Synthesis, Properties, and Application of Tetrakis(pentafluoroethyl)gallate, [Ga(C 2 F 5 ) 4 ] −

First Main-Group Element Lewis Acid Thionyl Chloride Adduct and its Chemistry

Although the adduct of aluminum trichloride with thionyl chloride has been reported, no thionyl chloride adduct of a main group element Lewis acid or organometallic compound has been structurally characterized. In this communication we present the synthesis and reactivity of the structurally ascertained adduct of thionyl chloride with tris(pentafluoroethyl)gallane as a representative of a main group element Lewis acid. Gallium and indium compounds with electron withdrawing groups, e.g. the pentafluoroethyl ligand, display versatile properties. While gallates and indates, [MR4]-, behave as weakly coordinating anions, neutral gallanes and indanes, MR3, are strong Lewis acids. Salts with the tetrakis(pentafluoroethyl)gallate and -indate, [M(C2F5)4]- (M=Ga, In), have recently been studied in detail. In contrast to this, work on the syntheses of the free Lewis superacids M(C2F5)3 (M=Ga, In) is scarce and underdeveloped. The hydrates [M(C2F5)3(OH2)2] proved to be suitable starting materials, particularly due to their thermal stability. Herein we report on synthesis and characterization of reactive adducts, [M(C2F5)3D], with the weak donor molecules (D) SOCl2 and Me3SiF. The effective Lewis acidities of Ga(C2F5)3 and In(C2F5)3 were experimentally determined by the (modified) Gutmann-Beckett method and their catalytic potential is showcased.

A Rare Example of a Gallium-Based Lewis Superacid: Synthesis and Reactivity of Ga(OTeF5)3

The Lewis superacid Ga(OTeF5)3 has been synthesized and characterized, revealing a monomeric structure in solution and a dimeric structure in the solid state. Isolated adducts of Ga(OTeF5)3 with strong and weak Lewis bases have been characterized spectroscopically as well as by single-crystal X-ray diffractometry. The Lewis acidity of this new species has been evaluated by means of different experimental and theoretical methods, which has allowed to classify it as one of only a few examples of a gallium-based Lewis superacid. The high Lewis acidity of Ga(OTeF5)3 was used to amplify the strength of the Brønsted acid HOTeF5, leading to the protonation of diethyl ether. Furthermore, Ga(OTeF5)3 was utilized to access the strong oxidizing system Ga(OTeF5)3/Xe(OTeF5)2 in SO2ClF, which was successfully employed in the synthesis of the dimethyl chloronium salt [Cl(CH3)2][Ga(OTeF5)4], a strong electrophilic methylation reagent.

Chlorodefluorination Induced by Gallium-Based Lewis Acids

The reaction of [Ga(C2F5)3(FSiMe3)] with chlorotrimethylsilane leads to the selective substitution of the α-fluorine atoms by chlorine atoms and thus to the formation of [Ga(CCl2CF3)3(FSiMe3)]. The corresponding chlorogallate ion, [Ga(CCl2CF3)3Cl]-, could be isolated and structurally characterized as its tetraphenylphosphonium salt. The catalytic potential of [Ga(CCl2CF3)3(FSiMe3)] was demonstrated via the chlorodefluorination of Me3SiCF3, 2,2-difluoropropane and [PPh4][Ga(C2F5)4].

Carbon-Based Weakly Coordinating Anions: Molecular Design, Synthesis and Applications

Although carbanions, which are usually regarded as reactive species and powerful metalation reagents, can be stabilized through choice of the substitution pattern, they have rarely been considered for the design of weakly coordinating anions (WCA). Here, we report on an evaluation of the potential of a series of differently substituted carbanions to serve as WCA by computational methods. This led us to the synthesis of the water- and air-stable allyl anion 1 with triflyl and 3,5-bis(trifluoromethyl)phenyl (ArF) moieties, which can be isolated in high yields even on a gram-scale. Single crystal X-ray crystallography and NMR studies confirmed the weak coordination ability of the anion by showing negligible or only weak interactions with different cations. This property enabled the application of 1 in the stabilization of reactive group 14 and 15 cations. In addition to the crystallization of a phosphenium cation, the first all-carbon salt with a non-aromatic carbanion is reported, which revealed to be a convenient reagent for hydride abstraction such as from silanes. Overall, this work demonstrates the so far untapped potential of carbanions as WCA, that are accessible with a variety of different cations for various applications.

Lewis Acid Decorated Hexacyanodiborane(6) Dianion

First examples of diborane(6) dianions decorated with weakly coordination B(C6F5)3 (BCF) groups and SiEt3 + moieties have been synthesized demonstrating the synthetic potential of the [B2(CN)6]2- dianion. [B2{CNB(C6F5)3}6]2- (1) was isolated as potassium and tetrabutylammonium salt. 1 is a rare example for a weakly coordinating dianion and it was used for the stabilization of the carbocation [Ph3C]+ and the oxonium acid [H(OEt2)2]+. Reaction of [Ph3C]21 with HSiEt3 resulted in the silylated neutral diborane(6) [B2{CNB(C6F5)3}4(CNSiEt3)2] (2) in which two BCF groups have been selectively replaced by SiEt3 + substituents, underscoring the stability and chemical versatility of the [B2(CN)6]2- dianion. The chemical properties and physicochemical data of 1 and 2 provide insight into electronic, coordinating, and steric properties of theses novel diborane(6) compounds.

The perfluoroadamantoxy aluminate as an ideal weakly coordinating anion? - synthesis and first applications

Weakly coordinating anions (WCAs) facilitate the stabilization and isolation of highly reactive and almost "naked" cations. Alkoxyaluminate-based WCAs such as [Al(OC(CF₃)₃)₄]^- ([pf]^-) are widely used due to their synthetic accessibility and their high stability. However, small cations are still able to coordinate the oxygen atoms of the [pf]^- anion or even to abstract an alkoxy ligand. The novel WCA [Al(OC₁₀F₁₅)₄]^- ([pfAd]^-; OC₁₀F₁₅ = perfluoro-1-adamantoxy) is characterized by a very rigid core framework, thus indicating a higher stability towards fluoride-ion abstraction (DFT calculations) and providing hope to generate less disordered crystal structures. The [pfAd]^- anion was generated by the reaction of the highly acidic alcohol perfluoro-1-adamantanol C₁₀F₁₅OH with LiAlH₄ in o-DFB. Li[pfAd] could not be synthesized free of impurities (and still contains unreacted alcohol). Yet, starting from contaminated Li[pfAd], the very useful pure salts Ag[pfAd], [Ph₃C][pfAd] and [H(OEt₂)₂][pfAd] could be synthesized. The salts were characterized by NMR spectroscopy, single-crystal X-ray diffraction and IR spectroscopy. Additionally, [NO][pfAd] could be synthesized containing alcohol impurities but nonetheless enabled the synthesis of the salt P₉⁺[pfAd]^-. The synthesis of Tl[pfAd] in a mixture of H₂O/acetone/o-DFB demonstrated the water stability of the [pfAd]^- anion.

An easy-to-perform evaluation of steric properties of Lewis acids

Steric and electronic effects play a very important role in chemistry, as these effects influence the shape and reactivity of molecules. Herein, an easy-to-perform approach to assess and quantify steric properties of Lewis acids with differently substituted Lewis acidic centers is reported. This model applies the concept of the percent buried volume (%V _Bur) to fluoride adducts of Lewis acids, as many fluoride adducts are crystallographically characterized and are frequently calculated to judge fluoride ion affinities (FIAs). Thus, data such as cartesian coordinates are often easily available. A list of 240 Lewis acids together with topographic steric maps and cartesian coordinates of an oriented molecule suitable for the SambVca 2.1 web application is provided, together with different FIA values taken from the literature. Diagrams of %V _Bur as a scale for steric demand vs. FIA as a scale for Lewis acidity provide valuable information about stereo-electronic properties of Lewis acids and an excellent evaluation of steric and electronic features of the Lewis acid under consideration. Furthermore, a novel LAB-Rep model (Lewis acid/base repulsion model) is introduced, which judges steric repulsion in Lewis acid/base pairs and helps to predict if an arbitrary pair of Lewis acid and Lewis base can form an adduct with respect to their steric properties. The reliability of this model was evaluated in four selected case studies, which demonstrate the versatility of this model. For this purpose, a user-friendly Excel spreadsheet was developed and is provided in the ESI, which works with listed buried volumes of Lewis acids %V _{Bur_LA} and of Lewis bases %V _{Bur_LB}, and no results from experimental crystal structures or quantum chemical calculations are necessary to evaluate steric repulsion in these Lewis acid/base pairs.

Gallium(III)- and Indium(III)-Containing Ionic Liquids as Highly Active Catalysts in Organic Synthesis

The chemical industry still requires development of environmentally friendly processes. Acid-catalysed chemical processes may cause environmental problems. Urgent need to replace conventional acids has forced the search for sustainable alternatives. Metal-containing ionic liquids have drawn considerable attention from scientists for many years. These compounds may exhibit very high Lewis acidity, which is usually dependent on the composition of the ionic liquid with the particular content of metal salt. Therefore, metal-containing ionic liquids have found a lot of applications and are successfully employed as catalysts, co-catalysts or reaction media in various fields of chemistry, especially in organic chemistry. Gallium(III)- and indium(III)-containing ionic liquids help to transfer the remarkable activity of metal salts into even more active and easier-to-handle forms of ionic liquids. This review highlights the wide range of possible applications and the high potential of metal-containing ionic liquids with special focus on Ga(III) and In(III), which may help to outline the framework for further development of the presented research topic and synthesis of new representatives of this group of compounds.

Synthesis and Characterization of Tetrakis(pentafluoroethyl)indate Salts

In the rising field of organoindium chemistry little is known about the perfluoroorganyl-substituted indium compounds. The increasing use of indium reagents is explained by their high stability and tolerance towards water and functional groups. Here we report on the synthesis of the novel tetrakis(pentafluoroethyl)indate, [In(C₂ F₅ )₄ ]^- , and its characterization in salts with the cations [Li(thf)₃ ]⁺ , Cs⁺ , [PPh₄ ]⁺ and [H₁₄ O₆ ]²⁺ (thf=tetrahydrofuran). To the best of our knowledge, these salts constitute the first perfluoroalkylindates and, in general, the first isolated perfluoroalkylindium compounds which are fully characterized by NMR spectroscopy, mass spectrometry, elemental analysis and X-ray diffraction analysis.

Repurposing of F-gases: challenges and opportunities in fluorine chemistry

Fluorinated gases (F-gases) are routinely employed as refrigerants, blowing agents, and electrical insulators. These volatile compounds are potent greenhouse gases and consequently their release to the environment creates a significant contribution to global warming. This review article seeks to summarise: (i) the current applications of F-gases, (ii) the environmental issues caused by F-gases, (iii) current methods of destruction of F-gases and (iv) recent work in the field towards the chemical repurposing of F-gases. There is a great opportunity to tackle the environmental and sustainability issues created by F-gases by developing reactions that repurpose these molecules.

Pentafluoroethylation of Carbonyl Compounds by HFC-125 via the Encapsulation of the K Cation with Glymes

A simple protocol to overcome the explosive pentafluoroethylation of carbonyl compounds by HFC-125 is described. The use of potassium (K) bases with triglyme or tetraglyme as a solvent safely yields the pentafluoroethylation products in good to high yields. The experimental results suggest that an encapsulation of the K cation by glymes as K(glyme)₂ inhibits the contact between the K cation and the reactive anionic pentafluoroethyl counterion, preventing their transformation into KF and explosive tetrafluoroethylene (TFE). The generation of sterically demanding [K(G3)₂]⁺ and [K(G4)₂]⁺ is an effective way as an unstable pentafluoroethyl anion reservoir.

Bis(perchlorocatecholato)germane: Hard and Soft Lewis Superacid with Unlimited Water Stability

Previously described Lewis superacids are moisture sensitive and predominantly hard in character-features that severely limit their widespread use in orbital-controlled reactions and under non-inert conditions. Described here are adducts of bis(perchlorocatecholato)germane, the first hard and soft Lewis superacid based on germanium. Remarkably, the synthesis of this compound is performed in water, and the obtained H2 O adduct constitutes a strong Brønsted acid. If applied as an adduct with aprotic donors, it displays excellent activity in a diverse set of Lewis acid catalyzed transformations, covering hydrosilylation, hydrodefluorination, transfer hydrogenation, and carbonyl-olefin metathesis. Given the very straightforward synthetic access from two commercially available precursors, the unlimited water stability and the soft Lewis acidic character, it promotes the transfer of Lewis superacidity into organic synthesis and materials science.

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.

Synthesis and Application of a Perfluorinated Ammoniumyl Radical Cation as a Very Strong Deelectronator

The perfluorinated dihydrophenazine derivative (perfluoro-5,10-bis(perfluorophenyl)-5,10-dihydrophenazine) ("phenazine^F ") can be easily transformed to a stable and weighable radical cation salt by deelectronation (i.e. oxidation) with Ag[Al(OR^F )₄ ]/ Br₂ mixtures (R^F =C(CF₃ )₃ ). As an innocent deelectronator it has a strong and fully reversible half-wave potential versus Fc⁺ /Fc in the coordinating solvent MeCN (E°'=1.21 V), but also in almost non-coordinating oDFB (=1,2-F₂ C₆ H₄ ; E°'=1.29 V). It allows for the deelectronation of [Fe^III Cp*₂ ]⁺ to [Fe^IV (CO)Cp*₂ ]²⁺ and [Fe^IV (CN-^t Bu)Cp*₂ ]²⁺ in common laboratory solvents and is compatible with good σ-donor ligands, such as L=trispyrazolylmethane, to generate novel [M(L)_x ]ⁿ⁺ complex salts from the respective elemental metals.

Hydrated Metal Ion Salts of the Weakly Coordinating Fluoroanions PF6-, TiF62-, B12F122-, Ga(C2F5)4-, B(3,5-C6H3(CF3)2)4-, and Al(OC(CF3)3)4-. In Search of the Weakest HOH···F Hydrogen Bonds

FTIR spectra of microcrystalline samples of 11 metal ion salt hydrates of a variety of weakly coordinating fluoroanions are reported. The compounds studied were Li(H₂O)₄(Al(OC(CF₃)₃)₄), Li(H₂O)(B(3,5-C₆H₃(CF₃)₂)₄), Li(H₂O)_n(Ga(C₂F₅)₄), Li(H₂O)(PF₆), Li₂(H₂O)₂(TiF₆), Li₂(H₂O)₄(B₁₂F₁₂), Na(H₂O)(PF₆), Na₂(H₂O)₂(B₁₂F₁₂), K₂(H₂O)₂(B₁₂F₁₂), Rb₂(H₂O)₂(B₁₂F₁₂), Cs₂(H₂O)(B₁₂F₁₂), and their partially or completely deuterated isotopologs and isotopomers. The O-D···F hydrogen bonds in Li(HOD)(H₂O)₃(Al(OC(CF₃)₃)₄) (ν(OD) = 2706 cm^-1), Li(HOD)(B(3,5-C₆H₃(CF₃)₂)₄) (ν(OD) = 2705 cm^-1), and Li(HOD)(H₂O)_n(Ga(C₂F₅)₄) (ν(OD) = 2697 cm^-1) rival HOD absorbed in polyvinylidene difluoride (ν(OD) = 2696 cm^-1) and HOD···FCH₃ in a frozen Ar matrix (ν(OD) = 2685 cm^-1) for the weakest hydrogen bonds between a water molecule and an F atom in any compound. As weak as they are, minor differences in O-H···F hydrogen bonds in the same fluoroanion salt can be distinguished spectroscopically. Uncoupled HOD molecules in asymmetric F···HOD···F' hydrogen bonding environments in Rb⁺, Cs⁺, Mg²⁺, and Co²⁺ hydrates of B₁₂F₁₂^2- gave rise to two observable ν(OD) bands even though the two R(O···F) distances differ by only 0.010(4) Å (Mg²⁺), 0.033(2) Å (Co²⁺), 0.074(4) Å (Rb⁺), and 0.106(6) Å (Cs⁺). A plot of ν(OD) for hydrates with a single uncoupled HOD molecule per metal ion (e.g., Li(HOD)(H₂O)₃(Al(OC(CF₃)₃)₄)) vs R(O···F) distance from single-crystal X-ray or neutron diffraction structures was prepared. The ν(OD) values range from 2305 to 2706 cm^-1 and the R(O···F) distances range from 2.58 to 3.17 Å. The plot consists of 53 {ν(OD), R(O···F)} data points, 23 of which are new and have ν(OD) > 2600 cm^-1, in contrast to a 1994 ν(OD) vs R(O···F) plot with 28 data points, none of which had ν(OD) > 2600 cm^-1. There is a clear and significant difference between the new ν(OD) vs R(O···F) plot and a literature ν(OD) vs R(O···O) plot for hydrates containing O-D···O hydrogen bonds. For a given ν(OD) stretching frequency, the exponential regression curves show that R(O···F) is typically 0.1-0.2 Å shorter than R(O···O), in harmony with the lower basicity and smaller size of F atoms vs O atoms.

Generation and Applications of the Hydroxide Trihydrate Anion, [OH(OH2 )3 ]- , Stabilized by a Weakly Coordinating Cation

The reaction of a strongly basic phosphazene (Schwesinger base) with water afforded the corresponding metastable hydroxide trihydrate [OH(OH₂)₃]⁻ salt. This is the first hydroxide solvate that is not in contact with a cation and furthermore one of rare known water‐stabilized hydroxide anions. Thermolysis in vacuum results in the decomposition of the hydroxide salt and quantitative liberation of the free phosphazene base. This approach was used to synthesize the Schwesinger base from its hydrochloride salt after anion exchange in excellent yields of over 97 %. This deprotonation method can also be used for the phosphazene‐base‐catalyzed preparation of the Ruppert–Prakash reagent Me₃SiCF₃ using fluoroform (HCF₃) as the trifluoromethyl building block and sodium hydroxide as the formal deprotonation agent.

Oxygen-Bridged Ga2 (Et)3 (OTeF5 )3 and the Weakly Coordinating Anions [Ga(Et)(OTeF5 )3 ]- and [Ga(OTeF5 )4 ]. Chemistry 2019;25:10441-10449. [PMID: 31090983 DOI: 10.1002/chem.201901651]

Salts of the weakly coordinating anions [Ga(OTeF₅ )₄ ]^- as well as [Ga(Et)(OTeF₅ )₃ ]^- and the neutral Ga₂ (Et)₃ (OTeF₅ )₃ were synthesized and characterized by spectroscopic methods and single-crystal X-ray diffraction. Ga₂ (Et)₃ (OTeF₅ )₃ was formed by treating GaEt₃ with pentafluoroorthotelluric acid (HOTeF₅ ) and reacted with PPh₄ Cl and CPh₃ Cl to [PPh₄ ][Ga(Et)(OTeF₅ )₃ ] and [CPh₃ ][Ga(Et)(OTeF₅ )₃ ]. In contrast, Ag[Ga(OTeF₅ )₄ ] was prepared from AgOTeF₅ and GaCl₃ and was used as a versatile starting material for further reactions. Starting with Ag[Ga(OTeF₅ )₄ ] the substrates [PPh₄ ][Ga(OTeF₅ )₄ ] and [CPh₃ ][Ga(OTeF₅ )₄ ] were formed from PPh₄ Cl and CPh₃ Cl.