Hypervalent Silicon Compounds by Coordination of Diphosphine-Silanes to Gold

Coordination of ambiphilic diphosphine-silane ligands [o-(iPr(2)P)C6H4](2)Si(R)F (R=F, Ph, Me) to AuCI affords pentacoordinate neutral silicon compounds in which the metal atom acts as a Lewis base. X-ray diffraction analyses, NMR spectroscopy, and OFT calculations substantiate the presence of Au -> Si interactions in these complexes, which result in trigonal-bipyramidal geometries around silicon. The presence of a single electron-withdrawing fluorine atom is sufficient to observe coordination of the silane as a sigma-acceptor ligand, provided it is positioned trans to gold. The nature of the second substituent at silicon (R = F, Ph, Me) has very little influence on the magnitude of the Au -> Si interaction, in marked contrast to N -> Si adducts. According to variable-temperature and 2D EXSY NMR experiments, the apical/equatorial positions around silicon exchange in the slow regime of the NMR timescale. The two forms, with the fluorine atom in trans or cis position to gold, were characterized spectroscopically and the activation barrier for their interconversion was estimated. The bonding and relative stability of the two isomeric structures were assessed by DFT calculations.