Comparative Lewis Acidity in Fluoroarylboranes: B(<i>o</i>‑HC₆F₄)₃, B(<i>p</i>‑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.