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Cp*Co(III) Catalysts with Proton-Responsive Ligands for Carbon Dioxide Hydrogenation in Aqueous Media

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Yosra M. Badiei †, Wan-Hui Wang ‡§, Jonathan F. Hull †, David J. Szalda †, James T. Muckerman *†,Yuichiro Himeda *‡§, and Etsuko Fujita *†

^† Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States

^‡ National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5-2, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

^§ Japan Science and Technology Agency, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

 Department of Natural Science, Baruch College, CUNY, New York, New York 10010, United States

Inorg. Chem., Article ASAP

DOI: 10.1021/ic401707u

Publication Date (Web): October 16, 2013

Copyright © 2013 American Chemical Society

*E-mail: muckerma@bnl.gov., *E-mail: himeda.y@aist.go.jp., *E-mail: fujita@bnl.gov.

Synopsis

New water-soluble pentamethylcyclopendadienyl cobalt(III) complexes with proton-responsive 4,4′- and 6,6′-dihydroxy-2,2′-bipyridine (4DHBP and 6DHBP, respectively) ligands have been prepared and were characterized by X-ray crystallography, UV−vis and NMR spectroscopy, mass spectrometry, and DFT calculations. The cobalt complexes containing 4DHBP ligands ([1−OH2]2+ and [1−Cl]+) display better thermal stability and exhibit notable catalytic activity for CO2 hydrogenation to formate in aqueous bicarbonate media at moderate temperature under a total 4−5 MPa (CO2:H2 1:1) pressure.

Abstract

New water-soluble pentamethylcyclopentadienyl cobalt(III) complexes with proton-responsive 4,4′- and 6,6′-dihydroxy-2,2′-bipyridine (4DHBP and 6DHBP, respectively) ligands have been prepared and were characterized by X-ray crystallography, UV–vis and NMR spectroscopy, and mass spectrometry. These cobalt(III) complexes with proton-responsive ligands predominantly exist in their deprotonated [Cp*Co(DHBP–2H⁺)(OH₂)] forms with stronger electron-donating properties in neutral and basic solutions, and are active catalysts for CO₂ hydrogenation in aqueous bicarbonate media at moderate temperature under a total 4–5 MPa (CO₂:H₂ 1:1) pressure. The cobalt complexes containing 4DHBP ligands ([1–OH₂]²⁺and [1–Cl]⁺, where 1 = Cp*Co(4DHBP)) display better thermal stability and exhibit notable catalytic activity for CO₂ hydrogenation to formate in contrast to the catalytically inactive nonsubstituted bpy analogues [3–OH₂]²⁺ (3 = Cp*Co(bpy)). While the catalyst Cp*Ir(6DHBP)(OH₂)²⁺ in which the pendent oxyanion lowers the barrier for H₂ heterolysis via proton transfer through a hydrogen-bonding network involving a water molecule is remarkably effective (ACS Catal. 2013, 3, 856–860), cobalt complexes containing 6DHBP ligands ([2–OH₂]²⁺ and [2–Cl]⁺, 2 = Cp*Co(6DHBP)) exhibit lower TOF and TON for CO₂ hydrogenation than those with 4DHBP. The low activity is attributed to thermal instability during the hydrogenation of CO₂ as corroborated by DFT calculations.