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

人们对碱土金属氢化物的兴趣正在迅速增长。这主要是因为最近人们认识到，富含地球的、生物相容的第二族金属，如镁或钙，可以用于均相催化，但也受到较轻的镁氢化物的储氢能力的推动。尽管目前有40多种定义明确的镁氢化物络合物，但对较重的Ae氢化物金属络合物(Ae=钙、锶、钡)的了解还很少。由于有少量的氢化钙络合物的晶体结构，直到最近，人们基本上还不知道定义良好的锶和钡络合物。虽然部分原因是较重的第2族金属氢化物具有非常高的反应活性，但这一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形成不溶性的AeH2盐。随着第二族的深入，稳定具有多齿和大体积配体的较大的Ae金属氢化物的挑战变得越来越具有挑战性。最近，我们出人意料地发现，不需要大的配体。以广泛使用的酰胺前驱体Ae[N(SiMe3)2]2为原料，合成了第一个锶氢化物配合物。簇合物Sr6H9[N(SiMe_3)_2]_3·∙(PMDTA)_3已被中性三齿配位的PMDTA配体MEN(CH_2CH_2NMe_2)_2稳定。然而，最近我们发现了一条通往氢化钡簇合物的合成路线，其中稳定的中性配体甚至是多余的。络合物Ba7H7[N(SiMe3)2]7可以以较好的产率(51%)分离出来，并且具有惊人的稳定性(高达95℃)。初步的反应性研究表明，这种最重的2族金属氢化物络合物具有极高的反应性：在非常温和的条件下(20°C，1bar)，乙烯被还原为乙烷，该反应被认为是通过短暂的乙基中间体进行的。这类新的重金属氢化合物具有很高的反应活性，这促使人们对它们的合成、结构、反应活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：通过改变酰胺前体、中性配体、金属和反应条件来概括合成方案。b探索这些配合物的高反应性的范围和局限性，特别是烯烃还原和加氢脱氟。这些研究的最终目的是开发催化应用。C阳离子Ae氢化物的合成和反应活性筛选。最近的证据表明，阳离子作用显著提高了氢化钙络合物的反应活性，这一证据也将被用于较重的锶和钡络合物。D旨在了解结构和成键以及反应性的较重的Ae氢化物络合物的理论研究。

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.