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Crystallographic Evidence for a Free Silylium Ion

Science  02 Aug 2002:
Vol. 297, Issue 5582, pp. 825-827
DOI: 10.1126/science.1073540

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Abstract

Evidence for a three-coordinate silyl cation is provided by the crystal structure of [(Mes)3Si][H-CB11Me5Br6]·C6H6(where Mes is 2,4,6-trimethylphenyl). Free (Mes)3Si+ cations are well separated from the carborane anions and benzene solvate molecules. Ortho-methyl groups of the mesityl substituents shield the silicon atom from the close approach of nucleophiles, while remaining innocent as significant ligands themselves. The silicon center is three-coordinate and planar. The downfield 29Si nuclear magnetic resonance chemical shift in the solid state (226.7 parts per million) is almost identical to that in benzene solution and in “gas phase” calculations, indicating that three-coordination can be preserved in all phases.

  • * To whom correspondence should be addressed. E-mail: chris.reed{at}ucr.edu (C.A.R.); jlambert{at}northwestern.edu(J.B.L.)

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