Electrochemical synthetic methods

Mechanistic studies of homogenous chemical reactions involving formation of (P)Rh(R) from (P)Rh and RX demonstrate a radical pathway(9). These studies were carried out under different experimental conditions from those in the electrosynthesis. Thus, the difference between the proposed mechanism using chemical and electrochemical synthetic methods may be due to differences related to the particular investigated alkyl halides in the two different studies or alternatively to the different reaction conditions between the two sets of experiments. However, it should be noted that the electrochemical method for generating the reactive species is under conditions which allow for a greater selectivity and control of the reaction products. 

The modern state of electrochemical synthetic method [10,201,202,206] allows us to obtain all types of coordination compounds. At the same time, numbers of reported data for different types of complexes are considerably different one from another. 

Electrochemical synthetic methods were also attempted around this time, yet this approach was limited due to solubility issues of the polymer. Electrochemical synthesis of polythiophene was typically performed on metal substrates, the most common being platinum, silver, iron and copper electrodes... 

If polymer materials are given good solubility, these materials can be purified by dissolution and subsequent reprecipitation. Therefore, differences in the synfiietic methods become less important. Although the electrochemical synthetic methods developed by many researchers are applicable to the synthesis of the soluble polythiophenes, chemical synthesis may be more advantageous. This is partly because a higher yield of the polymer materials is expected from chemical synthesis. These approaches were first pursued by Elsenbaumer and co-workers [15]. 

The zinc and cadmium analogs of [Mn(CO)5]2Hg were synthesized by treating MnjlCO) with the corresponding metal [67] (Scheme 12.28). Alternatively, [MnCCO) Ljj M (M = Zn, Cd) could also be achieved by reacting [Mn(CO), L]H with dialkyl metal [68]. An electrochemical synthetic method for [Mn(CO)j]2 M bipy (M = Zn, Cd) was developed by Zhandire [69]. 

Electrochemical synthetic methods are usually easily controllable, selective and simple. The current density or potential of the working electrode can be easily adjusted resulting in a desired rate and/or the desired products formed. Fullerenes, which can undergo as many as six reversible one- electron reductions and one... 

The development of polythiophenes since the early 1980s has been extensive. Processible conducting polymers are available and monomer derivathation has extended the range of electronic and electrochemical properties associated with such materials. Problem areas include the need for improved conductivity by monomer manipulation, involving more extensive research using stmcture—activity relationships, and improved synthetic methods for monomers and polymers alike, which are needed to bring the attractive properties of polythiophenes to fmition on the commercial scale. 

Usually metal-free phthalocyanine (PcH2) can be prepared from phthalonitrile with or without a solvent. Hydrogen-donor solvents such as pentan-l-ol and 2-(dimethylamino)ethanol are most often used for the preparation.113,127 128 To increase the yield of the product, some basic catalyst can be added (e.g., DBU, anhyd NH3). When lithium or sodium alkoxides are used as a base the reaction leads to the respective alkali-metal phthalocyanine, which can easily be converted into the free base by treatment with acid and water.129 The solvent-free preparation is carried out in a melt of the phthalonitrile and the reductive agent hydroquinone at ca. 200 C.130 Besides these and various other conventional chemical synthetic methods, PcH2 can also be prepared electrochemically.79... 

The syntheses and spectroscopic and electrochemical characterization of the rhodium and iridium porphyrin complexes (Por)IVI(R) and (Por)M(R)(L) have been summarized in three review articles.The classical syntheses involve Rh(Por)X with RLi or RMgBr, and [Rh(Por) with RX. In addition, reactions of the rhodium and iridium dimers have led to a wide variety of rhodium a-bonded complexes. For example, Rh(OEP)]2 reacts with benzyl bromide to give benzyl rhodium complexes, and with monosubstituted alkenes and alkynes to give a-alkyl and fT-vinyl products, respectively. More recent synthetic methods are summarized below. Although the development of iridium porphyrin chemistry has lagged behind that of rhodium, there have been few surprises and reactions of [IrfPorih and lr(Por)H parallel those of the rhodium congeners quite closely.Selected structural data for rr-bonded rhodium and iridium porphyrin complexes are collected in Table VI, and several examples are shown in Fig. 7. ... 

According to Ref. [12], template for synthesis of nanomaterials is defined as a central structure within which a network forms in such a way that removal of this template creates a filled cavity with morphological or stereochemical features related to those of the template. The template synthesis was applied for preparation of various nanostructures inside different three-dimensional nanoporous structures. Chemically, these materials are presented by polymers, metals, oxides, carbides and other substances. Synthetic methods include electrochemical deposition, electroless deposition, chemical polymerization, sol-gel deposition and chemical vapor deposition. These works were reviewed in Refs. [12,20]. An essential feature of this... 

Related Polymer Systems and Synthetic Methods. Figure 12A shows a hypothetical synthesis of poly (p-phenylene methide) (PPM) from polybenzyl by redox-induced elimination. In principle, it should be possible to accomplish this experimentally under similar chemical and electrochemical redox conditions as those used here for the related polythiophenes. The electronic properties of PPM have recently been theoretically calculated by Boudreaux et al (16), including bandgap (1.17 eV) bandwidth (0.44 eV) ionization potential (4.2 eV) electron affinity (3.03 eV) oxidation potential (-0.20 vs SCE) reduction potential (-1.37 eV vs SCE). PPM has recently been synthesized and doped to a semiconductor (24). 

Figure 13 shows the irreversible conversion of a nonconjugated poly (p-phenylene pentadienylene) to a lithiun-doped conjugated derivative which has a semiconducting level of conductivity (0.1 to 1.0 S/cm) (29). Obviously, the neutral conjugated derivative of poly (p-phenylene pentadienylene) can then be reversibly generated from the n-type doped material by electrochemical undoping or by p-type compensation. A very similar synthetic method for the conversion of poly(acetylene-co-1,3-butadiene) to polyacetylene has been reported (30), Figure 14. This synthesis of polyacetylene from a nonconjugated precursor polymer containing isolated CH2 units in an otherwise conjugated chain is to be contrasted with the early approach of Marvel et al (6) in which an all-sp3 carbon chain was employed.

The synthesis of metalloporphyrins which contain a metal-carbon a-bond can be accomplished by a number of different methods(l,2). One common synthetic method involves reaction of a Grignardreagent or alkyl(aryl) lithium with (P)MX or (PMX)2 where P is the dianion of a porphyrin macrocycle and X is a halide or pseudohalide. Another common synthetic technique involves reaction of a chemically or electrochemically generated low valent metalloporphyrin with an alkyl or aryl halide. This latter technique is similar to methods described in this paper for electrosynthesis of cobalt and rhodium a-bonded complexes. However, the prevailing mechanisms and the chemical reactions... 

Rhodium Porphyrins. Chemical syntheses of [CPDRh32 and (P)Rh(R) complexes are well known(4-11). Electrochemical techniques have also been used to synthesize dimeric metal-metal bonded [(TPP)RhJ 2 as well as monomeric metal-carbon a-bonded (TPP)Rh(R) and (0EP)Rh(R)(12-16). The electrosynthetic and chemical synthetic methods are both based on formation of a highly reactive monomeric rhodium(II) species, (P)Rh. This chemically or electrochemically generated monomer rapidly dimerizes in the absence of another reagent as shown in Equation 1. 

Cobalt Porphyrins. The primary synthetic method for generating cobalt porphyrins with a metal carbon a-bond is to react a chemically or electrochemically generated cobalt(I) anion, [(P)Co] , with an alkyl or an aryl halide(19-26). [(P)Co] is stable and... 

Indirect electrochemical methods have been intensively studied, especially from the viewpoint of development of innovative synthetic methods in industrial organic chemistry. The indirect procedure is required when the direct method is unsuitable because (1) the desired reaction does not proceed sufficiently because of an extremely slow reaction or a very low current efficiency (2) the electrolysis lacks product-selectivity and thus offers only a low yield (3) tar and products cover the surface of the electrode, interrupting the electrolysis. Indirect electrochemical techniques involve the recycling of mediators (or electron carriers) in a redox system, as depicted in Fig. 1 [1-24]. 

A case in point involves the electrochemical oxidation of vicinal diacids. A standard synthetic method for the preparation of carbon-carbon double bonds occurs by the bis-decarboxylation of such diacids. Even relatively strained, synthetically inaccessible double bonds have been introduced in this way, e.g., eqn 6. 

The above conventional synthetic methods are based on the use of metal salts or carbonyls as complex-formers. At the same time, as far back as at the end of the nineteenth century [504], the possibility of use of compact elemental metals for obtaining complex compounds was shown. This circumstance served as a basis for development of the electrochemical [10,24,201,202,206,505-507], gas-phase [201,202,508-512], and liquid-phase [201,202,513] syntheses of metal complexes using zero-valent metals. All these syntheses are united as direct synthesis of metal complexes [201,202,513]. Much literature is devoted to this area, generalized in a series of reviews [505-507,510-513] and monographs [10,201,202,206,508]. In this respect, only principal aspects of the direct synthesis and the most recent achievements of its application for obtaining various types of coordination and organometallic compounds will be discussed in this section. 

The considerable role of synthetic methods—in particular, direct gas-phase [11,15-18] and electrochemical [11,17,19] techniques—in controlled creation of coordination compounds of various types (molecular and r n-Ti-complexes, chelates, and homo- and heteronuclear compounds) is discussed. Particular attention is given to the complex compounds having standard and nonstandard coordination modes of the most widespread (typical) ligands, for example a(N)- and ji(ri6-C)-complexes... 

In this respect, this review provides a comprehensive survey of synthetic methods and physicochemical properties of the porous carbon materials. Furthermore, as electrochemical applications of the porous carbons to electrode materials for supercapacitor, the effects of geometric heterogeneity and surface inhomogeneity on ion penetration into the pores during double-layer charging/ discharging are discussed in detail by using ac-impedance spectroscopy, current transient technique, and cyclic voltammetry. 

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