Electrochemical template production

The electrochemical template synthesis of MPc is a complicated multistep process. This is confirmed by the isolation and mass-spectrometric identification of a series of intermediate products in NiPc synthesis, which could be formed according to Scheme 2-36. 

There are many ingenious and successful routes now developed for nanocry stalline syntliesis some rely on gas phase reactions followed by product dispersal into solvents [7, 9,13,14 and 15]. Otliers are adaptations of classic colloidal syntlieses [16,17,18 and 19]. Electrochemical and related template metliods can also be used to fomi nanostmctures, especially tliose witli anisotropic shapes [20, 21, 22 and 23]. Ratlier tlian outline all of tlie available metliods, this section will focus on two different techniques of nanocrystal syntliesis which together demonstrate tlie general strategies. 

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. 

Porous aluminum oxide can be used as a template for the production of nanowires and nanotubes. For example, metals can be deposited on the pore walls by the following procedures deposition from the gas phase, precipitation from solution by electrochemical reduction or with chemical reducing agents, or by pyrolysis of substances that have previously been introduced into the pores. Wires are obtained when the pore diameters are 25 nm, and tubes from larger pores the walls of the tubes can be as thin as 3 nm. For example, nanowires and nanotubes of nickel, cobalt, copper or silver can be made by electrochemical deposition. Finally, the aluminum oxide template can be removed by dissolution with a base. 

Template reactions between malonaldehydes and diamines in the presence of copper(II), nickel(II) or cobalt(II) salts yield neutral macrocyclic complexes (equation 15).99-102 Both aliphatic102 and aromatic101 diamines can be used. In certain cases, non-macrocyclic intermediates can be isolated and subsequently converted into unsymmetrical macrocyclic complexes by reaction with a different diamine (Scheme ll).101 These methods are more versatile and more convenient than an earlier template reaction in which propynal replaces the malonaldehyde (equation 16).103 This latter method can also be used for the non-template synthesis of the macrocyclic ligand in relatively poor yield. A further variation on this reaction type allows the use of an enol ether (vinylogous ester), which provides more flexibility with respect to substituents (equation 17).104 The approach illustrated in equation (15), and Scheme 11 can be extended to include reactions of (3-diketones. The benzodiazepines, which result from reaction between 1,2-diaminobenzenes and (3-diketones, can also serve as precursors in the metal template reaction (Scheme 12).101 105 106 The macrocyclic complex product (46) in this sequence, being unsubstituted on the meso carbon atom, has been shown to undergo an electrochemical oxidative dimerization (equation 18).107... 

The direct template electrosynthesis of some metal chelates has been developed [615,616]. The electrochemical reactions were carried out in mild conditions that allowed isolation of not only the expected final products, but also complexes of initial ligand systems. In particular, the template reaction (3.255) takes place according to the following scheme [615] ... 

For copper and cadmium, two products 816 and 817 only were formed the first directly in the electrochemical cell and the second isolated after evaporation of the liquor to 50% of the initial volume. Template electrosynthesis with use of zinc led to the formation of the chelate 804, which was precipitated as in the electrochemical cell from partially evaporated and cooled mother liquor. 

Electrochemical reduction of Ni(taab)2+ [taab = (20), see p. 231] occurs in two one-electron steps, to complexes formulated as [Nim(taab)] + and [Niu(taab)]°. The relationship between the annulene taab and the two-electron reduction product, the porphyrin-like taab2-, is discussed.102 The preparation of macrocycles of type (87) by a template synthesis requires a minimum ring size of x = y = 3 and depends upon the strong complexing of the metal ion at the pH of the reaction, otherwise the metal hydroxide precipitates. The NiL(C104)2,nH20 (n = 0, 1, or 2) species have been prepared for x = 3, y = 4.479... 

Some simple biphenols equipped with methyl groups, e.g., 3,3, 5,5 -tetramethyl-2,2 -biphenol 38, have attracted attention as important components of highly potent ligand systems [75-86]. Remarkably, the sustainable synthesis of such biphenols is rather challenging despite their simple scaffolds. In particular, methyl-substituted phenols are prone to side reactions. This is especially the case when 2,4-dimethyl-phenol (37) is oxidatively treated. Upon anodic conversion 37 is preferably transformed into polycyclic architectures [87]. Direct electrolysis in basic media provided only traces of the desired biphenol 38 and the dominating components of the product mixture consisted of Pu in meter s ketone 39 and the consecutive pentacyclic spiro derivative 40 [88]. For an efficient electrochemical access to 3,3, 5,5 -tetramethyl-2,2,-biphenol (38) we developed a boron-based template strategy [89, 90]. This methods requires a multi-step protocol but can be conducted on a multi-kilogram scale (Scheme 17). 

The most commonly used hard templates are anodic aluminum oxide (AAO) and track-etched polycarbonate membranes, both of which are porous structured and commercially available. The pore size and thickness of the membranes can be well controlled, which then determine the dimension of the products templated by them. The pores in the AAO films prepared electrochemically from aluminum metals form a regular hexagonal array, with diameters of 200 nm commercially available. Smaller pore diameters down to 5 nm have also been reported (Martin 1995). Without external influences, capillary force is the main driving force for the Ti-precursor species to enter the pores of the templates. When the pore size is very small, electrochemical techniques have been employed to enhance the mass transfer into the nanopores (Limmer et al. 2002). 

Ruvl(0)2(L)]2 + 2649d being reported. The chemical and electrochemical interconversion of these products has been achieved, and the oxidative activity of Ruv analogues assessed.26496 The template synthesis of [Ru(L)(OH2)] (C104)2 (L = 464) has been reported although the exact nature of this product is unknown.26511 Reaction of [Ru2(OAc)4Cl] with LH2 (LH2 = 465) in ethanol yields the... 

Figure 1.11 (a) Stepwise production of metallic Chemical Society.) (b) Optical properties of gold nanotubules, wherethe internal surface area was nanotubes in alumina membranes at various modified with (2-cyanoethyl)triethoxysilane, sizes and lengths. (From [35] C.R. Martin, followed by electrochemical deposition and Science, 266, (1994), 1961—1966. Reprinted with removal of the alumina template. (Reprinted permission from AAAS.) with permission from Ref. [50], 1991 American... 

The electrochemical deposition is carried out through the pores on the metallic layer that coated the back side of the template and is used as a cathode for the electroplating. The micro- and nanomaterials that are produced in this way take the form of wires and tubules. After finishing of the deposition process, the polymer membrane is dissolved in the corresponding solvent, for example, in alkali solution at temperature 70°C-80°C for PET film or in dichloromethane for the PC membrane. The scheme of the ion-track membrane production in thin polymer film and electroplating of copper nanowires within the pores is given in Figure 18.1. 

Electrochemical deposition is ideal for the production of thin supported layers for applications such as photonic mirrors, because the surface of the electrochemically deposited film can be very uniform. Electrochemical deposition occurs from the electrode surface out through the overlying template, the first layer of templated material, deposited out to a thickness comparable... 

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