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Synthesis and Reactivity of Bi-, Tri-, and Tetrametallic Aluminum Tetraphenolate Complexes

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journal contribution
posted on 26.07.2002, 00:00 by Andrew Cottone, Michael J. Scott
A series of ligands, each with four phenoxide arms, have been prepared with the intent to place two Lewis acidic metal center in close proximity and thus foster cooperative binding of external Lewis bases. The ligands 15 incorporate a pair of 2,2‘-methylenebis(4-tert-butyl-6-alkylphenol) groups linked by three different spacers:  anthracene (1H4, 5H4), dibenzofuran (2H4, 4H4), and xanthene (3H4). The reaction of 1H4 and 2H4 with trimethylaluminum led to the formation of the C2-symmetric, tetranuclear aluminum compounds [(1)Al4Me8], 6, and [(2)Al4Me8], 7, respectively. In contrast, when 3H4 was treated with trimethylaluminum, a binuclear aluminum species, [(3)Al2Me2], 8, containing an Al2O2 bridging unit, was isolated. Heating solutions of the tetranuclear complexes 6 and 7 in the presence of excess ligand induced the elimination of 2 equiv of trimethylaluminum and afforded the binuclear aluminum complexes [(1)Al2Me2], 9, and [(2)Al2Me2], 10. Addition of NEt4Cl to solutions of 9 and 10 resulted in the isolation of the anionic, bimetallic aluminum species [NEt4][(1)Al2Me2Cl], 11, and [NEt4][(2)Al2Me2Cl], 12. Both complexes contain a symmetrical chloride bridge between the two metal centers. In contrast, the reaction of NEt4Cl with 8 produced the asymmetric dianionic compound [NEt4]2[(3)Al2Me2Cl2], 13. Compound 9 reacts with sodium phenoxide to afford [(1)Al2Me2(OPh)Na(OEt2)], 14, and this species contains a six-membered NaAl2O3 ring. When ketones or aldehydes are added to 9, the two aluminum centers in the resulting product each bind a carbonyl group in an η1 fashion. For example, the reaction of α,α,α-trimethylacetophenone with 9 produced the C2-symmtetric complex {(1)[AlMe(OC11H14)]2}, 15, while 10, upon addition of cyclopentanone, afforded the asymmetric, monometallic species [(2H)Al(OC5H8)], 16, with one free phenolic arm. The three remaining phenoxide groups in 16 all coordinate to the lone aluminum. To probe the influence of the steric environment of the ligand on the reactivity, methyl groups were incorporated at the ortho-position of the phenoxide, and this ligand, 4H4, reacted with trimethylaluminum to produce the trinuclear species [(4)Al3Me5], 17, with a stabilizing six-membered Al3O3 bridge. The slightly more sterically encumbered isopropyl-substituted ligand, 5H4, formed the binuclear aluminum species [(5)Al2Me2], 18, analogous to 8, 9, and 10. Addition of benzaldehyde to 18 afforded the asymmetric binuclear species {(5)[AlMe][AlMe(OCHPh)]}, 19, and the benzaldehyde substrate coordinates to only one aluminum in this complex.