Oxidation and Reduction of Cr III Complexes

Oxidation reaction kinetics of Cr(III) complexes are not common as reagents capable of producing higher oxidation states of chromium usually decompose the ligands. There are, however, a few systems, such as porphines, where higher oxidation states can be stabilized. Another class is complexes with macrocyclic ligands, and with Ce(IV) or OH as oxidants Cr(IV) species are produced (Table 6.9). This one-electron transfer is often followed by ligand oxidation or further oxidation to Cr(VI).  

Reduction of Cr(III) complexes with the hydrated electron gives the analogous Cr(II) complex. In many cases this species has only a transitory existence in aqueous solution due to ligand dissociation. With [Cr(polypyridyl)3] (poly-pyridyl = phen, bipy) complexes, however, ligand dissociation is slow and the resultant Cr(II) complexes can be characterized in solution (e.g., by UV absorption spectra). As the polypyridyl is successively displaced by oxalate, e.g., in the series [Cr(phen)3] , [Cr(phen)2(ox)], [Cr(phen)(ox)2] , the stability of the resultant Cr(II) product is significantly decreased (Table 6.9).  

The kinetics of the oxidation of [Cr(OH2)6] by Ce(IV) and IO4 as well as a series of aminocarboxylate [(edta, 2-hydroxy-ethylene diaminetriace-tate (toh), nta ] chromium(III) complexes by 10 have been reported. In all cases, the product is Cr(VI) [Eqs. (14) and (15)] and two of the studies describe the effect of mixed solvents(Table 6.9). An inner-sphere electron transfer process is proposed for reaction (14). 

With excess IO4, the oxidations proceed by a pseudo-second-order process ( 2), but the Ce(IV) oxidation, which is initially second order, is slowed by negative catalysis from the Ce(III) product, to give a first-order process (ki)/ Rate data are presented in Table 6.9. 

or MV as reactants are also given in Table 6.10. The redox potentials for a number of (diamino polycarboxylato) chromium II/III redox couples have recently been determined.  

A series of [(H20)5Cr(imXY)] complexes have been prepared by Fe reduction of the corresponding Cr (02)2(0)(imXY), and the rate of the reduction of these with C(CH3)20H radicals has been measured (Table 6.9). The radicals were produced from the decomposition of [(H20)5CrC(CH3)20H]2+ [6(311) = 2599] in a controlled manner, as aU the rate constants for the acid heterolysis, Eq. (5), and the reversible homolysis, Eq. (4), are known (the stored free radical technique). The rather narrow variation of rate constant with substitution pattern suggest the radical attacks via the coordinated imidazole, followed by a rapid intramolecular electron transfer (17). The sometimes faster reaction of HimXY with CfCHjlaOH-l-HiO is avoided by using excess (imidazole)chromium(III) and [Cr(H20)6] +. 

The photochemically excited ( E) state of tra 5-[Cr(cyclam)(NH3)2] can oxidize the methyl viologen cation (MV ) to give the corresponding Cr(II) macrocycUc complex, but this rapidly loses NH3 in acid solution, and the resulting diaqua complex is oxidized back to Cr(III) with according to 

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Liu et al. [21] investigated the simultaneous photocatalytic reduction of Cr(VI) and oxidation ofbisphenol A (BPA) in an aqueous solution in the presence of Fe(III)-OH complexes as catalysts, achieving a synergy effect of the simultaneous photocatalytic oxidation and reduction of both pollutants. Papadam et al. [55], instead, coupled the reduction of Cr( VI) to the oxidative degradation of an azodye, while in another study it was reported the simultaneous photocatalytic reduction of Fe(VI) and oxidation of ammonia [50]. 

Most reactions of coordination compounds cap be classified as either substitutions or oxidation-reductions. The classic book by Basolo and Pearson discusses both types in detail. The oxidation-reductions can occur either by simple electron transfer or by atom transfer. Taube s work on the reduction of cobalt(III) complexes by Cr is especially important in this regard. Among the many reactions which he has studied, the best known, perhaps, is ... 

Chapter 13 by Levina, Codd, and Lay, deals with possible medical applications of chromium compounds. Carcinogenicity and antidiabetic effects of chromium compounds, originally thought to be quite distinct biologically, are found to depend on varying amounts of Cr (VI) and reactive intermediates, such as Cr (V) species, formed in vivo either by reduction of Cr (VI) or by oxidation of Cr (III) complexes. The cartoon in Figure 1 should prove very helpful to those readers unfamiliar with chromium chemistry and its applications. 

Step 2 The electron transfer from Cr (II) to Co (III) in the bridged activated complex occurs through the chloro bridge. This results into oxidation of Cr (II) to Cr (III) and reduction of Co (III) to Co (II). Step 3 The Cr (III) attracts the Cl ion more strongly, as compared to Co (II). Due to this, the chloro ligand becomes a part of the chromium complex in the final product. 

Masking by oxidation or reduction of a metal ion to a state which does not react with EDTA is occasionally of value. For example, Fe(III) (log K- y 24.23) in acidic media may be reduced to Fe(II) (log K-yyy = 14.33) by ascorbic acid in this state iron does not interfere in the titration of some trivalent and tetravalent ions in strong acidic medium (pH 0 to 2). Similarly, Hg(II) can be reduced to the metal. In favorable conditions, Cr(III) may be oxidized by alkaline peroxide to chromate which does not complex with EDTA. 

Chromium(II) is a very effective and important reducing agent that has played a significant and historical role in the development of redox mechanisms (Chap. 5). It has a facile ability to take part in inner-sphere redox reactions (Prob. 9). The coordinated water of Cr(II) is easily replaced by the potential bridging group of the oxidant, and after intramolecular electron transfer, the Cr(III) carries the bridging group away with it and as it is an inert product, it can be easily identified. There have been many studies of the interaction of Cr(II) with Co(III) complexes (Tables 2.6 and 5.7) and with Cr(III) complexes (Table 5.8). Only a few reductions by Cr(II) are outer-sphere (Table 5.7). By contrast, Cr(edta) Ref. 69 and Cr(bpy)3 are very effective outer-sphere reductants (Table 5.7). 

The ion Cu" is extremely labile. Rate constants for the formation of maleate or fumarate complexes are =10 M s Ref. 281. It can be prepared in an acid perchlorate solution by reaction of Cu with a one-electron reducing agent such as Cr, or Eu Ref. 282. Although there is a marked tendency for disproportionation, solutions of Cu are metastable for hours in the absence of oxygen, particularly when concentrations of Cu(I) are low and the acidity is high. Espenson has capitalized on this to study the rates of reduction by Cu of some oxidants, particularly those of Co(III), Table 5.7 (see Prob. 6(c) Chap. 5). 

The configuration of the macrocyclic ligand affects the electrochemical properties of Ni(II) complexes (Table I) (56a, 54). For example, the oxidation and reduction potentials of CR,S,R,S)-[Ni(14)]2+ are shifted by +0.14and +0.13 V, respectively, compared with those of the Rfi,S,S isomer. Similar trends are also observed for a series of R,Sfi,S and Rfi,S,S isomers of -methylated cyclam derivatives (61a, 61b). The anodic shift of the redox potentials for the i ,S ,S-Ni(II) complex indicates that the complex is more difficult to oxidize to Ni(III) but easier to reduce to Ni(I), compared with the RJl,S,S complex. This may be related to the reduced ligand field strength of the R,Sfi,S complex, which stabilizes the antibonding -orbitals and thus makes addition of an electron more favorable while removal of an electron is less favorable. 

The chemistry in this area (6-8) has been approached from two different directions reduction of Cr(VI) or oxidation of Cr(II) it is only recently that an overall, self-consistent picture has emerged (7,9). The key experiment is the observation that the reaction between Cr(VI) and alcohols in acid solution under an 02 atmosphere yields [(H20)5 Cijii02]2+, a Cr(III) superoxo complex,1 according to the sequence (1). 

In the case of chromium it is uncertain, whether the oxidation state (III) or (VI) is predominant in seawater. This is mainly due to the observed very slow oxidation rate of Cr(III) (Elderfield, 1970 Nakayama et al.1981 Gould, 1982 van der Weyden and Reith, 1982). Interesting is the possibility of the existence of Cu(I) in marine surface waters. Reduction of Cu(II) would be initiated by photochemically produced H202- The back oxidation is so slow that Cu(I) species have a sufficiently long lifetime to form complexes,... 

The kinetics of the oxidation of Cr(III) and Cu(I) have been discussed before. Cr(VI) is reduced by dissolved organic matter, the slow re-oxidation resulting in a large enough ti for an existence of Cr(III). Also the existence of Cu(I) in seawater is a steady state between the reduction- and back-oxidation reactions. The lifetime is dependent on pH, PC>2, complexing ligands and redox intermediates such as H2O2 (Moffet and Zika, 1983). 

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

N-methyl derivative resulted in oxidation of the ligand with concomitant reduction of Co (III) to Co (II). The preparation of tris (benzohydroxa-mato) chromium (III), Cr(benz)3, was successful and resulted in the separation and characterization of its two geometric isomers (2). The half-lives for isomerization of these complexes near physiological conditions is on the order of hours. To facilitate the separation of all four optical isomers of a simple model tris (hydroxamate) chromium (III) complex, we prepared (using Z-menthol as a substituent) the optically active hydroxamic acid, N-methyl-Z-menthoxyacethydroxamic acid (men). This resulted in the separation of the two cis diastereoisomers of tris(N-methyl-Z-menthoxyacethydroxamato) chromium (III) from the trans diastereoisomers and their characterization by electronic absorption and circular dichroism spectra. 

Masking can be achieved by precipitation, complex formation, oxidation-reduction, and kinetically. A combination of these techniques may be employed. For example, Cu " can be masked by reduction to Cu(I) with ascorbic acid and by complexation with I . Lead can be precipitated with sulfate when bismuth is to be titrated. Most masking is accomplished by selectively forming a stable, soluble complex. Hydroxide ion complexes aluminum ion [Al(OH)4 or AlOa"] so calcium can be titrated. Fluoride masks Sn(IV) in the titration of Sn(II). Ammonia complexes copper so it cannot be titrated with EDTA using murexide indicator. Metals can be titrated in the presence of Cr(III) because its EDTA chelate, although very stable, forms only slowly. 

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