Iron III halides

3H20 gives a smaller chemical isomer shift than FeFj at room temperature but shows a substantial quadrupole splitting (see Table 6.8). The shift decreases regularly and reversibly with increase in pressure up to 175 kbar, and at the same time the quadrupole splitting increases from 0-48 mm s to 0-722 mm s [9]. The electron density at the nucleus and the electric field gradient thus both increase with increase in pressure. 

X-ray studies on FeCls at room temperature indicate a hexagonal unit cell. The chlorine atoms are hexagonally close-packed with the iron atoms occupying two-thirds of the possible octahedral sites in a layered array. The iron environment is not perfectly octahedral although no quadrupole splitting is resolved in the paramagnetic Mossbauer spectrum. The chemical isomer shift at room temperature [94] is 0-436 mm s which is to be compared with 0-442 mm s for FeCla-bHaO and 0-489 mm s for FeFa  

A recent study has revealed a previously unknown phase transition in FeCla at about 250 K [94]. There is a small discontinuous decrease in the chemical isomer shift with decreasing temperature of 0-003 mm s which has been confirmed by extremely careful measurement. A change in the close-packed chlorine lattice to a face-centred cubic one by a shearing motion of adjacent chlorine layers was postulated and subsequently verified by X-ray diffraction. There is also the suggestion of a return to the hep structure at 165 K so the cubic phase may be metastable. 

The zero-field spectra with gross asymmetry are a common feature of high-spin iron(III) compounds. A difference has been noted between the crystalline 

Application of pressure causes an apparent reversible reduction to an Fe electronic configuration by charge transfer of an electron from a nonbonding ligand orbital on to the central iron atom. The related [Fe(NH3)6]Cl3 complex shows a different pressure dependence between 15-25 kbar and above 

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Iron(III) chloride forms numerous addition compounds, especially with organic molecules which contain donor atoms, for example ethers, alcohols, aldehydes, ketones and amines. Anhydrous iron(III) chloride is soluble in, for example, ether, and can be extracted into this solvent from water the extraction is more effective in presence of chloride ion. Of other iron(III) halides, iron(III) bromide and iron(III) iodide decompose rather readily into the +2 halide and halogen. 

Lewis acid catalysts such as aluminum chloride and iron(III) halides also bond to nitrogen to strongly deactivate the ring toward Friedel-Crafts reactions and halogenation. 

Terminal aliphatic alkynes (e.g., 1-octyne) react with iron(III) halides (FeCls and FeBrs) to give the corresponding 2-halovinyl derivatives (route A, Scheme 10). The moderate yields were remarkably improved upon addition of stoichiometric amounts of carboxylic acids. 

Experiments with terminal acetylenes, isolation of an intermediate acetal, alkyne hydratation studies, and ab initio calculations provide substantiation of a unified mechanism that rationalizes the reactions in which the complex formation between the alkyne and the iron(III) halides is the activating step (Scheme 12) [27]. 

Scheme 21 Plausible mechanism for Prins-type cyclization promoted by iron(III) halides...

Anhydrous iron(III) halides catalyse coupling of alkynes and aldehydes.211 Simple terminal alkynes, R CH, react with aldehydes, R2CHO, to give ( ,Z)-1,5-dihalo-1,4-dienes (55). In contrast, non-terminal arylalkynes give ( ,)-o, /3-unsaturated ketones. The catalysts also promote standard Prins cyclization of homoallylic alcohols. Studies of intermediates and of alkyne hydration - together with calculations - all support FeX3 complex formation with alkyne as the activating step. 

Oxidation of ]V-MeTTPFenCl (46, 52). Catalytic alkene oxidation by iron N-alkylporphyrins requires that the modified heme center can form an active oxidant, presumably at the HRP compound I level of oxidation. To show that iron N-alkyl porphyrins could form highly oxidized complexes, these reactive species were generated by chemical oxidation and examined by NMR spectroscopy. Reaction of the (N-MeTTP)FenCl with chlorine or bromine at low temperatures results in formation of the corresponding iron(III)-halide complex. Addition of ethyl- or t-butyl-hydroperoxide, or iodosylbenzene, to a solution of N-MeTTPFenCl at low temperatures has no effect on the NMR spectrum. However, addition of m-chloroperoxybenzoic acid (m-CPBA) results in the formation of iron(III) and iron(IV) products as well as porphyrin radical compounds that retain the N-substituent. 

Iron(III) halides are obtained by direct halogenation of Fe but FeBr3 and Fel3 are best prepared by a photochemical reaction 4... 

Iron(III) halides Pep3 is green PeCb is black PeBrs is dark red-brown. 

Compounds (6-8) contain the basic structural unit [Fe(py) —S2 2] they have three unpaired electrons and are probably penta-coordinate Fe(IIl) S = i compounds (structure V) similar to the bis-(N,N -dithiocarbamato)-iron(III) halides discussed in the preceding section. Both series show an approximately systematic variation in A with change in the ligand which is not matched by a corresponding variation in the chemical isomer shift, so that it seems unlikely that large changes in delocalisation are occurring. The very small temperature dependence of A in the S = i complexes makes it difficult to determine the electronic level separations. 

Iron(iii).—Halides and Pseudohalides. The preparation of FeC in very high purity has been reported.It has been found that photo-reduction of FeCl3 to iron(ii) in aqueous solution involves [Fe(OH)] ", not [FeCl], as the photoactive species.The structure of (MeNH3)2[FeBr4]Br shows the complex anion to be tetrahedral. I.r. data have been reported for the related complex anions [FeCl Br4 ] (n = 0—4). ... 

Any electron-d cient atom can act as a Lewis acid. Many compounds containing group IIIA elements such as boron and aluminum are Lewis acids because group IIIA atoms have only a sextet of electrons in their outer shell. Many other compounds that have atoms with vacant orbitals also act as Lewis acids. Zinc and iron(III) halides (ferric halides) are frequently used as Lewis acids in organic reactions. 

In the halogenation of benzene, an iron catalyst is used. The iron is converted to iron(ni) halide (FeXs). Iron(III) halide reacts further to polarize the X2 molecule. 

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