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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 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.

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)

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.

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.

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.

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.

Complexation of C 6 Me 6 by Fe 2+ in (C 6 Me 6 ) 2 Fe 2+ followed by (i) addition of hydride, (ii) subsequent addition of a carbanion, (iii) removal of hydride by Ph 3 C + by an electron-transfer pathway, (iv) deprotonation, (v) acylation, (vi) deprotonation, and (vii) decomplexation by Al 2 O 3 /O 2 gives a regio- and stereo-specifically controlled cyclohexadiene.