Molecular Copper Bipyridine Complex as a Catalyst for Electrochemical Nitrate Reduction

The electrochemical reduction of nitrate to ammonia presents a sustainable alternative to conventional nitrogen fixation methods, yet developing molecular model systems that can reveal fundamental mechanistic principles remains a key challenge. Here, we report a well-defined copper bipyridine chloride complex, [Cu(bpy)₂Cl]⁺, that catalyzes nitrate reduction in aqueous phosphate buffer (pH = 7), achieving ∼100% Faradaic efficiency for ammonia at -0.89 V vs RHE and a partial current density exceeding 30 mA cm^-2. Structure-activity relationships were investigated by introducing substituents at the 4-position of the pyridine units (R= H, ^tBu, OMe, CF₃). This study revealed that a sterically accessible and electron deficient copper center exhibits enhanced activity toward nitrate reduction. Detailed mechanistic insights were obtained via a combination of in situ Fourier transform infrared (FTIR), electrochemical mass spectrometry, and ultraviolet-visible (UV-vis) spectroscopy, revealing stepwise N-O bond cleavage and proton-coupled electron transfer through NO₂, NO, and hydroxylamine intermediates. To expand the catalytic scope beyond nitrate, [Cu(bpy)₂Cl]⁺ was also shown to selectively reduce organonitro substrates, achieving up to 90% Faradaic efficiency for aniline formation from nitrobenzene. This electrochemical and spectroscopic study highlights the crucial role of molecular design, advancing the understanding of nitrate electroreduction pathways and guiding future catalyst development.