Overuse of nitrogenous fertilizers has led to massive eutrophication caused by unassimilated nitrate (NO3-). Nitrate reductase enzymes feature molybdenum centers that reduce nitrate to nitrite (NO2-). While different Fe or Cu based enzymes reduce nitrite to nitric oxide (NO) in denitrification or directly to ammonium (NH4+), nitrate reduction is particularly challenging due to the weak coordinating ability of this polyoxoanion to transition metals. Particularly attractive would be nitrate reduction pathways that take advantage of the large biological pool of thiols (RSH) and hydrogen sulfide (H2S) as reductants, perhaps revealing unexpected roles for nitrate in NO signaling. We report a β-diketiminato copper(II) nitrate complex [CuII](κ2-O2NO) that undergoes reduction to NO upon reaction with 4 equiv. thiols RSH. Reduction of nitrate (Nox = +5) to NO (Nox = +2) along with copper(II) to copper(I) occurs in an overall 4-electron process that produces 2 equiv. RS-SR. Detailed mechanistic studies reveal that [CuII](κ2-O2NO) undergoes 2-electron reduction to [CuII](κ2-O2N) with RSH to generate the corresponding sulfenic acid RSOH that rapidly reacts with excess thiol RSH to form RS-SR and H2O. Subsequent reaction of the copper(II) nitrite [CuII](κ2-O2N) with RSH results in transnitrosation to give the S-nitrosothiol RSNO along with [CuII]-OH that rapidly reacts with RSH to give [CuII]-SR that reacts with RSNO to give RS-SR, NO, and [CuI]. Computational studies underscore the favorability of O-atom transfer to thiols RSH due to the thermodynamic favorability of the RSOH sulfenic acid tautomer. Importantly, H2S also generates NO from [CuII](κ2-O2NO) , revealing a new role for nitrate in H2S/NO crosstalk.

Reduction of nitrate to nitric oxide at copper(II) by thiols and H2S

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