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

Transition-metal-mediated reductive coupling of nitric oxide (NO_(g)) to nitrous oxide (N₂O_(g)) 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_(g) reductive coupling at a copper-ion center, [(tmpa)Cu^I(MeCN)]⁺ (1) {tmpa = tris(2-pyridylmethyl)amine}. At −110 °C in EtOH (<−90 °C in MeOH), exposing 1 to NO_(g) leads to a new binuclear hyponitrite intermediate [{(tmpa)Cu^II}₂(μ-N₂O₂^2-)]²⁺ (2), exhibiting temperature-dependent irreversible isomerization to the previously characterized κ²-O,O′-trans-[(tmpa)₂Cu₂^II(μ-N₂O₂^2-)]²⁺ (OO^Xray) complex. Complementary stopped-flow kinetic analysis of the reaction in MeOH reveals an initial mononitrosyl species [(tmpa)Cu(NO)]⁺ (1-(NO)) that binds a second NO molecule, forming a dinitrosyl species [(tmpa)Cu^II(NO)₂] (1-(NO)₂). The decay of 1-(NO)₂ requires an available starting complex 1 to form a dicopper-dinitrosyl species hypothesized to be [{(tmpa)Cu}₂(μ-NO)₂]²⁺ (D) bearing a diamond-core motif, en route to the formation of hyponitrite intermediate 2. In contrast, exposing 1 to NO_(g) in 2-MeTHF/THF (v/v 4:1) at <−80 °C leads to the newly observed transient metastable dinitrosyl species [(tmpa)Cu^II(NO)₂] (1-(NO)₂) prior to its disproportionation-mediated transformation to the nitrite product [(tmpa)Cu^II(NO₂)]⁺. Our study furnishes a near-complete profile of NO_(g) 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.