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] 1a (R = tBu) and 1b (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 pKa values for the conjugate acids of 1a and 1b 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 N-heterocyclic phosphines. 1a and 1b 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 1a reveals the role of steric ligands in optimizing catalytic performance.