Superacids Based on Pentafluoroorthotellurate Derivatives of Aluminum

Perfluorooxosulfate Salts as SOF4-Gas-Free Precursors to Multidimensional SuFEx Electrophiles

Sulfur(VI) Fluoride Exchange (SuFEx) chemistry stands as a well-established method for swiftly constructing complex molecules in a modular fashion. An especially promising segment of this toolbox is reserved for multidimensional SuFEx hubs: three or more substituents pluggable into a singular SVI centre to make 'beyond-linear' clicked constructions. Sulfurimidoyl difluorides (RNSOF2) stand out as the prime example of this, however their preparation from the scarcely available thionyl tetrafluoride (SOF4) limits this chemistry to only a few laboratories with access to this gas. In this work, we identify silver pentafluorooxosulfate (AgOSF5) as a viable SuFEx hub with reactivity equal to SOF4. The AgF2-mediated oxidation of SOCl2 gives rise to the hexacoordinate AgOSF5 adduct, which in contact with primary amines produces the sulfurimidoyl fluorides in high yields. In addition, we have found this workflow to be fully extendable to the trifluoromethyl homologue, AgOSF4CF3, and we propose the use of AgOSF4X salts as a general route to azasulfur SuFEx electrophiles from commercial starting materials.

Introducing AFS ([Al(SO3F)3]x) - a thermally stable, readily available, and catalytically active solid Lewis superacid

Common Lewis superacids often suffer from low thermal stability or complicated synthetic protocols, requiring multi-step procedures and expensive starting materials. This prevents their large-scale application. Herein, the easy and comparably cheap synthesis of high-purity aluminium tris(fluorosulfate) ([Al(SO3F)3]x, AFS) is presented. All starting materials are commercially available and no work-up is required. The superacidity of this thermally stable, polymeric Lewis acid is demonstrated using both theoretical and experimental methods. Furthermore, its synthetic and catalytic applicability, e.g. in bond heterolysis reactions and C-F bond activations, is shown.

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.

[Xe(OTeF5)(pyF)]+: a strong oxidizing xenonium(II) teflate cation with N-donor bases

Herein we report on the formation of the adduct salts [Xe(OTeF5)(pyF)][Al(OTeF5)4] (pyF = C5F5N, C5H3F2N) by abstraction of an -OTeF5 group from Xe(OTeF5)2 with the Lewis superacid Al(OTeF5)3 and subsequent adduct formation of the generated [XeOTeF5]+ cation with fluorinated pyridines. These salts represent the first xenonium cations with the weakly coordinating [Al(OTeF5)4]- anion. The strong oxidizing property of these compounds is further assessed.

The Field of Main Group Lewis Acids and Lewis Superacids: Important Basics and Recent Developments

New developments in the field of Lewis acidity are highlighted, with the focus of novel Lewis acids and Lewis superacids of group 2, 13, 14, and 15 elements. Several important basics, illustrated by modern examples (classification of Donor-Acceptor (DA) complexes, amphoteric nature of any compound in terms of DA interactions, reorganization energies of main group Lewis acids and the role of the energies of frontier orbitals) are presented and discussed. It is emphasized that the Lewis acidity phenomena are general and play vital role in different areas of chemistry: from weak "atomophilic" interactions to the complexes of Lewis superacids.

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.

Noncovalent Interactions in Halogenated Pyridinium Salts of the Weakly Coordinating Anion [Al(OTeF5 )4 ]. Chemistry 2023;29:e202202749. [PMID: 36268910 PMCID: PMC10107151 DOI: 10.1002/chem.202202749]

The synthesis and the first structural characterization of the halogenated pyridinium salts [C₅ F₅ NH]⁺ , [C₅ F₄ ClNH]⁺ , [(C₅ F₅ N)₂ H]⁺ , [(C₅ Cl₅ N)₂ H]⁺ of the weakly coordinating anion (WCA) [Al(OTeF₅ )₄ ]^- , showing noncovalent interactions in the solid state, are presented. The salts were characterized by the multinuclear NMR and IR spectroscopy as well as X-ray diffraction. Hirshfeld surface analysis and solid state structures reveal various intermolecular anion-π and σ-hole interactions between the corresponding halogenated pyridinium cations and the anion [Al(OTeF₅ )₄ ]^- .

Gold Teflates Revisited: From the Lewis Superacid [Au(OTeF5 )3 ] to the Anion [Au(OTeF5 )4 ]. Chemistry 2023;29:e202203634. [PMID: 36598847 DOI: 10.1002/chem.202203634] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

A new synthetic access to the Lewis acid [Au(OTeF₅ )₃ ] and the preparation of the related, unprecedented anion [Au(OTeF₅ )₄ ]^- with inorganic or organic cations starting from commercially available and easy-to-handle gold chlorides are presented. In this first extensive study of the Lewis acidity of a transition-metal teflate complex by using different experimental and quantum chemical methods, [Au(OTeF₅ )₃ ] was classified as a Lewis superacid. The solid-state structure of the triphenylphosphine oxide adduct [Au(OPPh₃ )(OTeF₅ )₃ ] was determined, representing the first structural characterization of an adduct of this highly reactive [Au(OTeF₅ )₃ ]. Therein, the coordination environment around the gold center slightly deviates from the typical square planar geometry. The [Au(OTeF₅ )₄ ]^- anion shows a similar coordination motif.

Unravelling the Role of the Pentafluoroorthotellurate Group as a Ligand in Nickel Chemistry

The pentafluoroorthotellurate group (teflate, OTeF₅ ) is able to form species, for which only the fluoride analogues are known. Despite nickel fluorides being widely investigated, nickel teflates have remained elusive for decades. By reaction of [NiCl₄ ]^2- and neat ClOTeF₅ , we have synthesized the homoleptic [Ni(OTeF₅ )₄ ]^2- anion, which presents a distorted tetrahedral structure, unlike the polymeric [NiF₄ ]^2- . This high-spin complex has allowed the study of the electronic properties of the teflate group, which can be classified as a weak/medium-field ligand, and therefore behaves as the fluoride analogue also in ligand-field terms. The teflate ligands in [NEt₄ ]₂ [Ni(OTeF₅ )₄ ] are easily substituted, as shown by the formation of [Ni(NCMe)₆ ][OTeF₅ ]₂ by dissolving it in acetonitrile. Nevertheless, careful reactions with other conventional ligands have enabled the crystallization of nickel teflate complexes with different coordination geometries, i.e. [NEt₄ ]₂ [trans-Ni(OEt₂ )₂ (OTeF₅ )₄ ] or [NEt₄ ][Ni(bpyMe₂ )(OTeF₅ )₃ ].

Insights on the Lewis Superacid Al(OTeF5 )3 : Solvent Adducts, Characterization and Properties

Preparation and characterization of the dimeric Lewis superacid [Al(OTeF₅ )₃ ]₂ and various solvent adducts is presented. The latter range from thermally stable adducts to highly reactive, weakly bound species. DFT calculations on the ligand affinity of these Lewis acids were performed in order to rank their remaining Lewis acidity. An experimental proof of the Lewis acidity is provided by the reaction of solvent-adducts of Al(OTeF₅ )₃ with [PPh₄ ][SbF₆ ] and OPEt₃ , respectively. Furthermore, their reactivity towards chloride and pentafluoroorthotellurate salts as well as (CH₃ )₃ SiCl and (CH₃ )₃ SiF is shown. This includes the formation of the dianion [Al(OTeF₅ )₅ ]^2- .

Molybdenum(VI) Bis(imido) Complexes: From Frustrated Lewis Pairs to Weakly Coordinating Cations

Molybdenum(VI) bis(imido) complexes [Mo(NtBu)₂ (L^R )₂ ] (R=H 1 a; R=CF₃ 1 b) combined with B(C₆ F₅ )₃ (1 a/B(C₆ F₅ )₃ , 1 b/B(C₆ F₅ )₃ ) exhibit a frustrated Lewis pair (FLP) character that can heterolytically split H-H, Si-H and O-H bonds. Cleavage of H₂ and Et₃ SiH affords ion pairs [Mo(NtBu)(NHtBu)(L^R )₂ ][HB(C₆ F₅ )₃ ] (R=H 2 a; R=CF₃ 2 b) composed of a Mo(VI) amido imido cation and a hydridoborate anion, while reaction with H₂ O leads to [Mo(NtBu)(NHtBu)(L^R )₂ ][(HO)B(C₆ F₅ )₃ ] (R=H 3 a; R=CF₃ 3 b). Ion pairs 2 a and 2 b are catalysts for the hydrosilylation of aldehydes with triethylsilane, with 2 b being more active than 2 a. Mechanistic elucidation revealed insertion of the aldehyde into the B-H bond of [HB(C₆ F₅ )₃ ]^- . We were able to isolate and fully characterize, including by single-crystal X-ray diffraction analysis, the inserted products Mo(NtBu)(NHtBu)(L^R )₂ ][{PhCH₂ O}B(C₆ F₅ )₃ ] (R=H 4 a; R=CF₃ 4 b). Catalysis occurs at [HB(C₆ F₅ )₃ ]^- while [Mo(NtBu)(NHtBu)(L^R )₂ ]⁺ (R=H or CF₃ ) act as the cationic counterions. However, the striking difference in reactivity gives ample evidence that molybdenum cations behave as weakly coordinating cations (WCC).

Air-stable aryl derivatives of pentafluoroorthotellurate

We report on two different sets of air-stable derivatives of pentafluoroorthotellurate containing fluorinated and non-fluorinated aryl groups. The acid cis-PhTeF₄OH was obtained in gram scale and further transformed to Ag[cis-PhTeF₄O], which was used as a cis-PhTeF₄O transfer reagent to obtain [PPh₄][cis-PhTeF₄O]. Furthermore, the synthesis of trans-(C₆F₅)₂TeF₃OH was achieved by a selective hydrolysis of trans-(C₆F₅)₂TeF₄ in the presence of KF and subsequent protonation by aHF. Quantum-chemical calculations show a higher acidity and robustness against fluoride abstraction for trans-(C₆F₅)₂TeF₃OH compared to cis-PhTeF₄OH.

The synthesis of air-stable aryl derivatives of OTeF₅ has been achieved through the use of convenient procedures. Pentafluorophenyl derivatives are stronger acids and more robust against electrophiles in comparison to phenyl analogues.

The Tris(pentafluorophenyl)methylium Cation: Isolation and Reactivity

Herein, we present two different routes for the synthesis of the perfluorinated trityl cation, which allowed the handling of the free, uncoordinated species in organic solvents for the first time. The usage of the weakly coordinating anion [Al(OTeF₅)₄]⁻ and its derivatives allows the characterization of this species by NMR spectroscopy and most importantly by single‐crystal X‐ray diffraction. The high hydride ion affinity of the cation is shown by hydrogen abstraction from isobutane. Furthermore, cyclic voltammetry reveals its oxidative potential which is supported by the reaction with tris(4‐bromophenyl)amine, giving rise to the formation of the ammoniumyl radical cation, also known as “magic blue”.

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.

The Power of the Proton: From Superacidic Media to Superelectrophile Catalysis

Superacidic media became famous in connection with carbocations. Yet not all reactive intermediates can be generated, characterized, and eventually isolated from these Brønsted acid/Lewis acid cocktails. The counteranion, that is the conjugate base, in these systems is often too nucleophilic and/or engages in redox chemistry with the newly formed cation. The Brønsted acidity, especially superacidity, is in fact often not even crucial unless protonation of extremely weak bases needs to be achieved. Instead, it is the chemical robustness of the aforementioned counteranion that determines the success of the protolysis. The advent of molecular Brønsted superacids derived from weakly coordinating, redox-inactive counteranions that do withstand the enormous reactivity of superelectrophiles such as silicon cations completely changed the whole field. This Perspective summarizes general aspects of medium and molecular Brønsted acidity and shows how applications of molecular Brønsted superacids have advanced from stoichiometric reactions to catalytic processes involving protons and in situ generated superelectrophiles.

One-step methylation of aromatic phosphorus heterocycles: synthesis and crystallographic characterization of a 1-methyl-phosphininium salt

For the first time, the direct synthesis of 1-methyl-phosphininium salts has been achieved by reacting aromatic λ³,σ²-phosphinines with the readily available dimethyl chloronium salt [(CH₃)₂Cl]⁺[Al(OTeF₅)₄]^-. The remarkably high electrophilicity of the alkylation reagent in combination with the weakly coordinating pentafluoro-orthotelluratoaluminate anion offers excellent conditions for this one-step approach. Our simple and quantitative access to 1-methyl-phosphininium salts will pave the way to explore the chemistry of such reactive species in more detail.

A Versatile Silver(I) Pentafluorooxosulfate Reagent for the Synthesis of OSF5 Compounds

Herein, we report the preparation of silver(I) pentafluorooxosulfate from commercially available AgF and OSF₄. The compound is surprisingly stable in a MeCN solution. Apart from that, AgOSF₅ has been stabilised by the addition of 2,2′‐bipyridine ligands. Starting from solutions of the unstabilised silver(I) salt, OSF₅ complexes with Ni^II, Cu^I, and Cu^II‐centres have been obtained. In addition, AgOSF₅ has proven to be generally capable of mediating the transfer of OSF₅ groups to aryne moieties, thus furnishing a new and safe method for the preparation of OSF₅‐substituted arenes. X‐ray crystal structure analysis of selected transition‐metal OSF₅ compounds have revealed distorted octahedral [OSF₅]⁻ anions which are extensively stabilised by hydrogen bonding.

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.

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.

Friedel-Crafts Type Methylation with Dimethylhalonium Salts

The dimethylchloronium salt [Me2 Cl][Al(OTeF5 )4 ] is used to methylate electron-deficient aromatic systems in Friedel-Crafts type reactions as shown by the synthesis of N-methylated cations, such as [MeNC5 F5 ]+ , [MeNC5 F4 I]+ , and [MeN3 C3 F3 ]+ . To gain a better understanding of such fundamental Friedel-Crafts reactions, the role of the dimethylchloronium cation has been evaluated by quantum-chemical calculations.

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.

An Extensive Set of Accurate Fluoride Ion Affinities for p-Block Element Lewis Acids and Basic Design Principles for Strong Fluoride Ion Acceptors

The computed fluoride ion affinity (FIA) is a valuable descriptor to assess the Lewis acidity of a compound. Despite its widespread use, the varying accuracy of applied computational models hampers the broad comparability of literature data. Herein, we evaluate the performance of selected methods (like DLPNO‐CCSD(T)) in FIA computations against CCSD(T)/CBS data and guide for the choice of suitable density functionals that allow the treatment of larger Lewis acids. Based on the benchmarked methods, we computed an extensive set of gas‐phase and solvation corrected FIA, that is covering group 13–16 elements featuring moderate to strong electron‐withdrawing substituents (190 entries). It permits an unbiased comparison of FIA over a significant fraction of the periodic table, serves as a source of reference for future synthetic or theoretical studies, and allows to derive some simple design principles for strong fluoride ion acceptors. Finally, the manuscript includes a tutorial section for the computation of FIA with and without the consideration of solvation.

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.

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.

A Very Strong Methylation Agent: [Me2 Cl][Al(OTeF5 )4 ]

A new chloronium-containing salt, [Me₂ Cl][Al(OTeF₅ )₄ ], was synthesized on multigram scale by means of a simple one-pot procedure. The isolated product can be handled at room temperature and used as a strong electrophilic methylation agent. This is demonstrated by the methylation of the very weak bases P(CF₃ )₃ , PF₃ , MeI, and MeBr.

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.

Nb-doped variants of high surface aluminium fluoride: a very strong bi-acidic solid catalyst

Novel aluminium Nb-doped fluoride catalysts were synthesized using an aluminium hydroxide precursor to afford solids where very strong Lewis acid sites coexist with Brønsted acid sites.

[P4H]+[Al(OTeF5)4]-: protonation of white phosphorus with the Brønsted superacid H[Al(OTeF5)4](solv)

A sustainable transformation of white phosphorus (P4) into chemicals of higher value is one of the key aspects in modern phosphorus research. Even though the chemistry of P4 has been investigated for many decades, its chemical reactivity towards the simplest electrophile, the proton, is still virtually unknown. Based on quantum-chemical predictions, we report for the first time the successful protonation of P4 by the Brønsted acid H[Al(OTeF5)4](solv). Our spectroscopic results are in agreement with acid-mediated activation of P4 under protonation of an edge of the P4-tetrahedron and formation of a three-center two-electron P-H-P bond. These investigations are of fundamental interest as they permit the activation of P4 with the simplest electrophile as a new prototype reaction for this molecule.

Facile and systematic access to the least-coordinating WCA [(RFO)3Al-F-Al(ORF)3]- and its more Lewis-basic brother [F-Al(ORF)3]- (RF = C(CF3)3)

By reaction of the Lewis acid Me3Si-F-Al(ORF)3 with a series of [PF6]- salts, gaseous PF5 and Me3Si-F are liberated and salts of the anion [F-Al(ORF)3]- ([f-al]-; RF = C(CF3)3) can be obtained. By addition of another equivalent of Me3Si-F-Al(ORF)3 to [f-al]-, gaseous Me3Si-F is released and salts of the least coordinating anion [(RFO)3Al-F-Al(ORF)3]- ([al-f-al]-) are formed. Both procedures work for a series of synthetically useful cations including Ag+, [NO]+, [Ph3C]+ and in very clean reactions with 5 g batch sizes giving excellent yields typically exceeding 90%. In addition, the synthesis of Me3Si-F-Al(ORF)3 has been optimized and scaled up to 85 g batches in an one-pot procedure. These anions could previously only be obtained by difficult to control decomposition reactions of [Al(ORF)4]- or by halide abstraction reactions with Me3Si-F-Al(ORF)3, generating relatively large countercations that are unsuited for further use as universal starting materials. Especially [al-f-al]- is of interest for the stabilization of reactive cations, since it is even weaker coordinating than [Al(ORF)4]- and more stable against strong electrophiles. This bridged anion can be seen as an adduct of [f-al]- and Al(ORF)3. Thus, it is similarly Lewis acidic as BF3 and eventually reacts with nucleophiles (Nu) from the reaction environment to yield Nu-Al(ORF)3 and [f-al]-. This prevents working with [al-f-al]- salts in ethereal or other donor solvents. By contrast, the [f-al]- anion is no longer Lewis acidic and may therefore be used for reactions involving stronger nucleophiles than the [al-f-al]- anion can withstand. Subsequently it may be transformed into the [al-f-al]- salt by simple addition of one equivalent of Me3Si-F-Al(ORF)3.

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

This Review gives a comprehensive overview of the most topical weakly coordinating anions (WCAs) and contains information on WCA design, stability, and applications. As an update to the 2004 review, developments in common classes of WCA are included. Methods for the incorporation of WCAs into a given system are discussed and advice given on how to best choose a method for the introduction of a particular WCA. A series of starting materials for a large number of WCA precursors and references are tabulated as a useful resource when looking for procedures to prepare WCAs. Furthermore, a collection of scales that allow the performance of a WCA, or its underlying Lewis acid, to be judged is collated with some advice on how to use them. The examples chosen to illustrate WCA developments are taken from a broad selection of topics where WCAs play a role. In addition a section focusing on transition metal and catalysis applications as well as supporting electrolytes is also included.

Further Development of Weakly Coordinating Cations: Fluorinated Bis(triarylphosphoranylidene)iminium Salts

So far unknown bis(triarylphosphoranylidene)iminium cations [PPN]⁺ with one fluorine atom in para ([PPN-1^F ]⁺ ), two in meta ([PPN-2^F ]⁺ ), or three in para and meta positions of the phenyl rings ([PPN-3^F ]⁺ ) were obtained by a newly developed one-pot reaction. These halogenated [PPN]⁺ cations were characterized by IR and Raman spectroscopy in comparison with quantum-chemical calculations, ESI⁺ mass spectrometry, NMR spectroscopy, and single-crystal X-ray diffraction. To assess their quality as weakly coordinating cations and the associated ability to stabilize labile anions, the electrostatic potential and fluoride-ion affinity were calculated and compared with those of the unsubstituted and so far unknown perfluorinated [PPN-5^F ]⁺ cations.

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.