Proton-Responsive Ligands Promote CO2 Capture and Accelerate Catalytic CO2/HCO2- Interconversion

The synthesis and investigation of [Rh(DHMPE)(2)][BF4] (1) are reported. 1 features proton-responsive 1,2-bis[(dihydroxymethyl)phosphino]ethane (DHMPE) ligands, which readily capture CO2 from atmospheric sources upon deprotonation. The protonation state of the DHMPE ligand was observed to have a significant impact on the catalytic reactivity of 1 with CO2. Deprotonation and CO2 binding to 1 result in a similar to 10-fold rate enhancement in catalytic degenerate CO2 reduction with formate, monitored by C-12/C-13 isotope exchange between (HCO2-)-C-12 and (CO2)-C-13. Studies performed using a similar complex lacking the hydroxyl ligand functionality ([Rh(DEPE)(2)][BF4] where DEPE = 1,2-bis(diethylphosphino)ethane) do not show the same rate enhancements when base is added. Based upon the cation-dependent activity of the catalyst, Eyring analysis, and cation sequestration experiments, CO2 binding to 1 is proposed to facilitate preorganization of formate/CO2 in the transition state via ligand-based encapsulation of Na+ or K+ cations to lower the activation energy and increase the observed catalytic rate. Incorporation of proton-responsive DHMPE ligands provides a unique approach to accelerate the kinetics of catalytic CO2 reduction to formate.