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-(t)Bu-5-Me-C6H2CH2)2] (H2[ONNO](2)), and C4H8N2[1,4-(2-OH-3,5-(t)Bu2-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-(t)Bu-C6H2OH (H[NO](2)), and 2-(CH2NC5H10)-4,6-(t)Bu2-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](2)2 (12) and AlEt[NO](3)2 (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.