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 ∼10-fold rate enhancement in catalytic degenerate CO₂ reduction with formate, monitored by ¹²C/¹³C isotope exchange between H¹²CO₂^- and ¹³CO₂. 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.