Low p<i>K</i><sub>a</sub> Phosphido-Boranes\nCapture Carbon Dioxide with Exceptional Strength: DFT Predictions\nFollowed by Experimental Validation

We have developed a class of phosphido-boranes (BoPh’s)\nwith formula X<sup>+</sup>[R<sub>2</sub>PBH<sub>3</sub><sup>–</sup>] that bind CO<sub>2</sub> with exceptional strength (Δ<i>G</i> = −8.2 to −24.0 kcal/mol) at ambient conditions.\nWe use quantum mechanics (QM) to determine how the choice of electron-donating\nversus electron-withdrawing ligand impacts the CO<sub>2</sub> binding\nstrength, in the presence of a donating borane moiety. We also examine\nthe role of the cation in CO<sub>2</sub> binding, finding that the\nion position relative to the bound CO<sub>2</sub> dramatically alters\nbinding strength. We find that the BoPh with two ethyl ligands Li[Et<sub>2</sub>PBH<sub>3</sub>] leads to Δ<i>G</i> = −24.0\nkcal/mol upon CO<sub>2</sub> binding while Li[Ph<sub>2</sub>PBH<sub>3</sub>] leads to Δ<i>G</i> = −12.8 kcal/mol.\nWe synthesized the BoPh with two phenyl ligands Li[Ph<sub>2</sub>PBH<sub>3</sub>] to validate the QM-predicted stability and predicted p<i>K</i><sub>a</sub>.