Paul Rey

TWO-DIMENSIONAL NITROXIDE-BASED MOLECULAR MAGNETIC MATERIALS

Manganese(II) complexes of imidazolyl-substituted chelating nitronylnitroxides are characterized by a stereoselective arrangement of the metal centers and the ligands to give a honeycomblike layer structure (shown on the right). The magnetic properties at high temperature indicate the presence of a ferrimagnetic structure within the layers. At low temperatures, ferromagnetic ordering is observed.

Control of the structural dimensionality in hydrogen-bonded self-assemblies of open-shell molecules. Extension of intermolecular ferromagnetic interactions in α-phenyl nitronyl nitroxide radicals into three dimensions

The variation of the number and position of OH substituents at the phenyl ring of α-phenyl nitronyl nitroxide radicals yields different hydrogen-bonded molecular self-assemblies with distinct dimensionalities; the mono-ortho-substituted isomer is most remarkable since it shows a three-dimensional network of weak hydrogen bonds exhibiting a bulk ferromagnetic transition at 0.45 K.

Nitronyl and imino nitroxides: improvement of Ullman's procedure and report on a new efficient synthetic route.

The synthesis of nitronyl and imino nitroxides has been reexamined with the aim of both increasing yields and of offering opportunities for new structures. The conditions for the formation of 2,3-bis(hydroxyamino)-2,3-dimethylbutane, the key intermediate of Ullmans route, have been carefully studied, and a new procedure is proposed, which affords the free base in a very pure form and up to 60% yield. Full characterization of this intermediate including an X-ray crystal structure is presented. An alternative synthetic route through 2,3-diamino-2,3-dimethylbutane and the corresponding imidazolidines which bypasses the delicate synthesis of the bis(hydroxyamino) compound is described. It is shown that 3-chloroperbenzoic acid is an effective oxidant for the transformation of adequately substituted imidazolidines into nitronyl nitroxides, which are obtained in high yield. An illustration of the potentialities of this new route, a new nitronyl nitroxide with two ethyl substituents in positions 4 and 5 of the imidazoline ring, is reported. The scope and limitations of the two routes are discussed.

Towards a Better Understanding of the Magnetic Interactions withinm-Phenyleneα-Nitronyl Imino Nitroxide Based Biradicals

An extensive investigation of the magnetic properties of three series of biradicals (bis-nitronyl nitroxides diNN-R, bis-imino nitroxides diIN-R and mixed INNN-R, where R is either hydrogen, a triple bond or trimethylsilylacetylenic group) has been carried out to give clear values of the intramolecular interactions through the m-phenylene coupling unit with alpha-nitronyl nitroxides (NN) or alpha-imino nitroxides (IN). An EPR study of the molecules in the isolated state is validated by ab initio calculations, which show the respective influence of spin polarisation and molecular conformation on the singlet-triplet gaps. All these results indicate that the triplet state is the ground state for such biradicals, except when the imidazolyl cycles are orthogonal to the phenyl ring. The magnetic properties of the biradicals in the solid state can be rationalised by examination of the short contacts produced between the ONCNO and ONCN groups. EPR studies on single crystals of the H-substituted series have confirmed the presence of a structural distortion for diNN-H whereas diIN-H and INNN-H do not exhibit such a peculiarity. The magnetic behaviour of diIN-H is described well by a four-spins model, with a strong intermolecular antiferromagnetic interaction of -90 K, whereas in the case of the two other compounds, a supplementary contact involves more complex interactions between the dimers. The compound diNN-tmsa exhibits a ferromagnetic intermolecular interaction of +11 K within the dimers, and this could be attributed to the relative disposition of the imidazolyl rings. Compound diNN-tr reveals a chain-like behaviour, whereas diIN-tr shows a predominant antiferromagnetic interaction within the dimers. The values for the intramolecular interactions in the solid state are in good agreement with those found for the isolated molecules.

A thorough investigation of the synthetic problems of vic-bis-hydroxylamine - the precursor of Ullman's nitroxides

Abstract The products formed in the course of the reduction of 2,3-dimethyl-2,3-dinitrobutane to 2,3-dihydroxylamino-2,3-dimethylbutane in a Zn/NH4Cl/EtOH-H2O system have been investigated. An improved synthesis of 2,3-dihydroxylamino-2,3-dimethylbutane (and its monosulphate salt) - the key precursor of Ullmans nitroxides - in a Zn/NH4Cl/THF-H2O system is proposed.

Synthesis of vicinal bishydroxylamine

Reduction of 2,3-dimethyl-2,3-dinitrobutane with Zn in aqueous ethanol in the presence of NH4Cl affords 2,3-bis(hydroxylamino)-2,3-dimethylbutane together with 2,3-diamino-2,3-dimethylbutane and complex Zn salts. A modified procedure was developed for the synthesis of bishydroxylamine, which involves reduction in a Zn/NH4Cl/THF−H2O system.

Intrinsic and environmental effects in the physico-chemical properties of nitroxides. The case of 2-phenyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide

Abstract The geometry, conformational behaviour and magnetic properties of 2-phenyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide, an important spin carrier in molecular magnetic materials, have been studied by a combined experimental and theoretical approach. The available structural data have been completed by the electronic spin resonance spectrum in aqueous solution, and we have utilised a hybrid Hartree–Fock/Kohn–Sham method coupled to a polarizable continuum representation of the solvent. The computations show that the molecule has a non-planar conformation in the gas phase and in solution, but that the molecular magnetic properties are significantly tuned by environmental effects.

Zweidimensionale, molekulare magnetische Materialien auf Nitroxidbasis

Mangan(II)-Komplexe von Imidazolyl-substituierten, chelatisierenden Nitronylnitroxiden sind durch die Zusammenlagerung der Metallzentren und der Nitroxidliganden mit hoher Stereoselektivitat zu einer Honigwabenschichtstruktur charakterisiert (Bild rechts). Die magnetischen Eigenschaften bei hohen Temperaturen sprechen fur eine ferrimagnetische Struktur innerhalb der Schichten. Bei niedrigen Temperaturen tritt ferromagnetische Ordnung auf.

The Hydrogen Bonding Strategy. A New Approach Towards Purely Organic/Molecular Ferromagnets

Abstract The variation of the number and position of OH substituents at the phenyl ring of α-phenyl nitronyl nitroxide radicals yields different H-bonded molecular self-assemblies in the solid state but do not change essentially their molecular spin density distributions. In accordance with the distinct structural dimensionalities, the obtained radicals show a large variety of magnetic behaviors, being the mono ortho substituted isomer the most remarkable one since it shows a 3-D network of weak H-bonds exhibiting a bulk ferromagnetic transition below 0.45K.

Le téréphthalate de di(tétraméthyl-2,2,6,6 pipéridinyl-4 oxyle-1) est-il un biradical à échange fort ou à échange faible?

La comparaison des intensites de signaux de resonance paramagnetique electronique de solutions de biradicaux a echange interelectronique faible, fort ou intermediaire, montre que le terephthalate est un biradical a echange faible.