Catalysis of the autoxidation of aquated sulfur dioxide by homogeneous metal-phthalocyanine complexes

Abstract

The autoxidation of sulfur dioxide to sulfate in aqueous solution has been examined over the pH range 4-13 in the presence of water-soluble transition metal-4,4’,4”,4”‘-tetrasulfophthaloc anine com lexes, MTSP (where M = Fe^(II), Mn^(II),Co^(II), Ni^(II), Cu^(II), and V^(IV). Experimental rate data have been analyzed in terms of a Michaelis-Menten kinetic expression derived from a bisubstrate kinetic model. The rate law and spectroscopic measurements indicate that the reaction proceeds via the formation of an activated complex in which O_2 and two molecules of SO_3^(2-) are reversibly bonded to the catalytic center. Relative reactivity is determined by the ability of the central metal atom to reversibly bind molecular oxygen. Analysis of the spectroscopic data suggests that photaassisted catalysis arises from the absorption of light by a ternary activated complex.