The versatile chemistry of the [B20H18]2- ions: novel reactions and structural motifs.

Among the polyhedral [closo-BnHn]2− ion series (n = 5–12 inclusive) the aromatic [closo-B10H10]2− ion is both readily available and quite reactive. Among its many reactions which retain its cage structure one finds the oxidative dimerization reaction in which two [closo-B10H12]2− ions each formally lose a hydride ion and undergo dimerization of the resulting [closo-B10H9]− ions to produce the [trans-B20H18]2− ion. The two-component [closo-B10H9]− ions of the latter are linked together by a pair of unique B–B–B bonds which provide unprecedented reactivity to the structure. Among these reactions are the two-electron reduction to a set of three interconvertible [B20H18]4− ions having intercage B–B bonds and the related reductive substitution reaction in which [trans-B20H18]2− undergoes attack by nucleophile, L, to produce [B20H18L]2−. The latter species is formally a substituted [B20H19]3− (L = H) ion formed by B–B bond protonation of one of the isomeric [B20H18]4− ions. These and a variety of novel reactions are described here along with interrelated reaction mechanisms considered for the first time.