Iron III complexes with chelating ligands

The study of frozen aqueous solutions of iron(III) compounds is only of limited value because of the tendency to hydrolyse, and the relaxation broadening [63, 128], and little work has been done. A potentially more profitable application is the study of solvent extracts. Frozen nitrobenzene extracts of iron(III) in HCl, HBr, and NaSCN solutions contain respectively the FeCl4 and FeBr4 anions and a 6-coordinate complex Fe(NCS)4X2 (X is either water or nitrobenzene) [129, 130]. The spin-spin relaxation behaviour is dependent on the solvent, and trioctylphosphine oxide extracts show partially resolved hyperfine structure. 

6 Iron(III) Complexes with Chelating Ligands Fe(acac)3 and Related Complexes 

Seven substituted-acetylacetonate complexes show very similar behaviour to the parent species, except that in some cases the relaxation broadening at 78 K is not as great and a quadrupole effect of up to 0-75 mm s is in evidence [131]. Further interpretation would require detailed knowledge of the spin interactions. 

The iron(III) compound Fe(acac)2Cl shows temperature-dependent spin-relaxation properties similar to those already found in iron(III) chloro-haemin, and the compound probably has the same square pyramidal structure [132]. The I i Kramers doublet lies lowest, and at low temperatures results in a fast relaxation and a narrow spectrum. Increasing temperature causes population of the f level, a slowing of the spin relaxation, and considerable line broadening. The compound is definitely in an 5 = f spin state (ji = 5-98 BM at 301 K) and makes an interesting contrast with the square pyramidal -S = compounds such as (R2NCS2)2FeX discussed in Chapter 8. 

A number of chelates derived from ethylenediaminetetraacetic acid, H4EDTA, and diethylenetriaminepentaacetic acid, HjDTPA, have been examined 

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