Hypervalent neutral O-donor ligand complexes of silicon tetrafluoride, comparisons with other group 14 tetrafluorides and a search for soft donor ligand complexes

Abstract

The hypervalent adducts of SiF₄, trans-[SiF₄(R₃PO)₂] (R = Me, Et or Ph), cis-[SiF₄{R₂P(O)CH₂P(O)R₂}] (R = Me or Ph), cis-[SiF₄(pyNO)₂] and trans-[SiF₄(DMSO)₂] have been prepared from SiF₄ and the ligands in anhydrous CH₂Cl₂, and characterised by microanalysis, IR and VT multinuclear (¹H, ¹⁹F, ³¹P) NMR spectroscopy. The NMR studies show extensive dissociation at ambient temperatures in non-coordinating solvents, but mixtures of cis and trans isomers of the monodentate ligand complexes were identified at low temperatures. Crystal structures are reported for trans-[SiF₄(R₃PO)₂] (R = Me or Ph), and cis-[SiF₄(pyNO)₂]. The GeF₄ analogues cis-[GeF₄{R₂P(O)(CH₂)_nP(O)R₂}] (R = Me or Ph, n = 1; R = Ph, n = 2) were similarly characterised and the structures of cis-[GeF₄{R₂P(O)CH₂P(O)R₂}] (R = Me or Ph) determined. The reaction of R₃AsO (R = Me or Ph) with SiF₄ does not give simple adducts, but forms [R₃AsOH]⁺ cations as fluorosilicate salts. SiF₄ adducts of some ether ligands (including THF, 12-crown-4) were also characterised by ¹⁹F NMR spectroscopy in solution at low temperatures (∼190 K), but are fully dissociated at room temperature. Attempts to isolate, or even to identify, SiF₄ adducts with phosphine or thioether ligands in solution at 190 K were unsuccessful, contrasting with the recent isolation and detailed characterisation of GeF₄ analogues. The chemistry of SiF₄ with these oxygen donor ligands, and with soft donors (P, As, S or Se), is compared and contrasted with those of GeF₄, SnF₄ and SiCl₄. The key energy factors determining stability of these complexes are discussed.