Iron diimine complexes substitution reactions

Substitution reactions of hexadentate diimine complexes of iron(II) are generally slow, thanks to the combination of the strongly binding diimine groups and the chelate effect, even when the ligand contains only two diimine units of the less strongly bonding py—CH=N— type k2(OH)... 

Over recent years there have been a number of publications concerned with formation constants of [Fe(a,a -diimine)3]2+ complexes at various temperatures. Consequently additional AH° and AS° values are now available (Table 13),425428 and some data concerning mixed solvents have also been reported.424 459 4,1 Most of the substitution reactions of the tris ligand, low-spin, intensely coloured complexes proceed at rates conveniently monitored by conventional spectrophotometric techniques, and a considerable body of literature dealing with these kinetic and mechanistic aspects has been published. The most important a,a -diimine ligands are 2,2 -bipyridine and 1,10-phenanthroline, and their iron(II) complexes are dealt with first before considering complexes of other a,a -diimines. 

Although rate constants for high-spin low-spin interconversion for [Fe(pyim)3], pyim = 2-(2 -pyridyl)imidazole (29), are extremely fast, they are relevant to discussions of racemization and dissociation of iron(II) diimine complexes, as in both cases significant participation by the high-spin form is often implicated in reactions involving the substitution-inert low-spin form. Thus... 

The dipolar addition reaction between DMAD or dimethyl maleate and the diimine species [Fe(CNR)3(Pr N=C HCH=NPr>)] (R = Bu , CH2Ph, Cy, 2,6-xylyl etc.) occurs to give complexes of type (4) in the case of [Fe(CO)2(CNR)(Pr N=CHCH=NPri)] there is a competition between CO and CNR insertion into the metallacycle. Compounds containing fused pyrrole rings arc also formed which on heating release substituted 2-vinylpyrroles. -64 similar cycloaddition also occurs between [Fe(C0)3(Bu N=CHCR=0)] (R = Me, Ph) and DMAD. 5 Purther details of the insertion of alkynes into vinylketene iron tricarbonyl complexes have appeared thermolysis of the insertion products gives cyclopentenediones or phenols. -... 

Substitution and oxidation can often both be involved in reactions of tris(diimine)-iron(II) complexes with oxidizing agents. Thus, for example, reaction with hydrogen peroxide involves rate-determining dissociation as the first step. Similarly, initial dissociation seems to be the first step in the predominant pathway for superoxide oxidation of the [Fe(phen)3] cation. Dissociation may also be involved in reactions of diimine-iron(II) complexes with nitrous acid. Here and elsewhere it is recognized that these complexes react with nitric acid—in the initial stages aquation may be the only important path, but autocatalytic redox processes usually become dominant before aquation is complete, especially for the more easily oxidizable ligands and complexes. ... 

As the majority of substitution reactions of low-spin complexes of co-balt(III) and iron(II) appear to proceed by dissociative mechanisms, the well-established existence of second-order pathways for some such reactions, especially of diimine—iron(II) species, has prompted considerable research and discussion. The operation of the much-debated mechanism of initial attack by the nucleophile at the coordinated diimine has proved remarkably difficult to prove or disprove with any certainty. Progress in this field has been limited of late, with various key experiments proving impossible to execute or merely providing further equivocal evidence (Section 5.4.2). 

In solution yellow iron(II) species containing one a,a -diimine ligand can be obtained from an excess of Fe + in an acidified solution of the ligand (which controls the amount of available free ligand via protonation). Formation constants for many such complexes of substituted 1,10-phenanthrolines have been reported and tabulated. The rate of formation of [Fe(phen)(H20)4] is rapid and has a very unfavourable entropy of activation ( = 53.6 kJ mol , A5 = — 66.9 J K mor Kinetics of the corresponding reactions with 2,2 -bipyridine have also... 

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