The Acidity of the HBr/AlBr3 System: Stabilization of Crystalline Protonated Arenes and Their Acidity in Bromoaluminate Ionic Liquids

Bulk protonated mesitylene, toluene, and benzene bromoaluminate salts were stabilized and characterized in the superacidic system HBr/n AlBr₃ with NMR spectroscopy and X‐ray analysis of [HC₆H₃(CH₃)₃]⁺[AlBr₄]^? ( 1 ), [HC₆H₅(CH₃)]⁺[AlBr₄]^? ( 2 ), and [C₆H₇]⁺[Al₂Br₇]^??C₆H₆ ( 3 ). Protonation attempts in bromoaluminate ILs led to a complete protonation of mesitylene, and a protonation degree of up to 15 % for toluene in the IL BMP⁺[Al₂Br₇]^?. Benzene could only be protonated in the more acidic IL BMP⁺[Al₃Br₁₀]^?, with a degree of 25 %. Protonation attempts on aromatics provide evidence that the bromoaluminate ILs tolerate superacidic environments. On the basis of the absolute Brønsted acidity scale, quantum chemical calculations confirmed the superacidic properties, and rank the acidities in ILs down to a pH_abs value of 164 with an error of less than one pH unit compared with experimental findings. The neat AlBr₃/HBr system even may reach acidities down to pH_abs 163.     

Superacidic or Not…︁? Synthesis,Characterisation, and Acidity of the Room‐Temperature Ionic Liquid [C(CH3)3]+ [Al2Br7]−

The room‐temperature ionic liquid (RT‐IL) [C(CH₃)₃]⁺ [Al₂Br₇]^? (m.p. 2 °C) was generated by bromide abstraction from tert‐butyl bromide with the Lewis acid aluminum bromide in the absence of solvent. The crystal structure of the tert‐butyl cation salt was determined by X‐ray diffraction. NMR, IR, and Raman spectroscopy, as well as quantum‐chemical and thermodynamic calculations, confirm the composition of this RT‐IL. Thus, one may consider this RT‐IL to be a readily accessible (and on a large scale) cationic Brønsted acid (protonated isobutene) with the potential for further reactivity. Based on the new absolute Brønsted acidity scale, we calculated an absolute pH_abs value of 171 for liquid bulk [C(CH₃)₃]⁺ [Al₂Br₇]^?. This value is about as acidic as 100 % sulfuric acid (pH_abs=171) and, thus, on the edge of superacidity.     

Nitration of Simple Aromatics with NO2 under Air Atmosphere in the Presence of Novel Brønsted Acidic Ionic Liquids

Abstract

Aromatics nitrate with NO₂/air catalyzed by novel Brønsted acidic ionic liquids (ILs) without any volatile chlorinated organic solvent under mild conditions. The ILs employed were caprolactam based, [Caprolactam]X (X^?=pTSO^?, BSO^?, BF₄ ^?, NO₃ ^?), which are of relatively lower cost and lower toxicity than traditional imidazolium‐based ILs. The nitration reactions were carried out at ?15 to ?0°C first, then at room temperature for a longer time with a little excessive NO₂ (ca. 1.4 eqv.) for moderate yield (for toluene). The IL could be reused four times.     

Role of C2 methylation and anion type on the physicochemical and thermal properties of imidazolium-based ionic liquids

This study focused on the effects of methylation and different anions (Br^? and Cl^?) on the physicochemical and thermal properties of [C₁₆MIM]X and [C₁₆MMIM]X, belonging to the imidazolium-based ionic liquid (IL) family. The effect of methylation on the transmittance in the fingerprint region of the Fourier transform infrared (FT-IR) spectrum was observed as a blue shift, and a new peak associated with the C-N stretching bond was obtained. In contrast, in the functional group region, the frequency shift was related to the change in the vibrational mode from C2-H-X to C2-methyl-X. In general, methylation resulted in an increase in decomposition temperature, an increase in melting temperature, and a decrease in melting enthalpy, leading to a reduction in entropy. The trends observed for the decomposition temperature, melting temperature, and melting enthalpy with different anions depended on the strength of the Brønsted acids and hydrogen bonds of the Br^? and Cl^? based anions. The thermal conductivity of the methylated ILs increased with an increase in temperature. In contrast, for the non-methylated (protonated) ILs, the thermal conductivity of [C₁₆MIM]Br decreased with an increase in temperature, while the opposite trend was observed for [C₁₆MIM]Cl. The data were compared with those of the short alkyl chain and weakly coordinating anion of NTf₂. The analysis was performed considering different phases, the prominent role and different behaviour in the hydrogen bonding at the C2 position of the imidazolium ring upon methylation, and the significant change in viscosity, which can influence the IL structure.     

Impacts of Composition and Post‐Treatment on the Brønsted Acidity of Steam‐Treated Faujasite: Insights from FTIR Spectroscopy

A detailed FTIR study of the effects of steaming and acid leaching on protonated Y faujasite (FAU) and EMT zeolites is provided and the results are thoroughly analysed. In particular, emphasis is placed on the Brønsted acidic evolution and acidic strength measurements for a large series of as‐modified zeolites using CO as a sensitive probe to distinguish various protonic sites. While an increase of acidity for framework OH groups is observed during the strengthening of dealumination for both FAU and EMT series, the steaming process also generates a large variety of additional Brønsted acidic groups. Regarding acidic strength, these heterogeneous OH groups are sensitive to post‐treatments and their existence strongly depends on the initial composition of the zeolites. The presence of residual Na⁺ cations in the starting materials induces dramatic Brønsted acidic changes after steaming. As a result, steamed zeolites that initially contain traces of sodium possess unusual acidic Brønsted groups with low acidity. This result contradicts the trend generally observed with framework OH groups, for which steaming results in an increase of Brønsted acidic strength. The study reveals that the situation is indeed more complex, as some compositions and post‐treatments strongly influence the Brønsted acidity of as‐steamed zeolites both in their nature and their corresponding acidic strength. By linking these IR‐compiled features to the as‐exposed modifications, a large acidity scale better suited to characterizing catalysts having Brønsted acidity expanding from lowest to highest strength is proposed.     

Bulk Gas‐Phase Acidity

The capability of a gaseous Brønsted acid HB to deliver protons to a base is usually described by the gas‐phase acidity (GA) value of the acid. However, GA values are standard Gibbs energy differences and refer to individual gas pressures of 1 bar for acid HB, base B^?, and proton H⁺. We show that the GA value is not suited to describe the bulk acidity of a gaseous acid. Here the pressure dependence of the activities of HB, H(HB)_n⁺, and B(HB)_m^? that result from gaseous autoprotolysis have to be considered. In this work, the pressure‐dependent absolute chemical potential of the proton in the representative gaseous proton acids CH₄, NH₃, H₂O, HF, and HCl was worked out and the general theory to describe bulk gas phase acidity—that can directly be compared with solution acidity—was developed.     

Synthesis and Structure of Solvated Protons Incorporating Weakly Coordinating Anions. Precursors of Superacids

ＩｎｔｒｏｄｕｃｔｉｏｎＩｔｉｓｕｎｉｖｅｒｓａｌｌｙｕｎｄｅｒｓｔｏｏｄｔｈａｔｗｒｉｔｉｎｇＨ+ ｉｓｓｈｏｒｔ ｈａｎｄｆｏｒａｓｏｌｖａｔｅｄｐｒｏｔｏｎ ,[Ｈ(ｓｏｌｖｅｎｔ) ｎ]+ ,ｔｈｅｖａｌｕｅｏｆｎａｎｄｔｈｅｄｅｔａｉｌｓｏｆｔｈｅｃｏｏｒｄｉｎａｔｉｏｎｅｎｖｉｒｏｎｍｅｎｔａｒｅｏｆｔｅｎｕｎｓｐｅｃｉｆｉｅｄ .Ｉｓｏｌａｔｉｏｎａｎｄｓｔｒｕｃｔｕｒａｌｃｈａｒａｃｔｅｒｉｚａｔｉｏｎｏｆｖａｒｉｏｕｓｓａｌｔｓｃｏｎｔａｉｎｉｎｇｒｅｐｒｅｓｅｎｔａｔｉｖｅ [Ｈ(ｓｏｌｖｅｎ…     

The Strongest Acid: Protonation of Carbon Dioxide

The strongest carborane acid, H(CHB₁₁F₁₁), protonates CO₂ while traditional mixed Lewis/Brønsted superacids do not. The product is deduced from IR spectroscopy and calculation to be the proton disolvate, H(CO₂)₂⁺. The carborane acid H(CHB₁₁F₁₁) is therefore the strongest known acid. The failure of traditional mixed superacids to protonate weak bases such as CO₂ can be traced to a competition between the proton and the Lewis acid for the added base. The high protic acidity promised by large absolute values of the Hammett acidity function (H₀) is not realized in practice because the basicity of an added base is suppressed by Lewis acid/base adduct formation.     

Effect of titanium source on structural properties and acidity of Ti-pillared bentonite

Ti-pillared bentonites (Ti-PBs) were synthesised using bentonite from the Hanç?l? region in Turkey. Ti(IV) chloride, Ti(IV) ethoxide and Ti(IV) propoxide were used as the titanium sources; the syntheses were carried out using different H⁺/Ti ratios, bentonite suspension percentages and calcination temperatures. Titanium was found in the form of titanium dioxide for all the sources. The Ti(IV) chloride source afforded a sample with a significantly higher specific BET surface area (by 323 m² g^?1), TiO₂ content of 50.5 mass % and a more microporous structure with a micropore volume of 0.112 cm³ g^?1; the Ti(IV) propoxide source afforded a more mesoporous structure with a higher total pore volume. The micropore region showed the formation of pores of different sizes, while prominent narrow peaks were obtained in the mesopore region. Ti-PBs, which exhibited only the anatase phase of titanium dioxide, yielded high Brønsted and Lewis acidities. When the rutile phase and the anatase phase occurred together, as a result of the lower TiO₂ content, the Brønsted and Lewis acidities of the Ti-PBs decreased. The use of Ti(IV) chloride and Ti(IV) propoxide sources at H⁺/Ti ratios of 4.0 and a bentonite suspension percentage of 2.0 resulted in samples exhibiting strong Brønsted acidity.     

Gas phase ion molecule reactions of organometallic compounds; protonation of selected η6-arenetricarbonylchromium complexes and η6-cycloheptatriene complexes of the Group VI metals with various Brønsted acid reagent ions

η⁶-Arenetricarbonylchromium(0) complexes, (with the η⁶-arene = benzene, toluene, methylbenzoate and acetophenone) and η⁶-cycloheptatrienetricarbonyl complexes of chromium(0), molybdenum(0) and tungsten(0) undergo reactions in the gas phase with the Brønsted acid reagent ions H₃⁺, CH₅⁺, t-C₄H₉⁺, (NH₃)_nH⁺ which depend on the Brønsted acid strengths of these ions and also on the basicity of the metal complexes. Processes which involve either metal or ligand proton attachment, as well as charge exchange, electrophillic addition and rearrangement reactions have been identified. Some comparisons are drawn between these gas phase observations and the solutions phase behaviour of these compounds.     

Aqueous Brønsted–Lowry Chemistry of Ionic Liquid Ions

Ionic liquids have become commonplace materials found in research laboratories the world over, and are increasingly utilised in studies featuring water as co‐solvent. It is reported herein that proton activities, a_H⁺, originating from auto‐protolysis of H₂O molecules, are significantly altered in mixtures with common ionic liquids comprised of Cl^?, [HSO₄]^?, [CH₃SO₄]^?, [CH₃COO]^?, [BF₄]^?, relative to pure water. pa_H⁺ values, recorded in partially aqueous media as ?log(a_H⁺), are observed over a wide range (～0–13) as a result of hydrolysis (or acid dissociation) of liquid salt ions to their associated parent molecules (or conjugate bases). Brønsted–Lowry acid–base character of ionic liquid ions observed is rooted in equilibria known to govern the highly developed aqueous chemistry of classical organic and inorganic salts, as their well‐known aqueous pKs dictate. Classical salt behaviour observed for both protic and aprotic ions in the presence of water suggests appropriate attention need be given to relevant chemical systems in order to exploit, or avoid, the nature of the medium formed.     

Effect of Brønsted/Lewis Acid Ratio on Conversion of Sugars to 5‐Hydroxymethylfurfural over Mesoporous Nb and Nb‐W Oxides

A series of mesoporous Nb and Nb‐W oxides were employed as highly active solid acid catalysts for the conversion of glucose to 5‐hydroxymethylfurfural (HMF ). The results of solid state ³¹P MAS NMR spectroscopy with adsorbed trimethylphosphine as probe molecule show that the addition of W in niobium oxide increases the number of Brønsted acid sites and decreases the number of Lewis acid sites. The catalytic performance for Nb‐W oxides varied with the ratio of Brønsted to Lewis acid sites and high glucose conversion was observed over Nb₅W₅ and Nb₇W₃ oxides with high ratios of Brønsted to Lewis acid sites. All Nb‐W oxides show a relatively high selectivity of HMF , whereas no HMF forms over sulfuric acid due to its pure Brønsted acidity. The results indicate fast isomerization of glucose to fructose over Lewis acid sites followed by dehydration of fructose to HMF over Brønsted acid sites. Moreover, comparing to the reaction occurred in aqueous media, the 2‐butanol/H₂O system enhances the HMF selectivity and stabilizes the activity of the catalysts which gives the highest HMF selectivity of 52% over Nb₇W₃ oxide. The 2‐butanol/H₂O catalytic system can also be employed in conversion of sucrose, achieving HMF selectivity of 46% over Nb₅W₅ oxide.     

Ionic Association of the Ionic Liquids [C4mim][BF4], [C4mim][PF6], and [Cnmim]Br in Molecular Solvents

Considering the ionic nature of ionic liquids (ILs), ionic association is expected to be essential in solutions of ILs and to have an important influence on their applications. Although numerous studies have been reported for the ionic association behavior of ILs in solution, quantitative results are quite scarce. Herein, the conductivities of the ILs [C_nmim]Br (n=4, 6, 8, 10, 12), [C₄mim][BF₄], and [C₄mim][PF₆] in various molecular solvents (water, methanol, 1‐propanol, 1‐pentanol, acetonitrile, and acetone) are determined at 298.15 K as a function of IL concentration. The conductance data are analyzed by the Lee–Wheaton conductivity equation in terms of the ionic association constant (K_A) and the limiting molar conductance (Λ_m⁰). Combined with the values for the Br^? anion reported in the literature, the limiting molar conductivities and the transference numbers of the cations and [BF₄]^? and [PF₆]^? anions are calculated in the molecular solvents. It is shown that the alkyl chain length of the cations and type of anion affect the ionic association constants and limiting molar conductivities of the ILs. For a given anion (Br^?), the Λ_m⁰ values decrease with increasing alkyl chain length of the cations in all the molecular solvents, whereas the K_A values of the ILs decrease in organic solvents but increase in water as the alkyl chain length of the cations increases. For the [C₄mim]⁺ cation, the limiting molar conductivities of the ILs decrease in the order Br^?>[BF₄]^?>[PF₆]^?, and their ionic association constants follow the order [BF₄]^?>[PF₆]^?>Br^? in water, acetone, and acetonitrile. Furthermore, and similar to the classical electrolytes, a linear relationship is observed between ln K_A of the ILs and the reciprocal of the dielectric constants of the molecular solvents. The ILs are solvated to a different extent by the molecular solvents, and ionic association is affected significantly by ionic solvation. This information is expected to be useful for the modulation of the IL conductance by the alkyl chain length of the cations, type of anion, and physical properties of the molecular solvents.     

Quantification of the Activation Capabilities of Lewis/Brφnsted Acid for Electrophilic Trifluoromethylthiolating Reagents

Summary of main observation and conclusion Electrophilic trifluoromethylthiolation has emerged as an important and efficient methodology for installing the SCF3 moiety onto an array of organic molecules.Due to the low reactivities of trifluoromethylthiolating reagents,these transformations often require activation through an exogenous Lewis/Br0nsted acid.We report herein the quantification of the activation capabilities of Lewis/Br0nsted acids for trifluoromethylthiolating reagents through computing the differenee in trifluoromethylthio cation donor ability(Tt+DA)between the"activated"and"unactivated"reagent.A moderate correlation is found to exist between the activation capability and Lewis acidity.     

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.     

Regioselective Hydration of Alkynes by Iron(III) Lewis/Brønsted Catalysis

The triflimide iron(III) salt [Fe(NTf₂)₃] promotes the direct hydration of terminal and internal alkynes with very good Markovnikov regioselectivities and high yields. The enhanced carbophilic Lewis acidity of the Fe^III cation mediated by the weakly‐coordinating triflimide anion is crucial for the catalytic activity. The iron(III) metal salt can be recycled in the form of the OPPh₃/[Fe(NTf₂)₃] system with similar activity and selectivity. However, spectroscopic and kinetic studies show that [Fe(NTf₂)₃] hydrolyzes under the reaction conditions and that catalytically less active Brønsted species are formed, which points to a Lewis/Brønsted co‐catalysis. This triflimide‐based catalytic system is regioselective for the hydration of internal aryl‐alkynes and opens the door to a new synthetic route to alkyl ketophenones. As a proof of concept, the synthesis of two antipsychotics Haloperidol and Melperone, with general butyrophenone‐like structure, is shown.     

Enantioselective Addition Reaction of Azlactones with Styrene Derivatives Catalyzed by Strong Chiral Brønsted Acids

An enantioselective intermolecular addition reaction of azlactones, as carbon nucleophiles, with styrene derivatives, as simple olefins, was demonstrated using a newly developed chiral Brønsted acid catalyst, namely, F₁₀BINOL‐derived N‐triflyl phosphoramide. Addition products having vicinal tetrasubstituted carbon centers, one of which is an all‐carbon quaternary stereogenic center, were formed in good yields with moderate to high stereoselectivities. Extremely high acidity of the new chiral Brønsted acid was confirmed by its calculated pK_a value based on DFT studies and is the key to accomplishing not only high catalytic activity but also efficient stereocontrol in the intermolecular addition.     

Phosphonate‐Modified UiO‐66 Brønsted Acid Catalyst and Its Use in Dehydra‐Decyclization of 2‐Methyltetrahydrofuran to Pentadienes

Phosphorus‐modified all‐silica zeolites exhibit activity and selectivity in certain Brønsted acid catalyzed reactions for biomass conversion. In an effort to achieve similar performance with catalysts having well‐defined sites, we report the incorporation of Brønsted acidity to metal–organic frameworks with the UiO‐66 topology, achieved by attaching phosphonic acid to the 1,4‐benzenedicarboxylate ligand and using it to form UiO‐66‐PO₃H₂ by post‐synthesis modification. Characterization reveals that UiO‐66‐PO₃H₂ retains stability similar to UiO‐66, and exhibits weak Brønsted acidity, as demonstrated by titrations, alcohol dehydration, and dehydra‐decyclization of 2‐methyltetrahydrofuran (2‐MTHF). For the later reaction, the reported catalyst exhibits site‐time yields and selectivity approaching that of phosphoric acid on all‐silica zeolites. Using solid‐state NMR and deprotonation energy calculations, the chemical environments of P and the corresponding acidities are determined.     

Configuration of MFI-type zeolite interacting with the probe molecule P(CH<Subscript>3</Subscript>)<Subscript>3</Subscript> – a theoretical study

With P(CH₃)₃ as the probe molecule adsorbed on titanium silicalite (TS-1) zeolite, the special and important role of T12 site in MFI-type zeolite was clearly elucidated. There are altogether three active sites present in TS-1 zeolite with Ti at the T12 site. Owing to the preferential adsorption of probe molecules on the first Brönsted acidic site, the Ti12 center will probably fail to show Lewis acidity. The ionic [HP(CH₃)₃]⁺ species can be stabilized by the first or second Brönsted acidic site, with the former energetically favored. The latter was formed through the transfer of the ionic [HP(CH₃)₃]⁺ species from the first to the second Brönsted acidic site.     

Chiral Brønsted Acid Directed Iron‐Catalyzed Enantioselective Friedel–Crafts Alkylation of Indoles with β‐Aryl α′‐Hydroxy Enones

A cooperative catalytic system established by the combination of an iron salt and a chiral Brønsted acid has proven to be effective in the asymmetric Friedel–Crafts alkylation of indoles with β‐aryl α′‐hydroxy enones. Good to excellent yields and enatioselectivities were observed for a variety of α′‐hydroxy enones and indoles, particularly for the β‐aryl α′‐hydroxy enones bearing an electron‐withdrawing group at the para position of the phenyl ring (up to 90 % yield and 91 % ee). The proton of the chiral Brønsted acid, the Lewis acid activation site, as well as the inherent basic site for the hydrogen‐bonding interaction of the Brønsted acid are responsible for the high catalytic activities and enantioselectivities of the title reaction. A possible reaction mechanism was proposed. The key catalytic species in the catalytic system, the phosphate salt of Fe^III, which was thought to be responsible for the high activity and good enantioselectivity, was then confirmed by ESIMS studies.