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

对碱土（Ae）金属氢化物配合物的普遍兴趣正在迅速增加。这主要是由于最近人们认识到，地球上丰富的、生物相容的2族金属，如Mg或Ca，可以用于均相催化，但也被较轻的Mg氢化物的储氢能力所推动。虽然目前有40多种明确的Mg氢化物配合物，但对较重的Ae氢化物金属配合物（Ae = Ca, Sr, Ba）缺乏了解。由于氢化钙配合物的晶体结构很少，而明确的锶和钡配合物直到最近才基本为人所知。虽然部分原因是由于较重的2族金属氢化物具有很高的反应活性，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的AeH2盐的形成。在第2族，用多齿状和大体积配体稳定较大的Ae金属氢化物配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(SiMe3)2]2开始，可以得到第一个Sr氢化物配合物。该簇Sr6H9[N(SiMe3)2]3∙(PMDTA)3被中性三齿PMDTA配体MeN(CH2CH2NMe2)2稳定。虽然可以得到类似的Ca配合物，但分离出类似的Ba氢化物仍然是难以捉摸的。然而，最近我们发现了一种合成Ba氢化物簇的方法，其中稳定的中性配体甚至是多余的。配合物Ba7H7[N(SiMe3)2]7的产率相当高（51%），并且稳定（高达95°C）。初步的反应性研究表明，这种最重的2族金属氢化物配合物具有极强的反应性：在非常温和的条件下（20°C, 1bar），乙烯被还原为乙烷，该反应被提议通过一个短暂的ba -乙基中间体进行。这类新型重Ae金属氢化物配合物具有很高的反应活性，迫切需要对其合成、结构、反应性和应用等方面进行全面的研究。提出的研究项目旨在解决以下挑战：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探索这些配合物的高反应性的范围和局限性，特别侧重于烯烃还原和氢脱氟。这些研究的最终目的是开发催化应用。c -阳离子氢化物配合物的合成和反应性筛选。最近的证据表明，阳离子化显著提高了Ca氢化物配合物的反应活性，也将探讨较重的Sr和Ba配合物。D对较重的Ae氢化物配合物进行理论研究，旨在了解其结构、成键和反应性。

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.