Lewis Acid Character Research Articles

B3H3Be ← F-: A Case of Extremely Strong Dative Bond.

Chemical bonding has attracted chemists since its inception. Dative bonding between a donor and acceptor moiety is also an important phenomenon, which results in stabilization of many chemical compounds. Herein, we show that an extremely strong dative bond is possible between a fluoride ion and a beryllium center which is a part of a half-sandwich complex, B3H3Be. Quantum chemical calculations have shown the possibility of formation of a half-sandwich complex of Be with a B3H32- ring. Calculations reveal that the complex is stable toward dissociation. The half-sandwich complex features a very low-lying lowest unoccupied molecular orbital (LUMO) concentrated on the Be atom, thereby indicating a Lewis acidic character of the complex. This lower-lying LUMO at the beryllium center is responsible for forming an extremely strong dative bond with the fluoride donor.

The ‘Open’ Site of Lewis Acid Zeolites for Meerwein‐Ponndorf‐Verley‐Oppenauer Reactions in Biomass Catalysis

AbstractHeteroatom zeolites are efficient catalysts for activating oxygenates in Meerwein‐Ponndorf‐Verley‐Oppenauer (MPVO) reactions, due to the unique Lewis acid character of their tetrahedral heteroatom metal sites. Such Lewis acid zeolites, especially heteroatom Beta zeolites, are intensively investigated with increasing attention in the valorization of biomass‐derived oxygenates. Herein, we first briefly review the recent development and advantage of Lewis acid zeolites for MPVO reactions involved in biomass valorization, majorly based on the selected case studies of the most representative heteroatom Beta zeolites. Increasing recognization of the open framework sites as the intrinsic active sites for Lewis acid zeolites in MPVO reactions has been discussed, with static and dynamic viewpoints. Furthermore, the recent progress on the development of characterization technologies applied in recent the state‐of‐the‐art showcases of identification and quantification of open sites, has been assessed, with a special focus on indirect detection using probe molecules and direct detection approaches. Future opportunities and challenges regarding the development of complimentary in situ characterization technologies and in‐depth understanding of the true nature of active sites and correlative mechanisms are examined for the ultimate goal of providing stable MPVO catalysts in biomass valorization.

Ferrier Glycosylation Mediated by the TEMPO Oxoammonium Cation.

The TEMPO oxoammonium cation has been proven to be both an efficient oxidizing reagent and an electrophilic substrate frequently found in organic reactions. Here, we report that this versatile chemical reagent can also be used as an efficient promoter for C- and N-glycosylation reactions through a Ferrier rearrangement with moderate to high yields. This unprecedented reactivity is explained in terms of a Lewis acid activation of glycal by TEMPO+ forming a type of glycal-TEMPO+ mesomeric structure, which occurs through an extended vinylogous hyperconjugation toward the π*(O═N+) orbital [LP(O1) → π*(C1═C2), π*(C1═C2) → σ*(C3-O3), and LP(O6) → π*(O═N+)]. This enables the formation of the respective Ferrier glycosyl cation, which is trapped by various nucleophiles. The extended hyperconjugation (or double hyperconjugation) toward the π*(O═N+) orbital, which confers the Lewis acid character of the TEMPO cation, was supported by natural bond orbital analysis at the M06-2X/6-311+G** level of theory.

Zn, Cd and Cu Coordination Polymers for Metronidazole Sensing and for Ullmann and Chan-Lam Coupling Reactions.

Five compounds, [Zn2(bpe)(BPTA)2(H2O)2] ⋅ 2H2O (1); [Zn(bpe)(BPTA)] (2); [Cd(bpe)(BPTA)H2O] (3); [Cd(BPTA) (bpmh)] ⋅ 2H2O (4); and Cu2(BPTA)2(bpmh)3(H2O)2] ⋅ 2H2O (5) were prepared employing 2,5-bis(prop-2-yn-1-yloxy)terephthalic acid (2, 5 BPTA) as the primary ligand and 1,2-di(pyridin-4-yl)ethane (4, 4' bpe) (1-3) and 1,2-bis(pyridin-3-ylmethylene)hydrazine (bpmh) (4-5) as the secondary ligands. Single crystal studies indicated that the compounds 1, 3 and 5 have two-dimensional layer structures and compounds 2 and 4 three-dimensional structures. The luminescence behaviour of the compounds 2 and 3 were explored for the sensing of metronidazole in aqueous medium. The studies indicated that the compounds can detect metronidazole in ppm level both in solution as well as simple paper strips. The Cu compound 5 was found to lose the coordinated water molecule at 100 °C without any structural change. The coordinatively unsaturated Cu-centre were examined towards the Lewis acidic character by carrying out the Ullmann type C-C homocoupling reaction of the aromatic halide compounds. The compounds, 4 and 5, also have the Lewis basic functionality arising out the =N-N=, aza groups. The bifunctional nature of the coordination polymers (CP) was explored towards the Chan-Lam coupling reaction between phenyl boronic acid and aniline derivatives in the ethanol medium. In both the catalytic reactions, good yields and recyclability were observed. The present studies illustrated the rich diversity that the transition metal containing compounds exhibit in extended framework structures.

Sustainable biodiesel production via castor oil transesterification using organotin(IV) compounds as catalysts

The rapid reduction in fossil fuel reserves and their environmental effects has caused a serious threat to humanity. Meanwhile, the exponential increase in the usage of fossil fuels demands the exploration of new energy sources. Therefore, this work was designed keeping in view the today need of the World for renewable energy source. The 4-(4-chloro-2-fluorophenylamino)-4-oxobut-2-enoic acid based organotin(IV) carboxylates were prepared and characterized by FT-IR, NMR and single crystal XRD analysis. Due to the Lewis acid character of Sn(IV) centre confirmed by DFT computational study, the synthesized compounds were used as catalysts for the transesterification of castor oil for biodiesel production. Under the optimized experimental conditions, the catalytic performance of the tested compounds was found as follow: 1 > 3 > 2 > 4 > 6 > 5 > 7. The obtained biodiesel was confirmed by FT-IR, NMR and GC–MS. The fuel properties of the obtained biodiesel were also determined and compared with American Standard (ASTM D-6751). Based on their catalytic performance, these compounds might be the potential future candidates for the expansion of catalyst systems for efficient biodiesel production from the various feedstocks.

Mechanistic Insights Into the [3+1] Cycloadditions of Me3SiN3 on Bis‐Silylene and the Formation of Pseudo‐Silaazatriene

AbstractThe reaction mechanism for the formation of the pseudo‐silaazatriene 1 (LN(SiMe3)2−Si−N−Si(L)NSiMe3; L=PhC−(NtBu)2) by the reaction of 1 equivalent of bis‐silylene (LSi−SiL; L=PhC(NtBu)2) with 3 equivalents of azide Me3SiN3 has been explored at M06/def2‐TZVPP//BP86‐D3(BJ)/def2‐TZVPP level of theory. The reaction mechanism is guided by the high Lewis basicity of the silicon lone pair and the high Lewis acidic character of Si−N bonds, as well as by the high tendency to eliminate N2 from Me3SiN3. The first step of the reaction is the formation of [3+1] cycloaddition product (I1) by the synergetic interactions of the lone pair on the silylene silicon with the π* molecular orbital of Me3SiN3 which is majorly localized on the terminal nitrogen atom, and the donation of the σ‐type lone pair on the nitrogen atom connected to the SiMe3 group with the Si−N σ* orbital on silylene. The elimination of N2 from I1 results in the formation of pseudo‐silaimine intermediate I2 having a dicoordinated, monovalent nitrogen atom. The most favourable pathway involves the second addition of Me3SiN3 followed by N2 elimination resulting bis‐silaimine intermediate I4‐P2. The highly reactive lone pair on the dicoordinated, monovalent nitrogen atom can be coordinated to the Si−N σ* MO of the second silylene moiety resulting in Si−N bond formation and Si−Si bond cleavage. This step is similar to the SN2′ reaction where the incoming nucleophile and the leaving group are on the same side resulting in the formation of silaimine I5. Further, the [3+1] cycloaddition of one more molecule of Me3SiN3 followed by N2 elimination results in the formation of silaimine intermediate I7. I7 undergoes 1,3‐silyl migration leading to the final product 1. The lone pairs on di‐ and tricoordinated nitrogen centres of all the intermediates are stabilized by hyperconjugative interactions. The delocalized hyperconjugative interaction stabilizes the lone pairs in 1.

Molar excess of coordinating N-heterocyclic carbene ligands triggers kinetic digestion of gold nanocrystals.

There has been much interest in evaluating the strength of the coordination interactions between N-heterocyclic carbene (NHC) molecules and transition metal ions, nanocolloids and surfaces. We implement a top-down core digestion test of Au nanoparticles (AuNPs) triggered by incubation with a large molar excess of poly(ethylene glycol)-appended NHC molecules, where kinetic dislodging of surface atoms and formation of NHC-Au complexes progressively take place. We characterize the structure and chemical nature of the generated PEG-NHC-Au complexes using 1D and 2D 1H-13C NMR spectroscopy, supplemented with matrix assisted laser desorption/ionization mass spectrometry, and transmission electron microscopy. We further apply the same test using thiol-modified molecules and find that though etching can be measured the kinetics are substantially slower. We discuss our findings within the classic digestion of transition metal ores and colloids induced by interactions with sodium cyanide, which provides an insight into the strength of coordination between the strong σ-donating (soft Lewis base) NHC and Au surfaces (having a soft Lewis acid character), as compared to gold-to-gold covalent binding.

Structure, stability and reactivity of inverse sandwich complexes M‐Be3‐M and M‐Zn3‐M (M = Li, Na, K, Cu, Ag, Au)

AbstractQuantum chemical calculations have b‐een carried out on the inverse sandwich type complexes formed between coinage and alkali metals with Be3 and Zn3 rings. Calculations reveal that the complexes are stable toward dissociation. Their Lewis acidic character has been evaluated using fluoride ion affinity (FIA) calculation, which reveals their strong acidic character. Topological analysis of electron density has also been performed to investigate the nature of the bond between the metal and the fluoride ion. Calculations reveal non covalent nature in case of alkali series and covalent nature in case of coinage metal series.

Cobalt perchlorate hexahydrate catalyzed one‐pot synthesis of dihydropyrimidin‐ones/‐thiones through sonochemistry and its mechanistic study using density functional theory calculations

AbstractA cobalt(II) perchlorate hexahydrate coordinated synthesis of dihydropyrimidin‐ones and ‐thiones through sonochemistry is developed. Herein, the reaction was demonstrated as a simple, efficient, and one‐pot method for synthesizing a series of interesting 3,4‐dihydropyrimidin‐2(1H)‐ones. Further, the reaction mechanism was investigated using mass spectrometry and density functional theory calculations suggesting that a Lewis acid character of cobalt(II) perchlorate hexahydrate plays a crucial role in coordinating carbonyl functionalities and stabilizing the polar intermediates like imine–enamine which further led to cyclized dihydropyrimidin‐ones and ‐thiones. This methodology was further explored to synthesize a gram‐scale synthesis of monastrol drug, a kinase inhibitor.

Application of organo-tin based complexes in transesterification reaction for biodiesel production- A review

The primary energy source utilized worldwide is derived from fossil fuels. Among all fields, the transportation sector accounts 54% of total energy consumption. The overuse and scarcity of fossil fuels demand the researchers to seek out a sustainable energy source. Among various renewable resources like solar, water, wind etc., biofuel is also an alternative pollution free energy source. Biodiesel are the fatty acid of methyl esters obtained from animal fat or vegetable oil. To achieve a similar viscosity as petrodiesel, the raw oil or fat is trans esterified to biodiesel. Transesterification is a well-known chemical process wherein the reaction takes place between oil and alcohol, facilitated by the presence of acid or base catalysts. Although the base catalyst has a high conversion rate, soap is generated as a by-product. As a result, acid catalysts are a feasible alternative source for transesterification, but with lower conversion and a high corrosion rate. As traditional acid catalysts, organotin catalysts possess good Lewis acidic character for the production of biodiesel. In this study, we have studied how different substituents on organotin (IV) complexes influence the catalytic activity in biodiesel production as well as in converting by-products of biodiesel into value added products.

Catalytic Conversion of Castor Oil into Biodiesel by Tri-organotin(IV) Catalysts: Chromatographic and Spectroscopic Characterization with Theoretical Support

Economic concern over fossil fuel reserves, extensive increase in CO2 emission, and change in the world climate due to the combustion of carbon sources have been driving the attention of both commercial and academic researchers towards new sustainable fuel routes to encounter rapidly growing worldwide population demands. In the present study, we have focused on catalytic transesterification towards the environmentally friendly biodiesel synthesis which is low cost, easily implemented and best alternative source of fossil fuels. Herein three triorganotin(IV) carboxylates derivatives namely trimethylstannyl cyclopentanecarboxylate, tributylstannyl cyclopentanecarboxylate and triphenylstannyl cyclopentanecarboxylate were resynthesized according to our reported procedure and theoretically investigated through DFT by applying LANL2DZ as functional with B3LYP basis set (level of theory) to calculate Molecular Electrostatic Potential (MEP) to determine electrophilic center of complexes and Lewis acidity. Owing to the Lewis acid character, the synthesized complexes were then used as catalysts in the transesterification reaction of castor oil. Different reaction parameters were also optimized to obtain maximum biodiesel yield. Synthesized castor oil biodiesel (COB) was characterized and confirmed by employing multitude spectroscopic techniques. The present study evaluated that these complexes can be potential candidates for biodiesel conversion from non-edible and cheap feedstock.

Tin-Free Synthesis of 2,2′-Biphenyl Gold(III) Complexes Using Dibenzosilol

Gold(III) complexes with dianionic 2,2′-biphenyl ligands stand out for their oxophilic Lewis acid character in combination with a stable and well-defined square planar geometry. They have been demonstrated to be unique and efficient chiral (pre)catalysts. Furthermore, their chemical and physical properties make them interesting for stoichiometric studies as well as organic electronics. These complexes are commonly prepared by transmetallating the bph ligand from undesirable and potentially toxic organotin compounds onto gold salts, making their synthesis problematic for health and the environment. Here, we present a tin-free synthesis using stable and nontoxic dibenzosilol as a transmetallating agent, accessing the same range of gold complexes as the dibenzostannanol-based synthesis. Furthermore, we confirm that corresponding isonitrile gold(III) complexes can be transformed into carbene gold(III) complexes by the nucleophilic attack of amines, opening up the route to new gold(III) catalyst design strategies.

Stable tetranuclear copper cage as efficient heterogeneous catalyst for C−C and C−N bond forming reactions

Two tetranuclear copper cages with tubular type, [Cu4(BMBA)4((CH3)2N)2(H2O)2] (1) and [Cu4(BMBA)4(Bipy)(H2O)2] (2), were successfully constructed by the reaction of Cu(NO3)2 with dicarboxylic acid 4,4′-[(ethylazanediyl)bis(methylene)]-bis[(1,1′-biphenyl)-2-carboxylic acid] (H2BMBA) under solvothermal conditions. The complexes 1 and 2 exhibit a similar cage structure, formed by four ligands connecting two binuclear copper paddle-wheel secondary building units (SBUs). Interestingly, the complex 2 was obtained by appropriately binding of 4, 4′-bipyridine (Bipy) molecule on the axial positions of two Cu paddle-wheels inside the cage. For this reason, the complex 2 exhibits excellent stability in some common organic solvents at room temperature and aqueous solutions with a pH of 2 to 12. Upon thermal activation of complex 2, the coordinated water molecules can be removed, and its exposed metal active-sites show Lewis acid character, which can be used an efficient heterogeneous catalysis of Knoevenagel condensation (CC) and Clauson-Kaas (CN) reactions.

Novel NHC-Based Au(I) Complexes as Precursors of Highly Pure Au(0) Nuggets under Oxidative Conditions

The Lewis-acidic character and robustness of NHC-Au(I) complexes enable them to catalyze a large number of reactions, and they are enthroned as the catalysts of choice for many transformations among polyunsaturated substrates. More recently, Au(I)/Au(III) catalysis has been explored either by utilizing external oxidants or by seeking oxidative addition processes with catalysts featuring pendant coordinating groups. Herein, we describe the synthesis and characterization of N-heterocyclic carbene (NHC)-based Au(I) complexes, with and without pendant coordinating groups, and their reactivity in the presence of different oxidants. We demonstrate that when using iodosylbenzene-type oxidants, the NHC ligand undergoes oxidation to afford the corresponding NHC=O azolone products concomitantly with quantitative gold recovery in the form of Au(0) nuggets ~0.5 mm in size. The latter were characterized by SEM and EDX-SEM showing purities above 90%. This study shows that NHC-Au complexes can follow decomposition pathways under certain experimental conditions, thus challenging the believed robustness of the NHC-Au bond and providing a novel methodology to produce Au(0) nuggets.

Lewis Acidic Zinc(II) Complexes of Tetradentate Ligands as Building Blocks for Responsive Assembled Supramolecular Structures

This review presents representative examples illustrating how the Lewis acidic character of the Zn(II) metal center in Zn(salen)-type complexes, as well as in complexes of other tetradentate ligands, and the nature of the medium govern their supramolecular aggregation, leading to the formation of a variety of supramolecular structures, either in solution or in the solid state. Stabilization of these Lewis acidic complexes is almost always reached through an axial coordination of a Lewis base, leading to a penta-coordinated square-pyramidal geometry around the metal center. The coverage is not exhaustive, mainly focused on their crystallographic structures, but also on their aggregation and sensing properties in solution, and on their self-assembled and responsive nanostructures, summarizing their salient aspects. The axial ligands can easily be displaced, either in solution or in the solid state, with suitable Lewis bases, thus being responsive supramolecular structures useful for sensing. This contribution represents the first attempt to relate some common features of the chemistry of different families of Zn(II) complexes of tetradentate ligands to their intrinsic Lewis acidic character.

Imine-stabilized silylium ions: synthesis, structure and application in catalysis.

Novel norbornene-based imine-stabilized silylium ions 2 have been synthesized via the simple reaction of sulfide-stabilized silylium ion 1 with carbonyl derivatives. Those silylium ions were fully characterized in solution and in the solid state by NMR spectroscopy and X-ray diffraction analysis as well as DFT calculations. Unlike the previously reported phosphine-stabilized silylium ion VI, behaving as a Lewis pair, calculations show that 2 have a strong Lewis acid character. Indeed, imine-stabilized silylium ions 2 are able to activate Si-H bonds and catalyzed the hydrosilylation of carbonyl derivatives under mild conditions.

Bulky anion effect on the architecture of chiral dysprosium single-molecule magnets.

The interest for chiral tris(β-diketonato)lanthanide complexes in coordination chemistry is huge due to its Lewis acid character, optical activity, and the control of the final compound architecture. The reaction of equimolar quantities of [Dy((-)/(+)hfc)3 (H2 O)] (hfc- = 3-(heptafluoropropylhydroxymethylene)-(+/-)-camphorate) and L led to the formation of a pair of enantiomers for dinuclear complexes [Dy((-)/(+)hfc)3 (L)]2 ⋅C7 H16 ([(-)/(+)1]⋅C7 H16 ) (L = 4'-(4'''-pyridyl-N-oxide)-1,2':6'1''-bis-(pyrazolyl)pyridine]). Starting from the previous experimental protocol with the addition of bulky BArF anions, a partial dissociation of the chiral [Dy((-)/(+)hfc)3 (H2 O)] was observed leading to the isolation of a mono-dimensional cationic chiral polymer {[Dy((-)/(+)hfc)2 (L)][BarF]}n ⋅nCH3 NO2 ([(-)/(+)2]n ⋅nCH3 NO2 ). Natural circular dichroism (NCD) highlighted an exciton CD couplet for [(-)/(+)2]n but not for (-)/(+)1. The latter behaves as a single-molecule magnet (SMM) with a blocking temperature up to 4K, whereas [(-)/(+)2]n is a 1D assembly of field-induced SMMs with a magnetic relaxation occurring through a Raman process only.

Surface energetics of graphene oxide and reduced graphene oxide determined by inverse gas chromatographic technique at infinite dilution at room temperature.

Graphene is of great interest for many far-reaching applications that involve interparticle interactions in adsorbents, coatings, and composites. A deep understanding of the surface components has been crucial but achieving the most accurate and reliable values of these, unaffected by experimental conditions or the analytical techniques used, remains a major challenge. To this end, we have proposed in this paper a novel approach for the first time, to the best of our knowledge, to determine London dispersive and specific (polar) components including the Lewis acid-base character of the surface free energy of graphene materials (graphene oxide (GO), reduced graphene oxide (rGO), and graphite) using inverse gas chromatography (IGC) technique at an infinite dilution. We have estimated the London dispersive surface energy values of graphite, GO, and rGO as van der Waals interaction to be 156-179, 89-106, and 110-119mJm-2, respectively, in the temperature range of 320-360K. These are attributable to the surface properties impacted by the oxygen moieties, defects, and micropores on the carbon frameworks. Further, the acceptor (KA) and donor (KD) parameters of GO were found to be 0.71 and 0.96, respectively, while those of rGO were 0.54 and 1.05. Notably, the GO is more of the Lewis acid character that could be amphoteric, while the Lewis base characteristics of both GO and rGO are not significantly changed. These results provide foundational knowledge to understand the physicochemical properties of graphene surfaces, which should be helpful to designing interface engineering in various applications.

(Keynote) N-Heterocyclic Carbene-coated Gold Nanoparticles and Luminescent Quantum Dots

N-heterocyclic carbenes (NHCs) have generated much interest for use as versatile metal-coordinating groups, since they were first synthesized by Arduengo and coworkers in 1991. NHC molecules can be considered as L-type ligands, because they share their non-bonding electron pairs with the σ-accepting orbital of transition metals, and this endows them with strong coordination interactions. NHC-appended molecules have recently been actively exploited as potentially effective ligands for the surface passivation of various colloidal nanomaterials.We investigate the coordination interactions between a few representative colloidal nanocrystals, including gold nanoparticles (AuNPs) and luminescent quantum dots (QDs), and a NHC-based polymer ligand. The latter presents multiple NHC groups and several short poly (ethylene glycol) (PEG) chains as solubilizing blocks. We find that our NHC-decorated ligands rapidly coordinate onto both sets of nanocrystals, which we attribute to their soft Lewis base nature. These ideally match the soft Lewis acid character of transition metal colloid surfaces, promoting strong coordination bonding through soft‐soft interaction. We combine NMR spectroscopy, fluorescence spectroscopy, high-resolution transmission electron microscopy supplemented with dynamic light scattering to characterize the nature of the binding interactions. Furthermore, the long-term stability of the NHC-stabilized nanocolloids have been tested after phase transfer to water, a highly challenging chemical venue for such groups, due to the moisture sensitive nature of NHC molecules. Data show that our NHC-polymer-stabilized AuNPs and QDs exhibit long-term colloidal stability in buffer media while preserving their optical and fluorescing properties, with no sign of degradation or aggregation build up for at least one year of storage. We will discuss the ligand design and synthesis, characterization of the polymer-stabilized nanocrystals under various conditions, with a particular focus on the beneficial effects of ligand multi-coordination interactions onto the nanocolloid surfaces.

Aryldiazonium salts can serve as nitrogen-based Lewis acid catalysts and their applications in the formation of photoactive charge transfer complexes

We report the Lewis acid catalysis of aryldiazonium salts, and their Lewis acidity applications in photogeneration of aryl radicals under additive-, photocatalyst- and transition metal-free conditions. In this visible light-mediated transformation, the Lewis acidic character of aryldiazonium salts enables access to the photoactive charge transfer complex with dichalcogenides. The usefulness and versatility of this new protocol are demonstrated through the chalcogenation of a variety of aryldiazonium salts.