Aluminium complexes containing bidentate and symmetrical tridentate pincer type pyrrolyl ligands: synthesis, reactions and ring opening polymerization

Abstract

A series of aluminium derivatives containing substituted bidentate and symmetrical tridentate pyrrolyl ligands, [C₄H₃NH(2-CH₂NH^tBu)] and [C₄H₂NH(2,5-CH₂NH^tBu)₂], in toluene or diethyl ether were synthesized. Their reactivity and application for the ring opening polymerization of ε-caprolactone have been investigated. The reaction of AlMe₃ with one equiv. of [C₄H₃NH(2-CH₂NH^tBu)] in toluene at room temperature affords [C₄H₃N(2-CH₂NH^tBu)]AlMe₂ (1) in 70% yield by elimination of one equiv. of methane. Interestingly, while reacting AlMe₃ with one equiv. of [C₄H₃NH(2-CH₂NH^tBu)] in toluene at 0 °C followed by refluxing at 100 °C, [{C₄H₃N(2-CH₂N^tBu)}AlMe]₂ (2) has been isolated via fractional recrystalliztion in 30% yield. Similarly, reacting AlMe₃ with two equiv. of C₄H₃NH(2-CH₂NH^tBu) generates [C₄H₃N(2-CH₂NH^tBu)]₂AlMe (3) in a moderate yield. Furthermore, complex 1 can be transformed to an aluminium alkoxide derivative, [C₄H₃N(2-CH₂NH^tBu)][OC₆H₂(-2,6-^tBu₂-4-Me)]AlMe (4) by reacting 1 with one equiv. of HOC₆H₂(-2,6-^tBu₂-4-Me) in toluenevia the elimination of one equiv. of methane. The reaction of AlR₃ with one equiv. of [C₄H₂NH(2,5-CH₂NH^tBu)₂] in toluene at room temperature affords [C₄H₂N(2,5-CH₂NH^tBu)₂]AlR₂ (5, R = Me; 6, R = Et) in moderate yield. Surprisingly, from the reaction of two equiv. of [C₄H₂NH(2,5-CH₂NH^tBu)₂] with LiAlH₄ in diethyl ether at 0 °C, a novel complex, [C₄H₂N(2-CH₂N^tBu)(5-CH₂NH^tBu)]₂AlLi (7) has been isolated after repeating re-crystallization. Furthermore, reacting one equiv. of C₄H₂NH(2,5-CH₂NH^tBu)₂ with AlH₃·NMe₃ in diethyl ether generates an aluminium dihydride complex, [C₄H₂N(2,5-CH₂NH^tBu)₂]AlH₂ (8), in high yield. Additionally, treating 8 with one equiv. of HOC₆H₂(-2,6-^tBu₂-4-Me) in methylene chloride produces [C₄H₂N(2,5-CH₂NH^tBu)₂][OC₆H₂(-2,6-^tBu₂-4-Me)]AlH (9) with the elimination of one equiv. of H₂. The aluminium alkoxide complex 4 shows moderate reactivity toward the ring opening polymerization of ε-caprolatone in toluene.