Synthesis, structure, and reactivity of 13- and 14-vertex carboranes.

Carboranes are a class of polyhedral boron hydride clusters in which one or more of the BH vertices are replaced by CH units. Their chemistry has been dominated by 12-vertex carboranes for over half a century. In contrast, knowledge regarding supercarboranes (carboranes with more than 12 vertices) had been limited merely to possible cage geometries predicted by theoretical work before 2003. Only in recent years has significant progress been made in synthesizing supercarboranes. Such a breakthrough relied on the use of Carbon-Atoms-Adjacent (CAd) nido-carborane dianions or arachno-carborane tetraanions as starting materials. In this Account, we describe our work on constructing and elucidating the chemistry of supercarboranes. Earlier attempted insertions of the formal [BR](2+) unit into Carbon-Atoms-Apart (CAp) 12-vertex nido-[7,9-C2B10H12](2-) did not produce the desired 13-vertex carboranes. Such failure is often attributed to the extraordinary stability of the B12 icosahedron (the "icosahedral barrier"). However, the difference in reducing power between CAp and CAd 12-vertex nido-carborane dianions had been overlooked. Our results have shown that CAd nido-carborane dianions are weaker reducing agents than the CAp isomers, allowing a capitation to prevail over a redox reactivity. This finding provides an entry point to the synthesis of supercarboranes and a series of 13- and 14-vertex closo-carboranes have been prepared and structurally characterized. They share some chemical properties with those of 12-vertex carboranes; on the other hand, they have their own unique characteristics. For example, a 13- vertex closo-carborane can undergo single electron reduction to give a stable carborane radical anion with [2n + 3] framework electrons, which can accept one additional electron to form a 13-vertex CAd nido-carborane dianion. 13-Vertex closo-carborane can also react with various nucleophiles to afford the cage carbon and/or boron extrusion products closo-CB11(-), nido-CB10(-), closo-CB10(-), and closo-C2B10, depending on the nature of the nucleophiles. Studies of supercarboranes remain a relative young research area, particularly in comparison to the rich literature of icosahedral carboranes with 12-vertices. Other supercarboranes are expected to be prepared and structurally characterized as the field progresses, and the results detailed here will further these efforts.