Group 4 metallocenes incorporating constrained-geometry carboranyl ligands

Ligand modifications have played a crucial role in developing new catalyst precursors for optimizing polymerization activity as well as polymer properties. o-Carborane is a highly versatile molecule and can be converted into the closo-C2B10H102−, nido-C2B9H112−, nido-C2B10H122−, and arachno-C2B10H124− ligands which are capable of being bonded to metal ions in σ-, η5-, η6-, and η7-fashion, respectively. Such unique features make the replacement possible of either a cyclopentadienyl or an amido unit in the traditional CpSiN constrained-geometry ligands, by a carboranyl moiety. These modifications lead to a novel class of constrained-geometry ligands bearing a carboanion functionality and to a new version of constrained-geometry ligand frameworks incorporating a dicarbollyl moiety. These ligands provide interesting opportunities for the design of metallocenes with new metal/charge and π/σ component combinations and for the study of the role of carborane in catalysis. Achievements, problems and perspectives in this new and rapidly growing field are discussed in this article.