Mechanism of reactions of cobalt(II) protoporphyrin IX dimethyl ester in protic and aprotic, co-ordinating solvents

Abstract

Cobalt(II) protoporphyrin IX dimethyl ester, [Co^IIP], when dissolved (5 × 10^–6 mol dm^–3) in a co-ordinating, aprotic, highly dielectric solvent(S), in air, rapidly gives [Co^IIP(S)] and [Co^IIP(S)(O₂)] at equilibrium. The Soret absorption maxima of these solutions gradually shift bathochromically, due probably to the formation of [(S) PCo^III(O₂)Co^IIIP(S)] peroxo-dimers. The Soret shifts (nm) and the reaction rate constant k_obs./s^–1(25 °C) are as follows: dimethyl sulphoxide (dmso), 403 to 427, 2.6 × 10^–6; dimethyl formamide (dmf), 402 to 424, 1.3 × 10^–5; hexamethylphosphoramide (hmpa), 402 to 426, 3.0 × 10^–5; acetonitrile, 399 to 424, 1.3 × 10^–5; pyridine (py), 402 to 426, 1.5 × 10^–4. On the other hand, the addition of py to a solution of [Co^IIP] in CH₃OH (in air) accelerates the formation of [Co^IIIP(py)₂]⁺ until k_obs.(25 °C) reaches 1.5 × 10^–3 s^–1, which occurs at ca. 1 mol dm^–3 py. The disappearance of [Co^IIP] then slows to a constant rate, which occurs at ca. 9 mol dm^–3 py. Analogous rate maxima appear with all alcohois tested (methanol, ethanol, n-propyl alcohol, ethylene glycol) and with formamide, upon addition of py (or piperidine). However, addition of py to the solutions of [Co^IIP] in dmso or hmpa causes continuous increase of the rate of disappearance of [Co^IIP], indicating that, in these solvents, there is no formation of a bis(ligand) Co^III P species.