Polyhedral dicarbaborane chemistry: a convenient synthesis of the [nido-7,8-C2B9H12]− ‘dicarbollide’ anion via the Brellochs reaction

It is known that the reaction of B10H14 in alkaline solution with HCHO yields the monocarbaborane anion [arachno-CB9H14]−. We now find that, in stronger alkaline solution with a large excess of HCHO, the dicarbaborane anion [nido-7,8-C2B9H12]− can be obtained in an experimentally convenient 52% yield: this is of high synthetic utility; for example, the essentially quantitative conversion to the COSAN anion [Co(C2B9H11)2]− hence gives an overall yield of COSAN of 49% from B10H14 in two simple steps.