Iron complexes tetraphenylporphyrin

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

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],... 

Figure 12.9 Structural model for deoxymyoglobin based on the formation of a five-coordinate iron complex of tetraphenylporphyrin (12.23, R = Ph), stabilised by 2-methyl imidazole.

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. 

Tajima, K., K. Mikami, K. Tada, S. Oka, K. Ishizu, and H. Ohya-Nishiguchi (1992). Autoreduction of a six-coordinate iron(lll) tetraphenylporphyrin-peroxide complex by heterolytic iron-oxygen bond cleavage. Inorg. Chim. Acta 194, 57-65. 

The likelihood that alkyl radicals, like their aryl counterparts, bind to the iron before shifting to the nitrogen is supported by the observation that the type II complexes formed between alkyldiazenes and P450 in the absence of oxygen are converted, in the presence of limited amounts of oxygen, to complexes with an absorption maximum at 480 nm characteristic of iron-carbon o-bonded complexes Furthermore, alkyl diazene-iron tetraphenylporphyrin complexes can be prepared under anaerobic conditions. However, the alkyl-iron complexes are much less stable and less well characterized than the aryl-iron complexes and their involvement in heme iV-alkylation reactions remains to be demonstrated. 

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... 

Evans, D. R. et al. Jt-Arene/cation structure and bonding. Solvation versus ligand binding in iron(III) tetraphenylporphyrin complexes of benzene, toluene, p-xylene, and [60]fullerene. J. Am. Chem. Soc. 121, 8466-8474, 1999. 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

In order to prepare and isolate solid-state, crystalline, oxygenated iron-heme model complexes, chemists learned to synthesize (by self-assembly methods) and oxygenate many types of hindered porphyrins. For instance, capped porphyrins were synthesized by direct condensation of a suitable tetraaldehyde with four pyrrole molecules.37 Picket-fence porphyrins such as I e(TPP)((V-MeIm) (where TPP = meso-tetraphenylporphyrin and /V-Melm = (V-methylimidazole)... 

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. " ... 

Synthetic iron porphyrin complexes such as Fe(TPP) (tetraphenylporphyrin = TPP), Fe(TMP) (Tetramesitylporphyrin = TMP), and Fe(TDCPP) (tetrakis (dichlorophenyl)porphyrin = TDCPP) (Fig. 9) have been used as models for P450 and peroxidase (9, 50-54). Early pioneering work showed that epoxida-tion catalyzed by Feln(TPP) was successfully carried out by the use of iodosylbenzene (Ph—1=0) as an oxidant (50). A very interesting feature of this model epoxidation is that the cis olefin is readily oxidized while the trans olefin is hardly oxidized (e.g., d.v-stylbene can be oxidized in 80% yield, but fraws-stylbene gave only a trace amount of the epoxide under the same conditions) (50, 55). Most of the model reactions are carried out in homogeneous organic solvents such as chloroform, dichloromethane, and acetonitrile, thus, the c/.v-epoxidation is expected to be a kinetically favorable process over the trans-epoxidation. 

An iron(III) porphyrin complex, (P8 )Fein(H20)2 (Px = tetra-ferf-butyl-tetrakis [2,2-bis(carboxylato)ethyl]-5,10,15,20-tetraphenylporphyrin), reacts reversibly with NO39 to form the nitrosyl complex. 

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. 

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