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)₆P₃N₃ (=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 ³¹P 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)₄AlMe, 4H(AlMe₂)₅, 4(AlMe₂)₆, {5H(AlMe₂)₄}₂AlMe, 5(AlMe₂)₆, 6(AlMe₂)₆, {7(AlMe₂)₄AlMe}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.