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

Four novel bridged-amidines H2L (1,4-R1[C(NR2)(NHR2)]2 (R1 = C6H4, R2 = 2,6-iPr2C6H3 (H2L1), R1 = C6H4, R2 = 2,6-Me2C6H3 (H2L2), R1 = C6H10, R2 = 2,6-iPr2C6H3 (H2L3), R1 = C6H10, R2 = 2,6-Me2C6H3 (H2L4)) 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 AlMe3 with 0.5 equivalent of the bridged-amidines (H2L) in THF at room temperature afforded the corresponding bimetallic aluminum methyl complexes Me2AlLAlMe2 (L1 (1), L2 (2), L3 (3), L4 (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), L6 (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-iPr2C6H3)2]H (HL5), [PhC(N-2,6-Me2C6H3)2]H (HL6), [CyC(N-2,6-iPr2C6H3)2]H (HL7), [CyC(N-2,6-Me2C6H3)2]H (HL8)) in THF at room temperature. All these complexes were characterized by elemental analysis, FT-IR, and NMR spectroscopy. The crystal structures of H2L3, 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.