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

Tatsuya Suzuki, Hiromasa Tanaka, Yoshihito Shiota, P. K. Sajith, Yasuhiro Arikawa, Kazunari Yoshizawa

Research output: Contribution to journalArticle

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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 - Ni = 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.

Original languageEnglish
Pages (from-to)7181-7191
Number of pages11
JournalInorganic Chemistry
Volume54
Issue number15
DOIs
Publication statusPublished - Aug 3 2015

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Ruthenium
ruthenium
Protons
retraining
Protonation
protons
cleavage
elimination
activation energy
Activation energy
ligands
Ligands
nitric oxide
proposals
electron transfer
Dimers
dimers
activation
Density functional theory
density functional theory

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

Cite this

Proton-Assisted Mechanism of NO Reduction on a Dinuclear Ruthenium Complex. / Suzuki, Tatsuya; Tanaka, Hiromasa; Shiota, Yoshihito; Sajith, P. K.; Arikawa, Yasuhiro; Yoshizawa, Kazunari.

In: Inorganic Chemistry, Vol. 54, No. 15, 03.08.2015, p. 7181-7191.

Research output: Contribution to journalArticle

Suzuki, Tatsuya ; Tanaka, Hiromasa ; Shiota, Yoshihito ; Sajith, P. K. ; Arikawa, Yasuhiro ; Yoshizawa, Kazunari. / Proton-Assisted Mechanism of NO Reduction on a Dinuclear Ruthenium Complex. In: Inorganic Chemistry. 2015 ; Vol. 54, No. 15. pp. 7181-7191.
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AU - Arikawa, Yasuhiro

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N2 - 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 - Ni = 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.

AB - 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 - Ni = 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.

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