Zinc and Aluminum Complexes Supported by Quinoline-Based N,N,N-Chelate Ligands: Synthesis, Characterization, and Catalysis in the Ring-Opening Polymerization of ε-Caprolactone and rac-Lactide

A series of zinc and aluminum complexes supported by quinoline-based N,N,N-chelate ligands were synthesized and characterized. The reaction of 2-PyCH2PPh2 or ArNC(Ph)CH2PPh2 (Ar = Ph, p-MeC6H4, p-MeOC6H4) with 8-azidoquinoline in dichloromethane gave the iminophosphoranes 2-PyCH2P(Ph2)N(8-C8H6N) (1; C8H6N = quinolyl) and ArNC(Ph)CH2P(Ph2)N(8-C8H6N), respectively. Treatment of the iminophosphoranes with 1 equiv of ZnEt2 afforded the corresponding zinc complexes [Zn(Et){2-PyCHP(Ph2)N(8-C8H6N)}] (2) and [Zn(Et){ArNC(Ph)CHP(Ph2)N(8-C8H6N)}] (5a, Ar = Ph; 5b, Ar = p-MeC6H4; 5c, Ar = p-MeOC6H4). Similar reactions between the iminophosphoranes and an equimolar amount of AlMe3 generated the aluminum complexes [Al(Me2){2-PyCHP(Ph2)N(8-C8H6N)}] (3) and [Al(Me2){ArNC(Ph)CHP(Ph2)N(8-C8H6N)}] (6a, Ar = Ph; 6b, Ar = p-MeC6H4). Compounds 1–6 were all characterized by 1H, 13C, and 31P NMR spectroscopy and elemental analysis. The molecular structures of complexes 2, 3, 5a, and 6b were determined by single-crystal X-ray diffraction techniques. In the presence of benzyl alcohol (BnOH) each of the zinc and aluminum complexes is the active catalyst in the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL), leading to polymers with good molecular weight control and narrow molecular weight distribution. The zinc complexes catalyze the ROP of rac-lactide (rac-LA) efficiently in the presence of BnOH, and the polymerizations are well controlled. However, the aluminum complexes are inactive toward the ROP of rac-lactide under the same conditions.