Functional metallomacrocycles and their polymers. 25. Kinetics and mechanism of the biomimetic oxidation of thiol by oxygen catalyzed by homogeneous (polycarboxyphthalocyaninato)metals

The oxidation of 2-mercaptoethanol (RSH) catalyzed by (polycarboxyphthalocyaninato)metal acid (M-papc, M = Fe(III), Co(II), Ni(II), Cu(II), pa = -(COOH)₂, -(COOH)₄, -(COOH)₈) in the presence of molecular oxygen has been studied in aqueous homogeneous solution at pH 7.0 and at 25°C. The catalytic effects of M-papc have been determined the oxygen consumption in the reaction mixture with a Warburg respirometer. The central metal ions and the number of carboxylate ions on M-papc affected the catalytic activities for the aerobic oxidation of RSH. (Octacarboxyphthalocyaninato)iron(III) and -cobalt(II) are remarkably effective catalysts. (Dicarboxyphthalocyaninato)metals and Ni(II)- as well as Cu(II)-phthalocyanine derivatives are less effective catalysts. The kinetics of the aerobic oxidation catalyzed by (octacarboxyphthalocyaninato)iron(III) and -cobalt(II) have been characterized in terms of a bisubstrate Michaelis-Menten rate law (k₅ = 600-800 min^-1, at pH 7.0, 25°C). The rate law and spectral experiments indicate that the catalyzed reactions proceed via the formation of a ternary activated complex where RS^- and O₂ reversibly coordinate with the central metal on the phthalocyanine rings. The activation parameters were also determined for this reaction at pH 7.0 and at 25°C. The catalytic activities followed the order of Co(II)-oapc > Fe(III)-oapc > Co(II)-tapc = Fe(III)-tapc. Differences in catalytic ability have been explained in terms of the aggregation and spin state of metallophthalocyanine derivatives in aqueous solution at pH 7.0. The (tetracarboxyphthalocyaninato)cobalt(II) covalently bound to poly(2-vinylpyridine-co-styrene), despite of being a heterogeneous system, is a remarkably effective catalyst (k₅ = 2210 min^-1) because of polymer effects.