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
(L1H), 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 L1H in good yield. The Al complexes [L1AlMe2] (1), [L1AlMe­(Cl)]
(2), and [L1AlCl2] (3) are prepared by treating L1H with a slight excess of
AlMe3, AlMe2Cl, and AlMeCl2, respectively,
in toluene. Further, the aluminum complexes [L2AlMe2] (5), [L2AlMe­(Cl)] (6), and [L2AlCl2] (7) are obtained
in good yields from 1,3-bis­(2-pyridylimino)­isoindoline (L2H) in a similar fashion. The ligand L1H 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 ε-caprolactone. It has been found that
[L1AlMe2], upon the addition of cocatalyst (benzyl
alcohol), gives a tetranuclear Al alkoxide (8), which
is highly efficient in catalyzing the ROP of ε-caprolactone.
[L2AlMe2] 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.