Iron porphin complexes

J.T. Groves, T. E. Nemo, R. S. Myers, Hydroxylation and epoxidation catalyzed by iron-porphine complexes-oxygen-transfer from iodosylbenzene, /. Am. Chem. Soc. 101 (1979) 1032. 

The phthalocyanine [1-4] system is structurally derived from the aza-[18]-annulene series, a macrocyclic hetero system comprising 18 conjugated n-electrons. Two well known derivatives of this parent structure, which is commonly referred to as porphine, are the iron(III)complex of hemoglobin and the magnesium complex of chlorophyll. Both satisfy the Htickel and Sondheimer (4n + 2)- electron rule and thus form planar aromatic systems. 

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. 

Phthalocyaninato(2-)] iron(II) is a dark blue, thermally stable solid that can be sublimed in vacuo at 300°. It is very soluble in pyridine, giving deep blue solutions of the bis(pyridine) adducts. It also forms an unstable purple hexaaniline adduct when dissolved in aniline. It is soluble in concentrated sulfuric add and dimethyl sulfoxide (slightly) but is insoluble in most other organic solvents. The iron(II) complex, unlike the corresponding iron(II) porphines, is relatively stable toward oxidation to the iron (III) state. The electronic spectrum shows the following absorption bands (1-chloronaphthalene solution) 595 (e = 16,000), 630 (e = 17,000), 658 (e = 63,000) (pyridine solution) 333 (e = 45,000), 415 (e = 15,000), 395 (e = 2000), 658 nm (e = 8000). 

P.C. Wilkins and R.G. Wilkins, Rapid Pulse Radiolytic Reduction of Iron(III) Complexes of Tetrakis(4-sulfonatophenyl)porphine Anion and Tetrakis(N-methylpyrid-4-yl)porphine Cation, Inorg. Chem., 25 (1986) 1908. 

Self-exchange rates of the three tetrakis-(4-pyridyl)porphine complexes [Fe(P)(imidazole)a] +/ +, [Fe(P)(OH)(OH2)] +/ +, and [Fe(P)(OHa)]6+/ + have been estimated as > 10 , > 10 , and 1.2 x 10 respectively. Of the iron(iii) complexes in this series, the first is low-spin, the second is believed to be an equilibrium mixture of high-spin and low-spin forms, and the third is high-spin. These results support a previous suggestion that electron transfer to or from a low-spin iron porphyrin system has a negligible Franck-Condon barrier. ... 

Porphine itself does not occur in nature, but analogous compounds with various side chains on the pyrrole rings are some of the most important life-sustaining compounds of nature. One example is heme, the iron-porphyrin complex responsible for the red color of arterial blood. 

The kinetic parameters for water exchange with the iron(III) complexes of the water-soluble porphyrins m 5o-tetrakis(A -methyl-4-pyridyl)porphine (TMpyP) and me50-tetrakis(p-sulfonatophenyl)porphine (TPPS) have been recorded they are, respectively, kx (per H2O) at 25°C = 7.8 x lO s" and 1.4 x 10 s- A// = 13.8 and 13.7 kcal mol" = 14.7 and 20.2 cal K" mol" The exchange rate is enhanced by a factor of 10" -10 over unsubstituted Feaq, A/f decreasing from 18 to 14 kcal mol Presumably the extra labilization produced by TPPS compared to TMpyP reflects a greater degree of charge donation to the iron. 

Manganese nitrosyl porphyrins [215] are considered good models for the iron-nitric oxide analogs, which are relatively unstable but very vital to many biological operations. A six-coordinate manganese nitrosyl porphyrin of the form (por)Mn(NO)(L), where por can be TTP (TTP = tetra(4-methylphenyl)porphine) and L = piperidine, methanol, 1-methyhmidazole, has been prepared [216] in moderate yields by the reductive nitrosylation of the (por)MnCl complex with NO in piperidine. The crystal structures of these compounds give indication of a linear Mn-NO bond [215]. 

Porphyrins and their derivatives play critical roles in many biological functions. Some of the most remarkable examples are protoporphyrin IX and its iron complex that constitutes the heme prosthetic group, and the magnesium complexes of pheophytin a and bacteriopheophytin a that are known as chlorophyll a and bacteriochlorophyll a, respectively. These natural compounds are illustrated in Fig. 1 together with the structure of porphin,... 

Fig. 8. Energy level diagram for Pyridine-Co complexes of iron(II) porphine, chlorin, iBC and BC (Taken from Ref. 93)...

The porphyrin metallo complexes in crude oils, asphaltenes and other natural bitumens are chiefly those of vanadium and nickel although copper, iron and even uranium have been suggested. Recently in a Precambrian shale, porphins were found to chelate with iron, zinc and copper in addition to vanadium and nickel.The origin of these complexes is still uncertain, although several theories have been advanced. Some of these theories could be verified or possibly even disproved if the porphyrin type bound to each metal was known. Furthermore, since these heavy metals are harmful to both health and catalysts, a systematic study of demetallation of metalloporphyrins should prove useful. 

From the point of view of coordination chemistry, ferric haems, and chlorophylls are spectacular cases of very stable complexes that are formed at much slower rates than the usual complexes of iron and magnesium. Falk (1) has summarized the earlier work done on the kinetics of metalloporphyrin formation when there was little information available on the behaviour of different metal ions under comparable conditions. Nevertheless, the scattered studies showed that the rates depend on the metal ion, the substituents at the porphin nucleus (I), the solvent, and the ligands of the metal ion. The last point was made for the first time by Lowe and Phillips (2) who showed that bidentate ligands could induce a spread... 

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