Lewis Superacids Research Articles

Taming the Lewis Superacidity of Non‐Planar Boranes: C−H Bond Activation and Non‐Classical Binding Modes at Boron

AbstractThe rational design of a geometrically constrained boron Lewis superacid featuring exceptional structure and reactivity is disclosed. It enabled the formation of non‐classical electron deficient B−H−B type of bonding, which was supported by spectroscopic and structural parameters as well as computational studies. Taming the pyramidal Lewis acid electrophilicity through weak coordinating anion dissociation enabled a series of highly challenging chemical transformations, such as Csp2−H and Csp3−H activation under a frustrated Lewis pair regime and the cleavage of Csp3−Si bonds. The demonstration of such rich chemical behaviour and flexibility on a single molecular compound makes it a unique mediator of chemical transformations generally restricted to transition metals.

Taming the Lewis Superacidity of Non‐Planar Boranes: C−H Bond Activation and Non‐Classical Binding Modes at Boron

AbstractThe rational design of a geometrically constrained boron Lewis superacid featuring exceptional structure and reactivity is disclosed. It enabled the formation of non‐classical electron deficient B−H−B type of bonding, which was supported by spectroscopic and structural parameters as well as computational studies. Taming the pyramidal Lewis acid electrophilicity through weak coordinating anion dissociation enabled a series of highly challenging chemical transformations, such as Csp2−H and Csp3−H activation under a frustrated Lewis pair regime and the cleavage of Csp3−Si bonds. The demonstration of such rich chemical behaviour and flexibility on a single molecular compound makes it a unique mediator of chemical transformations generally restricted to transition metals.

Synthesis of high molecular weight polyisobutylene with Al(C6F5)3-based initiating systems under mild conditions

We report here by using a binary initiating system containing a sterically bulky Lewis super acid Al(C6F5)3 together with cumyl chloride (CumCl), the cationic polymerization of isobutylene was realized at temperatures varying from −60 °C to 15 °C in a non-halogenated solvent to afford polymers with medium molecular weights from 0.9 × 104 to 14.8 × 104 g·mol−1. Moreover, addition of EDs (EDs = OPh2, SPh2 and NPh3) to the above system generated ternary systems, leading to obvious improvement of the catalytic performances. Among these systems, Al-CumCl-OPh2 was mostly efficient to transfer up to 20,000 equiv. IB into PIBs with high molecular weights up to 35.5 × 104 g·mol−1 at −40 °C. The activation energies (Ep) for the polymerization degree with different initiating systems were determined, suggesting that the molecular weight of PIB was strongly dependent on the polymerization temperature.

Lewis acid-base adducts of Al(N(C6F5)2)3 and Ga(N(C6F5)2)3 - structural features and dissociation enthalpies.

Herein we present the molecular structures of six neutral Lewis acid-base adducts of the Lewis superacid Al(N(C6F5)2)3 and its higher homolog Ga(N(C6F5)2)3 with the electron pair donors MeCN, CNtBu, THF and PMe3. The crystal structures reveal crucial structural changes compared to the free Lewis acids as a consequence of the adduct formation. Furthermore, we calculated the corresponding dissociation enthalpies of the adducts which lie between 69 and 141 kJ mol-1 and are therefore considerably lower compared to the values for the formation of the anionic fluoride or chloride metallates.

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.

Highly Perfluorinated Covalent Triazine Frameworks Derived from a Low‐Temperature Ionothermal Approach Towards Enhanced CO 2 Electroreduction

AbstractPerfluorinated covalent triazine frameworks (F‐CTFs) have shown unique features and attractive performance in separation and catalysis. However, state‐of‐the‐art F‐CTFs synthesized via the ZnCl2‐promoted procedure have quite low fluorine contents due to C−F bond cleavage induced by chloride (a Lewis base) and the harsh conditions deployed (400–700 °C). Fabricating F‐CTFs with high fluorine contents (>30 wt %) remains challenging. Herein, we present a low‐temperature ionothermal approach (275 °C) to prepare F‐CTFs, which is achieved via polymerization of tetrafluoroterephthalonitrile (TFPN) over the Lewis superacids, e.g., zinc triflimide [Zn(NTf2)2] without side reactions. With low catalyst loading (equimolar), F‐CTFs are afforded with high fluorine content (31 wt %), surface area up to 367 m2 g−1, and micropores around 1.1 nm. The highly hydrophobic F‐CTF‐1 exhibits good capability to boost electroreduction of CO2 to CO, with faradaic efficiency of 95.7 % at −0.8 V and high current density (−141 mA cm−2) surpassing most of the metal‐free electrocatalysts.

Highly Perfluorinated Covalent Triazine Frameworks Derived from a Low-Temperature Ionothermal Approach Towards Enhanced CO2 Electroreduction.

Perfluorinated covalent triazine frameworks (F-CTFs) have shown unique features and attractive performance in separation and catalysis. However, state-of-the-art F-CTFs synthesized via the ZnCl2 -promoted procedure have quite low fluorine contents due to C-F bond cleavage induced by chloride (a Lewis base) and the harsh conditions deployed (400-700 °C). Fabricating F-CTFs with high fluorine contents (>30 wt %) remains challenging. Herein, we present a low-temperature ionothermal approach (275 °C) to prepare F-CTFs, which is achieved via polymerization of tetrafluoroterephthalonitrile (TFPN) over the Lewis superacids, e.g., zinc triflimide [Zn(NTf2 )2 ] without side reactions. With low catalyst loading (equimolar), F-CTFs are afforded with high fluorine content (31 wt %), surface area up to 367 m2 g-1 , and micropores around 1.1 nm. The highly hydrophobic F-CTF-1 exhibits good capability to boost electroreduction of CO2 to CO, with faradaic efficiency of 95.7 % at -0.8 V and high current density (-141 mA cm-2 ) surpassing most of the metal-free electrocatalysts.

Lewis Superacidic Tellurenyl Cation-Induced Electrophilic Activation of an Inert Carborane.

The aryltellurenyl cation [2‐(tBuNCH)C6H4Te]+, a Lewis super acid, and the weakly coordinating carborane anion [CB11H12]−, an extremely weak Brønsted acid (pK a=131.0 in MeCN), form an isolable ion pair complex [2‐(tBuNCH)C6H4Te][CB11H12], in which the Brønsted acidity (pK a 7.4 in MeCN) of the formally hydridic B−H bonds is dramatically increased by more than 120 orders of magnitude. The electrophilic activation of B−H bonds in the carborane moiety gives rise to a proton transfer from boron to nitrogen at slightly elevated temperatures, as rationalized by the isolation of a mixture of the zwitterionic isomers 12‐ and 7‐[2‐(tBuN{H}CH)C6H4Te(CB11H11)] in ratios ranging from 62 : 38 to 80 : 20.

Lewis Superacidic Catecholato Phosphonium Ions: Phosphorus-Ligand Cooperative C-H Bond Activation.

A series of catecholato phosphonium ions, including the first stable bis(catecholato)-substituted derivatives, are isolated and fully characterized. The cations rank among the most potent literature-known Lewis acids on the Gutmann-Beckett and ion affinity scales. In contrast to halogenated or multiply charged phosphorus cations, Lewis superacidity is imparted by structural constraints, as disclosed by energy decomposition analysis. The modular access provides a tunable scaffold while maintaining extreme affinity, demonstrated by the synthesis of a chiral Lewis superacid. The combination of electrophilic phosphorus and basic oxygen substituents leverages new reactivity modes by phosphorus-ligand cooperativity. With this, a phosphorus-mediated C-H bond activation is accomplished.

Synthesis and Catalytic Activity of Atrane-type Hard and Soft Lewis Superacids with a Silyl, Germyl, or Stannyl Cationic Center.

The synthesis and isolation of atrane-type molecules 1E+ (E=Si, Ge, or Sn) having a cationic group 14 elemental center are reported. The cations 1E+ act as hard and soft Lewis superacids, which readily interact with various hard and soft Lewis basic substrates. The rigid atrane framework stabilizes the localized positive charge on the elemental center and assists the formation of the well-defined highly coordinated states of 1E+ . The cations were applied to the hydrodefluorination, Friedel-Crafts reaction, alkyne cyclization, and carbonyl reduction as Lewis acid catalysts. Most notably, [1Si][ClO4 ] exhibits unique chemoselectivity that depends on a solvent in the competitive reaction of silyl enol ether with a mixture of benzaldehyde dimethyl acetal and benzaldehyde. Our findings indicate the potential of hard and soft Lewis superacids in organic synthesis.

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, H2catCF3, and adducts of its silicon complex Si(catCF3)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.

Synthesis, Characterization, and Density Functional Theory Studies of Three-Dimensional Inorganic Analogues of 9,10-Diboraanthracene-A New Class of Lewis Superacids.

The three-dimensional inorganic analogues of 9,10-diboraanthracene, B2X2(C2B10H10)2 (X = Cl, 1; X = Br, 2), were attained by salt elimination of Li2C2B10H10 and trihaloboranes. The methyl- and phenyl-substituted compounds B2Me2(C2B10H10)2 (3) and B2Ph2(C2B10H10)2 (4) were obtained by treating 1 or 2 with the corresponding Grignard reagents. These compounds were fully characterized by NMR, cyclic voltammetry (CV), IR, and single-crystal X-ray diffraction analyses. Experimental (CV and Gutmann-Beckett method) and computational (fluoride ion affinity, hydride ion affinity and LUMO energy) results suggest that the order of Lewis acidity is 2 > 1 > 4 > 3 > SbF5. Treatment of 1 or 2 with HSiEt3 gave a rare neutral borane-silane adduct, (Et3SiH)2B2H2(C2B10H10)2 (5). The equilibrium of 5 in solution was thoroughly investigated by spectroscopy and quantum calculations.

Silicon Tetrakis(trifluoromethanesulfonate): A Simple Neutral Silane Acting as a Soft and Hard Lewis Superacid.

A facile synthesis and isolation of pristine silicon tetrakis(trifluoromethanesulfonate), Si(OTf)4, is reported, acting as the first neutral silicon‐based Lewis superacid suitable towards soft and hard Lewis bases. Its OTf groups have a dual function: they are excellent leaving groups and modulate the degree of reactivity towards soft and hard Lewis bases. Exposed to soft Lewis donors, Si(OTf)4 leads to [L2Si(OTf)4] complexes (L=isocyanide, thioether and carbonyl compounds) with retention of all Si−OTf bonds. In contrast, it can cleave C−X bonds (X=F, Cl) of hard organic Lewis bases with a high tendency to form SiX4 (X=F, Cl) after halide/triflate exchange. Most notable, Si(OTf)4 allows a gentle oxydefluorination of mono‐ and bis(trifluoromethyl)benzenes, resulting in the formation of the corresponding benzoylium species, which are stabilized by the weakly coordinating [Si(OTf)6] dianion.

Zintl Lewis Superacids: Al(Ge9L3)3 (L = H, CH3, CHO, CN).

In quest of a Zintl ion-based Lewis superacids, Al(Ge9L3)3 {L= H, CH3, CHO and CN} compounds have been designed and their properties have been studied within the framework of conceptual density functional theory-based reactivity descriptors. Superacid property has been identified for these complexes as per the fluoride ion affinity (FIA) values. Studies reveal that Al[Ge9(CN)3]3 and Al[Ge9(CH3)3]3 behave like superacids as their FIA exceeds the value of SbF5, which is considered as the strongest Lewis acid. It has been observed that the ligand plays an important role in reactivity as well as in Lewis acidic property.

Caralumane Superacids of Lewis and Brønsted Character.

Carborane Brønsted superacids have proven to be useful reagents in a variety of organic and inorganic synthetic processes. In this work, analogs in which the icosahedral CB11 carborane core is replaced by a CAl11 core are studied using ab initio electronic structure tools. Each so-called caralumane Brønsted acid is formed by adding HF, HCl, or HH to a corresponding caralumane Lewis acid possessing a vacant Al-centered orbital that acts to accept an electron pair from the HF, HCl, or HH. The Lewis acid strengths of the species involved, as measured by their F- ion affinities, are all found to exceed the threshold for labeling them Lewis superacids. Also, the deprotonation Gibbs free energies of the Brønsted acids are found to be small enough for them to be Brønsted superacids. When HF or HCl is bound to a caralumane Lewis acid to form the Brønsted acid, the HF or HCl is bound datively to a single Al atom, and hydrogen bonds can be formed between this molecule's H atom and nearby F or Cl atoms attached to other Al atoms. In contrast, when HH is bound to the Lewis acid to form the Brønsted acid, two novel low-energy structures arise, both of which are Brønsted superacids. One has an essentially intact HH molecule attached to a single Al atom in a η2 fashion. In the other, the HH molecule is heterolytically cleaved to generate a hydride ion that attaches to a single Al atom and a proton that binds in a multicenter manner to other Al atoms. The structures and relative energies of a multitude of such caralumane Lewis and Brønsted superacids are provided and discussed.

Reversible Silylium Transfer between P‐H and Si‐H Donors

AbstractThe Mo=PR2 π* orbital in a Mo phosphenium complex acts as acceptor in a new PIII‐based Lewis superacid. This Lewis acid (LA) participates in electrophilic Si‐H abstraction from E3SiH to give a Mo‐bound secondary phosphine ligand, Mo‐PR2H. The resulting Et3Si+ ion remains associated with the Mo complex, stabilized by η1‐P‐H donation, yet undergoes rapid exchange with an η1‐Si‐H adduct of free silane in solution. The equilibrium between these two adducts presents an opportunity to assess the role of this new LA in catalytic reactions of silanes: is the LA acting as a catalyst or as an initiator? Preliminary results suggest that a cycle including the Mo‐bound phosphine‐silylium adduct dominates in the catalytic hydrosilylation of acetophenone, relative to a putative cycle involving the silane‐silylium adduct or “free” silylium.

Reversible Silylium Transfer between P-H and Si-H Donors.

The Mo=PR2 π* orbital in a Mo phosphenium complex acts as acceptor in a new PIII -based Lewis superacid. This Lewis acid (LA) participates in electrophilic Si-H abstraction from E3 SiH to give a Mo-bound secondary phosphine ligand, Mo-PR2 H. The resulting Et3 Si+ ion remains associated with the Mo complex, stabilized by η1 -P-H donation, yet undergoes rapid exchange with an η1 -Si-H adduct of free silane in solution. The equilibrium between these two adducts presents an opportunity to assess the role of this new LA in catalytic reactions of silanes: is the LA acting as a catalyst or as an initiator? Preliminary results suggest that a cycle including the Mo-bound phosphine-silylium adduct dominates in the catalytic hydrosilylation of acetophenone, relative to a putative cycle involving the silane-silylium adduct or "free" silylium.

Completing the Redox-Series of Silicon Trisdioxolene: ortho-Quinone and Lewis Superacid Make a Powerful Redox Catalyst.

Quinones are mild oxidants, the redox potentials of which can be increased by supramolecular interactions. Whereas this goal has been achieved by hydrogen bonding or molecular encapsulation, a Lewis acid‐binding strategy for redox amplification of quinones is unexplored. Herein, the redox chemistry of silicon tris(perchloro)dioxolene 1 was studied, which is the formal adduct of ortho‐perchloroquinone QCl with the Lewis superacid bis(perchlorocatecholato)silane 2. By isolating the anionic monoradical 1 .−, the redox‐series of a century‐old class of compounds was completed. Cyclic voltammetry measurements revealed that the redox potential in 1 was shifted by more than 1 V into the anodic direction compared to QCl, reaching that of “magic blue” or NO+. It allowed oxidation of challenging substrates such as aromatic hydrocarbons and could be applied as an efficient redox catalyst. Remarkably, this powerful reagent formed in situ by combining the two commercially available precursors SiI4 and QCl.

Isolable Silicon-Based Polycations with Lewis Superacidity.

Molecular silicon polycations of the types R2Si2+ and RSi3+ (R=H, organic groups) are elusive Lewis superacids and currently unknown in the condensed phase. Here, we report the synthesis of a series of isolable terpyridine‐stabilized R2Si2+ and RSi3+ complexes, [R2Si(terpy)]2+ (R=Ph 1 2+; R2=C12H8 2 2+, (CH2)3 3 2+) and [RSi(terpy)]3+ (R=Ph 4 3+, cyclohexyl 5 3+, m‐xylyl 6 3+), in form of their triflate salts. The stabilization of the latter is achieved through higher coordination and to the expense of reduced fluoride‐ion affinities, but a significant level of Lewis superacidity is nonetheless retained as verified by theory and experiment. The complexes activate C(sp3)−F bonds, as showcased by stoichiometric fluoride abstraction from 1‐fluoroadamantane (AdF) and the catalytic hydrodefluorination of AdF. The formation of the crystalline adducts [2(F)]+ and [5(H)]2+ documents in particular the high reactivity towards fluoride and hydride donors.

Bis(alizarinato)silane: In Silico Design and Synthesis of a Powerful Chromogenic Lewis Acid as a Dual-Gated Fluoride Ion Probe

Previously reported Lewis superacids are colorless and do not exhibit visual changes upon adduct formation. Herein, we combine the features of near Lewis superacidity with light absorption and emis...