Crystal Structures of the Carborane Dianions [ 1,4-(PhCB10H10C) 2C6H4] 2-and [ 1,4-( PhCB10H10C) 2C6F4] 2-and the Stabilizing Role of the para- Phenylene Unit on 2n+3 Skeletal Electron Clusters

While carboranes with 2n+2 and 2n+4 (n=number of skeletal atoms) skeletal electrons (SE) are widely known, little has been reported on carboranes with odd SE numbers. Electrochemical measurements on two-cage assemblies, where two C-phenyl-ortho-carboranyl groups are linked by a para-phenylene or a para-tetrafluorophenylene bridge, revealed two well separated and reversible two-electron reduction waves indicating formation of stable dianions and tetraanions. The salts of the dianions were isolated by reduction with sodium metal and their unusual structures were determined by X-ray crystallography. The diamagnetic dianions contain two 2n+3 SE clusters where each cluster has a notably long carboraneC-carboraneC distance of ca 2.4 angstrom. The conjugation within the phenylene bridge plays an important role in the stabilization of these carboranes with odd SE counts.