Small carboranes

Usually carboranes are defined as carbon–boron clusters of four or more vertexes in which nonclassical (electrondelocalized multicenter) bonding plays a significant role. In all but a few cases, this definition clearly separates the carboranes from conventional organoboranes whose hydrocarbon-like structures can be described entirely in terms of 2c2e bonds. Some 4- and 5-vertex cages straddle the borderline between classical and nonclassical systems, but otherwise there is no difficulty in distinguishing carboranes from other compound types. Two nonclassical CB3 derivatives have been isolated and characterized, and several such systems have been explored theoretically. A 4-vertex carborane, nido-carbatetraborane, has been prepared by reaction of the tetraborane anion B4H3(CMe3)4 with iodomethane. Among the 5-vertex open clusters, no compound having a confirmed nido-CB4 cage framework has been characterized at present; however, several arachno-CB4 clusters are known to be prepared by very different routes. Nido-1,2-dicarbapentaborane, a structural and electronic analogue of B5H9, is an unusual compound. Quenched gas-phase reactions of higher alkynes with B4H10 at 70ºC generate a series of C(1)-, C(2)-, and C(1,2)-substituted derivatives of nido-C2B3H7. At present, 1,5-C2B3H5 is the only definitively characterized closo-C2B3 carborane. Successive formal replacement of BHbridge units in hexaborane (B6H10) with C atoms (or of B with C+) generates the series of 6-vertex nido species. This chapter discusses the structure, properties, synthesis, and chemistry of small carboranes including four-, five-, and six-vertex clusters.