Design, synthesis, reactivity, and magnetic properties of uranium based oxide and nitride complexes

The activation of small molecules is a paramount challenge in modern chemistry. The use of cheap and abundant molecules such as N2, H2, CO2, or CO as energy supplies or as precursors for fine chemicals production is highly desirable. In particular, the only industrial process known so far that uses the ubiquitous molecule N2 is the Haber-Bosch process for the production of NH3, which consumes about 2% of the world energy yearly. Low valent uranium complexes have been proven to be ideal candidates in the field of small molecule activation such as N2. The activation of N2 in the Haber-Bosch process is thought to proceed through the formation of metal nitrides. The study of molecular nitride species, thus, is extremely important in light of the possibility to achieve N-functionalization processes. Uranium nitride species, indeed, are able to effect the activation of small molecules by N-functionalization. Low valent uranium species are of interest also for their magnetic properties. The magnetic anisotropy and the unpaired electron in U(III) species make them, indeed, ideal candidates as Single Molecule Magnets (SMM). The main objective of this thesis is to synthesize novel uranium species for the activation of small molecules and to synthesize low valent uranium species of interest for their magnetic properties. In the first part the synthesis of a dimeric U(III) complex bridged by an oxo linker and its reactivity with N2 will be reported. The functionalization of the activated N2 shows striking differences compared to previously reported complexes. The difference in reactivity is accompanied by a difference in the magnetic properties of the complexes, which reflects their different electronic structure and binding scheme. Since the full splitting of N2 could not be achieved so far with U(III) complexes, the synthesis of a bis-µ-nitride complex, which would serve as a model for cleaved N2, was targeted. Its synthesis and characterization are reported in Chapter 3. We proved that such a bis nitride complex is reactive and is able to activate small molecules, forming N-C bonds by reaction with molecules like CO2, CS2, or CO. More importantly, H2 is activated by the bis nitride complex in an unprecedented oxidative cleavage. While uranium nitride complexes supported by the siloxide ligand were reported to exhibit nucleophilic reactivity, other nitride complexes are instead unreactive or even electrophilic. In order to rationalize this behaviour, the synthesis of different uranium nitride complexes bearing different coligands has been targeted and their synthesis is described in Chapter 4. The importance of the coligand has been experimentally proven in closely related systems. Despite several bridging nitride complexes have been reported, a paucity of terminal uranium nitride complexes are, instead, isolable. In particular, the photochemical reactivity of terminal uranium azide complexes has so far failed to yield the desired terminal nitride species. In this work we report the first photochemical synthesis of an isolable uranium terminal nitride. The terminal nitride complex is remarkably stable and its functionalization with CO is reported. Lastly, the synthesis of low valent heteroleptic complexes is reported. These compounds can serve as starting materials for the synthesis of novel uranium complexes and are of interest for their magnetic properties.

Small Molecule Activation by Multimetallic Uranium Complexes

Marta Falcone

The development of new processes for the selective and sustainable transformation of abundant small molecules constitutes one of the major research areas in inorganic and organometallic chemistry. These molecules as CO2, CO, N2 and H2 are abundant reservoirs of chemical energy, readily available sources of carbon, nitrogen and hydrogen. In a patent from 1909, Haber reported that uranium and uranium nitride were very efficient catalysts for ammonia synthesis. In order to perform multi-electron transfer, multimetallic complexes need to be designed. Reactivity of uranium bridging nitride remained unexplored since we investigated the nitride based reactivity of a dinuclear uranium(IV) bridging nitride complex towards small molecules such as CO2, yielding the first example of a uranium complex containing a bridging dicarbamate ligand, or CO, resulting in the complete cleavage in mild conditions of one of the strongest bonds in nature, or H2, yielding one of the few examples in f-elements of metal hydride, which is readily transferred to substrates such as CO2. Beyond the ligand based reactivity, we were interested in reducing the two metal centers to access a diuranium(III) bridging nitride, to pursue nitrogen activation. This complex was shown to have high flexibility, provided by the siloxide ligands, high reducing power and a bridging atom, the nitride, that holds together the uranium centers, with the possibility to bend upon nitrogen activation. This approach led us to the isolation of a hydrazido complex, resulting from the four-electron reduction of dinitrogen by the two metal centers. Moreover, complete cleavage of the NâN single bond was achieved by addition of CO to give two cyanate ligands or by addition of H2 to give ammonia. The nitride atom linker is nonetheless, too reactive to act as a spectator and it is involved in reactivity. Hence, we target the isolation of the analogue dinuclear U(III) complex with an oxide ligand as linker. This complex showed the same reactivity towards nitrogen but DFT revealed a more ionic character of the UâO bond nature with respect to the UâN bond, leading to different reactivity of the hydrazido fragment upon addition of CO and upon addition of H2, showing a smaller extent of activation in the oxo complex with respect to the nitride. The diuranium(III) bridging oxide complex showed interesting reactivity with H2, forming a bis-hydride complex capable of reducing CO2 beyond the formate level, to methanol.

EPFL2019

Stereoselective synthesis of alkenes via base-metal-catalyzed addition reactions to alkynes and activation of hydrogen peroxide and carbon dioxide by iron complexes

Fedor Zhurkin

Alkene functionality can be found in the majority of natural products, drugs, catalysts and organic materials. Therefore, methods of C-C double bond formation constitute a cornerstone of organic synthesis. Selective formation of either (Z)- or (E)-isomer is especially desirable. 1,2-Addition to alkynes unites a large arsenal of methods of C-C double bond formation, among which the addition of carbon-centered species is the most interesting, since it offers a possibility of construction of the carbon skeleton. The first part of the present dissertation is devoted to the development of methods of selective addition of carbon reagents to alkynes. Chapter 1 reviews the existing methods of addition of carbon-centered species, generated from organometallic reagents, organic halides or carbonyl compounds to C-C triple bond with the accent on their stereoselectivity. Carbometalative mechanisms lay in the background of the majority of the reactions in this chemistry. The Z/E selectivity of 1,2-addition to triple bond can be controlled via predominance of either syn- or anti-addition or via Z/E isomerization of the addition intermediates. In Chapter 2 a novel method of disubstituted alkene synthesis via reductive addition of alkyl halides to terminal arylalkynes is described. The remarkable feature of this method is highly selective formation of (Z)-alkenes (generally Z/E > 10:1). Minor modifications of the reducing agent allowed the extension of the substrate scope to the primary alkyl iodides. Mechanistic studies revealed that this reaction proceeds as a formal anti-carbozincation. The resulting organozinc species form disubstituted (Z)-alkenes upon aqueous work-up. On the other hand, it would be interesting to utilize these species in cross-coupling reactions to allow the selective synthesis of trisubstituted alkenes. In Chapter 3 a catalytic system for highly regioselective copper-catalyzed allylation of these reagents is described. In Chapter 4 the results on stereoselective intermolecular addition of aldehydes and ketones to alkynes in the presence of zinc and trimethylchlorosilane are summarized. This reaction gives 1-chloro-1,3-dienes as products. The activation of small molecules has become an increasingly attractive area of research over the last years. In Chapter 5 an outline is given of the literature examples of small-molecule activation, namely activation of carbon dioxide and hydrogen peroxide, by iron complexes. In Chapter 6 the studies on small-molecule activation by iron-sulfur cubane clusters are presented. Electrochemical reduction of carbon dioxide and hydrogenation of alkenes were studied. As an additional result, synthesis and characterization of an unprecedented organic iron(I) ate-complex is reported. Structural modification of a non-heme iron complex with hydrogen-bond-donating substituents is described in Chapter 7. The deep impact of these modifications on the structure and reactivity of the resulting complex is discussed.

EPFL2017

Activation of small molecules promoted by multimetallic complexes supported by redox active ligands

Davide Toniolo

The activation of small molecule is a topic of high current interest. A variety of homogeneous end heterogeneous catalysts have been studied to be able to promote chemical transformation of industrial relevance under mild condition and therefore decrease the costs. Often the most performant catalysts are composed by rare and precious metals such as Pd, Pt, Rh, Ir, making the catalyst expensive and not suitable for large scale reactions. Lots of effort is put in researching ways to substitute precious and rare metal while keeping catalytic efficiency and selectivity. F-elements have shown to be able to perform many chemical transformations that are common to d-metals, promoting also in some cases unusual chemical reactivity. Differently from transition metals, f-elements present a unique coordina-tion chemistry, governed by electrostatic interaction with the ligand environment and steric constrain. During my PhD I focused my attention on the synthesis of highly reactive, low valent metal complex-es based of f-elements for the activation of small molecules. In particular, two main pathways have been followed to promote the multi-electrons transfer necessary for the transformation of the substrate: The first approach consists in the use of redox active ligands to support the metal centre and act as electron reservoir in order to increase the number of electrons exchanged during the reaction. This method has been used for d-, f- and d- mixed f-elements complexes, resulting to be an elegant way for the preparation of multimetallic compounds capable of promoting multi-electron transfer reaction. In the second approach, divalent classic lanthanides have been stabilized thanks to the bulky tris(tert-butoxide)siloxide ligand that resulted a to be a useful tool for the formation of highly reactive bimetal-lic complexes. The reactivity of the synthetized complexes towards small molecules such as CO2, CS2 and arenes has been tested, proving the ability of those compounds to act as multielectron reducing reagents. Moreo-ver, we proved that multidentate Schiff base ligands are capable of stabilizing low valent d-metal com-plexes allowing the formation bonding interaction between different metals with the enhancement of the magnetic properties.

EPFL2020