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 [(C 6 F 5 ) 3 B(OH 2 )]·H 2 O, the aqua species that exists in acetonitrile solutions of B(C 6 F 5 ) 3 in the presence of water. NMR spectroscopic analysis of the deprotonation of [(C 6 F 5 ) 3 B(OH 2 )]·H 2 O with 2,6-Bu t 2 C 5 H 3 N in acetonitrile allows a p K value of 8.6 to be determined for the equilibrium [(C 6 F 5 ) 3 B(OH 2 )]·H 2 O ⇄ [(C 6 F 5 ) 3 B(OH)] - + [H 3 O] + . On the basis of a calculated value for the hydrogen bond interaction in [(C 6 F 5 ) 3 B(OH 2 )]·H 2 O, the p K a for (C 6 F 5 ) 3 B(OH 2 ) is estimated to be 8.4 in acetonitrile. Such a value indicates that (C 6 F 5 ) 3 B(OH 2 ) 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 (C 6 F 5 ) 3 B(OH 2 ) and (C 6 F 5 ) 3 B(NCMe) are labile, with dissociation of H 2 O 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 = H 2 O, ROH, MeCN) and hydrogen bonds involving B−OH 2 and B−O(H)R derivatives.