Synthèse et réactivité de complexes de diazote des groupes 6 et 7

Nitrogen is an almost inexhaustible source of nitrogen. Its transformation into value-added organonitrogen compounds is a challenge for chemists. Indeed, the very high stability of this molecule makes its activation and transformation difficult. Today, only the Haber-Bosch process is capable of transforming nitrogen into ammonia industrially. This process is also very energy-intensive and contributes to almost 2% of the world's CO2 emissions per year. In this context, chemists are trying to find alternatives to transform nitrogen under milder and greener conditions. One method of choice is to activate this small molecule at a metal center. In this thesis, different ways are explored to functionalize dinitrogen, via its coordination to group 6 and 7 metals. The first strategy is based on the addition of a nucleophile on a dinitrogen ligand. This objective led us to revise a mechanism proposed more than 40 years ago for the functionalization of dinitrogen with organolithium reagents, thanks to a combined study between experiment (real-time infrared spectroscopy, nuclear magnetic resonance, X-ray diffraction) and theory (quantum chemistry calculations). The work presented in the second chapter derives from the first one and explores the addition of electrophiles to manganese ate-complexes. Finally, in the third and last chapter, the synthesis of several heterobimetallic complexes with a bridging dinitrogen ligand is described. In these edifices, each nitrogen atom of the small molecule is bound to a different metal. These results enabled us to demonstrate the influence of the Lewis acidity of a metal on the cooperative activation of the dinitrogen ligand, thanks to the structures obtained by X-ray diffraction. A rapid evaluation of the reactivity of one of the most interesting complexes in terms of dinitrogen functionalization was then carried out.Le diazote est une source quasi inépuisable d'azote. Sa transformation en composés organo-azotés à valeur ajouté est un défi pour les chimistes. En effet, la très grande stabilité de cette molécule rend son activation et sa transformation difficile. Aujourd'hui, seul le procédé Haber-Bosch est capable de transformer industriellement le diazote en ammoniac. Ce procédé est, également, très énergivore et contribue à près de 2 % des émissions annuelles de CO2 dans le monde. C'est dans ce contexte que les chimistes tentent de trouver des alternatives pour transformer le diazote dans des conditions plus douces et plus vertes. Une méthode de choix est d'activer cette petite molécule à un centre métallique. Dans cette thèse, différentes voies sont explorées pour fonctionnaliser le diazote, via sa coordination à des métaux des groupes 6 et 7. La première stratégie repose sur l'addition d'un nucléophile sur un ligand diazote. Cet objectif nous a amené à réviser un mécanisme proposé il y a plus de 40 ans pour la fonctionnalisation du diazote par des organolithiens, grâce à une étude combinée entre expérience (spectroscopie infrarouge en temps réel, résonance magnétique nucléaire, diffraction des rayons X) et théorie (calculs de chimie quantique). Les travaux présentés dans le second chapitre découlent du premier. Y est exploré l'ajout d'électrophiles sur des ate-complexes au manganèse. Enfin, dans le troisième et dernier chapitre, la synthèse de plusieurs complexes hétérobimétalliques avec un ligand diazote pontant est décrite. Dans ces édifices, chaque atome d'azote de la petite molécule est lié à un métal différent. Ces résultats nous ont permis de démontrer l'influence de l'acidité de Lewis d'un métal sur l'activation coopérative du ligand diazote, grâce aux structures obtenues par diffraction des rayons X. Une rapide évaluation de la réactivité d'un des complexes les plus intéressants en terme de fonctionnalisation du diazote a ensuite été menée

Syntheses of N2-bridged heterobimetallic complexes, their structural and qualitative bonding analyses

International audienceA series of N2-bridged (μ-η1:η1-N2) heterobimetallic complexes were prepared by the reaction of the dinitrogen complex [Mo(depe)2(N2)2] (depe = 1,2-bis(diethylphosphino)ethane) with the group IV-VI high valent metal halides [Cp2ZrCl2], NbCl5, TaCl5, MoCl5 and WCl4 via chloride substitution. Compounds [MoCl(depe)2(μ-N2)ZrCp2Cl] (1) and [MoCl(depe)2(μ-N2)M'Cl4(THF)] (2–5, M’ = Nb, Ta, Mo or W) were crystallized from the reaction mixtures. In the case of the reaction with WCl4, disproportionation of W(IV) was suspected but not evidenced. Single crystal X-ray diffraction analyses on all compounds allowed a systematic structural study, and the sketching of qualitative molecular orbital diagrams helped discussing the bonding situation and bolster the structural data

An Experimental and Computational Investigation Rules Out Direct Nucleophilic Addition on the N2 Ligand in Manganese Dinitrogen Complex [Cp(CO)2Mn(N2)]

International audienceWe have re-examined the reactivity of the manganese dinitrogen complex [Cp(CO)2Mn(N2)] (1, Cp = η5-cyclopentadienyl, C5H5) with phenylithium (PhLi). By combining experiment and density functional theory (DFT), we have found that, unlike previously reported, the direct nucleophilic attack of the carbanion onto coordinated dinitrogen does not occur. Instead, PhLi reacts with one of the CO ligand to provide an anionic acylcarbonyl dinitrogen metallate [Cp(CO)(N2)MnCOPh]Li (3) that is stable only below –40 °C. Full characterization of 3 (including single crystal X-ray diffraction) was performed. This complex decomposes quickly above –20 °C with N2 loss to give a phenylate complex [Cp(CO)2MnPh]Li (2). The latter compound was erroneously formulated as an anionic diazenido compound [Cp(CO)2MnN(Ph)=N]Li in earlier reports, ruling out the claimed and so-far unique behavior of the N2 ligand in 1. DFT calculations were run to explore both the hypothesized and the experimentally verified reactivity of 1 with PhLi and are fully consistent with our results. Direct attack of a nucleophile on metal-coordinated N2 remains to be demonstrated

Assessing Combinations of B(C6F5)3 and N2-Derived Molybdenum Nitrido Complexes for Heterolytic Bond Activation

Two different dinitrogen-derived molybdenum nitrido complexes varying by their geometry, ligand spheres and oxidations states were shown to engage their N ligand in dative bonding with the strong Lewis acid B(C6F5)3. The stable adducts were assessed for frustrated Lewis pair-type heterolytic E–H bond splitting of hydrosilanes (E = Si) and HB(C6F5)2. Whereas Si–H bond activation was achieved, HB(C6F5)2 was shown to substitute B(C6F5)3 in a quantitative or equilibrated fashion, depending on the nature of the nitrido complex. No B–H bond splitting was observed. Thermodynamics of these reactions, computed by DFT, are in agreement with the experimental outcomes