Synthesis of tris- and tetrakis(pentafluoroethyl)silanes

A neutral germanium-centred hard and soft lewis superacid and its unique reactivity towards hydrosilanes

The germanium-centred Lewis superacid Ge(pinF)2 (1) was isolated as acetonitrile mono-adduct 1·MeCN and thoroughly characterized by NMR spectroscopy, X-ray crystallography and quantum chemical calculations. Ion abstraction and NMR experiments revealed the hard as well as soft Lewis superacidic nature of 1·MeCN. The title compound readily activates hydrosilanes such as Et3SiH, which is not feasible for its harder silicon homologue 2·MeCN, and even reacts with Et3SiF. The strongly coordinating acetonitrile could be abstracted by B(C6F5), giving the donor-free Ge(pinF)2 (1) and Si(pinF)2 (2) which are Lewis superacids. Unlike 1·MeCN, the donor-free 1 efficiently catalyses hydrosilylation of α-methylstyrene by Et3SiH. For this process, an inverse temperature dependence was observed, i.e. a complete conversion was achieved rapidly when the reaction was cooled to -35 °C, but the reaction stopped at elevated temperatures. Mechanistic investigations, including stoichiometric experiments and quantum chemical calculations, outlined the formation of germylene Ge(pinF) (3), which acts as the active catalyst. The germylene is formed by reductive elimination of the silylated pinacol from the hydrogermane intermediate, which is obtained by the initial reaction of 1 with Et3SiH. The inverse temperature dependence of the catalytic reaction could be explained by low entropy associated with the complexation of two cooperating germylenes and the substrates. With this example we introduce an in situ generated Lewis acidic germylene complex for catalytic hydrosilylation of olefins and again exemplify the great potential of main-group-element-based complexes in catalysis.

Bis(perfluoropinacolato)silane: A Neutral Silane Lewis Superacid Activates Si-F Bonds

Despite the earth abundance and easy availability of silicon, only few examples of isolable neutral silicon centered Lewis superacids are precedent in the literature. To approach the general drawbacks of limited solubility and unselective deactivation pathways, we introduce a Lewis superacid, based on perfluorinated pinacol substituents. The compound is easily synthesized on a gram‐scale as the corresponding acetonitrile mono‐adduct 1⋅(MeCN) and was fully characterized, including single crystal X‐ray diffraction analysis (SC‐XRD) and state‐of‐the‐art computations. Lewis acidity investigations by the Gutmann‐Beckett method and fluoride abstraction experiments indicate a Lewis superacidic nature. The challenging Si−F bond activation of Et₃SiF is realized and promising catalytic properties are demonstrated, consolidating the potential applicability of silicon centered Lewis acids in synthetic catalysis.

Salts of Tris(pentafluoroethyl)silylchalcogenolates [Si(C2F5)3E]- with E = S, Se, and Te: Synthesis, Structure, and Reactivity

Unlike silanolates [SiR₃O]^- (R = alkyl, aryl), which have been intensely studied, the heavier derivatives [SiR₃E]^- (E = S, Se, Te) have been much less examined. Among such species, virtually nothing is known about perfluoroalkyl-substituted silylchalcogenolates. In this contribution, a convenient synthesis of tris(pentafluoroethyl)silylchalcogenolate salts [{(Et₂N)₃P═N}₃PN(H)^tBu][Si(C₂F₅)₃E] (E = S, Se, Te; ^tBu = tert-butyl) is presented. All representatives were isolated and fully characterized by multinuclear NMR spectroscopy, IR spectroscopy, mass spectrometry, elemental analysis, and X-ray diffraction studies. Furthermore, first reactivity studies of these novel species toward selected metal halide complexes were performed. In this course, metal complexes [HgPh{SSi(C₂F₅)₃}] (2) and [Au(PPh₃){SSi(C₂F₅)₃}] (3) were isolated and characterized.

Tris(pentafluoroethyl)silanol Derivatives and the Lewis Amphoteric Tris(pentafluoroethyl)silanolate Anion, [Si(C2 F5 )3 O]. Chemistry 2021;27:11041-11044. [PMID: 34061416 PMCID: PMC8453507 DOI: 10.1002/chem.202101845]

While alkyl‐substituted siloxanes are widely known, virtually nothing is known about perfluoroalkyl siloxanes and their congener species, the silanols and silanolates. We recently reported on the tris(pentafluoroethyl)silanide ion, [Si(C₂F₅)₃]⁻, which features Lewis amphoteric character deriving from the pentafluoroethyl substituents and their strong electron‐withdrawing properties. Transferring this knowledge, we investigated the Lewis amphoteric behavior of the tris(pentafluoroethyl)silanolate, [Si(C₂F₅)₃O]⁻. In order to examine such Lewis amphoteric behavior, we first developed a strategy for the synthesis of the corresponding silanol Si(C₂F₅)₃OH, which readily condenses at room temperature to the hexakis(pentafluoroethyl)disiloxane, (C₂F₅)₃SiOSi(C₂F₅)₃. Deprotonation of Si(C₂F₅)₃OH employing a sterically demanding phosphazene base allows the characterization of the first example of a dimeric triorganosilanolate: the dianionic hexakis(pentafluoroethyl)disilanolate, [{Si(C₂F₅)₃O}₂]²⁻, implies Lewis amphoteric character of the monomeric [Si(C₂F₅)₃O]⁻ anion.

Bis(pertrifluoromethylcatecholato)silane: Extreme Lewis Acidity Broadens the Catalytic Portfolio of Silicon

Given its earth abundance, silicon is ideal for constructing Lewis acids of use in catalysis or materials science. Neutral silanes were limited to moderate Lewis acidity, until halogenated catecholato ligands provoked a significant boost. However, catalytic applications of bis(perhalocatecholato)silanes were suffering from very poor solubility and unknown deactivation pathways. In this work, the novel per(trifluoromethyl)catechol, H₂ cat^CF3 , and adducts of its silicon complex Si(cat^CF3 )₂ (1) are described. According to the computed fluoride ion affinity, 1 ranks among the strongest neutral Lewis acids currently accessible in the condensed phase. The improved robustness and affinity of 1 enable deoxygenations of aldehydes, ketones, amides, or phosphine oxides, and a carbonyl-olefin metathesis. All those transformations have never been catalyzed by a neutral silane. Attempts to obtain donor-free 1 attest to the extreme Lewis acidity by stabilizing adducts with even the weakest donors, such as benzophenone or hexaethyl disiloxane.

Evidence of the Lewis-Amphoteric Character of Tris(pentafluoroethyl)silanide, [Si(C2 F5 )3 ]. Angew Chem Int Ed Engl 2021;60:12124-12131. [PMID: 33617080 PMCID: PMC8252080 DOI: 10.1002/anie.202016455]

According to a first view on the geometrical and electronic structure of the tris(pentafluoroethyl)silanide, this anion appears as a Lewis base. Quantum chemical calculations on perfluoroalkylated silanides show significantly lower HOMO and LUMO energy levels in comparison to their non-fluorinated counterparts, which implies reduced Lewis basicity and increased Lewis acidity of the [Si(C2 F5 )3 ]- ion. With these findings and a HOMO-LUMO gap of 4.80 eV similar to N-heterocyclic silylenes (NHSis), perfluoroalkyl silanides are predestined to exhibit Lewis-amphoteric character similar to silylenes. Deprotonation of Si(C2 F5 )3 H with sterically demanding phosphazene bases afforded thermally stable phosphazenium salts of the [Si(C2 F5 )3 ]- anion, which add to benzaldehyde, benzophenone, CS2 , and CO2 in various manners. This behavior also mirrors the reactivity of silylenes towards ketones as well as heterocumulenes and is rationalized by Lewis amphotericity being inherent in these silanides.

Synthesis and Characterization of Tetrakis(pentafluoroethyl)aluminate

While perfluorinated aryl, aryloxy and alkoxy aluminum species are well-established as weakly coordinating anions (WCAs), corresponding perfluoroalkyl aluminum derivatives are virtually unknown. Reaction of Si(C2 F5 )3 CH3 with Li[AlH4 ] afforded the tetrakis(pentafluoroethyl)aluminate, [Al(C2 F5 )4 ]- . Several salts of the [Al(C2 F5 )4 ]- ion were synthesized and characterized by NMR spectroscopic methods, mass spectrometry, X-ray diffraction studies and elemental analysis.

Pentafluoroethylaluminates: A Combined Synthetic, Spectroscopic, and Structural Study

Salts of the tetrakis(pentafluoroethyl)aluminate anion [Al(C2 F5 )4 ]- were obtained from AlCl3 and LiC2 F5 . They were isolated with different counter-cations and characterized by NMR and vibrational spectroscopy and mass spectrometry. Degradation of the [Al(C2 F5 )4 ]- ion was found to proceed via 1,2-fluorine shifts and stepwise loss of CF(CF3 ) under formation of [(C2 F5 )4-n AlFn ]- (n=1-4) as assessed by NMR spectroscopy and mass spectrometry and supported by results of DFT calculations. In addition, the [(C2 F5 )AlF3 ]- ion was structurally characterized.

Bis(catecholato)silanes: assessing, rationalizing and increasing silicon's Lewis superacidity

Although bis(catecholato)silanes have been known for several decades, their substantial Lewis acidity is not yet well described in the literature. Herewith, the synthesis and characterization of multiple substituted bis(catecholato)silanes and their triethylphosphine oxide, fluoride and chloride ion adducts are reported. The Lewis acidity of bis(catecholato)silanes is assessed by effective (Gutmann-Beckett, catalytic efficiency), global (theoretical and relative experimental fluoride (FIA) and chloride (CIA) ion affinities) and intrinsic (electrophilicity index) scaling methods. This comprehensive set of experimental and theoretical results reveals their general Lewis acidic nature and provides a consistent Lewis acidity trend for bis(catecholato)silanes for the first time. All experimental findings are supported by high-level DLPNO-CCSD(T) based thermochemical data and the Lewis acidity is rationalized by complementary chemical bonding analysis tools. Against the common belief that inductive electron withdrawal is the most important criterion for strong Lewis acidity, the present work highlights the decisive role of π-back bonding effects in aromatic ring systems to enhance electron deficiency. Thus, bis(perbromocatecholato)silane is identified and synthesized as the new record holder for silicon Lewis superacids.

Synthesis of a Perfluorinated Phenoxyphosphorane and Conversion to Its Hexacoordinate Anions

We report the synthesis and structural characterization of a neutral P^V Lewis acid, P(OC₆ F₅ )₅ , and salts containing the six-coordinate anions [P(OC₆ F₅ )₅ F]^- and [P(OC₆ F₅ )₆ ]^- . The latter anion exhibits a rare example of F-π_arene interactions in both the solid and the solution phase, which has been quantitatively studied by variable-temperature (VT) NMR spectroscopy. The Lewis acid strength of P(OC₆ F₅ )₅ has been assessed through experimental fluoride ion competition experiments and quantum-chemical calculations of its fluoride ion affinity (FIA) and global electrophilicity index (GEI). Our findings highlight the importance of considering solvent effects in electrophilicity calculations, even when neutral Lewis acids are involved, and show a rare divergence between FIA and GEI trends. The coordinating abilities of the [P(OC₆ F₅ )₆ ]^- and [P(OC₆ F₅ )₅ F]^- anions towards the trityl cation, as a prototypical electrophile, have been assessed.

Pentafluoroethylated Compounds of Silicon, Germanium and Tin

In this contribution we present an account on pentafluoroethylated compounds of silicon, germanium and tin. The pronounced electron-withdrawing effect of the pentafluoroethyl group leads to a markedly increased Lewis acidity at the central atom which results in the stabilization of hypervalent complexes, anionic element(II) species as well as remarkable reactivities of element-element and element-hydrogen bonds. By addition to unsaturated C-C bonds or by reaction with organic halides as well as transition-metal complexes the molecules bearing a pentafluoroethyl-element group are readily accessible. Moreover, the utilization of pentafluoroethyl groups facilitates the formation of donor-stabilized germylenes and stannylenes. A series of such compounds serves as suitable pentafluoroethylation reagents. Conversely to the well-studied trifluoromethyl derivatives these compounds frequently exhibit a higher thermostability, which allows a more convenient handling.

Activation of Acetonitrile with (C2 F5 )3 PF2 and Amines

Acetonitrile is deprotonated by a combination of the strong Lewis acid (C₂ F₅ )₃ PF₂ and triethylamine. The resulting cyanomethyl function is bound to the phosphorus moiety, affording the highly stable salt [HNEt₃ ][P(C₂ F₅ )₃ F₂ (CH₂ CN)]. Salt metathesis reactions furnished the corresponding [Cu(MeCN)₂ ]⁺ and [Ag(MeCN)]⁺ derivatives in which the CH₂ CN substituent of the anion [P(C₂ F₅ )₃ F₂ (CH₂ CN)]^- coordinates to the metal. An investigation of the gas separation capability of the silver salt [Ag(MeCN)][P(C₂ F₅ )₃ F₂ (CH₂ CN)] shows an uptake of 1.7 equivalents of isobutene from a propane/isobutene mixture. The reaction of (C₂ F₅ )₃ PF₂ with acetonitrile and diethylamine furnishes [P(C₂ F₅ )₃ F₂ {NHC(CH₃ )NEt₂ }]-a phosphate featuring an amidine ligand which formally results from hydroamination of acetonitrile by HNEt₂ . Exchange of HNEt₂ with the primary amines H₂ NPh and H₂ NBu leads to comparable amidine salts that exhibit a solvent-dependent conformational isomerism.

Bis(perchlorocatecholato)silane-A Neutral Silicon Lewis Super Acid

No neutral silicon Lewis super acids are known to date. We report on the synthesis of bis(perchlorocatecholato)silane and verify its Lewis super acidity by computation (DLPNO-CCSD(T)) and experiment (fluoride abstraction from SbF₆^- ). The exceptional affinity towards donors is further demonstrated by, for example, the characterization of an unprecedented SiO₄ F₂ dianion and applied in the first hydrodefluorination reaction catalyzed by a neutral silicon Lewis acid. Given the strength and convenient access to this new Lewis acid, versatile applications might be foreseen.

Nucleophilic Transfer Reactions of the [Si(C2 F5 )3 ]- Moiety

The tris(pentafluoroethyl)silanide anion is accessible by the deprotonation of Si(C₂ F₅ )₃ H at low temperatures. Subsequent quenching of the resulting salt-like compounds with suitable electrophiles, such as transition-metal complexes or Group 14 element halides, leads to a plethora of novel tris(pentafluoroethyl)silane derivatives. This underlines the versatility of Li[Si(C₂ F₅ )₃ ] as a powerful nucleophilic transfer reagent.

The Tris(pentafluoroethyl)silanide Anion

Tris(pentafluoroethyl)silane, which is conveniently accessible by the treatment of Si(C₂ F₅ )₃ X (X=Cl, Br) with Bu₃ SnH, was successfully employed for hydrosilylation reactions. In the presence of a palladium catalyst, hydrosilylation of phenylacetylene with Si(C₂ F₅ )₃ H affords the product of an α-addition whereas the reaction of trimethylsilylacetylene with the silane leads to the β-trans product. Tris(pentafluoroethyl)silane can be deprotonated by sterically demanding bases such as lithium diisopropylamide at low temperatures to give the corresponding silanide ion. The addition of crown ethers or cryptands to this highly reactive species enabled the isolation and characterization of salt-like tris(pentafluoroethyl)silanide at room temperature.

Hypervalent Pentafluoroethylgermanium Compounds, [(C2 F5 )n GeX5-n ]- and [(C2 F5 )3 GeF3 ]2- (X=F, Cl; n=2-5)

This paper gives an account on hypervalent fluoro- and chloro(pentafluoroethyl)germanium compounds. The selective synthesis of the tris(pentafluoroethyl)dichlorogermanate salt [PNP][(C₂ F₅ )₃ GeCl₂ ] as well as its X-ray structural analysis is described. As a representative example for pentafluoroethylfluorogermanates, the synthesis and structure of 2,4,6-triphenylpyryliumtris(pentafluoroethyl)difluorogermanate [C₂₃ H₁₇ O][(C₂ F₅ )₃ GeF₂ ] is reported. Fluoride-ion affinities for pentafluoroethylgermanes were calculated using quantum chemical methods, disclosing (C₂ F₅ )₃ GeF as a weaker Lewis acid than (C₂ F₅ )₃ SiF or (C₂ F₅ )₃ PF₂ . The theoretical results were confirmed by experiments and give the basis of a synthetic protocol for (C₂ F₅ )₃ GeF. Pentakis(pentafluoroethyl)germanate [PPh₄ ][Ge(C₂ F₅ )₅ ] was detected as an intermediate during the synthesis of [PPh₄ ][(C₂ F₅ )₄ GeF] starting from tris(pentafluoroethyl)difluorogermanate and LiC₂ F₅ .

Synthesis of Functional Bis(pentafluoroethyl)silanes (C2 F5 )2 SiX2 , with X=H, F, Cl, Br, OPh, and O2 CCF3

As recently shown, the introduction of pentafluoroethyl functionalities into silicon compounds is of general interest due to an enhanced Lewis acidity of the resulting species. By this means, the synthesis of previously inaccessible hypervalent silicon derivatives is enabled. While an easy access to tris(pentafluoroethyl)silanes has already been published, synthetic strategies for the selective preparation of bis derivatives are yet unknown. In this contribution, a convenient protocol for the synthesis of functional bis(pentafluoroethyl)silicon compounds is presented. These compounds represent precursors for the synthesis of pentafluoroethylated polysiloxanes. Furthermore, they prove to be resistant to oxonium cations, which is a key feature for the preparation of stable pentafluoroethylsilic acids. Treatment of dichlorodiphenoxysilane with in situ generated pentafluoroethyl lithium leads to the corresponding bis(pentafluoroethyl)silane in high yields. (C₂ F₅ )₂ Si(OPh)₂ serves as a starting material for further functionalized bis(pentafluoroethyl)silanes. These silanes have been isolated and their reactivity towards N bases studied. The pronounced Lewis acidity of the obtained compounds has been documented by the formation of octahedral adducts with nitrogen donors such as 1,10-phenanthroline and acetonitrile.

Synthesis of chlorosilicates

Chlorosilicates represent important intermediates in S(N)2 reactions of chlorosilanes. They can be stabilized by the introduction of electron-withdrawing substituents. Salts of various (pentafluoroethyl)chlorosilicates have been isolated and structurally characterized.