Diimine ligands iron complexes

There are very few examples of asymmetric synthesis using optically pure ions as chiral-inducing agents for the control of the configuration at the metal center. Chiral anions for such an apphcation have recently been reviewed by Lacour [19]. For example, the chiral enantiomerically pure Trisphat anion was successfully used for the stereoselective synthesis of tris-diimine-Fe(ll) complex, made configurationally stable because of the presence of a tetradentate bis(l,10-phenanthroline) ligand (Fig. 9) [29]. Excellent diastereoselectivity (>20 1) was demonstrated as a consequence of the preferred homochiral association of the anion and the iron(ll) complex and evidence for a thermodynamic control of the selectivity was obtained. The two diastereoisomers can be efficiently separated by ion-pair chromatography on silica gel plates with excellent yields. 

Electronic and vibrational spectroscopy continues to be important in the characterization of iron complexes of all descriptions. Charge-transfer spectra, particularly of solvatochromic ternary diimine-cyanide complexes, can be useful indicators of solvation, while IR and Raman spectra of certain mixed valence complexes have contributed to the investigation of intramolecular electron transfer. Assignments of metal-ligand vibrations in the far IR for the complexes [Fe(8)3] " " were established by means of Fe/ Fe isotopic substitution. " A review of pressure effects on electronic spectra of coordination complexes includes much information about apparatus and methods and about theoretical aspects, though rather little about specific iron complexes. ... 

There is a brief but eomprehensively referenced section on iron in a review of homoleptie 2,2-bipyridyl eomplexes " iron-terpy (3 pages) and iron-quaterpy (1/2 page) complexes have also been briefly reviewed." Absorption spectra and photochemistry of apprcmriate iron diimine complexes are ineluded in a text on polypyridyl and porphyrin complexes." A review of the applieation of ehiral 2,2 -bipyridines, 1,10-phenanthrolines, and 2,2 6, 2"-terpyridines in homogeneous eatalysis eontains a little material on iron complexes of such ligands." ... 

Bithiazole, 4,4 -btz = (97), is not a diimine its iron(II) complex [Fe(4,4 -btz)3] is only weakly colored but its magnetic properties and Fe—N bond distances 1.970-1.973A suggest that the ligand field effect of 4,4 -btz is not enormously less than that of bipy. ... 

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). 

Diaza-1,3-butadiene (a-Diimine) Ligands Their Coordination Modes and the Reactivity of Their Metal Complexes, 21, 152 Diene-Iron Carbonyl Complexes, 1, 1... 

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.

A series of iron(II) complexes of the type [FeL2X2 ] has been described177), where L denotes the bidentate a-diimine ligand 2,2 -bi-4,5-dihydrothiazine (btz) or 2,2 -bi-2-thiazoline (bt) or its alkyl-substituted derivatives (bts, btn, and btne) X refers to... 

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. 

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. 

Tab. 3.1 H nmr shifts of diimine pyridine ligands 63-65 and iron complexes 66, 67, 69 in CD2CI2 (relative to solvent at d=5.25 ppm).

The infrared spectra in the rock-salt region of a comprehensive range of complexes have been tabulated (392, 520, 556, 626, 644). The spectra are very similar with the exception of those of the spin-paired iron(II) complexes (392). The spectra of iron(II) complexes with bipyridyl, phenanthroline, and related a-diimine ligands have been discussed by Busch and Bailar (122). 

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