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

Abstract

Transition-metal-mediated reductive coupling of nitric oxide (NO (g) ) to nitrous oxide (N 2 O (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(2pyridylmethyl)amine}. At −110 °C in EtOH (<−90 °C in MeOH), exposing)] 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 (NO 2 )] + . 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.

Elucidation of the Electrocatalytic Nitrite Reduction Mechanism by Bio-Inspired Copper Complexes

“…The slope of the double-logarithmic plot is 0.71 for both high and low nitrite concentrations, which is indicative of a side reaction that only affects the catalyst concentration (see Figure S4). Previously, nonelectrochemical studies on Cu I (tmpa) have shown that Cu(I) dimers can form in solution both in the absence , and presence of NO . Hence, we assume that similar Cu(I) dimeric species form upon reduction of the catalyst under our electrochemical conditions, resulting in an effective lower catalyst concentration.…”

“…For determination of the reaction order, we rationalized that Cu(tmpa) selectively catalyzes the one-electron reduction of nitrite. It is known that Cu I (tmpa) is also able to produce N 2 O during the reduction of nitrite, , due to the disproportionation of the generated NO to N 2 O in protic solvents. , However, this side reaction does not lead to the consumption of extra electrons and is suppressed in the presence of high concentrations of NO 2 – and low concentrations of NO, as binding of NO to Cu(tmpa) is only weak. , …”

“…Interestingly, repeating the experiments with a lower nitrite concentration range (0.2–100 mM) results in a linear trend (Figure S5a), but the slope of the double-logarithmic plot is 0.77 instead of 1.0 (see Figure S3b). We attribute this change to the disproportionation of NO, which will become a significant competitive reaction for low nitrite concentrations. ,, The disproportionation of NO is a redox-neutral process; however, it will indirectly affect the measured i cat . The i cat is inhibited by the formed NO product, as NO will compete with nitrite for binding to copper.…”

“…It is known that Cu I (tmpa) is also able to produce N 2 O during the reduction of nitrite, 11,52 due to the disproportionation of the generated NO to N 2 O in protic solvents. 56,57 However, this side reaction does not lead to the consumption of extra electrons and is suppressed in the presence of high concentrations of NO 2 − and low concentrations of NO, as binding of NO to Cu(tmpa) is only weak. 11,46 In measurements in which the concentration of NaNO 2 is stepwise increased from 100 mM to 1000 mM, the catalytic current increases with nitrite concentration (Figure 3a).…”

“…The calculated structure of [Cu­(tmpa)­NO 2 ] with η 2 -O,O-bonded nitrite has one elongated Cu–O bond of 3.0 Å, indicative that bidentate binding is not favored (Figure S19). The binding mode of nitrite to Cu I (tmpa) has not been reported before; nevertheless, our results are consistent with other N-bound mononuclear Cu­(I) nitrito species in the literature. ,− ,− Besides, only N-bound nitrosyl copper complexes have been reported thus far outside an enzymatic environment, ,,, suggesting that dehydration takes place from the N-bound nitrite.…”