[(CH3)Al(CH2)]12: Methylaluminomethylene (MAM‐12)

The molecular structure of enigmatic “poly(aluminium‐methyl‐methylene)” (first reported in 1968) has been unraveled in a transmetalation reaction with gallium methylene [Ga8(CH2)12] and AlMe3. The existence of cage‐like methylaluminomethylene moieties was initially suggested by the reaction of rare‐earth‐metallocene complex [Cp*2Lu{(μ‐Me)2AlMe2}] with excess AlMe3 affording the deca‐aluminium cluster [Cp*4Lu2(μ3‐CH2)12Al10(CH3)8] in low yield (Cp*=C5Me5). Treatment of [Ga8(CH2)12] with excess AlMe3 reproducibly gave the crystalline dodeca‐aluminium complex [(CH3)12Al12(μ3‐CH2)12] (MAM‐12). Revisiting a previous approach to “poly(aluminium‐methyl‐methylene” by using a (C5H5)2TiCl2/AlMe3 (1 : 100) mixture led to amorphous solids displaying solubility behavior and spectroscopic features similar to those of crystalline MAM‐12. The gallium methylene‐derived MAM‐12 was used as an efficient methylene transfer reagent for ketones. The next stage towards the cage: [(CH3)Al(CH2)]12 (MAM‐12) is accessible in high yield through Ga/Al transmetalation with homoleptic gallium methylene [Ga8(CH2)12] and trimethylaluminium. MAM‐12 adopts a cage‐like structure, similar to the structural motifs proposed for the elusive methylaluminoxane [(CH3)Al(O)]n (MAO), featuring three distinct aluminium coordinations. MAM‐12 is insoluble in aliphatic/aromatic solvents and disaggregates to MAM‐3 and [Me2Al‐CH2‐AlMe2] in a coordinating solvent like THF as suggested by a DOSY NMR experiments.