Synthesis of Aluminum Complexes of Triaza Framework Ligands and Their Catalytic Activity toward Polymerization of ε-Caprolactone

The synthesis and characterization of 1,5-bis(2,6-diisopropylphenyl)-2,4-diphenyl-1,3,5-triazapenta-1,3-diene ((LH)-H-1), a triaza ligand, and Al complexes of its anion are reported. A neat condensation reaction between N-(Dipp)benzamidine (Dipp = 2,6-diisopropylphenyl) and N-(Dipp)benzimidoyl chloride affords (LH)-H-1 in good yield. The Al complexes [(LAlMe2)-Al-1] (1), [(LAlMe)-Al-1(Cl)] (2), and [(LAlCl2)-Al-1] (3) are prepared by treating (LH)-H-1 with a slight excess of AlMe3, AlMe2Cl, and AlMeCl2, respectively, in toluene. Further, the aluminum complexes [(LAlMe2)-Al-2] (5), [(LAlMe)-Al-2(Cl)] (6), and [(LAlCl2)-Al-2] (7) are obtained in good yields from 1,3-bis(2-pyridylimino)isoindoline ((LH)-H-2) in a similar fashion. The ligand (LH)-H-1 and complexes 1, 2, and 4-6 have been structurally characterized. All of the complexes have been explored for their catalytic activity toward the ring-opening polymerization (ROP) of epsilon-caprolactone. It has been found that [(LAlMe2)-Al-1], upon the addition of cocatalyst (benzyl alcohol), gives a tetranuclear Al alkoxide (8), which is highly efficient in catalyzing the ROP of epsilon-caprolactone. [(LAlMe2)-Al-2] has also been found to be a good catalyst. The crystal structure of 8 and the catalytic activities of all the complexes in detail are reported.