Ionic covalent anchoring

In 2002 Mehnert and co-workers were the first to apply SILP-catalysis to Rh-catalysed hydroformylation [74], They described in detail the preparation of a surface modified silica gel with a covalently anchored ionic liquid fragment (Scheme 7.7). The complex N-3-(3-triethoxysilylpropyl)-4,5-dihydroimidazole was reacted with 1-chlorobutane to give the complex l-butyl-3-(3-triethoxysilylpropyl)- 4,5-dihydroimidazolium chloride. The latter was further treated with either sodium tetrafluoroborate or sodium hexafluorophosphate in acetonitrile to introduce the desired anion. In the immobilisation step, pre-treated silica gel was refluxed with a chloroform solution of the functionalised ionic liquid to undergo a condensation reaction giving the modified support material. Treatment of the obtained monolayer of ionic liquid with additional ionic liquid resulted in a multiple layer of free ionic liquid on the support. 

SILC was also used without covalently anchoring the ionic liquid fragment to the silica support. In this case, [bmim][PF6] was simply added to silica in acetone together with the catalyst. [Rh(norbornadiene)(PPh3)2]PF6 and the solvent evaporated to yield the supported catalyst-philic phase. Catalyst evaluation on the hydrogenation of model olefins showed enhanced activity in comparison to homogeneous and biphasic reaction systems, in analogy to Davis s observations. Also... 

An immobilized version of a Heck reaction catalyzed by Pd nanoparticles has been very recently described by Kaiimi and Enders [289]. The nanoparticles were obtained as a result of the covalent anchoring of a N-heterocyclic carbene palladium/ionic liquid matrix on a silica surfece and their nature was confirmed by TEM coupled with EDX analysis. The catalyst showed high thermal stability (up to 280 °C) and could be recycled four times for the reaction of bromobenzene with methylacrylate achieving a total turnover number of 36600. After carrying out a hot filtration process, the authors could not detect any Pd in the filtrate. The filtrate also showed no further reaction progress. Erom these findings the authors concluded that the reaction was, in their case, indeed catalyzed by the heterogeneous Pd particles and not from monomolecular Pd-complexes leached from the sur ce. 

Ionic Liquid Catalysts Supported through Covalent Anchoring 5.6.2.3.1 Supported Lewis acidic chlorometalate catalysts... 

Even though most of the supported ionic liquid catalysts prepared thus far have been based on silica or other oxide supports, a few catalysts have been reported where other support materials have been employed. One example involves a polymer-supported ionic liquid catalyst system prepared by covalent anchoring of an imidazolium compound via a linker chain to a polystyrene support [79]. Using a multi-step synthetic strategy the polymeric support (e.g. Merrifield resin among others) was modified with l-hexyl-3-methylimidazolium cations (Scheme 5.6-4) and investigated for nucleophilic substitution reactions including fluorina-tions with alkali-metal fluorides of haloalkanes and sulfonylalkanes (e.g. mesylates, tosylates and triflates). 

This concept was first applied for hydroformylation in 2002 by Mehnert at ExxonMobil [50]. This work involves a surface of a support material (silica gel) that is modified with a monolayer of covalently anchored fragments of l-n-butyl-3-[3-(triethoxysilanyl)propyl]imidazolium. Treatment of this surface with additional IL results in the formation of a multiple layer of free ionic liquid which serves as... 

Scheme 4.2 Covalent anchoring of ionic liquid on silica.

Similarly, Au NPs have also been supported on r-GO an N-doped r-GO and the materials have been tested as catalysts for the aerobic oxidation of benzyl alcohol [40-41]. It was found that the presence of nitrogen as dopant is beneficial to obtain Au NPs with small size, therefore exhibiting enhanced catalytic activity [41]. The use of Au NPs and Au-Pd NPs supported on r-GO has also been reported to promote the aerobic oxidation of alcohols and oxidation of methanol to methyl formate [42]. In one of these examples, r-GO has been functionalized with imidazolium ionic liquid covalently anchored to the r-GO in order to increase the affinity of the support for Au NPs. 

They should have a polar moiety or a set of polar functional groups that is responsible for the ion recognition process. The rest of the ionophore molecules should contain hydrophobic regions that are compatible with the surrounding membrane matrix. The historical argument that an ionophore molecule must also exhibit a certain mobility within the membrane has been largely disproved by the comparable analytical performance of a number of membrane materials where the ionophore is covalently anchored onto the polymeric backbone [35]. It seems beneficial, however, to at least either have mobile ionophores or mobile ionic sites to guarantee an acceptably low membrane resistance. 

In order to reduce the amount of precious ionic liquids and to overcome their high viscosity, which cause low diffusion coefficients for H2 and CO, the immobilization of ionic liquids hosting the catalyst on solid materials is of interest [107]. Immobilization can be achieved by physisorption, tethering, or covalent anchoring of ionic liquid fragments on a porous material. In the ideal case, a thin film is formed containing the catalyst and ionic liquid. 

Organocatalysts are often non-covalently anchored to an insoluble support such as ionic liquid modified-silica, polyelectrolytes, montmorillonite, or [i-cyclodejctrin. Although bond strength in non-covalently supported catalyst is weak, the catalyst itself is directly used for immobihzation without the need for modification or synthetic steps required for covalent attachment to support [113]. 

In the case of chemisoriDtion this is the most exothennic process and the strong molecule substrate interaction results in an anchoring of the headgroup at a certain surface site via a chemical bond. This bond can be covalent, covalent with a polar part or purely ionic. As a result of the exothennic interaction between the headgroup and the substrate, the molecules try to occupy each available surface site. Molecules that are already at the surface are pushed together during this process. Therefore, even for chemisorbed species, a certain surface mobility has to be anticipated before the molecules finally anchor. Otherwise the evolution of ordered stmctures could not be explained. 

Cross-linked gel-type functional polymers (CFPs) are organic materials built up with interconnected polymer chains [1]. Pendants hanging from the polymer chains may render CFPs reactive materials particularly suitable for anchoring metal centres removed from a liquid phase, by means of covalent or ionic bonds [2] ... 

Fig. 21.2 Nonferrocene diamine ligands (shown on the leji) can be anchored to the inner walls of mesoporous silica using either a covalent or an ionic immobilization approach (right)...

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