Big macrocyclic assemblies of carboranes (big MACs):: synthesis and crystal structure of a macrocyclic assembly of four carboranes containing alternate ortho- and meta-carborane icosahedra linked by para-phenylene units

The macrocyclic compound, [1,2-C2B10H10-1,4-C6H4-1,7-C2B10H10-1,4-C6H4](2) (5)-a novel cyclooctaphane, was prepared by condensation of the C,C'-dicopper(l) derivative of meta-carborane with 1,2-bis(4-iodophenyl)-ortho-carborane. The X-ray crystal structure of 5.C6H6-6C(6)H(12) was determined at 150 K, revealing an extremely loose packing mode. Molecule 5 has a crystallographic C-s and local C-2r symmetry; the macrocycle adopts a butterfly (dihedral angle 143degrees) conformation with the ortho-carborane units at the wingtips and the phenylene ring planes roughly perpendicular to the wing planes. Multinuclear NMR spectra suggest that molecule 5 in solution inverts rapidly via the planar D-2h geometry, which (from ab initio HF/6-31G* calculations) is only I kcal mol(-1) higher in energy than the C-2r. one. An attempt to prepare an even larger macrocycle, comprising three para-carborane and three ortho-carborane units linked by six para-phenylene units, was unsuccessful. (C) 2003 Elsevier Science B.V. All rights reserved.