Probing the influence of steric bulk on anion binding by triarylboranes: comparative studies of FcB(o-Tol)(2), FcB(o-Xyl)(2) and FcBMes(2)

Steric crowding brought about on pyramidalization at boron has been predicted computationally to be of central importance to the strength and selectivity of anion binding by triarylboranes. The role of steric factors in systems containing a ferrocenyl reporter unit has been systematically probed in the current study by comparison of the F-/CN- binding properties of FcB(o-Tol)(2) (1, o-Tol = C6H4Me-2), FcB(o-Xyl)(2) (2, o-Xyl = C6H3Me2-2,6) and FcBMes(2) (3, Mes = C6H2Me3-2,4,6)), both in solution and in the solid state. Somewhat surprisingly, the inclusion of an extra ortho-methyl aryl substituent (e.g. for 2/3 vs. 1) is found to have a relatively small effect on the binding affinities of these boranes (e.g. log(10)K(CN) = 5.94(0.02), 4.73(0.01), 5.56(0.02), for 1, 2 and 3 respectively). Consistent with this observation, the degree of pyramidalization at boron determined for the cyanide adducts [1 center dot CN](-), [2 center dot CN](-) and [3 center dot CN](-) in the solid state is also found to be essentially invariant (angle C-aryl B C-aryl = 338, 337, 337 degrees, respectively), as are the B-CN and mean B-C-aryl distances. In the solid state at least, it is apparent that the adverse steric effects potentially brought about by increasing ortho substitution are mitigated by a greater degree of synchronous rotation of the aryl substituents about the B-C-aryl bonds. Thus a mean inter-plane angle of 71 degrees is observed for [1 center dot CN](-) while the corresponding values for [2 center dot CN](-) and [3 center dot CN](-) are 78 degrees and 79 degrees.