Polyanionic Ligand Platforms for Methyl- and Dimethylaluminum Arrays.

Trimethylaluminum finds widespread applications in chemical and materials synthesis, most prominently in its partially hydrolyzed form of methylalumoxane (MAO), which is used as a cocatalyst in the polymerization of olefins. This work investigates the sequential reactions of trimethylaluminum with hexaprotic phosphazenes (RNH)6 P3 N3 (=XH6 ) equipped with substituents R of varied steric bulk including tert-butyl (1H6 ), cyclohexyl (2H6 ), isopropyl (3H6 ), isobutyl (4H6 ), ethyl (5H6 ), propyl (6H6 ), methyl (7H6 ), and benzyl (8H6 ). Similar to MAO, the resulting complexes of polyanionic phosphazenates [XH n ] n-6 accommodate multinuclear arrays of [AlMe2 ]+ and [AlMe]2+ . Reactions were monitored by 31 P NMR spectroscopy, and structures were determined by single-crystal X-ray diffraction. They included 1H4 (AlMe2 )2 , 1H3 (AlMe2 )3 , 2H3 (AlMe2 )3 , 3(AlMe2 )4 AlMe, 4H(AlMe2 )5 , 4(AlMe2 )6 , {5H(AlMe2 )4 }2 AlMe, 5(AlMe2 )6 , 6(AlMe2 )6 , {7(AlMe2 )4 AlMe}2 , and 8(AlMe2 )6 . The study shows that subtle variations of the steric properties of the R groups influence the reaction pathways, levels of aggregation, and fluxional behavior. While [AlMe2 ]+ is the primary product of the metalation, [AlMe]2+ is utilized to alleviate overcrowding or to aid aggregation. At the later stages of metalation, [AlMe2 ]+ groups start to scramble around congested sites. The ligands proved to be very robust and extremely flexible, offering a unique platform to study complex multinuclear metal arrangements.