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

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 Bronsted acids than CF3COOH, such as perchloric acid (HClO4) or trifluoromethanesulphonic acid (HOTf). In contrast, facile catalytic reduction of O-2 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 O-2 reduction under otherwise identical reaction conditions. While the O-2 reduction reactions in DMF are linearly dependent on the pK(a) of Bronsted acids, the acid dependence on catalytic O-2-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 O-2 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 O-2 reduction. Detailed kinetic studies indicate that 1-H+ undergoes reduction by two-electrons and the reduced species binds O-2 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 O-2 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 O-2 reduction with high [Fc*] proceeds via a [2e(-) + 2e(-)] mechanism, where the released H2O2 during catalysis is further reduced to water.