Synthetic and structural studies on C-ethynyl- and C-bromo-carboranes

Abstract

A high-yield preparation of the C-monoethynyl para-carborane, 1-Me₃SiCC-1,12-C₂B₁₀H₁₁, from C-monocopper para-carborane and 1-bromo-2-(trimethylsilyl)ethyne, BrCCSiMe₃ is reported. The low-yield preparation of 1,12-(Me₃SiCC)₂-1,12-C₂B₁₀H₁₀ from the C,C′-dicopper para-carborane derivative with 1-bromo-2-(trimethylsilyl)ethyne, BrCCSiMe₃, has been re-investigated and other products were identified including the C-monoethynyl-carborane 1-Me₃SiCC-1,12-C₂B₁₀H₁₁ and two-cage assemblies generated from cage–cage couplings. The contrast in the yields of the monoethynyl and diethynyl products is due to the highly unfavourable coupling process between 1-RCC-12-Cu-1,12-C₂B₁₀H₁₀ and the bromoalkyne. The ethynyl group at the cage carbon C(1) strongly influences the chemical reactivity of the cage carbon at C(12)—the first example of the ‘antipodal effect’ affecting the syntheses of para-carborane derivatives. New two-step preparations of 1-ethynyl- and 1,12-bis(ethynyl)-para-carboranes have been developed using a more readily prepared bromoethyne, 1-bromo-3-methyl-1-butyn-3-ol, BrCCCMe₂OH. The molecular structures of the two C-monoethynyl-carboranes, 1-RCC-1,12-C₂B₁₀H₁₁ (R = H and Me₃Si), were experimentally determined using gas-phase electron diffraction (GED). For R = H (R_G = 0.053) a model with C_5v symmetry refined to give a CC bond distance of 1.233(5) Å. For R = Me₃Si (R_G = 0.048) a model with C_s symmetry refined to give a CC bond distance of 1.227(5) Å. Molecular structures of 1,12-Br₂-1,12-C₂B₁₀H₁₀, 1-HCC-12-Br-1,12-C₂B₁₀H₁₀ and 1,12-(Me₃SiCC)₂-1,12-C₂B₁₀H₁₀ were determined by X-ray crystallography. Substituents at the cage carbon atoms on the C₂B₁₀ cage skeleton in 1-X-12-Y-1,12-C₂B₁₀H₁₀ derivatives invariably lengthen the cage C–B bonds. However, the subtle substituent effects on the tropical B–B bond lengths in these compounds are more complex. The molecular structures of the ethynyl-ortho-carborane, 1-HCC-1,2-C₂B₁₀H₁₁ and the ethene, trans-Me₃SiBrCCSiMe₃Br are also reported.