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

碱土金属氢化物配合物的研究日益受到人们的关注。这主要是由于最近认识到地球上丰富的、生物相容的第2族金属如Mg或Ca可以用于均相催化，但也可以通过较轻的Mg的储氢能力来提供燃料。虽然目前有超过40个定义明确的镁氢化物配合物，但缺乏对较重的Ae氢化物金属配合物（Ae = Ca，Sr，Ba）的了解。由于Ca氢化物络合物的晶体结构很少，直到最近才基本上不知道定义明确的Sr和Ba络合物。虽然部分是由于较重的第2族金属的非常高的反应性，但这种缺陷主要源于这些化合物的非常动态的行为，导致快速的配体交换和不溶性AeH 2盐的形成。在第2组中，稳定具有多齿和大体积配体的较大Ae金属氢化物络合物的挑战变得越来越具有挑战性。最近，我们意外地发现不需要大的配体。从广泛使用的酰胺前体Ae[N（SiMe 3）2]2开始，可以获得第一个Sr氢化物配合物。簇合物Sr 6 H9 [N（SiMe 3）2]3-[（PMDTA）3]被中性三齿PMDTA配体MeN（CH 2CH 2NMe 2）2稳定。虽然可以得到类似的钙配合物，但类似的钡氢化物的分离仍然难以捉摸。然而，最近，我们发现了一种合成Ba氢化物簇的方法，其中稳定中性配体甚至是多余的。络合物Ba 7 H7 [N（SiMe 3）2]7可以以相当的产率（51%）分离并且令人惊讶地稳定（高达95 °C）。初步的反应性研究证明了这种最重的第2族金属氢化物络合物的极端反应性：在非常温和的条件下（20 °C，1巴），乙烯被还原为乙烷，该反应被提议通过短暂的Ba-乙基中间体进行。这类新的Ae重金属氢化物配合物具有很高的反应活性，这促使人们对其合成、结构、反应性和应用进行全面的研究。拟议的研究项目旨在解决以下挑战：A通过酰胺前体、中性配体、金属和反应条件的变化来概括合成方案。B探索这些络合物的高反应性的范围和限制，特别关注烯烃还原和加氢脱烯烃。这些研究的最终目的是开发催化应用。阳离子Ae氢化物配合物的合成和反应活性筛选。最近的证据表明，阳离子化显着提高钙氢化物络合物的反应性也将探讨较重的Sr和Ba complex.D理论研究，旨在了解结构和键以及反应性较重的Ae氢化物络合物。

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.