Zinc(II) complex immobilized on amine functionalized silica gel: a novel,highly efficient and recyclable catalyst for multicomponent click synthesis of 1,4-disubstituted 1,2,3-triazoles

We report a highly efficient and recyclable heterogeneous zinc catalytic system via covalent immobilization of 2-hydroxyacetophenone (2-HAP) onto an amine functionalized silica gel followed by metallation with zinc chloride and its catalytic application in three component click synthesis of 1,4-disubstituted 1,2,3-triazoles. The structure of the synthesized organic–inorganic hybrid material (SiO₂@APTES@2HAP-Zn) has been confirmed by various physicochemical characterization techniques, such as solid-state ¹³C CPMAS NMR spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction analysis (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, scanning electron microscopy (SEM), atomic absorption spectroscopy (AAS), energy-dispersive X-ray fluorescence spectroscopy (ED-XRF), and elemental analysis. The newly designed catalyst works under mild reaction conditions and also exhibits excellent performance in terms of good product yield and high turnover number (TON). One of the most important attributes of the present methodology is that the catalyst can be recycled several times without appreciable loss in its activity as proved by FTIR spectroscopy and SEM analysis. Besides, the heterogeneity test also confirms that no leaching of active catalytic species occurs from the silica supported zinc catalyst which confirms its remarkable structural stability under the reaction conditions.     

Application of solid-state NMR (13C and 29Si CP/MAS NMR) spectroscopy to the characterization of alkenyltrialkoxysilane and trialkoxysilyl-terminated polyisoprene grafting onto silica microparticles

The solid-state Nuclear Magnetic Resonance (NMR) was used to characterize surfaces of silica gels chemically modified by alkenyltrialkoxysilanes and trialkoxysilyl terminated 1,4-polyisoprenes. The formation of covalent bonds created between alkoxy functional groups from alkenyltrialkoxysilane or trialkoxysilyl-terminated 1,4-polyisoprene and silanol groups on silica was clearly demonstrated by means of ¹³C and ²⁹Si CP/MAS NMR spectroscopy. Quantitative data, including calculation of the grafting yields in relation with the initial silanol concentrations, were also obtained by using solid-state ²⁹Si-NMR leading to a final well-defined characterization of the silica surfaces. A relatively good agreement was noticed between the grafting yields calculated from ²⁹Si-NMR spectra and those determined from other analytical techniques such as Wijs titration or elementary analysis. The reactivity of the various silica silanols towards each coupling agent was clearly characterized and estimated, as were the proportions of the various grafted structures formed at the silica surface. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36 : 437–453, 1998     

Grafting of model primary amine compounds to cellulose nanowhiskers through periodate oxidation

This study demonstrates regioselective oxidation of cellulose nanowhiskers using 2.80–10.02 mmols of sodium periodate per 5 g of whiskers followed by grafting with methyl and butyl amines through a Schiff base reaction to obtain their amine derivatives in 80–90 % yield. We found a corresponding increase in carbonyl content (0.06–0.14 mmols/g) of the dialdehyde cellulose nanowhiskers with the increase in oxidant as measured by titrimetric analysis and this was further evidenced by FT-IR spectroscopy. Grafting of amine compounds to the oxidized cellulose nanowhiskers resulted in their amine derivatives, which are found to be partially soluble in DMSO. Therefore, the reduction reaction between amines and carbonyl groups was confirmed through ¹³C NMR spectra, which was also supported by copper titration, XPS, and FT-IR spectroscopy. Morphological integrity and crystallinity of the nanowhiskers was maintained after the chemical modification as studied by AFM and solid-state ¹³C NMR, respectively.     

Synthesis and characterization of amino-functionalized silica nanoparticles

Monodispersed raw silica nanoparticles (RSNPs) with the particle size of 40 nm were successfully fabricated by condensation reaction of tetraethylorthosilicate in methanol with high concentration ammonia (1.2 M). The RSNPs were treated with the coupling agent 3-aminopropyltrimethoxysilane (APTMS) for grafting amine groups on the surface to obtain the amino-functionalized silica nanoparticles (ASNPs). The chemical structure and surface morphology of RSNPs and ASNPs were characterized by Fourier-transform infrared spectra, solid-state NMR spectra and scanning electron microscopy. In addition, a method to quantify the grafted amine groups on the surface of ASNPs was developed by using the ninhydrin assay. The ninhydrin analysis showed that 60 mol % of the APTMS molecules were immobilized on the surface, that is, 4.4 amine groups per nm2 of surface area were bonded on nonporous ASNPs. The weight loss of particles obtained from thermogravimetry analysis indicated the amount of grafted amine groups and was used as a reference to compare with the value determined from ninhydrin method.     

Surface modification of silica nanoparticles by UV-induced graft polymerization of methyl methacrylate

In this study we modified the surface of silica nanoparticles with methyl methacrylate by UV-induced graft polymerization. It is a surface-initiated polymerization reaction induced by ultraviolet irradiation. The resulting organic-inorganic nanocomposites were near-monodisperse and fabricated without homopolymerization of the monomer. Substantial increase in mean particle size was observed by SEM image analysis after UV-induced grafting of methyl methacrylate onto pure silica particles. FT-Raman spectroscopy and X-ray photoelectron spectroscopy studies of these materials revealed the successful grafting of methyl methacrylate onto the silica surface. The formation of a covalent bond between the grafted PMMA chains and silica surface was indicated by FT-Raman spectra. Thermogravimetric analysis of the PMMA-grafted silica particles indicated the polymer contents in good agreement with SEM photographs.     

Surface Chemistry of Nanohybrids with Fumed Silica Functionalized by Polydimethylsiloxane/Dimethyl Carbonate Studied Using 1H, 13C,and 29Si Solid-State NMR Spectroscopy

The investigation of molecular interactions between a silica surface and organic/inorganic polymers is crucial for deeper understanding of the dominant mechanisms of surface functionalization. In this work, attachment of various depolymerized polydimethylsiloxanes (PDMS) of different chain lengths, affected by dimethyl carbonate (DMC), to silica nanoparticles pretreated at different temperatures has been studied using ²⁹Si, ¹H, and ¹³C solid-state NMR spectroscopy. The results show that grafting of different modifier blends onto a preheated silica surface depends strongly on the specific surface area (SSA) linked to the silica nanoparticle size distributions affecting all textural characteristics. The pretreatment at 400 °C results in a greater degree of the modification of (i) A-150 (SSA = 150 m²/g) by PDMS-10/DMC and PDMS-1000/DMC blends; (ii) A-200 by PDMS-10/DMC and PDMS-100/DMC blends; and (iii) A-300 by PDMS-100/DMC and PDMS-1000/DMC blends. The spectral features observed using solid-state NMR spectroscopy suggest that the main surface products of the reactions of various depolymerized PDMS with pretreated nanosilica particles are the (CH₃)₃SiO-[(CH₃)₂SiO-]_x fragments. The reactions occur with the siloxane bond breakage by DMC and replacing surface hydroxyls. Changes in the chemical shifts and line widths, as shown by solid-state NMR, provide novel information on the whole structure of functionalized nanosilica particles. This study highlights the major role of solid-state NMR spectroscopy for comprehensive characterization of functionalized solid surfaces.     

Calix[4]arene-modified silica nanoparticles for the potentiometric detection of iron (III) in aqueous solution

This article aims to present the synthesis and the characterization of a novel hybrid material, based on silica nanoparticles, and which can be used as a sensitive element for the electrochemical detection of iron (III). This material was obtained by grafting an iron ligand-based calix[4]arene structure onto nanosized silica particles. The covalent anchoring of the chelator was obtained by nucleophilic coupling reaction between the modified calix[4]arene and the chloropropyl functionalized silica. The grafting reaction was confirmed by FTIR (DRIFT), solid-state ¹³C Cross-Polarization MAS (CPMAS) NMR and thermal analysis. The interaction between the material and the dissolved iron (III) was demonstrated though potentiometric measurements. A linear evolution of the open circuit potential, which can be used analytically for iron (III) detection, has been obtained.     

Saccharide modified silica particles by enzymatic grafting

The surface of silica particles has been chemically modified with oligo- or poly-(α,1←4)-D -glucopyranose (amylose) chains of various length by covalently attaching maltoheptaose derivatives to the solid support and enzymatic polymerization of glucose-1-phosphate with potato phosphorylase (E.C. 2.4.1.1) as catalyst. The characterization of the products by solid-state NMR spectroscopy showed an interesting dependence of the linewidth with the grafting density of the glucan chains. The modified silica particles showed chiral discrimination in liquid chromatography.     

Mononuclear-dinuclear equilibrium of grafted copper complexes confined in the nanochannels of MCM-41 silica

Following the structural concept of copper-containing proteins in which dinuclear copper centers are connected by hydroxide bridging ligands, a bidentate copper(II) complex has been incorporated into nano-confined MCM-41 silica by a multistep sequential grafting technique. Characterization by a combination of EPR spectroscopy, X-ray photoelectron spectroscopy (XPS), UV/Vis spectroscopy, IR spectroscopy , and solid-state (13)C and (29)Si cross-polarization magic-angle spinning (CP-MAS) NMR suggests that dinuclear Cu complexes are bridged by hydroxide and other counterions (chloride or perchlorate ions), similar to the situation for EPR-undetectable [Cu(II)···Cu(II)] dimer analogues in biological systems. More importantly, a dynamic mononuclear-dinuclear equilibrium between different coordination modes of copper is observed, which strongly depends on the nature of the counterions (Cl(-) or ClO(4)(-)) in the copper precursor and the pore size of the silica matrix (the so-called confinement effect). A proton-transfer mechanism within the hydrogen-bonding network is suggested to explain the dynamic nature of the dinuclear copper complex supported on the MCM-41 silica.     

Physico-chemical characterization of MCM-41 silica spheres made by the pseudomorphic route and grafted with octadecyl chains

This work reports on the first comprehensive characterization of octadecyl (C(18)) modified MCM-41 silica spheres, prepared via the pseudomorphic route, followed by grafting with mono- or trifunctional octadecyl (C(18)) alkyl chains and endcapping with hexamethyldisilazane. Small angle X-ray scattering (SAXS), nitrogen adsorption-desorption and scanning electron microscopy (SEM) measurements were performed to obtain information about the MCM-41 pore structure, surface properties and morphological features. The degree of grafting and cross-linking of the silanes were determined by (29)Si magic angle spinning NMR spectroscopy, while FTIR and (13)C NMR were employed to study the conformational behavior of the surface-immobilized alkyl chains. The SAXS pattern proved the existence of a hexagonal mesopore arrangement for both the ungrafted and the grafted MCM-41 silica spheres. In addition, there is evidence of some long-range distortion in the pore structure. SEM measurements revealed the same morphological features for the parent silica and the MCM-41 silica spheres before and after C(18) grafting. The achieved surface loading for the MCM-41 material is rather low. It was also shown that a substantial amount of the accessible surface silanol groups is endcapped by trimethylsilane which in turn results in a very low surface coverage due to the octadecyl chains. The nitrogen sorption studies provided values for the surface area, total pore volume and pore diameter which are very typical for mesoporous materials. The reduction in surface area and total pore volume upon surface grafting is related to the binding of trimethylsilane in the interior of the pores, while due to the spatial restrictions octadecyl chains are primarily attached near the pore entrance. The experimental FTIR and (13)C NMR data point to a very low conformational order of the C(18) chains which is in accordance with the observed low surface coverage and the resulting spatial freedom for these surface-immobilized alkyl chains.     

Analytical investigation of the chemical reactivity and stability of aminopropyl-grafted silica in aqueous medium

Various samples of aminopropyl-functionalized silica (APS) have been prepared by grafting an organosilane precursor 3-aminopropyl-triethoxysilane (APTES) onto the surface of silica gel. The amine group content of the materials has been adjusted by varying the amount of APTES in the reaction medium (toluene). The grafted APS solids have been characterized with using several analytical techniques (N₂ adsorption, X-ray photoelectron spectroscopy, infrared spectrometry) to determine their physico-chemical properties. Their reactivity in aqueous solutions was studied by acid-base titration, via protonation of the amine groups, and by way of complexation of these groups by Hg^II species. APS stability in aqueous medium was investigated at various pH and as a function of time, by the quantitative analysis of soluble Si- or amine-containing species that have been leached in solution upon degradation of APS. The chemical stability was found to increase when decreasing pH below the pK_a value corresponding to the RNH₃⁺/RNH₂ couple, but very low pH values were necessary to get long-term stability because of the high local concentration of the amine groups in the APS materials. Adsorption of mercury(II) ions on APS was also performed to confirm the long-term stability of the grafted solid in acidic medium. Relationship between solution pH and APS stability was discussed. For sake of comparison, the stability of APS in ethanol and that of mercaptopropyl-grafted silica (MPS) in water have been briefly considered and discussed with respect to practical applications of silica-based organic–inorganic hybrids, e.g., in separation science or in the field of electrochemical sensors.     

Fast and Selective Aqueous-Phase Oxidation of Styrene to Acetophenone Using a Mesoporous Janus-Type Palladium Catalyst

A heterogeneous Janus-type palladium interphase catalyst was obtained by selective surface modification of a hollow mesoporous silica material. The catalyst comprises hydrophobic octyl groups on one side of the silica nanosheets and single-site bis-imidazoline dichlorido palladium(II) complexes on the other. The structure of this composite material has been analyzed by means of elemental analysis, atomic absorption spectroscopy, BET surface analysis, TGA, SEM and solid-state CP-MAS ¹³C and ²⁹Si NMR spectroscopy. The catalyst showed extraordinary activity for the aqueous-phase oxidation of styrene to acetophenone using 30% hydrogen peroxide as the oxidant. An 88% yield of acetophenone could be achieved after 60 min.     

A novel green template for the synthesis of mesoporous silica

Mesoporous pure silicas and functionalized silica with a narrow pore size distribution centered at 3.8 nm were prepared by a novel template, amphiphilic dendritic polyglycerol. The resulting silica materials were characterized by electron microscopy; nitrogen adsorption; (1)H, (13)C, and (29)Si solid-state cross-polarization magic-angle spinning NMR spectroscopy. It was shown that the template could be completely removed from the pure and functionalized silica in an environmentally friendly way by means of a simple water extraction procedure. Furthermore, it was shown that these materials could be easily functionalized, for example, by employing aminopropyl groups. Thus, a new environmentally friendly pathway to this fascinating class of silica material has been opened.     

Effect of chemical modification on carbon dioxide adsorption property of mesoporous silica

Three adsorbents were prepared by different modification methods, which were grafting silica gel with (3-aminopropyl) trimethoxysilane, grafting silica gel with acrylamide polymer, and impregnating silica gel with acrylamide polymer, respectively. The characterization of materials was carried out by N(2) adsorption experiments, Fourier transform infrared spectroscopy, scanning electron microscopy, thermo-gravimetric analysis, and elemental analyses. The results showed that the amine group was successfully loaded on all three modified adsorbents; among that, the polymer-modified silica adsorbents had higher amine content and larger surface area than the aminopropyl-grafted silica adsorbent and displayed higher thermal stability than the other polymer-modified silica materials previously reported. The CO(2) adsorption/desorption experiments performed at 25°C by TGA-DSC method showed that the highest CO(2) adsorption capacity (0.98 mmol/g) was observed for the polymer-impregnated silica adsorbent. CO(2) adsorbed on all samples was completely desorbed by purging with inert gas at 60°C except for the aminopropyl-grafted silica material, which showed the highest enthalpy of CO(2) adsorption.     

Perhydrocarbyl ReVII complexes: comparison of molecular and surface complexes

The molecular complex [Re(=CtBu)(=CHtBu)(CH2tBu)2] 1 reacts with a silica partially dehydroxylated at 700 degrees C to give syn-2, [(=SiO)Re(=CtBu)(=CHtBu)(CH2tBu)], as a single isomer according to mass-balance analysis, IR, and solid-state NMR spectroscopy. 1D and 2D solid-state NMR (HETCOR and long-range HETCOR) on a 13C-labeled-2 has allowed us to observe the chemical shifts of all carbons (including those that are not labeled) and ascertain their assignments. Moreover, EXAFS data are consistent with the presence of two carbons at a relatively short distance (1.79 A), which cannot be deconvoluted, but which are consistent with the presence of alkylidene and alkylidyne carbons along with two other first neighbors at a longer distance (2.01 A), the alkyl carbon and the O atom by which the Re is attached to the surface. Moreover, the data also suggest the presence of a siloxane bridge of the silica surface at 2.4 A in the coordination sphere of the Re center. Thermal and photochemical treatment allow us to observe the anti isomer, which was also fully characterized by 1D and 2D solid-state NMR. This behavior parallels the reactivity of molecular Re complexes, and their respective 1H and 13C chemical shifts match those of the corresponding molecular analogues syn- and anti-2m and n. Finally, the grafting of 1 onto silica involves the reaction of both the alkyl and the alkylidene ligand with an equiprobability, leaving the alkylidyne as a spectator ligand. Noteworthy is the formation of 2 [(=SiO)Re(=CtBu)(=CHtBu)(CH2tBu)], rather than the corresponding trisneopentyl-neopentylidyne Re complex, monografted on silica, [(=SiO)Re(=CtBu)(CH2tBu)3], which would have been expected from the reactivity of 1 with various molecular Br?nsted acids and which also suggests that a proximal siloxane bridge forces the alpha-H abstraction process, leading to syn-2a.     

Controlled synthesis of stable poly(vinyl formamide-co-vinyl amine)/silica hybrid particles by interfacial post-cross-linking reactions

 The chemical synthesis and the physicochemical properties of stable poly(vinyl formamide-co-vinyl amine)/silica hybrid particles are presented. Copolymers of poly(vinyl formamide) (PVFA) and poly(vinyl amine) (PVAm) and their protonated forms were adsorbed onto silica from aqueous solutions. The influences of the pH strength and the ion concentration of the aqueous solution as well as the copolymer composition (degree of hydrolyzation of PVFA), and the molecular mass on the adsorption process were investigated by electrokinetic measurements, potentiometric titration, and quantitative elemental analyses. Silica surface-charge neutralization is achieved at a pH strength above 10 for highly hydrolyzed (95%) PVFA polymers. Decreasing the amino content in the PVAm chain shifts successively both the point of zero charge and the isoelectric point to lower pH values. PVFA-co-PVAm layers onto silica are adsorbed weakly. To fix these layers irreversibly, cross-linking reactions with (4,4′-diisocyanate)diphenyl methane were carried out on the surface of solid PVFA-co-PVAm/silica hybrid particles suspended in acetone. The cross-linking reaction, which is connected with the conversion of amino groups, is also a tool to control the surface charge of the PVFA-co-PVAm/silica hybrids. X-ray photoelectron spectroscopy and solid-state ¹³C cross-polarization magic-angle spinning NMR spectroscopy were used to obtain information on the number of and the structure of the functionalized polyelectrolyte layers on silica. The success of cross-linking was also shown by the results of these spectroscopic methods. Received: 28 June 1999 /Accepted: 27 August 1999     

Synthesis of fullerene-silica hybrid materials

Fullerene/silica hybrid materials were obtained by radiation grafting on silica surface of toluene or decalin solutions of C₆₀. As determined by thermogravimetric analysis, the amount of C₆₀ grafted on silica surface was dependent from the radiation dose administered and independent from the C₆₀ concentration and the nature of the organic solvent. In absence of air, a dose of 48 kGy was sufficient to ensure a grafting level of 30% by weight of C₆₀ in the hybrid material. The fullerene/silica hybrid material shows a remarkable thermal stability, since the early decomposition starts above 300 °C as measured by DTG and DTA. The chemical structure of the fullerene/silica hybrid material was determined by FT-IR spectroscopy and with solid state ¹³C CP-MAS NMR. The potential application of such materials has been outlined.     

Reactions of phosphinites with oxide surfaces: a new method for anchoring organic and organometallic complexes

When a pincer-ligated iridium complex with a phosphinite substituent in the para-position of the aromatic backbone is immobilized on γ-alumina, it becomes a highly effective supported catalyst for the transfer-dehydrogenation of alkanes. The nature of the interaction between the organometallic complex and the support was investigated using solid-state (31)P MAS NMR spectroscopy, solution-state (1)H and (31)P{(1)H} NMR spectroscopy, IR and GC/MS analysis of extracted reaction products. The phosphinite substituent is cleaved from the pincer ligand by its reaction with hydroxyl groups on the γ-alumina surface, resulting in covalent anchoring of the complex via the aryl ring. A similar reaction occurs on silica, allowing for ready grafting onto this support as well. A strategy for anchoring homogeneous catalysts on hydroxyl-terminated oxide supports though the selective cleavage of [POR]-containing ligand substituents is suggested.     

Influence of dispersing medium on grafting of aminopropyltriethoxysilane in swelling clay materials

Functionalization of montmorillonite clay has been done using 3-aminopropyltriethoxysilane in the presence of various solvent media. Qualitative evidence of the presence of aminosilane attached to the clay platelets have been identified using Fourier transform infrared spectroscopy (FTIR) and 29Si and 13C solid-state nuclear magnetic resonance (NMR) spectroscopy. Grafting yield has been calculated using thermogravimetric analysis and total grafting yield increases with the solvent surface energy. X-ray diffraction studies of the silane functionalized montmorillonite clay exhibits two peaks, which may be attributed to intercalation and surface interaction with the broken edge platelets. Functionalized clay has been characterized by surface area measurements to understand the influence of solvents on the surface area of the functionalized clay.     

硅胶接枝新型长链季铵盐抗菌材料制备及其抗菌性能

硅胶接枝季铵盐;水不溶抗菌材料;聚乙烯亚胺;抗菌机理