Porphyrin complexes of iron

Table I. Representative and 2H NMR Data for Iron Complexes of Porphyrins and ]V-Methylporphyrins... 

An obvious choice for the construction of artificial oxygen binding mimics is the use of oxidisible transition metal (particularly iron) complexes of porphyrin ligands such as 12.23, as in the natural system. In such cases, the O2 affinity is governed essentially by the reduction potential of the metal centre (Ej) as in the reaction + e = M +. Iron(ll) and cobalt(ll) are particularly favourable in... 

It is known that iron complexes of porphyrins are salts in which the metal complexes act as cations. When dissolved in dry hydrocarbon, these salts are undissociated. At the interface with water or in wet octane, where water is, presumably, present as a microemulsion, hydrolysis takes place ... 

The reduction electrochemistry of ECP porphyrin films furthermore responds to added axial ligands in the expected ways. We have tested this (2,6) for the ECP form of the iron complex of tetra(o-amino)phenyl)porphyrin by adding chloride and various nitrogeneous bases to the contacting solutions, observing the Fe(III/II) wave shift to expected potentials based on the monomer behavior in solution. This is additional evidence that the essential porphyrin structure is preserved during the oxidation of the monomer and its incorporation into a polymeric film. 

Better results for the porphyrin complex-catalyzed asymmetric epoxidation of prochiral olefins were achieved by Naruta et al.98 using iron complexes of chiral binaphthalene or bitetralin-linked porphyrin 128 as chiral catalysts. As shown in Scheme 4-45, asymmetric epoxidation of styrene or its analogs provided the product with good ee. Even better results were obtained with substrates bearing electron-withdrawing substituents. 

It is quite evident that the ferrous complexes of porphyrins, both natural and synthetic, have extremely high affinities towards NO. A series of iron (II) porphyrin nitrosyls have been synthesized and their structural data [11, 27] revealed non-axial symmetry and the bent form of the Fe-N=0 moiety [112-116]. It has been found that the structure of the Fe-N-O unit in model porphyrin complexes is different from those observed in heme proteins [117]. The heme prosthetic group is chemically very similar, hence the conformational diversity was thought to arise from the steric and electronic interaction of NO with the protein residue. In order to resolve this issue femtosecond infrared polarization spectroscopy was used [118]. The results also provided evidence for the first time that a significant fraction (35%) of NO recombines with the heme-Fe(II) within the first 5 ps after the photolysis, making myoglobin an efficient N O scavenger. 

B. Iron Complex of a Crown Ether-Porphyrin Conjugate Suitable for... 

The Fe(II)-NO complexes of porphyrins 66-68) and heme proteins 24, 49, 53, 69-76) have been studied in detail by EPR spectroscopy, which allows facile differentiation between five-coordinate heme—NO and six-coordinate heme—NO(L) centers. However, only a few reports of the Mossbauer spectra of such complexes have been published 68, 77-82), and the only Fe(III)-NO species that have been studied by Mossbauer spectroscopy include the isoelectronic nitroprusside ion, [FeCCNlsCNO)] (7S), the five-coordinate complexes [TPPFe(NO)]+ 68) and [OEPFe(NO)]+ 82), and two reports of the nitro, nitrosyl complexes of iron(III) tetraphenylporphjrrins, where the ligand L is NO2 82, 83). 

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." ... 

Nitric oxide and iron nitrosyl complexes have been observed in the reduction of nitrite by bacterial nitrite reductases, which contain iron chlorin or iron isobac-terichlorin [151]. A specific nitric oxide reductase also exists to convert NO to nitrous oxide [9]. Iron complexes of chlorins, isobacteriochlorins, and porphyrins, as well as ruthenium and osmium polypyridines, and cobalt and nickel... 

Weiss, R. Gold, A. Trautwein, A. X. Terner, J. High-valent iron and manganese complexes of porphyrins and related macrocycles, The Porphyrin Handbook. Volume 4. Biochemistry and Binding Activation of Small Molecules , Eds. Kadish, . M. Smith, . M. Guilard, R. Academic Press San Diego, 2000, pp. 65-96. 

Several porphyrin analogues with the pyrrolic nitrogens substituted by heteroatoms have been synthesized, but only the oxa analogues (23 X = O, Y = NH) are reported to form stable metal complexes (Figure 8). 60 Formation of a Zn complex of the thia analogue (23 X = S, Y = NH) requires the presence of a large excess of Zn" ion. An iron complex of dithiaporphyrin (23 X = Y = S) is also known. 

Iron porphines constitute the active site of hemoproteins and as such are among the most important and most widely studied series of metal complexes. The syntheses of iron protoporphyrin (heme) and its derivatives are important for the reconstitution of hemoproteins (particularly when one wishes to incorporate 57Fe for Mossbauer studies), as well as for model studies. In addition to the naturally occurring porphyrins, iron complexes of both 5,10,15,20-tetraphenyl-21//,23//-porphine (H2tpp) and octaethyl-21//,23//-porphine (H2oep) are widely used in model studies of the natural systems. 

Given the similarities in chemical shifts and linewidths, as well as the contributions of symmetry to the appearance of the spectrum, the electronic and molecular structure of new iron complexes of N-alkyl-porphyrins may be ascertained, to a first approximation, from NMR data. Thus for low-spin iron(III) complexes one would expect at least four sharp resonances upfield of the diamagnetic region. Iron(IV) complexes should have at least four resonances upfield of the diamagnetic region. Iron(III) can be differentiated from iron(IV) by measurement of the solution susceptibility (51). 

Formation of an Iron Complex of an N-alkyl Porphyrin ir-Radical (46, 52). In the absence of sodium methoxide, the reaction between (N-MeTTP)FenCl and m-CPBA takes two different paths. When 1.1 equivalents of the peroxy acid are added, the Fe11 complex is converted into a high-spin, five-coordinate Fem complex. However,... 

Several different sequences of reactions may be postulated for the conversion of the magnesium complexes of pheophytlns (chlorophylls) and Iron complexes of protoporphyrin IX and related porphyrins (hemes and hemlns) Into the nickel and vanadium porphyrins found In petroleum. One possible reason for the Isolation of only the nickel and oxovanadlum metalloporphyrlns Is that only they were resistant to degradation. While studies of Hodgson do Indicate that complexatlon of vanadium and nickel do Impart added thermal stability to porphyrins (12). Berezin has found that complexatlon of other metal Ions such as cobalt and copper also Imparts added thermal stability (13. 14). In addition, Hodgson s study Indicates that relatively little thermal degradation of the metalloporphyrlns has taken place In most crude oils (which would lead to unbound vanadium and nickel), One would expect that if little degradation of these metalloporphyrlns has occurred, complete disappearance of other metalloporphyrlns by thermal degradation Is an unreasonable assumption. 

Recent attention has turned to novel, porphyrin-related macrocycles such as the porphy-cenes82-85 and the nitrite reductase model iron complexes of porphinone, porphindione, and iso-bacteriochlorin.86... 

Naruta et al. [225, 226] designed the twin-coronet porphyrin ligands (62) and (63) with binaphthyl derivatives as chiral substituents (Figure 13). Each face of the macrocycle is occupied by two binaphthyl units and the ligand has C2 symmetry. Iron complexes of these compounds can be very effective catalysts in the epoxidation of electron-deficient alkenes. Thus, nitro-substituted styrenes are readily epoxidized in 76-96% ee [226]. The degree of enantioselectivity can be explained on the basis of electronic interactions between the substrate aromatic ring and the chiral substituents rather than on the basis of steric interactions. 

As there are many kinds of hemes which are different in their structures (Fig. 2.1), there are also many kinds in cytochromes which have hemes as the prosthetic group. Heme is a complex of porphyrin with iron. In Fig. 2.1, the structures of several hemes are shown which will appear in this book. 

Weiss, R., A. Gold, A.X. Trautwein, and J. Terner (2000). High-valent iron and manganese complexes of porphyrins and related macrocycles. In... 

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