Bis(triphenyl-λ5-phosphanylidene)ammonium fluoride: a reactive fluoride source to access the hypervalent silicates [MenSiF5−n]− (n = 0–3)

Abstract

A new synthesis of bis(triphenyl-λ⁵-phosphanylidene)ammonium fluoride ((Ph₃PNPPh₃)F, abbreviated as (PNP)F), is described. The title compound has been fully characterized by multinuclear NMR spectroscopy, vibrational spectroscopy, elemental analysis and single crystal and powder X-ray diffraction for the first time. In the solid state (PNP)F exists as a covalent molecular compound, in which the fluoride ion is asymmetrically bonded to the two phosphorus atoms of the [PNP]⁺ cation. The phosphorus–fluorine bond with 181.98(13) pm is surprisingly long and the longest P–F bond in any phosphorane. (PNP)F can be assumed to be a very good source of reactive fluoride. To investigate the fluoride ion donating properties, (PNP)F was reacted with a range of different fluoromethylsilanes Me_nSiF_4−n (n = 0–4). Reactions of (PNP)F with the fluoromethylsilanes were performed in aceto- or propionitrile and in 1,2-dimethoxyethane under inert conditions. The resulting hypervalent fluoromethylsilicates [Me_nSiF_5−n]⁻ (n = 0–3) were fully characterized by multinuclear NMR and vibrational spectroscopy and single crystal X-ray diffraction. From the reaction of (PNP)F with Me₄Si in acetonitrile, the starting materials were recovered unchanged. To aid the understanding of the experimental results the fluoride ion affinities (FIA) for these silanes have been calculated by DFT calculations on the PBE0/def2-TZVPP level of theory. The fluoride ion affinity in the series of Me_nSiF_4−n (n = 0–4) decreases with the number of methyl groups and is too low for Me₄Si to bind a fluoride ion under these reaction conditions.