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

A series of amidine ligands RAmDIPP (R is backbone substituent, R = 2-methylpyridine (HL1), N,N,2-tri-methylaniline (HL2), N,N-dimethylpropan-l-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 HL2-HL4 with Ca(N(SiHMe2)(2))(THF) or Ca[N(SiMe3)(2)](THF)(2), respectively, afforded complexes 1-8 (1: [L-1 (AmCa)-Ca-DIPP(SiHMe2)(2)](2), 2: (LAmCaN)-Am-1-Ca-DIPP (SiMe3)(2).(THF)(2), 3: L(2)AM(DIPP) CaN(SiHMe2)(2)center dot(THF)(2), 4: (LAmCaN)-Am-2-Ca-DIPP(SiMe3)(2)center dot(THF), 5: (LAmCaN)-Am-3-Ca-DIPP(SiHMe2)(2).(THF)(2), 6: (LAmCaN)-Am-3-Ca-DIPP(SiMe3)(2)center dot(THF)(2), 7: [(LCaN)-Ca-4(SiHMe2)(2)](2), 8: [(LCaN)-Ca-4(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.