A terpyridine-based copper complex for electrochemical reduction of nitrite to nitric oxide

In biological systems, nitrite reductase enzymes (NIRs) are responsible for reduction of nitrite (NO₂^-) to nitric oxide (NO). These NIRs have mostly Cu- or Fe-containing active sites, surrounded by amine-containing ligands. Therefore, mononuclear Cu complexes with N-donor ligands are highly relevant in the development of NIR model systems and in the mechanistic investigation of the nitrite reduction reaction. Herein, we report on a terpyridine-based Cu^II complex with square planar geometry for H⁺-assisted electrochemical reduction of NO₂^-. Through electrochemical measurements, spectroscopic characterization and isotope-labelling experiments we propose a mechanistic reaction pathway involving an unstable HNO₂ state. The Cu^I intermediate, formed electrochemically, was isolated and its molecular structure was deduced, showing linkage isomerism of the nitrite ligand. Moreover, qualitative and quantitative product analysis by GC-MS shows N₂O formed as a side product along with the main product NO. Furthermore, by obtaining single crystals and conducting structural analysis we were able to determine the structural arrangement and redox state of the complex after electrochemical treatment.