Iron II tetraphenylporphyrin

Until recently, despite the number of the examples of the formation of cobalt(II)- and iron(II)-tetraphenylporphyrins (MTPP) (107,108) from the direct reaction of the bare metal atoms (Co or Fe) and adsorbed tetraphenylporphyrin molecules (2HTPP), it was only possible to speculate (107-110) about the reaction mechanism, since the reaction is fast at 300 K, and thus no intermediates could be observed. 

As an example, the nature of the ground state of iron II tetraphenylporphyrin bistetrahydrofuran [TPP Fe(THF)2] was not well established by theoretical or spectroscopic methods the THF axial ligands are weakly bound and the complex is high spin. 

Trichloromethyl thioalkyl compounds of formula RSCXClj (a, b, X = Cl, R = Ph or CHjPh) as well as dichloromethyl benzyl thioether react with iron(II) tetraphenylporphyrin (TPP) (233), in the presence of a reducing agent (iron powder or Na2S204), to produce chlorocarbene complexes of type 234 (158). 

The capped porphyrins prepared by Baldwin et al. [56, 57] are other model systems designed to test the consequences of steric hindrance on CO binding (Scheme 4). These compounds were reported to discriminate against dioxygen in favor of carbon monoxide [62, 119-121]. The CO affinity of the capped porphyrins differs by less than a factor of three from that of unprotected iron(II) tetraphenylporphyrin, while the dioxygen affinity is more than a factor of 100 lower. Kinetic studies of CO binding show that the CO dissociation rate constants are very similar to those of unprotected hemes. Recently, the X-ray crystal structure of a carbonylated complex of the smallest capped porphyrin was obtained [122]. The CO ligand is reported to deviate 7° and 4° from the heme normal, respectively, for each independent molecule present in the unit cell. 

Hoshini, M., Bab, T. (1998). Laser photolysis of iron(III) tetraphenylporphyrin in methanol. A kinetic study on the formation of the superoxide anion radical from the dioxygen adduct of iron(II) tetraphenylporphyrin, J. Am. Chem. Soc., 120 6820. 

Cyclic voltammetry of iron(III) porphyrin-sulfate complexes has been described. Thiosulfate can add to iron(III) porphyrins to give an adduct which is high-spin at normal temperatures but low-spin at low temperatures. The tetraphenylporphyrin adduct undergoes decomposition slowly in DMF to give [Fe (tpp)] plus tetrathionate. In DMSO tetraphenylporphyrinatoiron(III) oxidizes thiosulfate by an autocatalytic process. Tetrathiotungstate complexes of iron(III)-tetra-phenylporphyrin undergo spontaneous reduction to iron(II) products with a half-life of about 30 minutes at ambient temperature. " ... 

Tetrathiotungstate complexes of iron(III)-tetraphenylporphyrin can be prepared from iron(III) tpp triflate they undergo slow spontaneous reduction to iron(II). " ... 

Robert and Meunier have isolated and characterized covalent adducts of metallopor-phyrins with artemisinin. Initial attempts using an iron(II) heme model were inconclusive, since the strong paramagnetism of the iron did not allow detailed characterization of the adduct and attempts at demetallation resulted in its degradation. Therefore, manganese(II) tetraphenylporphyrin (TPP) was used since it required milder demetallation conditions and the adduct 50a was formed. The suggested mechanism for the alkylation involves a pendent ethyl radical 50 (Scheme 13). 

Recently, syntheses of the model compounds 156 and 157 were reported [107,108], which are closely related to earlier approaches [103,109] (Fig. 25). In agreement with theoretical calculations the CO complexes of the Fe(II)por-phyrins 156 and 157 display a split Soret band at 370/446 nm and 383/456 nm, respectively, but no experiments with molecular oxygen were reported. But it was demonstrated that 157 catalyzed the formation of stable aryloxy radicals from the corresponding phenols in the presence of e.g. feri-butylhydroperoxide (TBHP). These results indicate a thiolate mediated 0-0 bond cleavage of TBHP accelerated 240 fold in comparison to iron(III)tetraphenylporphyrin [108],... 

For designing the enzyme model, it is important to consider the functional and structural similarity between the model and the active site of enzyme. With this in mind, we chose the iron(II) complexes of octaethylporphyrin and tetraphenylporphyrin, (OEP Fe(II) py2 and TPP Fe(II) py2), respectively, as models of the heme and benzene as the hydrophobic environment surrounding the heme. 

Shin gta/. " have shown that Fe complexes of tetraphenylporphyrins and etioporphyrin undergo autoreduction in DMSO solution by addition of hydroxide or alkoxide ions or solid sodium hydride. The autoreduction leads to the formation of the corresponding five-coordinate hydroxo-iron(II) (S = 2) complexes. This reduction can be followed easily by NMR spectroscopy. 

Models which allow interpretation of X-ray diffraction data in terms of orbital populations, radial dependence of the orbitals and LCAO coefficients are discussed. They are applied to experimental data on iron(II) phthalocyanine, iron(II) meso-tetraphenylporphyrin and its bis-pyridyl and bis-tetrahydrofurane derivatives. The diffraction studies indicate that the first two complexes are intermediate spin complexes with differing ground states while the last two are respectively low- and high-spin iron(II) compounds. A difference between the two intermediate spin complexes is thought to be related to the effect of the crystalline environment. This interpretation implies that the leading contributor to the ground... 

The bispyridine haeme iron(II) compounds are low-spin 5 = 0 complexes and give only a single quadrupole split spectrum [1]. The bis-adducts of iron(II) and iron(III) protoporphyrin, a, /9, y, d-tetraphenylporphyrin, and protoporphyrin dimethyl ester with bases such as imidazole and pyridine are also low-spin [2]. The quadrupole splittings at 80 K were found to be... 

One strategy to mimic the heme proteins is to embed the iron(II) porphyrins in a polymer. Bell et al. incorporated iron-5,10,15,20-tetraphenylporphyrin into films of a... 

R. Salzmann, C.J. Ziegler, N. Godbout, M.T. McMahon, K.S. Sushck, E. Oldfield, Carbonyl complexes of iron(ii), tuthenium(ii), and osminm(ii) 5,10,15,20-tetraphenylporphyrinates a comparative investigation by X-ray crystallography, sohd-state NMR spectroscopy, and density functional theory, J. Am. Chem. Soc. 

Iron(O) tetraphenylporphyrin is also a catalyst of the electrochemical carbon dioxide reduction to carbon monoxide in DMF electrolytes [29-31]. Under CO, the catalyst is, however, rapidly destroyed by either carboxylation or hydrogenation of the porphyrin ring. The mechanism proceeds through coordination of CO2 to Fe(0). At low temperature (-40°C), a second CO2 molecule adds to the first-coordinated one in an acid-base reaction fashion. This is followed by the cleavage of a C—O bond of the first-coordinated CO2 molecule and concomitant formation of the Fe(II)CO complex and a carbonate anion. After reduction of the Fe(ll)CO complex by the Fe(0) complex, CO is... 

In a comparative investigation, an iron(III) tetraphenylporphyrin-MWCNT hybrid gave better performances than a cobalt(II) tetraphenylporphyrin-MWCNT one, not only in oxygen, but also in proton and carbon dioxide rednc-tion reactions [200]. 

Substitution is, of course, much slower in [Ru(pc)L2] than in [Fe(pc)L2]. A D mechanism operates in both series of complexes the five-coordinate intermediates show very little discrimination. Variable-temperature proton nmr studies of axial ligand exchange in 1-methylimidazole- and 4-r-butylpyridine-benzyl isonitrile-ruthenium-tetraphenylporphyrin complexes show that tetraphenylporphyrin has a much smaller cis effect here than in analogous iron(II) systems. Again tt-bonding effects are important in determining kinetic parameters. ... 

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