Alkaline earth metal complexes stabilized by amidine and guanidine ligands: synthesis, structure and their catalytic activity towards polymerization of rac-lactide.

Abstract

A series of amidine ligands RAmDIPP (R is backbone substituent, R = 2-methylpyridine (HL1), N,N,2-trimethylaniline (HL2), N,N-dimethylpropan-1-amine (HL3); DIPP (2,6-diisopropylphenyl) is N substituent) and a (Z)-1,1-diethyl-2,3-bis(2-methoxyphenyl)guanidine ligand (HL4) have been synthesized. Reaction of HL1-HL4 with Ca[N(SiHMe2)2](THF) or Ca[N(SiMe3)2](THF)2, respectively, afforded complexes 1-8 (1: [L1AmDIPPCa(SiHMe2)2]2, 2: L1AmDIPPCaN(SiMe3)2·(THF)2, 3: L2AmDIPPCaN(SiHMe2)2·(THF)2, 4: L2AmDIPPCaN(SiMe3)2·(THF), 5: L3AmDIPPCaN(SiHMe2)2·(THF)2, 6: L3AmDIPPCaN(SiMe3)2·(THF)2, 7: [L4CaN(SiHMe2)2]2, 8: [L4CaN(SiMe3)2]2). All the complexes were well-defined by NMR spectrum analyses and the molecular structures of 3 and 5-8 were further determined by single crystal X-ray diffraction analyses. In combination with an excess of PhOH as the chain transfer agent, complexes 1-8 catalyzed immortal ring-opening polymerization of rac-lactide in a controlled manner and exhibited moderate catalytic activity at room temperature. The end-group fidelity of the resultant polymer was certified by NMR and MALDI-TOF mass spectra.