Transition Metal Templated Approaches

Until recently, azides were relatively rare precursors of interlocked molecules and most of the time they were meant to be linked to alkynes using 1,3-dipolar cycloaddition. In this review, we will restrict ourselves to their use in attachment of stoppers to pseudo-rotaxanes and present a selection of the most representative papers. They can be divided into purely organic molecular entanglements and into interlocked molecules prepared by transition metal templated approaches. 

We have not attempted to cover all or even most aspects of crown chemistry and some may say that the inclusions are eclectic. We felt that anyone approaching the field would need an appreciation for the jargon currently abounding and for the so-called template effect since the latter has a considerable bearing on the synthetic methodology. We have, therefore, included brief discussions of these topics in the first two chapters. In chapters 3—8, we have tried to present an overview of the macrocyclic polyethers which have been prepared. We have taken a decidedly organic tack in this attempting to be comprehensive in our inclusion of alkali and alkaline earth cation binders rather than the compounds of use in transition metal chemistry. Nevertheless, many of the latter are included in concert with their overall importance. 

Transition-metal nanopartides are of fundamental interest and technological importance because of their applications to catalysis [22,104-107]. Synthetic routes to metal nanopartides include evaporation and condensation, and chemical or electrochemical reduction of metal salts in the presence of stabilizers [104,105,108-110]. The purpose of the stabilizers, which include polymers, ligands, and surfactants, is to control particle size and prevent agglomeration. However, stabilizers also passivate cluster surfaces. For some applications, such as catalysis, it is desirable to prepare small, stable, but not-fully-passivated, particles so that substrates can access the encapsulated clusters. Another promising method for preparing clusters and colloids involves the use of templates, such as reverse micelles [111,112] and porous membranes [106,113,114]. However, even this approach results in at least partial passivation and mass transfer limitations unless the template is removed. Unfortunately, removal of the template may re-... 

New Supramolecular Approaches in Transition Metal Catalysis Template-Ligand Assisted Catalyst Encapsulation, Self-Assembled Ligands and Supramolecular Catalyst Immobilization... 

We started this templated approach to catalyst encapsulation using tris(mefa-pyridyl)phosphine and zinc(ii)TPP (TPP = tetraphenylporphyrin) [7]. From NMR and UV/Vis titration experiments we found a selective assembly process via coordination of the nitrogen to the zinc(ii)TPP, rendering the phosphine donor atom completely encapsulated by the three porphyrin components (Figure 8.4). The phosphine center is still available for coordination to transition metals, providing... 

Another approach is the introduction of some transition metals, such as Cr, V, Fe, Cu, Mn, Co, Ni, Mo, and La into the synthesis mixture of the MCM-41, MMS with a Si/Me ratio of 80 [85], It was then demonstrated that the presence of the transition metal salts in the gel during the hydrothermal synthesis process hinders the action of the template, which results in MCM-41 pores that are not well formed. These materials were then loaded with TiOz, via the solgel method, and the activity of the Ti02/TM-MCM-41 catalysts in the degradation of the 4-chlorophenol reaction was tested in the presence of UV and/or visible light. It was shown that although some metals are deleterious, others can improve the performance of photocatalysts and even enable them to utilize visible light [85],... 

With the above considerations in mind, it is instructive to ask what type of SO2 coordination is to be expected, given the known orbital templates of commonly encountered transition metal fragments. The reader is referred to the extensive work of Hoff-mann and coworkers and the references cited therein for discussions of these fragments and their theoretical basis. As examples of the approach, important valence orbitals for the metal fragments C4v-Cr(CO)s(d ), C2v-PtCl (d ), C2v-Fe(CO)4(d ), and C2V-Ni(CO)2(d ), as presented by Hoffmann, et al., are shown in Fig. 3. The frontier orbitals for these fragments have certain features in common pertinent to SO2 binding. Each... 

The imprinting of polymer supports is an exciting development in the immobilization of transition metal complexes. The process involves the copolymerization of an inorganic or an organic template into a crosslinked polymer network. In a subsequent step, the template is chemically removed leaving an imprint of molecular dimensions in the resin. Ideally, the imprint retains chemical information related to the size and shape of the template. This approach has been used to prepare chiral imprints in otherwise achiral polymer networks. The method is outlined in Scheme... 

However, most of these difficulties should not arise in the case of template-monomer complexes formed with very high association constants. Consequently a number of laboratories, including our own, are currently involved in the design of new polymerisable monomers which can form strong complexes with some of the common structural motifs possessed by many template molecules [5-8], (See also Chapter 4 for further examples.) The use of transition metal co-ordination complexes as templates in imprinting is another particularly promising approach (see Chapter 6 for more detail). 

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