Metal–Ligand Interplay Governs the Coordination Flexibility and Catalytic Performance of Ca (II) , Mg (II) , and Zn (II) Complexes Supported by Benzamidinate‐Oxazolinate Type Ligands

A series of six benzamidinate‐oxazolinate complexes of Ca 2+ , Mg 2+ , and Zn 2+ were synthesized and fully characterized by NMR spectroscopy, elemental analysis, and single‐crystal X‐ray diffraction. Their catalytic performances in the ring‐opening polymerization (ROP) of lactide and ε‐caprolactone were systematically investigated. These complexes exhibit distinct reactivities following the order Ca 2+ > Mg 2+ > Zn 2+ , which correlates with the ionic radius and coordination flexibility of the metal centers. Structural analyses reveal that variations in the M‐N coordination environment indirectly regulate M‐O interactions and, consequently, metal–monomer binding strength. The more flexible coordination sphere of Ca 2+ weakens M‐O bonding, thereby facilitating monomer insertion and enhancing polymerization efficiency. In contrast, Mg 2+ forms stronger M‐O interactions and displays reduced reactivity, while Zn 2+ , constrained by a more rigid coordination geometry, exhibits the lowest catalytic activity. Kinetic studies demonstrate the first‐order dependence on monomer concentration, and spectroscopic evidence supports a monomer‐activation mechanism. Overall, this work establishes clear structure–activity relationships linking coordination dynamics to catalytic performance and provides useful design principles for developing efficient divalent metal catalysts for sustainable cyclic ester polymerization.