Hypervalent hydridosilicates: synthesis, structure and hydride bridging

A range of hydridosilicate anions has been prepared and characterised by spectroscopic, structural and computational methods. The general approach involved reaction of KH with a neutral silane precursor in the presence of [18]crown-6. In this manner, [K([18]crown-6)](+) salts of [Ph3SiH2](-) (1), [Ph3SiF2](-) (9), and [(p-FC6H4)(3)SiHF]-/[(p-FC6H4)(3)SiH2](-) (12) were stabilised and characterized by NMR spectroscopy and X-ray diffraction. In each case, the anion adopts a trigonal bipyramidal (TBP) geometry with three equatorial phenyl groups eclipsing the axial Si-H/Si-F bonds. The Si-H center dot center dot center dot K distances, along with DFT calculations on 1, indicate an electrostatic interaction that does not dictate the geometry adopted by the anion. A [(H2SiOPr3)-Pr-i](-) salt (7) has also been crystallised in the same way; X-ray diffraction shows in this case a distorted TBP array with axial hydride ligands, and both Si-H center dot center dot center dot K and Si-O center dot center dot center dot K interactions. H-1 NMR exchange experiments show 1 to undergo facile hydride exchange with Ph3SiH. Compound 1 acts as a good hydride transfer reagent to a variety of substrates, but its high reactivity often results in redistribution and other side reactions.