Solution and solid state studies on the binding of isomeric carboranes C₂B₁₀H₁₂ by p-Bu^t-calix[5]arene

P-Bu^t-calix[5]arene forms crystalline inclusion complexes with o- and m-carboranes in toluene or dichloromethane–hexane, but not with the p-isomer, the extended structures being based on 1 : 1 host–guest supermolecules, with the p-Bu^t-substituents creating a snug fit for o- and m-carborane; p-carborane forms a highly hexane soluble complex, induced by grinding, which crystallizes as fibres. Solution phase studies showed the presence of a 1 : 1 host–guest stoichiometry with all three isomeric carboranes as determined from Job plots. The association constants for the o- and m-carborane complexes are 6.4 ± 0.3 M⁻¹ and 3.8 ± 0.1 M⁻¹ respectively, whereas the p-isomer is only weakly associated. Competition experiments involving all three isomers show rapid exchange on the NMR time scale, and no selectivity in solution is evident. Selective association involving the o- and m-isomers in the solid state is therefore remarkable, and it is a manifestation of crystal packing forces which embodies the differences in dipole moments of the carboranes.