Mechanistic Study of Reduction of Nitrite to NO by the Copper(II) Complex: Different Concerted Proton–Electron Transfer Reactivity between Nitrite and Nitro Complexes

The literature contains numerous reports of copper complexes for nitrite (NO2–) reduction. However, details of how protons and electrons arrive and how nitric oxide (NO) is released remain unknown. The influence of the coordination mode of nitrite on reactivity is also under debate. Kundu and co-workers have reported nitrite reduction by a copper(II) complex [J. Am. Chem. Soc. 2020, 142, 1726–1730]. In their report, the copper(II) complex reduced nitrite using a phenol derivative as a reductant, resulting in NO, a hydroxyl copper(II) complex, and the corresponding biphenol. Also, the involvement of proton-coupled electron transfer was proposed by mechanistic studies. Herein, density functional theory calculations were performed to determine a mechanism for reduction of nitrite by a copper(II) complex. As a result of geometry optimization of an initial complex, two possible structures were obtained: Cu–ONO and Cu–NO2. Two possible reaction pathways initiated from Cu–ONO or Cu–NO2 were then considered. The calculation results indicated that the Cu–ONO pathway is energetically favorable. When changes in the electronic structure were considered, both pathways were found to involve concerted proton–electron transfer (CPET). In addition, an intrinsic reaction coordinate analysis revealed that the two pathways were achieved by different types of CPET. Furthermore, an intrinsic bond orbital analysis clearly indicated that, in the Cu–ONO pathway, the chemical events involved proceeded concertedly yet asynchronously