Proton-Responsive Ligands Promote CO Capture and Accelerate Catalytic CO/HCO Interconversion.

The synthesis and investigation of [Rh(DHMPE)][BF] () are reported. features proton-responsive 1,2-bis[(dihydroxymethyl)phosphino]ethane (DHMPE) ligands, which readily capture CO from atmospheric sources upon deprotonation. The protonation state of the DHMPE ligand was observed to have a significant impact on the catalytic reactivity of with CO. Deprotonation and CO binding to result in a ∼10-fold rate enhancement in catalytic degenerate CO reduction with formate, monitored by C/C isotope exchange between HCO and CO. Studies performed using a similar complex lacking the hydroxyl ligand functionality ([Rh(DEPE)][BF] 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, CO binding to is proposed to facilitate preorganization of formate/CO 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 CO reduction to formate.