CH3ReO3. 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)2Co(S(O)CH2CH2NH2)2+, which is in turn more slowly oxidized to the sulfinato complex, (en)2Co(S(O)2CH2CH2NH2)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 H2O2/CH3ReO3 adduct (A). The reversible formation of a 2:1 H2O2/CH3ReO3 adduct (B) 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 (HClO4), for the formation of the 1:1 H2O2/CH3ReO3 adduct A (k1 = 77 +/- 1 L mol-1 s-1) and its dissociation (k-1 = 9.0 +/- 0.5 s-1) and for the oxidation by A of the thiolato complex, (4.2 +/- 0.3) X 10(5) L mol-1 s-1, and of the sulfenato complex, 265 +/- 7 L mol-1 s-1.