Solid‐State NMR Characterization of Wilkinson’s Catalyst Immobilized in Mesoporous SBA‐3 Silica

The Wilkinson’s catalyst [RhCl(PPh₃)₃] has been immobilized inside the pores of amine functionalized mesoporous silica material SBA‐3 and The structure of the modified silica surface and the immobilized rhodium complex was determined by a combination of different solid‐state NMR methods. The successful modification of the silica surface was confirmed by ²⁹Si CP‐MAS NMR experiments. The presence of the T_n peaks confirms the successful functionalization of the support and shows the way of binding the organic groups to the surface of the mesopores. ³¹P‐³¹P J‐resolved 2D MAS NMR experiments were conducted in order to characterize the binding of the immobilized catalyst to the amine groups of the linkers attached to the silica surface. The pure catalyst exhibits a considerable ³¹P‐³¹P J‐coupling, well resolvable in 2D MAS NMR experiments. This J‐coupling was utilized to determine the binding mode of the catalyst to the linkers on the silica surface and the number of triphenylphosphine ligands that are replaced by coordination bonds to the amine groups. From the absence of any resolvable ³¹P‐³¹P J‐coupling in off‐magic‐angle‐spinning experiments, as well as slow‐spinning MAS experiments, it is concluded, that two triphenylphosphine ligands are replaced and that the catalyst is bonded to the silica surface through two linker molecules.     

Characterization of C18 chemically modified silica-gel and porous glass phases by high resolution solid state NMR spectroscopy and other physico-chemical methods

Summary Two types of packing materials, porous glass (PG) and silica gel (SG) have been modified by mono- and difunctional octadecylchlorosilane. The packing surfaces before and after chemical modification have been characterized by CP/MAS NMR, SIMS, porosimetrical, elemental and chromatographic methods. On the basis of the physico-chemical and chromatographic data the PG and SG (of similar porosity) used as supports of chemically bonded phases for RP HPLC, have been compared. Presented at 13th Symposium on Column Liquid Chromatography, Stockholm, June 25–30, 1989.     

硅胶键合杯[4]芳烃固定相的合成和核磁共振表征

杯芳烃（Ｃａｌｉｘａｒｅｎｅｓ）［１］是由苯酚单体通过亚甲基单元连接起来的一类性质稳定的环状低聚物．由于它的独特结构特点以及由此表现出来的特有性能,使之已成为继冠醚和环糊精之后的第三代超分子［２］．近年来,杯芳烃在色谱中的应用已经引起人们的关注,有关...     

Pentacyanoferrates(II) on the surface of organomodified silica gel: The matrix effect

Pentacyanoferrate(II) absorbed on a silica gel surface previously modified with 3-aminopropyl and 3-imidazolylpropyl groups were characterized by¹³C MAS/NMR, FT-IR, Mössbauer spectroscopy, and cyclic voltammetry. FT-IR and¹³C MAS/NMR data indicated that the pentacyanoferrate(II) complex is bonded to the surface by the nitrogen atom of the functional group. The differences in the isomeric shifts, the quandrupole splittings and the midpoint potentials of the adsorbed complexes in comparison with the model complexes were attributed to the matrix polar effect—i.e., the interaction of the complex with polar groups on the silica surface.     

Graft polymerization of styrene initiated by covalently bonded peroxide groups on silica

The graft polymerization of styrene initiated by immobilized peroxide groups was investigated. Three different types of modification reactions were used to introduce peroxide groups which are directly attached onto the surface of two different silica supports. Silanol groups were chlorinated using thionyl chloride or tetrachlorosilane. In another reaction pathway 1,3,5-benzenetricarbonyl chloride enabled the introduction of free acid chloride residues bonded onto the surface of silica. tert-Butyl hydroperoxide (TBHP) was used to transform the chlorosilyl and the acid chloride groups into peroxide residues. In a further reaction step the covalently bonded peroxides initiated the polymerization of styrene to form grafted polystyrene directly attached onto the silica support. Solid-state 13C CP/MAS NMR spectroscopy, and thermogravimetric and scanning electron microscope measurements enabled a clear structure and property elucidation of the different bonded phases. The highest amount of grafted polystyrene was achieved employing the acid chloride synthesis pathway with silica-gel, whereas modification of spherical silica only led to minor amounts of grafted polymer. The results contribute to the evolving need to understand particle surface modifications and may have positive impact on development of new HPLC stationary phases for improved elutant resolution.     

New alkyl-phosphate bonded stationary phases for liquid chromatographic separation of biologically active compounds

A new type of bonded stationary phase for liquid chromatography, with the properties of immobilized artificial membranes, has been synthesized. Alkyl-phosphate adsorbents were obtained by modification of aminopropyl silica gel. The structures of the synthesized materials were confirmed by use of instrumental techniques--elemental analysis, infrared spectroscopy (FTIR), and (13)C and (29)Si CP/MAS NMR. Analysis revealed that the adsorbents mimic the phospholipids present in natural cell membranes. The new synthesized alkyl-phosphate stationary phases may be used for liquid chromatographic separation of biologically active compounds of different polarity.     

Preparation and Characterization of a New Quercetin‐bonded Stationary Phase for High Performance Liquid Chromatography

A quercetin‐bonded silica gel stationary phase (QUSP) containing natural flavonoid ligand was first prepared via γ‐glycidoxypropyltrimethoxysilane (KH‐560) as a coupling reagent for high‐performance liquid chromatography. Its chemical structure was characterized by Fourier infrared spectroscopy, elemental analysis, thermal thermogravimetry and ¹³C cross polarization/magic angle spinning nuclear magnetic resonance (CP/MAS NMR). The chromatographic property of QUSP was systematically evaluated by using neutral, basic and acidic aromatic compounds as probes. In order to clarify its retention mechanism, a comparative study of QUSP with conventional octadecylsilyl‐bonded stationary phase (ODS) was also carried out under the same conditions. The results showed that the new quercetin‐bonded phase exhibited an excellent reversed‐phase chromatographic property with relatively weak hydrophobicity. However, it has an advantage over ODS in the fast separation of polar aromatic compounds because the quercetin ligand could provide various sites besides hydrophobicity, such as hydrogen bonding, dipole‐dipole, π‐π staking and charge transfer interactions. QUSP was performed in the baseline separations of ionized polar basic or acidic compounds, including pyridines, anilines, pyrimidines, purines and phenols with symmetric peak shape in common mobile phases without buffer salt within relatively short time. The natural ligands from herbs are readily available and contain a variety of active sites, which facilitate the exploration of industrial chromatographic separation materials for green products.     

New synthesis of alkyldihydrosilanes and their applications in modification of silica gel

Summary A comparison of newly synthesized, reversed-phase bonded stationary phases is presented. The materials studied include monomeric n-octyl, n-octadecyl, aryl and n-octyldihydrogeno bonded-phases. Differences in reactivity of the silane functional groups are demonstrated and the high coverage obtained with the hydrogeno-bonded phases is shown. The structures of the bonded phases have been determined by²⁹Si and¹³C CP/MAS NMR spectroscopy.     

Effect of surface coverage on the conformation and mobility of C18-modified silica gels

C18-modified silica gels with surface coverages of 2 to 8.2 micromol m(-2), were prepared by different synthetic pathways and characterized by Fourier Transform infrared spectroscopy (FTIR), solid-state nuclear magnetic resonance (NMR) spectroscopy, and chromatographic measurements. The effects of temperature and bonding density on the conformational order of C18-modified silica gels were studied in detail by FTIR spectroscopy. The silane functionality and degree of cross-linking of silane ligands on the silica surface were evaluated by 29Si cross-polarization magic-angle spinning (CP/MAS) NMR and the structural order and mobility of the alkyl chains were investigated by 13C CP/MAS NMR spectroscopy. CH2 symmetric and anti-symmetric stretching bands and CH2 wagging bands were used as IR probes to monitor the conformational order and flexibility of the alkyl chains in the C18 phases. Qualitative information about the conformational order was obtained from frequency shifts of the CH2 symmetric and anti-symmetric stretching bands. The relative amounts of kink/gauche-trans-gauche, double-gauche, and end-gauche conformers in the alkyl chains were determined by analysis of CH2 wagging bands. These results indicate that surface coverage plays a dominant role in the conformational order of C18-modified silica gels. The FTIR and NMR data are discussed in the context of the chromatographic shape-selectivity differences.     

Preparation of Various Bonded Phases for HPLC Using Monochlorosilanes

Abstract

Monochlorosilanes have been prepared with yields of about 80% through a catalytic hydrosilylation of terminal olefins. Subsequently, the silanes are chemically bonded to silica to obtain: (i) n-octyldimethylsilyl bonded phases with reproducible surface coverage ranging from 0.8 to 3.5 μmol/m², (ii) propyldimethylsilyl bonded phases with different functional groups at the γ-position, all showing a nearly equal surface coverage of some 3.3 μmol/m², and (iii) n-alkyldimethylsilyl bonded phases with chainlengths ranging from 1 to 22 carbon atoms and with surface coverages ranging from 3.9 μmol/m² for RP-1 to 3.0 μmol/m² for the RP-22 bonded phase. A simplified model based on the pore structure of silica allows an explanation and estimation of the maximum surface coverage as a function of the chainlength of the bonded phase.     

Application of cationic polymerization to grafting and coating of silica particles

The cationic polymerization of electron rich monomers such as vinyl ethers, vinyl furane, and cyclopentadiene on silica surfaces can be initiated by aryl methyl halides. The reactions yield always soluble polymers (by heterogeneous catalysis) and novel polymer/silica hybrid materials. The link between polymer and solid is caused by covalent Si-O-C bonds, by network formation of the polymers during the chain growth, or by a combination of both of them. The analysis of the polymer structures on the surface by ¹H MAS NMR spectroscopy in suspension and by solid state ¹³C CP MAS NMR spectroscopy is described. Proof of Si-O-C bonds via DRIFT spectroscopy and ¹³C CP MAS NMR spectroscopy is given. The most effective method of irreversibly linking the polymer to the silica surface is the network formation. Polyvinyl ethers are bound strongly to the surface, as can be shown by FTIR measurements, but the linkage is not stable due to the Si-O-C bonds' susceptibility to hydrolysis. Poly-cyclopentadienes (PCPD) are linked to the surface by Si-O-C bonds, which show an extraordinary high resistance to acids and bases. Si-O-C bond formation of poly-2-vinyl furane could not yet be detected by ¹³C CP MAS NMR spectroscopy and DRIFT spectroscopy. In this case the high degree of coating derives from the bifunctionality of 2-vinyl furane: it may undergo Friedel-Crafts-alkylation at the 5-position of the furane ring as well as chain polymerization via the vinyl group at the 2-position.     

Influence of the Structure of Chemically Bonded C 18 Phases on HPLC Separation of Naproxen Glucuronide Diastereoisomers

Abstract

On the basis of Nucleosil-100 a series of materials with varying structure and different coverage density of chemically bonded C₁₈ phases (CBP) were prepared. The physico-chemical characteristics of these packings e.g. porosity and carbon content were studied by the BET method and CHN analysis. The structure of the C₁₈ CBP was determined by solid state CP/MAS NMR. The prepared packings and columns have been applied for HPLC separation of naproxen and its two diastereoisomeric conjugates with glucuronic acid. Material with monomeric C₁₈ CBP structure and with high coverage density has given good and reproducible separation results.     

New stationary phases of conjugated π-electron systems for HPLC—characterization of structure and dynamics by solid state NMR spectroscopy

Four new stationary phases for HPLC were prepared by modifying silica gel with a trifrnctional aminoalkyl silane. A conjugated π-electron system was linked to the amino group either directly or with a carboxylic anhydride as a spacer in between. Characterization and dynamic measurements of the new stationary phases were performed by solid state NMR spectroscopy. The results of the ²⁹si and ¹³C CP/MAS techniques were compared with the recently developed ¹H MAS-only technique. Despite strong homonuclear dipole-dipole interactions it was possible to obtain well resolved ¹H MAS spectra of those stationaru phases with a high degree of crosslinking. Limited mobility of the aromatic ligand fragments is common for all examined stationaryu phases. The chromatographic properties of the new phases were tested by their ability tio sepatate a mixture of eight PAHS. It could be shown that the π-π interaction mechanism is responsible for the separation of the eight PAHs, because the elution order of the PAHs did not changem despite the use of both a nonpolar mobile phase (n-heptane) and a polar mobile phase (methanol/ water mixture).     

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     

Structural Features of Phosphate and Sulfate Modified Zirconia Prepared by Sol–Gel Route

This paper describes the effect of sulfate, phosphate and nitrate complexing ligands on the structural features of amorphous xerogels and on the crystallization of metastable zirconia phases during the xerogel-ceramic conversion. Powdered samples were prepared by a sol–gel route using zirconyl chloride precursors chemically modified by complexing ligands. The structural evolution of ZrO₂ phases as function of firing temperature was analyzed by XRPD, EXAFS and ³¹P NMR/MAS. The experimental results show the formation of metastable t-ZrO₂ during the low firing temperature of xerogels modified by sulfate or phosphate groups. The martensitic tetragonal-monoclinic transformation occurs during desorption of sulfate groups. The largest temperature interval of stability of metastable tetragonal zirconia was observed for phosphate-modified xerogels.     

Extraction of copper ions using silica gel with chemically modified surface

The applicability of silica gels for the application in solid-phase extraction was tested. Silica was modified with ketoimine groups. Surface characteristics of the modified silica were determined by elemental analysis, NMR spectra of solid phases (²⁹Si CP MAS NMR), analysis of pore size distribution of the silica support, and nitrogen adsorption-desorption. Newly proposed sorbents with ketoimine groups were applied in the preconcentration of trace amounts of the Cu (II) ions from lake water, post-industrial water, and demineralized water unburdened back to the lake. Presented at the 34th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 21–25 May 2007.     

Coverage homogenity of chemically bonded C18 phases for HPLC. Investigation of the changes in the free surface energy of the packings

Summary Packings containing chemically bonded C₁₈ groups with different coverage density were prepared using monochlorooctadecylsilane. The method permitting the determination of the free surface energy on the basis of adsorption measurements of two different liquids, noctane and water, was utilized to evaluate the coverage homogeneity of the silica gel surface by the alkylsilyl ligands. It was found that the packing is characterized by a coverage greater than 3.8 mol/m², it is homogenous and the support surface is fully screened.     

The Adsorption of Methanol Vapors on Silica Gel Si-100 and Its Surface Containing Different Concentrations of Chemically Bonded Fluoride Ions

Synthesis of two types Si-100 silica gel with different concentrations of chemically bonded surface fluoride ions (2 and 4 μmol m⁻² ) was performed. Using gravimetric methods, adsorption isotherms of methanol as a function of its pressure in gas phase were measured for these adsorbents. It was found the strong surface deactivation of silica gel in relation to methanol with the increase of bonded fluoride ions concentration. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of synthetic routes on the conformational order and mobility of C18 and C30 stationary phases

Silica gels modified with n-alkyl chains (n = 18, 30) are prepared by two different synthetic routes and are examined by variable temperature FTIR and solid-state NMR spectroscopy. HPLC measurements of SRM 869, cis/trans ss-carotene isomers and xanthophylls isomers confirm the dependence of the separation mechanism on the alkyl chain length and the synthetic routes. The determination of the silane functionality and degree of cross-linking of silane ligands on the silica surface is achieved by 29Si CP/MAS NMR measurements. The structural order and mobility of the alkyl chains are investigated by means of variable temperature 13C CP/MAS NMR measurements. Variable temperature FTIR studies are performed where conformational order and flexibility of the alkyl chains in C18 and C30 phases are monitored through conformational sensitive CH2 symmetric, anti-symmetric stretching and wagging modes. In addition, the chromatographic properties of the C18 and C30 phases are determined. The results derived from the FTIR, NMR and HPLC measurements are discussed in the context of the applied synthetic routes and alkyl chain lengths.     

Liquid crystalline polymers as stationary phases. IV. Chemical bonding and immobilization of the polymer on silica characterization by solid-state nuclear magnetic resonance spectroscopy

Chemical bonding reaction and immobilization through low energy radiation (heating) have been investigated to fix a side-chain liquid crystalline polymer (SC-LCP) on silica particles in order to use the resulting modified silica in normal-phase HPLC. Highly stable chromatographic stationary phases are observed under excellent polymer solvent flow conditions (THF) for both methods and better column efficiencies are also exhibited towards PAHs' separation compared to the classical coated stationary phase. The characterization of these new stationary phases and the rationale for improved column stability have been investigated by solid state 13C and 29Si CP/MAS NMR spectroscopy. It is clearly shown that the chemical bonding is achieved by the classical hydrosilylation reaction between PHMS chains and vinyl modified silica. The bonded polymer is likely a copolymer than a homopolymer. The immobilization of the SC-LCP by heating results in the breaking of Si-O-Si bonds of the polysiloxane chain after the attack of the silica surface silanols. Applications to fullerenes and carotenes separation of these bonded stationary phases are compared to the separation power of a classical monomeric C18 stationary phase in NP-HPLC as n-hexane-toluene or methyl-tertiobutyl ether-methanol mixtures.