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

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 ([<b>1–OH</b>_<b>2</b>]²⁺ and [<b>1–Cl</b>]⁺, where <b>1</b> = 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 [<b>3–OH</b>_<b>2</b>]²⁺ (<b>3</b> = 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 (<i>ACS Catal.</i> <b>2013</b>, <i>3</i>, 856–860), cobalt complexes containing 6DHBP ligands ([<b>2–OH</b>_<b>2</b>]²⁺ and [<b>2–Cl</b>]⁺, <b>2</b> = 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