Other Iron II Complexes

The tris-ligand complex of iron(II) and 3-hydroxyiminopentane-2,4-dione (isonitrosoacetylacetone) (35) is low-spin, but must be near the spin-crossover region since it is formed from its components on the stopped-flow timescale.  

8 Inert-Metal Complexes Other Inert Centers 

Most substitutions at iron(III) are fast, and are therefore discussed elsewhere in this report (see Chapter 9), but several are slow enough to monitor by conventional techniques and are therefore mentioned here (though pentacyanoferrate(III) complexes are in Section 8.3.1). The first system bridges this and the preceding sections, for it involves relatively slow fac mer isomerization for tris-hydroxamato complexes of iron(II) and of iron(III). These complexes containing ligand (36) have been known for some time, but isomer details have only been sorted out in the course of the present kinetic study. Kinetics of formation of several iron(III)-hydroxamate complexes have also been reported.  

Kinetic parameters have been obtained for reaction of [Fe(par)2] , par = 4-(2-pyridylazo)resorcinol (37), with cyanide and the mechanism of formation of the product [Fe(CN)3(par)] discussed. A more complicated sequence of steps is involved in the fairly slow displacement of edta from [Fe(edta)] by hexadentate Schiff base ligands derived from trien and substituted salicaldehydes (38). Kinetic studies were carried out for two such ligands, and it was demonstrated the gallium(III) complexes of such ligands are also fairly inert (cf. Section 8.1).  

Rate constants and activation parameters have been obtained for dissociation of the axial ligands from tetraphenylporphyrin and tetramesitylporphyrin complexes [Fe (porph)L2]. Steric factors were assessed by the comparison of results 

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The standard redox potential is 1.14 volts the formal potential is 1.06 volts in 1M hydrochloric acid solution. The colour change, however, occurs at about 1.12 volts, because the colour of the reduced form (deep red) is so much more intense than that of the oxidised form (pale blue). The indicator is of great value in the titration of iron(II) salts and other substances with cerium(IV) sulphate solutions. It is prepared by dissolving 1,10-phenanthroline hydrate (relative molecular mass= 198.1) in the calculated quantity of 0.02M acid-free iron(II) sulphate, and is therefore l,10-phenanthroline-iron(II) complex sulphate (known as ferroin). One drop is usually sufficient in a titration this is equivalent to less than 0.01 mL of 0.05 M oxidising agent, and hence the indicator blank is negligible at this or higher concentrations. 

Other paramagnetic bis(amidinate) iron(II) complexes of the type [But(NR)2]2Fe (R = Cy, Pr ) have been prepared analogously from the lithium amidinate salts and FeCl2- The coordination geometry around Fe is distorted tetrahedral (Scheme 137). 

These results obtained in applied field clearly prove that the ST in the dinuclear compounds under study proceeds via [HS-HS] O [HS-LS] O [LS-LS]. Simultaneous ST in both iron centers of the [HS-HS] pairs, leading directly to [LS-LS] pairs, apparently can be excluded, at least in the systems discussed above. This is surprising in view of the fact that these dinuclear complexes are centrosymmetric, that is, the two metal centers have identical surroundings and therefore, experience the same ligand field strength and consequently, thermal ST is expected to set in simultaneously in both centers. In other dinuclear iron(II) complexes, however, thermally induced direct ST from [HS-HS] to [LS-LS] pairs does occur and, indeed, has been observed by Mossbauer measurements [30, 31]. 

Measurements of the proximal histidine-iron stretching frequency by Resonance Raman spectroscopy revealed that this bond is very weak in relation to other heme protein systems (vFe.His = 204 cm-1) (130). Formation of the sGC-NO complex labilizes this ligand resulting in the formation of a 5-coordinate high spin iron(II) complex, and the conformational change responsible for the several hundred-fold increase in catalytic activity (126,129,130). 

Iron(III) very readily forms complexes, which are commonly 6-coordinate and octahedral. The pale violet hexaaquo-ion [Fe(H20)6]3+ is only found as such in a few solid hydrated salts (or in their acidified solutions), for example Fe2(S04)3.9H20. Fe(C104)3.10H20. In many other salts, the anion may form a complex with the iron(III) and produce a consequent colour change, for example iron(III) chloride hydrate or solution, p. 394. Stable anionic complexes are formed with a number of ions, for example with ethanedioate (oxalate), C204, and cyanide. The redox potential of the ironll ironlll system is altered by complex formation with each of these ligands indeed, the hexacyanoferrate(III) ion, [Fe(CN)6]3. is most readily obtained by oxidation of the corresponding iron(II) complex, because... 

Iron(ii) complexes of ethylenedithiodiacetic acid, diethylenetrithioacetic acid, and ethylenetetrathiotetra-acetic acid have all been reported. " " Isonitrile and other complexes. Refluxing [Fe(CNMe) ](HSOj2 "hh excess methylamine for 12 h in methanol gives the cation [Fe(CNMe)gNH2Me] which can be precipitated as its PF salt. " The structure of this cation has been determined by X-ray methods and is shown in (52). The location of the protons was confirmed by n.m.r. and the suggested mechanism of formation is as shown. 

Fe(gmi)3] in glycol-water and a range of other binary aqueous solvent mixtures. These results, along with further results for AV for base hydrolysis of [Fe(phen)3] " and of [Fe(bipy)3] " in alcohol-water mixtures, have permitted the construction of a scheme combiniim solvent and ligand effects on AF for base hydrolysis of a range of diimine-iron(II) complexes. ... 

AF values for cyanide attack at [Fe(phen)3] +, [Fe(bipy)3] + and [Fe(4,4 -Me2bipy)3] " in water suggest a similar mechanism to base hydrolysis, with solvation effects dominant in both cases. Cyanide attack at [Fe(ttpz)2] , where ttpz is the terdentate ligand 2,3,5,6-tetrakis(2-pyridyl)pyr-azine, follows a simple second-order rate law activation parameters are comparable with those for other iron(II)-diimine plus cyanide reactions. Interferences by cyanide or edta in spectro-photometric determination of iron(II) by tptz may be due to formation of stable ternary complexes such as [Fe(2,4,6-tptz)(CN)3] (2,4,6-tptz= (66)). ... 

The sterically hindered dianionic bidentate phenoxide ligand (205) gives several tetrahedral iron(II) complexes, e.g., [Fe (205)(THF)2], [Fe (205)(py)2], [Fe (205)(bipy)], and [Fe (205)(2,6-xylylNC)2]. The first of these is prepared from FeCl2 and (205)H2 in tetrahydrofuran the others are prepared from the dimer [Fe2(205)2]. The 2,6-xylylNC complex is low-spin, the others high-spin. There is also a five-coordinate iron(III) complex, red-black [Fe(205)(bipy)Cl], whose structure is intermediate between trigonal bipyramidal and square pyramidal." ... 

Binuclear iron(II) complexes in which a hydroxide bridge is supported by the dinucleating bis-carboxylate ligand dibenzofuran-4,6-bis(diphenylacetate), (217), have proved useful models for hemerythrin. The nature of the binuclear iron center in hemerythrin itself, and in other metalloproteins, has been reviewed, the binding of O2, NO, N3, and NCS to the iron of hemerythrin discussed, " and the volume profile for hemerythrin reacting with O2 established. Bulky tolyl-substituted carboxylate ligands, both bridging and terminal, and... 

An analogous behavior extends to other species having small reorganization energies and appropriate potentials such as the iron(II) complexes Fe(DMB)32 + and Fe(DTB)32 + (Ey2 0.95 V versus SCE). When used in the presence of an excess of Co(DTB)32 + and in conjunction with suitable sensitizers like the heteroleptic dye Ru(dnbpy)(H2DCB)22+ (Em = 1.25 V versus SCE) (Fig. 17.28), the iron(II) comediators clearly enhance the performance of the Co(DTB)32+ and outperform the I /I3 redox couple, at least in terms of monochromatic photon to current conversion efficiency, with maximum values close to 85%. 

Reduction of the complex on Raney nickel yielded benzylamine, N-methyl-benzylamine, and N,N-dimethylbenzylamine but no / -phenylbenzylamine, a reduction product resulting under the same reaction conditions from benzyl cyanide. Hydrolysis with dilute sulfuric acid in acetic acid yielded benzylamine only, and oxidation of the complex with potassium permanganate gave 4.2 moles of benzoic acid per mole of complex. The bromide anion can be exchanged metathetically with various other anions such as perchlorate, iodide, and thiocyanate. When heated at 100° C. in vacuum, the complex lost one mole of benzyl bromide and yielded only one dicyanotetrakis(benzylisonitrile)iron(II) complex. 

Reaction of free-base porphyrin compounds with iron(II) salts in an appropriate solvent results in loss of the two N—H protons and insertion of iron into the tetradentate porphyrin dianion ligand. Five-coordinate iron(III) porphyrin complexes (hemins), which usually have the anion of the iron(II) salt for the fifth or axial ligand, are isolated if the reaction is carried out in the presence of air. Iron(II) porphyrin complexes (hemes) can be isolated if the reaction and workup is conducted under rigorously anaerobic conditions. Typically, however, iron(II) complexes are obtained from iron(III) porphyrin complexes by reduction with dithionite, thiolate, borohydride, chromous ion, or other reducing agents. 

Colorimetric. A sensitive method for the determination of small concentrations of dissolved iron is the spectrophotometric determination of the orange-red tris(l,10-phenanthroline)iron(II) complex. Other substituted phenanthrolines can be even more sensitive. Only the iron(II) complexes of these ligands are highly colored. The sample is first treated with an excess of reducing agent. The complexes are stable from pH 2 ndash 9 and analysis preferably is done at about pH 3.5. 

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