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Title: Positional effects of hydroxy groups on catalytic activity of proton-responsive half-sandwich Cp*Iridium(III) complexes

Journal Article · · Organometallics
DOI: https://doi.org/10.1021/om500832d · OSTI ID:1182539
 [1];  [2];  [2];  [3];  [1];  [1];  [2];  [4]
  1. National Inst. of Advanced Industrial Science and Technology, Higashi, Tsukuba, Ibaraki (Japan)
  2. Brookhaven National Lab. (BNL), Upton, NY (United States)
  3. National Inst. of Advanced Industrial Science and Technology, Higashi, Tsukuba, Ibaraki (Japan); Japan Science and Technology Agency, Honcho, Kawaguchi, Saitama (Japan); Univ. of Technology, Panjin (China)
  4. National Inst. of Advanced Industrial Science and Technology, Higashi, Tsukuba, Ibaraki (Japan); Japan Science and Technology Agency, Honcho, Kawaguchi, Saitama (Japan)
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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 pK a 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 the low pKa value of < 0.4 because the deprotonation is facilitated by the strong intramolecular hydrogen bond formed between the generated oxyanion and the remaining hydroxy group, which in turn leads to an elevated pKa 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), transfer hydrogenation reactions using formic acid/formate as a hydrogen source (pH 7.2 and 2.6) 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 electrondonating 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.

Research Organization:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI ID:
1182539
Report Number(s):
BNL--107640-2015-JA; KC0304030
Journal Information:
Organometallics, Journal Name: Organometallics Journal Issue: 22 Vol. 33; ISSN 0276-7333
Publisher:
American Chemical SocietyCopyright Statement
Country of Publication:
United States
Language:
English

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Development of Effective Catalysts for Hydrogen Storage Technology Using Formic Acid journal September 2018
Metall-Ligand-Kooperation journal September 2015
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Unprecedentedly High Formic Acid Dehydrogenation Activity on an Iridium Complex with an N , N ′-Diimine Ligand in Water journal July 2015
Bifunctional Ru II -Complex-Catalysed Tandem C−C Bond Formation: Efficient and Atom Economical Strategy for the Utilisation of Alcohols as Alkylating Agents journal October 2016
Development of an Iridium-Based Catalyst for High-Pressure Evolution of Hydrogen from Formic Acid journal August 2016
Iridium Complexes with Proton-Responsive Azole-Type Ligands as Effective Catalysts for CO 2 Hydrogenation journal November 2017
Mechanistic Considerations on Homogeneously Catalyzed Formic Acid Dehydrogenation: Mechanistic Considerations on Homogeneously Catalyzed Formic Acid Dehydrogenation journal April 2018
Platinum Complexes with a Phosphino-Oxime/Oximate Ligand: Platinum Complexes with a Phosphino-Oxime/Oximate Ligand journal July 2018
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Impact of {Os(pap) 2 } in fine-tuning the binding modes and non-innocent potential of deprotonated 2,2′-bipyridine-3,3′-diol journal January 2016
Optimum bifunctionality in a 2-(2-pyridyl-2-ol)-1,10-phenanthroline based ruthenium complex for transfer hydrogenation of ketones and nitriles: impact of the number of 2-hydroxypyridine fragments journal January 2016
Catalytic reactivity of an iridium complex with a proton responsive N-donor ligand in CO 2 hydrogenation to formate journal January 2018
Mechanistic insights into catalytic CO 2 hydrogenation using Mn( i )-complexes with pendant oxygen ligands journal January 2018
Mechanistic insights into HCO 2 H dehydrogenation and CO 2 hydrogenation catalyzed by Ir(Cp*) containing tetrahydroxy bipyrimidine ligand: the role of sodium and proton shuttle journal January 2018
Hydrogen generation from formic acid decomposition on a highly efficient iridium catalyst bearing a diaminoglyoxime ligand journal January 2018
Electronic effects on reactivity and anticancer activity by half-sandwich N,N-chelated iridium( iii ) complexes journal January 2018
Light-driven catalytic hydrogenation of carbon dioxide at low-pressure by a trinuclear iridium polyhydride complex journal January 2019
CO 2 hydrogenation by phosphorus–nitrogen PN 3 P-pincer iridium hydride complexes: elucidation of the deactivation pathway journal January 2019

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