Erschienen in:
Chemistry – A European Journal, 19 (2013) 26, Seite 8478-8489
Sprache:
Englisch
DOI:
10.1002/chem.201300684
ISSN:
0947-6539;
1521-3765
Entstehung:
Anmerkungen:
Beschreibung:
<jats:title>Abstract</jats:title><jats:p>A new tetranuclear magnesium hydride cluster, [{<jats:bold>NN</jats:bold>‐(MgH)<jats:sub>2</jats:sub>}<jats:sub>2</jats:sub>], which was based on a NN‐coupled bis‐β‐diketiminate ligand (<jats:bold>NN</jats:bold><jats:sup>2−</jats:sup>), was obtained from the reaction of [{<jats:bold>NN</jats:bold>‐(Mg<jats:italic>n</jats:italic>Bu)<jats:sub>2</jats:sub>}<jats:sub>2</jats:sub>] with PhSiH<jats:sub>3</jats: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 (<jats:italic>J</jats:italic>=8.5 Hz). To shed light on the relationship between the cluster size and H<jats:sub>2</jats:sub> release, the thermal decomposition of [{<jats:bold>NN</jats:bold>‐(MgH)<jats:sub>2</jats:sub>}<jats:sub>2</jats: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<jats:sub>2</jats:sub>‐desorption temperature with decreasing cluster size is observed, in line with previously reported theoretical predictions on (MgH<jats:sub>2</jats:sub>)<jats:sub><jats:italic>n</jats:italic></jats:sub> model systems. Deuterium‐labeling studies further demonstrate that the released H<jats:sub>2</jats: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 [{<jats:bold>NN</jats:bold>‐(MgH)<jats:sub>2</jats:sub>}<jats:sub>2</jats:sub>] reveals a counterintuitive interaction between two formally closed‐shell H<jats:sup>−</jats:sup> ligands that are separated by 3.106 Å. This weak interaction could play an important role in H<jats:sub>2</jats:sub> desorption. Although the molecular product after H<jats:sub>2</jats:sub> release could not be characterized experimentally, DFT calculations on the proposed decomposition product, that is, the low‐valence tetranuclear Mg(I) cluster [(<jats:bold>NN</jats:bold>‐Mg<jats:sub>2</jats:sub>)<jats:sub>2</jats:sub>], predict a structure with two almost‐parallel, localized MgMg bonds. As in a previously reported β‐diketiminate Mg<jats:sup>I</jats:sup> dimer, the MgMg 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 [(<jats:bold>NN</jats:bold>‐Mg<jats:sub>2</jats:sub>)<jats:sub>2</jats:sub>] are connected through a bond path that suggests that there is bonding between all four Mg<jats:sup>I</jats:sup> atoms.</jats:p>