Secondary-sphere hydrogen bonding promotes catalytic nitrate reduction at iron

Nitrate ( ${\mathrm{NO}}_3^{-}$ ) is a groundwater pollutant that is challenging to remediate. Here we report a bioinspired strategy to enhance nitrate binding and reduction through a network of secondary-sphere hydrogen-bonding interactions within a ligand scaffold. Using a series of redox-inactive Zn(II) complexes based on the tris(2-pyridylmethyl)amine ligand, we demonstrate that ligand variants bearing appended aniline hydrogen-bond donors exhibit up to seven orders of magnitude higher binding affinity for ${\mathrm{NO}}_3^{-}$ compared to analogues lacking such interactions. These hydrogen bonds induce substantial charge redistribution within ${\mathrm{NO}}_3^{-}$ , thereby activating it for reduction. Replacing Zn with a redox-active metal such as Fe results in nitrate deoxygenation. Stoichiometric experiments provide mechanistic insights into the role of dimeric intermediates and their conversion into more reactive monomeric species. Guided by these mechanistic insights, we achieve catalytic nitrate reduction under both thermal and photochemical conditions, producing NO and NH₃, respectively, with high turnover numbers.