Zinc(II) complexes of Triaza and Amidinate ligands: Efficient initiators for the ring‐opening polymerization of ε ‐Caprolactone and rac ‐Lactide

Zinc complexes supported by tertiary 1,3,5‐triazapenta‐1,3‐dienate ligand (L 1 ) and N ‐benzoyl‐ N′ ‐arylbenzamidinate [aryl =2,6‐diisopropylphenyl (L 2 ), phenyl (L 3 )] ligands have been synthesized and characterized. The reaction of L 1 H with ZnEt 2 affords a mononuclear zinc complex [L 1 ZnEt] ( 1 ) in good yield. Tetra nuclear zinc complex [(L 1 ) 2 Zn 4 O(OAc) 4 ] ( 2 ) is prepared by treating L 1 H with one equivalent of Zn(OAc) 2 in toluene. Further, dinuclear zinc complexes [L 2 ZnEt] 2 ( 3 ) and [L 3 ZnEt] 2 ( 4 ) are obtained in good yields from L 2 H and L 3 H with ZnEt 2 in toluene respectively . The complexes 1–4 have been characterized by 1 H/ 13 C NMR spectroscopy and single crystal X‐ray diffraction studies. All of the complexes have been explored for their catalytic activity toward the ring‐opening polymerization (ROP) of ε ‐caprolactone. It has been found that complex 1 is an active catalyst for the polymerization of ε ‐caprolactone in presence of a cocatalyst benzyl alcohol (BnOH). While complex 2 is as active as 1 there is no need for a cocatalyst for the polymerization to proceed. Dinuclear zinc complexes 3 and 4 show very high activity for the ROP of ε ‐caprolactone (CL) and rac ‐lactide (LA) without requiring a cocatalyst. The resultant polymers are found to have very high molecular weight ( M n  = 296 X 10 3  g mol −1 ) and relatively narrow polydispersity index compared to 1 and 2 .