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.