R3C−H→SiFR3 Agostic Interaction
datasetposted on 14.07.2008, 00:00 by Christopher L. Dorsey, François P. Gabbaï
As part of our investigations of main group complexes with unusual bridging interactions, we have set out to determine if fluorosilane Lewis acids could form σ-complexes with alkanes. In order to probe this possibility, we have synthesized the cationic fluorosilane 1-(dimethylfluorosilane)-8-(9-xanthylium)naphthalenediyl (+) as a tetrafluoroborate salt and converted it into 1-(dimethylfluorosilane)-8-(9H-xanthene)naphthalenediyl (3) by reaction with NaBH4. Both [BF4] and 3 have been fully characterized. Examination of the structure of 3 indicates the presence of an interaction involving the C−H bond at the 9-position of the xanthene unit and the silicon atom. This interaction, which is characterized by a Si−H separation of 2.32(2) Å and a F−Si−H angle of 177.0(5)°, leads the silicon atom to adopt a distorted trigonal-bipyramidal geometry. The nature of this interaction has been investigated experimentally by NMR and IR spectroscopy as well as computationally using density functional calculations, atom in molecules, and natural bond order analyses. These combined experimental and computational results suggest that the short Si−H contact in 3 corresponds to an agostic C−H → Si interaction.