Electron transfer mechanism of catalytic superoxide dismutation via Cu(ii/i) complexes: evidence of cupric–superoxo/–hydroperoxo species

Abstract

To understand the electron transfer mechanisms (outer versus inner sphere) of catalytic superoxide dismutation via a Cu(II/I) redox couple such as occur in the enzyme copper–zinc superoxide dismutase, the Cu(II/I) complexes [(L1)₂Cu](ClO₄)₂·CH₃CN, (1·CH₃CN) and [(L1)₂Cu](ClO₄), (2) supported by a bis-N₂S_thioether ligand, 2-pyridyl-N-(2′-methylthiophenyl)methyleneimine (L1) have been synthesized and structurally characterised. Both 1 and 2 display the same cyclic voltammogram (CV) featuring a quasireversible response at E_1/2 = +0.33 V vs. SCE that falls in the SOD potential window of −0.04 V to +0.99 V. These complexes catalytically dismutate superoxide radicals at 298 K in aqueous medium (the IC₅₀ for 1 is 2.15 μM). Electronic absorption spectra (233 K and 298 K), FTIR, ESI mass spectra, CV (233 K and 298 K) and DFT calculations collectively indicate formation of [(L1)₂Cu(O₂˙⁻)]⁺, [(L1)₂Cu(O₂²⁻)] and [(L1)₂Cu(OOH⁻)]⁺ species and help to elucidate the electron transfer mechanism for the SOD function of 1 and 2. Once O₂˙⁻ binds to Cu^II (evident at 233 K), the first step of the catalytic cycle (Cu^II + O₂˙⁻ → Cu^I + O₂) does not follow but the second step (Cu^I + O₂˙⁻ + 2H⁺ → H₂O₂ + Cu^II) does follow. Therefore, the catalytic disproportionation of superoxide radicals via1 and 2 at 298 K indicates that the first and second steps of the catalytic cycle proceed through outer and inner sphere electron transfer mechanisms, respectively. Feasibility of the first step to occur in pure aprotic solvent (where 18-crown-6-ether is used to solubilise KO₂) was tested and also supports the same notion of the electron transfer mechanisms as stated above.