Computational studies of Lewis acidities of tris(fluorophenyl)-substituted boranes: an additive relationship between Lewis acidity and fluorine position.

Computational studies of the binding energies of all possible tris(fluoroaryl)boranes B(C(6)H(x)F(5-x))(3) to NMe(3) and PMe(3) show that they (and by extension, the Lewis acidities of the boranes) can be tuned to a sizable range of values through judicious placement of fluorines. This holds despite the fact that the B-X bond distance changes little regardless of substitution, save when the aryl ring is 2,6-disubstituted. Fluorine substitution appears to affect the binding energies additively. Least-squares regression finds substitution at the 2-position to increase the binding energy by ca. 13 kcal·mol(-1), while substitution at the 3- and 5-positions increases it by ca. 3 kcal·mol(-1). Substitution at the 4-position has little to no impact, while substitution at the 6-position decreases the binding energy by ca. 3-6 kcal·mol(-1). The last observation arises from steric congestion because the 6-position can only be substituted in tandem with substitution at the 2-position. Models suggest that the pattern arises from polarization effects that decrease exponentially as the distance between boron and fluorine increases.