Three different mononuclear copper(II) complexes 1-3 bearing ditopic macrocyclic ligands (L-1 or L-2) have been prepared. Both ligands include two coordinating cores, namely tris(methylpyridyl) amine (TPA) and pyridinedicarboxamide (PydCA). Complexes 1-3 have been characterized in solid state, and in solution by UV-Vis and EPR spectroscopies, as well as by cyclic voltammetry. X-ray diffraction analyses of crystals of complexes 1 and 3 show that the Cu(II) ion is preferably coordinated in the TPA site. Moreover, the coordination sphere of the copper center fully depends on the Cu(II) salt used for the synthesis (CuCl2 for 1 and 2, Cu(OTf)(2) for 3). Hence, the tetracoordinated bis-chloro complex 1 adopts a distorted square-planar geometry at solid state, whereas the pentacoordinated bis-aqua complex 3 displays an almost perfect square pyramidal conformation. Both complexes 1 and 3 react with H2O2 in acetonitrile, leading to the formation of copper(II)- hydroperoxo species according to the UV-Vis spectroscopic studies.

Two novel tripodal ligands L1 and L2 based on a tris(methylpyridyl)amine (TPA) motif have been prepared and reacted with two different iron(ii) salts. The ligand L1 contains a bis(amino-phenyl)-TPA group whereas the macrocyclic ligand L2 displays two different coordinating cores, namely TPA and pyridine-dicarboxamide. The resulting mononuclear complexes 1-4 have been characterized in the solid state and in solution by spectroscopic and electrochemical methods. All complexes are high spin and mainly pentacoordinated. X-ray diffraction analyses of the crystals of complexes 2 and 3 demonstrate that the coordination sphere of the iron(ii) centre adopts either a distorted bipyramidal-trigonal or square pyramidal geometry. In the absence of an exogenous substrate, oxidation of complex 2 by H2O2 induces an intramolecular aromatic hydroxylation, as shown by the X-ray structure of the resulting dinuclear complex 2'. Catalytic studies in the presence of a substrate (cyclohexane) show that the reaction process is strongly impacted by the macrocyclic topology of the ligand and the nature of the counter-ion.

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.

The question of the conversion of nitrile groups into amides (nitrile hydration) by action of water in mild and eco-compatible conditions and in the presence of iron is addressed in this article. We come back to the only known example of hydration of a nitrile function into carboxamide by a ferrous [Fe(II)] center in particularly mild conditions and very efficiently and demonstrate that these unusual conditions result from the occurrence of steric stress at the reaction site and formation of a more stable end product. Two bis(cyano-substituted) (tris 2-pyridyl methyl amine) ligands have been prepared, and the structures of the corresponding FeCl2 complexes are reported, both in the solid state and in solution. These two ligands only differ by the position of the nitrile group on the tripod in the α and β position, respectively, with respect to the pyridine nitrogen. In any case, intramolecular coordination is impossible. Upon action of water, the nitrile groups are hydrated however only if they are located in the α position. The fact that the β-substituted β-(NC)2TPAFeCl2 complex is not water sensitive suggests that the reaction proceeds in an intramolecular way at the vicinity of the metal center. In the bis α-substituted α-(NC)2TPAFeCl2 complex, both functions are converted in a very clean fashion, pointing out that this complex exhibits ligand flexibility and is not deactivated after the first hydration. At a preparative scale, this reaction allows the one-pot conversion of the bis(cyano-substituted) tripod into a bis(amido-substituted) one in particularly mild conditions with a very good yield. Additionally, the XRD structure of a ferric compound in which the two carboxamido ligands are bound to the metal in a seven-coordinate environment is reported.

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.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

We report in this communication the preparation of the BrMPPA ligand, a new tris(2-pyridylmethyl)amine-type tripod in which di-alpha-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 Fe-II complexes in the solid state and in solution. The new tripod can potentially act as a kappa(5) 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 kappa(4) 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.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

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.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

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.

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 a-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 O-2 to the metal centres, (ii) intramolecular modifications of potentially reactive ligands upon reaction of the complexes with O-2 and (iii) attempts to extend the reactivity to exogenous substrates, i.e. to access biomimetic catalysis. The hypothesis of the inner sphere reduction of O-2 leading to the `` superoxide way'' as an early step is discussed.

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 α-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 ≈ 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 (≈ 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.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

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.

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.