Role of C,C‘-Linkage in the Formation and Stabilization of Supercarboranes. Synthesis and Structure of Carbon-Atoms-Apart 13-Vertex Carborane and 14-Vertex Metallacarborane

This work reports, for the first time, the synthesis and structural characterization of both carbon-atoms-adjacent and carbon-atoms-apart 13-vertex carboranes and 14-vertex metallacarborane without C,C‘-linkages using direct and indirect methods. The results show that the C,C‘-linkages do not have any obvious effect on the thermal and kinetic stability of 13-vertex carboranes, and the CAp isomer is thermodynamically more stable than the CAd one. The role of the linkages is just to lower the reducing power of the nido-carborane dianions, facilitating the capitation reaction. CAp 13-vertex carboranes can be directly prepared by the reaction of [nido-7,9-R2-7,9-C2B10H10]Na2 with dihaloborane reagents. However, the yield is rather low due to the very strong reducing power of the CAp nido-carborane dianions. This work supports our earlier argument that the prevention of the redox reactions is the key to obtaining successively larger carboranes.