Zn(II) and Cu(II) unsymmetrical formamidine complexes as effective initiators for ring‐opening polymerization of cyclic esters

Abstract

A series of Zn(II) and Cu(II) complexes were synthesized using unsymmetrical N,N′‐ diarylformamidine ligands, i.e. N‐(2‐methoxyphenyl)‐N′‐2,6‐dichorophenyl)‐formamidine (L1), N‐(2‐methoxyphenyl)‐N′‐phenyl)‐formamidine (L2), N‐(2‐methoxyphenyl)‐N′‐(2,6‐dimethylphenyl)‐formamidine (L3) and N‐(2‐methoxyphenyl)‐N′‐(2,6‐diisopropylphenyl)‐formamidine (L4). The complexes, [Zn2(L1)2(OAc)4] (1), [Zn2(L2)2(OAc)4] (2), [Zn2(L3)2(OAc)4] (3), [Zn2(L4)2(OAc)4] (4), [Cu2(L1)2(OAc)4] (5), [Cu2(L2)2(OAc)4] (6), [Cu2(L3)2(OAc)4] (7) and [Cu2(L4)2(OAc)4] (8), were prepared via a mechanochemical method with excellent yields between 95 ‐ 98% by reacting the metal acetates and corresponding ligands. Structural studies showed that both complexes are dimeric with a paddlewheel core structure in which the separation between the two metal centres are 2.9898 (8) and 2.6653 (7) Å in complexes 3 and 7, respectively. Complexes 1 – 8 were used in ring‐opening polymerization of ε‐caprolactone (ε‐CL) and rac‐lactide (rac‐LA). Zn(II) complexes were more active than Cu(II) complexes, with complex 1 bearing electron withdrawing chloro groups being the most active (kapp = 0.0803 h‐1). Low molecular weight poly‐(ε‐CL) and poly‐(rac‐LA) ranging from 1720 to 6042 g mol‐1, with broad molecular weight distribution (PDIs, 1.78 – 1.87) were obtained. Complex 2 gave reaction orders of 0.56 and 1.52 with respect to ε‐CL and rac‐LA, respectively.