Stabilizing Magnesium Hydride Complexes with Neutral Ligands

Abstract

The reaction of Mg[N(SiMe3)2]2 with PhSiH3 in benzene at room temperature gave a white precipitate of approximate constitution MgH1.5N′′0.5. This undefined hydride rich Mg salt can be dissolved by addition of well‐known neutral donors, such as PMDTA, DABCO or the N‐heterocyclic carbene IMeiPr (N,N′‐diisopropyl‐2,3‐dimethylimidazol‐2‐ylidine). This allowed for crystallization and structural characterization of five new magnesium hydride complexes: Mg2(µ‐H)2N′′2·(PMDTA) (1), Mg4(µ‐H)6N′′2·(PMDTA)2 (2), Mg5(µ‐H)7N′′3·(PMDTA)2 (3), Mg2(µ‐H)2N′′2·(DABCO)2 (4) and Mg2(µ‐H)2N′′2·(IMeiPr)2 (5). The PMDTA complexes 1–3 were only isolated as mixtures that could not be separated but complexes 4 and 5 have been fully characterized. The structures of the Mg hydride complexes 1, 4 and 5 are reproduced well by DFT calculations (B3PW91/6‐311++G**). Bonding between Mg and hydride is largely ionic (circa 80 %) and there is very little charge transfer from the neutral donor to Mg2+. The calculated solvation enthalpies (ΔHsolv) per ligand are –12.4 kcal/mol (PMDTA), –15.9 kcal/mol (DABCO) and –20.8 kcal/mol (IMeiPr). In complexes 4 and 5 Bond‐Critical‐Points are found between the two negatively charged hydride ligands. The very high electron densities of 0.15–0.16 e/Å3 in these unusual BCP's are related to the short H–···H– distances of circa 2.55 Å.