Electrochemical reduction transition metal ions

The 14-membered macrocycle 1,4,8,11-tetraazacyclotetradecane (cyclam or [14]aneN4), unlike cyclen, is capable of encircling most transition metal ions and in the case of Co111 the trans configuration is much preferred by comparison with the folded cis isomer. Electrochemical reduction of A,v-[Co(cyclam)(OI I)2]+ in 3M NaOH leads to rapid isomerization to the trans form, and the relative stabilities of the trans and cis isomers of the di- and trivalent complexes were determined from a thermodynamic cycle.702 This preference for trans orientation of the non-macrocyclic donors has enabled the isolation and investigation of many Co complexes without the complications of isomerization. Some novel examples include /r[Pg.61]

There has been considerable debate about whether intrazeolitic species are elec-trochemically accessible and recent results on zeolite-encapsulated metal complexes are of interest in this regard. Before we discuss this topic, the role of ion exchange in electrochemical response is discussed. Electroactive transition metal ions can readily be ion exchanged into zeolites. Baker and co-workers reported that a Co +-zeolite Y-ZME resulted in reduction of cobalt ions if Li" was used as the supporting cation in the electrolyte solution, whereas the voltammetric response was absent in the presence of the larger tetrabutylammonium ion (TBA) [166]. Figure 32 compares the voltammetric data for Li+ and TBA. There are two ways to interpret these data ... 

The electrochemical potential for redox reaction controls the situation where atoms of one element are available to be sorbed by a zeolite containing exchangeable cations of another element. Within the zeolite and even in the absence of water, aqueous reduction potentials are usually capable of deciding whether reaction will occur, with an error due to the difference between the zeolitic environment and aqueous solution of no more than 0.1 (or perhaps 0.2) V. Accordingly there is no question that alkali-metal vapors will reduce transition-metal ions within a zeolite, and that vapors of zinc, mercury, or sulfur will not reduce the cations of the alkali or alkaline-earth metals. 

The main aim is to acquire knowledge of the nature, structure and concentrations of the electroactive species present in the molten electrolytes, at the working temperatures, during electrochemical reduction of refractory m al and other transition metal ions in molten alkali chloride mixtures. 

Conversion reactions may circumvent these limitations and calculated theoretical capacities are above those of lithium cobalt oxide, lithium iron phosphate, or graphite by a factor of 5 to 10. This is due to a complete electrochemical reduction with the transfer of several electrons per 3d transition metal ion t3qjically leading to metal nanoparticles which are embedded in a host matrbc of the lithium compound. 

The simplest electrochemical reactions, which can be foxmd among the different kinds of electrode processes, are those where electrons are exchanged across the interface by flipping oxidation states of transition metal ions in the electrolyte adjacent to the electrode surface (Bamford Compton, 1986), i.e. an ET (electron transfer) mechanism. The electrode acts as the source or sink of electrons for the redox reaction and is supposed to be inert. The reduction of ferricyanide to ferrocyanide (Angell Dickinson, 1972 Bamford Compton, 1986 Bruce et al., 1994 Iwasita et al., 1983) is an example of such a mechanism ... 

Polynuclear transition metal cyanides such as the well-known Prussian blue and its analogues with osmium and ruthenium have been intensely studied Prussian blue films on electrodes are formed as microcrystalline materials by the electrochemical reduction of FeFe(CN)g in aqueous solutionThey show two reversible redox reactions, and due to the intense color of the single oxidation states, they appear to be candidates for electrochromic displays Ion exchange properties in the reduced state are limited to certain ions having similar ionic radii. Thus, the reversible... 

Twenty years ago the main applications of electrochemistry were trace-metal analysis (polarography and anodic stripping voltammetry) and selective-ion assay (pH, pNa, pK via potentiometry). A secondary focus was the use of voltammetry to characterize transition-metal coordination complexes (metal-ligand stoichiometry, stability constants, and oxidation-reduction thermodynamics). With the commercial development of (1) low-cost, reliable poten-tiostats (2) pure, inert glassy-carbon electrodes and (3) ultrapure, dry aptotic solvents, molecular characterization via electrochemical methodologies has become accessible to nonspecialists (analogous to carbon-13 NMR and GC/MS). 

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