Aluminum Complexes Stabilized by Piperazidine‐Bridged Bis(phenolate) Ligands: Syntheses, Structures, and Application in the Ring‐Opening Polymerization of ε ‐Caprolactone

The synthesis and characterization of aluminum alkoxide and alkyl complexes stabilized by piperazidine‐bridged bis(phenolate) ligands are described. Treatment of ligand precursors H 2 [ONNO] 1 {H 2 [ONNO] 1 =1,4‐bis(2‐hydroxy‐3‐ tert ‐butyl‐5‐methylbenzyl)piperazidine} and H 2 [ONNO] 2 {H 2 [ONNO] 2 =1,4‐bis(2‐hydroxy‐3,5‐di‐ tert ‐butylbenzyl)piperazidine} with AlEt 2 (OCH 2 Ph) and AlEt 2 (OPr‐ i ), which were generated in situ by the reactions of AlEt 3 with equivalent of the corresponding alcohols, in a 1:1 molar ratio in THF gave the corresponding aluminum alkoxide complexes [ONNO] 1 Al(OCH 2 Ph) ( 1 ) and [ONNO] 2 Al(OPr‐ i ) ( 2 ), respectively. The reaction of H 2 [ONNO] 1 with AlEt 2 (OCH 2 Ph) in a 1:2 molar ratio in THF afforded a mixture of monometallic aluminum ethyl complex [ONNO] 1 AlEt ( 3 ) and complex 1 , which can be isolated by stepwise crystallization. Similarly, H 2 [ONNO] 2 reacted with AlEt 2 (OPr‐ i ) in a 1:2 molar ratio in THF to give a mixture of aluminum ethyl complex [ONNO] 2 AlEt ( 4 ) and complex 2 . Complexes 1 and 2 were also available via treatment of complexes 3 and 4 with 1 equiv. of benzyl alcohol and isopropyl alcohol, respectively. All of these complexes were fully characterized including X‐ray structural determination. It was found that complexes 1 to 4 can initiate the ring‐opening polymerization of ε ‐caprolactone, and complexes 1 and 2 showed higher catalytic activity in comparison with complexes 3 and 4 .