Redox property

The redox chemistry of macrocyclic ligand complexes has received much attention. There are several reasons for this. 

In this chapter, for convenience of discussion, the redox behaviour of cyclic systems is divided into two categories. Initially, those reactions for which there is no marked alteration of the unsaturation pattern of the ligand are discussed. Subsequently, systems in which the final product involves an alteration in the ligand s unsaturation are treated. However, it is emphasized that a continuum exists between redox behaviour which is completely metal-centred and that which is solely ligand-based. 

The capacity of cyclic ligands to stabilize less-common oxidation states of a coordinated metal ion has been well-documented. For example, both the high-spin and low-spin Ni(n) complexes of cyclam are oxidized more readily to Ni(m) species than are corresponding open-chain complexes. Chemical, electrochemical, pulse radiolysis and flash photolysis techniques have all been used to effect redox changes in particular complexes (Haines McAuley, 1982) however the major emphasis has been given to electrochemical studies. 

It needs to be pointed out that E values may also be quite sensitive to the nature of the solvent and supporting electrolyte used for an electrochemical study. Apart from solvation effects of the non-specific type, solvent molecules may occupy coordination sites in either the starting complex or the products and hence influence redox behaviour (Fabbrizzi, 1985). Similarly, the nature of the anion present may also strongly influence the redox potential if it has ligating properties (Zeigerson etal., 1982). Because of such effects, caution needs to be exercised in attempting to compare electrochemical data which have not been obtained under similar conditions. 

In the promotion of less-common oxidation states, much attention has been focused on the redox behaviour of transition metal ions such as nickel and copper although many other metal types have also had unusual oxidation states stabilized by macrocyclic ligands. However, within the limitations of a single chapter it is not possible to attempt a wide ranging 

There is, at present, very little redox information available for APX, which is as much a reflection on the difficulties associated with electrochemical measurements on complex metalloproteins as a whole (Armstrong et al., 1997 Armstrong et al., 1993) as on any special difficulty with peroxidases (or APX) itself. Hence, detailed redox information for 

FIGURE 6. Thin-layer spectra (sodium phosphate, pH 7.0, 25.0 C, I = O.IOM) and corresponding Nemst plot of soybean APX at various applied potentials, E,pp(mV vs SHE). The fully oxidized (O) and fully reduced (R) spectra are indicated. For clarity, the visible region has been expanded. Reprinted with permission from Jones et al., 1998. 

Although one eleetron reduetion potentials for the Compound 1/ Compound II and Compound Il/ferrie eouples for A. ramosus (915 and 982mV respeetively, pH 7.0), (Farhangrazi et al., 1994)) and HRP (878 and 869mV respeetively (Farhangrazi et al., 1995), 879 and 903 respeetively (He et al, 1996), and 880 and 900 respeetively (Hayashi and Yamazaki, 1979) (all pH 7.0)) have been reported, these are not yet available for APX. 

Should one wish to pursue it and given favorable conditions, one can, with varying amounts of success, inactivate oneis enzyme, although the mechanistic details are not clear in all cases. 

Such simple HOMO-LUMO arguments, however, do not seem applicable to even slightly larger metal clusters. The redox potentials of a series of tetracobalt carbonyl clusters with different ligands were found to correlate well with the total 

2 Electronic Structures of Metal Clusters and Cluster Compounds 

All the vinylogous systems 22-32 exhibit the same pattern of redox properties. The position of the potentials Ei and E2, or better E = (Ei + E2 /2 of a certain vinyl-ogue is mainly determined by the heterocyclic end groups (cf. Table 8). 

Since aromaticity is lost on reduction, quinoline derivatives 24qx and 25qx are reduced more easily than the pyridine systems 22qx and 23ox (compare also 5qx and 6ox)- Only in 31 red tlie situation is reversed. On oxidation it loses aromaticity and therefore is most difficult to oxidize (potentials rather positive). 

32 in which aromaticity is neither lost nor gained during the redox process serves as a neutral example. 

With increasing numbers of vinylene groups partly the positions of E, and partly those of E2 are more influenced. The reason for this phenomenon is not clear. Different solvation energies may be involved. 

The most important result in Table 8, however, concerns Ksem The longer the vinylene chain the smaller sem becomes. The highest Ksem in each series is always connected with the first member, provided that it is planar. These vinylogous series confirm that the much too low Ksem s of (n = 0) systems, 22 (=2), 24, 27 =20) and 28 =21) are due to lack of planarity. Therefore they have to be substituted by the planar derivatives, e.g. 2B2,20B and 2iB. 

In self-doped polyaniline, sulfonic groups induce changes in the geometry of the polyaniline backbone [152], affecting the physicochemical properties of the polymer. Comparative electronic absorption 

In table 1, the one-electron reduction potentieils of some mono- and multisubstituted benzene radical cations measured by pulse radiolysis (or estimated) in aqueous solution are given. The one-electron reduction potentials of monosubstituted benzene radical cations range from 

The intercept of equation (7), 2.24 0.04 V vs. NHE, corresponds to the one-electron reduction potential of the unsubstituted benzene radical cation. Due to its very short life-time in solution, the thermodynamical one-electron reduction potential for this radical cation has not been possible to measure directly by pulse radiolysis or any other technique. The values given for the unsubstituted benzene radical cation in table 1 are thermochemical estimates giving the upper and lower linuts. Recently, the limiting values were confirmed by Mohan and Mittal based on a pulse radiolysis study. [18] 

Equation (7) also constitutes a basis for further empirical modeling of substituent effects on benzene radical cations since it gives the intercepts for the linear relationships between one-electron reduction potentials of 1,4-substituted benzene radical cations and of the 4-substituent given a specific 1-substituent. The 1-substituent defines the family, e.g., phenol, anisole, aniline, etc. Since the 1,4-substituents can belong to either the 1-substituent family or the 4-substituent family, the general empirical relationship must be symmetrical, i.e., the resulting one-electron reduction potential must be the same regardless of choice of family. 

One-electron Reduction Potentials of Some Substituted Benzene Radical Cations 

The linear relationship for an arbitrary family is then given by the equation (8) 

Spectrophotomeric study of the voltammetric oxidation of [Ptj (pop)4] in aqueous phosphate buffer solution in the presence of an excessive amount of various halide anions (X = Cl , Br , or I ) by use of the OTTLE cell technique indicated the quantitative formation of [Pt2(pop)4X2] with expected isosbestic points (228). The intermediate mixed-valence state was not detected. Cyclic voltammetric study employing similar conditions revealed that the oxidation potential depends significantly on the kind of coexisting halide ions. It was suggested that a small amount of [Pt2(pop)4X] in equilibrium with [Pt2(pop)4] in the vicinity of the electrode undergoes oxidation. 

The reduction potentials of complexes [Pt2(pyt)4X2l depend on the kind of axial ligand X-(X- = Cr, -0.44 V Br-, -0.32 V SCN-, -0.24 V I , - 0.23 V). However, the reoxidation potentials of the reduced species are constant (+ 0.28 V) irrespective of the kind of coexisting anions and are the same as the oxidation potential of [Pt2(pyt)4l. Addition of methanol to the electrolized DMF solution revealed that the solvent-coordinated species is formed first and then the coordinated solvent is rapidly substituted by X . The redox processes are schematized as follows. 

The redox processes became quasi-reversible in the presence of Cl or Br , with E 2 values of +0.129 and +0.150 V, respectively (Fig. 23). From a plot of [Pd2(pyt)4]/[X ] against oxidation current, it was concluded that one X per complex is involved in the redox process. However, the redox process is likely to be a two-electron one on the basis of the A p value. 

In contrast to [Pd2(pyt)4l, [Pd2(form)4] exhibits two reversible one-electron oxidation processes in dichloromethane ( [,2 = +0.81 and + 1.19 V vs Ag/AgCl ( , 2(Fc °) = +0.49 V)) (Fig. 24) 78, 79). In this case, the oxidation occurs at the ligand site at least in the first oxidation step. However, [Ni2(form)4] exhibits a reversible oxidation and an irreversible oxidation process in dichloromethane at ,2 = +0.73 V and pa = +1. 25 V, respectively. It is worth noting that the form complexes do not require axial ligands in their oxidation processes. 

In general the extreme sensitivity of the whole class of cobalt(II) amine complexes toward dioxygen underscores their reducing character. A quantitative measure of this property is the reduction potential  

Owing to the very facile substitution of the amine ligands in the re- 

The published Ev2 values for the complexes in question are shown in Table 2. 

If [Colenla] is taken as a reference complex for the tris-bidentate complexes, one observes that the unstrained complexes with five-membered rings have rather similar reduction potentials. The complexes of [Colpnlal , in acetone at least, become progressively easier to reduce with increasing ob content. The reduction potential of [Co(t-menlal is 450 mV more positive than [Co(en)3], indicating the destabilizing effect that axially oriented substituents have on the Co(III) complex. 

Complexes of sexadentate ligands display a wide range of reduction potentials. The lowest reduction potentials reported for any cobalt(III) hexaamine are -0.61 V for the diamcyclam complex and -0.62 V for/be 063 [Co(diAmpnsar) Both of these complexes are expected to have unusually short Co-N bond lengths this has been confirmed for the diamcyclam complex 149). 

Semiquinone stability in solution is defined by the equilibrium constant Kf) of the reverse of the semiquinone dismutation reaction 

In a hydrophobic milieu such as the inner membrane has been estimated to be about 10 [246], which makes SQ of the membranous pool undetectable by EPR. 

The functionally relevant redox potentials of the semiquinone couples are obviously those of quinone-enzyme complexes, not of free quinone molecules. For instance, the effective E of the QHj/Q couple bound to the enzyme is shifted from the corresponding value of free couple by an amount that is determined by the relative affinities of binding of quinone and quinol to the binding-site. The effect of binding on the E is given by the equation (see Ref. 247) 

and are the dissociation constants of quinol, quinone and semiquinone, the and E, 2(B) l e bound one-electron couples may be derived, as 

The stability of semiquinone is an equilibrium property of the reversed dismu-tation reaction (Eqn. 2). It is stressed that the above treatment refers to equilibrium conditions. Tight binding to a proteinaceous site could mean that a (semi)quinone molecule would not dissociate out of it before some catalytically important event has taken place. Kinetically determined midpoint potentials may in such cases be functionally more relevant, and could be considerably distorted from the equilibrium values. Lack of equiUbration of protons between the proteinaceous site and the bulk 

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Suzuki T, Maruyama Y, Akasaka T, Ando W, Kobayashi K and Nagase S 1994 Redox properties of organofullerenes J. Am. Chem. Soc. 116 1359-63... 

The redox properties have already been considered. A number of reactions of soluble (alkali metal) sulphites are noteworthy ... 

Color and redox properties of a dye are related to the two mo-lecular orbitals of i v and They are both sensitive to the nature and position of the substituent. A perturbation of both orbitals occurs generally, but in some cases the effect is much more important for one of them. 

Enclosure also changes the redox properties of a compound, its color, and other physical properties (1,2). On this basis nonlinear optical materials, luminescence markers, controlled light switches, and other high-tech devices might be designed and prepared (15,17,137). 

PMo220 4q, is analytically usehil, being formed in the molybdenum test for phosphate ion. Poly- and heteropolymolybdate ions are used in the precipitation of dyes. The protonated forms of the ions are strongly acidic and many poly- and heteropolymolybdate compounds have catalytic activity that is attributable to their acid—base or redox properties. 

The quiaones have excellent redox properties and are thus important oxidants ia laboratory and biological synthons. The presence of an extensive array of conjugated systems, especially the a,P-unsaturated ketone arrangement, allows the quiaones to participate ia a variety of reactioas. Characteristics of quiaoae reactioas iaclude nucleophilic substitutioa electrophilic, radical, and cycloaddition reactions photochemistry and normal and unusual carbonyl chemistry. 

A closer analysis of die equilibrium products of the 1 1 mixture of methane and steam shows the presence of hydrocarbons as minor constituents. Experimental results for die coupling reaction show that the yield of hydrocarbons is dependent on the redox properties of the oxide catalyst, and the oxygen potential of the gas phase, as well as die temperamre and total pressure. In any substantial oxygen mole fraction in the gas, the predominant reaction is the formation of CO and the coupling reaction is a minor one. 

Computer simulations of electron transfer proteins often entail a variety of calculation techniques electronic structure calculations, molecular mechanics, and electrostatic calculations. In this section, general considerations for calculations of metalloproteins are outlined in subsequent sections, details for studying specific redox properties are given. Quantum chemistry electronic structure calculations of the redox site are important in the calculation of the energetics of the redox site and in obtaining parameters and are discussed in Sections III.A and III.B. Both molecular mechanics and electrostatic calculations of the protein are important in understanding the outer shell energetics and are discussed in Section III.C, with a focus on molecular mechanics. 

The treatment of electrostatics and dielectric effects in molecular mechanics calculations necessary for redox property calculations can be divided into two issues electronic polarization contributions to the dielectric response and reorientational polarization contributions to the dielectric response. Without reorientation, the electronic polarization contribution to e is 2 for the types of atoms found in biological systems. The reorientational contribution is due to the reorientation of polar groups by charges. In the protein, the reorientation is restricted by the bonding between the polar groups, whereas in water the reorientation is enhanced owing to cooperative effects of the freely rotating solvent molecules. 

The electrochemistry of S-N and Se-N heterocycles has been reviewed comprehensively. The emphasis is on the information that electrochemical studies provide about the redox properties of potential neutral conductors. To be useful as a molecular conductor the 4-1, 0, and -1 redox states should be accessible and the neutral radical should lie close to the centre of the redox spectrum. The chalcogen-nitrogen heterocycles that have been studied in most detail from this viewpoint... 

The second step involves the transfer of electrons from the reduced [FMNHg] to a series of Fe-S proteins, including both 2Fe-2S and 4Fe-4S clusters (see Figures 20.8 and 20.16). The unique redox properties of the flavin group of FMN are probably important here. NADH is a two-electron donor, whereas the Fe-S proteins are one-electron transfer agents. The flavin of FMN has three redox states—the oxidized, semiquinone, and reduced states. It can act as either a one-electron or a two-electron transfer agent and may serve as a critical link between NADH and the Fe-S proteins. 

The redox properties of the elements also show interesting trends. In common with several... 

The production of CIO2 obviously hinges on the redox properties of oxochlorine species (p. 853)... 

Ref. 23, Chemical properties of the halogens — redox properties aqueous solutions, pp. 1188-95 Oxoacids and oxoacid salts of the halogens, pp. 1396-1465. 

The Creutz-Taube anion, [(NH3)5Ru- N(CH=CH)2N Ru(NH3)5] + displays more obvious redox properties, yielding both 4+ and 6- - species, and much interest has focused on the extent to which the pyrazine bridge facilitates electron transfer. A variety of spectroscopic studies supports the view that low-energy electron tunnelling across the bridge delocalizes the charge, making the 5- - ion symmetrical. Other complexes, such as the anion [(CN)5Ru (/z-CN)Ru (CN)5] , are asymmetric... 

FP) in the aqueous phase. Subsequent separation of U and Pu depends on their differing redox properties (Fig. 31.3). The separations are far from perfect (see p. 1097), and recycling or secondary purification by ion-exchange techniques is required to achieve the necessary overall separations. 

Quinones are an interesting and valuable class of compounds because of their oxidation-reduction, or redox, properties. They can be easily reduced to hydroquinones (g-dihydroxybenzenes) by reagents such as NaBH4 and SnCl2/ and hydroquinones can be easily reoxidized back to quinones by Fremy s salt. 

The redox properties of quinones are crucial to the functioning of living cells, where compounds called ubiquinones act as biochemical oxidizing agents to mediate the electron-transfer processes involved in energy production. Ubiquinones, also called coenzymes Q, are components of the cells of all aerobic organisms, from the simplest bacterium to humans. They are so named because of their ubiquitous occurrence in nature. 

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