Oxirane synthesis from diazocarbonyl compounds via NHC-Ag catalysis

A new method was developed to synthesize oxirane products from the reaction of diazocarbonyl substrates with aryl aldehydes by using Ag(I) N-heterocyclic carbene complex as the catalyst. A combination of N-heterocyclic carbene silver complex (IPrAgCl) with another silver salt (AgOTf) generated the catalytic active IPr-Ag⁺ intermediate, which then catalyzed the epoxidation reaction.     

Gold(I)-catalyzed direct C-H arylation of pyrazine and pyridine with aryl bromides

An efficient procedure for the direct C-H arylation of electron-poor aromatics such as pyrazine and pyridine with aryl bomides is described. In the presence of catalytic amount of Cy₃PAuCl and with the use of t-BuOK as base, pyrazine undergoes the direct C-H arylation with aryl bromides at 100 °C, and the yields of the arylated products depend on the nature of aryl bromides. In the cases of electron-rich aryl bromides used, the arylated pyrazines can be obtained in good to high yields. For electron-poor aryl bromides, the addition of AgBF₄ is the crucial point to accelerate the coupling reaction to give the arylated products in moderate yields. Pyridine also reacts with electron-rich aryl bromides catalyzed by Cy₃PAuCl to give a mixture of arylated regioisomers in moderate yield. However, in order to realize the direct C-H arylation of pyridine with electron-poor aryl bromides, the addition of silver salt as additive and a milder reaction temperature (60 °C) are required.     

Solvation behaviour of silver(I) iodate in methanol—acetonitrile and ethanol—acetonitrile mixtures

The solvation behaviour of silver(I) iodate in methanol—acetonitrile (AN) and ethanol—acetonitrile mixtures has been studied at 30°C by solubility and emf measurements. The solubility of the salt increases with the addition of AN and passes through a maximum at X_AN = 0.3 and 0.6 in the case of MeOH-AN and EtOH-AN mixtures, respectively, and then decreases with further addition of AN. The transfer free energy of silver ion decreases while that of iodate ion increases with the addition of AN in both the solvent mixtures. The solvent transport number, Δ of AN is positive with a maximum at X_AN = 0.45 (Δ = 0.45 (Δ = 5.4) and at X_AN = 0.55 (Δ = 2.4) in the case of MeOH-AN and EtOH-AN mixtures, respectively. These results have been interpreted in terms of the heteroselective solvation of the salt, the silver ion being preferentially solvated by AN and the iodate ion by the amphiprotic solvent component in these mixtures.     

Silver Salts of the Weakly Coordinating Anion [Me3NB12Cl11]–

Silver(I) salts of weakly coordinating anions (WCA) are commonly applied as oxidizing agents or halide abstracting reagents. The feasibility of a particular silver salt for such applications strongly depends on the “nakedness“ of the silver cation. In this study the reactivity of Ag[Me₃NB₁₂Cl₁₁] in different solvents was investigated. Crystal structures of a variety of complexes were obtained. In several crystal structures two boron clusters are bridged by Ag–Cl contacts. This leads to polymeric structures (e.g. for Ag[Me₃NB₁₂Cl₁₁]·0.5CH₂Cl₂ and Ag[Me₃NB₁₂Cl₁₁]·SO₂). Sterically demanding aromatics like mesitylene, pyrene, and acenaphthene are η¹‐ or η²‐bonded to the silver atom and also form coordination polymers, whereas benzene as a ligand leads to a molecular structure, in which two benzene molecules are η²‐coordinated to the silver cation. In contrast, strong σ donor ligands like pyridine and triphenylphosphine give homoleptic silver complexes and thus cation and anion are separated. Furthermore, the ability of Ag[Me₃NB₁₂Cl₁₁] for performing metathesis reactions was investigated. The reaction with Au^ICl gave the [Au(NCMe)₂]⁺ cation.     

Synthesis of Nitrogen-Containing Polyaromatics by Aza-Annulative π-Extension of Unfunctionalized Aromatics

Nitrogen-containing polycyclic aromatic compounds (N-PACs) are an important class of compounds in materials science. Reported here is a new aza-annulative π-extension (aza-APEX) reaction that allows rapid access to a range of N-PACs in 11–84 % yields from readily available unfunctionalized aromatics and imidoyl chlorides. In the presence of silver hexafluorophosphate, arenes and imidoyl chlorides couple in a regioselective fashion. The follow-up oxidative treatment with p-chloranil affords structurally diverse N-PACs, which are very difficult to synthesize. DFT calculations reveal that the aza-APEX reaction proceeds through the formal [4+2] cycloaddition of an arene and an in situ generated diarylnitrilium salt, with sequential aromatizations having relatively low activation energies. Transformation of N-PACs into nitrogen-doped nanographenes and their photophysical properties are also described.     

An inorganic complex of silver (III): K5Ag(IO5OH)2 · 8H2O

The chemical preparation and crystal structure of the trivalent silver salt K₅Ag(IO₅OH)₂ · 8H₂O are described (monoclinic, space group Cc; a = 21.79(4), b = 6.320(3), c = 15.16(3) Å, β = 96.14(4); four formula units per unit cell). The structure is refined until R = 0.033 for 2718 reflections. Isolated Ag(IO₅OH)^5?₂ units occur, which contain trivalent silver ions surrounded by four oxygen atoms from two IO₆ octahedra in rectangular configuration. The differences in the IO bond lengths, as well as the small deviations of the crystal structure from centrosymmetry, are in agreement with an antiperiplanar position of the OH groups in both octahedra.     

4-N,N-Dimethylaminopyridine promoted selective oxidation of methyl aromatics with molecular oxygen

4-N,N-Dimethylaminopyridine (DMAP) as catalyst in combination with benzyl bromide was developed for the selective oxidation of methyl aromatics. DMAP exhibited higher catalytic activity than other pyridine analogues, such as 4-carboxypyridine, 4-cyanopyridine and pyridine. The sp3 hybrid carbon-hydrogen (C-H) bonds of different methyl aromatics were successfully oxygenated with molecular oxygen. The real catalyst is due to the formation of a pyridine onium salt from the bromide and DMAP. The onium salt was well characterized by NMR and the reaction mechanism was discussed.     

Electrical transport properties of poly(aniline-co-p-phenylenediamine) and its composites with incorporated silver particles

Statistical copolymers of aniline and p-phenylenediamine, poly(aniline-co-p-phenylenediamine)s, were synthesised by oxidative polymerisation using various oxidants, ammonium peroxydisulphate or silver nitrate. Depending on the choice of oxidant, copolymers or composites with silver particles were obtained. Different molar concentrations of p-phenylenediamine in the reaction mixture provided materials of different conductivities. The influence of both the copolymer composition and the presence of discrete silver particles on the electric and dielectric properties of the system was studied. The results showed a decrease in the conductivity of copolymers and their composites with the silver content compared with the content of standard polyaniline salt. The reduction in conductivity was described in terms of the decreased density of hopping centres due to defects in the copolymer structure. The dielectric relaxations observed were described in terms of their activation energies and were linked to the corresponding conduction mechanism.     

Synthesis of bis(N-heterocyclic carbene) palladium complexes derived from (S,S)-1,2-bis(1-hydroxypropyl)benzene

New o-xylylene-linked bis(benzimidazolium) salts were synthesized in six-steps from C₂-symmetric chiral 1,4-diol, 1,2-bis(1-hydroxypropyl)benzene, as a starting material. The silver complex of bis(benzimidazol-2-ylidene) was obtained on treatment of bis(benzimidazolium) salt with silver oxide. The reaction of the silver bis-NHC with [PdCl₂(PhCN)₂] afforded the bis-NHC complex of palladium. The X-ray diffraction studies on Pd complexes revealed that these complexes have distorted square planar geometry around the Pd center coordinating the NHC ligand in mutually cis-position. The arene ring of o-xylylene unit hanged over the Pd center and thus these complexes showed C₁-symmetric structures. The variable temperature NMR spectroscopy revealed that these Pd complexes showed fluxional behavior between C₁- and C₂-symmetric structures in solution state.     

Michael-type addition of hydroxide to alkynylselenonium salt: practical use as a ketoselenonium ylide precursor

A novel synthetic method of ketodiphenylselenonium ylide from alkynylselenonium salt is described. A reaction of alkynylselenonium salt, hydroxide ion, and aldehyde in the presence of silver triflate and triethylamine gave oxiranylketones just as a trans-isomer in moderate to good yields, whereas benzoyl aziridine derivatives were obtained from the reaction with sodium p-toluenesulfonamide instead of a hydroxide ion.     

A convenient reduction of functionalized polycyclic aromatics into parent hydrocarbons

Polycyclic aromatics with various functional group (e.g. OH, OR, SR, and halogen) were easily defunctionalized by aluminum chloride and ethanethiol to give parent aromatics in high yields under mild conditions. This reaction proceeds through sulfide as the intermediate, hence it is also useful for the synthesis of sulfides of polycyclic aromatics.     

Effect of the polymer matrix on the formation of silver nanoparticles in polymer–silver salt complex membranes

When polymer–silver salt complex membranes were exposed to UV irradiation, the separation performances of both the permeance and selectivity for propylene–propane decreased, which was primarily attributed to the reduction of the silver ions in the membranes to silver nanoparticles. Here, the effect of the polymer matrix on the formation of silver nanoparticles in the polymer–silver salt complex membranes was investigated. This effect was assessed for the complexes of two kinds of silver salts (AgBF₄ and AgCF₃SO₃) with several polymeric ligands containing three different carbonyl groups, including poly(vinyl pyrrolidone) (PVP) with an amide group, poly(vinyl methyl ketone) (PVMK) with a ketone group, and poly(methyl methacrylate) (PMMA) with an ester group. UV–vis spectra and transmission electron microscopy (TEM) images clearly indicated that the reduction rate of the silver ions has the following order in the various polymer matrices: PVP > PVMK > PMMA, whereas the size and the distribution of the nanoparticles exhibited the reverse order. The tendency to form silver nanoparticles was explained in terms of the differences between the comparative strengths of the interactions of the silver ions with the different carbonyl oxygens in the matrices, as well as that of the silver ions with counteranions, which was characterized by X‐ray photoelectron spectroscopy (XPS) and FT‐Raman spectroscopy. It was concluded that when the concentration of free silver ions was low due to weak polymer–silver ion and strong silver ion–anion interactions, as found with PMMA, the reduction rate of silver ions to silver nanoparticles was slow. Therefore, the PMMA–silver complex membranes were less sensitive to decreases in separation performance upon UV irradiation than compared to the PVP membranes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1168–1178, 2006     

Novel scorpionate-type triscarbene ligands and their silver and gold complexes

New silver(I) carbene complexes were obtained starting from the N-heterocyclic carbene ligand precursors {[HB(RImH)₃]Br₂} (R = Bn, Mes and t-Bu) and {[HC(MeBImH)₃](BF₄)₃}, by treatment of the imidazolium salt with Ag₂O. Use of the tris-imidazolylborate precursors resulted in stable, well-characterized trimetallic complexes of general formula {Ag₃[HB(RIm)₃]₂}Br, which were successfully employed as carbene transfer reagents in the synthesis of related gold(I) complexes by transmetallation. The silver complexes also proved to be active catalysts of the coupling of aryl iodides with terminal alkynes (the Sonogashira reaction), although related bimetallic silver complexes were found to exhibit enhanced reactivity.     

Synthesis of Nitrogen‐Containing Polyaromatics by Aza‐Annulative π‐Extension of Unfunctionalized Aromatics

Nitrogen‐containing polycyclic aromatic compounds (N‐PACs) are an important class of compounds in materials science. Reported here is a new aza‐annulative π‐extension (aza‐APEX) reaction that allows rapid access to a range of N‐PACs in 11–84 % yields from readily available unfunctionalized aromatics and imidoyl chlorides. In the presence of silver hexafluorophosphate, arenes and imidoyl chlorides couple in a regioselective fashion. The follow‐up oxidative treatment with p‐chloranil affords structurally diverse N‐PACs, which are very difficult to synthesize. DFT calculations reveal that the aza‐APEX reaction proceeds through the formal [4+2] cycloaddition of an arene and an in situ generated diarylnitrilium salt, with sequential aromatizations having relatively low activation energies. Transformation of N‐PACs into nitrogen‐doped nanographenes and their photophysical properties are also described.     

Efficient silver-free gold(I)-catalyzed hydration of alkynes at low catalyst loading

The use of [(IPr)AuOH] as versatile, air- and moisture-stable pre-catalyst permits the in situ generation of the cationic gold(I) species [(IPr)Au]X after reaction with a Brønsted acid. This catalytic system presents as a main advantage the lack of use of a silver salt activator or co-catalyst which is often air-, light- and moisture-sensitive. A general gold(I)-catalyzed procedure using this in situ activation at very low catalyst loading is reported for the hydration of a broad range of internal and terminal alkynes.     

Synthesis of silver/poly(2-hydroxyethyl methacrylate) particles via a combination of inverse miniemulsion and silver ion reduction in a "nanoreactor"

Silver salt/poly(2-hydroxyethyl methacrylate) (poly(HEMA)) hybrid particles were first prepared by inverse miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA) with silver tetrafluoroborate (AgBF(4)) as a lipophobe. High silver salt loads of up to 13% with respect to the disperse phase were achieved. The silver/poly(HEMA) hybrid particles were subsequently formed via a gas-phase in situ reduction of AgBF(4) by hydrazine on the surfaces of silver salt/poly(HEMA) particles. The formation of silver nanoparticles was confirmed by UV-vis spectroscopy and X-ray diffraction. The morphology of the hybrid particles of silver salt/poly(HEMA) and silver/poly(HEMA) was fully characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), and dynamic light scattering (DLS). The influence of the reaction parameters including the type and amount of cosolvent, salt content, and type of surfactant on the particle properties and colloidal stability during the reduction process was thoroughly investigated.     

Structural changes of silver polymer electrolytes: Comparison between poly(2‐ethyl‐2‐oxazoline) and poly(N‐vinyl pyrrolidone) complexes with silver salt

The difference between the polymer matrices of poly(2‐ethyl‐2‐oxazoline) (POZ) and poly(N‐vinyl pyrrolidone) (PVP) does not have a significant effect on the facilitated propylene transport and propylene solubility in 1:1 polymer/silver salt complex membranes, according to our previous work. In this article, its origin is investigated in terms of both microstructures of silver polymer electrolytes and the coordinative interaction of silver ion with polymer and with the counteranion. Initially different microstructures of POZ and PVP become similar to each other upon dissolving a large amount of silver salt, as evidenced by propane transport properties, specific volume, and Bragg d‐spacing. The dissolution of the silver salt in the polymer solvent strongly depends on the coordinative interaction between silver ion and carbonyl oxygen of POZ and PVP. Thus, the structural similarity upon dissolving silver salts in POZ and PVP is primarily determined by the coordinative interaction between silver ion and carbonyl oxygen, which was confirmed by theoretical structure calculation based on density functional theory and by IR and Raman spectroscopy. Therefore, facilitated olefin transport for silver polymer electrolyte membranes does not strongly depend on the polymeric matrix at high silver concentrations. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 232–237, 2004     

Rapid Biosynthesis of Silver Nanoparticles Using Areca Nut (Areca catechu) Extract Under Microwave-Assistance

In this research paper, we report on the rapid synthesis of silver nanoparticles using dried areca nut (Areca catechu). The microwave exposed aqueous areca nut powder when treated with the aqueous silver salt solution yielded irregular shaped silver nanoparticles. The formation and morphology of the nanoparticles are studied using UV–visible spectroscopy, field emission scanning electron microscopy, transmission electron microscopy and atomic force microscopy. The X-ray diffraction studies and energy dispersive X-ray analysis indicate that the particles are crystalline in nature. The understanding of capping of biological moiety is derived from Fourier transform infrared spectroscopy and the thermogravimetric analysis. The green chemistry approach for the synthesis of silver nanoparticles is modest, amenable for large scale commercial production. Further the biologically synthesized silver nanoparticles are known for their potential antibacterial activity.     

Simultaneous Grafting Polymerization of Acrylic Acid and Silver Aggregates Formation by Direct Reduction Using γ Radiation onto Silicone Surface and Their Antimicrobial Activity and Biocompatibility

The modification of medical devices is an area that has attracted a lot of attention in recent years; particularly, those developments which search to modify existing devices to render them antimicrobial. Most of these modifications involve at least two stages (modification of the base material with a polymer graft and immobilization of an antimicrobial agent) which are both time-consuming and complicate synthetic procedures; therefore, as an improvement, this project sought to produce antimicrobial silicone (PDMS) in a single step. Using gamma radiation as both an energy source for polymerization initiation and as a source of reducing agents in solution, PDMS was simultaneously grafted with acrylic acid and ethylene glycol dimethacrylate (AAc:EGDMA) while producing antimicrobial silver nanoparticles (AgNPs) onto the surface of the material. To obtain reproducible materials, experimental variables such as the effect of the dose, the intensity of radiation, and the concentration of the silver salt were evaluated, finding the optimal reaction conditions to obtain materials with valuable properties. The characterization of the material was performed using electronic microscopy and spectroscopic techniques such as ¹³C-CPMAS-SS-NMR and FTIR. Finally, these materials demonstrated good antimicrobial activity against S. aureus while retaining good cell viabilities (above 90%) for fibroblasts BALB/3T3.     

Role of anions for the reduction behavior of silver ions in polymer/silver salt complex membranes

The reduction of silver ions to silver nanoparticles is an essential issue in polymer/silver salt complex membranes for facilitated olefin transport, because it has a critical influence on the long-term stability of membrane performance. In this study, the role of anions for the formation of silver nanoparticles in polymer/silver salt complexes was investigated. This role was assessed for the complexes of poly(N-vinyl pyrrolidone) (PVP) with three silver salts including AgBF₄, AgCF₃SO₃, and AgNO₃. Especially, UV irradiation to the membranes was used to clearly investigate the reduction behavior of silver ions. Separation performance test, UV–vis spectroscopy and transmission electron microscopy (TEM) clearly show that the reduction rate of silver ions strongly depends on the counteranions of salt, and has the following order: AgBF₄ > AgCF₃SO₃ > AgNO₃. This behavior of the formation of silver nanoparticles in polymer/silver salt complex membranes is explained in terms of the interaction strength of silver ions with the carbonyl oxygens of polymer, and that of silver ions with counteranions. It is concluded that when the former interaction is strong and the latter one is weak, the reduction rate of silver ions to silver nanoparticles is fast, and vice versa. These interactions were characterized using FT-IR, FT-Raman spectroscopy, and theoretical ab initio calculation.