Aluminum alkoxide, amide and halide complexes supported by a bulky dipyrromethene ligand: synthesis, characterization, and preliminary ε-caprolactone polymerization activity

Aluminum halide, alkoxide and amide complexes 2-6 of the form (N, N)AlX2- nYn (n = 0, 1 and (N, N) = 1,9-dimesityl-5-phenyldipyrromethene (1)) were synthesized and characterized by NMR spectroscopy and X-ray crystallography. The in situ generated lithium salt of dipyrromethene 1 was reacted with AlX3 to afford aluminum halide complexes (N, N)AlX2 (X = Cl (2), I (3)) which were isolated as dichroic crystals. Salt metathesis reactions were employed to produce alkoxide complexes (N, N)Al(Cl)((OBu)-Bu-t) (4) and (N, N)Al((OBu)-Bu-t)(2) (5) from compound 2. The dimethylamide complex (N, N)Al(NMe2)(2) (6) was prepared by reaction of dipyrromethene 1 with [Al(NMe2)(3)](2). Crystallographic data revealed that the dipyrromethene is non-planar when bulky coligands are present as in compounds 3-6, while in the dichloride complex 2 the dipyrromethene is planar. Halide complexes 2 and 3 reacted with adventitious moisture in toluene to afford crystalline acid-base adducts (N, N)H center dot HX, (X = Cl (7), I (8)), which adopted structures reminiscent of anion receptors. Alkoxide and dimethylamide complexes 5 and 6 were also applied as precatalysts for the ring-opening polymerization of epsilon-caprolactone and preliminary results are reported.