Synthesis and spectroscopic characterization of cobalt porphyrins and their catalytic activity towards dioxygen reduction

The synthesis and spectroscopic characterization of cobalt(Ⅱ) 5-(4-pyridyl)-10,15,20-triphe-nylporphyrin,cobalt(Ⅱ) 5-(4-N-hexadecylpyridiniumyl)-10,15,20-triphenylporphyrin bromide andcobalt(Ⅱ) 5-(2-aminophenyl)-10,15,20-triphenyl-porphyrin are reported.The corresponding copperand vanadyl derivatives ((TriP)Cu,[(hTriP)Cu]~+Br~- and [(hTriP)VO]~+Br~-) were also studied.Eachmetalloporphyrin was characterized by UV-visible,ESR and ~1H NMR spectroscopy.These me-talloporphyrins can be firmly adsorbed on the glassy carbon (GC) surface.The catalytic reduction ofdioxygen at GC electrodes modified by these catalysts was studied by cyclic voltammetry (CV).Thekinetic process of dioxygen reduction at the cobalt porphyrin-modified electrodes was studied with arotating ring disk electrode.     

Synthesis, purification, and characterization of phosphine oxides and their hydrogen peroxide adducts

Reactions of the tertiary phosphines R(3)P (R = Me, Bu, Oct, Cy, Ph) with 35% aqueous H(2)O(2) gives the corresponding oxides as the H(2)O(2) adducts R(3)P=O·(H(2)O(2))(x) (x = 0.5-1.0). Air oxidation leads to a mixture of products due to the insertion of oxygen into one or more P-C bonds. (31)P NMR spectroscopy in solution and in the solid state, as well as IR spectroscopy reveal distinct features of the phosphine oxides as compared to their H(2)O(2) adducts. The single crystal X-ray analyses of Bu(3)P=O and [Cy(3)P=O·(H(2)O(2))](2) show a P=O stacking motif for the phosphine oxide and a cyclic structure, in which the six oxygen atoms exhibit a chair conformation for the dimeric H(2)O(2) adduct. Different methods for the decomposition of the bound H(2)O(2) and the removal of the ensuing strongly adsorbed H(2)O are evaluated. Treating R(3)P=O·(H(2)O(2))(x) with molecular sieves destroys the bound H(2)O(2) safely under mild conditions (room temperature, toluene) within one hour and quantitatively removes the adsorbed H(2)O from the hygroscopic phosphine oxides within four hours. At 60 °C the entire decomposition/drying process is complete within one hour.     

Synthesis and structural characterization of cross-linked histidine-phenol Cu(ii) complexes as cytochrome c oxidase active site models

Tridentate cross-linked histidine-phenol Cu(ii) ether and ester complexes, chemical analogs of the active site of several heme-copper oxidases, have been synthesized and crystallized.     

Synthesis and characterization of two binuclear iron(III) complexes of aminoethanol derivatives of aminophenol as models for non-heme iron enzymes active sites

Two aminoethanol derivatives of aminophenol ligands were synthesized and characterized by IR and ¹H NMR spectroscopies. The binuclear iron(III) complexes of these ligands have been prepared and characterized by IR, ¹H NMR and UV-Vis spectroscopic techniques, cyclic voltammetry, single crystal X-ray diffraction and magnetic susceptibility studies. X-ray analysis revealed binuclear complexes, Fe₂(L₂), in which Fe(III) centers are surrounded by two phenolate and hydroxyl oxygen atoms, and amine nitrogens of the ligands. The metal active sites of both complexes are held together by the two above mentioned hydroxyl bridges. Variable temperature magnetic susceptibility indicates antiferromagnetic coupling between the iron centers of both complexes. This exchange coupling is stronger for Fe₂(L^ae)₂, such that it shows a room temperature strong coupling between the two iron centers. The investigated complexes undergo irreversible electrochemical oxidation and reduction.     

Formation and characterization of carbon monoxide adducts of iron "twin coronet" porphyrins. Extremely low CO affinity and a strong negative polar effect on bound CO

The carbon monoxide (CO) adducts of iron "twin coronet" porphyrins (TCPs) are characterized by UV-vis, resonance Raman (RR), IR, and 13C NMR spectroscopies. A superstructured porphyrin, designated as TCP, was used as a common framework for the four different types of iron complexes. TCP bears two binaphthalene bridges on each side and creates two hydrophobic pockets surrounded by the bulky aromatic rings. In the CO-binding cavities, the hydroxyl groups are oriented toward the center above the heme. The iron complexes investigated are as follows: TCP (which is without a covalently linked axial ligand), TCP-PY (which has a linked pyridine ligand), and TCP-TB and TCP-TG (both of which have a linked thiolate ligand). These complexes were synthesized as ferric forms and identified by the various spectroscopic methods. The UV-vis spectra of TCP-CO and TCP-PY-CO exhibit lambda(max) at 432, 546 and 428, 541 nm, respectively. On the other hand, the CO adducts of TCP-TB and TCP-TG show typical hyperporphyrin spectra for a thiolate-ligated iron(II) porphyrin-CO complex. In the RR spectra, the nu(Fe-CO) bands were observed at 506, 489 cm(-1) (TCP), 465 cm(-1) (TCP-PY), 458, 437 cm(-1) (TCP-TG) and 429 cm(-1) (TCP-TB). Compared with the reported nu(Fe-CO) frequencies of hemoproteins and their model systems, these observed values are unusually low. Further, abnormally high nu(C-O) bands are observed at 1990 cm(-1) (TCP-CO) and 2008 cm(-1) (TCP-PY-CO) in IR spectra. The lower nu(Fe-CO) and the higher nu(C-O) frequencies can be ascribed to the strong negative polar effect caused by the vicinal hydroxyl groups in the cavity. This prediction is further supported by the observation of significant 13C shieldings exhibited by TCP-CO (delta = 202.6 ppm) and TCP-PY-CO (delta = 202.3 ppm), in comparison to hemoproteins and other heme models. The CO affinity of TCP-PY (P1/2CO = 0.017 Torr at 25 C) is unusually lower than other heme models. The unique behavior of these CO adducts is discussed in context of the TCP structures.     

Synthesis and properties of binuclear nitride-bridged iron octaphenyltetraazaporphyrin. E.p.r. studies of dioxygen adduct formation

Summary The synthesis and characterization are reported for (-nitrido)bis[(octaphenyltetraazaporphyrinato)iron] ([OPTAP)Fe]₂N). The [(OPTAP)Fe]₂N dimer is paramagnetic with one unpaired electron per dimer unit and shows a typical axially symmetric e.p.r. spectrum in non-polar solvents at liquid N₂ temperature (g=2.126, g=2.001), with well-resolved superhyper-fine splitting resulting from the -nitrido bridge (A ^N =2.48mT, A ^N =2.60mT).A pyridine monoadduct of [(OPTAP)Fe]₂N, formed in frozen glasses has been characterised by changes in e.p.r. spectra. Electronic spectra show no pyridine adduct formation at 298 K. Mössbauer parameters (IS=–0.042 mm/s, QS=1.769mm/s at 298K) are indicative of the predominant iron(IV) character in [(OPTAP)Fe]₂N.The pyridine monoadduct binds O₂ reversibly. The O₂ adduct was characterized by e.p.r. spectroscopy (g₁=2.062, g₂=2.007, g₃=2.000). O₂ titration as monitored by e.p.r. shows that the adduct stoichiometry is one O₂ per dimer. A spin-pairing model for O₂ binding accounts for the observed O₂-[(OPTAP)Fe]₂N interaction.     

Formation of the active species of cytochrome p450 by using iodosylbenzene: a case for spin-selective reactivity

The generation of the active species for the enzyme cytochrome P450 by using the highly versatile oxygen surrogate iodosylbenzene (PhIO) often produces different results compared with the native route, in which the active species is generated through O(2) uptake and reduction by NADPH. One of these differences that is addressed here is the deuterium kinetic isotope effect (KIE) jump observed during N-dealkylation of N,N-dimethylaniline (DMA) by P450, when the reaction conditions change from the native to the PhIO route. The paper presents a theoretical analysis targeted to elucidate the mechanism of the reaction of PhIO with heme, to form the high-valent iron-oxo species Compound I (Cpd I), and define the origins of the KIE jump in the reaction of Cpd I with DMA. It is concluded that the likely origin of the KIE jump is the spin-selective chemistry of the enzyme cytochrome P450 under different preparation procedures. In the native route, the reaction proceeds via the doublet spin state of Cpd I and leads to a low KIE value. PhIO, however, diverts the reaction to the quartet spin state of Cpd I, which leads to the observed high KIE values. The KIE jump is reproduced here experimentally for the dealkylation of N,N-dimethyl-4-(methylthio)aniline, by using intra-molecular KIE measurements that avoid kinetic complexities. The effect of PhIO is compared with N,N-dimethylaniline-N-oxide (DMAO), which acts both as the oxygen donor and the substrate and leads to the same KIE values as the native route.     

Studies of tailed metalloporphyrins VI. Synthesis,characterization and catalysis of benzimidazole-linked iron(III) porphyrins

Transition Metal Chemistry -     

Unusual efficiency of a non-heme iron complex as catalyst for the hydroxylation of aromatic compounds by hydrogen peroxide: comparison with iron porphyrins

The non-heme iron complex, Fe(TPAA = tris-〚N-(2-pyridylmethyl)-2-aminoethyl〛amine)(ClO₄)₂, is a bad catalyst for the epoxidation of alkenes such as cyclooctene, cyclohexene and cis-stilbene and for the hydroxylation of alkanes such as adamantane by H₂O₂, when compared to the iron porphyrin Fe〚TDCPN₅P = meso-tetra-(2,6-dichlorophenyl)-β-pentanitroporphyrin〛Cl. At the opposite, Fe(TPAA)(ClO₄)₂ is a much better catalyst for the hydroxylation of arenes by H₂O₂; in its presence, anisole, toluene, ethylbenzene, benzene and chlorobenzene are transformed into the corresponding phenols, with respective yields of 53, 17, 24, 22 and 13% based on H₂O₂. Interestingly, in Fe(TPAA)-catalysed oxidations of anisole, toluene and ethylbenzene by H₂O₂, hydroxylation of the aromatic ring is by far the major reaction, even when compared to usually favoured reactions such as benzylic oxidation and oxidative demethylation.     

Synthesis and catalytic properties of a series of cobalt porphyrins as cytochrome P450 model: the effect of substituents on the catalytic activity

A series of cobalt porphyrins derived from hemin was prepared as cytochrome P450 models. Effects of substituents at the cobalt deuteroporphyrin-propionate side chains are investigated in oxidation of toluene with air to benzaldehyde and benzyl alcohol without the use of solvent and sacrificial co-reductant. The catalytic activity of cobalt porphyrins depends on the type of substituents. When the electron-withdrawing groups like –Cl, –Br, were introduced into the double propionate side chains, they can increase the catalyst stability and selectivity to benzaldehyde. In comparison with these electron-withdrawing groups, the electron-donor groups, such as –CH₃, –S–S– and –NH₂ groups, can improve their catalytic activities. Moreover, the electron-donor group containing an unpaired electron (such as –S–S–, –NH₂) is benefit for improving its catalytic efficiency and promoting the electron delivery. It can be concluded that the double propionate side chains in the deuteroporphyrin complex may participate in oxidation process and effect electron transfer from the high-valent metalloporphyrin species to the substrate.     

Pyrene.pyrene complexes at the active site of cytochrome P450 3A4: evidence for a multiple substrate binding site

Cytochrome P450 monooxygenases (CYPs) metabolize nearly all drugs and toxins. Recently, it has become clear that CYPs exhibit both homotropic and heterotropic allosteric kinetics for many substrates. However, the mechanism of cooperative kinetics has not been established for any specific human CYP/substrate combination. Suggested mechanisms include binding of multiple substrates within distinct, static, subsites of a single large active site or binding of multiple substrates within a single fluid active site. CYP3A4 hydroxylates pyrene with positive cooperativity. Therefore, experiments were designed to exploit the fluorescence properties of pyrene, which diagnostically distinguish between pyrene.pyrene complexes versus spatially separated pyrene substrates. Pyrene complexes (excimers) yield an emission spectrum clearly distinct from pyrene monomers. In lipid-free aqueous/glycerol solutions of CYP3A4, addition of pyrene affords a concentration-dependent low-spin to high-spin conversion of the CYP3A4 heme prosthetic group, indicating occupancy of the active site by pyrene. Under the same conditions, in the presence of CYP3A4 but not other heme proteins, the excimer/monomer ratio (E/M) of pyrene was decreased in emission spectra, compared to pyrene alone. However, excitation spectra indicate a CYP3A4-dependent increase in the wavelength shift for the excimer excitation spectrum versus the monomer excitation spectrum, as well as changes in the excimer excitation peak shape and vibronic structure. These changes are reversed by the CYP3A4 substrate testosterone. Together, the results demonstrate that pyrene.pyrene ground-state complexes occupy the CYP3A4 active site, and they provide the first spectroscopic evidence for substrate complexes within a single fluid active site. Functional implications include the possibility that turnover rate, regioselectivity, and stereoselectivity of the reaction are determined by the substrate.substrate complex rather than individual substrates.     

Syntheses of manganese and iron tetraspirobifluorene porphyrins as new catalysts for oxidation of alkenes by hydrogen peroxide and iodosylbenzene

The condensation of pyrrole with 9,9′-spirobifluorene aldehyde yields new the 5,10,15,20-tetra(spirobifluorene) porphyrins; epoxidation of cyclooctene and styrene derivatives catalyzed by the manganese and iron complexes is reported using H₂O₂ and PhIO as oxygen atom donors.     

Synthesis and structural characterization of nitrite-coordinating CoII and CoIII complexes as models for the reaction center of Co-substituted nitrite reductase

Co^II and Co^III complexes containing nitrite and tridentate aromatic amine compounds [bis(6-methyl-2-pyridylmethyl)amine (Me₂bpa) and bis(2-pyridylmethyl)amine (bpa)] have been prepared as models of the catalytic center in Co-substituted nitrite reductase: [Co^II(Me₂bpa)(NO₂)Cl]₂ · acetone (2), Co^II(Me₂bpa)(NO₂)₂ (3), Co^II(bpa)(NO₂)Cl (4), Co^II(bpa)(NO₂)₂ (5), Co^III(Me₂bpa)(NO₂)(CO₃) (6), and Co^III(bpa)(NO₂)₃ (7). The X-ray crystal structure analyses of these Co^II and Co^III complexes indicated that the geometries of the cobalt centers are distorted octahedral and the Me₂bpa and bpa with three nitrogen donors exhibit mer- (2, 3, and 7) and fac-form (4 and 6). The coordination mode of nitrite depends on the cobalt oxidation state, to Co^II through the oxygen (nitrito coordination, O- and O,O-coordination) and to Co^III through nitrogen (nitro coordination, N-coordination mode). These findings are consistent with the results of their IR spectra, except that another oxygen of the O-coordinated nitrito group in 3 might interact weakly with Co^II according to its IR spectrum. Reductions of the nitrite in 2, 3, 4, and 5 to nitrogen monoxide were not accelerated in the presence of proton, perhaps due to the nitrito coordination in these Co^II complexes.     

Synthesis and characterization of pyridine amide cation radical complexes of iron: stabilization due to coordination with low-spin iron(III) center

We reported the synthesis and characterization of peptide complexes of low-spin iron(III) [Fe(bpb)(py)2][ClO4] (1) and Na[Fe(bpb)(CN)2] (2) [H2bpb = 1,2-bis(pyridine-2-carboxamido)benzene; py = pyridine], where iron is coordinated to four nitrogen donors in the equatorial plane with two amide nitrogen anions and two pyridine nitrogen donors (Ray, M.; Mukherjee, R.; Richardson, J. F.; Buchanan, R. M. J. Chem. Soc., Dalton Trans. 1993, 2451). Chemical oxidation of 2 and a new low-spin iron(III) complex Na[Fe(Me6bpb)(CN)2].H2O (4) [synthesized from a new iron(III) complex [Fe(Me6bpb)(py)2][ClO4] (3) (S = 1/2)] [H2Me6bpb = 1,2-bis(3,5-dimethylpyridine2-carboxamido)-4,5-dimethylbenzene) by (NH4)2Ce(NO3)6 afforded isolation of two novel complexes [Fe(bpb)-(CN)2] (5) and [Fe(Me6bpb)(CN)2].H2O (6). All the complexes have been characterized by physicochemical techniques. While 1-4 are brown/green, 5 and 6 are violet/bluish violet. The collective evidence from infrared, electronic, M?ssbauer, and 1H NMR spectroscopies, from temperature-dependent magnetic susceptibility data, and from cyclic voltammetric studies provides unambiguous evidence that 5 and 6 are low-spin iron(III) ligand cation radical complexes rather than iron(IV) complexes. Cyclic voltammetric studies on isolated oxidized complexes 5 and 6 display identical behavior (a metal-centered reduction and a ligand-centered oxidation) to that observed for complexes 2 and 4, respectively. The M?ssbauer data for 6 are almost identical with those of the parent compound 4, providing compelling evidence that oxidation has occurred at the ligand in a site remote from the iron atom. Strong antiferromagnetic coupling (-2J > or = 450 cm(-1)) of the S = 1/2 iron atom with the S = 1/2 ligand pi-cation radical leads to an effectively S = 0 ground state of 5 and 6. The oxidized complexes display 1H NMR spectra (in CDCl3 solution), characteristic of diamagnetic species.