Magnesium(I) Reduction of Aluminum(III) Hydride Complexes: Generation of Mixed Valence Aluminum (AlI/Al0) Hydride Cluster Compounds, [Al6H8(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, [{(ArNacnac)Mg}2] (ArNacnac=[HC(MeCNAr)2]−, Ar=mesityl (Mes) or 2,6‐xylyl (Xyl)), have afforded deep red mixed valence aluminum hydride cluster compounds, [Al6H8(NR3)2{Mg(ArNacnac)}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, [(ArNacnac)(Me3N)Mg−Al(μ‐H)3[{Mg(ArNacnac)}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. Reductions of simple amido‐ and aryloxy−aluminum dihydride complexes with dimagnesium(I) reagents have afforded deep red mixed‐valence aluminum hydride cluster compounds, which have an average Al oxidation state of +0.66 (see picture). These clusters extend the limited parallels that exist between the chemistry of aluminum hydrides and polyhedral boranes.