Redox coupling organometallic species

Modern electrochemical methods provide the coordination chemist with a powerful means of studying chemical reactions coupled to electron transfer and exploiting such chemistry in electrosynthesis. In addition, the electrochemical generation of reactive metallo intermediates can provide routes for the activation of otherwise inert molecules, as in the reduction of N2 to ammonia,50 and for electrocatalyzing redox reactions, such as the reduction of C02 to formate and oxalate,51 the oxidation of NH3 to N02-,52 and the technologically important oxidation of water to 02 or its converse, the reduction of 02 to water.53 Electrochemical reactions involving coordination compounds and organometallic species have been extensively reviewed.54-60... 

Anion receptors incorporating cobaltocenium have been studied extensively due to the combination of an accessible redox couple and potential favourable electrostatic interactions of the cationic organometallic metallocene complex with anions. The first anion receptor based on this species was reported by Beer and co-workers in 1989 [6]. The macrocyclic bis-cobaltocenium receptor 1 was shown to bind (via electrostatic interaction) and to electro chemically sense bromide in acetonitrile solvent media. 

A number of other metal-based redox-active centers have been incorporated into supramolecular receptors, representative examples of which are displayed in Fig. 5 (Compounds 24-28). Many of these receptors electro-chemically respond to cations. but species that respond to anions and neutral molecules are also known. A number of the cation binders are organometallic crown ether and metallocrown or metallothiacrown derivatives, for example. Compound 24. Flow-ever, in many cases, the redox processes are not particularly reversible, and relatively small anodic shifts in the metal-centered redox couples are observed. A series of self-assembled [12]metallocrown-3 complexes, two of which are 25 and 26, were found by Severin to bind halide salts of small Group 1 metals strongly in organic solvents, with affinities similar to those of the cryptands. X-ray crystal structures revealed that the metal cation was... 

Ei/2 values are now most generally reported at room temperature (whatever that is) but earlier measurements were often made using the thermodynamically standard temperature of 25 °C. Low temperature values of E1/2 have also been reported for a variety of inorganic and organometallic species. This is generally the case when a coupled chemical reaction is involved and when reversible E1/2 values are needed to characterize a given redox couple in the absence of associated chemistry which might render the overall process irreversible. 

Three important concurrent reactions involving electron transfer may occur in ATRP (Figure 8.22 E is the redox potential of the couple shown in parentheses) (i) disproportionation of the ATRP (usually Cu -based) activator, (ii) oxidation or reduction of organic radicals to carbocations or carbanions, respectively, and (iii) formation of organometallic species via a reaction between radicals and the (usually lower oxidation state) complexes catalyzing the ATRP process. 

Oxovanadium(V) compounds are potential Lewis acids with oxidation ability to induce one-electron oxidation reactions based on their characteristics. Oxidation capability and redox potential are effectively controlled by the substituent of oxovanadium(V) compounds. A catalytic system is allowed to be realized by the redox interaction with molecular oxygen. The oxidative ligand coupling proceeds via the intermetallic interaction between vanadium species and main-group organometallics. 

Figure 7 illustrates the electrochemial redox chemistry in acetronitrile for several coordination complexes of iron [Fe (MeCN)4, Fe CL, and Fe (acac)s (acac = acetylacetonate)] in relation to that for two iron organometallics [Fe (Cp)2 and Fe (CO)s (iron-pentacarbonyl) both stable 18-electron systems]. In MeCN, Fe (MeCN)4" is the only charged species of the group. It is reversibly oxidized (II/III couple E1/2, -I-1.6 V vs SCE). The uncharged Fe Cb molecule is reversibly reduced (Ill/n couple Ei/2, -1-0.2 V vs SCE) to giveFe Cl, which is reduced by an irreversible two-electron process to iron metal (Ep,c -L5 V vs SCE). The more basic Fe (acac)3 molecule is reversibly reduced (ni/n couple Ei/2, -0.7 V vs SCE), but does not exhibit a second reduction peak. The III/II reduction potentials for these three coordination complexes are a measure of their relative electrophilicity (Lewis acidity). 

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