Boron(8) substituted nitrilium and ammonium derivatives, versatile cobalt bis(1,2-dicarbollide) building blocks for synthetic purposes

Abstract

The reaction of the cobalt bis(dicarbollide)(1−)(1) ion in the presence of t-butylbromide, acting as a potent Lewis acid activator, leads to the clean substitution of 1 by the N-atom of acetonitrile (or benzonitrile), thus resulting in the smooth formation of [(8-RCN)-1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)-3,3′-Co(III)]⁰ (R = CH₃ or C₆H₅) (2 and 3). These compounds can serve as versatile precursors for the generation of a variety of other synthetically useful functional groups. The nitrogen atom of the nitrile CN– bond in 2 and 3 is prone to the facile addition of nucleophiles. Thus, alkaline hydrolysis of 2 and 3 in aqueous alcohols furnishes high yields of the corresponding B(8)-N-alkylamides [(8-RC(O)=NH)-1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)-3,3′-Co(III)]⁻, where R = CH₃ or C₆H₅ (4 and 5). The reactions with butyl and diethyl amine result in the high-yield formation of 8-alkylamidine derivatives [(8-³R²RNH-¹RCN)-1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)-3,3′-Co(III)]⁰, where ¹R = CH₃, ²R = H, ³R = C₄H₉, (6), ¹R = C₆H₅, ²R = ³R = C₂H₅ (7) or ¹R = C₆H₅, ²R = ³R = C₂H₅ (8). Hydrazinolysis of 2 provides a high yield of the zwitterionic [(8-H₃N)-1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)-3,3′-Co(III)]⁰ (9), whereas reduction of the nitrile group in 2 and 3 using BH₃·SMe₂ affords the respective alkylammonium derivatives [(8-RH₂N)-1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)-3,3′-Co(III)]⁰, where R = –C₂H₅ (10) or –CH₂C₆H₅ (11) in moderate yields. Compound 11 can also be prepared by alkylation of 9 along with dibenzylderivative (12). A further example of alkylation is the ring cleavage of the 3-hydroxypropanesulfonic acid γ-sultone producing a compound with alkyl ammonium sulfonic acid substitution [(8-O₃SC₃H₆NH₂)-1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)-3,3′-Co(III)]⁻ (13) All new compounds were characterized by multinuclear NMR spectroscopy and mass spectrometry and the structures of compounds 2, 6, 7 and 12 were established by X-ray crystallography.