Magnesium(I) Reduction of Aluminum(III) Hydride Complexes: Generation of Mixed Valence Aluminum (AlI /Al0 ) Hydride Cluster Compounds, [Al6 H8 (NR3 )2 {Mg(β-diketiminate)}4 ].

Reduction of a range of amido- and aryloxy-aluminum dihydride complexes, e.g. [AlH2 (NR3 ){N(SiMe3 )2 }] (NR3 =NMe3 or N-methylpiperidine (NMP)), with β-diketiminato dimagnesium(I) reagents, [{(Ar Nacnac)Mg}2 ] (Ar Nacnac=[HC(MeCNAr)2 ]- , Ar=mesityl (Mes) or 2,6-xylyl (Xyl)), have afforded deep red mixed valence aluminum hydride cluster compounds, [Al6 H8 (NR3 )2 {Mg(Ar Nacnac)}4 ], which have an average Al oxidation state of +0.66, the lowest for any well-defined aluminum hydride compound. In the solid-state, the clusters are shown to have distorted octahedral Al6 cores, having zero-valent Al axial sites and mono-valent AlH2 - equatorial units. Several novel by-products were isolated from the reactions that gave the clusters, including the Mg-Al bonded magnesio-aluminate complexes, [(Ar Nacnac)(Me3 N)Mg-Al(μ-H)3 [{Mg(Ar Nacnac)}2 (μ-H)]]. Computational analyses of one aluminum hydride cluster revealed its Al6 core to be electronically delocalized, and to possess one unoccupied, and six occupied, skeletal molecular orbitals.