pH-dependent H2-activation cycle coupled to reduction of nitrate ion by Cp*Ir complexes

Seiji Ogo, Hidetaka Nakai, Yoshihito Watanabe

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49 Citations (Scopus)


This paper reports a pH-dependent H2-activation {H2 (pH 1-4) → H+ + H- (pH -1) → 2H+ + 2e-} promoted by Cp*Ir complexes {Cp* = η5-C5(CH3)5}. In a pH range of about 1-4, an aqueous HNO3 solution of [Cp*III(H2O)3]2+ (1) reacts with 3 equiv of H2 to yield a solution of [(Cp*IrIII)2(μ- H)3]+ (2) as a result of heterolytic H2-activation {2[1] + 3H2 (pH 1-4) → [2] + 3H+ + 6H2O}. The hydrido ligands of 2 display protonic behavior and undergo H/D exchange with D+: [M-(H)3-M]+ + 3D+ ⇌ [M-(D)y3-M]+ + 3H+ (where M = Cp*Ir). Complex 2 is insoluble in a pH range of about -0.2 (1.6 M HNO3/H2O) to -0.8 (6.3 M HNO3/H2O). At pH -1 (10 M HNO3/H2O), a powder of 2 drastically reacts with HNO3 to give a solution of [Cp*IrIII(NO3)2] (3) with evolution of H2, NO, and NO2 gases. D-labeling experiments show that the evolved H2 is derived from the hydrido ligands of 2. These results suggest that oxidation of the hydrido ligands of 2 {[2] + 4NO3- (pH -1) → 2[3] + H2 + H+ + 4e-} couples to reduction of NO3- (NO3- → NO2- → NO). To complete the reaction cycle, complex 3 is transformed into 1 by increasing the pH of the solution from.-1 to 1. Therefore, we are able to repeat the reaction cycle using 1, H2, and a pH gradient between 1 and -1. A conceivable mechanism for the H2-activation cycle with reduction of NO3- is proposed.

Original languageEnglish
Pages (from-to)597-601
Number of pages5
JournalJournal of the American Chemical Society
Issue number4
Publication statusPublished - Jan 30 2002
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry


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