CO2 and CO/H-2 Conversion to Methoxide by a Uranium(IV) Hydride

Here we show that a scaffold combining siloxide ligands and a bridging oxide allows the synthesis and characterization of the stable dinuclear uranium(IV) hydride complex [K-2{U(OSi((OBu)-Bu-t)(3))(3)(mu-O)(mu-H)(2)}], 2, which displays high reductive reactivity. The dinuclear bis-hydride 2 effects the reductive coupling of acetonitrile by hydride transfer to yield [K-2{U(OSi((OBu)-Bu-t)(3))(3)(mu-O)(mu-kappa(2)-NC(CH3)NCH2CH3)}], 3. Under ambient conditions, the reaction of 2 with CO affords the oxomethylene(2-) reduction product [K-2{U(OSi((OBu)-Bu-t)(3))(3)(mu-CH2O)(mu-O)}], 4, that can further add H 2 to afford the methoxide hydride complex [K-2{U(OSi((OBu)-Bu-t)(3))(3)(mu-OCH3)(mu-O)(mu-H)}], 5, from which methanol is released in water. Complex 2 also effects the direct reduction of CO2 to the methoxide complex 5, which is unprecedented in f element chemistry. From the reaction of 2 with excess CO2, crystals of the bis-formate carbonate complex [K-2{U(OSi((OBu)-Bu-t)(3))(3)(mu-CO3)(mu-HCOO)(2)}], 6, could also be isolated. All the reaction products were characterized by X-ray crystallography and NMR spectroscopy.

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

Small Molecule Activation by Multimetallic Uranium Complexes

Marta Falcone

EPFL2019