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-PyCH 2 PPh 2 or ArN═C(Ph)CH 2 PPh 2 (Ar = Ph, p -MeC 6 H 4, p -MeOC 6 H 4 ) with 8-azidoquinoline in dichloromethane gave the iminophosphoranes 2-PyCH 2 P(Ph 2 )═N(8-C 8 H 6 N) ( 1; C 8 H 6 N = quinolyl) and ArN═C(Ph)CH 2 P(Ph 2 )═N(8-C 8 H 6 N), respectively. Treatment of the iminophosphoranes with 1 equiv of ZnEt 2 afforded the corresponding zinc complexes [Zn(Et){2-PyCHP(Ph 2 )═N(8-C 8 H 6 N)}] ( 2 ) and [Zn(Et){ArNC(Ph)═CHP(Ph 2 )═N(8-C 8 H 6 N)}] ( 5a, Ar = Ph; 5b, Ar = p -MeC 6 H 4; 5c, Ar = p -MeOC 6 H 4 ). Similar reactions between the iminophosphoranes and an equimolar amount of AlMe 3 generated the aluminum complexes [Al(Me 2 ){2-PyCHP(Ph 2 )═N(8-C 8 H 6 N)}] ( 3 ) and [Al(Me 2 ){ArNC(Ph)═CHP(Ph 2 )═N(8-C 8 H 6 N)}] ( 6a, Ar = Ph; 6b, Ar = p -MeC 6 H 4 ). Compounds 1 – 6 were all characterized by 1 H, 13 C, and 31 P 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.