Iron diimine complexes oxidation

Activation volumes for dissociation, base hydrolysis, cyanide attack, and peroxodisulfate oxidation (see following pages) of iron(II)-diimine complexes are collected together in Table 9. 

The solvatochromic behavior of the closely related iron(0) complexes Fe(CO)3(diimine) is mentioned under lower oxidation states (Section 5.4.3.5.6). 

Reaction kinetics and mechanisms for oxidation of [Fe(diimine)2(CN)2], [Fe(diimine)(CN)4] (diimine = bipy or phen) (and indeed [Fe(CN)6] ) by peroxoanions such as (S20g, HSOs", P20g ) have been reviewed. Reactivity trends have been established, and initial state— transition state analyses carried out, for peroxodisulfate oxidation of [Fe(bipy)2(CN)2], [Fe(bipy)(CN)4] , and [Fe(Me2bsb)(CN)4] in DMSO—water mixtures. Whereas in base hydrolysis of iron(II)-diimine complexes reactivity trends in binary aqueous solvent mixtures are generally determined by hydroxide solvation, in these peroxodisulfate oxidations solvation changes for both partners affect the observed pattern. ... 

When the basic solutions are allowed to stand, the product ironCII) complexes dissociate and eventually iron(III)hydroxide precipitates. From such solutions, free diimine ligand and a free oxidized ligand product, which has not been further characterized, were isolated. In solutions in which dioxygen was formed (cf. Fig. 2), free diimine-iV-oxide was also detected (4). 

The iron(II) diimine complex (30) displays unusual stability towards acids, alkali, reducing and oxidizing agents and has been termed quasi-aromatic .44 Complex (30) undergoes bromination in acetic acid to give the dibromo analogue (31) (equation 20).45... 

Figure 9-21. The oxidation of the iron(n) complex of 2,2 -bipiperidine by dioxygen gives the iron(n) complex of the corresponding diimine ligand.

Presumably, the oxidation of the iron(n) diimine complex derived from glyoxal and methylamine (encountered previously) by cerium(iv) occurs by a related mechanism (Fig. 9-37). This also provides an interesting example of the metal ion stabilising a particular tautomer of a ligand. The free ligand would be expected to exist as the amide tautomer 9.22. 

Figure 9-37. The oxidation of an iron(n) diimine complex with cerium(iv) results in an oxygen atom transfer to give a complex of the hydroxyimine tautomer of amide 9.22,...

Complexes with other Bidentate 1,4-Diimines. The thermodynamic stability of the five-membered iron(II) 1,4-diimine chelate ring in low-spin complexes is illustrated by the facile autoxidation of saturated amine complexes to the corresponding colored 1,4-diimine complexes. The first example of this oxidative dehydrogenation is shown in equation (14), and the more recently studied " reaction (15) is known to involve iron(III) species and radical intermediates. Interestingly, the complex [Fe(CN)4(HN=CHCH=NH)] can be obtained by oxidizing [Fe(CN)4(en)] , whereas the tris-ligand complex of this smallest diimine ligand has yet to be prepared. 

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