Proton-Assisted Mechanism of NO Reduction on a Dinuclear Ruthenium Complex

Abstract

Density-functional-theory (DFT) calculations are performed for the proposal of a plausible mechanism on the reduction of NO to N2O by a dinuclear ruthenium complex, reported by Arikawa and co-workers [J. Am. Chem. Soc. 2007, 129, 14160]. On the basis of the experimental fact that the reduction proceeds under strongly acidic conditions, the role of protons in the mechanistic pathways is investigated with model complexes, where one or two NO ligands are protonated. The reaction mechanism of the NO reduction is partitioned into three steps: reorientation of N2O2 (cis-NO dimer), O-N bond cleavage, and N2O elimination. A key finding is that the protonation of the NO ligand(s) significantly reduces the activation barrier in the rate-determining reorientation step. The activation energy of 43.1 kcal/mol calculated for the proton-free model is reduced to 30.2 and 17.6 kcal/mol for the mono- and diprotonated models, respectively. The protonation induces the electron transfer from the Ru(II)Ru(II) core to the O═N-N═O moiety to give a Ru(III)Ru(III) core and a hyponitrite (O-N═N-O)(2-) species. The formation of the hyponitrite species provides an alternative pathway for the N2O2 reorientation, resulting in the lower activation energies in the presence of proton(s). The protonation also has a marginal effect on the O-N bond cleavage and the N2O elimination steps. Our calculations reveal a remarkable role of protons in the NO reduction via N2O formation and provide new insights into the mechanism of NO reduction catalyzed by metalloenzymes such as nitric oxide reductase (NOR) that contains a diiron active site.

Hydrogen Bond-Induced Nitric Oxide Dissociation on Cu(110)

“…Nitric oxide (NO) emission from the exhaustive gas of combustion process has caused negative impacts on the environment, e.g, acid rain, photochemical smog, and ozone depletion. Therefore, NO reduction to harmless substances is an important task to mitigate the environmental pollution. − The catalysts of expensive precious metals − such as Pt, Rh, and Pd are often employed to remove NO as well as CO and unburned hydrocarbon gases simultaneously from the exhaust gas. However, the practice requires that the catalysts to be abundant, of low cost, and have high catalytic activity.…”

A trans-Hyponitrite Intermediate in the Reductive Coupling and Deoxygenation of Nitric Oxide by a Tricopper–Lewis Acid Complex

“…The mechanism of the reduction of NO to N 2 O has been probed in synthetic model systems based on the metals involved in biological NO reduction (Fe, Cu). ,, Complexes of other transition metals have also been reported to display NO reduction activity. ,, Despite these many investigations, only a handful of metal complexes that contain the hyponitrite motif have been reported, in which hyponitrite ligands adopt a variety of binding modes to metal centers (Figure ). Formation of hyponitrite from NO has been reported for Pt, Co, Ru, Ni, and Y compounds. − In some cases, treatment of hyponitrite complexes with H + sources results in the formation of N 2 O. ,− ,, A single hyponitrite complex featuring biologically relevant metals (based on the precedent of CcO and NOR) has been reported in which a trans -hyponitrite ligand bridges two Fe-porphyrin units. , This complex also generates N 2 O upon reaction with HCl. However, the hyponitrite ligand is obtained from hyponitrous acid rather than from reduction of NO.…”

Conversion of Nitric Oxide into Nitrous Oxide as Triggered by the Polarization of Coordinated NO by Hydrogen Bonding

“…The use of 2‐ 15 NO shifted this peak from 1622 to 1577 cm −1 (Δ ν =45 cm −1 ; see Figure S13). This isotope‐sensitive peak at 1622 cm −1 is close to the value of ν NO ≈1605 cm −1 observed for bridging neutral [N 2 O 2 ] coordination in diruthenium complexes that produce N 2 O by protonation‐induced electron transfer from the diruthenium core to the [N 2 O 2 ] unit ,. Furthermore, the concomitant decrease in the bands at 1542 and 1513 cm −1 in [D 8 ]THF/[D 4 ]MeOH (Figure a), which signifies the consumption of 2 and 2⋅⋅⋅CD 3 OD , and the increase in the peak at 2224 cm −1 in THF/MeOH (2155 cm −1 when 2‐ 15 NO was used; see Figure S13) from the accumulation of N 2 O occur concurrently with the intensity change at 1622 cm −1 .…”