Positional effects of hydroxy groups on catalytic activity of proton-responsive half-sandwich Cp∗iridium(III) complexes

Yuki Suna, Mehmed Z. Ertem, Wan Hui Wang, Hide Kambayashi, Yuichi Manaka, James Muckerman, Etsuko Fujita, Yuichiro Himeda

Research output: Contribution to journalArticle

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Abstract

Proton-responsive half-sandwich Cp∗Ir(III) complexes possessing a bipyridine ligand with two hydroxy groups at the 3,3′-, 4,4′-, 5,5′-, or 6,6′-positions (3DHBP, 4DHBP, 5DHBP, or 6DHBP) were systematically investigated. UV-vis titration data provided average pKa values of the hydroxy groups on the ligands. Both hydroxy groups were found to deprotonate in the pH 4.6-5.6 range for the 4-6DHBP complexes. One of the hydroxy groups of the 3DHBP complex exhibited a low pKa value of a value of ∼13.6 for the second deprotonation step. The crystal structures of the 4- and 6DHBP complexes obtained from basic aqueous solutions revealed their deprotonated forms. The intramolecular hydrogen bond in the 3DHBP complex was also observed in the crystal structures. The catalytic activities of these complexes in aqueous phase reactions, at appropriate pH, for hydrogenation of carbon dioxide (pH 8.5), dehydrogenation of formic acid (pH 1.8), and transfer hydrogenation reactions using formic acid/formate as a hydrogen source (pH 2.6 and 7.2) were investigated to compare the positional effects of the hydroxy groups. The 4- and 6DHBP complexes exhibited remarkably enhanced catalytic activities under basic conditions because of the resonance effect of the strong electron-donating oxyanions, whereas the 5DHBP complex exhibited negligible activity despite the presence of electron-donating groups. The 3DHBP complex exhibited relatively high catalytic activity at low pH owing to the one strong electron-donating oxyanion group stabilized by the intramolecular hydrogen bond. DFT calculations were employed to study the mechanism of CO2 hydrogenation by the 4DHBP and 6DHBP complexes, and comparison of the activation free energies of the H2 heterolysis and CO2 insertion steps indicated that H2 heterolysis is the rate-determining step for both complexes. The presence of a pendent base in the 6DHBP complex was found to facilitate the rate-determining step and renders 6DHBP a more effective catalyst for formate production.

Original languageEnglish
Pages (from-to)6519-6530
Number of pages12
JournalOrganometallics
Volume33
Issue number22
DOIs
Publication statusPublished - Nov 24 2014

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formic acid
catalytic activity
Protons
Catalyst activity
Hydrogenation
protons
Electrons
Hydrogen bonds
hydrogenation
Crystal structure
Ligands
formates
Deprotonation
Dehydrogenation
Titration
Discrete Fourier transforms
Carbon Dioxide
Free energy
Hydrogen
hydrogen bonds

ASJC Scopus subject areas

  • Organic Chemistry
  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

Cite this

Positional effects of hydroxy groups on catalytic activity of proton-responsive half-sandwich Cp∗iridium(III) complexes. / Suna, Yuki; Ertem, Mehmed Z.; Wang, Wan Hui; Kambayashi, Hide; Manaka, Yuichi; Muckerman, James; Fujita, Etsuko; Himeda, Yuichiro.

In: Organometallics, Vol. 33, No. 22, 24.11.2014, p. 6519-6530.

Research output: Contribution to journalArticle

Suna, Yuki ; Ertem, Mehmed Z. ; Wang, Wan Hui ; Kambayashi, Hide ; Manaka, Yuichi ; Muckerman, James ; Fujita, Etsuko ; Himeda, Yuichiro. / Positional effects of hydroxy groups on catalytic activity of proton-responsive half-sandwich Cp∗iridium(III) complexes. In: Organometallics. 2014 ; Vol. 33, No. 22. pp. 6519-6530.
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T1 - Positional effects of hydroxy groups on catalytic activity of proton-responsive half-sandwich Cp∗iridium(III) complexes

AU - Suna, Yuki

AU - Ertem, Mehmed Z.

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AU - Kambayashi, Hide

AU - Manaka, Yuichi

AU - Muckerman, James

AU - Fujita, Etsuko

AU - Himeda, Yuichiro

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