Showing papers on "Lewis acids and bases published in 2014"

Enhanced photoluminescence and solar cell performance via Lewis base passivation of organic-inorganic lead halide perovskites.

Abstract: Organic-inorganic metal halide perovskites have recently emerged as a top contender to be used as an absorber material in highly efficient, low-cost photovoltaic devices. Solution-processed semiconductors tend to have a high density of defect states and exhibit a large degree of electronic disorder. Perovskites appear to go against this trend, and despite relatively little knowledge of the impact of electronic defects, certified solar-to-electrical power conversion efficiencies of up to 17.9% have been achieved. Here, through treatment of the crystal surfaces with the Lewis bases thiophene and pyridine, we demonstrate significantly reduced nonradiative electron-hole recombination within the CH(3)NH(3)PbI(3-x)Cl(x) perovskite, achieving photoluminescence lifetimes which are enhanced by nearly an order of magnitude, up to 2 μs. We propose that this is due to the electronic passivation of under-coordinated Pb atoms within the crystal. Through this method of Lewis base passivation, we achieve power conversion efficiencies for solution-processed planar heterojunction solar cells enhanced from 13% for the untreated solar cells to 15.3% and 16.5% for the thiophene and pyridine-treated solar cells, respectively.

Lewis acid-base interactions between polysulfides and metal organic framework in lithium sulfur batteries

Abstract: Lithium-sulfur (Li-S) battery is one of the most promising energy storage systems because of its high specific capacity of 1675 mAh g(-1) based on sulfur. However, the rapid capacity degradation, mainly caused by polysulfide dissolution, remains a significant challenge prior to practical applications. This work demonstrates that a novel Ni-based metal organic framework (Ni-MOF), Ni6(BTB)4(BP)3 (BTB = benzene-1,3,5-tribenzoate and BP = 4,4'-bipyridyl), can remarkably immobilize polysulfides within the cathode structure through physical and chemical interactions at molecular level. The capacity retention achieves up to 89% after 100 cycles at 0.1 C. The excellent performance is attributed to the synergistic effects of the interwoven mesopores (∼2.8 nm) and micropores (∼1.4 nm) of Ni-MOF, which first provide an ideal matrix to confine polysulfides, and the strong interactions between Lewis acidic Ni(II) center and the polysulfide base, which significantly slow down the migration of soluble polysulfides out of the pores, leading to the excellent cycling performance of Ni-MOF/S composite.

Catalytic amide formation from non-activated carboxylic acids and amines

Abstract: The amide functionality is found in a wide variety of biological and synthetic structures such as proteins, polymers, pesticides and pharmaceuticals. Due to the fact that synthetic amides are still mainly produced by the aid of coupling reagents with poor atom-economy, the direct catalytic formation of amides from carboxylic acids and amines has become a field of emerging importance. A general, efficient and selective catalytic method for this transformation would meet well with the increasing demands for green chemistry procedures. This review covers catalytic and synthetically relevant methods for direct condensation of carboxylic acids and amines. A comprehensive overview of homogeneous and heterogeneous catalytic methods is presented, covering biocatalysts, Lewis acid catalysts based on boron and metals as well an assortment of other types of catalysts.

Tetrel bond–σ-hole bond as a preliminary stage of the SN2 reaction

Abstract: MP2/aug-cc-pVTZ calculations were carried out on complexes of ZH4, ZFH3 and ZF4 (Z = C, Si and Ge) molecules with HCN, LiCN and Cl− species acting as Lewis bases through nitrogen centre or chlorine ion. Z-Atoms in these complexes usually act as Lewis acid centres forming σ-hole bonds with Lewis bases. Such noncovalent interactions may adopt a name of tetrel bonds since they concern the elements of the group IV. There are exceptions for complexes of CH4 and CF4, as well as for the F4Si⋯NCH complex where the tetrel bond is not formed. The energetic and geometrical parameters of the complexes were analyzed and numerous correlations between them were found. The Quantum Theory of ‘Atoms in Molecules’ and Natural Bonds Orbital (NBO) method used here should deepen the understanding of the nature of the tetrel bond. An analysis of the electrostatic potential surfaces of the interacting species is performed. The electron charge redistribution, being the result of the tetrel bond formation, is the same as that of the SN2 reaction. The energetic and geometrical parameters of the complexes analyzed here correspond to different stages of the SN2 process.

Lewis Acid-Assisted Formic Acid Dehydrogenation Using a Pincer-Supported Iron Catalyst

Abstract: Formic acid (FA) is an attractive compound for H2 storage. Currently, the most active catalysts for FA dehydrogenation use precious metals. Here, we report a homogeneous iron catalyst that, when used with a Lewis acid (LA) co-catalyst, gives approximately 1,000,000 turnovers for FA dehydrogenation. To date, this is the highest turnover number reported for a first-row transition metal catalyst. Preliminary studies suggest that the LA assists in the decarboxylation of a key iron formate intermediate and can also be used to enhance the reverse process of CO2 hydrogenation.

Direct observation of a borane–silane complex involved in frustrated Lewis-pair-mediated hydrosilylations

Abstract: Perfluorarylborane Lewis acids catalyse the addition of silicon-hydrogen bonds across C=C, C=N and C=O double bonds. This 'metal-free' hydrosilylation has been proposed to occur via borane activation of the silane Si-H bond, rather than through classical Lewis acid/base adducts with the substrate. However, the key borane/silane adduct had not been observed experimentally. Here it is shown that the strongly Lewis acidic, antiaromatic 1,2,3-tris(pentafluorophenyl)-4,5,6,7-tetrafluoro-1-boraindene forms an observable, isolable adduct with triethylsilane. The equilibrium for adduct formation was studied quantitatively through variable-temperature NMR spectroscopic investigations. The interaction of the silane with the borane occurs through the Si-H bond, as evidenced by trends in the Si-H coupling constant and the infrared stretching frequency of the Si-H bond, as well as by X-ray crystallography and theoretical calculations. The adduct's reactivity with nucleophiles demonstrates conclusively the role of this species in metal-free 'frustrated-Lewis-pair' hydrosilylation reactions.

Simple,Chemoselective, Catalytic Olefin Isomerization

Abstract: Catalytic amounts of Co(SaltBu,tBu)Cl and organosilane irreversibly isomerize terminal alkenes by one position. The same catalysts effect cycloisomerization of dienes and retrocycloisomerization of strained rings. Strong Lewis bases like amines and imidazoles, and labile functionalities like epoxides, are tolerated.

Halogen bonding in water results in enhanced anion recognition in acyclic and rotaxane hosts

Abstract: The ability to achieve strong molecular recognition in water is a key challenge for supramolecular chemistry. Now, halogen bonding — the attractive interaction between an electron-deficient halogen atom and a Lewis base — has been shown to be superior to hydrogen bonding for strong anion binding in water. Ripple image: © PhotoDisc/Getty Images.

Synthesis of terephthalic acid via Diels-Alder reactions with ethylene and oxidized variants of 5-hydroxymethylfurfural

Abstract: Terephthalic acid (PTA), a monomer in the synthesis of polyethylene terephthalate (PET), is obtained by the oxidation of petroleum-derived p-xylene. There is significant interest in the synthesis of renewable, biomass-derived PTA. Here, routes to PTA starting from oxidized products of 5-hydroxymethylfurfural (HMF) that can be produced from biomass are reported. These routes involve Diels-Alder reactions with ethylene and avoid the hydrogenation of HMF to 2,5-dimethylfuran. Oxidized derivatives of HMF are reacted with ethylene over solid Lewis acid catalysts that do not contain strong Bronsted acids to synthesize intermediates of PTA and its equally important diester, dimethyl terephthalate (DMT). The partially oxidized HMF, 5-(hydroxymethyl)furoic acid (HMFA), is reacted with high pressure ethylene over a pure-silica molecular sieve containing framework tin (Sn-Beta) to produce the Diels-Alder dehydration product, 4-(hydroxymethyl)benzoic acid (HMBA), with 31% selectivity at 61% HMFA conversion after 6 h at 190 °C. If HMFA is protected with methanol to form methyl 5-(methoxymethyl)furan-2-carboxylate (MMFC), MMFC can react with ethylene in the presence of Sn-Beta for 2 h to produce methyl 4-(methoxymethyl)benzenecarboxylate (MMBC) with 46% selectivity at 28% MMFC conversion or in the presence of a pure-silica molecular sieve containing framework zirconium (Zr-Beta) for 6 h to produce MMBC with 81% selectivity at 26% MMFC conversion. HMBA and MMBC can then be oxidized to produce PTA and DMT, respectively. When Lewis acid containing mesoporous silica (MCM-41) and amorphous silica, or Bronsted acid containing zeolites (Al-Beta), are used as catalysts, a significant decrease in selectivity/yield of the Diels-Alder dehydration product is observed.

Improved Postsynthesis Strategy to Sn-Beta Zeolites as Lewis Acid Catalysts for the Ring-Opening Hydration of Epoxides

Abstract: Nanocrystalline Sn-Beta zeolites have been successfully prepared via an improved two-step postsynthesis strategy, which consists of creating vacant T sites with associated silanol groups by dealumination of parent H-Beta and subsequent dry impregnation of the resulting Si-Beta with organometallic dimethyltin dichloride. Characterization results from UV−vis, XPS, Raman, and 119 Sn solid-state MAS NMR reveal that most Sn species have been successfully incorpo- rated into the framework of Beta zeolite through the postsynthesis process and exist as isolated tetrahedral Sn(IV) in open arrangement. The creation of strong Lewis acid sites upon Sn incorporation is confirmed by FTIR spectroscopy with pyridine adsorption. The Sn-Beta Lewis acid catalysts are applied in the ring-opening hydration of epoxides to the corresponding 1,2-diols under near ambient and solvent-free conditions, and remarkable activity can be obtained. The impacts of Lewis acidity, preparation parameters, and reaction conditions on the catalytic performance of Sn-Beta zeolites are discussed in detail.

Enabling catalytic ketone hydrogenation by frustrated Lewis pairs.

Abstract: Hydrogenation of alkyl and aryl ketones using H2 is catalytically achieved in 18 examples using 5 mol % B(C6F5)3 in an ethereal solvent. In these cases the borane and ether behave as a frustrated Lewis pair to activate H2 and effect the reduction.

Recent Advances on the Lewis Acid-Catalyzed Cascade Rearrangements of Propargylic Alcohols and Their Derivatives

Abstract: Propargylic alcohols and their derivatives are important classes of organic compounds that can be easily accessible from terminal alkynes and aldehydes or ketones. They are attractive and have been extensively applied as synthetic intermediates in modern organic synthesis. Recent investigations on the chemistry of propargylic alcohols and their derivatives disclosed a variety of highly efficient Lewis acid-catalyzed cascade reactions based on Meyer–Schuster rearrangement of propargylic alcohols and [3,3] rearrangement of propargylic esters and propargyl vinyl ethers. A tremendous number of structurally versatile enones, carbocycles, and heterocycles have been constructed through these methodologies. These advances, including our recent researches in this field, are summarized in this review.

Nonmetal Catalyzed Hydrogenation of Carbonyl Compounds

Abstract: Solutions of the Lewis acid B(C6F5)3 in 1,4-dioxane are found to effectively catalyze the hydrogenation of a variety of ketones and aldehydes These reactions, the first to allow entirely metal-free catalytic hydrogenation of carbonyl groups under relatively mild reaction conditions, are found to proceed via a “frustrated Lewis pair” mechanism in which the solvent, a weak Bronsted base yet moderately strong donor, plays a pivotal role

Reducing CO2 to Methanol Using Frustrated Lewis Pairs: On the Mechanism of Phosphine–Borane-Mediated Hydroboration of CO2

Abstract: The full mechanism of the hydroboration of CO2 by the highly active ambiphilic organocatalyst 1-Bcat-2-PPh2–C6H4 (Bcat = catecholboryl) was determined using computational and experimental methods. The intramolecular Lewis pair was shown to be involved in every step of the stepwise reduction. In contrast to traditional frustrated Lewis pair systems, the lack of steric hindrance around the Lewis basic fragment allows activation of the reducing agent while moderate Lewis acidity/basicity at the active centers promotes catalysis by releasing the reduction products. Simultaneous activation of both the reducing agent and carbon dioxide is the key to efficient catalysis in every reduction step.

Friedel–Crafts Reaction of Benzyl Fluorides: Selective Activation of C ? F Bonds as Enabled by Hydrogen Bonding

Abstract: A Friedel–Crafts benzylation of arenes with benzyl fluorides has been developed. The reaction produces 1,1-diaryl alkanes in good yield under mild conditions without the need for a transition metal or a strong Lewis acid. A mechanism involving activation of the CF bond through hydrogen bonding is proposed. This mode of activation enables the selective reaction of benzylic CF bonds in the presence of other benzylic leaving groups.

New bonding modes of carbon and heavier group 14 atoms Si–Pb

Abstract: Recent theoretical studies are reviewed which show that the naked group 14 atoms E = C–Pb in the singlet 1D state behave as bidentate Lewis acids that strongly bind two σ donor ligands L in the donor–acceptor complexes L→E←L. Tetrylones EL2 are divalent E(0) compounds which possess two lone pairs at E. The unique electronic structure of tetrylones (carbones, silylones, germylones, stannylones, plumbylones) clearly distinguishes them from tetrylenes ER2 (carbenes, silylenes, germylenes, stannylenes, plumbylenes) which have electron-sharing bonds R–E–R and only one lone pair at atom E. The different electronic structures of tetrylones and tetrylenes are revealed by charge- and energy decomposition analyses and they become obvious experimentally by a distinctively different chemical reactivity. The unusual structures and chemical behaviour of tetrylones EL2 can be understood in terms of the donor–acceptor interactions L→E←L. Tetrylones are potential donor ligands in main group compounds and transition metal complexes which are experimentally not yet known. The review also introduces theoretical studies of transition metal complexes [TM]–E which carry naked tetrele atoms E = C–Sn as ligands. The bonding analyses suggest that the group-14 atoms bind in the 3P reference state to the transition metal in a combination of σ and π∥ electron-sharing bonds TM–E and π⊥ backdonation TM→E. The unique bonding situation of the tetrele complexes [TM]–E makes them suitable ligands in adducts with Lewis acids. Theoretical studies of [TM]–E→W(CO)5 predict that such species may becomes synthesized.

State of the art of Lewis acid-containing zeolites: lessons from fine chemistry to new biomass transformation processes

Abstract: The former synthesis of TS-1 opened new catalytic opportunities for zeolites, especially for their application as selective redox catalysts in several fine chemistry processes. Interestingly, isolated Ti species in the framework positions of hydrophobic zeolites, such as high silica zeolites, offer unique Lewis acid sites even in the presence of protic polar solvents (such as water). Following this discovery, other transition metals (such as Sn, Zr, V, Nb, among others) have been introduced in the framework positions of different hydrophobic zeolitic structures, allowing their application in new fine chemistry processes as very active and selective redox catalysts. Recently, these hydrophobic metallozeolites have been successfully applied as efficient catalysts for several biomass-transformation processes in bulk water. The acquired knowledge from the former catalytic descriptions in fine chemistry processes using hydrophobic Lewis acid-containing zeolites has been essential for their application in these novel biomass transformations. In the present review, I will describe the recent advances in the synthesis of new transition metal-containing zeolites presenting Lewis acid character, and their unique catalytic applications in both fine chemistry and novel biomass-transformations.

Role of Lewis and Brønsted Acid Sites in the Dehydration of Glycerol over Niobia

Abstract: The role of Lewis and Bronsted sites in the dehydration of glycerol on niobium oxide and Na+-exchanged niobium oxide is investigated using FTIR spectroscopy supported by DFT calculations. Glycerol is impregnated on the catalysts at room temperature using an ex-situ method. Under high vacuum conditions, glycerol forms a stable multidentate alkoxy species through its primary hydroxyl groups with the Lewis sites. When coordinated this way, the primary C–O bonds are polarized, favoring dehydration in this position to form hydroxyacetone. In contrast, dehydration of the secondary alcohol group is kinetically favored over Bronsted acid sites in the absence of steric constraints. The primary product of this reaction, 1,3-propenediol, is further dehydrated to acrolein. When more than a monolayer of glycerol is impregnated on niobia, monoaromatic compounds are also formed on the surface upon heating.

Selective Base-Catalyzed Isomerization of Glucose to Fructose

Abstract: Fructose is a key intermediate in the conversion of cellulosic biomass to biofuels and renewable platform chemicals. Biomass-derived glucose can be isomerized to fructose using either Lewis acid or Bronsted base catalysts. Lewis acids are typically preferred as alkaline conditions promote a large number of side reactions. It is widely admitted that only low fructose yields, below 10%, are achievable with inorganic bases. Here, fructose was synthesized with 32% yield using commercially available organic amines. Glucose conversion and fructose selectivity were comparable to Lewis acids, which opens new perspectives for the base-catalyzed pathway.

Organocatalytic Lewis base functionalisation of carboxylic acids, esters and anhydrides via C1-ammonium or azolium enolates

Abstract: This tutorial review highlights the organocatalytic Lewis base functionalisation of carboxylic acids, esters and anhydrides via C1-ammonium/azolium enolates. The generation and synthetic utility of these powerful intermediates is highlighted through their application in various methodologies including aldol-lactonisations, Michael-lactonisations/lactamisations and [2,3]-rearrangements.

Photodissociation of B-N Lewis adducts: a partially fused trinaphthylborane with dual fluorescence.

Abstract: The synthesis of a planarized trinaphthylborane with partially fused structure is presented. This compound shows not only high chemical and thermal stability but also sufficient Lewis acidity to form Lewis adducts with pyridine derivatives in solution. The B–N Lewis adducts exhibit unprecedented photodissociation behavior in the excited state, reminiscent of the photogeneration of carbenium ions from triarylmethane leuco dyes. Consequently, these B–N Lewis adducts exhibit dual fluorescence emission arising from the initial tetracoordinate B–N adducts and the photodissociated tricoordinate boranes.

Structure–activity relationship of VOx/CeO2 nanorod for NO removal with ammonia

Abstract: A series of vanadia supported on ceria nanorods are prepared by impregnation method for selective catalytic reduction (SCR) of NO with ammonia. Two kinds of vanadia species (VOx) (oligomeric and polymeric VOx) and CeVO4 are dispersed on the ceria surface according to the vanadium surface density. These species slightly suppress the catalyst reducibility and concentration of surface oxygen defects rather than distort the ceria cubic lattice or enlarge the BET surface areas. Polymeric VOx and CeO2 create the Lewis acid sites and CeVO4 could be served as the Bronsted acid sites. Polymeric VOx provide new active sites compared with pure CeO2 for the SCR reaction and CeVO4 enhance the number of active sites. Moreover, part of the Lewis acid sites might be converted into the Bronsted acid sites at high temperature under the SCR gas flow. According to the investigations of the reaction mechanism, both Lewis and Bronsted acid sites are reactive with gaseous NO. At low temperature, cis-N2O22− and dimer (NO)2 are active, while surface nitrite or nitrate species are active at high temperature.

Dealuminated Beta zeolite as effective bifunctional catalyst for direct transformation of glucose to 5-hydroxymethylfurfural

Abstract: To improve the catalytic performance of Beta zeolite in the direct transformation of glucose into 5-hydroxymethylfurfural (HMF), effects of calcination and steam treatment on the structure of Al atoms in the framework and acid properties of Beta zeolite were examined in detail 27Al MAS NMR measurement and IR observation revealed that a part of Si–O–Al bonds in the framework were cleaved to form Al species out of the * BEA framework during the treatments and these species showed Lewis acidity Especially, when the ammonium-type Beta was calcined over 700 °C or treated with steam (50 kPa in N2 balance) over 500 °C, the amount of Lewis acid sites was increased at the expense of Bronsted acid sites Thus prepared Beta zeolite catalysts having a sufficient amount of Lewis acid sites were found to be effective bifunctional catalysts in synthesis of HMF from glucose; for example, Beta zeolite prepared by the calcination at 750 °C showed 55% selectivity to HMF at 78% conversion of glucose We clarified the roles of Lewis and Bronsted acid sites on the Beta zeolite in the direct transformation of glucose to HMF Furthermore, the reaction mechanism for the isomerization of glucose was investigated by means of isotope experiment using deuterated glucose Finally, reusability of the Beta zeolite was also investigated

Atomistic explanation of shear-induced amorphous band formation in boron carbide.

Abstract: Boron carbide (B_4C) is very hard, but its applications are hindered by stress-induced amorphous band formation. To explain this behavior, we used density function theory (Perdew-Burke-Ernzerhof flavor) to examine the response to shear along 11 plausible slip systems. We found that the (011 ¯ 1 ¯ )/⟨1 ¯ 101⟩ slip system has the lowest shear strength (consistent with previous experimental studies) and that this slip leads to a unique plastic deformation before failure in which a boron-carbon bond between neighboring icosahedral clusters breaks to form a carbon lone pair (Lewis base) on the C within the icosahedron. Further shear then leads this Lewis base C to form a new bond with the Lewis acidic B in the middle of a CBC chain. This then initiates destruction of this icosahedron. The result is the amorphous structure observed experimentally. We suggest how this insight could be used to strengthen B_4C.

Gas uptake, molecular sensing and organocatalytic performances of a multifunctional carbazole-based conjugated microporous polymer

Abstract: A multifunctional carbazole-based conjugated microporous polymer MFCMP-1 is successfully prepared by oxidative coupling polymerization using a single monomer and structurally characterized. A new three-dimensional π-conjugated polymer framework can be combined with permanent microporous, highly luminescent properties and abundant nitrogen activated sites in the skeleton. It possesses a large BET surface area of over 840 m2 g−1 with a pore volume of 0.52 cm3 g−1, and displays a high carbon dioxide uptake capacity (up to 3.69 mmol g−1) at 273 K and 1 bar, with good selectivity towards CO2 over N2 and CH4. MFCMP-1 exhibits also strong fluorescent emission at 529 nm after excitation at 380 nm in THF solution and works as a luminescent chemosensor towards hazardous and explosive molecules, such as nitrobenzene, 2-nitrotoluene, and 2,4-dinitrotoluene. In addition, MFCMP-1 features a high concentration of Lewis base nitrogen sites on its internal surfaces; it thus acts as a highly efficient recyclable heterogeneous organocatalyst towards Knoevenagel reaction of malononitrile with aromatics, heterocyclic aldehydes, and cyclic ketones. Furthermore, we further highlight that the ease of synthesis and low cost, coupled with multifunctional properties, make MFCMP-1 an attractive functional material in practical applications.