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

Mark A. Fox*, Audrey M. Cameron, Paul J. Low, Michael A.J. Paterson, Andrei S. Batsanov, Andrés E. Goeta, David W.H. Rankin, Heather E. Robertson, Julien T. Schirlin

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

A high-yield preparation of the C-monoethynyl para-carborane, 1-Me 3SiCC-1,12-C2B10H11, from C-monocopper para-carborane and 1-bromo-2-(trimethylsilyl)ethyne, BrCCSiMe 3 is reported. The low-yield preparation of 1,12-(Me 3SiCC)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-Me 3SiCC-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 (RG = 0.053) a model with C5v symmetry refined to give a CC bond distance of 1.233(5) Å. For R = Me3Si (RG = 0.048) a model with Cs symmetry refined to give a CC bond distance of 1.227(5) Å. Molecular structures of 1,12-Br2-1,12-C2B10H 10, 1-HCC-12-Br-1,12-C2B10H10 and 1,12-(Me3SiCC)2-1,12-C2B10H 10 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-C2B 10H11 and the ethene, trans-Me3SiBrCCSiMe 3Br are also reported.

Original languageEnglish
Pages (from-to)3544-3560
Number of pages17
JournalDalton Transactions
Issue number29
DOIs
Publication statusPublished - 27 Apr 2006

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