Synthetic routes to carbon substituted cobalt bis(dicarbollide) alkyl halides and aromatic amines along with closely related irregular pathways

Abstract

Carbon substituted cobalt bis(dicarbollide) alkyl halides [(1-X-(CH₂)_n-1,2-C₂B₉H₁₀)(1,2-C₂B₉H₁₁)-3,3′-Co]Me₄N (X = Br, I; n = 1–3) are prepared in high yields (>90%) from their corresponding alcohols without side skeletal substitutions. These species offer access to the synthesis of aromatic cobalt bis(dicarbollide) amines, however only for particular terminal halogen substitution, the propylene pendant arm, and under appropriately controlled reaction conditions. Thus, the compounds substituted at cage carbon atoms with a propylene linker and terminal aromatic amine groups could be prepared. In other cases, numerous irregular reaction pathways occur, undoubtedly as a consequence of the bulky anionic boron cage in close proximity to the reaction site. Among them, an unusual intramolecular hydroboration forming rigidified carbon-to-boron bridged isomeric anions with an asymmetric structure that correspond to formulae [(1,8′-μ-C₂H₄)-(1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₀)-3,3′-Co]⁻ and [(1,7′-μ-C₂H₄)-(1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₀)-3,3′-Co]⁻ is described herein and the former isomer is structurally characterized. This product with a restrained geometry is widely accessible through nucleophile and/or thermally induced decomposition of (pseudo)halides attached to the cage via an ethylene linker. Surprisingly enough, also doubly bridged isomeric species [(1,8-μ-C₂H₄-1,2-C₂B₉H₉)₂-3,3′-Co]⁻ and [(1,7-μ-C₂H₄-1,2-C₂B₉H₉)₂-3,3′-Co]⁻ are available in good yield using these methods. Furthermore, other more typical side reactions are discussed, i.e. nucleophilic reactions of propyl halides with Me₃N formed apparently by disproportionation of Me₄N⁺ at higher temperatures or with pyridine used as a base.