Radical Substitution at Boron in Small Cobaltacarboranes. Conversion of η⁵-C₅Me₅ to η⁴-C₅Me₅R Complexes¹

As an alternative approach to metal-promoted cross-coupling reactions for the preparation of B-substituted organo derivatives of small cobaltacarboranes, synthesis via radical substitution reactions has been explored. Reduction of neutral Cp*Co^III(2,3-Et₂C₂B₄H₃-5-I) (1) and Cp*Co^III(2,3-Et₂C₂B₄H₄) (4) to generate the respective 19-electron anionic Co(II) complexes 1^- and 4^-, followed by radical reactions with nucleophiles and electrophiles, led to substitution at boron and/or the Cp* (η⁵-C₅Me₅) ligand. Reaction of 1 with Rieke Mg* in THF formed Cp*Co(Et₂C₂B₄H₃)-5-O(CH₂)₄-(η⁴-C₅Me₅)CoH(Et₂C₂B₄H₃-5-I) (2), whose two cobaltacarborane units are linked through a tetramethyleneoxy linking group. In contrast, 1^- and 4^- reacted with MeOSO₂CF₃ to afford (η⁴-C₅Me₆)CoH(Et₂C₂B₄H₃-5-I) (3) or (η⁴-C₅Me₆)CoH(Et₂C₂B₄H₃-5-Me) (5) and (η⁴-C₅Me₆)CoH(Et₂C₂B₄H₄) (6). The somewhat reactive 1^- was stabilized with the introduction of 18-crown-6 to form the paramagnetic salt K(18-crown-6)⁺Cp*Co^II(Et₂C₂B₄H₃-5-I)^- (10). Treatment of 1 and 10 with the nucleophile LiNMe₂ in the presence of K(Hg) gave respectively Cp*Co(Et₂C₂B₄H₃-5-NMe₂) (8) and Cp*Co(Et₂C₂B₄H₃-7-NMe₂) (11). Compound 8 was decapped on exposure to air to give Cp*Co(Et₂C₂B₃H₄-5-NMe₂) (9); its apically substituted isomer 11 cannot undergo deborylation. A tert-butoxide group was introduced at B(5) in moderate yield by reaction of K O-t-Bu with 1^-, affording 12. Treatment of 12 with acidified methanol produced the B(5)−OH derivative 14. X-ray diffraction analyses confirmed the structures of 2, 10, and 11.