Catalysis of mononuclear aquaruthenium complexes in oxygen evolution from water: A new radical coupling path using hydroxocerium(IV) species

Masaki Yoshida, Shigeyuki Masaoka, Jiro Abe, Ken Sakai

Research output: Contribution to journalArticle

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Abstract

The mechanism of O2 evolution from water catalyzed by a series of mononuclear aquaruthenium complexes, [Ru(terpy)(bpy)(OH2)] 2+, [Ru(tmtacn)(R2bpy)(OH2)]2+ (R=H, Me, and OMe; R2bpy=4,4'-disubstituted-2,2'-bipyridines), and [Ru(tpzm)(R2bpy)(OH2)]2+ (R=H, Me, and OMe), is investigated, where terpy=2,2':6',2″-terpyridine, bpy=2,2'-bipyridine, tmtacn=1,4,7-trimethyl-1,4,7-triazacyclononane, and tpzm=tris(1-pyrazolyl) methane. The kinetics of O2 evolution is investigated as a function of either the catalyst concentration or the oxidant concentration by employing Ce(NH4)2(NO3)6 as an oxidant; these catalysts can be classified into two groups that have different rate laws for O2 evolution. In one class, the rate of O2 evolution is linear to both the catalyst and Ce4+ concentrations, as briefly reported for [Ru(terpy)(bpy)(OH2)]2+ (S. Masaoka, K. Sakai, Chem. Lett. 2009, 38, 182). For the other class, [Ru(tmtacn)(R 2bpy)(OH2)]2+, the rate of O2 evolution is quadratic to the catalyst concentration and independent of the Ce4+ concentration. Moreover, the singlet biradical character of the hydroxocerium(IV) ion was realized by experimental and DFT investigations. These results indicate that the radical coupling between the oxygen atoms of a RuV=O species and a hydroxocerium(IV) ion is the key step for the catalysis of [Ru(terpy)(bpy)(OH2)]2+ and [Ru(tpzm)(R 2bpy)(OH2)]2+, while the well-known oxo-oxo radical coupling among two RuV=O species proceeds in the catalysis of [Ru(tmtacn)(R2bpy)(OH2)]2+. This is the first report demonstrating that the radical character provided by the hydroxocerium(IV) ion plays a crucial role in the catalysis of such ruthenium complexes in the evolution of O2 from water. Water oxidation: The reaction mechanism of O2 evolution catalyzed by mononuclear aquaruthenium complexes can be classified into two groups that exhibit different paths of O-O bond formation. The radical character provided by the hydroxocerium(IV) ion plays a crucial role in the catalysis by such ruthenium complexes in the evolution of O2 from water.

Original languageEnglish
Pages (from-to)2369-2378
Number of pages10
JournalChemistry - An Asian Journal
Volume5
Issue number11
DOIs
Publication statusPublished - Nov 2 2010

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Catalysis
Ions
Oxygen
2,2'-Dipyridyl
Catalysts
Ruthenium
Water
Oxidants
Discrete Fourier transforms
Atoms
Oxidation
Kinetics
bis(bipyridyl)ruthenium(II)

All Science Journal Classification (ASJC) codes

  • Biochemistry
  • Organic Chemistry

Cite this

Catalysis of mononuclear aquaruthenium complexes in oxygen evolution from water : A new radical coupling path using hydroxocerium(IV) species. / Yoshida, Masaki; Masaoka, Shigeyuki; Abe, Jiro; Sakai, Ken.

In: Chemistry - An Asian Journal, Vol. 5, No. 11, 02.11.2010, p. 2369-2378.

Research output: Contribution to journalArticle

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N2 - The mechanism of O2 evolution from water catalyzed by a series of mononuclear aquaruthenium complexes, [Ru(terpy)(bpy)(OH2)] 2+, [Ru(tmtacn)(R2bpy)(OH2)]2+ (R=H, Me, and OMe; R2bpy=4,4'-disubstituted-2,2'-bipyridines), and [Ru(tpzm)(R2bpy)(OH2)]2+ (R=H, Me, and OMe), is investigated, where terpy=2,2':6',2″-terpyridine, bpy=2,2'-bipyridine, tmtacn=1,4,7-trimethyl-1,4,7-triazacyclononane, and tpzm=tris(1-pyrazolyl) methane. The kinetics of O2 evolution is investigated as a function of either the catalyst concentration or the oxidant concentration by employing Ce(NH4)2(NO3)6 as an oxidant; these catalysts can be classified into two groups that have different rate laws for O2 evolution. In one class, the rate of O2 evolution is linear to both the catalyst and Ce4+ concentrations, as briefly reported for [Ru(terpy)(bpy)(OH2)]2+ (S. Masaoka, K. Sakai, Chem. Lett. 2009, 38, 182). For the other class, [Ru(tmtacn)(R 2bpy)(OH2)]2+, the rate of O2 evolution is quadratic to the catalyst concentration and independent of the Ce4+ concentration. Moreover, the singlet biradical character of the hydroxocerium(IV) ion was realized by experimental and DFT investigations. These results indicate that the radical coupling between the oxygen atoms of a RuV=O species and a hydroxocerium(IV) ion is the key step for the catalysis of [Ru(terpy)(bpy)(OH2)]2+ and [Ru(tpzm)(R 2bpy)(OH2)]2+, while the well-known oxo-oxo radical coupling among two RuV=O species proceeds in the catalysis of [Ru(tmtacn)(R2bpy)(OH2)]2+. This is the first report demonstrating that the radical character provided by the hydroxocerium(IV) ion plays a crucial role in the catalysis of such ruthenium complexes in the evolution of O2 from water. Water oxidation: The reaction mechanism of O2 evolution catalyzed by mononuclear aquaruthenium complexes can be classified into two groups that exhibit different paths of O-O bond formation. The radical character provided by the hydroxocerium(IV) ion plays a crucial role in the catalysis by such ruthenium complexes in the evolution of O2 from water.

AB - The mechanism of O2 evolution from water catalyzed by a series of mononuclear aquaruthenium complexes, [Ru(terpy)(bpy)(OH2)] 2+, [Ru(tmtacn)(R2bpy)(OH2)]2+ (R=H, Me, and OMe; R2bpy=4,4'-disubstituted-2,2'-bipyridines), and [Ru(tpzm)(R2bpy)(OH2)]2+ (R=H, Me, and OMe), is investigated, where terpy=2,2':6',2″-terpyridine, bpy=2,2'-bipyridine, tmtacn=1,4,7-trimethyl-1,4,7-triazacyclononane, and tpzm=tris(1-pyrazolyl) methane. The kinetics of O2 evolution is investigated as a function of either the catalyst concentration or the oxidant concentration by employing Ce(NH4)2(NO3)6 as an oxidant; these catalysts can be classified into two groups that have different rate laws for O2 evolution. In one class, the rate of O2 evolution is linear to both the catalyst and Ce4+ concentrations, as briefly reported for [Ru(terpy)(bpy)(OH2)]2+ (S. Masaoka, K. Sakai, Chem. Lett. 2009, 38, 182). For the other class, [Ru(tmtacn)(R 2bpy)(OH2)]2+, the rate of O2 evolution is quadratic to the catalyst concentration and independent of the Ce4+ concentration. Moreover, the singlet biradical character of the hydroxocerium(IV) ion was realized by experimental and DFT investigations. These results indicate that the radical coupling between the oxygen atoms of a RuV=O species and a hydroxocerium(IV) ion is the key step for the catalysis of [Ru(terpy)(bpy)(OH2)]2+ and [Ru(tpzm)(R 2bpy)(OH2)]2+, while the well-known oxo-oxo radical coupling among two RuV=O species proceeds in the catalysis of [Ru(tmtacn)(R2bpy)(OH2)]2+. This is the first report demonstrating that the radical character provided by the hydroxocerium(IV) ion plays a crucial role in the catalysis of such ruthenium complexes in the evolution of O2 from water. Water oxidation: The reaction mechanism of O2 evolution catalyzed by mononuclear aquaruthenium complexes can be classified into two groups that exhibit different paths of O-O bond formation. The radical character provided by the hydroxocerium(IV) ion plays a crucial role in the catalysis by such ruthenium complexes in the evolution of O2 from water.

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