Bimetallic amidinate aluminum methyl complexes: Synthesis, crystal structure and activity for ε-caprolactone polymerization

摘要

Four novel bridged-amidines H2L (1,4-R~1[C(NR ~2)(NHR~2)]_2 (R~1 = C_6H _4, R~2 = 2,6-iPr_2C_6H _3 (H_2L_1), R~1 = C_6H _4, R~2 = 2,6-Me_2C_6H_3 (H_2L_2), R~1 = C_6H_(10), R~2 = 2,6-iPr_2H_3 (H _2L_3), R~1 = C6_H_(10), R ~2 = 2,6-Me2C6H3 (H2L 4)) were prepared in 65-78% isolated yields by the condensation reaction of dicarboxylic acid with four equimolar amounts of amines in the presence of PPSE (polyphosphoric acid trimethylsilyl ester) at 180°C. Alkane elimination reaction of AlMe_3 with 0.5 equivalent of the bridged-amidines (H2L) in THF at room temperature afforded the corresponding bimetallic aluminum methyl complexes Me_2AlLAlMe _2 (L~1 (1), L~2 (2), L~3 (3), L~ 4 (4)) in 83-93% isolated yields. In order to understand whether the bimetallic complexes could show cooperative effect in the polymerization, their related mononuclear analogues LAlMe2 (L5 (5), L 6 (6), L7 (7), L8 (8)) had also been synthesized in 85-91% isolated yields by treatment of AlMe3 with 1 equivalent of amidines HL ([PhC(N-2,6-iPr_2C _6H_3)_2]H (HL_5), [PhC(N-2,6-Me _2C_6H_3)_2]H (HL_6), [CyC(N-2,6-iPr_2C_6H_3)_2]H (HL_7), [CyC(N-2,6-Me_2C_6H_3)_ 2]H (HL_8)) in THF at room temperature. All these complexes were characterized by elemental analysis, FT-IR, and NMR spectroscopy. The crystal structures of H_2L_3, 1, 3, 5 and 7 were determined by X-ray diffraction. In all these amidinate aluminum methyl complexes, Al is four-coordinated by two nitrogen atoms from the dihapto-amidinate ligand and two methyl groups to adopt a distorted tetrahedral geometry. These complexes showed activity towards the ring-opening polymerization of ε-caprolactone at room temperature to give unimodal molecular weight distribution polyesters.