Porous xerogels with a bifunctional surface layer of the composition ≡Si(CH<Subscript>2</Subscript>)<Subscript>3</Subscript>SH/≡Si(CH<Subscript>2</Subscript>)<Subscript>2</Subscript>H<Subscript>3</Subscript>

The sol-gel method with ethanol as a solvent and fluoride ion as a catalyst was used to prepare polysiloxane xerogels containing both 3-mercaptopropyl and n-propyl groups in the surface layer. An increase in the relative amount of n-propyltriethoxysilane in the initial reaction solution was found to result in the formation of xerogels with developed porous structures, which was accompanied by an increase in the specific surface area from 370 to 550 m²/g; simultaneously, other porous structure parameters such as sorption volume and pore size exhibited a tendency to increase. Atomic-force microscopy was used to show that the xerogels synthesized comprised aggregates of mean size 30 nm. An analysis of the IR and ¹³C cross-polarization magic angle spinning NMR data led us to conclude that the surface layer of bifunctional xerogels contained not only 3-mercaptopropyl and n-propyl groups but also silanol groups, part of nonhydrolyzed alkoxy groups, and H-bonded water molecules. The ²⁹Si cross-polarization magic angle spinning NMR spectra revealed the presence of structural units of the compositions T¹ [(≡SiO)Si(OR’)₂(CH₂CH₂CH₃) and/or (≡SiO)Si(OR’)₂(CH₂)₃SH, R’ = H, OCH₃, or OC₂H₅], T² [(≡SiO)₂Si(OR’)(CH₂CH₂CH₃) and (≡SiO)₂Si(OR’)(CH₂)₃SH], and T³ [(≡SiO)₃SiCH₂CH₂CH₃ and (≡SiO)₃Si(CH₂)₃SH] in the xerogels synthesized.     

The preparation of polysiloxane xerogels containing amide derivatives of phosphonic and thiophosphonic acids in the surface layer

Xerogels containing residues of amide derivatives of phosphonic and thiophosphonic acids, ≡Si(CH₂)₃NHP(S, O)(OC₂H₅)₂ (functional group concentration of 1.3–2.2 mmol/g) have been prepared by a sol-gel method. It has been shown that xerogels having a developed porous structure (with specific surface areas of 240–485 m²/g, pore volumes of 0.20–0.50 cm³/g, and pore diameters of 3.6–6.5 nm) are formed at tetraethoxysilane-to-trifunctional silane ratios of 4: 1 (and above) and 6: 1 (and above) for the derivatives of phosphonic and thiophosphonic acids, respectively. The IR and ¹³C CP/MAS NMR spectroscopy data have demonstrated that the surface layer of the xerogels contains not only (thio)phosphonic acid residues, but also silanol groups and water molecules participating in hydrogen bonding. The ²⁹Si CP/MAS NMR spectroscopy data have indicated that structural groups are, for the most part, contained in structural units T³ [(≡SiO)₃Si(CH₂)₃NHP(O, S)(OC₂H₅)₂] and T² [(≡SiO)₂Si(OR)[(CH₂)₃NHP(O, S)(OC₂H₅)₂] (R = H or C₂H₅).     

Bridged Polysilsesquioxane Xerogels Functionalizated by Amine- and Thiol- Groups: Synthesis, Structure, Adsorption Properties

Bridged polysilsesquioxane xerogels containing amine (–NH₂; –NH(CH₂)₂NH₂; —NH) and thiol (–SH) groups were synthesized by hydrolytic polycondensation of 1,2-bis(triethoxysilyl)ethane, 1,4-bis(triethoxysilyl)benzene and appropriate trifunctionalized silanes in the presence of a fluoride-ion catalyst in an ethanol solution. ²⁹Si CP/MAS NMR give indication of the molecular framework of these materials formed by structural T¹, T² and T³ units. 3-aminopropyl or 3-mercaptopropyl groups accessible to proton or metal ions are fixed to the xerogel surface by the siloxane bonds. IR and ¹³C CP/MAS NMR data clearly show that 3-aminopropyl groups form hydrogen bonds. The same data testify that all xerogels contain non-condensed silanol groups and some fraction of non-hydrolyzed ethoxygroups. Functionalized polysilsesquioxane xerogels obtained by means of organic spacers have a porous structure (500–1000 m²/g) and a high content of functional groups (1.0–2.7 mmol/g). AFM data indicate that xerogels are formed by aggregating primary particles—the size of such aggregates is in the range 30–65 nm. It was established that the main factors influencing the structure and adsorption properties considered hybrid materials are: the nature and geometrical size of the functional groups, spacer flexibility and, in some cases, the ratio of the reacting alkoxysilanes and the ageing time of the gel.     

The influence of the Si(OC2H5)4/(CH3O)3Si(CH2)3SH ratio on the structure-adsorption characteristics of xerogels formed and accessibility of functional groups in their surface layers

It was shown for the example of the Si(OC₂H₅)₄/(CH₃O)₃Si(CH₂)₃SH system that successively increasing the fraction of tetraethoxysilane in it (from 1: 1 to 5: 1 (mol)) successively decreased the content of 3-mercaptopropyl groups in xerogels synthesized by the sol-gel method (in the presence of methanol as a solvent and fluorine ions as a catalyst) from 5.0 to 1.9 mmol/g, whereas the specific surface area of such xerogels simultaneously increased from 13 to 631 m²/g. The sorption volume of pores also increased, their mean diameter varying insignificantly. The mean diameter of pores (2.2–2.5 nm) was close to the boundary between meso-and micropores, which was in agreement with the form of nitrogen adsorption isotherms (type I according to the IUPAC classification). It was shown by scanning electron microscopy that virtually nonporous xerogels formed at a 1: 1 ratio between alkoxysilanes consisted of spherical partially united particles 2.5–3 μm in diameter. All the 3-mercaptopropyl groups of this and other samples were, however, accessible to silver(I) ions. It follows that these groups are situated in the surface layer of xerogels. The number of thiol groups per 1 nm² of the surface of nonporous xerogels was 1.7–7.0 groups/nm² and depended on the ratio between reacting alkoxysilanes and s _sp.     

Trivalent-pentavalente Phosphorverbindungen/Phosphazene. IV. Darstellung und Eigenschaften neuer N-silylierter Diphosphazene

Trivalent-Pentavalent Phosphorus Compounds/Phosphazenes. IV. Preparation and Properties of New N-silylated Diphosphazenes Phosphazeno-phosphanes, R₃P = N? P(OR′) ₂ (R = CH₃, N(CH₃)₂; R′ = CH₂? CF₃) react with trimethylazido silane to give N-silylated diphosphazenes, R₃P = N? P(OR′)₂ = N? Si(CH₃)₃ compounds decompose by atmospherical air to phosphazeno-phosphonamidic acid esters, R₃ P?N? P(O)(O? CH₂? CF₃)(NH₂). Thermolysis of diphosphazene R₃P = N? P(OR′) ₂ = N? Si(CH₃)₃ (R = CH₃, R′ = CH₂? CF₃) produces phosphazenyl-phosphazenes [N?P(N?P(CH₃)₃)OR′] _n. The compounds are characterized by elementary analysis, IR-, ¹H_-, ²⁹Si-, ³¹P-n.m.r., and mass spectroscopy.     

Complex formation involving Hg<Superscript>2+</Superscript> ions on the surface of the polysiloxane xerogels functionalized by 3-mercaptopropyl groups

The influence of parameters of the porous structure and the surface layer composition of the xerogels containing 3-marcaptopropyl and alkyl groups on their sorption properties toward the Hg²⁺ ions and the stability constants of the formed complexes, which are calculated using the model of chemical reactions, is studied. An increase in the overall surface concentration of the functional groups is shown to induce a change in the composition of the formed complexes. At the concentration of the functional groups lower than 0.01 mmol/m² the [HgS(CH₂)₃Si≡]⁺ complexes are formed, and above 0.01 mmol/m² the composition of the complexes depends on the mercury(II) content in the starting solution: at low contents the [Hg{S(CH₂)₃Si≡}₂] complexes are formed, whereas at higher concentrations the composition of the complexes becomes simpler. Only the [Hg{S(CH₂)₃Si≡}₂] complexes are formed on the nearly nonporous xerogel with the polymeric structure of the surface layer (the functional group concentration is 0.38 mmol/m²). This, in turn, leads to the situation that the maximum static sorption capacity (590–620 (mg of Hg²⁺)/(g of sorbent)) is observed for the xerogels with a rather low content of the 3-mercaptopropyl groups (3.0–3.8 mmol/g). The stability of the formed complexes also depends on the surface concentration of the functional groups: the stability constant of the 1: 1 Hg(II) complexes decreases with an increase in the concentration of thiol groups. The introduction of alkyl groups into the surface layer also decreases the stability of the complexes formed. The [Hg{S(CH₂)₃Si≡}₂] complexes formed in the surface layer of the xerogels are characterized by similar stability constants.     

A magic angle spinning NMR study of xerogels functionalized by 3-mercaptopropyl groups

¹H magic angle spinning NMR spectroscopy was used to study xerogels containing the 3-mercaptopropyl group. These xerogels were synthesized using tetraethoxysilane, 1,2-bis(triethoxysilyl)ethane, and 1,4-bis(triethoxysilyl)benzene as structuring agents. The assignment of the NMR signals observed showed the presence of thiol groups introduced during syntheses and organic bridges in the frame of polysilsesquioxane samples. An analysis of the ¹H magic angle spinning NMR spectra also showed the presence of small amounts of alcohols, water participating in H-bonding, and nonhydrolyzed alkoxyl groups in the xerogels. In several instances, the structural units of T ⁿ and Q ^m types present in the xerogels were identified. The ¹H magic angle spinning NMR spectroscopy combined with ¹³C and ²⁹Si solid-state NMR spectroscopy allows the composition of xerogels and the nature of the structural units they contain to be identified more thoroughly and reliably.     

Accessibility and Solid State NMR Studies on Sol‐Gel Processed Diphosphine Ligands

Novel xerogels X1 a–d were obtained by sol‐gel processing of the monomeric T‐functionalized diphosphine ligand (MeO)₃Si(CH₂)₆CH[CH₂PPh₂]₂ [1(T⁰)] with various amounts of the co‐condensing agents MeSi(OMe)₂(CH₂)₆(OMe)₂SiMe (D⁰–C₆–D⁰) and MeSi(OMe)₂(CH₂)₃(C₆H₄)(CH₂)₃(OMe)₂SiMe [Ph(1,4‐C₃D⁰)₂] . ²⁹Si CP/MAS NMR spectroscopic investigations were applied to probe the matrices and their degree of condensation. The integrity of the hydrocarbon backbone and diphosphine moiety was examined by means of solid state NMR spectroscopy (¹³C, ³¹P). To study the dynamics of the matrices and the phosphorus centers detailed measurements of relaxation time (T_1ρH) and cross polarization constants (T_SiH, T_PH) were carried out. The accessibility of the polysiloxane‐supported diphosphines was scrutinized by some typical phosphine reactions. It was found that reagents such as H₂O₂, MeI as well as bulky molecules like (NBD)Mo(CO)₄ or (COD)PdCl₂ are able to reach all phosphorus centers independent on the kind of the backbone of the matrix. SEM micrographs show the morphology of the hybrid materials and energy dispersive X‐ray spectroscopy (EDX) suggest that the distribution of the elements agree with the applied composition.     

Trivalent-pentavalente Phosphorverbindungen/Phosphazene. V. Darstellung,Eigenschaften und Reaktionen neuer Phosphonig- und Phosphinigsäureester

Trivalent-Pentavalent Phosphorus Compounds/Phosphazenes. V. Preparation, Properties, and Reactions of New Phosphonous- and Phosphinous Acid Esters Phosphonous diesters R? P(OR′)₂ (R ? CH₃, Ph and R′ ? CH₂? CF₃) have been synthetized by reaction of phosphonous dichlorides with 2, 2, 2-trifluoroethanol in the presence of a base. These diesters react with trimethyl(trimethylsilylimino)phosphorane, (CH₃)₃P?N? Si(CH₃)₃ by desilylation and N? P-linking to phosphinous acid esters (CH₃)₃P?N? P(OR′)R. The phosphinous acid esters react with methyl iodide to the quaternary salts [(CH₃)₃P? N? P(OR′)(R)CH₃]⁺I^?. The compounds are characterized by elementary analysis and spectroscopical methods.     

Synthesis and properties of polysiloxane xerogels containing tetrasulfide groups

Polysiloxane xerogels with a functional group content of 1.1?C1.9 mmol/g have been obtained by the hydrolytic condensation of the alkoxysilanes Si(OC₂H₅)₄ and [(C₂H₅O)₃Si(CH₂)₃S₂]₂ in the 2 : 1, 4 : 1, and 8 : 1 ratios. It has been demonstrated by ¹³C and ²⁹Si CPMAS NMR spectroscopy that the xerogels have a polysiloxane framework with dipropyl tetrasulfide bridges, silanol groups, unhydrolyzed ethoxyl groups, and hydrogen-bonded water molecules on the surface. The xerogels have a porous structure. As the molar ratio of the reacting alkoxysilanes is increased in the above-specified range, the specific surface area of the xerogel increases (from 89 to 312 m²/kg) and the same is valid for other structure-adsorption characteristics. The synthesized polysiloxane xerogels readily sorb Hg²⁺ from acidified solutions. Their static sorption capacity can be as high as 1.5 g Hg per gram of sorbent. However, in the course of time, the 1 : 1 complexes forming on the xerogel surface undergo transformations accompanied by the release of mercury sulfide and/or Hg²⁺ reduction to mercury metal.     

Novel D- and T-functionalized Polysiloxane Matrices for Reactions in Interphases

New hydrocarbon bridged co-condensation agents of the type RSi(OMe)₂(CH₂)_zC₆H₄(CH₂)_z(OMe)₂SiR { 3[Ph(1,4-C₃D⁰)₂] , z = 3, R = Me; 3[Ph(1,4-C₃T⁰)₂] , z = 3, R = OMe; 4[Ph(1,4-C₃D⁰)₂] , z = 4, R = Me} were synthesized by hydrosilylation of the corresponding α,ω-dienes CH₂=CH–(CH₂)_z–2–C₆H₄–(CH₂)_z–2–CH=CH₂ [z = 3 ( 1 ), 4 ( 2 )] with HSiR(OMe)₂ (R = Me, OMe). These silane monomers were sol-gel processed, partially with MeSi(OMe)₃ ( T ⁰) to give the polysiloxanes 3 a , 3 b , 4 c , 3 d , 3 e , 4 f , and 3 ab (Table 1, Schemes 2 and 3); D = D type silicon atom (two oxygen neighbors), T = T type of silicon atom (three oxygen neighbors). The relative amounts of T and D silyl species and the degrees of condensation were determined by ²⁹Si and ¹³C CP/MAS NMR spectroscopic investigations. ²⁹Si and ¹³C CP/MAS NMR relaxation time studies (T_SiH, T_CH, T_1ρH), and 2 D WISE NMR experiments were applied to get knowledge about the polymer dynamics. For the first time protons of such polysiloxane systems were detected by ¹H SPE/MAS NMR measurements in suspension. Mobility studies were carried out in different solvents. Furthermore the swelling capacities of the polymers 3 a , 3 b , and 4 c in different solvents and the BET surface areas of all materials were investigated. SEM micrographs show the morphology of 3 a and 3 b .     

Trägerfixierte Organometallkomplexe. IV. Strukturuntersuchungen an neutralen und kationischen (Ether-phosphan)palladium(II)-Komplexen

Supported Organometallic Complexes. IV. Structural Investigations on Neutral and Cationic (Ether-phosphane)palladium(II) Complexes . Reaction of the (ether-phosphane) ligands PhP(R)CH₂—D ( 2a?c ) [D=CH₂OCH₃: R=Ph ( a ), (CH₂)₃Si(OCH₃)₃ ( b ), (CH₂)₃SiO_3/2 ( b ′); D= R=(CH₂)₃Si(OCH₃)₃ ( c ), (CH₂)₃SiO_3/2 ( c ′)] with Cl₂Pd(COD) ( 1 ) results in the formation of Cl₂Pd(P — O)₂ ( 3a?c ). Cleavage of Cl^? from 3 with AgSbF₆ yields the cationic, monochelated complexes [ClPd(P — O)(P ∩ O)]⁺ ( 4 a—c ) (P — O: η¹-P-coordinated; P ∩ O: η²-O ∩ P-coordinated). 4 a crystallizes in the monoclinic space group P2₁/c with the lattice constants a=1 062,4(2), b=1 912,2(4) und c=1 635,5(3) pm, β=101,22(3)° and Z=4 (R=0,0341; R_w=0,033). With water 3 b, c and 4 b, c are subjected to polycondensation to give the supported complexes 3 b′, c′, 4 b′, c ′. The structure 3 b′, c′, 4 b′, c ′ is investigated by comparison of their ³¹P CP MAS data with the ³¹P{¹H} NMR spectra of their soluble precursors 3 b, c, 4 b, c . ¹³C CP MAS NMR spectra of 3 b′, c ′ and 4 b′, c ′ indicate η¹-P- and η²-O ∩ P-coordination of the ligands. The polysiloxane network of 4 b ′ was inspected by contact time variation of the ²⁹Si CP MAS NMR spectra and it appeared that 77% of the Si—O units are crosslinked, corresponding to a ratio T₄:T₃:T₁=67:100:10.     

29Si MAS-NMR Study of Silica Gels and Xerogels: Influence of the Catalyst

Four silica gels were prepared by hydrolysis of tetraethoxysilane (TEOS) in ethanol, using different catalysts: HCl, NaOH, NH₃, and NBu₄F. Nitrogen adsorption-desorption isotherms indicated that the HCl-catalyzed xerogel was purely microporous, whereas the other samples exhibited a very broad distribution of mesopores and a variable amount of micropores. ²⁹Si MAS NMR spectroscopy of the wet gels (before drying) pointed to a low degree of condensation for the HCl-catalyzed gel, and to the presence of unhydrolyzed TEOS monomer in the NaOH-catalyzed gel. Comparison with the ²⁹Si MAS NMR spectra of the xerogels indicated a significant increase of the degree of condensation during the drying procedure (3 hrs at 120°C under vacuum) for the HCl-catalyzed gel.     

Proton conductivity of composites based on Nafion and silica matrices with a chemically modified surface

The surface of aerosilogel and macroporous and microporous glasses was modified with aromatic sulfo groups (≡Si(CH₂)₂-C₆H₄-SO₃H) by the chemical assembly method. Composite solid electrolytes containing Nafion were prepared from the silica matrices with a chemically modified surface. The proton conductivity of the materials was studied by impedance spectroscopy. Composite materials based on Nafion and aerosilogel matrices and macroporous glasses with grafted ≡Si(CH₂)₂-C₆H₄-SO₃H groups showed the highest proton conductivity.     

Formation of borosilicate glasses from silicon alkoxides and metaborate esters in dry non-aqueous solvents

The reaction of metaborate esters (RO)₃B₃O₃ [R = Me, Et, ClCH₂CH₂–, Cl₃CCH₂–, ClCH₂CH₂CH₂–, (ClCH₂)₂CH–] with Si(OR)₄ (R = Me, Et), either neat or in dry propan-2-one or dry THF at room temperature, led to gels which when dried and heated in air for 20 mins at 600°C afforded borosilicate glasses in high ceramic yields. The dried gels and glasses were characterized by elemental analysis, TGA, IR, and powder XRD, and solid-state MAS ²⁹Si and ¹¹B NMR. The gelling reaction was investigated by solution ¹¹B and ²⁹Si NMR. These NMR studies indicated B–O–Si reaction intermediates and a mechanism involving alkoxy exchange and various condensation/elimination reactions of the borosilicate esters have been proposed.     

Investigating the two inequivalent NH2(CH3)2 ions in [NH2(CH3)2]2CuCl4 using magic angle spinning nuclear magnetic resonance

The structural change near the phase transition temperatures of [NH₂(CH₃)₂]₂CuCl₄ is discussed in terms of the chemical shifts and the spin-lattice relaxation times T_1ρ in the rotating frame for ¹H MAS NMR and ¹³C CP/MAS NMR. The ¹H T_1ρ undergoes molecular motion near the phase-transition temperature (T_C2 = 253 K). In addition, the two inequivalent [NH₂(CH₃)₂] (1) and [NH₂(CH₃)₂] (2) sites were distinguishable by the ¹³C chemical shift. And, the most significant change was observed at T_C2 for the ¹³C CP/MAS NMR spectrum; this temperature corresponds to a ferroelastic phase transition with different orientations.     

Characterisation of hybrid xerogels synthesised in acid media using methyltriethoxysilane (MTEOS) and tetraethoxysilane (TEOS) as precursors

The mild synthetic conditions provided by the sol-gel process and the versatility of the colloidal state allow for the mixing of inorganic and organic components at the nanometre scale in virtually any ratio for the preparation of hybrid materials. Our interest in hybrid xerogels focuses on combining their porosity with other properties to prepare optic-fibre sensors. The specific aim of this paper is to synthesise hybrid xerogels in acid media using methyltriethoxysilane (MTEOS) and tetraethoxysilane (TEOS) as silica precursors and to investigate the effect of the MTEOS molar ratio on the structure and porous texture of xerogels. Gelation time exponentially increased as the MTEOS molar ratio increased. Increasing the MTEOS molar ratio yielded xerogels with lower density and lower particle size. The incorporation of MTEOS resulted in new FTIR bands at 1276 and 791 cm⁻¹, which was attributed to vibrational modes of methyl group. The band around 1092 cm⁻¹ associated with siloxane bonds shifted to lower wavenumbers and split into two bands. The ²⁹Si spectra only showed the Q ⁿ (n=2, 3, 4) signal in xerogels with 0% MTEOS and the T ⁿ (n=2, 3) signal in xerogels with 100% MTEOS; hybrid xerogels showed both Q and T signals. From XRD peaks at 2θ around 9°, we inferred that xerogels (>70% MTEOS) consisted of nanocrystalline CH₃–SiO_3/2 species. Increasing the MTEOS molar ratio produced xerogels with lower pore volumes and lower average pore size. The integration of methyl groups on the surface decreased the surface polarity and, in turn, the characteristic energy.     

Successive heterolytic cleavages of H2 achieve N2 splitting on silica-supported tantalum hydrides: a DFT proposed mechanism

DFT(B3PW91) calculations have been carried out to propose a pathway for the N(2) cleavage by H(2) in the presence of silica-supported tantalum hydride complexes [(≡SiO)(2)TaH(x)] that forms [(≡SiO)(2)Ta(NH)(NH(2))] (Science 2007, 317, 1056). The calculations, performed on the cluster models {μ-O[(HO)(2)SiO](2)}TaH(1) and {μ-O[(HO)(2)SiO](2)}TaH(3), labelled as (≡SiO)(2)TaH(x) (x = 1, 3), show that the direct hydride transfers to coordinated N-based ligands in (≡SiO)(2)TaH(η(2)-N(2)) and (≡SiO)(2)TaH(η(2)-HNNH) have high energy barrier barriers. These high energy barriers are due in part to a lack of energetically accessible empty orbitals in the negatively charged N-based ligands. It is shown that a succession of proton transfers and reduction steps (hydride transfer or 2 electron reduction by way of dihydride reductive coupling) to the nitrogen-based ligands leads to more energetically accessible pathways. These proton transfers, which occur by way of heterolytic activation of H(2), increase the electrophilicity of the resulting ligand (diazenido, N(2)H(-), and hydrazido, NHNH(2)(-), respectively) that can thus accept a hydride with a moderate energy barrier. In the case of (≡SiO)(2)TaH(η(2)-HNNH), the H(2) molecule that is adding across the Ta-N bond is released after the hydride transfer step by heterolytic elimination from (≡SiO)(2)TaH(NH(2))(2), suggesting that dihydrogen has a key role in assisting the final steps of the reaction without itself being consumed in the process. This partly accounts for the experimental observation that the addition of H(2) is needed to convert an intermediate, identified as a diazenido complex [(≡SiO)(2)TaH(η(2)-HNNH)] from its ν(N-H) stretching frequency of 3400 cm(-1), to the final product. Throughout the proposed mechanism, the tantalum remains in its preferred high oxidation state and avoids redox-type reactions, which are more energetically demanding.     

A review on polysiloxane-immobilized ligand systems: Synthesis, characterization and applications

The immobilized silica gel ligand systems made by modification of silica surfaces have been briefly summarized. Short background was described based on the synthesis methods and their applications. In this review more attention towards the functionalized polysiloxane xerogels and their postmodification has been given. Polysiloxane-immobilized ligand systems bearing organofunctionalized ligand groups of general formula P-(CH₂)₃-X (where P represents a three-dimensional silica like network-matrix and X is an organofunctional group) were prepared through the sol-gel process by hydrolytic polycondensation of Si(OR)₄ and the appropriate silane coupling agent (RO)₃Si(CH₂)₃X (where R is an alkyl group, e.g CH₃ or C₂H₅). There are many other immobilized ligand systems, which were prepared by treatment of post-polysiloxane precursors with an appropriate organofunctional ligand. Variety of functionalized materials ranging from simple up to macrocyclic immobilized ligand systems were prepared and well characterized. These materials have the advantage over the functionalized silica, as they can be prepared using different molar ratios of Si(OR)₄ and (RO)₃Si(CH₂)₃X silane agents, and therefore their metal uptake capacities can be altered. A mixture of two different ligand groups can also be achieved on the same matrix. Analytical and environmental applications of these materials have been reported including extraction, separation and preconcentration of metal ions. A variety of physical chemistry techniques that were employed to characterize the surface and the bulk of the immobilized systems were reported. These included high-resolution solid-state nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR).     

Cationic Silica‐Supported N‐Heterocyclic Carbene Tungsten Oxo Alkylidene Sites: Highly Active and Stable Catalysts for Olefin Metathesis

Designing supported alkene metathesis catalysts with high activity and stability is still a challenge, despite significant advances in the last years. Described herein is the combination of strong σ‐donating N‐heterocyclic carbene ligands with weak σ‐donating surface silanolates and cationic tungsten sites leading to highly active and stable alkene metathesis catalysts. These well‐defined silica‐supported catalysts, [(≡SiO)W(=O)(=CHCMe₂Ph)(IMes)(OTf)] and [(≡SiO)W(=O)(=CHCMe₂Ph)(IMes)⁺][B(Ar^F)₄^?] [IMes=1,3‐bis(2,4,6‐trimethylphenyl)‐imidazol‐2‐ylidene, B(Ar^F)₄=B(3,5‐(CF₃)₂C₆H₃)₄] catalyze alkene metathesis, and the cationic species display unprecedented activity for a broad range of substrates, especially for terminal olefins with turnover numbers above 1.2 million for propene.