Synthesis of Aluminum Complexes of Triaza Framework
Ligands and Their Catalytic Activity toward Polymerization of ε‑Caprolactone
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Abstract
The synthesis and characterization
of 1,5-bis(2,6-diisopropylphenyl)-2,4-diphenyl-1,3,5-triazapenta-1,3-diene
(L<sup>1</sup>H), 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 L<sup>1</sup>H in good yield. The Al complexes [L<sup>1</sup>AlMe<sub>2</sub>] (<b>1</b>), [L<sup>1</sup>AlMe(Cl)]
(<b>2</b>), and [L<sup>1</sup>AlCl<sub>2</sub>] (<b>3</b>) are prepared by treating L<sup>1</sup>H with a slight excess of
AlMe<sub>3</sub>, AlMe<sub>2</sub>Cl, and AlMeCl<sub>2</sub>, respectively,
in toluene. Further, the aluminum complexes [L<sup>2</sup>AlMe<sub>2</sub>] (<b>5</b>), [L<sup>2</sup>AlMe(Cl)] (<b>6</b>), and [L<sup>2</sup>AlCl<sub>2</sub>] (<b>7</b>) are obtained
in good yields from 1,3-bis(2-pyridylimino)isoindoline (L<sup>2</sup>H) in a similar fashion. The ligand L<sup>1</sup>H 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
[L<sup>1</sup>AlMe<sub>2</sub>], 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.
[L<sup>2</sup>AlMe<sub>2</sub>] 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