Reversible iron-nitrogen migration of alkyl, aryl, or vinyl groups in iron porphyrins: a possible passage between .sigma. FeIII(porphyrin)(R) and FeII(N-R)(porphyrin)complexes

Iron(iv) alkyl complexes: electronic structure contributions to Fe-C bond homolysis and migration reactions that form N-C bonds from N2

High-valent iron alkyl complexes are rare, as they are prone to Fe-C bond homolysis. Here, we describe an unusual way to access formally iron(iv) alkyl complexes through double silylation of iron(i) alkyl dinitrogen complexes to form an NNSi2 group. Spectroscopically validated computations show that the disilylehydrazido(2-) ligand stabilizes the formal iron(iv) oxidation state through a strongly covalent Fe-N π-interaction, in which one π-bond fits an "inverted field" description. This means that the two bonding electrons are localized more on the metal than the ligand, and thus an iron(ii) resonance structure is a significant contributor, similar to the previously-reported phenyl analogue. However, in contrast to the phenyl complex which has an S = 1 ground state, the ground state of the alkyl complex is S = 2, which places one electron in the π* orbital, leading to longer and weaker Fe-N bonds. The reactivity of these hydrazido(2-) complexes is dependent on the steric and electronic properties of the specific alkyl group. When the alkyl group is the bulky trimethylsilylmethyl, the formally iron(iv) species is stable at room temperature and no migration of the alkyl ligand is observed. However, the analogous complex with the smaller methyl ligand does indeed undergo migration of the carbon-based ligand to the NNSi2 group to form a new N-C bond. This migration is followed by isomerization of the hydrazido ligand, and the product exists as two isomers that have distinct η1 and η2 binding of the hydrazido group. Lastly, when the alkyl group is benzyl, the Fe-C bond homolyzes to give a three-coordinate hydrazido(2-) complex which is likely due to the greater stability of a benzyl radical compared to that for methyl or trimethylsilylmethyl. These studies demonstrate the availability of a hydrocarbyl migration pathway at formally iron(iv) centers to form new N-C bonds directly to N2, though product selectivity is highly dependent on the identity of the migrating group.

Migration Reactivities of sigma-Bonded Ligands of Organoiron and Organocobalt Porphyrins Depending on Different High Oxidation States

Migration reactivities of sigma-bonded organo-iron and -cobalt porphyrins were examined as a function of the compound oxidation state. Migration rates were determined for both the one-electron and two-electron oxidized species produced in the electron-transfer oxidation with different oxidants in acetonitrile at 298 K. The investigated compounds are represented as [(OETPP)Fe(R)](n)()(+), where n = 1 or 2, OETPP = the dianion of 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin, and R = C(6)H(5), 3,5-C(6)F(2)H(3), or C(6)F(5), and as [(TPP)Co(R)](n)()(+), where n = 1 or 2, TPP = the dianion of 5,10,15,20-tetraphenylporphyrin, and R = CH(3) or C(6)H(5). The rapid two-electron oxidation of (OETPP)Fe(III)(R) occurs with [Ru(bpy)(3)](3+) (bpy = 2,2'-bipyridine) to produce [(OETPP)Fe(IV)(R)](2+). The formation of this species is followed by a slow migration of the sigma-bonded R group to a nitrogen of the porphyrin ring to give [(N-ROETPP)Fe(II)](2+) and then by a rapid electron-transfer oxidation of the migrated product with [Ru(bpy)(3)](3+) to yield [(N-ROETPP)Fe(III)](3+) as a final product. When [Ru(bpy)(3)](3+) is replaced by a much weaker oxidant such as ferricenium ion, only the one-electron oxidation of (OETPP)Fe(R) occurs to produce [(OETPP)Fe(IV)(R)](+). A migration of the R group also occurs in the one-electron oxidized porphyrin species, [(OETPP)Fe(IV)(R)](+), to produce [(N-ROETPP)Fe(II)](+), which is rapidly oxidized by ferricenium ion to yield [(N-ROETPP)Fe(III)](2+). The migration rate of the R group in [(OETPP)Fe(IV)(R)](+) is about 10(4) times slower than the migration rate of the corresponding two-electron oxidized species, [(OETPP)Fe(IV)(R)](2+). The migration rate of the sigma-bonded ligand of [(TPP)Co(IV)(R)](+), produced by the one-electron oxidation of (TPP)Co(III)(R) with [Fe(phen)(3)](3+) (phen = 1,10-phenanthroline) is also about 10(4) times slower than the migration rate of the R group in the corresponding two-electron oxidized species, [(TPP)Co(IV)(R)](2+), which is produced by the two-electron oxidation with [Ru(bpy)(3)](3+). A comparison of the migration rates with the oxidation states of the porphyrins indicates that the migration occurs via an intramolecular electron transfer from the R group to the Fe(IV) or Co(IV) metal of the organometallic porphyrin.

Arylhydrazines as probes of hemoprotein structure and function

The reactions of arylhydrazines (ArNHNH2) or aryldiazenes (ArN = NH) with simple iron porphyrins or with hemoproteins that have relatively open active sites, including hemoglobin, myoglobin, cytochrome P450, chloroperoxidase, catalase, prostaglandin synthase, and indoleamine-2,3-dioxygenase yield sigma-bonded aryl-iron complexes. Denaturation of the protein complexes under aerobic, acidic conditions shifts the aryl group to the porphyrin nitrogens and produces mixtures of the four possible N-arylprotoporphyrin IX regioisomers. The regioisomers are obtained in approximately equal amounts if the iron-to-nitrogen shift occurs outside of the protein but the ratio of isomers differs if the rearrangement is controlled by the protein. Only in the case of cytochrome P450 enzymes can the shift be induced to occur without denaturation of the protein. The isomer ratios obtained when the shift occurs in the intact active site provide direct experimental information on the active site topology and dynamics. Topological information has thus been obtained for cytochromes P450 1A1, 1A2, 2B1, 2B2, 2B4, 2B10, 2B11, 2E1, 11A1, 51, 101, 102, and 108. In contrast to hemoproteins with open active sites, conventional peroxidases react with arylhydrazines to give delta-meso-aryl adducts and covalent protein adducts. Reaction with the delta-meso edge but not the heme iron provides key evidence that restricting access of substrates to the ferryl oxygen helps direct the reaction towards peroxidase rather than peroxygenase catalysis.

Enzymatic oxidation of xenobiotic chemicals

Studies with biomimetic models can yield considerable insight into mechanisms of enzymatic catalysis. The discussion above indicates how such information has been important in the cases of flavoproteins, hemoproteins, and, to a lesser extent, the copper protein dopamine beta-hydroxylase. Some of the moieties that we generally accept as intermediates (i.e., high-valent iron oxygen complex in cytochrome P-450 reactions) would be extremely hard to characterize were it not for biomimetic models and more stable analogs such as peroxidase Compound I complexes. Although biomimetic models can be useful, we do need to keep them in perspective. It is possible to alter ligands and aspects of the environment in a way that may not reflect the active site of the protein. Eventually, the model work needs to be carried back to the proteins. We have seen that diagnostic substrates can be of considerable use in understanding enzymes and examples of elucidation of mechanisms through the use of rearrangements, mechanism-based inactivation, isotope labeling, kinetic isotope effects, and free energy relationships have been given. The point should be made that a myriad of approaches need to be applied to the study of each enzyme, for there is potential for misleading information if total reliance is placed on a single approach. The point also needs to be made that in the future we need information concerning the structures of the active sites of enzymes in order to fully understand them. Of the enzymes considered here, only a bacterial form of cytochrome P-450 (P-450cam) has been crystallized. The challenge to determine the three-dimensional structures of these enzymes, particularly the intrinsic membrane proteins, is formidable, yet our further understanding of the mechanisms of enzyme catalysis will remain elusive as long as we have to speak of putative specific residues, domains, and distances in anecdotal terms. The point should be made that there is actually some commonality among many of the catalytic mechanisms of oxidation, even among proteins with different structures and prosthetic groups. Thus, we see that cytochrome P-450 has some elements of a peroxidase and vice versa; indeed, the chemistry at the prosthetic group is probably very similar and the overall chemistry seems to be induced by the protein structure. The copper protein dopamine beta-hydroxylase appears to proceed with chemistry similar to that of the hemoprotein cytochrome P-450 and, although not so thoroughly studied, the non-heme iron protein P. oleovarans omega-hydroxylase.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo formation of sigma-methyl- and sigma-phenyl-ferric complexes of hemoglobin and liver-cytochrome P-450 upon treatment of rats with methyl- and phenylhydrazine

Ferric sigma-phenyl complexes of hemoglobin and liver cytochrome P-450 are formed in vivo upon administration of C6H5NHNH2 to rats. Small amounts of the sigma-methyl complex of hemoglobin were also detected in vivo upon treatment of rats with CH3NHNH2. At the doses used for CH3NHNH2 (25 and 50 mg/kg) the states and levels of hemoglobin in the blood and spleen, and of cytochrome P-450 in the liver were almost unchanged. On the contrary, C6H5NHNH2 (25-100 mg/kg) led to a decrease of the HbO2 blood level (10-50%), together with an increase in the HbFe(III) level and the appearance of the HbFe(III)-C6H5 complex. The concentration of this complex reaches its maximum value (2 mM) 1 h after C6H5NHNH2 administration (20% of total hemoglobin). At the same time large amounts of HbO2, HbFe(III) and HbFe(III)-C6H5 appeared in the spleen, and remained high up to 24 h after treatment. Treatment of rats with C6H5NHNH2 (25-100 mg/kg) led to a significant decrease in the level of liver cytochrome P-450 (a 70% decrease 2 h after treatment with 100 mg/kg C6H5NHNH2). About 15% of the remaining cytochrome P-450 existed as a cyt.-P-450-Fe(III)-C6H5 complex, a new example of cytochrome P-450-Fe-metabolite complex which is stable in vivo.

Reaction of monosubstituted hydrazines and diazenes with rat-liver cytochrome P450. Formation of ferrous-diazene and ferric sigma-alkyl complexes

The alkyldiazenes RN = NH (R = CH3 or C2H5) react with reduced microsomal cytochrome P450 leading to complexes exhibiting a Soret peak at 446 nm. Upon oxidation of the [cytochrome P450-Fe(II)(CH3N = NH)] complex with limited amounts of dioxygen, a new complex characterized by a Soret peak at 486 nm is formed. The latter complex was also formed upon slow reaction of methyldiazene with microsomal cytochrome P450-Fe(III) or in situ oxidation of methylhydrazine by limited amounts of O2 or ferricyanide. This complex is rapidly destroyed by O2 or ferricyanide in excess and more slowly by excess dithionite in the presence of CO. Reactions of ethyldiazene or benzyldiazene with cytochrome P450-Fe(III) afforded similar complexes characterized by Soret peaks around 480 nm. These results, when compared to those recently described on reactions of monosubstituted hydrazines RNHNH2 and diazenes RN = NH with hemoglobin and iron-porphyrins, are consistent with a [cytochrome P450-Fe(II)(RN = NH)] structure for the 446-nm-absorbing complexes and a sigma-alkyl cytochrome P450-Fe(III)-R structure for the complexes characterized by a Soret peak around 480 nm. They also suggest a sigma-cytochrome P450-Fe(III)-Ph structure for the complex derived from phenylhydrazine oxidation, recently described in the literature. Finally, they provide the first evidence that cytochrome P450-Fe(III)-R complexes are formed upon microsomal oxidation of alkyl or phenylhydrazines.