Alkaline earth metal amide complexes containing a cyclen-derived (NNNN) macrocyclic ligand: synthesis, structure, and ring-opening polymerization activity towards lactide monomers

Abstract

Cyclic tetra-amine 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane, (Me₃TACD)H (= Me₃[12]aneN₄) was reacted with [M{N(SiMe₃)₂}₂(thf)_x] (M = Mg, x = 0; M = Ca, x = 2) in n-pentane to give thf-free complexes [{Me₃TACD}M{N(SiMe₃)₂}] (M = Mg (1); M = Ca (2)). Both compounds are monomeric in solution and X-ray diffraction studies showed the metal centers to be five-coordinated. The proligand (Me₃TACD)H reacted with [Ca{N(SiHMe₂)₂}₂(thf)₂] in benzene-d₆ to give [{(Me₃TACD)SiMe₂N(SiHMe₂)}Ca{N(SiHMe₂)₂}] (3) with hydrogen evolution. Single-crystal X-ray diffraction showed 3 to be monomeric with a six-coordinated calcium center in the solid state. The alkaline earth metal amide complexes 1 to 3 catalyzed the ring-opening polymerization of lactide monomers to give syndiotactic-rich polylactides from meso-lactide and isotactic-rich polylactides from rac- and L-lactide.