Reductive Coupling of Nitric Oxide by Cu(I): Stepwise Formation of Mono- and Dinitrosyl Species to a Cupric Hyponitrite Intermediate

Transition-metal-mediated reductive coupling of nitric oxide (NO) to nitrous oxide (NO) has significance across the fields of industrial chemistry, biochemistry, medicine, and environmental health. Herein, we elucidate a density functional theory (DFT)-supplemented mechanism of NO reductive coupling at a copper-ion center, [(tmpa)Cu(MeCN)] () {tmpa = tris(2-pyridylmethyl)amine}. At -110 °C in EtOH (<-90 °C in MeOH), exposing to NO leads to a new binuclear hyponitrite intermediate [{(tmpa)Cu}(μ-NO)] (), exhibiting temperature-dependent irreversible isomerization to the previously characterized κ-O,O'--[(tmpa)Cu(μ-NO)] () complex. Complementary stopped-flow kinetic analysis of the reaction in MeOH reveals an initial mononitrosyl species [(tmpa)Cu(NO)] () that binds a second NO molecule, forming a dinitrosyl species [(tmpa)Cu(NO)] (). The decay of requires an available starting complex to form a dicopper-dinitrosyl species hypothesized to be [{(tmpa)Cu}(μ-NO)] () bearing a diamond-core motif, to the formation of hyponitrite intermediate . In contrast, exposing to NO in 2-MeTHF/THF (v/v 4:1) at <-80 °C leads to the newly observed transient metastable dinitrosyl species [(tmpa)Cu(NO)] () prior to its disproportionation-mediated transformation to the nitrite product [(tmpa)Cu(NO)]. Our study furnishes a near-complete profile of NO activation at a reduced Cu site with tripodal tetradentate ligation in two distinctly different solvents, aided by detailed spectroscopic characterization of metastable intermediates, including resonance Raman characterization of the new dinitrosyl and hyponitrite species detected.