Well‐Defined Molecular Magnesium Hydride Clusters: Relationship between Size and Hydrogen‐Elimination Temperature. Issue 26 (9th May 2013)

• Record Type: Journal Article
• Authors: Intemann, Julia; Spielmann, Jan; Sirsch, Peter; Harder, Sjoerd
• Is Part Of: Chemistry. Volume 19:Issue 26(2013)
• Page Start: 8478
• Page End: 8489
• Publication Date: 2013-05-09
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201300684
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>A new tetranuclear magnesium hydride cluster, [{<bold>NN</bold>‐(MgH)<sub>2</sub>}<sub>2</sub>], which was based on a NN‐coupled bis‐β‐diketiminate ligand (<bold>NN</bold><sup>2−</sup>), was obtained from the reaction of [{<bold>NN</bold>‐(Mg<italic>n</italic>Bu)<sub>2</sub>}<sub>2</sub>] with PhSiH<sub>3</sub>. Its crystal structure reveals an almost‐tetrahedral arrangement of Mg atoms and two different sets of hydride ions, which give rise to a coupling in the NMR spectrum (<italic>J</italic>=8.5 Hz). To shed light on the relationship between the cluster size and H<sub>2</sub> release, the thermal decomposition of [{<bold>NN</bold>‐(MgH)<sub>2</sub>}<sub>2</sub>] and two closely related systems that were based on similar ligands, that is, an octanuclear magnesium hydride cluster and a dimeric magnesium hydride species, have been investigated in detail. A lowering of the H<sub>2</sub>‐desorption temperature with decreasing cluster size is observed, in line with previously reported theoretical predictions on (MgH<sub>2</sub>)<sub><italic>n</italic></sub> model systems. Deuterium‐labeling studies further demonstrate that the released H<sub>2</sub> solely originates from the oxidative coupling of two hydride ligands and not from other hydrogen sources, such as the β‐diketiminate ligands. Analysis of the DFT‐computed electron density in [{<bold>NN</bold>‐(MgH)<sub>2</sub>}<sub>2</sub>] reveals a counterintuitive interaction between two formally closed‐shell H<sup>−</sup> ligands that are separated by 3.106 Å. This weak interaction could play an important role in H<sub>2</sub> desorption. Although the molecular product after H<sub>2</sub> release could not be characterized experimentally, DFT calculations on the proposed decomposition product, that is, the low‐valence tetranuclear Mg(I) cluster [(<bold>NN</bold>‐Mg<sub>2</sub>)<sub>2</sub>], predict a structure with two almost‐parallel, localized MgMg bonds. As in a previously reported β‐diketiminate Mg<sup>I</sup> dimer, the MgMg bond is not characterized by a bond critical point, but instead displays a local maximum of electron density midway between the atoms, that is, a non‐nuclear attractor (NNA). Interestingly, both of the NNAs in [(<bold>NN</bold>‐Mg<sub>2</sub>)<sub>2</sub>] are connected through a bond path that suggests that there is bonding between all four Mg<sup>I</sup> atoms.</p> </abstract>