Reductive Coupling of Nitric Oxide by Cu(I)

Nitric oxide (NO(g)) plays a ubiquitous role as a signaling molecule and strengthens the mammalian host defense response by effecting oxidative destruction of harmful pathogen biomolecules (i.e., by oxidation and/or nitration). Toxic overproduction of NO(g) and reactive NOx products such as peroxynitrite (OONO) and nitrogen dioxide (NO2) are mitigated by the oxidation of NO(g) to NO3 by iron-containing metalloenzymes, or by the reductive degradation of toxic NO(g) through Fe/Cu-mediated reductive coupling of two NO(g) to benign N2O(g). In enzymes with binuclear active sites, reductive coupling of NO(g) is thought to involve two key steps: i) N-N bond formation between two metal-nitrosylated reduced units forming a hyponitrite anion (N2O22-HN) and ii) N-O bond scission from the HN intermediate to release N2O(g). Although enzymatic studies and model chemistries have provided useful mechanistic insights, there remains a great deal of ambiguity around the order and timing of transfer of electrons from the metal centers to NO(g), formation and coordination modes of the M-NO unit (end-on, κ1-O, κ1-N; side-on, κ2-O,N) and M-HN (cis, trans, monoanionic, dianionic, neutral) moieties and the timing and pathway of proton-shuttling events.

In a recent study, researchers from an international team (USA, Germany and Spain) elucidated details of solvent-dependent reaction pathways for the reductive-coupling (and/or disproportionation) chemistry of NO(g) reactivity with [(tmpa)CuI(MeCN)](1). Through a combination of spectroscopy (UV-vis, rRaman), stopped-flow kinetics and computational analysis (at S12g/TZ2P with COSMO and ZORA included self-consistently), the team was able to characterize new intermediates, going from a cupric nitrosyl species 1-(NO) towards a dicupric-hyponitrite species 2 with novel HN coordination geometry; the latter subsequently isomerizes irreversibly in MeOH to a trans-μ-O,O’-hyponitrite dicopper(II) complex (OOxray), whose X-ray structure was obtained in 2017.

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M. Bhadra, T. Albert, A. Franke, V. Josef, I. Ivanović-Burmazović, M. Swart, P. Moënne-Loccoz, K. Karlin, Reductive Coupling of Nitric Oxide by Cu(I): Stepwise Formation of Mono and Dinitrosyl Species en route to a Cupric Hyponitrite Intermediate, J. Am. Chem. Soc. 2023, 145, 2230-2242

Key concepts