Templated synthesis, thermodynamically

Eurlan RLE, Otto S, Sanders JKM. Supramolecular templating in thermodynamically controlled synthesis. Ptoc Natl Acad Sci USA 2002 99 4801-4804. 

The subject matter of this chapter will be subdivided into sections concerning template synthesis of the complexes structural and thermodynamic properties of the complexes with synthetic cyclic polyamines complexes with mixed-donor macrocycles reactivity of the complexes cryptates and complexes with phthalocyanines and porphyrins. 

In contrast, the thermodynamic template effect in macrocycle synthesis is a process by which the presence of a metal ion template stabilises thermodynamically, or removes (e.g. by precipitation) one particular (usually cyclic) product from an equilibrating mixture, driving the equilibrium towards this thermodynamic minimum. This leads us to the conclusion that any thermodynamically stabilising influence may drive an equilibrium mixture towards a particular product according to Le Chatalier s Principle (in an equilibrating situation, the system will react to diminish the effects of externally applied changes in conditions). 

At the same time, this method has a series of disadvantages. Among them, we note the possibiity of contamination of the final product not only by the excess of one of the reactants [2], but also by complexes of the components of the ligand system. So, to carry out strictly template synthesis experiments, it is necessary to take into account a comparative stability of coordination compounds, obtained on the basis of initial components-precursors and the ligand itself. Not only the thermodynamic characteristics of complex-formation processes should be taken into consideration [326,327], but also the influence of solvolysis processes (especially hydrolysis) and the type of atmosphere (air oxygen). 

The kinetics and mechanism of synthesis and decomposition of macrocyclic compounds are regarded as one of the most important aspects in the chemistry of these compounds. The majority of papers concern metal ions complexing with preliminarily synthesized macrocyclic ligands and metal ion substitutions by other metal ions in the preliminarily prepared complexes. Template synthesis, the most promising approach to the directed preparation of macrocyclic compounds with desired structures [17], plays a still more decisive role in the chemistry of macrobicyclic complexes with encapsulated metal ion. However, the literature contains only scarce data on the kinetics and the mechanism of the template synthesis of macrocyclic compounds because of the difficulties encountered in experimental determinations of kinetic and thermodynamic parameters, such as low product yields, nonaqueous media, high temperatures, and side reactions. 

The precatenate intermediate can be regarded as a threaded complex, which is thermodynamically stabilized by coordination bonds, from this simple consideration, a transition metal-templated synthesis of rotaxanes was devised, the principle of which is shown in Figure 28 [117]. The threading step (i) is a complexation reaction. 

Scheme 12 Template synthesis of the first-formed bis (P-N-NH ) complex and thermodynamic P-N-N-P product complex...

The most general definition of a template is as a structure-directing agent. In surfactant solutions the final templated polymers can be either discrete nanoparticles or mesostructured bulk materials as a consequence of polymerization, respectively, in the non-continuous or continuous domains of the template. Thermodynamically stable media, such as microemulsions, equilibrium vesicles, or lyotropic mesophases are especially useful as templates because of their structural definition and reproducible morphologies. The mesostructure of a thermodynamically stable template is defined by composition and temperature, but this same feature makes the structure unstable to changes in temperature, pH, or concentration. The aim of template synthesis is to transfer the self-organized template structure into a mechanically and chemically stable, durable, and processable material. 

Figure 3-9 Metal-templated synthesis of the [2]catenane 36 under thermodynamic control.

Although this concept was only recently articulated as a general approach to the synthesis of chemical species exhibiting molecular recognition capabilities, many of the principles and practices that characterize DCC had been in place for several decades. DCC may be viewed as the intersection of two pre-edsting approaches to synthesis thermodynamically controlled templated synthesis and combinatorial chemistry. 

Scheme 1.9 [3]Catenane 20 has been generated using thermodynamically controlled templated synthesis [37].

Thermodynamic control (Figure 1, right) is based on adsorption of substances until quasi-equilibrium stage. In this case, the surface ratio of the adsorbed species is defined by the ratio of products of their concentration and binding constant. This deposition is much less influenced by poorly controllable fluctuations of external conditions and provides much better reproducibility. The total coverage can be almost 100%. Because of these reasons, the thermodynamic control is advantageous for preparation of mixed nanostructured monolayers for electrochemical applications including a formation of spreader-bar structures for their application as molecular templates for synthesis of nanoparticles. 

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