2,2 -Bipyridinium salts oxidation

Organic Molecules It can be seen from our earlier discussion that the presence of a transition metal ion is not always required for an electrochromic effect. Indeed, many organic molecules can yield colored products as a result of reversible reduction or oxidation. 4,4 -Bipyridinium salts are the best known example of such compounds. These compounds can be prepared, stored, and purchased in colorless dicationic form (bipm +). One electron reduction of the dication leads to the intensely colored radical cation (bipm+ ). Such radical cations exist in equilibrium with their dimers (bipm ). In the case of methyl viologen, the radical cation is blue and the dimer is red. By varying the substient group in the molecule, different colors can be obtained. 

Some bipyridinium salts are remarkable herbicides. They rapidly desiccate all green plant tissue with which they come into contact, and they are inactivated by adsorption on to clay minerals in the soil. This potent herbicidal activity is found only in quaternary salts, e.g. diquat (254) and paraquat (255), with redox potentials for the first reduction step between -300 and -500 mV (equations 158 and 159) (B-80MI20504). The first reduction step, which is involved in herbicidal activity, involves a completely reversible, pH independent, one-electron transfer to yield the resonance stabilized radicals (256) and (257). The second reduction step, (256 -> 258) and (257 -> 259), is pH dependent and the p-quinoid species formed are good reducing agents that may readily be oxidized to diquatemary salts. 

It was found, that also Ru and Os colloids can act as catalysts for the photoreduction of carbon dioxide to methane [94, 95]. [Ru(bpy)3]2+ plays a role of a photosensitizer, triethanolamine (TEOA) works as an electron donor, while bipyridinium electron relays (R2+) mediate the electron transfer process. The production of hydrogen, methane, and small amounts of ethylene may be observed in such a system (Figure 21.1). Excited [Ru(bpy)3]2+ is oxidized by bipyridinium salts, whereas formed [Ru(bpy)3]3+ is reduced back to [Ru(bpy)3]2+ by TEOA. The reduced bipyridinium salt R + reduces hydrogen and C02 in the presence of metal colloids. Recombination of surface-bound H atoms competes with a multi-electron C02 reduction. More selective reduction of C02 to CH4, ethylene, and ethane was obtained using ruthenium(II)-trisbipyrazine, [Ru(bpz)3]2+/TEOA/Ru colloid system. The elimination of hydrogen evolution is thought to be caused by a kinetic barrier towards H2 evolution in the presence of [Ru(bpz)3]2+ and noble metal catalysts [96]. 

Aromatic hydrocarbons are called 7r bases because of their rather low ionization potentials. On the other hand, aromatic compounds containing a nitrogen atom possess lower MO energy than the parent hydrocarbons, and they show less electron-donating character or electron-accepting character. The mode of action of the herbicide paraquat 300 (1,1 -dimethyl-4,4 -bipyridinium salt) is thought to be connected with its reversible reduction-oxidation reactivity. The compounds with a lower level of activity possess higher reduction potentials, and their one-electron transfer is not quite quantitatively reversed. 

Other materials being investigated include ferrocene with a bipyridinium salt,234 niobium oxide,235 nickel oxo-hydroxide,236 and cobalt oxohydroxide.237 The last is pale yellow in the reduced state and dark gray in the oxidized state. A typical electrolyte is lithium perchlorate in propylene carbonate. Solid electrolytes, such as a lithium salt (perchlorate, tetrafluoroborate, or triflate), in a polyepoxide238 or in a polyvinyl chloride gel in ethylene carbonate-propylene carbonate,239 lithium iodide in polyvinyl bu-tyral,240 and Naflon H (a polymeric perfluorocarbon-sulfonic acid),241 have also been tested. Some other systems use suspended particles between two panes of glass.242 When the particles are aligned by an electric field, the window becomes transparent. Combination photo-voltaic-electrochromic devices are under study.243... 

Conductive polymer films on electrodes have been prepared by electrochemical polymerization of electroactive monomers such as a pyrrole-substituted mediator, or by evaporating solutions containing preformed polymer. Examples of electrocatalyses reported include the oxidation of alcohols by pyrrole-substituted 2,2,5,5-tetramethyl-3-pyrroline-l-oxyl [26] and organohalide de-halogenation by pyrrole-substituted 4,4 -bipyridinium salt [27]. The preparation of mediator-modified electrode by evaporating solutions of preformed polymers was carried out by dip-coating polymers including mediators on electrode surface or by covalent attachment of mediators to dip-coated polymers on electrode surfaces. Examples of the former electrocatalyses are selected from the several reports on the oxidation of NADH by dopamine... 

Oxidative quenching by electron acceptors such as bipyridinium salts (also known as viologens), polypyridine complexes of Rh(III) and Co(IIl) and cage complexes have been widely studied [38-40] ... 

A number of electron acceptors have been identified for reaction (40) that can effectively mediate H2 evolution from water in the presence of Pt-group based redox catalysts. Typical examples are bipyridinium salts such as methyl viologen (MV " "), polypyridine complexes of Rh(III), Co(II) and cage complexes of Co(II). For this reason, the oxidative quenching of various transition metal polypyridine complexes, M(LL)3]" by these electron acceptors have been extensively studied ... 

Typical electrochromic materials are bipyridinium salts (viologens). Dyes like viologens often consist of differenf chemical species in an electrolyte which are either oxidized or reduced at the electrodes to form radical cations and anions that recombine charges after diffusion (Figure 10.38 and Figure 10.39). In such systems a small but significant current will be necessary to maintain the colored state. Electrochromism with viologens as electrochromics and liquid electrolytes may be combined with FEDOT as a transparent electrode material. ... 

Ammonium salts with two different alkyl chains were prepared directly via subsequent alkylations of dimethylamine with primary bromides and crystallization. Commercial hexadecyl-methylamine can be conveniently applied in the same way in order to convey functionality to cationic synkinons. A recent example describes subsequent alkylations with a small functional and a long-chain primary bromide (Scheme 2.4). A-acylated / -phenylenediamine was also alkylated at the second nitrogen atom which had two different alkyl chains, with or without extra functionality . After deacylation, this head group can be diazotized or coupled oxidatively with various heterocycles in water (Scheme 2.4). Photoactive and coloured membrane surfaces are thus obtained. Phenylene-diamine, pyridine and in particular A-methyl-4,4-bipyridinium chloride are relatively weak nucleophiles. Substitution of bromides is slow and the more reactive iodides can rarely be obtained commercially, but the selection of nitromethanes as solvent for bromide substitution is of great help as well as the addition of sodium iodide to enforce a Finkelstein reaction or a combination of both. 

OTHER COMMENTS Paraquat is a cation (l,l-dimethyl-4,4 -bipyridinium), CAS No. 4685-14-7 commercial product is the dichloride salt of paraquat, CAS No. 1910-42-5 a dimethyl sulfate salt, a yellow solid, CAS No. 2074-50-2, has also been commercialized has been used as an agricultural herbicide, desiccant, and defoliation agent during pre-harvest used to control aquatic weeds dichloride has been utilized as a biological oxidation-reduction indicator use has been restricted. 

Other studies utilizing (nanosecond) time-resolved spectroscopy have been carried out on ion pairs, such as those based on donors consisting of Zn, Cd, and Hg bis-dithiolene anions and bipyridinium dications (viologens) [192-194] as acceptors (e.g., methylviologen with Ered° = - 0-41 V). These salts give rise to the characteristic bands of the reduced viologen (MV +, max = 391 and 608 nm) and that of the oxidized dithiolene complex (Xmax = 450 nm), e.g. 

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