Is there a future for the continued development of the chemistry of small cage C2B4-carborane? A historical perspective

The organometallic chemistry of the small-cage nido-C2B4-carborane ligands is unique and offers potential advantages over both the larger-cage C2B9 and cyclopentadienide systems. However, the study of the chemistry of this ligand system has been limited by the difficulties inherent in their large scale preparation. Initially, the carboranes were produced in limited quantities from the high temperature pyrolysis of gas phase alkyne–pentaborane(9) mixtures, usually in the presence of an amine catalyst. Synthetic methods have evolved so that it is now possible to produce multi-gram quantities of the trimethylsilyl-substituted carbons adjacent, nido-2-(SiMe3)3-(R)-2,3-C2B4H6 (R=SiMe3, H, or an alkyl group). Efficient methods have also been developed for the conversion of these compounds into the corresponding carbons apart, nido-2,4-C2B4-carborane isomers and for changing the cage carbon substituents. One of the greatest assets in fostering the study of these systems was the almost limitless supply of the pentaborane(9) precursor, supplied free-of-charge from an extensive government surplus. That source is no longer available nor is there a commercial source to take its place. These considerations have led us to review the history of the development of the methods used in the preparation of the small cage carboranes and to speculate on the future of the study of these most interesting ligands.