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)₃SiHF]^–/[(p-FC₆H₄)₃SiH₂]^– (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–HK distances, along with DFT calculations on 1, indicate an electrostatic interaction that does not dictate the geometry adopted by the anion. A [H₂SiOⁱPr₃]^– 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–HK and Si–OK interactions. ¹H 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.