Well‐Defined Molecular Magnesium Hydride Clusters: Relationship between Size and Hydrogen‐Elimination Temperature

Abstract A new tetranuclear magnesium hydride cluster, [{ NN ‐(MgH) 2 } 2 ], which was based on a NN‐coupled bis‐β‐diketiminate ligand ( NN 2− ), was obtained from the reaction of [{ NN ‐(Mg n Bu) 2 } 2 ] with PhSiH 3 . 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 ( J =8.5 Hz). To shed light on the relationship between the cluster size and H 2 release, the thermal decomposition of [{ NN ‐(MgH) 2 } 2 ] 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 2 ‐desorption temperature with decreasing cluster size is observed, in line with previously reported theoretical predictions on (MgH 2 ) n model systems. Deuterium‐labeling studies further demonstrate that the released H 2 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 [{ NN ‐(MgH) 2 } 2 ] reveals a counterintuitive interaction between two formally closed‐shell H − ligands that are separated by 3.106 Å. This weak interaction could play an important role in H 2 desorption. Although the molecular product after H 2 release could not be characterized experimentally, DFT calculations on the proposed decomposition product, that is, the low‐valence tetranuclear Mg(I) cluster [( NN ‐Mg 2 ) 2 ], predict a structure with two almost‐parallel, localized MgMg bonds. As in a previously reported β‐diketiminate Mg I 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 [( NN ‐Mg 2 ) 2 ] are connected through a bond path that suggests that there is bonding between all four Mg I atoms.