Hypervalent Pentafluoroethylgermanium Compounds, [(C2F5)nGeX5−n]− and [(C2F5)3GeF3]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][(C₂F₅)₃GeCl₂] 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 [C₂₃H₁₇O][(C₂F₅)₃GeF₂] is reported. Fluoride‐ion affinities for pentafluoroethylgermanes were calculated using quantum chemical methods, disclosing (C₂F₅)₃GeF as a weaker Lewis acid than (C₂F₅)₃SiF or (C₂F₅)₃PF₂. The theoretical results were confirmed by experiments and give the basis of a synthetic protocol for (C₂F₅)₃GeF. Pentakis(pentafluoroethyl)germanate [PPh₄][Ge(C₂F₅)₅] was detected as an intermediate during the synthesis of [PPh₄][(C₂F₅)₄GeF] starting from tris(pentafluoroethyl)difluorogermanate and LiC₂F₅.     

Tris(pentafluoroethyl)silanol Derivatives and the Lewis Amphoteric Tris(pentafluoroethyl)silanolate Anion, [Si(C2F5)3O]−

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.     

Synthesis of Five‐ and Six‐Coordinate Tris(pentafluoroethyl)fluorosilicates

The research area of perfluoroalkylsilanes is still in its infancy. Although there are already many examples of difluorotriorganylsilicates, the first example of a completely characterized trifluorotriorganylsilicate is presented, the dianion [Si(C₂F₅)₃F₃]^2?. The strongly electron‐withdrawing influence of the pentafluoroethyl groups appears to be a fundamental cause of the stability of this compound. This dianion is also the first structurally characterized example of a tris(pentafluoroethyl)silicon compound. The synthesis and complete characterization of [PPh₄]₂[Si(C₂F₅)₃F₃] and [PPh₄][Si(C₂F₅)₃F₂] along with the precursor [H(OEt₂)₂][Si(C₂F₅)₃F₂] was achieved from SiCl₄ and LiC₂F₅.     

Synthesis of Functional Bis(pentafluoroethyl)silanes (C2F5)2SiX2, with X=H,F, Cl,Br, OPh,and O2CCF3

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. 1 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. 2 Furthermore, they prove to be resistant to oxonium cations, which is a key feature for the preparation of stable pentafluoroethylsilic acids. 3 Treatment of dichlorodiphenoxysilane with in situ generated pentafluoroethyl lithium leads to the corresponding bis(pentafluoroethyl)silane in high yields. (C₂F₅)₂Si(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 Tris(pentafluoroethyl)germanes

The synthesis of tris(pentafluoroethyl)germanium derivatives is described. The reaction of germanium tetrachloride with three equivalents of the pentafluoroethylation reagent LiC₂F₅ does not lead selectively to the formation of tris(pentafluoroethyl)chlorogermane, (C₂F₅)₃GeCl. Here the introduction of a diethylamino function as a protecting group was beneficial. Thus, treatment of Cl₃GeNEt₂ with LiC₂F₅ smoothly afforded (C₂F₅)₃GeNEt₂. The replacement of the amino substituent by halides was accomplished by reaction with HBr or HCl on a multigram scale. The combination of (C₂F₅)₃GeCl with Ag₂CO₃ gave rise to the formation of the digermoxane [(C₂F₅)₃Ge]₂O. An obtuse Ge‐O‐Ge angle of 150.2(1)° was determined by X‐ray diffraction. Attempted hydrolysis of the digermoxane leads to an equilibrium mixture of the precursor, (C₂F₅)₃GeOH, and water.     

Nucleophilic Transfer Reactions of the [Si(C2F5)3]− Moiety

The tris(pentafluoroethyl)silanide anion is accessible by the deprotonation of Si(C₂F₅)₃H 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(C₂F₅)₃] as a powerful nucleophilic transfer reagent.     

The New Fluoro‐tris(pentafluoroethyl)borate Anion [(C2F5)3BF]—, Unexpected Formation of [(C2F5)3BH]— as a By‐Product. Das neue Fluoro‐tris(pentafluoroethyl)borat Anion [(C2F5)3BF]—, unerwartete Bildung des Nebenprodukts [(C2F5)3BH]—

Dimethylamine‐tris(pentafluoroethyl)borane (C₂F₅)₃B · NHMe₂ ( 1 ) has been obtained from C₂F₅I, Br₂BNMe₂ and tris(diethylamino)phosphane in sulfolane. Alkylation using CH₃I/KOH yielded the trimethylamine‐tris(pentafluoroethyl)borane (C₂F₅)₃B · NMe₃ ( 2 ). Compound 2 reacts with NEt₃×3HF at 200—204 °C under replacement of the trimethylamine ligand to form the novel fluoro‐tris(pentafluoroethyl)borate anion [(C₂F₅)₃BF]^— ( 3 ) in good yield. Side products are the hydroxy‐tris(pentafluoroethyl)borate [(C₂F₅)₃BOH]^— ( 4 ) and the hydrido‐tris(pentafluoroethyl)borate [(C₂F₅)₃BH]^— ( 5 ).     

Replacement of Trimethylamine in Trimethylamine‐trispentafluoroethyl‐borane. Structures of [(C2F5)3BNMe3], [NMe4][(C2F5)3BOH], and K[(C2F5)3BCH2(CO)C6H5]

Trimethylamine‐tris(pentafluoroethyl)borane [(C₂F₅)₃BNMe₃] ( 1 ) reacts at 190 °C with water under displacement of the trimethylamine ligand to yield the hydroxy‐tris(pentafluoroethyl)borate [(C₂F₅)₃BOH]^? ( 2 ). In tributylamine 1 reacts with alkynes HC≡CR to form novel ethynyl‐tris(pentafluoroethyl)borate anions [(C₂F₅)₃BC≡CR]^? – R = C₆H₅ ( 3 ), C₆H₄CH₃ ( 4 ), Si(CH(CH₃)₂)₃ ( 5 ) – in moderate yields. Compound 3 adds water across the triple bond to form the novel anion [(C₂F₅)₃BCH₂(CO)C₆H₅]^? ( 6 ). The structures of [(C₂F₅)₃BNMe₃], [NMe₄][(C₂F₅)₃BOH] and K[(C₂F₅)₃BCH₂(CO)C₆H₅] have been determined by x‐ray crystallography.     

The Tris(pentafluoroethyl)silanide Anion

Tris(pentafluoroethyl)silane, which is conveniently accessible by the treatment of Si(C₂F₅)₃X (X=Cl, Br) with Bu₃SnH, was successfully employed for hydrosilylation reactions. In the presence of a palladium catalyst, hydrosilylation of phenylacetylene with Si(C₂F₅)₃H 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.     

Synthesis,Properties, and Application of Tetrakis(pentafluoroethyl)gallate, [Ga(C2F5)4]−

Weakly coordinating anions (WCAs) are important for academic reasons as well as for technical applications. Tetrakis(pentafluoroethyl)gallate, [Ga(C₂F₅)₄]^?, a new WCA, is accessible by treatment of [GaCl₃(dmap)] (dmap=4‐dimethylaminopyridine) with LiC₂F₅. The anion [Ga(C₂F₅)₄]^? proved to be reluctant towards deterioration by aqueous hydrochloric acid or lithium hydroxide. Various salts of [Ga(C₂F₅)₄]^? were synthesized with cations such as [PPh₄]⁺, [CPh₃]⁺, [(O₂H₅)₂(OH₂)₂]²⁺, and [Li(dec)₂]⁺ (dec=diethyl carbonate). Thermolysis of [(O₂H₅)₂(OH₂)₂][Ga(C₂F₅)₄]₂ gives rise to a dihydrate of tris(pentafluoroethyl)gallane, [Ga(C₂F₅)₃(OH₂)₂]. All products were characterized by NMR and IR spectroscopy, mass spectrometry, X‐ray diffraction, and elemental analysis. Furthermore, an outlook for the application of [Li(dec)₂][Ga(C₂F₅)₄] as a conducting salt in lithium‐ion batteries is presented.     

Synthesis of Bis(pentafluoroethyl)germanes

The chemistry of bis(pentafluoroethyl)germanes (C₂F₅)₂GeX₂ is presented. The synthesis of such species requires Br₂GePh₂, wherein the phenyl substituents function as suitable protecting groups. After treatment with two equivalents of LiC₂F₅, (C₂F₅)₂GePh₂ is produced. The replacement of the phenyl rings is smoothly effected by gaseous HBr or HCl in the presence of a Lewis acidic catalyst. The trigermoxane [(C₂F₅)₂GeO]₃ results from the reaction of (C₂F₅)₂GeBr₂ with Ag₂CO₃. Its crystalline 1,10‐phenanthroline adduct was fully characterised by X‐ray diffraction. The combination of (C₂F₅)₂GeBr₂ with Bu₃SnH gave rise to the formation of (C₂F₅)₂GeH₂.     

Pentafluoroethylaluminates: A Combined Synthetic,Spectroscopic, and Structural Study

Salts of the tetrakis(pentafluoroethyl)aluminate anion [Al(C₂F₅)₄]⁻ were obtained from AlCl₃ and LiC₂F₅. They were isolated with different counter-cations and characterized by NMR and vibrational spectroscopy and mass spectrometry. Degradation of the [Al(C₂F₅)₄]⁻ ion was found to proceed via 1,2-fluorine shifts and stepwise loss of CF(CF₃) under formation of [(C₂F₅)_4−nAlF_n]⁻ (n=1–4) as assessed by NMR spectroscopy and mass spectrometry and supported by results of DFT calculations. In addition, the [(C₂F₅)AlF₃]⁻ ion was structurally characterized.     

Tuning the Lewis Acidity of Neutral Silanes Using Perfluorinated Aryl- and Alkoxy Substituents

The emerging field of Lewis acidic silanes demonstrates the versability of molecular silicon compounds for catalytic applications. Nevertheless, when compared to the multifunctional boron Lewis acid B(C₆F₅)₃, silicon derivatives still lack in terms of reactivity. In this regard, we demonstrate the installation of perfluorotolyl groups (Tol^F) on neutral silicon atoms to obtain the respective tetra- and trisubstituted silanes Si(Tol^F)₄ and HSi(Tol^F)₃. These compounds were fully characterized including SC-XRD analysis but unexpectedly showed no significant Lewis acidity. By using strongly electron-withdrawing perfluorocresolato groups (OTol^F) the tetrasubstituted silane Si(OTol^F)₄ was obtained, bearing an 8 % increased Δδ(³¹P) shift when applying the Gutmann-Beckett method, compared to literature-known Si(OPh^F)₄. Ultimately the heteroleptic Si(Ph^F)₂pin^F was successfully synthesized and fully characterized including SC-XRD analysis, introducing a highly Lewis acidic silicon atom holding two silicon-carbon bonds.     

Reactions of salicylaldehyde with tris(pentaf luorophenyl)silanes and secondary amines

Reactions of tris(pentafluorophenyl)silanes RSi(C₆F₅)₃ with salicylaldehyde and secondary amines were studied. The reactions afforded α-pentafluorophenyl-substituted amines. Silanes RSi(C₆F₅)₃ (R = Me, Ph, C₆F₅, CH₂CH=CH₂, and CH=CH₂) were found to be efficient reagents for transfer of the C₆F₅ group to the iminium cation generated from salicylaldehyde and amine. However, tris(pentafluorophenyl)phenylethynyl-and tris(pentafluorophenyl)silanes were not able to serve as a source of a fluorinated substituent because of competitive transfer of acetylenide fragment or hydride. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 498–503, March, 2006.     

Synthesis of Unsymmetrically Substituted Phosphane Oxides (R1R2P(O)H) and Phosphinous Acids (R1R2POH)

This paper describes the synthesis of unsymmetrically substituted phosphinous acids and phosphane oxides featuring at least one electron‐withdrawing pentafluoroethyl group. The presence of a diethylamino function as a protecting group allows a selective reaction of RClPNEt₂ (R=CF₃, C₆F₅, C₆H₅) with LiC₂F₅. On treatment with para‐toluenesulfonic acid the isolated aminophosphanes R(C₂F₅)PNEt₂ are readily converted into the corresponding phosphinous acids or phosphane oxides, respectively. Investigation of the tautomeric equilibrium between oxide and acid tautomer revealed (CF₃)(C₂F₅)POH as a stable phosphinous acid, whereas the pentafluorophenyl and phenyl derivatives constitute a solvent dependent equilibrium between the acid and the oxide tautomer.     

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(C₂F₅)₃CH₃ with Li[AlH₄] afforded the tetrakis(pentafluoroethyl)aluminate, [Al(C₂F₅)₄]⁻. Several salts of the [Al(C₂F₅)₄]⁻ ion were synthesized and characterized by NMR spectroscopic methods, mass spectrometry, X-ray diffraction studies and elemental analysis.     

A Neutral Germanium/Phosphorus Frustrated Lewis Pair and Its Contrasting Reactivity Compared to Its Silicon Analogue

Chlorogermane (C₂F₅)₃GeCl with very electronegative pentafluoroethyl groups was converted with LiCH₂P(tBu)₂ to obtain the intramolecular frustrated Lewis pair (FLP) (C₂F₅)₃GeCH₂P(tBu)₂, a neutral, germanium-based FLP. Its reactivity was compared to its silicon homologue (C₂F₅)₃SiCH₂P(tBu)₂. Both FLPs cleave NO but give cyclic (Si) and open-chain oxides (Ge). In reactions with HCl both FLPs gave the same adduct type in the solid state, while the proton seems more mobile in solution in the germanium case. Reactions with PhCNO and Me₃SiCHN₂ result in ring-type adducts. The structures of (C₂F₅)₃GeCH₂P(tBu)₂ and of five adducts with substrates were elucidated by X-ray diffraction. The study clearly showed the germanium compound to have a more moderate Lewis acidity compared to the silicon analogue.     

NHC→SiCl4: An Ambivalent Carbene‐Transfer Reagent

The addition of BCl₃ to the carbene‐transfer reagent NHC→SiCl₄ (NHC=1,3‐dimethylimidazolidin‐2‐ylidene) gave the tetra‐ and pentacoordinate trichlorosilicon(IV) cations [(NHC)SiCl₃]⁺ and [(NHC)₂SiCl₃]⁺ with tetrachloroborate as counterion. This is in contrast to previous reactions, in which NHC→SiCl₄ served as a transfer reagent for the NHC ligand. The addition of BF₃ ? OEt₂, on the other hand, gave NHC→BF₃ as the product of NHC transfer. In addition, the highly Lewis acidic bis(pentafluoroethyl)silane (C₂F₅)₂SiCl₂ was treated with NHC→SiCl₄. In acetonitrile, the cationic silicon(IV) complexes [(NHC)SiCl₃]⁺ and [(NHC)₂SiCl₃]⁺ were detected with [(C₂F₅)SiCl₃]^? as counterion. A similar result was already reported for the reaction of NHC→SiCl₄ with (C₂F₅)₂SiH₂, which gave [(NHC)₂SiCl₂H][(C₂F₅)SiCl₃]. If the reaction medium was changed to dichloromethane, the products of carbene transfer, NHC→Si(C₂F₅)₂Cl₂ and NHC→Si(C₂F₅)₂ClH, respectively, were obtained instead. The formation of the latter species is a result of chloride/hydride metathesis. These compounds may serve as valuable precursors for electron‐poor silylenes. Furthermore, the reactivity of NHC→SiCl₄ towards phosphines is discussed. The carbene complex NHC→PCl₃ shows similar reactivity to NHC→SiCl₄, and may even serve as a carbene‐transfer reagent as well.     

Tris(pentafluoroethyl)difluorophosphorane: A Versatile Fluoride Acceptor for Transition Metal Chemistry

Fluoride abstraction from different types of transition metal fluoride complexes [L_nMF] (M=Ti, Ni, Cu) by the Lewis acid tris(pentafluoroethyl)difluorophosphorane (C₂F₅)₃PF₂ to yield cationic transition metal complexes with the tris(pentafluoroethyl)trifluorophosphate counterion ( FAP anion, [(C₂F₅)₃PF₃]⁻) is reported. (C₂F₅)₃PF₂ reacted with trans-[Ni(iPr₂Im)₂(Ar^F)F] (iPr₂Im=1,3-diisopropylimidazolin-2-ylidene; Ar^F=C₆F₅, 1 a ; 4-CF₃-C₆F₄, 1 b ; 4-C₆F₅-C₆F₄, 1 c ) through fluoride transfer to form the complex salts trans-[Ni(iPr₂Im)₂(solv)(Ar^F)] FAP ( 2 a - c[solv] ; solv=Et₂O, CH₂Cl₂, THF) depending on the reaction medium. In the presence of stronger Lewis bases such as carbenes or PPh₃, solvent coordination was suppressed and the complexes trans-[Ni(iPr₂Im)₂(PPh₃)(C₆F₅)] FAP ( trans -2 a[PPh₃] ) and cis-[Ni(iPr₂Im)₂(Dipp₂Im)(C₆F₅)] FAP ( cis -2 a[Dipp₂Im] ) (Dipp₂Im=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were isolated. Fluoride abstraction from [(Dipp₂Im)CuF] ( 3 ) in CH₂Cl₂ or 1,2-difluorobenzene led to the isolation of [{(Dipp₂Im)Cu}₂]²⁺2 FAP⁻ ( 4 ). Subsequent reaction of 4 with PPh₃ and different carbenes resulted in the complexes [(Dipp₂Im)Cu(LB)] FAP ( 5 a – e , LB=Lewis base). In the presence of C₆Me₆, fluoride transfer afforded [(Dipp₂Im)Cu(C₆Me₆)] FAP ( 5 f ), which serves as a source of [(Dipp₂Im)Cu)]⁺. Fluoride abstraction of [Cp₂TiF₂] ( 7 ) resulted in the formation of dinuclear [FCp₂Ti(μ-F)TiCp₂F] FAP ( 8 ) (Cp=η⁵-C₅H₅) with one terminal fluoride ligand at each titanium atom and an additional bridging fluoride ligand.     

Boron Lewis Acid‐Catalyzed Hydroboration of Alkenes with Pinacolborane: BArF3 Does What B(C6F5)3 Cannot Do!

The transition‐metal‐free hydroboration of various alkenes with pinacolborane (HBpin) initiated by tris[3,5‐bis(trifluoromethyl)phenyl]borane (BAr^F₃) is reported. The choice of the boron Lewis acid is crucial as the more prominent boron Lewis acid tris(pentafluorophenyl)borane (B(C₆F₅)₃) is reluctant to react. Unlike B(C₆F₅)₃, BAr^F₃ is found to engage in substituent redistribution with HBpin, resulting in the formation of Ar^FBpin and the electron‐deficient diboranes [H₂BAr^F]₂ and [(Ar^F)(H)B(μ‐H)₂BAr^F₂]. These in situ‐generated hydroboranes undergo regioselective hydroboration of styrene derivatives as well as aliphatic alkenes with cis diastereoselectivity. Another ligand metathesis of these adducts with HBpin subsequently affords the corresponding HBpin‐derived anti‐Markovnikov adducts. The reactive hydroboranes are regenerated in this step, thereby closing the catalytic cycle.