Kinetic and mechanistic investigations of dioxygen reduction by a molecular Cu(ii) catalyst bearing a pentadentate amidate ligand

Abstract

A pentadentate Cu(II) complex, [Cu^II(dpaq)](ClO₄) (1), featuring a redox active ligand, H-dpaq (H-dpaq = 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), catalyses four-electron reduction of dioxygen by decamethylferrocene (Fc*) in the presence of trifluoroacetic acid (CF₃COOH) in acetone at 298 K. No catalytic oxygen reduction was observed in the presence of stronger Brønsted acids than CF₃COOH, such as perchloric acid (HClO₄) or trifluoromethanesulphonic acid (HOTf). In contrast, facile catalytic reduction of O₂ occurs by Fc* with 1 and HClO₄ or HOTf in dimethylformamide (DMF). The use of CF₃COOH as the proton source in DMF results in the suppression of O₂ reduction under otherwise identical reaction conditions. While the O₂ reduction reactions in DMF are linearly dependent on the pK_a of Brønsted acids, the acid dependence on catalytic O₂-reduction reactivity by 1 in acetone showed complete reversal. Cyclic voltammetry studies using p-chloranil as the probe substrates in the presence of acids in the solvents reveal that the strengths of the protonic acids increase significantly in acetone compared to that in DMF. The amidate-N in [Cu^II(dpaq)](ClO₄) (1) undergoes protonation in the presence of HClO₄ or HOTf in DMF to form [Cu^II(H-dpaq)]²⁺ (1-H⁺), but not in the presence of CF₃COOH. Enhanced acid strength of CF₃COOH in acetone, however, effectively protonates 1 and triggers O₂ reduction. Protonation of 1 with HClO₄ or HOTf in acetone results in the change of its coordination environment, and this protonated species does not trigger O₂ reduction. Detailed kinetic studies indicate that 1-H⁺ undergoes reduction by two-electrons and the reduced species binds O₂ to form a Cu(II)-superoxo intermediate. This is followed by a rate-determining proton-coupled electron-transfer (PCET) reduction to generate the Cu(II)-hydroperoxo intermediate. While catalytic O₂ reduction in acetone occurs predominantly via a 4e⁻/4H⁺ pathway, product selectivity (H₂O vs. H₂O₂) in DMF depends upon the concentration of the reductant (Fc*). While dioxygen reduction to H₂O₂ is favoured at low [Fc*], mechanistic studies suggest that O₂ reduction with high [Fc*] proceeds via a [2e⁻ + 2e⁻] mechanism, where the released H₂O₂ during catalysis is further reduced to water.