Synthesis, structure, and reactivity of β-diketiminato aluminum complexes

The preparation and reaction chemistry of beta-diketiminato aluminum complexes are described, (TTP)AlCl2 (1) (TTPH = 2-(p-tolylamino)-4-(p-tolylimino)-2-pentene is formed by the treatment of AlCl3 with LiTTP. Sequential alkylation of 1 with CH3Li results in the formation of the mono- and dimethyl aluminum complexes (TTP)AlMeCl (2) and (TTP)-AlMe2 (3), respectively. Only monoalkyl complexes are produced when more hindered alkyllithium reagents are used. Compounds 2 and 3 are more conveniently prepared by treating Al(CH3)(3) with TTPH.HCl and TTPH, respectively. The more sterically hindered beta-diketimine Ligand 2-((2,6-diisopropylphenyl)amino)-4-((2,6-diisopropylphenyl)imino)-2-pentene (DDPH) also reacts smoothly with Al(CH3)(3) to yield (DDP)Al(CH3)(2) (4). Compound 3 undergoes methyl abstraction reactions upon addition of B(C6F5)(3) or AgOTf. Cationic species formed from 3 and B(C6F5)3 are unstable and decompose to (TTP)Al(CH3)(C6F5) and MeB(C6F5)(2). In contrast, (TTP)Al(CH3)(OTf) (6) is thermally stable, but the triflate group is surprisingly inert toward displacement by Lewis bases. Compounds 1, 3, 4, and 6 were crystallographically characterized. The structures all indicate that the beta-diketiminato backbone is essentially planar. The pseudotetrahedral aluminum center is displaced from the plane formed by the ligand backbone in 4 by 0.72 Angstrom.