Soft Lewis Acid Research Articles

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.

Fusing Triphenylbismuth and PnPh3 (Pn = P-Bi): Synthesis, Isolation, and Characterization of 9-Bisma-10-Pnictatriptycenes.

The first series of 9-bisma-10-pnictatriptycenes Bi(C6H4)3Pn (2-Pn, Pn = P-Bi; see graphic) has been synthesized in a two-step procedure via suitable tris(2-bromophenyl)pnictanes 1-Pn and characterized in solution as well as in the solid state. DFT calculations suggest preferential interactions between 2-Pn and soft Lewis acids via the lighter pnictogen donor atom. Experimental studies demonstrate that even the weakest Lewis base in the series of 2-Pn, namely the dibismatriptycene 2-Bi, interacts with Lewis acidic [BiMe2(SbF6)] in solution. Analytical techniques include (VT-)NMR spectroscopy, DOSY NMR spectroscopy, high-resolution mass spectrometry, single-crystal X-ray diffraction analyses, and DFT calculations.

Domino Three-Component N-Acylation/[4 + 2] Cycloaddition/Alder-ene Synthesis of Polysubstituted Benzo[f]isoindole-4-carboxylic Acids.

Diversely substituted, partially saturated benzo[f]isoindole-4-carboxylic acids were synthesized by a new three-component reaction (3CR) starting from cinnamic amines (3-arylallylamines), maleimides, and maleic anhydride. The process consists of N-acylation of the amines by maleic anhydride, intramolecular [4 + 2] cycloaddition in vinylarenes (the IMDAV reaction), and the concluding Alder-ene reaction between Diels-Alder intermediates and maleimides. All of the reaction steps proceed in a highly regio- and stereoselective manner, furnishing five adjacent chiral centers and leading to a single diastereoisomer of the title compound. The efficiency of the transformation is secured by thermal conditions or utilization of soft Lewis acids (Yb(OTf)3) as catalysts. The kinetics and mechanism of the 3CR were studied by using dynamic 19F NMR. Based on the NMR data and density functional theory (DFT) calculations, the IMDAV, not the Alder-ene, reaction is the rate-limiting step of the entire process.

4‐Vinylpyridine derived physically crosslinked hydrogel as efficient mercury indicator

AbstractHere, we report a photoclicked crystalline monomer DiPy which forms polymeric hydrogel Polym‐DiPyC3. The monomer DiPy and polymer Polym‐DiPyC3 are well characterized using spectroscopic and analytical techniques. The rheometric curve revealed the hydrogel Polym‐DiPyC3 to exhibit viscoelasticity with a damping factor (tanδ) in the range between 0 and 1. The considered hydrogel film on exposure to mercury concentration ≥2 ppm showed a loss of transparency and swellability. Energy dispersive x‐ray and mapping confirmed the mercury presence on the surface of Polym‐DiPyC3. The material behaves as a naked‐eye indicator due to gain in opacity and loss of swellability. The DFT parameter supported the soft Lewis acid Hg2+ to guest onto the soft thioether site of the polymer film Polym‐DiPyC3.Highlights The monomer DiPy was synthesized via thiol‐alkene photoclick reaction. The ionic polymer Polym‐DiPyC3 is physically crosslinked. The polymer Polym‐DiPyC3 exhibits viscoelastic property. The mercury presence causes Polym‐DiPyC3 to lose its hydrogel property. The observed affinity of mercury for sulfur was well supported with DFT.

Diiminium Nucleophile Adducts Are Stable and Convenient Strong Lewis Acids.

Strong Lewis acids are essential tools for manifold chemical procedures, but their scalable deployment is limited by their costs and safety concerns. We report a scalable, convenient, and inexpensive synthesis of stable diiminium-based reagents with a Lewis acidic carbon centre. Coordination with pyridine donors stabilises these centres; the 2,2'-bipyridine adduct shows a chelation effect at carbon. Due to high fluoride, hydride, and oxide affinities, the diiminium pyridine adducts are promising soft and hard Lewis acids. They effectively produce acylpyridinium salts from carboxylates that can acylate amines to give amides and imides even from electronically intractable coupling partners.

Supported Noble Metal Catalysts and Adsorbents with Soft Lewis Acid Functions.

Heterogeneous noble metal catalysts exhibit various functions. Although their redox functions have been extensively studied, we focused on their soft Lewis acid functions. Supported Au, Pt, and Pd catalysts electrophilically attack the π-electrons of soft bases such as alkynes, alkenes, and aromatic compounds to perform addition and substitution reactions. Hydroamination, intramolecular cyclization of alkynyl carboxylic acids, isomerization of allylic esters, vinyl exchange reactions, Wacker oxidation, and oxidative homocoupling of aromatics are introduced based on a discussion of the active species and reaction mechanisms. Furthermore, the adsorption of sulfur compounds, which are soft bases, onto the supported AuNPs is discussed. The adsorption and removal of 1,3-dimethyltrisulfane (DMTS), which is the compound responsible for the stale odor of "hine-ka" in alcoholic beverages, particularly Japanese sake, is described.

(N-Benzoyl-N-phenyl-hydroxy-laminato)carbon-yl(tri-phenyl-arsine)rhodium(I).

The mol-ecule of the title compound, [Rh(C13H10NO2){As(C6H5)3}(CO)] or [Rh(BPHA)(AsPh3)(CO)] (BPHA is the N-benzoyl-N-phenyl-hydroxy-laminate anion), comprises a bidentate N-benzoyl-N-phenyl-hydroxy-laminate anion coordinating through the O atoms to the soft Lewis acid, rhodium(I), and two monodentate ligands, viz. tri-phenyl-arsine and carbonyl. The resulting CO2As coordination environment around the central RhI atom is distorted square planar.=.

Tuning Intermolecular Interactions to Enhance the Cyclability of Non-Aqueous, Organic Redox Flow Batteries.

Enhancing the electrochemical stability and reversibility of redox-active organic molecules is crucial to improve the performance of non-aqueous redox flow batteries (RFBs). Compared with the widely adopted strategy of molecular engineering, we show in this study that tuning the intermolecular interaction between the active material with the supporting electrolyte is another feasible way to address the performance of non-aqueous organic RFBs. Combined with theoretical and experimental investigations, the influence of Lewis acidity of the supporting electrolyte cations and anions on the electrochemical stability and reversibility of bipyridine-based anode material is revealed. As a demonstration, a redox flow cell based on 4,4'-bipyridine anolyte and ferrocene catholyte shows greatly enhanced performance by using supporting electrolyte composed of soft Lewis acid and soft Lewis base. This study provides an alternative, yet highly effective way to addressing the cyclability of an organic compound for non-aqueous RFBs.

(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.

Superior Iodine Uptake Capacity Enabled by an Open Metal-Sulfide Framework Composed of Three Types of Active Sites

Superior Iodine Uptake Capacity Enabled by an Open Metal-Sulfide Framework Composed of Three Types of Active Sites

Exposed carboxyl functionalized MIL-101 derivatives for rapid and efficient extraction of heavy metals from aqueous solution

Exploring efficient adsorbent for heavy metals remediation from wastewater and contaminated soil is a pursuing issue. Robust functionalized metal-organic frameworks (MOFs) represent one of burgeoning scavengers because of their tunable structural topology, designable functionalities and extraordinary affinity for heavy metal ions. In this present work, we report a facile and versatile strategy to synthesize a series of carboxyl-functionalized MIL-101(Fe) derivatives by microwaving a solution of organic ligands with varying carboxyl number and Fe3+ ions. The resulting MIL-101(Fe) derivatives were characterized using various techniques including scanning electron microscope (SEM), powder X-ray diffraction, Fourier transformed infrared, Brunauer–Emmett–Teller specific surface area and X-ray photoelectron spectroscopy. The intrinsically carboxyl-rich characteristic and large surface area of MIL-101-(COOH)2 afford fast adsorption kinetics (< 15 min) and high saturation capacity (467.6 mg/g) for Pb(II). Strikingly, it can efficiently capture Pb(II) possessing impressive adsorption efficiencies (> 90%) over a wide pH range of 4.5–11.0. Moreover, the resulting MIL-101-(COOH)2 simultaneously exhibited high affinity toward different types of heavy metal ions, including borderline Lewis acid (Pb(II), Cu(II) and Ni(II)), soft Lewis acid (Cd(II) and Hg(II)) and hard Lewis acid (Eu(III) and Cr(III)). Such impressive treatment for Pb(II)-contaminated lead-acid battery industrial wastewater (more than 96.7%) can be attributed to the coordination arising from abundant carboxyl groups in the pores of MIL architecture with target heavy metal ions. Significantly, these findings validate the exceptional potential of carboxyl-functionalized MIL-101(Fe) derivatives in the application of heavy metal ions remediation.

Investigation of reusability and deactivation mechanism of supported platinum catalysts in the practical isomerization of allylic esters

Metal oxides supported platinum catalysts with residual chloride are known to be reusable soft Lewis acid catalysts for the isomerization of allylic esters. To deepen the understanding of the industrial application of the catalytic system and optimize the catalyst design, an investigation on the reusability and deactivation mechanism of supported platinum catalysts in the isomerization of but-3-ene-1,2-diyl diacetate was conducted. A great reactivation effect was observed by treating the used catalysts with HCl, rather than that of calcination. Pt-Cl/CeO2 showed the best reusability among the catalysts prepared with other metal oxides, and it was uncovered that the reduction of PtOxCly species should be the main reason for deactivation with the assistance of X-ray absorption fine structure (XAFS) analysis. Besides, a relatively stable state was also observed from the XAFS spectra of Pt-Cl/CeO2 catalysts in the reusing test, and it could effectively prevent the active species from further deactivation. Furthermore, in an extended evaluation of the stability of the catalysts, reusability, and a limited deactivation were also verified through a 10-run batch reaction. Additionally, the continuous flow reaction was also proven to be feasible.

Comparison of toxic effects of lead and copper and protective power of glutathione on oxidative stress parameters

IntroductionLead as an industrial pollutant can be detected at all stages of the working and living environment. Lead, based on its properties, solubility and mobility, accumulates in the soil, so that the average concentrations of oil in the soil are between 15 and 25 mg/kg (Radojević at all., 1999). Due to increased human activity, the amount of copper in the air, soil and water has increased. Glutathione (GSH) is an essential cofactor of many enzymes, such as: formaldehyde dehydrogenase, glyoxalase, prostaglandin endoperoxide isomerase, dehydrochlorinase and others. GSH is a biological redox in the metabolism of erythrocytes, it also plays a role in the transport of amino acids. Reactive forms of oxygen cause oxidative biomolecules (lipids, proteins, DNA) (Freidovich, 1999; Massaad i Klann, 2010).ObjectivesThe aim of this research was to examine the protective role of supplements GSH in conditions of chronic intoxication with sublethal doses of lead acetate and copper II sulfate.MethodsThe preparation of biomaterials for testing and making homogenates of brain tissue of albino rats of Wistar strain was performed and the activity of acid and alkaline DNase was measured spectrophotometrically (Kocić i sar., 1998).ResultsLead otherwise “as soft Lewis acid” has a pronounced affinity for interaction with “soft bases” such as S-atoms of the thiol group in antioxidants, natural biomolecules and supplements in this case in glutathione.ConclusionsIt can be said that GSH is a desirable supplement and antioxidant in the detoxification of reactive oxygen species in rats exposed to lead poisoning.DisclosureNo significant relationships.

Impact of anion polarizability on ion pairing in microhydrated salt clusters.

Despite longstanding interest in the mechanism of salt dissolution in aqueous media, a molecular level understanding remains incomplete. Here, cryogenic ion trap vibrational action spectroscopy is combined with electronic structure calculations to track salt hydration in a gas phase model system one water molecule at a time. The infrared photodissociation spectra of microhydrated lithium dihalide anions [LiXX'(H2O) n ]- (XX' = I2, ClI and Cl2; n = 1-3) in the OH stretching region (3800-2800 cm-1) provide a detailed picture of how anion polarizability influences the competition among ion-ion, ion-water and water-water interactions. While exclusively contact ion pairs are observed for n = 1, the formation of solvent-shared ion pairs, identified by markedly red-shifted OH stretching bands (<3200 cm-1), originating from the bridging water molecules, is favored already for n = 2. For n = 3, Li+ reaches its maximum coordination number of four only in [LiI2(H2O)3]-, in accordance with the hard and soft Lewis acid and base principle. Water-water hydrogen bond formation leads to a different solvent-shared ion pair motif in [LiI2(H2O)3]- and network formation even restabilizes the contact ion pair motif in [LiCl2(H2O)3]-. Structural assignments are exclusively possible after the consideration of anharmonic effects. Molecular dynamics simulations confirm that the significance of large amplitude motion (of the water molecules) increases with increasing anion polarizability and that needs to be considered already at cryogenic temperatures.

Headspace µ–solid phase extraction of 1,4–dioxane and 2–methyl–1,3–dioxolane from shampoo samples in a home–mode device and large volume injection of deep eutectic solvent: Theoretical and experimental studies

In the current study, a headspace micro solid–phase extraction was developed for extraction of 1,4–dioxane and 2–methyl–1,3–dioxolane from shampoo samples. After extraction, the quantities of analytes were determined using the large volume injection method followed by gas chromatography–flame ionization detector. Octadecylsilane, menthol: formic acid deep eutectic solvent, and 1,3–dioxane were used as adsorbent, eluent, and internal standard, respectively. Under optimum conditions, linear ranges for target analytes were from 0.30 to 5000 ng mL−1, and 0.43–5000 ng mL−1 with correlation coefficients ≥ 0.9972. The limits of detection were 0.08 and 0.11 ng mL−1 for 1,4–dioxane and 2–methyl–1,3–dioxolane, respectively. The limits of quantification were 0.28 and 0.43 ng mL−1 for 1,4–dioxane and 2–methyl–1,3–dioxolane, respectively. The intra– and inter–day precisions for target analytes at the spiked level of 25 ng mL−1 were ≤ 4.2%. The feasibility of the recommended procedure was successfully tested for the extraction and determination of 1,4–dioxane and 2–methyl–1,3–dioxolane in shampoo samples. The extraction recoveries were ranged from 89 to 90% with relative standard deviations<5.1%. The nature of the interactions between studied chemical species was investigated in the light of conceptual density functional theoretical calculations. As to conclude, it was seen that experimental observations are compatible with the theoretical explanations made according to “hard and soft (Lewis) acids and bases” Principle.

Synthesis, Characterization, and Reaction Studies of Pd(II) Tripeptide Complexes.

The aqueous synthesis of Pd(II) complexes with alkylated tripeptides led to the hydrolysis of the peptides at low pH values and mixtures of complexed peptides were formed. A non-aqueous synthetic route allowed the formation and isolation of single products and their characterization. Pd(II) complexes with α-Asp(OR)AlaGly(OR), β-Asp(OR)AlaGly(OR), and TrpAlaGly(OR) (R = H or alkyl) as tri and tetradentate chelates were characterized. The tridentate coordination mode was accompanied by a fourth monodentate ligand that was shown to participate in both ligand exchange reactions and a direct removal to form the tetradentate coordination mode. The tetradentate coordination revealed a rare a hemi labile carbonyl goup coordination mode to Pd(II). Reactivity with small molecules such as ethylene, acids, formate, and episulfide was investigated. Under acidic conditions and in the presence of ethylene; acetaldehyde was formed. The Pd(II) is a soft Lewis acid and thiophilic and the complexes abstract sulfur from episulfide at apparent modest catalytic rates. The complexes adopt a square planar geometry according to a spectroscopic analysis and DFT calculations that were employed to evaluate the most energetically favorable coordination geometry and compared with the observed infrared and NMR data.

Novel Bifunctional [16]aneS4-Derived Chelators for Soft Radiometals

The field of targeted radionuclide therapy is rapidly growing, highlighting the need for wider radionuclide availability. Soft Lewis acid ions, such as radioisotopes of platinum, rhodium and palladium, are particularly underdeveloped. This is due in part to a lack of compatible bifunctional chelators. These allow for the practical bioconjugation to targeting vectors, in turn enabling radiolabeling. The [16]andS4 macrocycle has been reported to chelate a number of relevant soft metal ions. In this work, we present a procedure for synthesizing [16]andS4 in 45% yield (five steps, 12% overall yield), together with a selection of strategies for preparing bifunctional derivatives. An ester-linked N-hydroxysuccimide ester (NHS, seven steps, 4% overall yield), an ether-linked isothiocyanate (NCS, eight steps, 5% overall yield) and an azide derivative were prepared. In addition, a new route to a carbon-carbon linked carboxylic acid functionalized derivative is presented. Finally, a general method for conjugating the NHS and NCS derivatives to a polar peptide (octreotide) is presented, by dissolution in water:acetonitrile (1:1), buffered to pH 9.4 using borate. The reported compounds will be readily applicable in radiopharmaceutical chemistry, by facilitating the labeling of a range of molecules, including peptides, with relevant soft radiometal ions.

Highly Selective Ortho-Directed Dicarboxylation of Cyclopentadiene by Methylcarbonates and CO2 or COS - First Insight into Co-ordination Chemistry of New Ambident Ligands.

This research presents the highly regioselective syntheses of 1,2‐dicarboxylated cyclopentadienide salts [Cat]2[C5H3(CO2)2H] by reaction of a variety of organic cation methylcarbonate salts [Cat]OCO2Me (Cat=NR4 +, PR4 +, Im+) with cyclopentadiene (CpH) or by simply reacting organic cation cyclopentadienides Cat[Cp] (Cat=NR4 +, PR4 +, Im+) with CO2. One characteristic feature of these dianionic ligands is the acidic proton delocalized in an intramolecular hydrogen bridge (IHB) between the two carboxyl groups, as studied by 1H NMR spectroscopy and XRD analyses. The reaction cannot be stopped after the first carboxylation. Therefore, we propose a Kolbe‐Schmitt phenol‐carboxylation related mechanism where the acidic proton of the monocarboxylic acid intermediate plays an ortho‐directing and CO2 activating role for the second kinetically accelerated CO2 addition step exclusively in ortho position. The same and related thiocarboxylates [Cat]2[C5H3(COS)2H] are obtained by reaction of COS with Cat[Cp] (Cat=NR4 +, PR4 +, Im+). A preliminary study on [Cat]2[C5H3(CO2)2H] reveals, that its soft and hard coordination sites can selectively be addressed by soft Lewis acids (Mo0, Ru2+) and hard Lewis acids (Al3+, La3+).

Pt/CeO2 with residual chloride as reusable soft Lewis acid catalysts: Application to highly efficient isomerization of allylic esters

Heterogeneous acid and base catalysts play a crucial role in many important chemical processes. Hard Lewis and Brønsted acid catalysts like silica-alumina and zeolites have been widely applied in numerous reactions; however, developing reusable and active soft Lewis acid catalysts remain a challenge. Herein, we demonstrate for the first time that a highly dispersed supported platinum catalyst can act as a heterogeneous soft Lewis acid. High turnover numbers and reusability were observed in the isomerization of allylic esters under solvent-free conditions. Moreover, the as-prepared catalysts are characterized by X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), revealing that the highly dispersed Pt clusters with Pt–Cl bonds play a key role in the high activity. The residual chloride anion enhances the Lewis acidity of the Pt metal center and thus improves the catalytic activity. Simultaneously, the high catalytic activity of Pt/CeO2 with residual chloride and the soft Lewis acid mechanism are also proved by density functional theory (DFT) calculations based on the model reaction.

Hydrogen-Bonding-Assisted Cationic Aqua Palladium(II) Complex Enables Highly Efficient Asymmetric Reactions in Water.

Metal-bound water molecules have recently been recognized as a new facet of soft Lewis acid catalysis. Herein, a chiral palladium aqua complex was constructed that enables carbon-hydrogen bonds of indoles to be functionalized efficiently. We embraced a chiral 2,2'-bipyridine as both ligand and hydrogen-bond donor to configure a robust, yet highly Lewis acidic, chiral aqua complex in water. Whereas the enantioselectivity could not be controlled in organic solvents or under solvent-free conditions, the use of aqueous environments allowed the σ-indolylpalladium intermediates to react efficiently in a highly enantioselective manner. This work thus describes a potentially powerful new approach to the transformation of organometallic intermediates in a highly enantioselective manner under mild reaction conditions.