Synthesis and X-ray characterization of 15-and 16-vertex closo-carboranes

Carboranes are a class of carbon-boron molecular clusters with three-dimensional aromaticity, and inherent robustness. These endowments enable carboranes as valuable building blocks for applications ranging from functional materials to pharmaceuticals. Thus, the chemistry of carboranes has received tremendous research interest, and significant progress has been made in the past decades. However, many attempts to the synthesis of carboranes with more than 14 vertices had been unsuccessful since the report of a 14-vertex carborane in 2005. The question arises as to whether these long sought-after molecules exist. We describe in this article the synthesis and structural characterization of 15- and 16-vertex closo-carboranes as well as 16-vertex ruthenacarborane. Such a success relies on the introduction of silyl groups to both cage carbons, stabilizing the corresponding nido-carborane dianions and promoting the capitation reaction with HBBr2.SMe2. This work would shed some light on the preparation of carboranes with 17 vertices or more, and open the door for studying supercarborane chemistry. The synthesis of closo-carboranes with more than 14 vertexes is challenging, and no examples have been reported to date. Herein, the authors present the long-sought 15- and 16-vertex closo-carboranes, in which the introduction of silyl groups to the two cage carbons is crucial; this finding might enable the synthesis of even larger carborane analogs in the future.