Facile N-functionalization and strong magnetic communication in a diuranium(v) bis-nitride complex

Uranium nitride complexes are of high interest because of their ability to effect dinitrogen reduction and functionalization and to promote magnetic communication, but studies of their properties and reactivity remain rare. Here we have prepared in 73% yield the diuranium(V) bis-nitride complex [K2{[U(OSi(OtBu)3)3]2(μ-N)2}], 4, from the thermal decomposition of the nitride-, azide-bridged diuranium(IV) complex [K2{[U(OSi(OtBu)3)3]2(μ-N)(μ-N3)}], 3. The bis-nitride 4 reacts in ambient conditions with 1 equiv. of CS2 and 1 equiv. of CO2 resulting in N–C bond formation to afford the diuranium(V) complexes [K2{[U(OSi(OtBu)3)3]2(μ-N)(μ-S)(μ-NCS)}], 5 and [K2{[U(OSi(OtBu)3)3]2(μ-N)(μ-O)(μ-NCO)}], 6, respectively. Both nitrides in 4 react with CO resulting in oxidative addition of CO to one nitride and CO cleavage by the second nitride to afford the diuranium(IV) complex [K2{[U(OSi(OtBu)3)3]2(μ-CN)(μ-O)(μ-NCO)}], 7. Complex 4 also effects the remarkable oxidative cleavage of H2 in mild conditions to afford the bis-imido bridged diuranium(IV) complex [K2{[U(OSi(OtBu)3)3]2(μ-NH)2}], 8 that can be further protonated to afford ammonia in 73% yield. Complex 8 provides a good model for hydrogen cleavage by metal nitrides in the Haber–Bosch process. The measured magnetic data show an unusually strong antiferromagnetic coupling between uranium(V) ions in the complexes 4 and 6 with Neel temperatures of 77 K and 60 K respectively, demonstrating that nitrides are attractives linkers for promoting magnetic communication in uranium complexes.

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

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

Luciano Barluzzi

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.

EPFL2021

Small Molecule Activation by Multimetallic Uranium Complexes

Marta Falcone

EPFL2019

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

Luciano Barluzzi

EPFL2021