Probing the influence of steric bulk on anion binding by triarylboranes: comparative studies of FcB(o-Tol)2, FcB(o-Xyl)2 and FcBMes2

Abstract

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 FcBMes₂ (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 2/3vs.1) is found to have a relatively small effect on the binding affinities of these boranes (e.g. log₁₀K_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·CN]⁻, [2·CN]⁻ and [3·CN]⁻ in the solid state is also found to be essentially invariant (∠C_aryl–B–C_aryl = 338, 337, 337°, 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° is observed for [1·CN]⁻ while the corresponding values for [2·CN]⁻ and [3·CN]⁻ are 78° and 79°.