Nitric oxide (NO) and nitrous oxide (N2O) are two gaseous substrates that play significant regulatory roles in the global nitrogen cycle. Conversion of the former to the latter premieres N-N bond formation, via reductive coupling of two NO molecules, mediated by two protons and two electrons. Various nitric oxide reductases (NORs) that allow such a conversion possess a bi-nuclear center as observed in non-heme flavodiiron NO reductase (FNOR), heme diiron NO reductase and the related heme-Cu oxidase (HCO) with mitigated efficacy. The mono-nuclear copper nitrite reductase (CuNiR), evolutionarily tuned for the reduction of NO2- to NO, reduces it further to N2O under high NO flux. At binuclear active sites, hyponitrite formation (that is often associated as the intermediate step in NO coupling) necessitates one e- oxidation at each metal center. Conversely, mono-metallic systems rely on an exogenous e- source along with one e- oxidation of the active site-transition metal for hyponitrite formation. Biomimetics of CuNiR portraying N-N bond formation at Cu, although paltry in literature, illustrates symmetric trans-hyponitrites formed via bimolecular coupling of two Cu-NO species. Using low-coordinate bidentate Cu(I) complexes, we account for reductive NO assembly at a single Cu site, isolating the copper bound cis-hyponitrite, {[CuII](κ2-O2N2)}- intermediate, stabilized by a Cu(I) Lewis acid coordinated to the nitrogen atom of the hyponitrite moiety. Addition of electrophiles to {[CuII](κ2-O2N2)}- led to facile N2O generation. Moreover, one e- oxidation of {[CuII](κ2-O2N2)}- result in hydrogen atom transfer (HAT) reactivity with substrates such as 9,10-dihydroanthracene.

Forming N-N bonds: Isolation of a copper cis-hyponitrite complex

ACS Spring Meeting 2021

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