Heavy Alkaline-Earth Metal Hydride Complexes: Syntheses, Structures and Applications as Super Reducing Agents

• Applicant: Professor Dr. Sjoerd Harder
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 401106702

The general interest in alkaline-earth (Ae) metal hydride complexes is rapidly increasing. This is mainly due to the recent recognition that earth-abundant, biocompatible, group 2 metals like Mg or Ca can be used in homogeneous catalysis but also fuelled by the hydrogen storage capabilities of the lighter Mg hydrides. Although there are currently more than 40 well-defined Mg hydride complexes, there is a lack of knowledge on heavier Ae hydride metal complexes (Ae = Ca, Sr, Ba). With a handful of crystal structures of Ca hydride complexes, well-defined Sr and Ba complexes were until very recently essentially unknown. Although partially due to the very high reactivity of the heavier group 2 metal hydrides this deficiency mainly originates from the very dynamic behaviour of these compounds, resulting in fast ligand exchange and formation of insoluble AeH2 salts. Going down group 2, the challenge to stabilize larger Ae metal hydride complexes with multi-dentate and bulky ligands becomes increasingly challenging. Very recently, we unexpectedly found that large ligands are not needed. Starting from a widely used amide precursor, Ae[N(SiMe3)2]2, the first Sr hydride complex could be obtained. The cluster, Sr6H9[N(SiMe3)2]3∙(PMDTA)3, has been stabilized by the neutral tridentate PMDTA ligand MeN(CH2CH2NMe2)2. Although the analogue Ca complex could be obtained, isolation of a similar Ba hydride remained elusive. Most recently, however, we found a synthetic route to a Ba hydride cluster in which stabilizing neutral ligands are even superfluous. Complex Ba7H7[N(SiMe3)2]7 could be isolated in a fair yield (51%) and is surprisingly stable (up to 95 °C). Preliminary reactivity studies demonstrate the extreme reactivity of this heaviest group 2 metal hydride complex: Under very mild conditions (20 °C, 1 bar), ethylene was reduced to ethane, a reaction that is proposed to proceed through a fleeting Ba-ethyl intermediate. The very high reactivity of this new group of heavy Ae metal hydride complexes urges comprehensive investigations on their syntheses, structures, reactivities and applications. The proposed research project aims to tackle the following challenges: A Generalization of the synthetic protocol by variation of the amide precursors, the neutral ligand, the metal and reaction conditions.B Exploring scope and limitations of the high reactivity of these complexes with a special focus on alkene reduction and hydrodefluorination. The final aim of these studies is to develop catalytic applications.C Syntheses of cationic Ae hydride complexes and reactivity screening. Recent evidence that cationization markedly boosts the reactivity of Ca hydride complexes will also be explored for the heavier Sr and Ba complexes.D Theoretical studies on heavier Ae hydride complexes aiming to understand structure and bonding as well reactivity.

• DFG Programme: Research Grants