Synthesis and characterization of µ-nitrido, µ-carbido and µ-oxo dimers of iron octapropylporphyrazine.

Three µ-X bridged diiron octapropylporphyrazine complexes having Fe(III)-O-Fe(III), Fe(+3.5)-N[double bond, length as m-dash]Fe(+3.5) and Fe(IV)[double bond, length as m-dash]C[double bond, length as m-dash]Fe(IV) structural units have been prepared and characterized by UV-vis, EPR, X-ray absorption spectroscopy and electrochemical methods. Single crystals of all the complexes were obtained from benzene-acetonitrile and their structures were determined by X-ray diffraction. In contrast to µ-oxo complex (), µ-nitrido () and µ-carbido () dimers crystallized with one benzene molecule per two binuclear complex molecules arranged cofacially to the porphyrazine planes at Fe-Cbenzene distances of 3.435-3.725 Å and 3.352-3.669 Å for and , respectively. The short distances suggest an interaction between the iron sites and the benzene π-system which is stronger in the case of the Fe(IV)[double bond, length as m-dash]C[double bond, length as m-dash]Fe(IV) unit with a higher Lewis acidity. The Fe-X-Fe angle increases in the sequence -- from 158.52° to 168.5° and 175.10°, respectively, in agreement with the Fe-X bond order. However, the lengths of the Fe-X bonds do not follow this trend: Fe-O = 1.75/1.76 Å > Fe-C = 1.67/1.67 Å > Fe-N = 1.65/1.66 Å indicating unexpectedly long Fe-C bonds. This observation can be explained by back π-donation from the µ-carbido ligand to the Fe-C antibonding orbital thus decreasing the bond order which is confirmed by DFT calculations.

Efficient epoxidation of olefins by H2O2 catalyzed by iron "helmet" phthalocyanines.

High yields of epoxides were obtained in the oxidation of a large range of olefins using 1.2-2 equiv. of H2O2 in the presence of iron helmet phthalocyanines. The involvement of high-valent iron oxo species was evidenced using cryospray mass spectrometry.

An N-bridged high-valent diiron-oxo species on a porphyrin platform that can oxidize methane.

High-valent oxo-metal complexes are involved in key biochemical processes of selective oxidation and removal of xenobiotics. The catalytic properties of cytochrome P-450 and soluble methane monooxygenase enzymes are associated with oxo species on mononuclear iron haem and diiron non-haem platforms, respectively. Bio-inspired chemical systems that can reproduce the fascinating ability of these enzymes to oxidize the strongest C-H bonds are the focus of intense scrutiny. In this context, the development of highly oxidizing diiron macrocyclic catalysts requires a structural determination of the elusive active species and elucidation of the reaction mechanism. Here we report the preparation of an Fe(IV)(µ-nitrido)Fe(IV) = O tetraphenylporphyrin cation radical species at -90 °C, characterized by ultraviolet-visible, electron paramagnetic resonance and Mössbauer spectroscopies and by electrospray ionization mass spectrometry. This species exhibits a very high activity for oxygen-atom transfer towards alkanes, including methane. These findings provide a foundation on which to develop efficient and clean oxidation processes, in particular transformations of the strongest C-H bonds.

Generation and characterization of high-valent iron oxo phthalocyanines.

The first high-valent iron oxo complex on the phthalocyanine platform has been prepared from iron tetra-tert-butyl-phthalocyanine and m-chloroperbenzoic acid and characterized by low temperature UV-vis, cryospray MS, EPR, X-ray absorption and high resolution X-ray emission methods.

High-valent diiron species generated from N-bridged diiron phthalocyanine and H(2)O(2).

N-bridged diiron tetra-tert-butylphthalocyanine activates H(2)O(2) to form anionic hydroperoxo complex [(Pc)Fe(IV)=N-Fe(III)(Pc)-OOH](-) prone to heterolytic cleavage of O-O bond with the release of OH(-) and formation of neutral diiron oxo phthalocyanine cation radical complex, PcFe(IV)=N-Fe(IV)(Pc(+)˙)=O. ESI-MS data showed stability of the Fe-N-Fe binuclear structure upon formation of this species, capable of oxidizing methane and benzene via O-atom transfer. The slow formation kinetics and the high reactivity preclude direct detection of this oxo complex by low temperature UV-vis spectroscopy. However, strong oxidizing properties and the results of EPR study support the formation of PcFe(IV)=N-Fe(IV)(Pc(+)˙)=O. Addition of H(2)O(2) at -80 °C led to the disappearance of iron EPR signal and to the appearance of the narrow signal at g = 2.001 consistent with the transient formation of PcFe(IV)=N-Fe(IV)(Pc(+)˙)=O. In the course of this study, another high valent diiron species was prepared in the solid state with 70% yield. The Mössbauer spectrum shows two quadrupole doublets with δ(1) = -0.14 mm s(-1), ΔE(Q1) = 1.57 mm s(-1) and δ(2) = -0.10 mm s(-1), ΔE(Q2) = 2.03 mm s(-1), respectively. The negative δ values are consistent with formation of Fe(iv) states. Fe K-edge EXAFS spectroscopy reveals conservation of the diiron Fe-N-Fe core. In XANES, an intense 1s → 3d pre-edge feature at 7114.4 eV suggests formation of Fe(iv) species and attaching of one oxygen atom per two Fe atoms at the 1.90 Å distance. On the basis of Mössbauer, EPR, EXAFS and XANES data this species was tentatively assigned as (Pc)Fe(IV)=N-Fe(IV)(Pc)-OH which could be formed from PcFe(IV)=N-Fe(IV)(Pc(+)˙)=O by hydrogen atom abstraction from a solvent molecule. Thus, despite unfavourable kinetics, we succeeded in the preparation of the first dirion(iv) phthalocyanine complex with oxygen ligand, generated in the (Pc)Fe(IV)=N-Fe(III)(Pc) - H(2)O(2) system capable of oxidizing methane.

Synergistic effects of encapsulated phthalocyanine complexes in MIL-101 for the selective aerobic oxidation of tetralin.

Metal phthalocyanine complexes encapsulated in MIL-101, and used as "ship-in-a-bottle" catalysts, show outstanding TONs in the aerobic oxidation of tetralin.

Kinetics and mechanism of the Co(II)-assisted oxidation of L-ascorbic acid by dioxygen and nitrite in aqueous solution.

A detailed study of the oxidation of L-ascorbic acid by dioxygen and nitrite in water at pH 5.8 and 7.0, catalyzed by the octasulfophenyltetrapyrazinoporphyrazine complex of cobalt(II), was carried out using conventional spectrophotometric, low-temperature and high-pressure stopped-flow techniques. The Co(II) complex activates L-ascorbic acid through an intramolecular one-electron oxidation step that involves the reduction of the octasulfophenyltetrapyrazinoporphyrazine. The reaction rate strongly depends on pH due to the different redox behaviour of the L-ascorbic acid/ascorbate species present in solution. Kinetic parameters for the different reaction steps of the catalytic process were determined. The final product of the reaction between L-ascorbic acid and nitrite was found to be nitrous oxide.

Stable N-bridged diiron (IV) phthalocyanine cation radical complexes: synthesis and properties.

Oxidation of N-bridged diiron tetra-tert-butylphthalocyanine (FePc(t)Bu)(2)N with (t)BuOOH in halogenated solvents afforded mu-nitrido diiron (IV) phthalocyanine cation radical complexes, (X)PcFe(IV)-N-Fe(IV)Pc(+) (X) (X = Cl, Br), as evidenced by UV-vis, electron paramagnetic resonance (EPR), electrospray ionization mass spectrometry (ESI-MS), Mössbauer, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data. Continuous introduction ESI-MS showed the initial formation of the t-butylperoxo diiron complex (t)BuOO-(FePc(t)Bu)(2)N followed by intermediate formation of a monohalogenated adduct which gave the dihalogenated complex as final stable product. Fe K-edge EXAFS analysis of (X)PcFe(IV)-N-Fe(IV)Pc(+) (X) showed bridging N atom at a distance of 1.69(1) A. The distance between two equivalent hexacoordinated in-plane iron (IV) centers is 3.39(1) A. Fe-X bond lengths are 2.33 and 2.54 A for X = Cl and Br, respectively. The use of (Cl)PcFe(IV)-N-Fe(IV)Pc(+) (Cl) for the catalytic oxidation of organic substrates has been shown for the first time.

Preparation and characterization of mu-nitrido diiron phthalocyanines with electron-withdrawing substituents: application for catalytic aromatic oxidation.

Mu-nitrido-bis [tetra-(hexyl-sulfonyl)phthalocyaninatoiron] (3a) and mu-nitrido-bis [tetra-(tert-butylsulfonyl) phthalocyaninatoiron] (3b) complexes have been prepared and fully characterized by electrospray ionization mass spectrometry, UV-Vis, FTIR, EPR, Mössbauer techniques as well as by X-ray photoelectron and Fe K-edge X-ray absorption spectroscopies. Small changes at the periphery of the phthalocyanine ligand introduce a difference in the iron oxidation state. While 3b with tert-butyl substituents is a neutral complex with a mixed-valence Fe(3.5)-N-Fe(3.5) structural unit, 3a having n-hexyl substituents is an oxidized cationic Fe(IV)-N-Fe(IV) complex. The structural parameters of N-bridged diiron phthalocyanine with a Fe(3.5)-N-Fe(3.5) unit were determined for the first time. Iron atoms in 3b are displaced out of plane by 0.24 A and the Fe-N bond distance of the linear Fe-N-Fe fragment is equal to 1.67 A. Both complexes selectively catalyze benzylic oxidation of alkyl aromatic compounds by tBuOOH. Toluene was oxidized to benzoic acid with 80% selectivity, and the total turnover number was as high as 197. p-Toluic acid was the principal product of p-xylene oxidation. In this case the turnover number achieved 587 substrate molecules per molecule of catalyst. The described catalytic system is complementary to the recently reported system based on mu-nitrido diiron tetrabutylphthalocyanine-H2O2 which effectively oxidizes the benzene ring.

Bio-inspired oxidation of methane in water catalyzed by N-bridged diiron phthalocyanine complex.

A stable mu-nitrido diiron phthalocyanine activates H2O2 to oxidize CH4 in water at 25-60 degrees C to methanol, formaldehyde and formic acid as evidenced using 13C and 18O labelling.

Novel iron(III) porphyrazine complex. Complex speciation and reactions with NO and H2O2.

The complex [iron(III) (octaphenylsulfonato)porphyrazine] (5-), Fe (III)(Pz), was synthesized. The p K a values of the axially coordinated water molecules were determined spectrophotometrically and found to be p K a 1 = 7.50 +/- 0.02 and p K a 2 = 11.16 +/- 0.06. The water exchange reaction studied by (17)O NMR as a function of the pH was fast at pH = 1, k ex = (9.8 +/- 0.6) x 10 (6) s (-1) at 25 degrees C, and too fast to be measured at pH = 10, whereas at pH = 13, no water exchange reaction occurred. The equilibrium between mono- and diaqua Fe (III)(Pz) complexes was studied at acidic pH as a function of the temperature and pressure. Complex-formation equilibria with different nucleophiles (Br (-) and pyrazole) were studied in order to distinguish between a five- (in the case of Br (-)) or six-coordinate (in the case of pyrazole) iron(III) center. The kinetics of the reaction of Fe (III)(Pz) with NO was studied as a model ligand substitution reaction at various pH values. The mechanism observed is analogous to the one observed for iron(III) porphyrins and follows an I d mechanism. The product is (Pz)Fe (II)NO (+), and subsequent reductive nitrosylation usually takes place when other nucleophiles like OH (-) or buffer ions are present in solution. Fe (III)(Pz) also activates hydrogen peroxide. Kinetic data for the direct reaction of hydrogen peroxide with the complex clearly indicate the occurrence of more than one reaction step. Kinetic data for the catalytic decomposition of the dye Orange II by H 2O 2 in the presence of Fe (III)(Pz) imply that a catalytic oxidation cycle is initiated. The peroxide molecule first coordinates to the iron(III) center to produce the active catalytic species, which immediately oxidizes the substrate. The influence of the catalyst, oxidant, and substrate concentrations on the reaction rate was studied in detail as a function of the pH. The rate increases with increasing catalyst and peroxide concentrations but decreases with increasing substrate concentration. At low pH, the oxidation of the substrate is not complete because of catalyst decomposition. The observed kinetic traces at pH = 10 and 12 for the catalytic cycle could be simulated on the basis of the obtained kinetic data and the proposed reaction cycle. The experimental results are in good agreement with the simulated ones.

Kinetics and mechanism of the iron phthalocyanine catalyzed reduction of nitrite by dithionite and sulfoxylate in aqueous solution.

The reactions of sodium nitrite with sodium dithionite and sulfoxylate ion were studied in the presence of iron(III) tetrasulfophthalocyanine, Fe(III)(TSPc)3-, in aqueous alkaline solution. Kinetic parameters for the different reaction steps in the catalytic reduction by dithionite were determined. The final product of the reaction was found to be nitrous oxide. Contrary to this, the product of the catalytic reduction of nitrite by sulfoxylate was found to be ammonia. The striking difference in the reaction products is accounted for in terms of different structures of the intermediate complexes formed during the reduction by dithionite and sulfoxylate, in which nitrite is suggested to coordinate to the iron complex via nitrogen and oxygen, respectively. Sulfoxylate is shown to be a convenient reductant for the synthesis of the highly reduced iron phthalocyanine species Fe(I)(TSPc*)6- in aqueous solution. The kinetics of the reduction of Fe(I)(TSPc)5- to Fe(I)(TSPc*)6-, as well as the oxidation of the latter species by nitrite, was studied in detail.

Kinetics and mechanism of the Co(II)-assisted oxidation of thioureas by dioxygen.

Catalytic oxidation of N,N'-dimethylthiourea and thiourea by dioxygen in water using a new cobalt(II) complex of octasulfophenyltetrapyrazinoporphyrazine was performed under mild conditions. The reaction is shown to include the formation of an intermediate anionic five-coordinate complex followed by an unusual two-electron oxidation to produce the corresponding urea and elemental sulfur (S8). Kinetic and thermodynamic parameters for the different reaction steps of the process were determined. Drastic differences in catalytic activity of cobalt and iron octasulfophenyltetrapyrazinoporphyrazines were observed.

Kinetics and mechanism of water substitution in the low-spin Fe(II) complex of 4-octasulfophenylpyrazinoporphyrazine.

The substitution reaction of the axial-coordinated water by pyridine, pyrazine and 4-CN-pyridine in the low-spin Fe(II) complex of octasulfophenyltetrapyrazinoporphyrazine was studied. Kinetic and thermodynamic parameters for the different reaction steps of the process were determined. On the basis of NMR data and spectrophotometric titrations, a pronounced non-equivalence of the two coordinated N-donor ligands was observed. The substitution of water by pyridine and 4-CN-pyridine is shown to include the formation of a precursor outer-sphere complex, whereas substitution by pyrazine follows a limiting dissociative mechanism.

Kinetics and mechanism of the cobalt phthalocyanine catalyzed reduction of nitrite and nitrate by dithionite in aqueous solution.

The reaction of sodium nitrite with sodium dithionite was studied in the presence of cobalt(II) tetrasulfophthalocyanine, COII(TSPc)4-, in aqueous alkaline solution. The overall mechanism comprises the reduction of CoII(TSPc)4- by dithionite, followed by the formation of an intermediate complex between COI(TSPc)5- and nitrite, which undergoes two parallel subsequent reactions with and without nitrite as a reagent. Kinetic parameters for the different reaction steps of the catalytic process were determined. The final product of the reaction was found to be ammonia. Contrary to those found for the catalytic reduction of nitrite, the products of the catalytic reduction of nitrate were found to be dinitrogen and nitrous oxide. The possible catalytic reduction of nitrous oxide was confirmed by independent experiments. The striking differences in the reduction products of nitrite and nitrate are explained in terms of different structures of the intermediate complex between CoI(TSPc)5- and substrate, in which nitrite and nitrate are suggested to coordinate via nitrogen and oxygen, respectively.