Aqua, Alcohol, and Acetonitrile Adducts of Tris(perfluorophenyl)borane:  Evaluation of Brønsted Acidity and Ligand Lability with Experimental and Computational Methods

Equilibrium studies have been performed to determine the Brønsted acidity of [(C6F5)3B(OH2)]·H2O, the aqua species that exists in acetonitrile solutions of B(C6F5)3 in the presence of water. NMR spectroscopic analysis of the deprotonation of [(C6F5)3B(OH2)]·H2O with 2,6-But 2C5H3N in acetonitrile allows a pK value of 8.6 to be determined for the equilibrium [(C6F5)3B(OH2)]·H2O ⇄ [(C6F5)3B(OH)]- + [H3O]+. On the basis of a calculated value for the hydrogen bond interaction in [(C6F5)3B(OH2)]·H2O, the pK a for (C6F5)3B(OH2) is estimated to be 8.4 in acetonitrile. Such a value indicates that (C6F5)3B(OH2) must be regarded as a strong acid, with a strength comparable to that of HCl in acetonitrile. Dynamic NMR spectroscopic studies indicate that the aqua and acetonitrile ligands in (C6F5)3B(OH2) and (C6F5)3B(NCMe) are labile, with dissociation of H2O being substantially more facile than that of MeCN, by a factor of ca. 200 in rate constant at 300 K. Ab initio calculations were performed in the gas phase and with a dielectric solvent model to determine the strength of B−L bonds (L = H2O, ROH, MeCN) and hydrogen bonds involving B−OH2 and B−O(H)R derivatives.