Hydrogen-bonding and steric effects on NO2⁻ reduction by molecular Cu(II) complexes

Understanding the mechanistic aspects of NO₂^- reduction at the molecular level is fundamental due to its biological relevance. This work examines how hydrogen-bonding interactions and steric effects govern the spontaneous chemical reduction of NO₂^- to NO by phenanthroline-based Cu(II) complexes. Two complexes, (1) and (2), featuring an -NH group capable of forming hydrogen bonding, were synthesized. Additionally, the complex (2) bears a methyl substituent that exerts a steric effect. Control complexes employing ligands using alkyl substituents, lacking H-bonding capability, were also synthesized for comparison purposes. All complexes were characterized by X-ray diffraction, mass spectrometry and EPR. NO₂^- reduction was monitored by UV-Vis, and NO release was confirmed using Co(TPP) by UV-Vis and NMR. Variable-temperature UV-Vis studies provided kinetic and thermodynamic insight into the reaction pathway. Results showed that hydrogen bonding promotes NO₂^- binding, while the steric effect drives its spontaneous reduction to NO.