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

Cyclic tetra-amine 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane, (Me(3)TACD)H (= Me-3[12]aneN(4)) 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 [{Me(3)TACD}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 (Me(3)TACD)H reacted with [Ca{N(SiHMe2)(2)}(2)(thf)(2)] in benzene-d(6) to give [{(Me(3)TACD)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.