Zwitterionic Phosphorus(I) Compounds Supported by a Bis(carbene)borate: Synthesis, Protonation, and Thermodynamics of P–H Bonds

Carbones are carbon(0) species stabilized by two donor ligands and are strong neutral bases; yet, heavier analogues and the thermodynamics of the E-H bonds of their protonated species remain largely unexplored. We report the synthesis and structural characterization of zwitterionic, two-coordinate phosphorus(I) complexes [(^Ph,RBCB)P] <b>1a</b> (R = <i>^t</i>Bu) and <b>1b</b> (R = Me), supported by an anionic bis(carbene)borate ligand. These can be viewed as charge-neutral phosphorus(I) analogues of carbone derivatives. The Brønsted basicity of the P(I) center has been quantified for the first time: the p<i>K</i>_a values for the conjugate acids of <b>1a</b> and <b>1b</b> are 14.7 ± 0.7 and 14.8 ± 0.7 in CH₃CN, respectively, far weaker than those of classical carbones. Bordwell thermodynamic cycles give the P-H bond dissociation free energies (BDFEs) of 67-68 kcal/mol for the protonated species, higher than that in Ph₂PH but lower than those in many <i>N</i>-heterocyclic phosphines. <b>1a</b> and <b>1b</b> catalyze the hydroborylation of 4-tolyl isocyanate via a formal P(I)/P(III) redox cycle. The markedly higher activity of the more sterically encumbered <b>1a</b> reveals the role of steric ligands in optimizing catalytic performance.