Syntheses of Mononuclear and Dinuclear Aluminum Complexes Stabilized by Phenolato Ligands and Their Applications in the Polymerization of ε‑Caprolactone: A Comparative Study

A series of mono- and dinuclear aluminum alkyl complexes stabilized by phenolato ligands have been prepared through alkane elimination reactions. Treatment of piperazidine-bridged bis­(phenol)­s C4H8N2[1,4-(2-OH-3,5-Me2-C6H2CH2)2] (H2[ONNO]1), C4H8N2[1,4-(2-OH-3-tBu-5-Me-C6H2CH2)2] (H2[ONNO]2), and C4H8N2[1,4-(2-OH-3,5-tBu2-C6H2CH2)2] (H2[ONNO]3) with 2.5–3 equiv of AlR3 (R = Me, Et) afforded dinuclear aluminum complexes (AlMe2)2[ONNO]1 (1), (AlMe2)2[ONNO]2 (2), (AlMe2)2[ONNO]3 (3), (AlEt2)2[ONNO]1 (4), (AlEt2)2[ONNO]2 (5), and (AlEt2)2[ONNO]3 (6), respectively. In order to compare the catalytic activities of these bimetallic complexes with their mononuclear counterparts, mono­(phenolato) aluminum complexes AlMe2[ON]1 (7), AlMe2[ON]2 (8), AlMe2[ON]3 (9), AlEt2[ON]2 (10), and AlEt2[ON]3 (11) were synthesized from reactions of 1 equiv of AlMe3 or 2 equiv of AlEt3 with phenols that bear piperidine moieties, i.e., [2-(CH2NC5H10)-4,6-Me2-C6H2OH (H­[NO]1), 2-(CH2NC5H10)-4-Me-6-tBu-C6H2OH (H­[NO]2), and 2-(CH2NC5H10)-4,6-tBu2-C6H2OH (H­[NO]3)], respectively. In comparison, reactions of H­[NO]n (n = 2, 3) with 0.5 equiv of AlEt3 led to the isolation of mononuclear monoalkyl complexes AlEt­[NO]22 (12) and AlEt­[NO]32 (13), respectively. All complexes have been characterized by elemental analysis and NMR spectroscopy, and the solid state structures of 5 complexes have been determined by X-ray diffraction analysis. The activities of both binuclear and mononuclear aluminum complexes in initiating the ring-opening polymerization (ROP) of ε-caprolactone have also been investigated and compared. In general, these phenolato-Al complexes showed high activities in initiating the ROP in the absence of alcohols. More importantly, dinuclear complexes have been found to be 2–8 times more active than their mononuclear counterparts, which provides evidence for the cooperation between two metal centers in the former.