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

Abstract The macrocyclic compound, [1,2-C 2 B 10 H 10 -1,4-C 6 H 4 -1,7-C 2 B 10 H 10 -1,4-C 6 H 4 ] 2 ( 5 )—a novel cyclooctaphane, was prepared by condensation of the C,C′-dicopper(I) derivative of meta -carborane with 1,2-bis(4-iodophenyl)- ortho -carborane. The X-ray crystal structure of 5 ·C 6 H 6 ·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 2 v symmetry; the macrocycle adopts a butterfly (dihedral angle 143°) 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 2 h geometry, which (from ab initio HF/6-31G* calculations) is only 1 kcal mol −1 higher in energy than the C 2 v 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.