Synthesis and structure of large boron cages and multi-decker metal carborane sandwich complexes

Synthetic techiques developed in recent years, including the transition metal-promoted oxidative fusion of boron cage compounds, have allowed the systematic construction of new borones, carboranes, and metallabornes, some of which exhibit novel structural features and/or unusual properties such as cage fluxionality. Among these are carbon-rich carboranes and metallacarboranes having up to 14 cluster vertices. Although many of these materials have complicated cage geometries, the study of them is greatly facilitated by a powerful new technique, two-dimensional (2D) ¹¹B-¹¹B NMR spectroscopy, which reveals the pattern of bonding within the boron framework. The combination of 2D NMR in conjunction with X-ray crystallography of key species has proven particularly valuable in exploring this area. In related work, multiple-decked sandwich complexes incorporating R₂C₂B₄H₄^2- and/or R₂C₂B₃H₅^2- carborane units, transition metal ions, and planar aromatic hydrocarbons have been prepared and characterized. Species of this type exhibit high electron delocalization and may lead to new types of low-dimensioal electrically conducting polymers.