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Julie Andrez

Synthesis of molecular complexes for small molecule activation

Published on 11 November 2016
Thesis presented November 11, 2016

The redox chemistry of f-elements is drawing the attention of inorganic chemists due to their unusual reaction pathways. Notably low-valent f-element complexes have been shown to be able to activate small molecules such as CO2 and N2 in mild conditions. Compared to d- block metals, f-elements present a coordination chemistry dominated by electrostatic interactions and steric constraints. Molecular complexes of f-elements could thus provide new catalytic routes to transform small molecules into valuable chemicals. However the redox chemistry of low valent f-elements is dominated by single-electron transfers while the reductions of CO2 and N2 require multi-electronic processes.
Accordingly the first approach of this PhD work was the use of redox active ligands as electron reservoir to support f-element centers increasing the electron number available for reduction events. The coordination of uranium with tridentate Schiff base ligand was investigated and led to isolation of a dinuclear electron-rich species able to undertake up to eight-electron reduction combining the redox activity of the ligands and the uranium centers. In order to obtain electron-rich compounds potentially able to polarize the C=O bond of CO2, the synthesis of heterobimetallic species supported by salophen Schiff base ligand was also studied.
In a second approach we have used bulky ligands with strong donor-character to tune the reducing abilities of low valent f-elements. In this case a bimolecular electron-transfer process is often observed. The reactivity of the U(III) siloxide complex [U(OSi(OtBu)3)4K] was further investigated. Notably, reaction with Ph3PS led to the formation of a terminal U(IV) sulfide complex with multiple U-S bond which was analyzed by DFT studies to better understand the bonding nature. Preliminary studies on the role of the counter-cation (M) in the system [U(OSi(OtBu)3)4M] on the outcome of the reactivity with CS2 and CO2 have also been performed. The enhancement of the reducing power of the “classical” divalent lanthanide ions was studied using two different ligand systems. The coordination of siloxide ligands to divalent Eu, Yb and Sm metal centres led to highly reactive compounds featuring unique reactivity with small molecules such as azobenzene, CS2 and CO2. Finally, syntheses and electrochemistry analyses of a family of homoleptic lanthanide complexes supported by related tripodal aminophenolate ligands allowed us to quantify the influence of the ligand donor-character and charge on the reduction potential of the lanthanide center. The reactivity of an Eu(II) system with CS2 was also investigated.

f-elements, redox active ligand, heterobimetallic complexes, cobalt, siloxide, cooperative effect, aminophenolate, small molecules

On-line thesis.