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 complexesfornitrite (NO2 (-)) reduction. However, detailsof how protons and electrons arrive and how nitric oxide (NO) is releasedremain unknown. The influence of the coordination mode of nitriteon reactivity is also under debate. Kundu and co-workers have reportednitrite reduction by a copper(II) complex [J. Am. Chem. Soc. 2020,142, 1726-1730]. In their report, the copper(II) complex reducednitrite 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 bymechanistic studies. Herein, density functional theory calculationswere performed to determine a mechanism for reduction of nitrite bya copper(II) complex. As a result of geometry optimization of an initialcomplex, two possible structures were obtained: Cu-ONO andCu-NO2. Two possible reaction pathways initiatedfrom Cu-ONO or Cu-NO2 were then considered.The calculation results indicated that the Cu-ONO pathway isenergetically favorable. When changes in the electronic structurewere considered, both pathways were found to involve concerted proton-electrontransfer (CPET). In addition, an intrinsic reaction coordinate analysisrevealed that the two pathways were achieved by different types ofCPET. Furthermore, an intrinsic bond orbital analysis clearly indicatedthat, in the Cu-ONO pathway, the chemical events involved proceededconcertedly yet asynchronously. The reactionmechanism of nitrite reduction by a coppercomplex was theoretically determined using DFT calculations. Differentreaction pathways were elucidated depending on the coordination modeof nitrite to copper (O-coordination or N-coordination). The synchronicityof proton transfer and electron transfer was evaluated by intrinsicreaction coordinate and intrinsic bond orbital analyses, suggestingthat different CPET modes are involved in each pathway.

Automated summary

In this study, the reduction mechanism of nitrite by a copper(II) complex was investigated using density functional theory calculations. Two possible reaction pathways were proposed based on different coordination modes of nitrite to copper, and the Cu-ONO pathway was found to be energetically favorable. The involvement of different proton-coupled electron transfer modes in each pathway was revealed by intrinsic reaction coordinate and intrinsic bond orbital analyses.