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

The Lewis acidic fluoroarylborane B( o -HC 6 F 4 ) 3 ( 2 ) was prepared and its Lewis acid strength assessed in comparison to the known, related boranes B(C 6 F 5 ) 3 ( 1 ) and B( p -HC 6 F 4 ) 3 ( 3 ). Experimental methods based on spectroscopic probes and equilibrium measurements were used to show that B(C 6 F 5 ) 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.