Recent progress in the chemistry of supercarboranes

The chemistry of boron clusters has been dominated by icosahedral carboranes for over half a century. Only in recent years has significant progress been made in the chemistry of supercarboranes (carboranes with more than 12 vertices). A number of CAd (carbon-atoms-adjacent) 13- and 14-vertex carboranes, and CAp (carbon-atoms-apart) 13-vertex carboranes as well as their corresponding 14- and 15-vertex metallacarboranes have been successfully prepared and structurally characterized. This breakthrough relied on the use of CAd nido-carborane dianions as starting materials. These supercarboranes can undergo single-electron reduction to give stable supercarborane radical monoanions with [2n+3] framework electrons, and electrophilic substitution reaction to afford hexasubstituted supercarboranes. They can react with nucleophiles to offer monocarba-closo-dodecaborate monoanions from cage-carbon extrusion reactions. Their unique chemical properties make the chemistry of supercarboranes distinct from that of their 12-vertex analogues. These studies open up new possibilities for the development of polyhedral clusters of extraordinary size. This focus review offers an overview of recent advances in this growing research field.