Otaciro R. Nascimento

Catalytic activity of nitro- and carboxy-substituted iron porphyrins in hydrocarbon oxidation: Homogeneous solution and supported systems

Abstract A series of iron(III)porphyrins containing NO2-substituents in the meso-ortho-phenyl or COOH-substituents in the meso-para-phenyl rings (FeP) have been used to catalyse hydrocarbon oxidation by iodosylbenzene. The FeP series were efficient and selective catalysts for alkene epoxidation and alkane hydroxylation. The most promising iron porphyrin, 5,10,15-tri(2-nitrophenyl)20-mono-(4-carboxyphenyl)porphyrin iron(III) chloride, Fe(TNMCPP)Cl, was covalently bound to aminopropylated silica (APS) through covalent binding between COOH groups in the FeP (after activation by reaction with SOCl2) and NH2 groups on the funcionalised silica resulting in the anchored catalyst SiNH(TNMCPP)FeCl. This system proved to be a highly efficient catalyst for alkene epoxidation. The same iron porphyrin was also supported on APS through electrostatic binding, resulting the heterogeneous catalyst SiNH3+(TNMCPP)FeCl. The iron centres are in different surroundings on the two supports as demonstrated by EPR, UV/VIS and oxidation reactions results. The SiNH3+(TNMCPP)FeCl is not a good catalyst for hydrocarbon oxidation, which can be attributed to the higher polarity of this support and the bis-axial coordination by the free NH2 groups of the support with the iron centre. Both effects are unfavourable for the interaction between the non-polar substrates and the catalyst.

Factors which affect the catalytic activity of iron(III) meso tetrakis(2,6-dichlorophenyl) porphyrin chloride in homogeneous system

An optimization study of the reaction conditions of Fe(TDCPP)Cl when it is used as catalyst in the hydroxylation of cyclohexane by iodosylbenzene (PhIO) has been carried out. It was found that Fe(TDCPP)Cl follows the classical PhIO mechanism described for Fe(TPP)Cl, which involves the monomeric active species FeIV(O)P+ (I). In the optimized condition ([Fe(TDCPP) = 3.0 × 10−4 mol l−1 in 1,2-dichloroethane (DCE); ultrasound stirring at 0°C; PhIOFeP molar ratio = 100), this FeP led to a yield of cyclohexanol (C-ol) of 96% and a turnover number of 96. Therefore, Fe(TDCPP)Cl may be considered a good biomimetic model and a very stable, resistant and selective catalyst, which yields C-ol as the sole product. DCE showed to be a better solvent than dichloromethane (DCM), 1 DCE:1 MeOH mixture or acetonitrile (ACN). Since the FeIV(O)P+ is capable of abstracting hydrogen atom from DCM, MeOH or ACN, the solvent competes with the substrate. Presence of O2 lowers the yield of C-ol, as it can further oxidize this alcohol to carboxylic acid in the presence of radicals. Presence of H2O also causes a decrease in the yield, since it converts the active species I into FeIV(OH)P, which cannot oxidize cyclohexane. Addition of excess imidazole or OH− to the system results in a decrease in the yield of C-ol, due to the formation of the hexaccordinated complexes Fe(TDCPP)Im+2 (low-spin, β2 = 2.5 × 108 mol−2 l2) and Fe(TDCPP)(OH)−2 (high-spin, β2 = 6.3 × 107 mol−2 l2). The formation of both Fe(TDCPP)Im+2 and Fe(TDCPP)(OH)−2 complexes were confirmed by EPR studies. The catalytic activities of Fe(TDCPP)Cl and Fe(TFPP)Cl were compared. The unusually high yields of C-ol with Fe(TFPP)Cl obtained when ultrasound, DCM and O2 atmosphere were used, suggest that a parallel mechanism involving the μ-oxo dimer form, O2 and radicals may also be occurring with this FeP, besides the PhIO mechanism.

Supported iron(III)porphyrins pentafluorophenyl-derivatives as catalysts in epoxidation reactions by H2O2: the role of the silica-support and sulfonatophenyl residues in the activation of the peroxidic bond

Abstract Ionic Fe(III)porphyrins pentafluorophenyl-derivatives: 5,10,15-tris(pentafluorophenyl)-20-(4- N -methylpyridyl)porphyrin iron(III) ([Fe{M(4- N -MePy)TPFP}] 2+ ), 5,10,15-tris(4- N -methylpyridyl)-20-(pentafluorophenyl)porphyrin iron(III) ([Fe{T(4- N -MePy)MPFP}] 4+ ) and 5,10,15-tris(pentafluorophenyl)-20-(3-sulfonatophenyl)porphyrin iron(III) ([Fe{M(3-SO 3 P)TPFP}]) immobilised on silica gel (SiO 2 ) and modified silica with sulfonatophenyl (SiSO 3 − ), propylimidazole (IPG), sulfonatophenyl plus propylimidazole (SiSO 3 − (IPG)), propyltrimethylammonium (SiN + ) and propyltrimethylammonium plus propylimidazole (SiN + (IPG)) were studied in the epoxidation of ( Z )-cyclooctene by H 2 O 2 . The better yields of product, cis -epoxycyclooctane, were achieved with the catalytic systems, [Fe{M(4- N -MePy)TPFP}]-SiSO 3 (IPG), [Fe{M(3-SO 3 P)TPFP}]-IPG and [Fe{M(3-SO 3 P)TPFP}]-SiO 2 , where is more probable to have available sulfonatophenyl groups. That is, where there is no ionic binding Fe(III)porphyrin-support via sulfonatophenyl. A charge effect supplied by the available sulfonatophenyl residues and a more polar microenvironment provided by the silica-support promote the heterolytic cleavage of the OO bond, which is essential to obtain high yields of products in oxidation reactions. Effects of general acid–base catalysis, as occurs in enzymes, and higher reduction potential provided by electron-withdrawing groups, present on porphyrin ring or axial ligand, as recently reported by Nam and coworkers for homogeneous Fe(III)porphyrins, to promote the heterolysis of the OO bond are ruled out.

Charge- and pH-dependent binding sites for dibucaine in ionic micelles: a fluorescence study.

Binding of micromolar concentrations of the local anesthetic dibucaine to micelles of cationic, zwitterionic and anionic detergents was studied using the fluorescence emission of dibucaine. Difference in quantum yields for charged and neutral dibucaine allowed to obtain shifts of pKa values due to binding. Estimates for the electrostatic potential affecting the tertiary amine of dibucaine were obtained from the pKa shifts. Change of fluorescence emission upon binding allowed to obtain the binding constants of both charged and neutral dibucaine to the micelles. The binding constant for the neutral form is essentially independent of micelle charge and of specific differences in detergent structure. Consistency between the ratio of neutral to cationic dibucaine binding constants and the measured pKa shift was tested. For LPC micelles complete agreement was found. For CTAC, however, the ratio of binding constants does not explain the pKa shift. The discrepancy between the results is used to estimate the errors involved upon neglecting non-coulombic electrostatic interactions of drugs to charged membrane surfaces. Fluorescence quenching with sodium iodide and nitroxide stearic acid derivatives allowed a depth profiling of the drug in the micelles.

Modulation of cytochrome c spin states by lipid acyl chains: a continuous- wave electron paramagnetic resonance (CW-EPR) study of haem iron

This work is a systematic study, showing a clear correlation between the nature of the lipid acyl chain and the spin states of cytochrome c interacting with different types of lipid membranes. According to the lipid acyl chain type, and the head group charge present in the bilayer, three spin states of cytochrome c were observed in different proportions: the native cytochrome c low spin state with rhombic symmetry (spin 1/2, g axially=3.07 and g radially=2.23), a low spin state with less rhombic symmetry (spin 1/2, g(1)=2.902, g(2)=2.225, and g(3)=1.510) and the high spin state (spin 5/2, g axially=6.0 and g radially=2.0). The proportion of the spin states of cytochrome c bound to bilayers was also dependent on the lipid/protein ratio, suggesting the existence of two or more protein sites interacting with the lipids. The lipid-induced alterations in the symmetry and spin states of cytochrome c exhibited partial reversibility when the ionic strength was increased, which reinforces the crucial role played by the electrostatic interaction with the lipid bilayer. Different cytochrome c spin states exhibited corresponding modifications in the haemprotein UV/visible spectra, particularly in the Q-band associated with loss of the 695 nm band and appearance of a band in the region of 600-650 nm. The observed reactivity of cytochrome c with oxidized forms of unsaturated lipids reinforces the possibility of the acyl chain insertion in the haemprotein structure.

Meso-aryl substituted metalloporphyrins supported on imidazole propyl gel (IPG). Catalytic activity in the oxidation of cyclohexane and characterization of iron porphyrin—IPG systems

Abstract In this work, the catalytic activity of Fe and Mn porphyrins with meso-aryl electron withdrawing substituents in the hydroxylation of cyclohexane using iodosylbenzene as the oxygen donor were compared. The catalysts were used in three ways: in solution, supported on imidazole propyl gel (IPG) and supported on silica gel (SG). Mn-porphyrin-IPG systems are better catalysts: Mn(TDCPP)-IPG gives 67% of cyclohexanol yield, while the corresponding solution system gives 17%. With the Fe(TDCPP)-IPG system, there is a decrease in the catalytic activity (25%), when compared to the solution system. It was possible to understand, through UV-Vis and EPR spectra characterization, that an excess of imidazole in the iron (III) porphyrin-IPG systems promotes partial catalyst deactivation due to the bis-imidazolyl coordination and reduction to iron(II). If the imidazole/ Fe(TDCPP) ratio decreases in the Fe(TDCPP)-IPG, the yield increases to 60%. It was possible to recycle Mn(TDCPP)-IPG, Fe(TDCPP)-IPG and Fe(TFPP)IPG up to 5 times without leaching of the supported catalyst, which results in very efficient systems with high turnovers.

Spectroscopic, structural, and functional characterization of the alternative low-spin state of horse heart cytochrome C.

The alternative low-spin states of Fe(3+) and Fe(2+) cytochrome c induced by SDS or AOT/hexane reverse micelles exhibited the heme group in a less rhombic symmetry and were characterized by electron paramagnetic resonance, UV-visible, CD, magnetic CD, fluorescence, and Raman resonance. Consistent with the replacement of Met(80) by another strong field ligand at the sixth heme iron coordination position, Fe(3+) ALSScytc exhibited 1-nm Soret band blue shift and epsilon enhancement accompanied by disappearance of the 695-nm charge transfer band. The Raman resonance, CD, and magnetic CD spectra of Fe(3+) and Fe(2+) ALSScytc exhibited significant changes suggestive of alterations in the heme iron microenvironment and conformation and should not be assigned to unfold because the Trp(59) fluorescence remained quenched by the neighboring heme group. ALSScytc was obtained with His(33) and His(26) carboxyethoxylated horse cytochrome c and with tuna cytochrome c (His(33) replaced by Asn) pointing out Lys(79) as the probable heme iron ligand. Fe(3+) ALSScytc retained the capacity to cleave tert-butylhydroperoxide and to be reduced by dithiothreitol and diphenylacetaldehyde but not by ascorbate. Compatible with a more open heme crevice, ALSScytc exhibited a redox potential approximately 200 mV lower than the wild-type protein (+220 mV) and was more susceptible to the attack of free radicals.

Characterization and catalytic activity of iron(III) mono(4-n-methyl pyridyl)-tris(halophenyl) porphyrins in homogeneous and heterogeneous systems

Abstract The synthesis, characterization and catalytic activity of the cationic iron porphyrins Fe[M(4-N-MePy)TDCPP]Cl2 and Fe[M(4-N-MePy)TFPP]Cl2 in the epoxidation of (Z)-cyclooctene by PhIO in homogeneous solution and supported on silica gel (SG), imidazole propyl gel (IPG) or SG modified with 2-(4-sulfonatophenyl)ethyl groups (SiSO3) have been accomplished. When supported on IPG, both cationic FeP bind to the support via Fe–imidazole coordination. Fe[M(4-N-MePy)TDCPP]IPG contains a mixture of low-spin bis-coordinated FeIIIP and high-spin mono-coordinated FeIIIP species, whereas Fe[M(4-N-MePy)TFPP]IPG only contains high-spin mono-coordinated FeIIIP. These FePIPG catalysts also contain FeIIP species, whose presence was confirmed by EPR spectroscopy using NO as a paramagnetic probe. Both cationic FePs coordinate to SG through Fe–O ligation and they are present as high-spin FeIIIP species. The cationic FePs supported on SiSO3− are also high-spin FeIIIP species and they bind to the support via electrostatic interaction between the 4-N-methylpyridyl groups and the SO3− groups present on the matrix. In homogeneous solution, both Fe[M(4-N-MePy)TDCPP]Cl2 and Fe[M(4-N-MePy)TFPP]Cl2 have similar catalytic activity to Fe(TDCPP)Cl and Fe(TFPP)Cl, leading to cis-epoxycyclooctane yields of 92%. When supported on inorganic matrices, both FePs lead to epoxide yields comparable to their homogeneous analogues and their anchoring enables catalyst recovery and re-use. Recycling of Fe[M(4-N-MePy)TDCPP]SiSO3− shows that this FeP maintains its activity in a second reaction.

Manganese(III) porphyrins: catalytic activity and intermediate studies in homogeneous systems

Abstract We studied how solvent, stirring method, PhIO/MnP molar ratio, presence of water and axial ligand affect the catalytic activities of Mn(TPP)Cl, Mn(MNPP)Cl, Mn(TDCPP)Cl and Mn(TFPP)Cl in the oxidation of cyclohexane by PhIO. A study of the catalytic intermediates in the reaction between Mn(TPP)Cl or Mn(TDCPP)Cl and PhIO was also carried out by UV–Vis and EPR spectroscopies. The reaction of Mn(TPP)Cl with PhIO showed the formation of a mixture of species MnIV(O)P·+ and MnV(O)P as intermediates, which were confirmed by the deconvolution of the UV–Vis spectra. Addition of imidazole as cocatalyst favoured the formation of the intermediate species MnV(O)P, evidenced by the UV–Vis band at 408 nm. The corresponding EPR spectra gave evidence that in the presence of imidazole, MnIV(O)P·+ species are formed only in very low amounts. For Mn(TDCPP)Cl the dominating intermediate species is MnIV(O)P·+. Addition of imidazole to halogen-substituted MnP systems does not result in increase of the C-ol yields because very stable bis-imidazole-MnP complexes are formed. Anchoring of such MnP on imidazole propyl gel (IPG) results in better catalytic activity because in this case, the catalyst is mono-coordinated to the support and imidazole favours the formation of the intermediate species MnV(O)P.

EPR OF MICRONUTRIENTS-HUMIC SUBSTANCES COMPLEXES EXTRACTED FROM A BRAZILIAN SOIL

The technique of electron paramagnetic resonance was used to examine the formation of micronutrients-humic substances complexes extracted from a Brazilian soil. Results indicate that there are two types of copper (II) complexes, one with oxygen and another with nitrogen atoms, in humic acids while in fulvic acids only the copper complexed with oxygen was detected. The iron (III) complex signal with g = 4.3 was observed in both humic acid and fulvic acid. The concentration of free radicals (semiquinone) detected in humic acids (1015 spins/g) was bigger than in fulvic acids (1014 spins/g) due to a greater humification degree confirmed by elementary analysis and E4/E6 ratio measurements.