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 <i>N</i>-(Dipp)­benzamidine (Dipp = 2,6-diisopropylphenyl) and <i>N</i>-(Dipp)­benzimidoyl chloride affords L¹H in good yield. The Al complexes [L¹AlMe₂] (<b>1</b>), [L¹AlMe­(Cl)] (<b>2</b>), and [L¹AlCl₂] (<b>3</b>) are prepared by treating L¹H with a slight excess of AlMe₃, AlMe₂Cl, and AlMeCl₂, respectively, in toluene. Further, the aluminum complexes [L²AlMe₂] (<b>5</b>), [L²AlMe­(Cl)] (<b>6</b>), and [L²AlCl₂] (<b>7</b>) are obtained in good yields from 1,3-bis­(2-pyridylimino)­isoindoline (L²H) in a similar fashion. The ligand L¹H and complexes <b>1</b>, <b>2</b>, and <b>4</b>–<b>6</b> 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 (<b>8</b>), 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 <b>8</b> and the catalytic activities of all the complexes in detail are reported