לוח שנה משולב

Typically carbon-based materials with extended π systems, based on carbon, such as graphenic materials, are studied by Raman and XRD techniques. It is rather surprising that up to now Electron Paramagnetic Resonance (EPR) spectroscopy is little exploited to study such materials. The EPR technique is particularly convenient to study in detail the magnetic properties of graphitic and graphenic materials, as it selectivity allows to distinguish the contribution from the different paramagnetic species, namely free electrons, edge states and molecular-like radicals. All these contributions are responsible of important properties, like conduction and magnetism.

N2 Cleavage and NH3 Activation by Surface Organometallic Chemistry on silica: Mechanistic Relevance of Metal-hydride Bonds
and H2 Heterolytic Splitting

Elsje Alessandra Quadrelli,* Mostafa Taoufik and Hong-Peng Jia Université de Lyon, ICL, UMR 5265 C2P2 CNRS- Université Lyon 1- CPE-Lyon, Equipe COMS, 43 Blvd du 11 Novembre 1918, BP 2077 69616, Villeurbanne, France
e-mail: quadrelli@cpe.fr

A draft mechanism for nitrogenase catalytic conversion of N2 to ammonia is now available, assigning a crucial role to bridging iron-hydride bonds to start N2 reduction on the FeMo cofactor:1 two metal-hydride bonds function, one might say, as an intermediary “electron storage” device, the two electrons which are necessary for the first reduction of N2 to diazenido becoming available upon H2 release.

We have reported a heterogeneous system based on silica-supported tantalum hydrides capable of achieving N2 cleavage with H2 for which we have uncovered a molecular mechanism that entails the same mechanistic feature.3 This presentation will detail our studies leading to the proposed mechanism (thereincluded reaction hydrazido and diazenido intermediates, DFT studies,3 reaction with hydrazine), the reaction of the same hydrides with ammonia4 and the catalytic H/D exchanges studies with D2 and ND3 on final imido complexe4.

We will conclude with an attempt to compare and contrast our mechanism with the current proposal in nitrogenase, Haber-Bosch and homogeneous Schrock-type catalyses to highlight the role of dihydrogen heterolityc splitting and metal-hydride bonds in dinitrogen cleavage and ammonia activation reactions.5

References
1. [a] B. M. Hoffman, D. Lukoyanov, D. R. Dean, L. C. Seefeldt, Accounts of Chemical Research 2013, 46(2), 587. [b] B. M. Hoffman, D. Lukoyanov, Z-Y Yang, D. R. Dean, L. C. Seefeldt, Chem Rev 2014, doi 10.1021/cr4004641x.
2. P. Avenier, M. Taoufik, A. Lesage, X. Solans-Monfort, A. Baudouin, A. de Mallmann, L. Veyre, J.-M. Basset, O. Eisenstein, L. Emsley, E. A. Quadrelli Science 2007 317, 1056.
3. X. Solans-Monfort, C. Chow, E. Gouré, Y. Kaya, J.-M. Basset, M. Taoufik, E. A. Quadrelli, O. Eisenstein Inorganic Chemistry 2012, 51(13), 7237.
4. C. Chow, M. Taoufik, E. A. Quadrelli, Eur. J. Inorg. Chem. 2011, (9), 1349.
5. H.-P. Jia and E. A. Quadrelli Chem. Soc. Rev. (2014), 43, 547-564.

Magnetic resonance is used mostly to image the intense signal arising from the water molecules in vivo, yielding high resolution anatomical maps. However, by suppressing the water signal, it is possible to detect the much weaker signals of less abundant metabolites, including creatine, choline, GABA, glutamine/glutamate and several others: this is termed Magnetic Resonance Spectroscopy (MRS). I will attempt to provide a broad overview of how this metabolic information can be leveraged to study the human brain by presenting in-vivo data from our multiple sclerosis cohort, as well as discuss the main difficulties associated with MRS and how the research we conduct aims to rectify them.