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 pK value of 8.6 to be determined for the equilibrium [(C 6 F 5 ) 3 B(OH 2 )]‚H 2 O a [(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 pK 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.

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MesB(C6F5)2 (1) has been prepared from MesMgBr and FB(C6F5)2·OEt2, while Mes2B(C6F5) (2) is readily available from CuC6F5 and Mes2BBr. The reduction potential E° of 1 vs Cp2Fe0/+ in THF is −1.72 V, while that of 2 is −2.10 V, and that of Mes3B (3) is −2.73 V. 11B and 1H NMR show that neither 1 nor 2 binds THF significantly. These results have been used to estimate the reduction potential of B(C6F5)3 in THF as −1.17 V vs Cp2Fe0/+ or as −0.64 V vs SCE.