Oxygen reduction catalysis by [Cu(cyclen)Cl]ClH₂O: The relationship of selectivity to proton in nonaqueous solution and aqueous solution

The reaction of 1,4,7,10-tetraazacyclododecane (cyclen) with CuCl2.2H(2)O results in a novel five-coordinated mononuclear copper complex, [Cu(cyclen)Cl]Cl.H2O, which has been characterized using physicochemical and spectroscopic methods. This copper complex can serve as a catalyst for oxygen reduction reactions (ORRs). The selectivity for 4e(-)/4H(+) reduction to H2O nu ersus 2e(-)/2H(+) reduction to H2O2 is determined by the relationship of the pKa value of the proton donor in nonaqueous solutions. The dissociation of Cl- from [Cu(cyclen)Cl]Cl.H2O creates an empty position on the copper centre, enabling to binding of dioxygen (O-2) to form Cu(II)-superoxo intermediate, which further reacts with a strong acid as a proton donor to form H2O and with a weak acid as a proton donor to form H2O2. Moreover, the faradaic efficiency for H2O2 production increases in an aqueous environment with the increase of pH.