Iron II Systems

Iron(III) salts also activate the oxidation of sulphoxides by hydrogen peroxide in dry acetonitrile although the yields are typically lower than for the iron(II) system . With iron(III) salts the hydrogen peroxide seems to be activated by direct complexation between the metal ion and the peroxide moiety. 

The first report [11] of a spin transition in a synthetic iron(II) system seems to be the result of a well-planned, deliberate strategy to identify the singlet/ quintet crossover region by the systematic variation of the field strength of the anionic groups in the six-coordinate species [Fe(phen)2X2] [7]. One... 

There have been other reports of transitions in related iron(III) systems [123] as well as in five-coordinate adducts of bis(ethylenedithiolato)iron(III) derivatives [124]. Remarkably, in these latter systems the transitions occur at extremely low temperatures and their observation at such temperatures is an indication of the relatively rapid inter-conversion of the spin states compared to iron(II) systems for which thermally-driven transitions are only rarely encountered below liquid nitrogen temperature. 

The importance of hydrogen bonding on the SCO behaviour of the tris(picolylamine)iron(II) system was investigated through the effect of iso-... 

Iron(II) systems based on hexadentate ligands where all the donor functions are imines will generally be low-spin. One well-known example is the Schiff base ligand obtained by condensing tren with 2-pyridinecarbaldehyde (22). 

The inter-conversion of the spin states in many instances is so rapid that the separate contributions to the 57Fe Mossbauer spectra are not resolved. Thus this technique, which has proved so diagnostic in iron(II) systems, is frequently less suited to the derivation of spin transition curves for iron(III). A further corollary of the faster spin state inter-conversion is the rarity of the LIESST effect among iron(III) systems, in contrast to its ubiquitous occurrence in iron(II). 

Despite these differences, the similarities predominate and virtually all the features noted for spin crossover in iron(II) are also found for iron(III). Because of the great emphasis on the cooperative aspects of the spin crossover phenomenon, iron(II) systems have tended to dominate more recent research. However, there are very striking examples among the iron(III) systems which are of strong relevance to these aspects and there is certainly scope for future work in this area. This is evident in much of the very recent work where it can be seen that specific strategies to increase the cooperativity have been successful and have led, for example, to solid iron(III) systems which display the LIESST effect [137, 138]. The generation of polymeric species as a means of increasing cooperativity, an approach which has been widely adopted for iron(II), has received relatively little attention for iro-n(III) and this is an area which can be expected to be exploited further. 

The dithionite reduction of the micelle encapsulated aqua (hydroxo) ferric hemes at pH 10 (in inert atmosphere) gives an iron (II) porphyrin complex whose optical spectrum [21] shows two well-defined visible bands at 524 and 567 nm and a Soret band split into four bands (Fig. 10). Such spectral features are typical of four-coordinate iron (II) porphyrins. The magnetic moment (p = 3.8 + 0.2 Pb) of this sample in the micellar solution is also typical of intermediate spin iron(II) system and is similar to that reported for four-coordinate S = 1 iron(II) porphyrins and phthalocyanine [54-56]. The large orbital-contribution (ps.o. = 2.83 p for S = 1) observed in this iron(II) porphyrin... 

The results in Table 9.12 confirm that transition-metal complexes can facilitate multielectron reduction pathways for 02. Within the group of complexes the iron(II) systems are the most effective. Thus, Fen(bpy)2+ in MeCN catalyzes a two-electron process at a potential that is 0.73 V less negative than the uncatalyzed one-electron process (—0.87 V) ... 

A convenient preparation of Cr(bipy)3(0104)2 has been published (380) and methods of examining spectra of low valent compounds in general have been discussed (349). In contrast to the iron(II) system, spin pairing occurs on the chromium(II) cation on addition of the second molecule of bipyridyl, thus [Cr(bipy)2(H20)2](C104)2 is low spin (178). 

In this section the results on high-spin iron(II) systems are presented before those on iron(III). The latter dominate, and are ordered approximately as follows the major structural classes of Fe203/M203 solid solutions, MFe03 perovskites, MFe03 orthoferrites, M3FesOi2 garnets and other iron(III) oxides approximately in the periodic table classification of the second metal. Any quaternary oxides are included with the most appropriate ternary system. 

The d-d electron transitions for a low-spin iron(II) system cover the spin forbidden Ax = lAx -> TX and A2 = XAX - 3T2 transitions with low intensity, as well as the spin allowed (orbital forbidden) A3 = XAX - XTX and A4 = XAX 1T2 transitions which are well resolved (Fig. 9.20). Typically, Ax 10000cm-1, J3 18000-19 500cm-1 and A4 26000cm-1 so that the LS system is coloured (purple). In contrast, the HS system exhibits only the single spin-allowed d-d transition As = 5T2 -> SE with a typical value of d5 12000 cm-1, which implies that the HS system is colourless (no low-lying metal-to-ligand charge-transfer transitions are assumed). 

However, in their mechanism and in their action nature bacterial and enzymatic fuel cells have much in common. In bacterial fuel cells intermediate redox systems are often used, as well, to facilitate electron transfer to (or from) the substrate. As the effect of microorganisms is much less specific than that of enzymes, a much wider selection of redox systems can be used, in particular, the simplest iron(III)/iron(II) system. The working conditions of these two kinds of biological fuel cells are similar as well a solution with pH around 7.0 and a moderate temperature, close to room temperature. 

Using a continuous-flow system, an e.s.r. study has been made of the reaction of Sn with H2O2 in HCl. Two pathways are described (a) a reaction to yield Sn , HO, and OH, similar to the one-electron process described for the corresponding iron(ii) system, and b) a direct interaction to Sn and 20H. The contribution from (a) is small both in aqueous media and in the presence of organic solvents, the second-order rate constant for the process (T=25°C 0.5M-HC1) being 24 3 s. Some consideration is given to (b) resembling... 

Bajwa, R.S. and Anderson, J.A. (1975) Conversion of agroclavine to setoclavine and isosetoclavine in cell-free extracts from Claviceps species SD 58 and in a thiolglycolate-iron(II) system./. Pharm. Sci., 64, 343-344. 

Substitution is, of course, much slower in [Ru(pc)L2] than in [Fe(pc)L2]. A D mechanism operates in both series of complexes the five-coordinate intermediates show very little discrimination. Variable-temperature proton nmr studies of axial ligand exchange in 1-methylimidazole- and 4-r-butylpyridine-benzyl isonitrile-ruthenium-tetraphenylporphyrin complexes show that tetraphenylporphyrin has a much smaller cis effect here than in analogous iron(II) systems. Again tt-bonding effects are important in determining kinetic parameters. ... 

The fission and formation of a variety of mono-, di, and tri-bridged di-cobalt(III) complexes will be discussed in this section, in this order. At the end comparison will be made with a cobalt(III)-iron(II) system, very closely related in that again both metal ions are low-spin centers. 

Several references deal with slow or relatively slow substitution at iron(III), while a few are directly relevant to analogous iron(II) systems discussed in Sections 8.2.1 and 8.2.2. Indeed iron(III)-diimine complexes have already been mentioned in connection with covalent hydration (Section 8.2.2.4). " " Substitution at [Fe(phen)3] is a very much less popular area of study than that of its iron(II) analogue (Section 8.2.2). Dissociation of the iron(III) complex in aqueous acetone is claimed to be first order with respect to [Fe(phen)3] ", second order with respect to acetone, and reciprocal first order with respect to This information is derived from observations at 620 nm a more complicated picture emerged from kinetic studies carried out at 470 nm The interpretation offered rather conceals the key role of acetone in solvating the leaving 1,10-phenanthroline, though this is mentioned in the final sentence. " It is regrettable that the authors do not report their primary experimental results, namely their rate constants. 

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