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Michèle Cesario

Publications and source records attributed to Michèle Cesario.

12 recordsLinked to original sources
PubMed2006

Electron transfer in self-assembled orthogonal structures.

Two new molecular dyads, comprising pyrromethene (bodipy) and 2,2':6',2"-terpyridine (terpy) subunits, have been synthesized and fully characterized. Absorption and fluorescence spectral profiles are dominated by contributions from the bodipy unit. Zinc(II) cations bind to the vacant terpy ligand to form both 1:1 and 1:2 (cation:ligand) complexes, as evidenced by X-ray structural data, NMR and spectrophotometric titrations. Attachment of the cations is accompanied by a substantial decrease in fluorescence from the bodipy chromophore due to intramolecular electron transfer across the orthogonal structure. At low temperature, nuclear tunneling occurs and the rate of electron transfer is essentially activationless. However, activated electron transfer is seen at higher temperatures and allows calculation of the corresponding reorganization energy and electronic coupling matrix element. In both cases, charge recombination is faster than charge separation.

Boron Compounds↗
PubMed2006

Photochemical ring expansion of 4-azidouracil: a route to 5H-1,3,5-triazepin-2,4-dione in the nucleoside series.

Under aqueous conditions, 4-azidouracil/tetrazolo[1,5-c]pyrimidin-5(6H)-one nucleosides undergo a very efficient photochemical nitrogen elimination and ring expansion to 1,3,5-triazepin-2,4-dione nucleosides whose structure has been confirmed by X-ray crystallography. In contrast, when the photolysis was attempted under anhydrous conditions in the presence of a nucleophile, a ring contraction reaction occurred, affording 2-oxoimidazolone nucleosides. A mechanism to account for the formation of ring expansion and contraction reactions and involving a carbodiimide intermediate is proposed which is reminiscent of the known photochemical behavior of 2-azidopyridines/tetrazolo[1,5-a]pyridines.

Azepines↗
PubMed2004

Tuning organogels and mesophases with phenanthroline ligands and their copper complexes by inter- to intramolecular hydrogen bonds.

A novel family of highly functionalized molecules consisting of a central 4-methyl-3,5-diacylaminobenzene platform linked in close proximity to the methyl group by two lateral aromatic rings each equipped with two long alkoxy chains has been rationally designed. The presence of amide tethers and a chelating phenanthroline fragment connected via an ester dipole formed a new class of gelating reagents and mesomorphic materials. A few of these compounds have the tendency to form macromolecule-like aggregates through noncovalent interactions in hydrocarbon solvents and were found to exhibit thermotropic cubic mesophases. In light of the X-ray molecular structure of the methoxy ligand, an infinite network maintained by intermolecular hydrogen bonds as well as by pi-pi stacking of the phenyl subunits was evidenced. FT-IR studies confirm that the common driving force for aggregation in the organogels and microsegregation in the mesophase is the occurrence of a tight intermolecular H-bonded network that does not persist in diluted solution. This situation is switched when the ligands are interlocked by a copper(I) cation. A strong intramolecular H-bond confirmed by X-ray diffraction of a single crystal for the methoxy case provides very stable complexes but inhibits the gelation of the solvents. Heating the complexes bearing long paraffin chains (n = 12 and 16) in the dried state leads to a self-organization into a columnar liquid-crystalline phase in which the columns are arranged along a 2D oblique symmetry as deduced from powder XRD experiments. In this case, the complexes with the appended counteranions self-assemble in a specific way to form columns. A striking observation is that the intramolecular hydrogen bond persists in the mesophase as it does in solution without any evidence of an extended network. As far as we are aware, these ligands and complexes are rare examples in which organogelation and thermotropic mesomorphic behavior could be observed in parallel with molecules bearing a chelating platform. Due to the synthetic availability of the 4-methyl-3,5-diacylaminobenzene core and the simplicity by which the chelating platforms can be graphed, this methodology represents a practical alternative to the production of functionalized organogelators and mesomorphic materials.

Journal Article↗
PubMed2004

Selective oxidation of a single primary alcohol function in oligopyridine frameworks.

A variety of mono-oxidized pyridine, bipyridine, terpyridine, and pyridine/pyridazine are readily prepared under mild conditions using Pyrolusite MnO2. This phase has been characterized by means of X-ray powder diffraction and scanning electron microscopy. The oxidative activity is in keeping with the nature, morphology, and surface area of the MnO2 reagent.

Journal Article↗
PubMed2004

A convenient protocol for the synthesis of ligands from a 4-methyl-3,5-diacylaminophenyl platform.

The synthesis of stable and highly organized phenanthroline, terpyridine, and pyridino-oxazoline ligands bearing one or two 4-methyl-3,5-diacylaminophenyl modules equipped with two lateral dialkoxyphenyl groups has been performed using EDC.HCl and DMAP reagents in the final coupling reaction. Evidently, in the final ligands and in the solid state intermolecular hydrogen bonding maintains the coherence of the tridimensional structure as clearly evidenced by FT-IR and X-ray diffraction spectroscopy in the cases of the methoxy ligands. The supramolecular packing is also maintained by additional pi-pi stacking interactions.

Journal Article↗
PubMed2004

Imidazole and imidazolate iron complexes: on the way for tuning 3D-structural characteristics and reactivity. Redox interconversions controlled by protonation state.

X-ray structures for six Fe(II) and Fe(III) complexes from two closely heptadentate N-tripodal ligands, L1H(3) = tris[(imidazol-4-yl)-3-aza-3-butenyl]amine and L2H(3) = tris[(imidazol-2-yl)-3-aza-3-butenyl]amine, are described: three complexes in the L1 series (namely, [Fe(II)(L1H(3))](2+) and [Fe(III)(L1H(3))](3+) at low pH and [Fe(III)(L1)](0) at high pH) and three complexes in the L2 series (namely, [Fe(II)(L2H(3))](2+) at low pH and [Fe(II)(L2H)](0) and [Fe(III)(L2)](0) at high pH). Most of these complexes are stable in both Fe(II) and Fe(III) redox states and with the ligand in various protonation states. In the solid state, hydrogen bonds networks were obtained. Structural differences induced by 2- or 4-imidazole substitution are described and discussed. In solution, interconversions between different forms, with regard to oxidation and protonation states, were investigated by UV-visible spectroscopy, cyclic voltammetry, and potentiometry. The deprotonation pattern of these polyimidazole iron(II) and iron(III) complexes is described in detail. pK(a)s of the imidazolate/imidazole moieties in MeOH/H(2)O are reported. Two new species, namely, [Fe(II)(L1)](-) and [Fe(II)(L2)](-), were shown to be obtained in DMSO upon strong base addition and characterized by UV-vis spectroscopy and cyclic voltammetry. Half-wave potentials of Fe(III)/Fe(II) complexes with ligand moieties in several protonation states are reported, both in DMSO and in MeOH/H(2)O. Because of the presence of free imidazole groups coordinated to the iron, the potential of the iron(III)/iron(II) couples can be tuned by pH. A shift of DeltaE = E(deprot) - E(prot) ranging from -270 to -320 mV per exchanged proton in DMSO was measured. This study shows moreover that interconversions (with regard to both redox and protonation states) can be reversed several times. As the complexes have been isolated in order to be tested as superoxide dismutase mimics, preliminary reactions with dioxygen and with superoxide, considered as oxidant and reducer of biological importance, are reported. In these two series, O(2)(-) behaves either as a base or as a reducer and no adducts have been observed.

Journal Article↗
PubMed2003

Stereoselective ring opening of meso bicyclic hydrazines: a straightforward approach to hydrazino cyclopentenic cores.

The diastereoselective synthesis of hydrazinocyclopentenes 6 or 7 can be achieved in a straightforward manner from Diels-Alder adduct 4 using an acid-catalyzed rearrangement or a palladium-catalyzed allylic substitution reaction. In the latter case, enantioenriched compounds with ee values up to 58% can be obtained when an appropriate chiral ligand is used. [reaction: see text]

Journal Article↗
PubMed2003

Direct measurement of the hyperfine and g-tensors of a Mn(III)-Mn(IV) complex in polycrystalline and frozen solution samples by high-field EPR.

The g-tensors and hyperfine tensors of the S = (1)/(2) ground state of the mixed valence [LMn(IIImu-O)(2)Mn(IV)L](3+) complex (L = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine) was deter-mined in the solid-state and frozen acetonitrile solution by high-field EPR. Both samples exhibited complex anisotropic temperature behaviors that precluded the use of routine spectrum simulation procedures to extract the spin parameters. To circumvent this problem, the parameters were measured directly by using multifrequency techniques. In the case of the frozen solution, this approach yielded seven of the nine spin parameters with varying uncertainty, the two extreme principal g-values, the four hyperfine couplings associated with each of these two g-values, and the middle g-value. This latter parameter was obtained from a first moment analysis. Unlike simulations, the statistical errors associated with each value could be assigned in a straightforward and rigorous manner. The directly measured g-values were different in frozen solution and polycrystalline powder. The temperature dependence of the high-field EPR spectra of the polycrystalline powder revealed a spin-spin interaction between the neighboring binuclear complexes.

Biomimetic Materials↗
PubMed2003

Cation sensors based on terpyridine-functionalized boradiazaindacene.

A new class of highly luminescent dyes is reported. The characteristic feature of these compounds is that a terpyridine fragment is closely appended to a boradiazaindacene moiety in such a way that cation binding to the vacant terpyridine causes strong perturbations of the photophysical properties of the boradiazaindacene unit. In particular, these sensors are especially applicable to the fluorescence detection of trace quantities of zinc(II) ions in solution. The mechanism of the cation-induced quenching process has been investigated by a combination of electrochemistry, UV/Vis absorption, emission, and NMR spectroscopy. Highly luminescent arrays can be formed by doping transparent polymers with low concentrations of these new dyes. In such materials, the change in photophysical properties upon cation binding is so marked that "cation writing" becomes feasible under routine conditions.

Journal Article↗
PubMed2002

Application of the palladium-catalyzed borylation/Suzuki coupling (BSC) reaction to the synthesis of biologically active biaryl lactams.

The palladium-catalyzed, two-step, one-pot borylation/Suzuki coupling (BSC) reaction was developed to synthesize sterically hindered 2,2'-disubstituted biphenyl and phenyl-indole compounds in a short, simple, and efficient manner from two easily accessible aryl halides. High yields can be obtained by choosing properly both components according to their rough electronic properties. The illustration of the utility of this method was provided by the solution and solid-phase synthesis of seven- or eight-membered biphenyl lactams 5a-e, as well as paullone 3a. These compounds exhibit moderate albeit significant cytotoxicities and may serve as structural models for future medicinal chemistry developments.

Alkaloids↗
PubMed1997

Selective Metal-Assisted Oxidative Cleavage of a C-N Bond: Synthesis and Characterization of the Mononuclear Iron(III) [Fe(BPG)Cl(2)] Complex and Its Two [Fe(BPA)Cl(3)] and [Fe(BPE)Cl(3)] Derivatives.

A new example of a site-selective metal-assisted oxidative cleavage of a C-N bond is reported. This phenomenon occurs in the nitrogen-centered tetradentate tripodal BPG ligand (BPG = (bis(2-pyridylmethyl)amino)acetate), which combines one carboxylate and two pyridines as pendant groups, when coordinated to iron(III). The corresponding iron(III) [Fe(BPG)Cl(2)] complex (1) is transformed to the iron(III) [Fe(BPA)Cl(3)] complex (2) with the tridentate BPA ligand (BPA = bis(2-pyridylmethyl)amine) retaining only two pyridine pendant groups, while the initial BPG carboxylate group is transformed to glyoxylic acid. The [Fe(BPA)Cl(3)] complex (2) has been fully characterized as well as another, [Fe(BPE)Cl(3)] (3), formed with the tridentate BPE ligand (BPE = methyl (bis(2-pyridylmethyl)amino)acetate) which, in addition to two pyridines, presents an ester group. The crystal structures of these two complexes have been resolved. The asymmetric unit of complex 2 has been characterized by two mononuclear neutral molecules linked by two hydrogen bonds. (Crystal data for 2: orthorhombic, Pna2(1), a = 15.603(8) Å, b = 8.485(4) Å, c = 22.752(11) Å, alpha = beta = gamma = 90 degrees, V = 3012(3) Å(3), Z = 8, R = 0.0582 [2813 reflections with I > 2sigma(I)], and R(w) = 0.1430.) Complex 3 is symmetric with the iron center, the amine nitrogen, one choride and the three atoms of the carboxylate group in special positions in the mirror plane. (Crystal data for 3: orthorhombic, Pmn2(1), a = 10.418(4) Å, b = 12.952(5) Å, c = 7.353(3) Å, alpha = beta = gamma = 90 degrees, V = 992.2(7) Å(3), Z = 2, R = 0.0500 [513 reflections with I > 2sigma(I)], and R(w) = 0.1231.) Redox potentials corroborate the structural features (E degrees = -38 mV for 1, -19 mV for 2, and +145 mV for 3 vs SCE in acetonitrile). Preliminary studies of the [Fe(BPG)Cl(2)] complex have shown that it reacts in its reduced form with dioxygen.

Journal Article↗