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 ([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.