Aqua, alcohol, and acetonitrile adducts of tris(perfluorophenyl)borane: Evaluation of Bronsted acidity and ligand lability with experimental and computational methods

Equilibrium studies have been performed to determine the Bronsted acidity o f [(C6F5)(3)B(OH2)].H2O, the aqua species that exists in acetonitrile solut ions of B(C6F5)(3) in the presence of water. NMR spectroscopic analysis of the deprotonation of [(C6F5)(3)B(OH2)].H2O with 2,6-(Bu2C5H3N)-C-t in aceto nitrile allows a pK value of 8.6 to be determined for the equilibrium [(C6F 5)(3)B(OH2)].H2O reversible arrow [(C6F5)(3)B(OH)](-) + [H3O](+). On the ba sis of a calculated value for the hydrogen bond interaction in [(C6F5)(3)B( OH2)].H2O. the pK(a) for (C6F5)(3)B(OH2) is estimated to be 8.4 in acetonit rile. Such a value indicates that (C6F5)(3)B(OH2) must be regarded as a str ong acid, with a strength comparable to that of HCl in acetonitrile. Dynami c NMR spectroscopic studies indicate that the aqua and acetonitrile ligands in (C6F5)(3)B(OH2) and (C6F5)(3)B(NCMe) are labile, with dissociation of H 2O being substantially more facile than that of MeCN, by a factor of ca. 20 0 in rate constant at 300 K. Ab initio calculations were performed in the g as 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.