In bacterial denitrification, nitrite reductase (NIR) enzymes are responsible for the reduction of NO2- to form NO and H2O. Since high concentrations of NO are cytotoxic, nitric oxide reductase (NOR) enzymes convert NO to N2O which typically take place at diiron centered NOR enzymes. Interestingly, the T2Cu site in CuNIR is also capable of reducing NO under high concentrations of NO. This behavior has been previously modeled with tris(pyrazolyl)borate copper complexes TpCu(NO) which possess a {Cu(NO)}11 configuration and are involved in the conversion of NO to N2O. Based on recent work from our lab with β-diketiminato {Ni(NO)}11 complexes, some synthetic {M(NO)}11 species may possess novel η2 “side-on” M-NO bonding as well as react instantaneously with NO to form cis-hyponitrite complexes [M](κ2-O2N2). In this work we more generally explore {M(NO)}10 and {M(NO)}11 (M = Ni and Cu) species to identify factors that lead to cis-hyponitrite intermediates [M](κ2-O2N2) as well as their conversion to nitrites [M](κ2-O2N) and N2O upon reaction with excess NO. We employ tripodal tris(pyrazolyl)borate (Tp) and tris(pyrazolyl)methane (Tpm) ligands in a combined synthetic and computational study that considers three potential binding modes of nitric oxide to a metal center: linear, bent or a rare side-on form seen at the T2Cu site of reduced CuNIR. We hypothesize that access to this side-on bound orientation assists formation of monometallic cis-hyponitrite species [M](κ2-O2N2). We also examine reduction of a metal-bound NO ligand to [M](HNO) species via proton-coupled electron transfer (PCET) and subsequent reaction with NO to examine NOR-like pathways for N2O formation besides the disproportionation of NO observed with TpCu(NO) complexes.

Examination of NO reduction at monometallic sites

254th National Meeting (2017)

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