Iron complexes, cyclic voltammograms

Cyclic voltammetry of the ferrocenylboron-capped oximehydrazonate iron(II) complexes with the same ribbed substituents (Table 39) reveal a reversible one-electron oxidation of the ferrocenyl iron at 320-H340 mV vs SCE. Electrolytic oxidation of the ferrocenyl iron at a potential of 750 mV in solution produces a stable mixed-valence complex. Cyclic voltammograms at this potential the solution before and after electrolysis indicate that the integrity of the complexes is maintained as the ferrocenyl is cycled between the +2 and +3 oxidation states. The redox potentials associated with the encapsulated iron ion are not affected by the oxidation state of the ferrocenyl iron, i.e., there is no apparent interaction between the two... 

The difference in oxidation potentials (A ) detected for the two waves found for the poly(ferrocenylsilanes) 15 (R = R = Me, Et, -Bu, -Hex), which provides an indication of the degree of interaction between the iron sites, varies from 0.21 V (for 15 (R = R = Me)) to 0.29 V (for 15 (R = R = -Bu or -Hex)) (63). This indicates that the extent of the interaction between the ferrocenyl units in poly(ferrocenylsilanes) depends significantly on the nature of the substituents at silicon, which may be a result of electronic or conformational effects (63). Unsymmetrically substituted poly(ferrocenylsilanes) show similar electrochemical behavior (59). In addition, polymer 15 (R = Me, R = Fc) shows a complex cyclic voltammogram which indicates that interactions exist between the iron centers in the polymer backbone and the ferrocenyl side groups (59). 

The product is exclusively carbon monoxide, and good turnover numbers are found in preparative-scale electrolysis. Analysis of the reaction orders in CO2 and AH suggests the mechanism depicted in Scheme 4.6. After generation of the iron(O) complex, the first step in the catalytic reaction is the formation of an adduct with one molecule of CO2. Only one form of the resulting complex is shown in the scheme. Other forms may result from the attack of CO2 on the porphyrin, since all the electronic density is not necessarily concentrated on the iron atom [an iron(I) anion radical and an iron(II) di-anion mesomeric forms may mix to some extent with the form shown in the scheme, in which all the electronic density is located on iron]. Addition of a weak Bronsted acid stabilizes the iron(II) carbene-like structure of the adduct, which then produces the carbon monoxide complex after elimination of a water molecule. The formation of carbon monoxide, which is the only electrolysis product, also appears in the cyclic voltammogram. The anodic peak 2a, corresponding to the reoxidation of iron(II) into iron(III) is indeed shifted toward a more negative value, 2a, as it is when CO is added to the solution. 

Fig. 8.10 Cyclic voltammogram of tris(bipyridine)iron(II) complex at a glassy carbon electrode in 0.05 M Bu4NCI04-AN.

The decrease of electron density at the iron centers caused by protonating or alkylating thiolate donors strongly shifts the redox potentials of corresponding complexes, which were obtained from cyclic voltammograms. Table III summarizes the results (and assignments) for [Fe(CO)(NnS4)] and its alkylated derivatives (89). 

The cyclic voltammograms of the [FeDA03(BC6Hs)2] and [Fe2DA03(BC6H5)2](BFj)2 azineoximates with binucleating clathrochelate ligand are discussed in Ref. 188. A quasi-reversible wave was observed for a mononuclear complex with a vacant cavity from 700 to 800 mV. The oxidation of binuclear dication occurs in two steps at more positive potentials (1 200 and 1 540 mV, Table 38). These waves were attributed to stepwise oxidation of encapsulated iron(II) ions to form mixed valence iron(II)/iron(III) complex as an intermediate product [188]. 

Thus, the cyclic voltammograms of these complexes demonstrated both oxidation and reduction waves and indicated the formation of uncommon species such as copper(III), nickel(III), nickel(IV), and low-spin iron(III) complexes. The data for manganese(III)- and manganese(IV)-capped complexes showed that it is possible to stabilize the tris-dioximatometalate(II) tetraanions with the different ttnMn or ttnMn caps. 

Checkers report that cyclic voltammogram of this iron(II) complex in CH,CN gives quasireversible behavior with E, = + 0.35 V versus a silver wire reference electrode. 

Figure 16.2 shows the cyclic voltammograms of ttis(bpy) complexes of iron [4], ruthenium [5], nickel [6], manganese, and chromium [7] in an amide-type ionic liquid, 1-butyl-l-methylpyrroUdinium bis(ttiliuoromeihylsulfonyl)amide ([C4mPyr] [N(Tf)2]). in case of Fe, Ru, Mn, and Cr, there are four pairs of cathodic and anodic current peaks, which are attributed to the following reactions in order from positive to negative potentials. 

The cyclic voltammogram of complex 32 is shown in Figure 20. While the CVs of complexes 23 and 24 showed single redox processes corresponding to three concurrent one-electron reductions, the CV of complex 32 showed that two distinct redox processes were occurring. These two processes can be attributed to reduction of the inner three and outer three iron centers. The Eyx values for these two redox processes occurred at -1.20 and 1.30 y respectively. This cyclic voltammogram was obtained in DMF at 40°C with a sweep rate of 0.1 V/s. 

The cyclic voltammogram of the star-shaped complex containing 12 metallic moieties in its structure (42) is shown in Figure 22. The reduction wave is very broad because of overlapping redox processes however, the reduction peaks for this complex became narrower at higher scan rates. The Ey/2 value corresponding to reduction of the iron centers in this complex was -1.25 V at a scan rate of 0.1 V/s. 

The electrochemical properties of the trimetallic monomers were examined using cyclic voltammetry. It has been established that cationic cyclopentadienyliron complexes undergo reduction processes, while ferrocene and functionalized ferro-cenes undergo oxidation processes. Figure 25 shows the cyclic voltammogram of complex 49. The 1/2 value obtained for the oxidation of the neutral iron center was 0.994 y and the 1/2 value obtained for the reduction of the two terminal cationic iron centers was -1.12 V... 

The electrochemical properties of the polymethacrylates contaming oiganoiron groups in their sidechains were examined using cyclic voltammelry. Figure 4 shows the cyclic voltammogram of polymer 7b obtained at - 50°C with a scan rate of 1 V/s. There are two reduction steps that can be observed in this polymer. The first reduction Ey2 = -1.24 V) corresponds to reduction of the 18-electron cationic iron centers to neutral 19-electron species. The second reduction step ( 1/2=-1.94 V) corresponds to the reversible formation of the anionic 20-electron iron complexes. 

Figure 10b shows the spectral changes obtained during controlled potential electrolysis at peak lie, -1.3 V. The process at Ic corresponds to the reduction of complexed Fe II) to uncomplexed Fe(0) and free neutral adenine. Splitting of the peak into II and II is presumably due to the presence of a form of the Fe(II) complex in which Me2S0 has replaced perchlorate. Spectral parameters show the same pattern of response as the Cu(II) adenine complex which is reduced at the more positive potential of the two. Metallic iron is present on the electrode and the final spectra is markedly similar to that of neutral adenine. Cyclic voltammograms of the reduced solution reveal no peaks other than peak IIIc and Ilia which are due to the reduction of neutral adenine and dimer oxidation, respectively. 

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