Iron ion complexes

Hydroxypyridones are bidentate ligands, i.e. they can occupy only two coordination sites. Hence three molecules of hydroxypyridones are needed to complex one Fe(III) completely. Upon dilution, 3 1 hydroxypyridone iron ion complexes might dissociate into 2 1 complexes, leaving available coordination sites on the iron that could allow catalysis on free radical reactions. 

The pale blue tris(2,2 -bipyridine)iron(3+) ion [18661-69-3] [Fe(bipy)2], can be obtained by oxidation of [Fe(bipy)2]. It cannot be prepared directiy from iron(III) salts. Addition of 2,2 -bipyridine to aqueous iron(III) chloride solutions precipitates the doubly hydroxy-bridged species [(bipy)2Fe(. t-OH)2Fe(bipy)2]Cl4 [74930-87-3]. [Fe(bipy)2] has an absorption maximum at 610 nm, an absorptivity of 330 (Mem), and a formation constant of 10. In mildly acidic to alkaline aqueous solutions the ion is reduced to the iron(II) complex. [Fe(bipy)2] is frequentiy used in studies of electron-transfer mechanisms. The triperchlorate salt [15388-50-8] is isolated most commonly. 

In analogy to the situation for bipyridine, the blue tris(l,10-phenanthroline)iron(3+) ion [1347949-7], [Fe(phen)2], must be obtained by oxidation of the corresponding iron(II) ion. [Fe(phen)2] has an absorption maximum at 590 nm, an absorptivity of 600 (Mem), and a formation constant of 10 . In solutions of pH > 4, this species is reduced to the iron(II) complex. The reduction is instantaneous in alkaline solution. At pH < 2, protons compete with iron(III) for the phenanthroline nitrogens and coordination is incomplete. [Fe(phen)2] is used most often in solution as an oxidant, but the trichloride [40273-22-1] and the triperchlorate monohydrate [20774-81-6] salts have been prepared. 

One of the best oxidation-reduction indicators is the 1,10-phenanthroline-iron(II) complex. The base 1,10-phenanthroline combines readily in solution with iron(II) salts in the molecular ratio 3 base l iron(II) ion forming the intensely red l,10-phenanthroline-iron(II) complex ion with strong oxidising agents the iron(III) complex ion is formed, which has a pale blue colour. The colour change is a very striking one ... 

The reported preparations of enantiomerically pure chiral iron-acyl complexes have relied upon resolutions of diastereomers. One route1415 (see also Houben-Weyl, Vol. 13/9 a, p 421) employs a resolution of the diastereomeric acylmenlhyloxy complexes (Fe/ )-3 and (FeS )-3 prepared via nucleophilic attack of the chiral menlhyloxide ion of 2 at a carbon monoxide of the iron cation of 1. Subsequent nucleophilic displacement of menthyloxide occurs with inversion at iron to generate the enantiomerically pure iron-acyl complexes (i>)-4 and (f )-4. 

There are very few examples of asymmetric synthesis using optically pure ions as chiral-inducing agents for the control of the configuration at the metal center. Chiral anions for such an apphcation have recently been reviewed by Lacour [19]. For example, the chiral enantiomerically pure Trisphat anion was successfully used for the stereoselective synthesis of tris-diimine-Fe(ll) complex, made configurationally stable because of the presence of a tetradentate bis(l,10-phenanthroline) ligand (Fig. 9) [29]. Excellent diastereoselectivity (>20 1) was demonstrated as a consequence of the preferred homochiral association of the anion and the iron(ll) complex and evidence for a thermodynamic control of the selectivity was obtained. The two diastereoisomers can be efficiently separated by ion-pair chromatography on silica gel plates with excellent yields. 

A stopped flow technique coupled with spectrophotometric analysis of the iron (II) complex formed has been used to investigate - the reactions of some organic complexes of iron(III) with the ion Fe ". The iron(III) was complexed with 1,10-phenanthroline, various substituted 1,10-phenanthrolines (5-methyl-, 5-nitro-, 5-chloro-, 5-phenyl-, 5,6-dimethyl-, 4,7-diphenyl-, and 3,4,7,8-tetramethyl-) and 2,2 -dipyridine, 4,4 -dimethyl-2,2 -dipyridine, and 2,2, 2"-tripyridine. The wavelengths used for the analysis lay in the region 500-552 m/i. 

There is a discrepancy between the cyanide criteria for both aquatic and drinking water standards and the current analytical technology. The criteria are stated for free cyanide (which Includes hydrocyanic acid and the cyanide ion), but the EPA approved analytical methodology for total cyanide measures the free and combined forms (11). This test probably overestimates the potential toxicity. An alternative method (cyanides amenable to chlorination) measures those cyanide complexes which are readily dissociated, but does not measure the iron cyanide complexes which dissociate in sunlight. This method probably tends to underestimate the potential toxicity. Other methods have been proposed, but similar problems exist (12). The Department of Ecology used the EPA-approved APHA procedure which includes a distillation step for the quantification of total cyanide (13,14). A modification of the procedure which omits the distillation step was used for estimation of free cyanide. Later in the study, the Company used a microdiffusion method for free cyanide (15). 

Treatment via chelation has been observed for 2-acetylpyridine thiosemi-carbazone derivatives, which have been found to possess inhibitory activity for the RNA-polymerases of the influenza virus [133]. The iron(III) complexes were shown to be 3 to 6 times more active as inhibitors of partially purified ribonucleotide reductase (no added iron) compared to uncomplexed thiosemi-carbazone [128]. Raina and Srivastava [134] prepared and characterized low spin iron(III) complexes of 2-acetylpyridine thiosemicarbazone, [Fe(8-H)2A] (A = NO3, OH, Cl, N3, NCS or NO2), which were proposed as being seven-coordinate. However, all but the azide complex are 1 1 electrolytes in DMF and their solid ESR spectra are rhombic with the g-values being about 2.20,2.15 and 2.00. Of the six complexes, the azide ion seems to interact ihost strongly with the iron(III) center. 

Iron(III) complexes have also been prepared with 2-acetylpyridine N-phenylthiosemicarbazone [142], 14. Three have been formulated as square pyramidal [Fe(14-H)A2] (A = Cl, NCS and NOj) based, in part, on molar conductivities of 37-62 ohm cm mol . Their ESR spectra in frozen DMF are essentially the same as for other Af-substituted species, which likely indicates the presence of [FeL2] ions. More recently [138], [Fe(14-H)2]C104 has been isolated and its solid ESR spectrum is reported to be axial with g > g - Table 1 summarizes the g-values of iron(III) heterocyclic thiosemicarbazone complexes. 

Iron(II) complexes are often included in studies when complexes are prepared from a large number of different metal ions. 2-formylpyridine thiosemicarbazone, 5, forms brown [Fe(5)2A2] (A = Cl, Br) when prepared in ethanol and [Fe(5-H)2] from aqueous alcohol solution [156], All of these complexes are diamagnetic. The resonance Raman and infrared spectra of [Fe(5-H)2] were examined in detail [130] and coordination occurs via the pyridyl nitrogen, azomethine nitrogen and thiol sulfur. There is appreciable d-d sulfur-to-iron(II) Jt-bonding. Solution studies of iron(II) complexes of some 5-substituted-2-formylpyridine thiosemicarbazones have been reported [157], but no solids... 

Bond length differences between HS and LS isomers have been determined for a number of iron(II), iron(III) and cobalt(II) complexes on the basis of multiple temperature X-ray diffraction structure studies [6]. The available results have been collected in Table 17. Average values for the bond length changes characteristic for a particular transition-metal ion have been extracted from these data and are obtained as AR 0.17 A for iron(II) complexes, AR 0.13 A for iron(III) complexes, and AR = 0.06 A for cobalt(II) complexes. These values may be compared with the differences of ionic radii between the HS and LS forms of iron(II), iron(III) and cobalt(II) which were estimated some time ago [184] as 0.16, 0.095, and 0.085 A, respectively. 

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