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

A high-yield preparation of the C-monoethynyl para-carborane, 1-Me3SiC=C-1,12-C2B10H11, from C-monocopper para-carborane and 1-bromo-2-(trimethylsilyl) ethyne, BrC=CSiMe3 is reported. The low-yield preparation of 1,12-(Me3SiC=C)(2)-1,12-C2B10H10 from the C, C'-dicopper para-carborane derivative with 1-bromo-2-( trimethylsilyl) ethyne, BrC=CSiMe3, has been re-investigated and other products were identified including the C-monoethynyl-carborane 1-Me3SiC=C-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-RC=C-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, BrC=CCMe2OH. The molecular structures of the two C-monoethynyl-carboranes, 1-RC=C-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 C=C bond distance of 1.233(5) angstrom. For R= Me3Si (R-G = 0.048) a model with C-s symmetry refined to give a C=C bond distance of 1.227(5) angstrom. Molecular structures of 1,12-Br-2- 1,12-C2B10H10, 1-HC=C-12-Br-1,12-C2B10H10 and 1,12-(Me3SiC=C) 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-HC=C-1,2-C2B10H11 and the ethene, trans-Me3SiBrC=CSiMe3Br are also reported.