Metallacarboranes as building blocks for polyanionic polyarmed aryl-ether materials

Polyanionic species have been obtained in high yield by a new route in the ring-opening reaction of cyclic oxonium [3,3′-Co(8-C₄H ₈O₂-1,2-C₂B₉H₁₀) (1′,2′-C₂B₉H₁₁)] (2) by using carboxylic acids, Grignard reagents, and thiocarboranes as nucleophiles. The crystal structures of Na₃(H₂O)(C₂H ₅OH)[1″,3″,5″-{3,3′-Co(8-O(CH ₂CH₂O)₂-1,2-C₂B₉H ₁₀)(1′,2′-C₂B₉H₁₁)} ₃-C₆H₃] and Na(H₂O)[3,3′-Co(8- O(CH₂CH₂O)₂C(O)CH₃-1,2-C ₂B₉H₁₀)(1′,2′-C₂B ₉H₁₁)] show that the chain contributes three or two oxygen atoms for coordination to Na⁺, and interestingly, the [3,3′-Co(1,2-C₂B₉H₁₁)₂] ^- moiety provides extra B-H coordination sites. These B-H⋯Na interactions in the solid state have also been confirmed by dynamic NMR studies in solution. These new polyanionic compounds that contain multiple carborane or metallacarborane clusters at their periphery may prove useful as new classes of boron neutron capture therapy compounds with enhanced water solubility and as a core to make a new class of dendrimers.