Synthesis and Reactivity of Bi-, Tri-, and Hexametallic and Zwitterionic Methyl Aluminum Complexes Containing a Phenoxide/Imine Ligand System

Abstract

Addition of 3 equiv of trimethylaluminum to tris(3,5-di-tert-butyl-2-hydroxyphenyl)methane, 1H3, affords the simple, C3-symmetric, trimetallic species [1(AlMe2)3], 2. To disrupt the Al3O3 bridging unit, one of the ortho-alkyl substituents from the tris(2-hydroxyphenyl)methane platform of 1H3 was substituted with a reactive aldehyde group to afford the ligands 3-(2,2‘-methylenebis(4-methyl-6-tert-butylphenol)-5-methyl-2-hydroxybenzaldehyde, 3H3, and 3-(2,2‘-methylenebis(4,6-di-tert-butylphenol)-5-methyl-2-hydroxybenzaldehyde, 4H3, respectively. Benzylamine reacts independently with both 3H3 and 4H3 specifically at the aldehyde functionality, affording the mixed donor ligands 3-(2,2‘-methylenebis(4-methyl-6-tert-butylphenol)-5-methylsalicylidenebenzylimine, 5H3, and 3-(2,2‘-methylenebis(4,6-di-tert-butylphenol)-5-methylsalicylidenebenzylimine, 6H3, in high isolable yields. Furthermore, the Salen-type ligand, 7H6, can be prepared from the reaction of 1,2-ethylenediamine and 4H3. Two equivalents of trimethylaluminum will react with 5H3 and 6H3 to produce the binuclear aluminum compounds [(5)Al2Me3], 8, and [(6)Al2Me3], 9, respectively. If an additional equivalent of trimethylaluminum is added to 9, the trimetallic species [(6)Al3Me6], 10, albeit as a slightly impure material, is observed, while 7H6 reacts in an analogous manner to afford [(7)Al6Me12], 11. Treatment of 8 with 1 equiv of DMSO produces the new complex [(5){AlMe2}{AlMe(OSMe2)}], 12, wherein the substrate is coordinated to only one of a possible two aluminum centers. In contrast to this simple transformation, the separate reactions of 8 and 9 with either cyclopentanone or cyclohexanone afford the zwitterionic mononuclear complexes [(5H)+AlMe-], 13, and [(6H)+AlMe-], 14. Solid-state structures are reported for 2, 6H3, 7H6, and 9−13.