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)(O(t)Bu) (4) and (N,N)Al(O(t)Bu)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·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 ε-caprolactone and preliminary results are reported.