Multi-electron Transfer by U(ɪɪ) and Masked U(ɪɪ) Complexes

Complexes of uranium in low oxidation state have shown the ability of activating non-reactive small molecules such as N2. However, the multi-electron transfer required for such activation, remains limited in uranium chemistry. Here, we review our recent research on the use of different strategies to overcome this issue, which has led to the isolation of a diuranium(ɪɪɪ) bridging oxide complex that reacts as a U(ɪɪ) synthon able to effect one-electron transfer per uranium center to N-heterocycles and multi-electron transfer to diphenylacetylene and azobenzene. We also showed that a closely related molecular U(ɪɪ) complex effects the same reactions providing the first unambiguous example of a monouranium four-electron transfer.

Il collegio dei dottori di Urbino. Dalle origini alla devoluzione del ducato.

Una breve nota intitolata «Memoria concernente l’erezione del Collegio Rotale di Urbino», pubblicata anonima nel 1816, costituisce l’unica bibliografia specifica sulla Magistratura urbinate

Conception et réactivité de complexes mono- et polymétalliques d'éléments f en bas degré d'oxydation

Au-delà de son importance dans l industrie nucléaire, la chimie d oxydoréduction de l uraniumretient de plus en plus l attention des chercheurs. En effet, la capacité toute particulière descomplexes d uranium à bas degré d oxydation à promouvoir des réductions originales par desvoies inhabituelles suscite actuellement un grand intérêt, tout particulièrement leur aptitude àactiver dans des conditions douces des petites molécules telles CO, CO2, N2, ou encore descomposés aromatiques et des azotures. Les composés d uranium, de part leurs propriétés decoordination tout à fait uniques pourraient offrir une alternative aux métaux de transitionclassiques pour la conception de catalyseurs. Cependant, comparativement aux métaux du bloc d,les processus polyélectroniques sont rares dans la chimie de l uranium à bas degré d oxidation quiest dominée par les transferts monoélectroniques. C est pourquoi le développement de nouveauxcomplexes d uranium capables de réaliser des réductions poly-électroniques est particulièrementintéressant. Le premier objectif de ce travail était d associer à l uranium des ligands non-innocentsservant de réservoir d électrons. Ainsi nous avons utilisé des bases de Schiff p-conjuguées pourexplorer la chimie de cet élément à bas degree d oxydation. Cela nous a permis d isoler descomplexes riches en électrons dans lesquels des électrons sont stockés sur le ligand via laformation de liaisons C-C. Ces mêmes liaisons sont rompues en présence d agent oxydant, et lesélectrons sont libérés pour réaliser des transformations polyélectroniques. Ce procédé a étéobservé pour plusieurs bases de Schiff, permettant de moduler les propriétés des composés. Dansune seconde approche, nous nous sommes intéressés à la synthèse et à l étude de la réactivité denouveaux complexes d uranium trivalent supportés par des ligands silanolates. De nouveauxcomposés dinucléaires d uranium à basse valence ont été obtenus. Ces composés très réactifsdécomposent spontanément en clivant des groupements tertiobutyls des ligands, conduisant à laformation de complexes d uranium(IV). En parallèle, un complexe monoanionique mononucléaired U(III) a été isolé, nous permettant de comparer la réactivité de l uranium trivalent dans différentsenvironnements stériques et électroniques. Ces études de réactivité ont permis de stabiliser unexemple rare de dimère d uranium ponté par un groupement CS22- et ont mis en évidence lacapacité de l uranium trivalent à promouvoir la dismutation de CO2 en carbonate et CO. La réactionde ces composés d uranium trivalent vis-à-vis d azotures organiques et inorganiques a produit denouveaux nitrures et nitrènes d uranium originaux. Enfin, la capacité de ces agents réducteurspuissants à transférer des électrons au toluène a permis d isoler une famille de complexessandwiches inversés où deux cations uranium sont liés de part et d autre d un cycle aromatique.Beyond its importance in nuclear industry the redox chemistry uranium is attracting increasinginterest because complexes of low-valent uranium can promote unusual reductive chemistrythrough unusual reaction pathways, including attractive examples of CO, CO2, N2, arenes andazides activation in mild condition. Due to the unique coordination and bonding properties ofuranium, its compounds could provide an attractive alternative to transition metals for thecatalytic transformation of small molecules. However, metal-based multi-electron processesremain uncommon in uranium chemistry especially in comparison with the d-block metals, thechemistry of low-valent uranium being dominated by single-electron transfers. In this context, thefirst aim of this project was to investigate the association of low-valent uranium to a non-innocentligand acting as an independent electron reservoir at a same molecule. Accordingly, weinterrogated the use of highly p-delocalized Schiff bases ligands for supporting low-valent uraniumchemistry. This led to the isolation of electron-rich complexes which are stabilized by storingelectrons on the ligands through the formation of C-C bonds. Interestingly, these C-C bonds can becleaved by oxidizing agents and the electrons released to participate in multi-electron redoxreactions. This process was observed within different Schiff-base ligand scaffolds, allowing atuning of the properties of the compounds. The second part of this work was dedicated to thesynthesis of novel trivalent uranium complexes supported by siloxy ligands and the study of theirredox reactivity and coordination properties. Novel dinuclear highly-reactive low-valent uraniumassemblies were developed. The study of their limited stability revealed that these compounds arespontaneously decomposing through the cleavage of tBu groups from the supporting ligandsresulting in the formation of U(IV) species. In parallel, a mononuclear trivalent uranium atecomplex was obtained, allowing to compare the reactivity of U(III) in different steric and electronicenvironements. Hence we became interested in studying the redox reactivity of these compoundswith different substrates including CO2, CS2, azides and arenes. These investigations led to thestabilization of a rare CS22- sandwich complex of uranium, and highlited the ability of U(III) topromote reductive disproportionation of CO2 to carbonate and CO. The reaction of these trivalenturanium siloxide species with organic and inorganic azides produced original uranium imidos andnitridos compounds with original topologies. Finally the capacity of these strongly reducing agentsto transfer electrons to the toluene fragment lead to the isolation of a family of arenes invertedsandwich complexes.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Complexes polynucléaires d'Uranium (structure réactivité et propriétés)

L'étude et la compréhension de la chimie fondamentale des actinides constitue un axe de recherche privilégié notamment dans le cadre de la technologie nucléaire tant en amont pour le développement de nouveaux combustibles qu'en aval pour l'étude du retraitement des déchets nucléaires. Une des problématiques principales au cours de ces études réside dans la capacité que possèdent les actinides à subir des réactions rédox et à former des assemblages polynucléaires. Néanmoins, très peu d'assemblages polynucléaires peuvent être synthétisés de manière reproductible, la plupart des complexes polynucléaires d'actinides décrits dans la littérature sont formés de façon fortuite plutôt que par conception rationnelle. En outre, les assemblages polynucléaires s'uranium ont été identifiés comme particulièrement prometteurs pour l'élaboration de matériaux magnétiques et pour leur réactivité. L'objectif de ce travail réside dans la synthèse d'assemblages polymétalliques à base d'uranium en mettant à profit quelques aspects de sa réactivité redox et de sa chimie de coordination. De nouvelles voies de synthèse de composés polynucléaires d'uranium ont été développées, et l'étude des propriétés physico-chimique des composés a été réalisée. La première approche utilisée repose sur la synthèse d'assemblages d'uranyle pentavalent. L'uranyle pentavalent est connu pour sa facilité d'agrégation via des interactions entre groupement uranyles appelées interaction cation-cation, mais l'isolation de ce type de composé a été très largement limitée par l'instabilité de l'uranyle(V) vis-à-vis de la dismutation. L'utilisation de ligands base de Schiff de type salen a permis dans ces travaux l'isolation du premier assemblage polynucléaire d'uranyle(V). Sur la base de ce résultat, la variation des ligands et des contre-ions utilisés a permis l'isolation d'une large famille d'assemblages polynucléaires d'uranyle(V) et l'étude fine des paramètres régissant leur stabilité. Par ailleurs, l'étude des propriétés magnétiques de ces assemblages a mis en valeur de rares exemples couplages antiferromagnétiques. En outre, cette voie de synthèse a été exploitée pour synthétiser le premier cluster 5f/3d présentant des propriétés de molécule aimant. Le deuxième axe d'approche suivi dans ce travail repose sur l'isolation de clusters oxo/hydroxo d'uranium. La réactivité d'hydrolyse de complexes d'uranium trivalent en présence de ligand à pertinence environnementale à permis la synthèse d'assemblages d'uranium dont la taille à pu être variée en fonction des conditions de synthèse employées. Enfin, de nouveaux assemblages présentant des topologies originales ont été isolés en exploitant la réactivité de dismutation de précurseurs d'uranium pentavalent.The study and comprehension of actinide's fundamental chemistry have important implications both for the development of new nuclear fuel and for the nuclear fuel reprocessing. One of the major issues in these processes is the ease of uranium to undergo redox reactions and to form polynuclear assemblies, which largely perturb these processes. However, despite their relevance, only few synthetic routes towards polynuclear uranium assemblies are described in the literature, and most of the polynuclear complexes reported are formed by serendipity rather than by rational design. Moreover, polynuclear uranium compounds are highly promising for the design of magnetic materials with improved properties and for reactivity studies. The aim of this work is the synthesis of polynuclear uranium complexes and the study of their reactivity and coordination properties. New synthetic routes to uranium polynuclear assemblies were developed and the study of their physico-chemical properties was carried out. The first approach investigated was based on pentavalent uranyl chemistry. Uranyl(V) is known to form aggregates via an interaction between uranyl moieties often named cation-cation interaction, but the isolation of uranyl(V) complexes had been largely limited by its ease of disproportionation. We isolated the first stable uranyl(V) polynuclear assembly using salen-type Schiff base ligand. Based on this result, a fine tuning of the ligand and counterion properties resulted in the isolation of a large family of uranyl(V) polynuclear assemblies and in a better understanding of the parameters ruling their stability. Moreover, rare examples of clear antiferromagnetic couplings were observed with these complexes. In addition, this synthetic path was used to build the first 5f-3d cluster presenting single molecule magnet properties. The second approach used in this thesis consisted in the synthesis of oxo/hydroxo uranium clusters. The controlled hydrolysis of trivalent uranium in presence of an environmentally relevant ligand lead to the synthesis of clusters, which size could be varied in function of the reaction conditions employed. Finally, new uranium clusters with original topologies were synthesised through the induced disproportionation of pentavalent uranyl precursors.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Four-electron Reduction and Functionalization of N₂ by a Uranium(III) Bridging Nitride

N2 is a cheap and widely available but very unreactive molecule. Notably, the only industrial process for the conversion of dinitrogen is the Haber-Bosch process to form ammonia, but under very harsh conditions (high temperature and pressure). Here, we review our recent research leading to the reduction of dinitrogen by four electrons, under ambient conditions by a uranium(III) bridging nitride, K3 U-N-U, where two uranium(III) cations are linked by a nitride group and a flexible metal-ligand scaffold. We also show that the bound dinitrogen can be further functionalized under mild conditions: the addition of acid, hydrogen and protons or carbon monoxide to the uranium hydrazido complex yields to the cleavage of the N-N single bond and to the formation of new N-H and N-C bonds

Synthesis and Structure of Nitride-Bridged Uranium(III) Complexes

The reduction of the nitride-bridged diuranium(IV) complex Cs[{U(OSi((OBu)-Bu-t)(3))(3)}(2)(mu-N)] affords the first example of a uranium nitride complex containing uranium in the +III oxidation state. Two nitride-bridged complexes containing the heterometallic fragments Cs-2[U-III - N - U-IV] and Cs-3[U-III - N - U-III] have been crystallographically characterized. The presence of two or three Cs+ cations binding the nitride group is key for the isolation of these complexes. In spite of the fact that the nitride group is multiply bound to two uranium and two or three Cs+ cations, these complexes transfer the nitride group to CS2 to afford SCN- and uraniurn(IV) disulfide

CO Cleavage and CO2 Functionalization under Mild Conditions by a Multimetallic CsU2 Nitride Complex

Novel efficient chemical processes involving cheap and widely accessible carbon dioxide or carbon monoxide under mild conditions for the production of valuable chemical products are highly desirable in the current energetic context. Uranium nitride materials act as high activity catalysts in the Haber-Bosch process but the reactivity of molecular nitride compounds remains unexplored. Here we review recent results obtained in our group showing that a multimetallic nitride complex [Cs{[U(OSi(OtBu)(3))(3)](2)(mu-N)}] (1) with a CsUIV-N-U-IV core, is able to promote N-C bond formation due to its strong nucleophile behaviour. In particular, complex 1, in the presence of excess CO2 leads to a remarkable dicarbamate product. The multimetallic CsUIV-N-U-IV nitride also readily cleaves the C O bond under mild conditions

Synthesis and Characterization of a Water Stable Uranyl(V) Complex

We have identified a polydentate aminocarboxylate ligand that stabilizes uranyl(V) in water. The mononuclear [UO2(dpaea)]X, (dpaeaH2 = Bis(pyridyl-6-methyl-2-carboxylate)-ethylamine; X = CoCp2*+ or X = K(2.2.2.cryptand) complexes have been isolated from anaerobic organic solution, crystallographically and spectroscopically characterized both in water and organic solution. These complexes disproportionate at pH ≤ 6, but are stable in anaerobic water at pH 7–10 for several days

Nitrogen reduction and functionalization by a multimetallic uranium nitride complex

Molecular nitrogen (N-2) is cheap and widely available, but its unreactive nature is a challenge when attempting to functionalize it under mild conditions with other widely available substrates (such as carbon monoxide, CO) to produce value-added compounds. Biological N-2 fixation can do this, but the industrial Haber–Bosch process for ammonia production operates under harsh conditions (450 degrees Celsius and 300 bar), even though both processes are thought to involve multimetallic catalytic sites (1, 2). And although molecular complexes capable of binding and even reducing N-2 under mild conditions are known, with co-operativity between metal centres considered crucial for the N-2 reduction step (1-14), the multimetallic species involved are usually not well defined, and further transformation of N-2-binding complexes to achieve N–H or N–C bond formation is rare (2, 6, 8, 10, 15, 16). Haber noted (17), before an iron-based catalyst was adopted for the industrial Haber–Bosch process, that uranium and uranium nitride materials are very effective heterogeneous catalysts for ammonia production from N-2. However, few examples of uranium complexes binding N-2 are known (18-22), and soluble uranium complexes capable of transforming N-2 into ammonia or organonitrogen compounds have not yet been identified. Here we report the four-electron reduction of N-2 under ambient conditions by a fully characterized complex with two U-iii ions and three K+ centres held together by a nitride group and a flexible metalloligand framework. The addition of H-2 and/or protons, or CO to the resulting N-2(4-)complex results in the complete cleavage of N-2 with concomitant N-2 functionalization through N–H or N–C bond-forming reactions. These observations establish that a molecular uranium complex can promote the stoichiometric transformation of N-2 into NH3 or cyanate, and that a flexible, electron-rich, multimetallic, nitride-bridged core unit is a promising starting point for the design of molecular complexes capable of cleaving and functionalizing N-2 under mild conditions