Catalyzed autoxidation of hydrogensulfite by cobalt(II) (2,3,9,10-tetramethyl-1,4,8,11-tetraaza-cyclotetradeca-1,3,8,10-tetraene) (H₂O)₂²⁺

The catalytic autoxidation of hydrogensulfite (hydrogen-trioxosulfate(1-)) in the presence of Co(tim)(H₂O)₂²⁺ (tim = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) was studied. Dioxygen reacts with excess sulfite in the presence of Co(tim)(H₂O)₂²⁺ in a complex process (a minimum of five kinetic steps can be identified) to produce Co(tim)(SO₃)_x^(3-2x)+ (x = 1 or 2) (λ_max at 350 nm) and sulfate. Seventy turnovers over 4 h were attained in a system where dioxygen and sulfite were supplied constantly. The Co(III) product formed reacts only slowly with dioxygen to produce sulfate. A mechanistic analysis of the results implies the involvement of a Co(tim)-dioxygen adduct, stabilized by an axial sulfite ligand.