Proton-Coupled Electron Transfer Enhances the Electrocatalytic Reduction of Nitrite to NO in a Bioinspired Copper Complex

摘要

The selective and efficient electrocatalytic reduction of nitrite to nitric oxide (NO) is of tremendous importance, both for the development of NO-release systems for biomedical applications and for the removal of nitrogen oxide pollutants from the environment. In nature, this transformation is mediated by (among others) enzymes known as the copper-containing nitrite reductases. The development of synthetic copper complexes that can reduce nitrite to NO has therefore attracted considerable interest. However, there are no studies describing the crucial role of proton-coupled electron transfer during nitrite reduction when such synthetic complexes are used. Herein, we describe the synthesis and characterization of two previously unreported Cu complexes (bold3/bold and bold4/bold) for the electrocatalytic reduction of nitrite to NO, in which the role of proton-relaying units in the secondary coordination sphere of the metal can be probed. Complex bold4/bold bears a pendant carboxylate group in close proximity to the copper center, while complex bold3/bold lacks such functionality. Our results suggest that complex bold4/bold is twice as effective an electrocatalyst for nitrite reduction than is complex bold3/bold and that complex bold4/bold is the best copper-based molecular electrocatalyst for this reaction yet discovered. The differences in reactivity between bold3/bold and bold4/bold are probed using a range of electrochemical, spectroscopic, and computational methods, which shed light on the possible catalytic mechanism of bold4/bold and implicate the proton-relaying ability of its pendant carboxylate group in the enhanced reactivity that this complex displays. These results highlight the critical role of proton-coupled electron transfer in the reduction of nitrite to NO and have important implications for the design of biomimetic catalysts for the selective interconversions of the nitrogen oxides.