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We report in this communication the preparation of the BrMPPA ligand, a new tris(2-pyridylmethyl)amine-type tripod in which di-α-substitution by an amido group and a bromine atom, respectively, provide functionality and sterically induced ligand flexibility. The coordination versatility of this tripod is evidenced by the complete characterization of two dichlorido and di-triflato FeII complexes in the solid state and in solution. The new tripod can potentially act as a κ5 ligand with coordination of the carbonyl group of the amide function. This is true when there is little steric hindrance at the coordination site. With bulky chlorido ligands, however, decoordination occurs, and the tripod becomes a κ4 chelate. In this case, a state-dependent coordination mode is reported: A dangling pyridyl group is observed in solution, whereas the carbonyl group is noncoordinated in the solid state, the three pyridine groups remaining bound.

We have synthesized the fully unsymmetrical [(6-bromo 2-pyridylmethyl) (6-fluoro 2-pyridylmethyl) (2-pyridylmethyl)] amine tripod FBrTPA. The synthesis involves preparation of the already known [(6-bromo 2-pyridylmethyl) (2-pyridylmethyl)] amine BrDPA, which is obtained either by classical condensation of α-substituted pyridine carboxaldehyde with aminomethyl pyridine, or by a pathway involving the protection/deprotection sequence of this primary amine. This second way is useful for syntheses of monosubstituted DPAs in the cases where α-substituted pyridine carboxaldehydes are unavailable. The crystal structure of the FeCl2 complex shows that the ligand binds in a κ4-N fashion, with however relatively long ligand-to-metal distances. The spectroscopic and electrochemical studies support decoordination of the bromopyridyl substituent in solution, with κ3-N coordination mode of the tripod within a complex displaying a trigonal bipyramidal geometry at the metal centre. This complex reacts easily with dry oxygen to yield an unsymmetrical μ-oxo diferric complex, the structure of which is also reported.

International audience; We report that the oxygen sensitivity of some Fe(II) complexes with tripodal ligands can be used, with benefit, in the oxidation of cyclohexane under mild conditions. Depending on the solvent, two very different reaction pathways are involved, which share the coordination of O 2 to the metal as the common initial step. We have synthesized a series of a-chlorinated tripods in the tris(2-pyridylmethyl)amine series Cl n TPA (n = 1-3) and fully characterized the corresponding FeX 2 complexes (X = Cl, CF 3 SO 3). The single-crystal X-ray structure analyses of the FeCl 2 complexes are reported. In CH 3 CN, the FeCl 2 complexes react smoothly with O 2 , whereas the Fe(CF 3 SO 3) 2 complexes are non-sensitive. In CH 3 CN, the reaction of the oxygen-sensitive Cl n TPAFeCl 2 (n = 0-3) with O 2 , acetic acid and zinc amalgam, in the presence of cyclohexane, affords a mixture of cyclohexanol/one in an ª ol/one ratio of 3.1 and a selectivity of the C3 • /C2 • in the adamantane conversion that is consistent with a metal-oxo based oxidation. Limited efficiency (ª 2 TON) was observed for the parent TPAFeCl 2 complex and Cl 1 TPAFeCl 2 , whereas both other complexes turned out to be poorly active. The TPAFeCl 2 complex was used to address mechanistic questions: when the reaction was carried out in pyridine, the ol/one ratio shifted to 0.15 while efficiency was improved by 7-fold. In pyridine and in the presence of a spin trap (DMPO), the radical-based character of the reaction was definitely established, by contrast with acetonitrile, where no oxygenated radicals were detected. Thus, the reactivity differences arise from involvement of two distinct active species. The dichotomous radical/biomimetic pathway is discussed to interpret these results.

We report in this communication the easy preparation of the α-iodo substituted tripods within the series of tris(2-pyridylmethyl)amine ligands, I1TPA, I2TPA and I3TPA, respectively. The characterization of the corresponding FeCl2 complexes in solution is described and structural analysis by X-ray diffraction for I1TPAFeCl2 and I2TPAFeCl2 is also reported. The steric effect of the iodo substituent is evidenced: (i) by elongation of the metal to iodo-pyridine distance within I1TPAFeCl2, which however remains a very stable compound; (ii) by decoordination of one substituted pyridine in I2TPAFeCl2 and I3TPAFeCl3. In I2TPAFeCl2 and in the solid state, this uncoordinated pyridine strongly interacts with the same fragment of the neighbouring molecule, providing an overall dinuclear arrangement for this complex.

We report that the formation of mu-oxo diferric compounds from O(2) and FeCl(2) complexes within the tris(2-pyridylmethyl)amine series (N. K. Thallaj et al. Chem. Eur. J., 2008, 14, 6742-6753) involves coordination of O(2) to the metal centre and that this reaction occurs following initial dissociation of the bound equatorial chloride anion. We also report evidence of the formation of a reduced form of dioxygen by an inner-sphere mechanism, thus leading to modification of the ligand. The solid-state structures of [FeCl(2)L] complexes (L(1) = mono(alpha-pivalamidopyridylmethyl)bis(2-pyridylmethyl)amine, L(2) = mono(alpha-pivalesteropyridylmethyl)bis(2-pyridylmethyl)amine, L(3) = bis(alpha-pivalamidopyridylmethyl)mono(2-pyridylmethyl)amine are described, and spectroscopic data support the structural retention in solution. In [FeCl(2)L(3)], the two amide hydrogen atoms stabilise the equatorial chloride anion in such a way that its exchange by a weak ligand is impossible: [FeCl(2)L(3)] is perfectly oxygen-stable. In [FeCl(2)L(2)], the equatorial chloride anion is completely free to move and coordination of O(2) can take place. The reaction product with [FeCl(2)L(2)] is a mu-oxo diferric complex in which the ester function has been transformed into a phenol group. This conversion can be seen as a hydrolysis reaction in basic medium, hence supporting the initial formation of a reduced form of dioxygen in the medium. Complex [FeCl(2)L(1)] exhibits a very weak reactivity with O(2), in line with a semistabilised equatorial chloride counteranion.

We report in this article one of the first examples of a reaction of O-demethylation carried out at a Fe(II) center by molecular dioxygen, in the homogeneous phase in non-porphyrinic chemistry. This reaction parallels at the intramolecular level a very important process found in biology leading to the derivatization and elimination of drugs by oxygen-dependent enzymes that contain nonheme iron centers. To get insight into some reactivity aspects of this reaction, we have used dioxygen and iron complexes coordinated to ligands that are substituted by methoxy groups. We detail in this work the coordination chemistry of FeCl(2) to the series of mono- (L(1)), di- (L(2)), and tris(2,3-dimethoxyphenyl) (L(3)) alpha-substituted ligands in the tris(2-pyridylmethyl)amine series and the behavior of the complexes upon reaction with molecular dioxygen. As main outcomes of this study, we demonstrate that the methoxy group does not need to be coordinated to the metal center to undergo O-demethylation, but needs to be properly orientated close to an oxygenated form of the metal. We also demonstrate the importance of the environment in the reactivity with molecular dioxygen: whereas a regular 18-electron Fe(II) reacts with O(2), a five- coordinate, 16-electron center may be oxygen-stable, if the access of dioxygen to the reaction site is locked.

The new BPMPB ligand, namely, bis[1-bis(2-pyridylmethyl),1 (pyridyl)]butyne, can be very easily obtained as a side product in the known reaction of picolyl chloride and sodium acetylide (which major product is the known terminal alkyne-substituted tripod). This symmetrical ligand contains two identical coordination sites with two methylenepyridines and one pyridyl group on each side, linked by an alkyne function providing a semirigid segment. Together with the molecular structure of the ligand which is reported, we describe the preparation of complexes with Fe(II)Cl2, Co(II)Cl2, Ni(II)Cl2, Cu(I)Cl, and Zn(II)Cl2 salts. All complexes have been characterized by X-ray diffraction studies as well as by standard spectroscopic techniques. The striking point in this work is the diversity of the structures that are obtained. Co(II) and Zn(II) provide isostructural dinuclear complexes in which both coordination sites are occupied within a tetrahedral symmetry. The Cu(I) complex is also a dinuclear compound, but in that case, the copper atom is coordinated to the alkyne moiety, two pyridines, and a bridging chloride. The (13)C NMR spectrum of the copper complex confirms that the metal center is coordinated to the alkyne in solution. The coordination of Ni(II) results in the formation of a mononuclear complex in which a pyridine has fused with the alkyne moiety to generate an indolizinium group; the structure of the corresponding alkenyl complex is reported. Finally, the addition of FeCl2 to the ligand results in the formation of a mononuclear complex with a free, noncoordinated indolizinium. The sequence developed in the present work illustrates the possibility for the metal centers to adopt various coordination modes which may be relevant to the conversion of an alkyne and a pyridyl unit into indolizinium.

Reaction of a twofold molar excess of bis(2-pyridylmethyl)amine DPA with 8α-bromoAc4riboflavin in dry DMF over 24 h affords the 8α-[bis(2-pyridylmethyl)amine]-N-Ac4riboflavin ligand. This compound can be described as a tweezer in which the flavin moiety acts as a potential electron mediator. The trichloroferric complex has been prepared, and together with the low potential reduction wave of the flavin moiety, the presence of a reversible FeIII/FeII couple at a higher potential is observed. The deacetylated ligand 8α-[bis(2-pyridylmethyl)amine]-N-riboflavin can easily be obtained. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

The new tridentate title ligand has been synthesized by a very convenient and high yield method. The dichloroferrous derivative was prepared in order to provide spectroscopic bases for solution studies of trigonal bipyramidal derivatives with aminomethylpyridyl containing ligands. Its crystal structure is reported, together with UV–Vis, 1H NMR and molecular conductivity data. The absence of any MLCT in UV–Vis, significant broadening of the β and β′ resonances in 1H NMR and neutral electrolytic behavior are reported and taken together, these parameters can now unequivocally serve as criteria to detect any tridentate coordination mode of the tripod in the chemistry of dichloroferrous complexes with tris-(2-pyridylmethyl)amine type ligands.

We report the preparation and the X-ray crystal structure of the iron complex [Fe(L1)Cl][Cl] [L1 = N,N,N′-tris(2-pyridylmethyl)-o-phenylenediamine], and its reactivity towards molecular dioxygen. Upon exposure to dry O2, coordinated L1 is oxidatively dehydrogenated to the aldimine ligand L2. The ferric cation [Fe(L2)(CH3CN)]3+ is thus obtained, and characterized by UV/Vis and EPR spectroscopy, and X-ray diffraction analysis. The free L2 ligand can be recovered after decomplexation. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Novel diiron complexes with an Fe2(mu-OMe)2 core were studied as models of the active site of nonheme iron-containing enzymes. X-ray crystal structures of the complexes showed the existence of two types of ligand folding-parallel and twisted-both of which have four virtually equivalent phenolato groups sticking out from the Fe2O2 rhombic plane. Cyclic voltammetry measurements revealed two or more distinct redox waves in a region of relatively high potential, in addition to known Fe(II)/Fe(III) redox waves in a region of lower potential. These new peaks were assigned to the high-valence state of iron atoms, that is, Fe(III)Fe(IV) and Fe(IV)Fe(IV), resonating with the phenoxyl radical(s). The split width of the redox waves ranged from 0.14 to 0.20 eV, which may be a measure of the electronic interaction of the phenolate groups through the Fe2(mu-OMe)2 core.

The tris(2-fluoro-6-pyridylmethyl)amine ligand, F3TPA, can easily be prepared by reaction of 2-fluoro-6-bromomethylpyridine with NH4Cl in the presence of NaOH. Complexation to FeCl2 affords the high-spin F3TPAFe(II)Cl2 complex, the X-ray structure of which is reported. The three fluorine substituents provide enough steric hindrance to force the tripod to coordinate in the tridentate mode, affording a trigonal bipyramidal iron center. This complex is thermally stable, and it reacts instantaneously with molecular dioxygen to afford the unsymmetrical micro-oxo dimer F3TPAFe(III)ClOFe(III)Cl3 as the major product, together with small amounts of the mixed salt [F3TPAFe(II)Cl]2, [Fe(III)2OCl6]. These two complexes have been isolated and characterized by X-ray diffraction analysis. A mechanism by which they are obtained is suggested and seems to parallel the well-known process of autoxidation of ferrous porphyrins.

Nitric oxide (NO) is a potent intra- and intercellular messenger involved in the control of vascular tone, neuronal signalling and host response to infection. In mammals, NO is synthesized by oxidation of l-arginine catalysed by hemeproteins called NO-synthases with intermediate formation of Nω-hydroxy-l-arginine (NOHA). NOHA and some hydroxyguanidines have been shown to be able to deliver nitrogen oxides including NO in the presence of various oxidative systems. In this study, NOHA and a model compound, N-(4-chlorophenyl)-N′-hydroxyguanidine, were tested for their ability to generate NO in the presence of a haemprotein model, microperoxidase 8 (MP8), and hydrogen peroxide. Nitrite and nitrate production along with selective formation of 4-chlorophenylcyanamide was observed from incubations of N-(4-chlorophenyl)-N′-hydroxyguanidine in the presence of MP8 and hydrogen peroxide. In the case of NOHA, the corresponding cyanamide, Nδ-cyano-L-ornithine, was too unstable under the conditions used and l-citrulline was the only product identified. A NO-specific conversion of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide to 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl and formation of MP8–Fe–NO complexes were observed by EPR spectroscopy and low-temperature UV/visible spectroscopy, respectively. These results clearly demonstrate the formation of nitrogen oxides including NO from the oxidation of exogenous hydroxyguanidines by hydrogen peroxide in the presence of a minienzyme such as MP8. The importance of the bioactivation of endogenous (NOHA) or exogenous N-hydroxyguanidines by peroxidases of physiological interest remains to be established in vivo.

A series of dichloroferrous complexes with ligands derived from the tris(2-pyridylmethyl)amine tripod has been prepared and characterized. The X-ray crystal structures of the complexes [bis(2-bromo-6-pyridylmethyl)(2-pyridylmethyl)amine]Fe(II)Cl(2) ((Br(2)TPA)Fe(II)Cl(2)) and [bis(2-phenyl-6-pyridylmethyl)(2-pyridylmethyl)amine]Fe(II)Cl(2), ((Ph(2)TPA)Fe(II)Cl(2)) are reported. In these complexes, the tripod coordinates in the tridentate mode, with a substituted pyridyl arm dangling away from the metal. Both complexes have a trigonal bipyramidal iron center with two equatorial chloride ions. Their crystal structures are compared with those of the [tris(2-pyridylmethyl)amine]Fe(II)Cl(2) and [(2-bromo-6-pyridylmethyl)bis(2-pyridylmethyl)amine]Fe(II)Cl(2) complexes ((TPA)Fe(II)Cl(2) and (BrTPA)Fe(II)Cl(2), respectively) in which the ligand coordinates in the tetradentate mode. For all complexes, the metal to ligand distances are systematically above the value of 2.0 A, and (1)H NMR displays paramagnetically shifted resonances with short relaxation times. This indicates that the iron is in a high-spin state. Electric conductivity measurements show that, for all complexes, the measured values lie within the same range, significantly below those expected for ionic complexes. Together with the analysis of the UV-visible and NMR data, this strongly suggests that the coordination mode of the tripod is retained in solution.

In trichloroferric complexes with tris(2-pyridylmethyl)amine ligands α-substituted by bulky groups, tridentate coordination of the tripode is observed. The X-ray crystal structures of the complexes...

Oxoferryl porphyrin π-cation radical active sites of compound I intermediates which are found in enzymes such as peroxidases and catalases have been extensively modeled by oxidized synthetic metalloporphyrins. The electronic symmetry states of these compounds were initially assigned on the basis of electronic absorption data. In recent years new experimental and theoretical results have become available which have led to a re-evaluation and modification of the original assignments. A historical perspective of these developments is provided in the context of recent NMR, resonance Raman, and other spectroscopic data and theoretical calculations for the synthetic models and enzymatic systems.

This study describes the catalytic properties of manganese microperoxidase 8 [Mn(III)MP8] compared to iron microperoxidase 8 [Fe(III)MP8]. The mini-enzymes were tested for pH-dependent activity and operational stability in peroxidase-type conversions, using 2-methoxyphenol and 3,3'-dimethoxybenzidine, and in a cytochrome P450-like oxygen transfer reaction converting aniline to para-aminophenol. For the peroxidase type of conversions the Fe to Mn replacement resulted in a less than 10-fold decrease in the activity at optimal pH, whereas the aniline para-hydroxylation is reduced at least 30-fold. In addition it was observed that the peroxidase type of conversions are all fully blocked by ascorbate and that aniline para-hydroxylation by Fe(III)MP8 is increased by ascorbate whereas aniline para-hydroxylation by Mn(III)MP8 is inhibited by ascorbate. Altogether these results indicate that different types of reactive metal oxygen intermediates are involved in the various conversions. Compound I/II, scavenged by ascorbate, may be the reactive species responsible for the peroxidase reactions, the polymerization of aniline and (part of) the oxygen transfer to aniline in the absence of ascorbate. The para-hydroxylation of aniline by Fe(III)MP8, in the presence of ascorbate, must be mediated by another reactive iron-oxo species which could be the electrophilic metal(III) hydroperoxide anion of microperoxidase 8 [M(III)OOH MP8]. The lower oxidative potential of Mn, compared to Fe, may affect the reactivity of both compound I/II and the metal(III) hydroperoxide anion intermediate, explaining the differential effect of the Fe to Mn substitution on the pH-dependent behavior, the rate of catalysis and the operational stability of MP8.

The mechanism of spin‐lattice relaxation has been investigated in the “picket‐fence” porphyrin [Fe(CH3COO)(TPpivP)]-, a high‐spin iron(II) complex with unusual large quadrupole splitting of 4.25 mm s-1, by conventional Mossbauer spectroscopy as well as by nuclear resonant forward scattering (NFS). Superparamagnetism with a blocking temperature of about 8 K is observable by both methods in the spectra of bacterioferritin from S. olivaceus. From these two examples general conclusions about the merits of both methods can be drawn.

We report in this communication the facile synthesis of the new tris(thiazolylmethyl)amine TTA ligand and the full characterization in solution and in the solid state of two ferrous complexes, [(TTA)FeCl2] and [(TTA)Fe(OTf)2]. TTA, the first example of a simple tris(thiazolemethyl)amine chelate – the second one only within this class of tripods – exerts a weak ligand field and the tertiary amine is weakly bound to the metal centre.

A comparison of the exchange interactions arising in the peroxidase and catalase Compound I intermediates and their iron(IV)-oxo porphyrin π-cation radical models, both of which are two oxidizing equivalents above the ferric state, suggests that in the models the oxidizing equivalent is localized on the porphyrin ring, while in the proteins it is partly delocalized onto the proximal ligand. Thus, the proximal axial ligand of iron participates indirectly in the oxidation reactions catalyzed by the enzymes. Possible roles of the axial ligand in the catalytic mechanism of these heme-based enzymes are discussed.

The complexation of Fe(SO(3)CF(3))(2) to the series of fluoro α-substituted tris-(2-aminomethylpyridyl)amine tripods F(1-3)TPA yields the triflato F(1-3)TPAFe(SO(3)CF(3))(2) complexes which have firstly been characterized in solution. As expected, bis-acetonitrile charged species are present in CH(3)CN, and neutral bis-triflato complexes in CH(2)Cl(2). The X-ray diffraction analyses of F(1)TPAFe(SO(3)CF(3))(2) and F(2)TPAFe(SO(3)CF(3))(2) crystallized from CH(2)Cl(2) solutions show that their structure in solution is retained in the solid state, with coordination of both triflate ions and the κ(4) coordination mode of the tripod in each complex. The solid state structure of the [F(2)TPAFe(NCMe)(SO(3)CF(3))](SO(3)CF(3)) complex obtained from crystallization in acetonitrile of the bis-triflato precursor is also reported. The presence of a bound triflate in the solid state is unexpected and interpreted as the result of solid-state stabilization by a metal center which displays some Lewis acidity character because of its coordination to an electron-deficient tripod. The fourth compound whose solid state structure is reported is [F(2)TPAFe(H(2)O)(2)](SO(3)CF(3))(2), fortuitously obtained after the bis-triflato precursor was handled under aerobic conditions. In CH(3)CN, all complexes are oxygen stable. The gain in stability of the bis-acetonitrile adducts is certainly responsible for the lack of reactivity of all complexes in this solvent. In CH(2)Cl(2), the parent TPAFe(SO(3)CF(3))(2) complex reacts with O(2) to yield a compound belonging to the well-known class of μ-oxo diferric compounds. Whereas F(1)TPAFe(SO(3)CF(3))(2) is poorly reactive, F(2)TPAFe(SO(3)CF(3))(2) and F(3)TPAFe(SO(3)CF(3))(2) turn out to be completely inert. This strongly contrasts with the behavior of the known F(1-3)TPAFeCl(2) complexes for which an increased reactivity is observed upon ligand substitution. In CH(2)Cl(2), conductimetry measurements indicate extremely weak (if any!) dissociation of the ancillary ligands in all complexes. Comparative analysis of the structures reveals relatively invariant structural parameters within the series of Fe(SO(3)CF(3))(2) complexes, whereas FeCl(2) complexes display important metal to ligand elongations upon tripod substitution. The reactivity increase upon fluorination of the ligand in the FeCl(2) complexes is interpreted as resulting from sterically-induced pyridine flexibility. The opposite situation with Fe(SO(3)CF(3))(2) complexes is due to the lock of the coordination polyhedron in the absence of important steric stress, especially when the metal center becomes electron-deficient.

In order to study structural influences on the interaction of Fe(IV) (S=1) and porphyrin cation radical (S′=1/2) in high-valent iron porphyrin complexes of the type ¦X-(TMP)Fe=O¦+(Cl−), X=I, Br2, Br4 were generated by mCPBA oxidation of corresponding Fe(III) porphyrins. The halogen substitution at the peripheral positions of the porphyrin leads to distortion of the planar porphyrin ring of ¦(TMP)Fe=O¦+. The new species have beeen investigated by temperature-dependent EPR and field-dependent Mossbauer spectroscopy; for the evaluation of spectra, we adopted the spin-Hamiltonian formalism including exchange interaction explicitly. As in ¦(TMP)Fe=O¦+, strong ferromagnetic spin coupling was observed with|J0|D=0.9–1 and a zero-field spltting ofD∼32 cm−1. For consistent parametrization of EPR and Mossbauer results, anisotropic coupling had to be introduced. Compared to ¦(TMP)Fe=O¦+ [1], analysis of the spectroscopic data shows that zero-field splitting and spin coupling is only slightly affected by the halogen distortion of the porphyrin structure.

We have developed over the last ten years a coordination chemistry based on the study of FeCl2 complexes of small nitrogen-containing tripods derived from tris(2-pyridylmethyl)amine (TPA) ligands. Simple α-substituted ligands were prepared and the corresponding FeCl2 complexes characterized, at best in solution and in many cases also in the solid state. Depending on their structure as well as on the nature of their ligands, the complexes were found to exhibit a versatile behaviour vs. molecular oxygen, affording new species with generally good yields. This prompted us to try to understand what was hidden beyond the familiar term “oxygen-sensitivity” and to study in more detail the reaction of these simple mononuclear Fe(II) complexes with molecular oxygen. The following report is an overview of several years of our research in this field, and will focus along the following directions: (i) general principles in terms of reactivity and coordination of O2 to the metal centres, (ii) intramolecular modifications of potentially reactive ligands upon reaction of the complexes with O2 and (iii) attempts to extend the reactivity to exogenous substrates, i.e. to access biomimetic catalysis. The hypothesis of the inner sphere reduction of O2 leading to the “superoxide way” as an early step is discussed.

Oxoferryl porphyrin π cation radical species have been generated by the oxidative action of m-chloroperoxybenzoic acid on the triflato complexes of (tetrakis(2,6-dichlorophenyl)porphyrinato]- and (tetrakis(2,4,6-trimethoxyphenyl)porphyrinato]-iron(III) in methylene chloride/methanol-d 4 . The new high-valent complexes have been characterized by a variety of spectroscopic techniques.

The X-ray structures of the β-substituted-pyrrole, its nickel(II) complex (NiTMOBP), and the nickel(II) derivative are reported. The synthesis of the free base H 2 TPFPOBP is also described. All these molecules are nonplanar, displaying saddle-shaped conformations

We report that the formation of mu-oxo diferric compounds from O(2) and FeCl(2) complexes within the tris(2-pyridylmethyl)amine series (N. K. Thallaj et al. Chem. Eur. J., 2008, 14, 6742-6753) involves coordination of O(2) to the metal centre and that this reaction occurs following initial dissociation of the bound equatorial chloride anion. We also report evidence of the formation of a reduced form of dioxygen by an inner-sphere mechanism, thus leading to modification of the ligand. The solid-state structures of [FeCl(2)L] complexes (L(1) = mono(alpha-pivalamidopyridylmethyl)bis(2-pyridylmethyl)amine, L(2) = mono(alpha-pivalesteropyridylmethyl)bis(2-pyridylmethyl)amine, L(3) = bis(alpha-pivalamidopyridylmethyl)mono(2-pyridylmethyl)amine are described, and spectroscopic data support the structural retention in solution. In [FeCl(2)L(3)], the two amide hydrogen atoms stabilise the equatorial chloride anion in such a way that its exchange by a weak ligand is impossible: [FeCl(2)L(3)] is perfectly oxygen-stable. In [FeCl(2)L(2)], the equatorial chloride anion is completely free to move and coordination of O(2) can take place. The reaction product with [FeCl(2)L(2)] is a mu-oxo diferric complex in which the ester function has been transformed into a phenol group. This conversion can be seen as a hydrolysis reaction in basic medium, hence supporting the initial formation of a reduced form of dioxygen in the medium. Complex [FeCl(2)L(1)] exhibits a very weak reactivity with O(2), in line with a semistabilised equatorial chloride counteranion.

Despite similar ring deformations in solution and in the solid state, the chloroiron(III) derivative of 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin ([FeCl(oetpp)], shown schematically) prepared in this study exhibits only a very weak quantum-mechanical admixture of spin S=3/2 (only 4-10 %) with spin S=5/2. In contrast, for the variety of [FeCl(oetpp)] studied earlier by other researchers a 40 % contribution of the S=3/2 state was found.

Mild conversion of the nitrile substituent into an amide functional group is observed upon coordination of CNTPA [CNTPA = (6-cyano 2-pyridylmethyl)bis(2-pyridylmethyle)amine] to FeCl2 followed by reaction with water. The crystal structure of CNTPAFeCl2 is reported and reveals that the nitrile group does not coordinate but is activated in the vicinity of the ferrous center. Upon treatment with water under anaerobic conditions, CNTPAFeCl2 converts into H2NCOTPAFeCl2, [H2NCOTPA = (6-carboxamido 2-pyridylmethyl)bis(2-pyridylmethyle)amine], the X-ray crystal structure of which is reported. The new ligand H2NCOTPA can easily be obtained upon decomplexation.

Crystal data: [Fe(TP piv P)(OSO 2 CF 3 )(H 2 O)] (C 65 H 66 N 8 O 8 F 3 SFe); monoclinic; a=13.161 (3), b=19.196 (6), c=26.212 (6) A; β=103.34 (2) o ; Z=4; d calc =1.270 g cm −3 ; space group P2 1 /c. The six-coordinate iron atom is bonded to the four porphyrinato nitrogens (Fe-N p =2.021 (16) A), to an oxygen atom of the triflate ion (Fe-O(triflate)=2.188 (5) A), placed inside the molecular cavity of the picket-fence porphyrin, and to a water molecule (Fe-O(water)=2.133 (5) A). The susceptibility measurements in an external field of 1.5 T show that the effective magnetic moment varies from 4.0 to 5.7 μ B in the temperature range 2-300 K. The EPR data yield g ⊥ =5.7, which corresponds to a mixture of 85% spin sextet 6 A 1 and 15% spin quartet 4 A 2 in the lowest Kramers doublet. Mossbauer spectra were recorded at temperature varying from 4.2 to 300 K in fields of 0-6.21 T. They exhibit temperature-independent quadrupole splitting. ΔE Q 2.2 mm s −1 , which lies between the ΔE Q values characteristic for high-spin (S= 5 / 2 ) and intermediate-spin (S= 3 / 2 ) porphyrins. Spin-spin and spin-lattice relaxation effects are explicitly accounted for in the intermediate relaxation regime within the framework of the (S= 5 / 2 ) spin Hamiltonian

Magnetic properties of frozen solutions of highly oxidized iron porphyrin complexes were investigated by EPR and Mossbauer spectroscopy. The Mossbauer spectra, recorded at low temperatures in various magnetic fields, were analyzed on the basis of spin Hamiltonian simulations. Spin coupling between ferryl iron (FeIV) and porphyrin cation radical was taken into account explicitly. Hyperfine and spin-coupling parameters are given for several complexes, together with zero-field parameters. One of the complexes exhibits weak spin coupling, it is the first model system exhibiting properties comparable to those of the oxoferryl cation radical enzyme Horse Radish Peroxidase I.

La reaction du complexe [Fe(η 6 -C 6 Me 6 )(η 5 -C 6 Me 6 H)] + F 6 − avec le bromure de benzyl magnesium (ou le dithianne/butyl lithium) donne des complexes de type [Fe(η 5 -C 6 Me 6 H)(η 5 -C 6 Me 6 R)] R=benzyl, dithiannyl-1,3 qui reagissent ensuite avec l'hexafluorophosphate de tritylium pour donner les complexes Fe(η 6 -C 6 Me 6 )(η 5 -C 5 Me 6 R)] + PF 6 − qui peuvent etre deprotones et benzoyles

Coordination of FeCl3 to the title ligand yields a mononuclear iron(III) complex 1, which was characterized by spectroscopic techniques and X-ray diffraction. The ligand is (kappa3-N) tridentate and the metal, which lies in a pseudo-octahedral environment, is bound to a phenolate group from the catechol substituent. The dichloroiron(II) complex 2 was easily obtained by metalation of the ligand with FeCl2 and characterized by various spectroscopic techniques. In their cyclic voltammograms both 1 and 2 display the same reversible FeII/FeIII wave at E1/2=10 mV (vs. SCE). Reduction of compound 1 with Zn/Hg yields 2', which displays identical properties to 2. Taken together, these findings indicate that in spite of the different oxidation state of the metal in 2, no major geometrical/structural change is observed at the metal center with respect to 1. The reaction of 2 with dioxygen in the absence of organic substrates proceeds extremely rapidly and yields compound 3, which is a diiron(III) derivative whose X-ray crystal structure is also reported. The possibility of a radical-based mechanism is discussed. Compound 3 displays an unusual geometry: one iron(III) center is seven-coordinate, whereas the other lies in a square-pyramidal environment. The two iron atoms are bridged by the catecholato substituents. To the best of our knowledge, 3 is the first example of a seven-coordinate iron(III) derivative with tris(2-pyridylmethyl)amine ligands.

The title compound, C13H10O4S, crystallizes with two molecules in the asymmetric unit in which the rings make dihedral angles of 3.9 (1) and 6.0 (1)°; this planarity is due in part to the presence of an intramolecular O—H...O hydrogen bond, which generates an S(6) ring in each molecule. Both molecules represent E isomers with respect to the central C=C bond. In the crystal, molecules are linked by C—H...O interactions into a three-dimensional network.

Nitric oxide (NO) is a potent intra- and intercellular messenger involved in the control of vascular tone, neuronal signalling and host response to infection. In mammals, NO is synthesized by oxidation of l-arginine catalysed by hemeproteins called NO-synthases with intermediate formation of Nomega-hydroxy-l-arginine (NOHA). NOHA and some hydroxyguanidines have been shown to be able to deliver nitrogen oxides including NO in the presence of various oxidative systems. In this study, NOHA and a model compound, N-(4-chlorophenyl)-N'-hydroxyguanidine, were tested for their ability to generate NO in the presence of a haemprotein model, microperoxidase 8 (MP8), and hydrogen peroxide. Nitrite and nitrate production along with selective formation of 4-chlorophenylcyanamide was observed from incubations of N-(4-chlorophenyl)-N'-hydroxyguanidine in the presence of MP8 and hydrogen peroxide. In the case of NOHA, the corresponding cyanamide, Ndelta-cyano-L-ornithine, was too unstable under the conditions used and l-citrulline was the only product identified. A NO-specific conversion of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide to 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl and formation of MP8-Fe-NO complexes were observed by EPR spectroscopy and low-temperature UV/visible spectroscopy, respectively. These results clearly demonstrate the formation of nitrogen oxides including NO from the oxidation of exogenous hydroxyguanidines by hydrogen peroxide in the presence of a minienzyme such as MP8. The importance of the bioactivation of endogenous (NOHA) or exogenous N-hydroxyguanidines by peroxidases of physiological interest remains to be established in vivo.

Kinetic studies were performed with microperoxidase-8 (Fe(III)MP-8), the proteolytic breakdown product of horse heart cytochrome c containing an octapeptide linked to an iron protoporphyrin IX. Mn(III) was substituted for Fe(III) in Mn(III)MP-8. The mechanism of formation of the reactive metal-oxo and metal-hydroperoxo intermediates of M(III)MP-8 upon reaction of H(2)O(2) with Fe(III)MP-8 and Mn(III)MP-8 was investigated by rapid-scan stopped-flow spectroscopy and transient EPR. Two steps (k(obs1) and k(obs2)) were observed and analyzed for the reaction of hydrogen peroxide with both catalysts. The plots of k(obs1) as function of [H(2)O(2)] at pH 8.0 and pH 9.1 for Fe(III)MP-8, and at pH 10.2 and pH 10.9 for Mn(III)MP-8, exhibit saturation kinetics, which reveal the accumulation of an intermediate. Double reciprocal plots of 1/k(obs1) as function of 1/[H(2)O(2)] at different pH values reveal a competitive effect of protons in the oxidation of M(III)MP-8. This effect of protons is confirmed by the linear dependence of 1/k(obs1) on [H(+)] showing that k(obs1) increases with the pH. The UV-visible spectra of the intermediates formed at the end of the first step (k(obs1)) exhibit a spectrum characteristic of a high-valent metal-oxo intermediate for both catalysts. Transient EPR of Mn(III)MP-8 incubated with an excess of H(2)O(2), at pH 11.5, shows the detection of a free radical signal at g approximately equal to 2 and of a resonance at g approximately equal to 4 characteristic of a Mn(IV) (S = 3/2) species. On the basis of these results, the following mechanism is proposed: (i) M(III)MP-8-OH(2) is deprotonated to M(III)MP-8-OH in a rapid preequilibrium step, with a pK(a) = 9.2 +/- 0.9 for Fe(III)MP-8 and a pK(a) = 11.2 +/- 0.3 for Mn(III)MP-8; (ii) M(III)MP-8-OH reacts with H(2)O(2) to form Compound 0, M(III)MP8-OOH, with a second-order rate constant k(1) = (1.3 +/- 0.6) x 10(6) M(-1) x s(-1) for Fe(III)MP-8 and k(1) = (1.6 +/- 0.9) x 10(5) M(-1) x s(-1) for Mn(III)MP-8; (iii) this metal-hydroperoxo intermediate is subsequently converted to a high-valent metal-oxo species, M(IV)MP-8=O, with a free radical on the peptide (R(*+)). The first-order rate constants for the cleavage of the hydroperoxo group are k(2) = 165 +/- 8 s(-1) for Fe(III)MP-8 and k(2) = 145 +/- 7 s(-1) for Mn(III)MP-8; and (iv) the proposed M(IV)MP-8=O(R(*+)) intermediate slowly decays (k(obs2)) with a rate constant of k(obs2) = 13.1 +/- 1.1 s(-)(1) for Fe(III)MP-8 and k(obs2) = 5.2 +/- 1.2 s(-1) for Mn(III)MP-8. The results show that Compound 0 is formed prior to what is analyzed as a high-valent metal-oxo peptide radical intermediate.

The tetraaza macrocycles 2-oxa-3-oxotetramesitylporphine (|H(2) 1|) and 2-oxa-3-oxotetrakis(2,6-dichlorophenyl)porphine (|H(2) 2|) and the corresponding iron complexes (|Fe(III)(X) 1| and |Fe(III)(X) 2|; X= Cl(-), OH(-), or SO(3)CF(3)(-)) have been synthesized. These macrocycles are derived from porphyrins by transformation of one pyrrole ring to an oxazolone ring. The resulting lactone functionality serves to restrict but not completely block pi-conjugation around the periphery. These complexes thus share properties with both porphyrins and chlorins. The ferric and high-valent iron complexes have been characterized by a variety of spectroscopic techniques. The molecular structure of |Fe(III)(Cl) 2| has been obtained by X-ray crystallography and shows that the structural changes at the macrocycle periphery do not perturb the coordination sphere of iron relative to the corresponding porphyrin complexes. This is illustrated by the observation that Fe-O frequencies in the resonance Raman spectra of the porpholactone analogues of compounds I and II are not substantially different from those of porphyrins and by the axial appearance of the EPR signals of the high-spin ferric complexes. This is consistent with reports that the Fe=O unit of oxidized porphyrins and chlorins is relatively insensitive to alteration of macrocycle symmetry. Nevertheless, probes of properties of the porpholactone macrocycle ((1)H NMR, resonance Raman skeletal modes) show effects of the asymmetry induced by the oxazolone ring. On the basis of (1)H NMR, EPR, Mössbauer, and resonance Raman data, the singly occupied molecular orbital of oxoferryl porpholactone pi-cation radicals correlates with the a(1u) molecular orbital of porphyrins under D(4)(h)() symmetry. Moreover, the paramagnetic properties and the intramolecular exchange interaction of ferryl iron and the porpholactone pi-radical have been characterized by EPR and magnetic Mössbauer measurements and spin-Hamiltonian analyses. The values J(0) = 17 cm(-)(1) and J(0) = 11 cm(-)(1) obtained for the exchange coupling constants of the oxoferryl porpholactone pi-cation radical complexes |Fe(IV)=O 1|(+) and |Fe(IV)=O 2|(+), respectively, are among the lowest found for synthetic compound I analogues.