Abstract
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 (3 and 4) 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 4 bears a pendant carboxylate group in close proximity to the copper center, whi...
Highlights
The reduction of nitrite (NO2−) to nitric oxide (NO) in the bloodstream is potentially a physiological source of NO,[1] which plays an important role in neurotransmission and vasodilation.[2−4] Nitrite reduction to NO is a key step in the natural nitrogen cycle, being part of the process of bacterial denitrification.[5]
We report the synthesis and characterization of two new copper complexes (3 and 4) which allow the role of proton-coupled electron transfer (PCET) in the electrocatalytic reduction of nitrite to NO with synthetic, small-molecule platforms to be addressed for the first time
Our results suggest that the incorporation of PCET-competent groups in the secondary coordination sphere of the metal center gives rise to a considerable enhancement in the rate of electrocatalytic nitrite reduction and give insights into the workings of the natural CuNIR enzymes and suggest new avenues for the development of biomimetic catalysts for applications such as the removal of NO2− from wastewater streams[73] and NOrelease systems for biomedical applications.[74]
Summary
The reduction of nitrite (NO2−) to nitric oxide (NO) in the bloodstream is potentially a physiological source of NO,[1] which plays an important role in neurotransmission and vasodilation.[2−4] Nitrite reduction to NO is a key step in the natural nitrogen cycle, being part of the process of bacterial denitrification.[5]. The much more cathodic position of the Cu(II)/Cu(I) redox wave in comparison to that seen in complex 3 is further evidence that the deprotonated carboxylic acid group remains coordinated to the copper center in solution (increasing the effective electron density at the metal and making reduction harder), as seen in the solid state (Figure 3) and as suggested by EPR, mass spectrometry, and CHN analysis (see the Experimental Section). Computational optimization of [4H-NO2] leads spontaneously to a structure where the carboxylate proton has migrated to the adjacent nitro ligand to give a coordinated nitrous acid group, [4-HNO2] (Scheme 3) This underscores the key role played by the proton-relaying group in nitrite reduction, as the in silico process follows a path analogous to that observed experimentally. In Scheme 3, there are further energy gains leading to a Cu(I) center with an N-coordinated nitrous acid and an electron transfer from the Cu(I) to the nitrous acid, giving the reaction products nitric oxide and water and regenerating the precatalyst
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