Low p<i>K</i>_a Phosphido-Boranes Capture Carbon Dioxide with Exceptional Strength: DFT Predictions Followed by Experimental Validation

We have developed a class of phosphido-boranes (BoPh's) with formula X⁺[R₂PBH₃^-] that bind CO₂ with exceptional strength (Δ<i>G</i> = -8.2 to -24.0 kcal/mol) at ambient conditions. We use quantum mechanics (QM) to determine how the choice of electron-donating versus electron-withdrawing ligand impacts the CO₂ binding strength, in the presence of a donating borane moiety. We also examine the role of the cation in CO₂ binding, finding that the ion position relative to the bound CO₂ dramatically alters binding strength. We find that the BoPh with two ethyl ligands Li[Et₂PBH₃] leads to Δ<i>G</i> = -24.0 kcal/mol upon CO₂ binding while Li[Ph₂PBH₃] leads to Δ<i>G</i> = -12.8 kcal/mol. We synthesized the BoPh with two phenyl ligands Li[Ph₂PBH₃] to validate the QM-predicted stability and predicted p<i>K</i>_a.