A journey from 12-vertex to 14-vertex carboranes and to 15-vertex metallacarboranes

Icosahedral o-carboranes are readily reduced by group 1 metals to give carbon atoms apart (CAp) nido-carborane dianions that can be further reduced to the corresponding arachno-carborane tetraanions with the assistance of transition-metal ions. The geometries of the arachno species are dependent upon the electronic configurations of transition-metal ions. In contrast, carbon atoms adjacent (CAd) nido-carborane dianions can react with lithium metal, in the absence of any transition-metal ions, to produce CAd arachno-carborane tetraanions having a significantly different structure from their CAp counterparts. These CAd nido- and arachno-carborane anions possess a reducing power weaker than that of the CAp isomers, which can facilitate the capitation reactions of these anions with boron dihalides. This important discovery provides a very valuable entry point to the synthesis of 13- and 14-vertex carboranes. Subsequently, the largest heteroborane, 15-vertex metallacarborane, has been prepared. These studies open up new possibilities for the development of polyhedral clusters of extraordinary size. This review offers an overview of recent advances in this growing research field.