FAlen–Zn Complexes for the Cycloaddition of CO2 to Epoxide and the Ring-Opening Polymerization of Lactide

We report the synthesis of four bis(o,p-^tBu-phenoxy-amidine) ligands (FAlen) with dimethylamino-, pyrrolidine-, and piperidine-substituted amidines and a phenylene or a maleonitrile amidine bridge. We examined the coordination chemistry of these ligands toward Zn and the potential of this combination as a catalyst in the cycloaddition of CO₂ to epoxide and in the ROP of rac-LA. FAlen ligands were shown to greatly differ from their parent bis(salicylaldimine) ligands (aka salen) in how they coordinate to Zn. In the solid state, the complexes were found as dinuclear tetracoordinated Zn complexes with the two FAlen ligands bridging the metal centers in an asymmetric μ-κ¹,κ³ manner. In solution, these dinuclear structures most often vanished even in noncoordinating solvent to afford mononuclear tetracoordinated species. The FAlen–Zn complexes were found to be less active than their salen–Zn counterparts in the catalytic cycloaddition of CO₂ with styrene epoxide. Conversely, the FAlen–Zn complexes were shown to be highly active for the ROP of rac-LA, whereas the parent salen–Zn complex exhibited no activity. A kinetic study performed with the most active complex 1 showed a first order in rac-LA, Zn complex, and ⁱPrOH. DFT calculations revealed two possible low-energy-barrier ROP mechanisms: an amidine-assisted activated monomer mechanism or a very unusual zincate-amidinium hydrogen-bonding mechanism.