Comparative Lewis Acidity in Fluoroarylboranes: B(o-HC6F4)3, B(p-HC6F4)3, and B(C6F5)3

The Lewis acidic fluoroarylborane B(o-HC6F4)(3) (2) was prepared and its Lewis acid strength assessed in comparison to the known, related boranes B(C6F5)(3) (1) and B(p-HC6F4)(3) (3). Experimental methods based on spectroscopic probes and equilibrium measurements were used to show that B(C6F5)(3) is the strongest Lewis acid of the three; while the Lewis acidities of 2 and 3 are comparable, the p-H-substituted isomer is slightly stronger in the tests employed. This contrasts with predictions made on the basis of computed bond formation energies, as recently reported by Durfey and Gilbert.