Proton-responsive ligands promote atmospheric CO2 capture and accelerate catalytic CO2/HCO2‒ interconversion

The synthesis and investigation of [Rh(DHMPE)2][BF4] (1) is reported. 1 features proton-responsive 1,2-bis(di(hydroxymethyl)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 results 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 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. CO2 binding to 1 is proposed to facilitate pre-organization of formate/ CO2 in the transition state to lower the activation energy and increase the observed catalytic rate. Incorporation of proton-responsive DHMPE ligands not only permits CO2 capture from air, but also provides a unique approach to accelerate the kinetics of catalytic CO2 reduction to formate.