Pseudo‐Bridging Silanols as Versatile Brønsted Acid Sites of Amorphous Aluminosilicate Surfaces

Amorphization tunes acidity : Pseudo‐bridging silanols, suggested as versatile Brønsted acid groups by molecular modeling studies, are obtained by shifted hydrolysis of Si? O? Al bridges formed by the thermal treatment of silica deposited on γ‐Al₂O₃ (100), and appear under given pretreatment conditions. Demixing of part of the silica from the aluminosilicate phase is predicted upon excess water adsorption.

     

Isopropanol Dehydration on Amorphous Silica–Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo‐Bridging Silanols

The mechanism of isopropanol dehydration on amorphous silica–alumina (ASA) was unraveled by a combination of experimental kinetic measurements and periodic density functional theory (DFT) calculations. We show that pseudo‐bridging silanols (PBS‐Al) are the most likely active sites owing to the synergy between the Brønsted and Lewis acidic properties of these sites, which facilitates the activation of alcohol hydroxy groups as leaving groups. Isopropanol dehydration was used to specifically investigate these PBS‐Al sites, whose density was estimated to be about 10⁻¹ site nm⁻² on the silica‐doped alumina surface under investigation, by combining information from experiments and theoretical calculations.     

The Effect of Ge Incorporation on the Brønsted Acidity of ZSM‐5

The effect of the isomorphous substitution of some of the Si atoms in ZSM‐5 by Ge atoms on the Brønsted acid strength has been investigated by i) DFT calculations on cluster models of the formula ((HO)₃SiO)₃‐Al‐O(H)‐T‐(OSi(OH)₃)₃, with T=Si or Ge, and ((HO)₃SiO)₃‐Al‐O(H)‐Si‐(OGe(OH)₃)(OSi(OH)₃)₂, ii) a ³¹P NMR study of zeolite samples contacted with trimethyl phosphine oxide probe molecules and iii) a X‐ray photoelectron spectroscopy (XPS) study of ZSM‐5 and Ge‐ZSM‐5 samples. The calculations reveal that the effect of Ge incorporation on the framework acidity strongly depends on the degree of substitution and on the exact T‐atom positions that are occupied by Ge. High Ge concentrations allow for enhanced stabilisation of the deprotonated Ge‐ZSM‐5 through structural relaxation, resulting in a slightly higher acidity as compared to ZSM‐5. This structural relaxation is not achievable in Ge‐ZSM‐5 with a low Ge content, which therefore has a slightly lower acidity than ZSM‐5. The NMR study indicates no difference between the Brønsted acidity of ZSM‐5(47) and Ge(0.09)ZSM‐5(36). Instead, evidence for the presence of a substantial amount of Ge? OH groups in the Ge‐containing samples was obtained from the NMR results, which is consistent with earlier FTIR studies. The XPS results do not point to an effect of Ge on the framework acidity of ZSM‐5(47), instead, the results can be best interpreted by assuming the presence of additional Ge? OH and Si? OH groups near the surface of the Ge(0.08)ZSM‐5(47) sample.     

Iron/Brønsted Acid Catalyzed Asymmetric Hydrogenation: Mechanism and Selectivity‐Determining Interactions

Hydrogenation catalysts involving abundant base metals such as cobalt or iron are promising alternatives to precious metal systems. Despite rapid progress in this field, base metal catalysts do not yet achieve the activity and selectivity levels of their precious metal counterparts. Rational improvement of base metal complexes is facilitated by detailed knowledge about their mechanisms and selectivity‐determining factors. The mechanism for asymmetric imine hydrogenation with Knölker’s iron complex in the presence of chiral phosphoric acids is here investigated computationally at the DFT‐D level of theory, with models of up to 160 atoms. The resting state of the system is found to be an adduct between the iron complex and the deprotonated acid. Rate‐limiting H₂ splitting is followed by a stepwise hydrogenation mechanism, in which the phosphoric acid acts as the proton donor. C?H ??? O interactions between the phosphoric acid and the substrate are involved in the stereocontrol at the final hydride transfer step. Computed enantiomeric ratios show excellent agreement with experimental values, indicating that DFT‐D is able to correctly capture the selectivity‐determining interactions of this system.     

Framework Stability and Brønsted Acidity of Isomorphously Substituted Interlayer‐Expanded Zeolite COE‐4: A Density Functional Theory Study

COE‐4 zeolites possess a unique two‐dimensional ten‐ring pore structure with the Si(OH)₂ hydroxyl groups attached to the linker position between the ferrierite‐type layers, which has been demonstrated through the interlayer‐expansion approach in our previous work (H. Gies et al. Chem. Mater.­ 2012 , 24, 1536). Herein, density functional theory is used to study the framework stability and Brønsted acidity of the zeolite T‐COE‐4, in which the tetravalent Si is isomorphously substituted by a trivalent Fe, B, Ga, or Al heteroatom at the linker position. The influences of substitution energy and equilibrium geometry parameters on the stability of T‐COE‐4 are investigated in detail. The relative acid strength of the linker position is revealed by the proton affinity, charge analysis, and NH₃ adsorption. It is found that the range of the 〈T‐O‐Si〉 angles is widened to maintain the stability of isomorphously substituted T‐COE‐4 zeolites. The smaller the 〈O1‐T‐O2〉 bond angle is, the more difficult is to form the regular tetrahedral unit. Thus, the substitution energies at the linker positions increase in the following sequence: Al‐COE‐4 < Ga‐COE‐4 < Fe‐COE‐4 < B‐COE‐4. The adsorption of NH₃ as a probe molecule indicates that the acidity can affect the hydrogen‐bonding interaction between (N?H???O2) and (N???H?O2). The relative Brønsted‐acid strength of the interlayer‐expanded T‐COE‐4 zeolite decreases in the order of Al‐COE‐4 > Ga‐COE‐4 > Fe‐COE‐4 > B‐COE‐4. These findings may be helpful for the structural design and functional modification of interlayer‐expanded zeolites.     

Protic Anions [H(B12X12)]− (X=F,Cl, Br,I) That Act as Brønsted Acids in the Gas Phase

The acidity of protic cations and neutral molecules has been studied extensively in the gas phase, and the gas‐phase acidity has been established previously as a very useful measure of the intrinsic acidity of neutral and cationic compounds. However, no data for any anionic acids were available prior to this study. The protic anions [H(B₁₂X₁₂)]^? (X=F, Cl, Br, I) are expected to be the most acidic anions known to date. Therefore, they were investigated in this study with respect to their ability to protonate neutral molecules in the gas phase by using a combination of mass spectrometry and quantum‐chemical calculations. For the first time it was shown that in the gas phase protic anions are also able to protonate neutral molecules and thus act as Brønsted acids. According to theoretical calculations, [H(B₁₂I₁₂)]^? is the most acidic gas‐phase anion, whereas in actual protonation experiments [H(B₁₂Cl₁₂)]^? is the most potent gas‐phase acidic anion for the protonation of neutral molecules. This discrepancy is explained by ion pairing and kinetic effects.     

Absolute Brønsted Acidities and pH Scales in Ionic Liquids

Although receiving large interest over the last years, some fundamental aspects of Brønsted acidity in ionic liquids (ILs) have up to now been insufficiently highlighted. In this work, standard states, activity, and activity coefficient definitions for IL solvent systems were developed from general thermodynamic considerations and then extended to a general mixed solvent standard state. By using the bromide/bromoaluminate systems as representative ILs, formulae for thermodynamically consistent pH scales for ILs with simple (Br^?) and complex ([Al_nBr_3n+1]^?) anions were derived on the basis of the chemical potential of the proton. Supported by quantum chemical [ccsd(t)/MP2/DFT/COSMO‐RS] calculations, Gibbs solvation energies of the proton were calculated, which allowed the ILs to be ranked in absolute acidity, that is, pH_abs or μ_abs(H⁺, IL), and additionally allowed their acidity to be compared with molecular Brønsted acid systems. It was shown that bromoaluminate ILs are suited for reaching superacidic conditions. The complexity of autoprotolysis processes in C₆MIM⁺[AlBr₄]^? (C₆MIM=1‐hexyl‐3‐methylimidazolium) with or without the addition of basic (i.e. Br^?) or acidic (AlBr₃ and/or HBr) solutes was examined in detail by model calculations, and they indicated a large thermodynamic influence of small deviations from the exact stoichiometric composition.     

p‐Nitrobenzoic Acid Adsorption on Nanostructured Gold Surfaces Investigated by Combined Experimental and Computational Approaches

Adsorption of guest molecules on host surfaces can lead to dramatic changes in the spectral properties of the guest. One such effect is surface‐enhanced infrared absorption (SEIRA), observed when the guest is adsorbed on, for example, thin films, metal surfaces, or nanotubes. p‐Nitrobenzoic acid (p‐NBA) exhibits a SEIRA effect when adsorbed on Ag and Au. Herein, the IR spectra of p‐NBA adsorbed on a homemade rough Au surface, recorded in reflection mode with an angle of incidence of 16.5°, are reported. This SEIRA experiment reveals more bands than found by previous SEIRA studies. The intensities of both symmetric and asymmetric COO^? and NO₂ stretching, in‐plane CH, and C?C ring stretching modes are enhanced. Theoretical models constructed on the basis of density functional theory reveal the binding mode of p‐NBA to gold “particles”. The p‐NBA anion binds to gold much more strongly than the neutral form, and interaction via the carboxylic oxygen atoms is preferred over the nitro group–gold contact. A significant charge transfer during chemisorption is found, which is considered to be crucial in leading to a high SEIRA enhancement factor.     

Metal‐Supported Aluminosilicate Ultrathin Films as a Versatile Tool for Studying the Surface Chemistry of Zeolites

The application of a variety of “surface‐science” techniques to elucidate surface structures and mechanisms of chemical reactions at zeolite surfaces has long been considered as almost impossible because of the poor electrical and thermal conductivity of those materials. Here, we show that the growth of a thin aluminosilicate film on a metal single crystal under controlled conditions results in adequate and well‐defined model systems for zeolite surfaces. In principle, silicate films that contain metals other than Al (e.g. Ti, Fe, etc) may be prepared in a similar way. We believe that this approach opens up a new playground for experimental and theoretical modeling of zeolites, aimed at a fundamental understanding of structure–reactivity relationships in such materials.     

Recent Developments in the Chiral Brønsted Acid‐catalyzed Allylboration Reaction with Polyfunctionalized Substrates

Asymmetric allylboration has played a central role in organic synthesis ever since the pioneering work by Hoffman and Brown, having found applications in the total synthesis of many natural products. A new dawn for this 40 year‐old reaction occurred with the beginning of the new century when the first catalytic asymmetric methods came into play. In less than one decade, several methodologies, able to achieve the desired homoallylic alcohols with ee ranges in the high 90s, were developed. Among them, in the present account, we will disclose our contribution to the development of the chiral binolphosphoric‐derived Brønsted acid‐catalyzed allylboration of aldehydes originally reported by Antilla in 2010. Our contribution to this field lies in its application to polyfunctionalized systems, both on the aldehyde and the allylboronate in question, which enables the rapid construction of molecular diversity and complexity. Parts of the work described herein have been carried out in collaboration with the groups of Profs. Akiyama and Houk.

     

Regioselective Long‐Range Proton Transfer in New Rifamycin Antibiotics: A Process in which Crown Ethers Act as Stronger Brønsted Bases than Amines

Water‐mediated proton transfer in six new derivatives of 3‐formylrifamycin SV that contain crown, aza‐crown, and benzo‐crown ether rings were investigated by FTIR and NMR spectroscopy. ¹H–¹H COSY couplings provide evidence for the formation of zwitterionic structures of the aza‐crown and crown ether derivatives of rifamycin, in which a proton from one of the phenolic groups is transferred to tertiary and secondary nitrogen atoms. The increased intensity of the continuous absorption in the mid‐infrared region together with the NMR data indicate proton transfer from the phenol group of the rifamycin core to the cavity of the benzo‐crown ether ring. This proton transfer is achieved by formation of hydronium (H₃O⁺) or Zundel ions (H₅O₂⁺), which form intermolecular hydrogen bonds with the oxygen atoms of the crown ether. DFT calculations are in agreement with the spectroscopic data and allow visualization of the structures of all new rifamycin derivatives, characterized by different intramolecular protonation sites.     

Brønsted Acidity and the Medium: Fundamentals with a Focus on Ionic Liquids

Fundamental aspects of Brønsted acidity in ionic liquid systems, in relation to those of simple protic molecules in the gas phase, pure protic molecules in the condensed phase and solutions of protic molecules in molecular systems, are presented. The variety of acidities possible, beyond those observed in aqueous systems, is emphasised and discussed in terms of differences of solvent levelling, ionisation, dissociation, homo‐/hetero‐conjugate ion speciation and the stabilisation of proton‐transfer products from solvent to solvent. It is argued that data regarding aqueous systems do not necessarily explain acid/base behaviour in other liquids satisfactorily. Methods of measuring acidity are reviewed, particularly by spectrophotometry and electrochemistry and recommendations proffered for estimating speciation and acidity of ionic liquids of various complexities.     

DFT Simulations of Water Adsorption and Activation on Low‐Index α‐Ga2O3 Surfaces

Density functional theory (DFT) calculations are used to explore water adsorption and activation on different α‐Ga₂O₃ surfaces, namely (001), (100), (110), and (012). The geometries and binding energies of molecular and dissociative adsorption are studied as a function of coverage. The simulations reveal that dissociative water adsorption on all the studied low‐index surfaces are thermodynamically favorable. Analysis of surface energies suggests that the most preferentially exposed surface is (012). The contribution of surface relaxation to the respective surface energies is significant. Calculations of electron local density of states indicate that the electron‐energy band gaps for the four investigated surfaces appears to be less related to the difference in coordinative unsaturation of the surface atoms, but rather to changes in the ionicity of the surface chemical bonds. The electrochemical computation is used to investigate the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) on α‐Ga₂O₃ surfaces. Our results indicate that the (100) and (110) surfaces, which have low stability, are the most favorable ones for HER and OER, respectively.     

Brønsted Acid‐Mediated Hydrative Arylation of Unactivated Alkynes

The Brønsted acid‐mediated reaction of unactivated alkynes with aryl sulfoxides leads to simultaneous hydration and intermolecular C?C bond formation. This solvent‐ and metal‐free transformation directly delivers α‐arylated carbonyl compounds as the products of a formal hydrative arylation in an atom‐economical manner. The products bear useful synthetic handles for further functionalization.     

Adsorption of 1,3‐Benzenedithiol and 1,3‐Benzenedimethanethiol on Gold Surfaces

The adsorption characteristics of 1,3‐benzenedithiol (1,3‐BDT) and 1,3‐benzenedimethanethiol (1,3‐BDMT) on Au surfaces are investigated by means of surface‐enhanced Raman scattering, UV/Vis absorption spectroscopy, and cyclic voltammetry (CV). 1,3‐BDMT is found to adsorb via two S–Au linkages at concentrations below monolayer coverage, but to have an upright geometry as the concentration increases on Au nanoparticles. On the other hand, 1,3‐BDT is found to adsorb by forming two S–Au linkages, regardless of concentration, based on the disappearance of the ν(SH)_free stretching band. Because of the absence of the methylene unit, 1,3‐BDT appeares not to self‐assemble efficiently on Au surfaces. The UV/Vis absorption spectroscopy and CV techniques are also applied to check the formation of self‐assembled monolayers of 1,3‐BDT and 1,3‐BDMT on Au. Density functional theory calculations based on a simple adsorption model using an Au₈ cluster are performed to better understand the nature of the adsorption characteristics of 1,3‐BDT and 1,3‐BDMT on Au surfaces.     

Brønsted Acid Catalyzed Tandem Defunctionalization of Biorenewable Ferulic acid and Derivates into Bio‐Catechol

An efficient conversion of biorenewable ferulic acid into bio‐catechol has been developed. The transformation comprises two consecutive defunctionalizations of the substrate, that is, C?O (demethylation) and C?C (de‐2‐carboxyvinylation) bond cleavage, occurring in one step. The process only requires heating of ferulic acid with HCl (or H₂SO₄) as catalyst in pressurized hot water (250 °C, 50 bar N₂). The versatility is shown on a variety of other (biorenewable) substrates yielding up to 84 % di‐ (catechol, resorcinol, hydroquinone) and trihydroxybenzenes (pyrogallol, hydroxyquinol), in most cases just requiring simple extraction as work‐up.