Synthetic macrocycles, iron complexes

History. Braun and Tschemak [23] obtained phthalocyanine for the first time in 1907 as a byproduct of the preparation of o-cyanobenzamide from phthalimide and acetic anhydride. However, this discovery was of no special interest at the time. In 1927, de Diesbach and von der Weid prepared CuPc in 23 % yield by treating o-dibromobenzene with copper cyanide in pyridine [24], Instead of the colorless dinitriles, they obtained deep blue CuPc and observed the exceptional stability of their product to sulfuric acid, alkalis, and heat. The third observation of a phthalocyanine was made at Scottish Dyes, in 1929 [25], During the preparation of phthalimide from phthalic anhydride and ammonia in an enamel vessel, a greenish blue impurity appeared. Dunsworth and Drescher carried out a preliminary examination of the compound, which was analyzed as an iron complex. It was formed in a chipped region of the enamel with iron from the vessel. Further experiments yielded FePc, CuPc, and NiPc. It was soon realized that these products could be used as pigments or textile colorants. Linstead et al. at the University of London discovered the structure of phthalocyanines and developed improved synthetic methods for several metal phthalocyanines from 1929 to 1934 [1-11]. The important CuPc could not be protected by a patent, because it had been described earlier in the literature [23], Based on Linstead s work the structure of phthalocyanines was confirmed by several physicochemical measurements [26-32], Methods such as X-ray diffraction or electron microscopy verified the planarity of this macrocyclic system. Properties such as polymorphism, absorption spectra, magnetic and catalytic characteristics, oxidation and reduc-... 

Nearly all iron complexes of synthetic macrocyclic ligands contain nitrogen either as the only ligand atom or as the major donor present. Moreover, most macrocyclic ligands are tetradentate usually presenting a roughly planar N4 donor set to the centrally complexed metal ion. The comprehensive review by Melson390 of the coordination chemistry of macrocyclic compounds should be consulted for work published up until 1978. 

In many cases, synthetic iron porphyrins have been investigated primarily as models of hemes in various proteins, but also as potentially useful magnetic materials, or as metal coordination complexes, since the porphyrin Macrocycle often confers upon the metal unique electronic, magnetic, and redox properties see Redox Properties Processed and chemical reactivities that are interesting in their own right. The properties and chemical reactivities of iron porphyrins will be summarized in this article. Iron complexes of related macrocycles, including reduced hemes (chlorins, isobacteriochlorins, etc.)... 

Though some diflBculties persist, it has been possible to synthesize a large number of iron complexes with synthetic macrocycles. Brief attention will be given first to the derivatives of TAAB (40). As summarized in Figure 19, o-aminobenzaldehyde condenses in the presence of ferrous chloride to form a TAAB complex. Because of the ease of oxida-... 

This group of compounds is widely found in nature as metal complexes in the chlorophylls, the haem groups of many iron proteins and the corrinoids. They have in common a macrocyclic structure which provides four N donor atoms at the comers of a square plane. Metal coordination to the N atoms results in the displacement of two H+ ions. An extremely important feature of these molecules is their extensive -electron delocalization. The complexation of these and synthetic analogues has been the subject of a number of texts.143-145 Some of these aspects are also covered by Dolphin (Chapter 21,1), and biological related properties by Hughes (Chapter 62.1). 

The nickel(II) ions can be removed from the above range of macrocyclic complexes and the resulting free ligands can then be converted into iron(II) complexes, 130 which have been shown to exhibit reversible oxygen binding.131,132 Similar characteristics have been observed for iron(II) complexes such as (60), which contain bridging from methyl carbon atoms rather than nitrogen atoms.132 Synthetic information has not been disclosed, but several routes are possible. 

Given the observation of catalysis of alkene epoxidation by iron N-alkyl porphyrins, it is likely that these complexes may yield synthetically useful catalysts (32, 65). The possibility of chiral induction by using either a chiral N-alkyl group (66) or a chiral macrocycle such as N-Me etioporphyrin (67) is an area that should prove fruitful in the near future. 

There is a particularly extensive and rich coordination chemistry associated with iron(II) N donor macrocycles. The coordination chemistry of the biologically important iron porphyrin complexes has been of interest since the classic studies of Fischer, and over recent years there has been a resurgence of work on these and also on a wide range of related synthetic macrocyclic complexes. This section concentrates on their coordination chemistry, and where appropriate highlights enhanced ligand reactivity specifically induced by the iron(II) centre. First saturated ligands are discussed and then the unsaturated systems, with the particularly well-studied porphyrins and phthalocyanines being dealt with in separate subsections. 

The lacunar cyclidene ligands are the first family of totally synthetic ligands to form both iron(II) and cobalt(II) Oj carriers. The molecular structure of the lacunar cyclidene complexes is most simply represented by the flat projection (la), while the stereochemistry is shown in Ib. Cyclidene refers to the parent macrocycle encircling the metal ion and the lacuna is the permanent void created by bridging the cleft arising from the saddle shape of the cyclidene macrocycle. 

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