Carbon-Substituted Amines of the Cobalt Bis(dicarbollide) Ion: Stereochemistry and Acid–Base Properties

Organic amines are found to be abundant in natural living systems. They also constitute an inestimable family of building blocks available in drug design. Considering the man-made cluster [(1,2-C₂B₉H₁₁)₂-3,3'-Co(III)]^- ion (<b>1-</b>) and its application as an emerging unconventional pharmacophore, the availability of the corresponding amines has been limited and those with amino groups attached directly to carbon atoms have remained unknown. This paper describes the synthesis of compounds containing one or two primary amino groups attached to the carbon atoms of the cobaltacarborane cage that are accessible via the reduction of newly synthesized azides or via the Curtius rearrangement of the corresponding acyl azide. This substitution represents the first members of the series of azides and primary amines with functional groups bound directly to the carbon atoms of the cage. As expected, the absence of the linker along with the presence of the bulky anionic polyhedral ion leads to a significant alteration of the chemical and physicochemical properties. On a broader series of amines of the ion <b>1</b>^<b>-</b> we have thus observed significant differences in the acidity of the amino groups, depending on whether these are attached to the carbon or boron atoms of the cage, or the C-substituted amines contain an aliphatic linker of variable length. The compounds are relevant for potential use as cobalt bis(dicarbollide) structural blocks in medicinal chemistry and material science. Our study includes single-crystal X-ray diffraction (XRD) structures of both amines and a discussion of their stereochemical and structural features.