Redox behaviour reductions

A major concern of this review is the tailoring of the redox behaviour of organic compounds, i.e. the optimization of such systems for electron storage and electron hopping. While the emphasis is on reduction and thus on anion formation, it has been shown on many occasions that oxidative cation formation leads to analogous conclusions (Meerholz and Heinze, 1990 Lewis and Singer, 1965). The structure of this text is thus obvious. 

Many model systems which mimic both the redox behaviour [for example, ready reduction to Co(i) species] and the alkyl binding ability of vitamin Bu derivatives have been investigated. The most studied of these has involved bis(dimethylgloximato)cobalt systems of type (307), known as the cobaloximes (Bresciani-Pahor et al., 1985). Other closely related... 

The ability of thioether macrocyclic complexes (and especially those of [9]aneS3) to support multi-redox behaviour at the coordinated metal centre is particularly notable. This allows a series of reversible stepwise one-electron oxidation and/or reduction processes, and stabilization of highly unusual transition metal oxidation states e.g. mononuclear [Pd([9]aneS3)2]2+/3+/4+,149 [Au([9]aneS3)2]+/2+/3+,150 [Ni([9]aneS3)2]2+/3+,151 and [Rh([9]aneS3)2]+/2+/3+.152 It appears to be the ability of the crown thioethers to readily adjust their... 

Interestingly, the isostructural Co(II) dication [Co(L)2]2+ (Ar = C6H4-p-OMe) displays a rather different redox behaviour. In fact, as illustrated in Figure 59, it undergoes either a one-electron oxidation (E2+/3+ = +0.01 V vs. Fc/Fc+), or two separated one-electron reductions ( 2+/+ = -0.93 V E+jq = —1.92 V), the first of which occurs at notably less negative potential values than the above-mentioned ligand-centred processes.11013... 

Complementary to the redox behaviour of the series [Ru3C18(PR3)4], the extra electron present in [Ru3C16(PR3)6] + causes the occurrence of two reversible one-electron oxidations and a reversible one-electron reduction, Table 5.29... 

As illustrated in Figure 35, the same redox behaviour is exhibited by the positively charged blue protein pseudoazurin (FW=13 400 p/ = 8.9), a class of proteins devoted to reduction of NCV to NO in denitrifying bacteria. 

As the HiPIPred and Fdox forms are experimentally indistinguishable in terms of the cluster, it is clear that their redox behaviour must be controlled by the protein, which prevents further oxidation of Fdox or further reduction of HiPIPred. No single iron-sulfur protein is known which... 

A related series of mixed-metal face to face porphyrin dimers (192) has been studied by Collman et al.506 A motivation for obtaining these species has been their potential use as redox catalysts for such reactions as the four-electron reduction of 02 to H20 via H202. It was hoped that the orientation of two cofacial metalloporphyrins in a manner which permits the concerted interaction of both metals with dioxygen may promote the above redox reaction. Such a result was obtained for the Co11 /Co" dimer which is an effective catalyst for the reduction of dioxygen electrochemic-ally.507 However for most of the mixed-metal dimers, including a Con/Mnn species, the second metal was found to be catalytically inert with the redox behaviour of the dimer being similar to that of the monomeric cobalt porphyrin. However the nature of the second metal ion has some influence on the potential at which the cobalt centre is reduced. 

Several studies2206-2210 have been published on the redox behaviour of [Ru(S2CNR2)3], A one electron reversible reduction to [Ru(S2CNR2)3]- is observed but the one electron oxidation is... 

In the solid state, iron complexes can display interesting internal redox behaviour as a result of applying high pressure.54 Reduction of iron(III) to iron(II) on the application of pressure is well documented and is usually monitored by Mossbauer or optical spectroscopy. For instance, conversion to iron(II) in [Fe(acac)3] proceeds continuously with pressure over the range above 30kbar,... 

The reduction of apical nitro substituents to amino groups is a precursor to the synthesis of a large variety of sarcophaginate complexes. The redox behaviour of these nitro substituents is very diverse and complexes with nitro radical anions, nitroso, hydroxylamine and amine substituents were obtained as primary products from such reductions, depending on the t e of reductant, pH, solvent, and the amount of reductant supplied [414]. 

Within the large number of multiredox arrays containing metalloporphyrins/covalently bound (conjugate) fullerene-metalloporphyrin dyads have gained enormous interest in the last ten years, mainly due to their potential application as artificial antennae Due to the multiredox behaviour of the fullerenes (up to six reversible one-electron reductions and at least one reversible one-electron oxidation), the porphyrin ligands and the incorporated metals, the assignment of electron-transfer steps in such systems is difficult. Recently, spectroelectrochemical characterisation has been carried out on a number of fullerene-[(TPP)Co] dyads shown in Scheme 4.3, which exhibit rather complex redox behaviour (Figure 4.19). 

We can see from these equilibria that L-ascorbic acid is a moderately weak acid in the loss of the first hydrogen ion and a very weak acid in the loss of the second. The pA"a for dehydroascorbic acid is only a rough value. The radical, HA, however, is a strong acid, comparable with the mineral acids. Of course all these species have their own redox behaviour values of the reduction potentials which have been determined or estimated for these moieties are shown in equations... 

Besides the use as a model system, copper exchanged zeolites themselves are of technical interest, e.g. as hydrocracking catalyst components [1]. The copper ion redox behaviour is important for cocatalytic effects during catalyst activation (reduction of Fe-zeolites ) and catalyst regeneration. 

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