Triethylsilyl Perfluoro-Tetraphenylborate, [Et₃Si⁺][F₂₀-BPh₄⁻], a widely used Non-Existent Compound

Abstract

The commonly used triethylsilyl per-fluoro-tetraphenylborate salt, [Et₃Si⁺][F₂₀-BPh₄⁻], has been misidentified. As prepared, the cation is a hydride-bridged silane adduct [R₃Si-H-SiR₃⁺]. Under favorable circumstances it can be an effective source of the triethylsilylium ion Et₃Si⁺ but in the absence of a stabilizing base the potent electrophilicity of Et₃Si⁺ decomposes the “inert” F₂₀-BPh₄⁻ counterion.

Over the past decade, R₃Si⁺ silylium ion chemistry has progressed from controversy over their existence^1–6 to an exploitation of their potent electrophilicity in stoichiometric reactions⁷ and, most impressively, in catalytic chemistry.⁸ These advances owe much to the availability of very weakly coordinating anions,^9,10 particularly carborane anions⁷ and the perfluorinated tetraphenylborate anion¹¹ (abbrev. F₂₀-BPh₄⁻ or PFTPB⁻).

A frequently used reagent in this silylation chemistry is the so-called triethylsilyl perfluorotetraphenylborate salt [Et₃Si⁺][F₂₀-BPh₄⁻], first reported by Lambert.^12,13 It is a versatile reagent for the abstraction of halide ions from transition metal complexes and from main group element compounds to give reactive, coordinatively unsaturated cations with F₂₀-BPh₄⁻ counterions. As liquid clathrates or “swirls”, such salts have high effective solubilities in low dielectric solvents making them very suitable for cationic catalysis. The reagent is readily prepared by reaction of the trityl salt of the F₂₀-BPh₄⁻ anion with triethylsilane, both starting materials being commercially available (Eq. 1).

$${\text{Et}}_3\text{SiH}+[{\text{Ph}}_3{\text{C}}^{+}][{\text{PFTPB}}^{-}]\to [{\text{Et}}_3{\text{Si}}^{+}][{\text{PFTPB}}^{-}]+{\text{Ph}}_3\text{CH}$$ (1)

Given that all solvents, even those as weakly coordinating as benzene,¹³ dichlorobenzene and liquid SO₂¹⁴ coordinate to the Et₃Si⁺ ion forming [Et₃Si(solvent)]⁺ cations, [Et₃Si⁺][F₂₀-BPh₄⁻] was prepared in the “absence” of solvent, or more correctly, using excess triethylsilane reagent as solvent.¹³ Evaporation of the excess silane gave a white powder which was characterized by CPMAS NMR as [Et₃Si⁺][F₂₀-BPh₄⁻]. The lack of any detectable difference in the ¹⁹F NMR of the anion between the presumed [Et₃Si⁺][F₂₀-BPh₄⁻] and the corresponding trityl ion salt of F₂₀-BPh₄⁻ led to the conclusion that the F₂₀-BPh₄⁻ anion was not coordinated to silicon.¹³ Lending some support to this ionic formulation is the more recent report of the X-ray crystal structure of the Me₃Si⁺ moiety partnered with the most weakly coordinating carborane anion, the undeca-fluorinated RCB₁₁F₁₁⁻ ion.¹⁵ The sum of the three C-Si-C angles in the Me₃Si moiety in Me₃Si(RCB₁₁F₁₁) is 354.4°, only ca. 6° short of the planarity expected for a Me₃Si⁺ ion. The Si atom is 0.25 Å out of the plane of the three C atoms towards the carborane anion. While perhaps not strictly ionic, this compound is certainly ion-like.

However, during routine characterization of the presumed [Et₃Si⁺][F₂₀-BPh₄⁻] salt we noticed that its ATR-IR spectrum contained an unanticipated strong absorption at ca. 1900 cm⁻¹. We had noticed this band earlier while characterizing the hydride-bridged disilyl cation [Et₃Si-H-SiEt₃]⁺ as a CHB₁₁Cl₁₁⁻ carborane salt and assigned it to ν_asSiHSi.¹⁴ Even the distinctive shape of the band was reproduced (see Fig. 1). It is immediately evident that the so-called silylium ion salt [Et₃Si⁺][F₂₀-BPh₄⁻] salt is, in fact, isolated as a silane adduct thereof: [Et₃Si-H-SiEt₃][F₂₀-BPh₄]. Once again, the solvent is coordinated to the silylium ion (Eq. 2).

Fig. 1.

ATR IR spectra of [Et₃Si-H-SiEt₃]⁺Y⁻ for Y = F₂₀-BPh₄⁻ (red) and CHB₁₁Cl₁₁⁻ (black) showing the distinctive ν_asSiHSi band near 1900 cm⁻¹.

$$2{\text{Et}}_3\text{SiH}+{\text{Ph}}_3\text{C}[{\text{F}}_{20}-{\text{BPh}}_4]\to [{\text{Et}}_3\text{Si}-\text{H}-{\text{SiEt}}_3][{\text{F}}_{20}-{\text{BPh}}_4]+{\text{Ph}}_3\text{CH}$$ (2)

The same result is obtained whether pure silane or a silane/hexane mixture is used as solvent, as long as at least 2 equiv. of silane are present. When the formation reaction is carried out with only one equivalent of Et₃SiH, ¹H NMR indicates that only half of the trityl ion is consumed. The reaction appears to be general. When trimethyl- and tri-i-propylsilane are substituted for triethylsilane, the products have the same characteristic IR band of a [R₃Si-H-SiR₃]⁺ cation (see Supp. Info.). The products have the correct elemental analyses for [R₃Si-H-SiR₃][F₂₀-BPh₄] salts. In addition, the presence of a displaceable equivalent of volatile silane was established by ¹H NMR for the isolated triethylsilane product by dissolving it in benzene and integrating [Et₃Si(benzene)]⁺ against a hexamethylbenzene internal standard.

This finding offers a note of caution to those using so-called “[Et₃Si][F₂₀-BPh₄]” in synthesis.^16–33 When prepared using excess silane as the sole solvent (or when using excess silane mixed with a non-coordinating solvent for the Et₃Si⁺ ion, e.g. an alkane) the isolated product will be [Et₃Si-H-SiEt₃][F₂₀-BPh₄] not [Et₃Si][F₂₀-BPh₄]. Because of its higher molecular weight, stoichiometries in subsequent metathesis reactions may therefore be miscalculated. Fortunately, the “extra” equivalent of silane is usually displaced during a silylation reaction and escapes the reaction as an innocent volatile byproduct. Indeed, those^22–33 who pre-dissolve [Et₃Si-H-SiEt₃][F₂₀-BPh₄] in a solvent that is coordinating to the Et₃Si⁺ ion, e.g. arenes, ethers etc., will displace the silane forming R₃Si(solvent)⁺ cations as the active silylating agents. Silane may be seen bubbling off.

These observations beg the question “Does [Et₃Si][F₂₀-BPh₄] really exist?” The answer appears to be no. When a film of [Et₃Si-H-SiEt₃][F₂₀-BPh₄] is deposited on the windows of an evacuable IR cell and pumped at 10⁻⁶ torr for 6 hours, no loss of intensity of the ν_asSiHSi band is observed, i.e., silane is not removed from the [Et₃Si-H-SiEt₃]⁺ cation under high vacuum at room temperature. When heated under vacuum to 65 °C the colorless product turns black and tris(pentafluorophenyl)boron³⁴ is an identifiable sublimate. The cleavage of a pentafluorophenyl group from the F₂₀-BPh₄⁻ anion is consistent with a growing number of reports that this anion is unstable towards strong electrophiles including H⁺,³⁵ “naked” Ag⁺,³⁶ and R₃Si⁺.¹⁷ Nevertheless, there is one report of [Me₃Si][F₂₀-BPh₄] as a colorless solid with Mp. 137 °C, no IR band in the region expected for ν_asSiHSi, and an acceptable C,H elemental analysis.³⁷ When we repeated the described synthetic procedure we observed only the formation of [Me₃Si-H-SiMe₃][F₂₀ -BPh₄].³⁸

In summary, there is no evidence for the existence of [Et₃Si][F₂₀-BPh₄]. The “as prepared” material is the silane adduct [Et₃Si-H-SiEt₃][F₂₀-BPh₄] and our attempts to remove the silane under vacuum lead to decomposition of the F₂₀-BPh₄⁻ anion. Only halogenated carborane anions are stable to the fierce electrophilicity of the Et₃Si⁺ silylium ion.^7,15,39 A truly free trialkylsilylium ion exists only in the gas phase.

ABBREVIATIONS

F₂₀-BPh₄⁻

perfluorinated tetraphenylborate anion