Electron spin resonance analysis of paramagnetic Pd(I) species in Pd(II)-exchanged H-rho zeolite

Zeolite rho was synthesized and Pd(II) exchanged into it. Pd(II) was reduced to paramagnetic Pd(1) by a thermal activation process. The interactions of Pd(I) in zeolite H-rho with oxygen, water, methanol, ammonia, carbon monoxide and ethylene have been studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. The ESR spectrum of an activated sample shows the formation of one Pd(I) species. Pd(I) interacts with water vapor or molecular oxygen to form Pd(II)–O₂, indicating decomposition of water. Equilibration with methanol results in a broad isotropic ESR signal which is attributed to the formation of small palladium clusters. ESEM shows that the Pd clusters coordinate one molecule of methanol. Adsorption of ammonia produces a Pd(I) complex containing four molecules of ammonia based upon resolved nitrogen superhyperfine coupling. Adsorption of carbon monoxide results in a Pd(I) complex containing two molecules of carbon monoxide based upon resolved¹³C superhyperfine coupling. ESR and ESEM results indicate that exposure to ethylene leads to two new Pd(I) species each of which coordinates one molecule of ethylene.     

Mechanistic studies on the binding of nitric oxide to a synthetic heme-thiolate complex relevant to cytochrome p450

The synthetic heme-thiolate complex (SR) in methanol binds nitric oxide (k(on) = (2.7 +/- 0.2) x10(6) M(-)(1) s(-)(1) at 25 degrees C) to form SR(NO). The binding of NO to the SR complex in a noncoordinating solvent, such as toluene, was found to be almost 3 orders of magnitude faster than that in methanol. The activation parameters DeltaH(), DeltaS(), and DeltaV() for the formation of SR(NO) in methanol are consistent with the operation of a limiting dissociative mechanism, dominated by dissociation of methanol in SR(MeOH). In the presence of an excess of NO, the formation of SR(NO) is followed by subsequent slower reactions. The substantially negative activation entropy and activation volume values found for the second observed reaction step support an associative mechanism which involves attack of a second NO molecule on the thiolate ligand in the initially formed SR(NO) complex. The following slower reactions are strongly accelerated by a large excess of NO or by the presence of NO(2)(-) in the SR/NO reaction mixture. They can be accounted for in terms of dynamic equilibria between higher nitrogen oxides (NO(x)()) and reactive SR species, which lead to the formation of a nitrosyl-nitrite complex of SR(Fe(II)) as the final product. This finding is clearly supported by laser flash photolysis studies on the SR/NO reaction mixture, which do not reveal simple NO photolabilization from SR(Fe(III))(NO), but rather involve the generation of at least three photoinduced intermediates decaying with different rate constants to the starting material. The species formed along the proposed reaction pathways were characterized by FTIR and EPR spectroscopy. The results are discussed in terms of their relevance for the biological function of cytochrome P450 enzymes and in context of results for the reaction of NO with imidazole- and thiolate-ligated iron(III) hemoproteins.     

Immobilization of heme complex by radiation polymerization

Transparent films containing an iron-porphyrin complex were synthesized by ⁶⁰Co γ-ray irradiation of aqueous solutions of ironporphyrin complex and a water soluble monomer such as 2-hydroxyethylmethacrylate or 1-vinyl-2-pyrrolidone. The iron(II)porphyrin complex was immobilized in the film by covalent bonding without any denaturation, under anaerobic condition or by protection of the ironporphyrin complex with carbon monoxide. After the irradiation of iron(III)porphyrin, the central iron ion was reduced spontaneously to the ferrous state. The films containing the iron(II)porphyrin adsorbed quantitatively carbon monoxide gas.     

Reactions of nitrogen oxides with heme models. Spectral and kinetic study of nitric oxide reactions with solid and solute Fe(III)(TPP)(NO3)

The reaction(s) of nitric oxide (nitrogen monoxide) gas with sublimed layers containing the nitrato iron(III) complex Fe(III)(TPP)(eta(2)-O(2)NO) (1, TPP = meso-tetraphenyl porphyrinate(2)(-)) leads to formation of several iron porphyrin species that are ligated by various nitrogen oxides. The eventual products of these low-temperature solid-state reactions are the nitrosyl complex Fe(TPP)(NO), the nitro-nitrosyl complex Fe(TPP)(NO(2))(NO), and 1 itself, and the relative final quantities of these were functions of the NO partial pressure. It is particularly notable that isotope labeling experiments show that the nitrato product is not simply unreacted 1 but is the result of a series of transformations taking place in the layered material. Thus, the nitrato complex formed from solid Fe(TPP)(eta(2)-O(2)NO) maintained under a (15)NO atmosphere was found to be the labeled analogue Fe(TPP)(eta(2)-O(2)(15)NO). The reactivities of the layered solids are compared to the behaviors of the same species in ambient temperature solutions. To interpret the reactions of the labeled nitrogen oxides, the potential exchange reactions between N(2)O(3) and (15)NO were examined, and complete isotope scrambling was observed between these species under the reaction conditions (T = 140 K). Overall it was concluded from isotope labeling experiments that the sequence of reactions is initiated by reaction of 1 with NO to give the nitrato nitrosyl complex Fe(TPP)(eta(1)-ONO(2))(NO) (2) as an intermediate. This is followed by a reaction in the presence of excess NO that is equivalent to the loss of the nitrate radical NO(3)(*)( )()to give Fe(TPP)(NO) as another transient species. A plausible pathway involving NO attack on the coordinated nitrate of 2 resulting in the release of N(2)O(4) concerted with electron transfer to the metal center is proposed.     

Reaction of an oxoiron(IV) complex with nitrogen monoxide: oxygen atom or oxide(•1-) ion transfer?

Reaction of [FeO(tmc)(OAc)](+) with the free radical nitrogen monoxide afforded a mixture of two Fe(II) complexes, [Fe(tmc)(OAc)](+) and [Fe(tmc)(ONO)](+) (where tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and AcO(-) = acetate anion). The amount of nitrite produced in this reaction (ca. 1 equiv with respect to Fe) was determined by ESI mass spectrometry after addition of (15)N-enriched NaNO(2). In contrast to oxygen atom transfer to PPh(3), the NO reaction of [FeO(tmc)(OAc)](+) proceeds through an Fe(III) intermediate that was identified by UV-vis-NIR spectroscopy and ESI mass spectrometry and whose decay is dependent on the concentration of methanol. The observations are consistent with a mechanism involving oxide(?1-) ion transfer from [FeO(tmc)(OAc)](+) to NO to form an Fe(III) complex and NO(2)(-), followed by reduction of the Fe(III) complex. Competitive binding of AcO(-) and NO(2)(-) to Fe(II) then leads to an equilibrium mixture of two Fe(II)(tmc) complexes. Evidence for the incorporation of oxygen from the oxoiron(IV) complex into NO(2)(-) was obtained from an (18)O-labeling experiment. The reported reaction serves as a synthetic example of the NO reactivity of biological oxoiron(IV) species, which has been proposed to have physiological functions such as inhibition of oxidative damage, enhancement of peroxidase activity, and NO scavenging.     

Thermodynamic and kinetic studies on the binding of nitric oxide to a new enzyme mimic of cytochrome p450

A new model for the P450 enzyme carrying a SO(3)(-) ligand coordinated to iron(III) (complex 2) reversibly binds NO to yield the nitrosyl adduct. The rate constant for NO binding to 2 in toluene is of the same order of magnitude as that found for the nitrosylation of the native, substrate-bound form of P450(cam) (E.S-P450(cam)). Large and negative activation entropy and activation volume values for the binding of NO to complex 2 support a mechanism that is dominated by bond formation with concomitant iron spin change from S = (5)/(2) to S = 0, as proposed for the reaction between NO and E.S-P450(cam). In contrast, the dissociation of NO from 2(NO) was found to be several orders of magnitude faster than the corresponding reaction for the E.S-P450(cam)/NO system. In a coordinating solvent such as methanol, the alcohol coordinates to iron(III) of 2 at the distal position, generating a six-coordinate, high-spin species 5. The reaction of NO with 5 in methanol was found to be much slower in comparison to the nitrosylation reaction of 2 in toluene. This behavior can be explained in terms of a mechanism in which methanol must be displaced during Fe-NO bond formation. The thermodynamic and kinetic data for NO binding to the new model complexes of P450 (2 and 5) are discussed in reference to earlier results obtained for closely related nitrosylation reactions of cytochrome P450(cam) (in the presence and in the absence of the substrate) and a thiolate-ligated iron(III) model complex.     

Reactions of nitrogen oxides with the five-coordinate Fe(III)(porphyrin) nitrito intermediate Fe(Por)(ONO) in sublimed solids

Detailed experimental studies are described for reactions of several nitrogen oxides with iron porphyrin models for heme/NxOy systems. It is shown by FTIR and optical spectroscopy and by isotope labeling experiments that reaction of small increments of NO2 with sublimed thin layers of the iron(II) complex Fe(Por) (Por = meso-tetraphenylporphyrinato dianion, TPP, or meso-tetra-p-tolylporphyrinato dianion, TTP) leads to formation of the 5-coordinate nitrito complexes Fe(Por)(eta1-ONO) (1), which are fairly stable but very slowly decompose under vacuum giving mostly the corresponding nitrosyl complexes Fe(Por)(NO). Further reaction of 1 with new NO2 increments leads to formation of the nitrato complex Fe(Por)(eta2-O2NO) (2). The interaction of NO with 1 at low temperature involves ligand addition to give the nitrito-nitrosyl complexes Fe(Por)(eta1-ONO)(NO) (3); however, these isomerize to the nitro-nitrosyl analogs Fe(Por)(eta1-NO2)(NO) (4) upon warming. Experiments with labeled nitrogen oxides argue for an intramolecular isomerization ("flipping") mechanism rather than one involving dissociation and rebinding of NO2. The Fe(III) centers in the 6-coordinate species 3 and 4 are low spin in contrast to 1, which appears to be high-spin, although DFT computations of the porphinato models Fe(P)(nitrite) suggest that the doublet nitro species and the quartet and sextet nitrito complexes are all relatively close in energy. The nitro-nitrosyl complex 4 is stable under an NO atmosphere but decomposes under intense pumping to give a mixture of the ferrous nitrosyl complex Fe(Por)(NO) and the ferric nitrito complex Fe(Por)(eta1-ONO) indicating the competitive dissociation of NO and NO2. Hence, loss of NO from 4 is accompanied with nitro --> nitrito isomerization consistent with 1 being the more stable of the 5-coordinate NO2 complexes of iron porphyrins.     

Simultaneous anodic stripping voltammetric determination of lead and cadmium with a carbon paste electrode modified by tributyl phosphate

A novel chemiluminescence (CL) flow sensor has been developed for the monitoring of iron(III). The analytical reagents involved in the CL reaction, including luminol and hexacyanoferrate(II) were both immobilized on an anion-exchange resin column. When sodium sulfate solution was passed through the column, these two reagents were eluted from the resins and then mixed with an iron(III) stream. By the fast reaction between iron(III) and hexacyanoferrate(II), the complex Prussian Blue was generated, which could catalyse the luminol oxidation by dissolved oxygen in alkaline aqueous solution to produce CL. The CL emission intensity was correlated with the standard iron(III) concentration in the range 0.01-Smgl^–1, and the detection limit was 7 × 10^–3mgl^–1 iron(III). A complete analysis, including sampling and washing, could be performed in l min with a relative standard deviation of less than 5%. The sensor was stable for over 200 times and has been applied successfully to the determination of iron in blood samples.     

Modification of zeolites usingin situ generated active species in the gas phase: II. Redox processes over the Mo5+ isolated sites in zeolites ZSM-5 and beta

The redox ability of isolated Mo⁵⁺ cations in model MoH-beta and MoH-ZSM-5 systems is studied byin situ ESR. The oxidation of reduced samples by nitrogen monoxide at 20°C proceeds much faster than oxidation by oxygen. The interaction between Mo⁵⁺ ions and propene at 20°C results in the formation of a complex in which the oxidizability of Mo(V) by NO is substantially enhanced. UV irradiation increases the oxidation rate for Mo⁵⁺ ions in HZSM-5 by nitrogen monoxide at 20°C, indicating the possibility for the photochemical activation of the process. The step of active site reduction during the interaction of the samples with the H₂ + NO + He gas mixtures of various compositions at 500°C is fast, and the dynamic equilibrium of the redox Mo(V) ai Mo(VI) reaction is shifted to the left Deceased.     

Computational investigation of the concerted dismutation of chlorite ion by water-soluble iron porphyrins

A detailed density functional theory examination of the reaction of an iron porphyrin chlorite dismutase model complex with chlorite was performed. We find that the molecular oxygen production observed occurs via the formation of η(1)-Fe(III) chlorite species, followed by the formation of O═Fe(IV) (compound II) and chlorine monoxide through homolytic bond cleavage. Chlorine monoxide then rebounds to form Fe(III)-peroxyhypochlorite followed by subsequent loss of chloride and loss of dioxygen accompanied by spin conversion to produce the Fe(III) complex and complete the catalytic cycle.     

O-alkenyl hydroxylamines: a new concept for cyclofunctionalization

[reaction: see text] Treatment of O-homoallylhydroxylamines with palladium(II) and copper(II) in the presence of a base, methanol, and carbon monoxide results in the formation of isooxazolidines. An electron-withdrawing group on the hydroxylamine nitrogen is essential. When carbamate groups are used the products are formed exclusively as their cis isomers.     

Effect of Ligand Fields on the Reactivity of O2‐Activating Iron(II)‐Benzilate Complexes of Neutral N5 Donor Ligands

Three new iron(II)‐benzilate complexes [(N4Py)Fe^II(benzilate)]ClO₄ ( 1 ), [(N4Py^Me2)Fe^II(benzilate)]ClO₄ ( 2 ) and [(N4Py^Me4)Fe^II(benzilate)]ClO₄ ( 3 ) of neutral pentadentate nitrogen donor ligands have been isolated and characterized to study their dioxygen reactivity. Single‐crystal X‐ray structures reveal a mononuclear six‐coordinate iron(II) center in each case, where benzilate binds to the iron center in monodentate mode via one carboxylate oxygen. Introduction of methyl groups in the 6‐positions of the pyridine rings makes the N4Py^Me2 and N4Py^Me4 ligand fields weaker compared to that of the parent N4Py ligand. All the complexes ( 1 – 3 ) react with dioxygen to decarboxylate the coordinated benzilate to benzophenone quantitatively. The decarboxylation is faster for the complex of the more sterically hindered ligand and follows the order 3 > 2 > 1 . The complexes display oxygen atom transfer reactivity to thioanisole and also exhibit hydrogen atom transfer reactions with substrates containing weak C?H bonds. Based on interception studies with external substrates, labelling experiments and Hammett analysis, a nucleophilic iron(II)‐hydroperoxo species is proposed to form upon two‐electron reductive activation of dioxygen by each iron(II)‐benzilate complex. The nucleophilic oxidants are converted to the corresponding electrophilic iron(IV)‐oxo oxidant upon treatment with a protic acid. The high‐spin iron(II)‐benzilate complex with the weakest ligand field results in the formation of a more reactive iron‐oxygen oxidant.     

NO* release from MbFe(II)NO and HbFe(II)NO after oxidation by peroxynitrite

In this work, we showed that the reaction of peroxynitrite with MbFe(II)NO, in analogy to the corresponding reaction with HbFe(II)NO (Herold, S. Inorg. Chem. 2004, 43, 3783-3785), proceeds in two steps via the formation of MbFe(III)NO, from which NO* dissociates to produce iron(III)myoglobin (Mb = myoglobin; Hb = hemoglobin). The second-order rate constants for the first steps are on the order of 10(4) and 10(3) M(-1) s(-1), for the reaction of peroxynitrite with MbFe(II)NO and HbFe(II)NO, respectively. For both proteins, we found that the values of the second-order rate constants increase with decreasing pH, an observation that suggests that HOONO is the species responsible for oxidation of the iron center. Nevertheless, it cannot be excluded that the pH-dependence arises from different conformations taken up by the proteins at different pH values. In the presence of 1.2 mM CO2, the values of the second-order rate constants are larger, on the order of 10(5) and 10(4) M(-1) s(-1), for the reaction of peroxynitrite with MbFe(II)NO and HbFe(II)NO, respectively. The pH-dependence of the values for the reaction with MbFe(II)NO suggests that ONOOCO2- or the radicals produced from its decay (CO3*-/NO2*) are responsible for the oxidation of MbFe(II)NO to MbFe(III)NO. In the presence of large amounts of nitrite (in the tens and hundreds of millimoles range), we observed a slight acceleration of the rate of oxidation of HbFe(II)NO by peroxynitrite. A catalytic rate constant of 40 +/- 2 M(-1) s(-1) was determined at pH 7.0. Preliminary studies of the reaction between nitrite and HbFe(II)NO showed that this compound also can oxidize the iron center, albeit at a significantly slower rate. At pH 7.0, we obtained an approximate second-order rate constant of 3 x 10(-3) M(-1) s(-1).     

THE KINETICS OF ELECTRON TRANSFER FOR A SERIES OF IRON(II) CYCLIDENE COMPLEXES

Abstract

The kinetics of oxidation of a series of iron(II) cyclidene complexes by tris(1,10-phenanthroline)cobalt(III) in methanol have been measured by stopped flow spectrophotometry. The reactions obey a first order rate law when the cobalt(III) complex is present in large excess. The corresponding second order rate constants fall in the range 5.0–130 × 10⁵ M^?1s^?1 (25°C). A linear correlation between the logarithm of the rate constant and the iron(III)/(II) redox potential indicates that the reactions behave as simple outer-sphere electron transfer processes. The self-exchange rate constants for the iron cyclidene complexes have been estimated from the Marcus equation and found to vary between 0.7 × 10⁷ M¹s¹ and 9 × 10⁷ M¹s¹. The dependence of the self-exchange rate constant on ligand structure is discussed.     

Mechanistic studies of nitric oxide reactions with water soluble iron(II), cobalt(II), and iron(III) porphyrin complexes in aqueous solutions: implications for biological activity.

The reactions of nitric oxide and carbon monoxide with water soluble iron and cobalt porphyrin complexes were investigated over the temperature range 298-318 K and the hydrostatic pressure range 0.1-250 MPa [porphyrin ligands: TPPS = tetra-meso-(4-sulfonatophenyl)porphinate and TMPS = tetra-meso-(sulfonatomesityl)porphinate]. Large and positive DeltaS(double dagger) and DeltaV(double dagger) values were observed for NO binding to and release from iron(III) complexes Fe(III)(TPPS) and Fe(III)(TMPS) consistent with a dissociative ligand exchange mechanism where the lability of coordinated water dominates the reactivity with NO. Small positive values for Delta and Delta for the fast reactions of NO with the iron(II) and cobalt(II) analogues (k(on) = 1.5 x 10(9) and 1.9 x 10(9) M(-1) s(-1) for Fe(II)(TPPS) and Co(II)(TPPS), respectively) indicate a mechanism dominated by diffusion processes in these cases. However, reaction of CO with the Fe(II) complexes (k(on) = 3.6 x 10(7) M(-1) s(-1) for Fe(II)(TPPS)) displays negative Delta and Delta values, consistent with a mechanism dominated by activation rather than diffusion terms. Measurements of NO dissociation rates from Fe(II)(TPPS)(NO) and Co(II)(TPPS)(NO) by trapping free NO gave k(off) values of 6.3 x 10(-4) s(-1) and 1.5 x 10(-4) s(-1). The respective M(II)(TPPS)(NO) formation constants calculated from k(on)/k(off) ratios were 2.4 x 10(12) and 1.3 x 10(13) M(-1), many orders of magnitude larger than that (1.1 x 10(3) M(-1)) for the reaction of Fe(III)(TPPS) with NO.     

Selective separations by reactive ion exchange: Part II. preconcentration and determination of complex iron cyanides in waters

Reactive ion exchange has been applied to the determination of p.p.b. concentrations of hexacyanoferrate(II) and hexacyanoferrate(III) in various water matrices. The in situ precipitation of copper hexacyanoferrate(II) or hexacyanoferrate(III) preconcentrates the complex cyanides on shallow beds of sulfonated cation-exchange resin in the copper(II) form. Hydrochloric acid reactively elutes other cations including concomitant iron species from the resin bed and, finally, aqueous ammonia reactively releases and elutes the hexacyanoferrate(II) (or III) species through the formation of the copper-ammine complex. Preconcentration factors of 100 or more are possible when 1-1 samples are used. Final determination of the complex cyanides is performed by atomic absorption spectrometry (for iron).     

Chelating ion-exchangers containing N-substituted hydroxylamine functional groups: Part I. N-aroylphenylhydroxylamines

Chelating ion exchangers containing N-carbonylphenylhydroxylamine functional groups have been synthesized and their exchange behaviour with copper, cobalt, iron, vanadium and uranium investigated. Of the two polymers described, a linear oxime-carbonyl polymer exhibited chelating capacity as a function of pH analogous to the chelates formed by BPHA. An oxime-carbonyl polymer based on polyethyleneimine had high capacities for the metal ions studied, but the principal mode of reaction was by electron donation from nitrogen atoms. The absence of co-ion in metal ion capacity studies indicates the possibility of formation of 1:2 and 1:3 metal complexes with the resin. Separations of iron(II)-iron(III) and vanadium-iron appear possible.     

Kinetic and mechanistic studies on the oxidation of hydroxylamine, semicarbazide, and thiosemicarbazide by iron(III) in the presence of triazines

In the presence of 3-(2-pyridyl)-5,6-bis(4-phenyl-sulphonicacid)-1,2,4-triazine disodium salt (PDTS), 3-(4-(4-phenylsulphonic-acid)-2-pyridyl)-5,6-bis(4-phenylsulphonic-acid)-1,2,4-triazine trisodium salt (PPDTS), or 2,4-bis(5,6-bis(4-phenylsulphonic-acid)-1,2,4-triazin-3-yl)pyridine tetra sodium salt (BDTPS), iron(III) oxidizes hydroxylamine to nitrogen gas, semicarbazide to CO₂ and NH₃ and thiosemicarbazide to a disulfide. The corresponding iron product is the 1:3 complex of iron(II) and PDTS, PPDTS, or BDTPS. The kinetics of these reactions was studied by monitoring the iron(II) product by conventional spectrophotometry. The reaction is first order in iron(III). Kinetic evidence was obtained for the formation of 1:1:2 ternary complexes of iron(III), substrate, and sulfonated triazine. Evidence for the ternary intermediate complexes was obtained by ion-exchange studies using ⁵⁹Fe-labeled iron(III) solutions. The dissociation of the ternary complex is identified as the rate-determining step.     

Ferrocytochrome c and deoxyhemoglobin in the reaction with the iron cysteamine nitrosyl complex {Fe2[S(CH2)2NH3]2(NO)4}SO4·2.5H2O

By an example of the iron cysteamine nitrosyl complex {Fe₂[S(CH₂)₂NH₃]₂(NO)₄}SO₄··2.5H₂O (CAC), it was shown for the first time that the hydrolysis of this NO donor in the presence of ferrocytochrome c (cyt c ²⁺) affords the iron nitrosyl complex NO-cyt c ²⁺, which serves as the NO depot. The rate constant of NO release from CAC was determined from the kinetics of the formation of NO-cyt c ²⁺. At pH 3.0 the rate constant is (2.7±0.1)·10⁻³ s⁻¹. Ferrocytochrome c produces a less stabilizing effect on CAC than deoxyhemoglobin (Hb). Thus in the presence of cyt c ²⁺, the reaction is completed in 1 h, whereas NO is released from a solution of CAC (2·10⁻⁴ mol L⁻¹) in the presence of Hb during 40 h. The previously unknown stabilization of iron nitrosyl complexes by hemoglobin was found.