Highly Active and Iso-Selective Catalysts for the Ring-Opening Polymerization of Cyclic Esters using Group 2 Metal Initiators

Article abstract of DOI:10.1002/chem.201700672

A series of alkali and alkaline earth (Ae) metal complexes bearing 1,2-phenylene(bis-diphenylphosphinothioic/selenoic amine) [{Ph₂P(E)NH}₂C₆H₄] (E=S (1-H2); Se (2-H2) ligands are reported. Alkali metal complexes [{Ph₂P(S)N}₂C₆H₄]Na(THF)₄ (3 a) [{Ph₂P(Se)N}₂C₆H₄]Na(THF)₄ (3 b), and [{Ph₂P(Se)N}₂C₆H₄]K(THF)₅ (4 b) were obtained in good yield by treating protic ligands 1-H2 or 2-H2 with metal hexamethyldisilazides [MN(SiMe₃)₂] (M=Na or K) at ambient temperature. The Ae metal complexes formulated as [{Ph₂P(E)N}₂C₆H₄]M(THF)₃ [E=S, M=Ca (5 a), Sr (6 a), Ba (7 a); E=Se, M=Ca (5 b), Sr (6 b), Ba (7 b)] can be synthesized by using two routes. The molecular structures of the free ligand 1-H2 and metal complexes 5 a,b–7 a,b in their solid states were established. Complexes 3 a and 3 b are isostructural; however, in complex 4 b, an attachment different from ligand 2 was observed. The complexes 5 a,b–7 a,b are isostructural and each metal ion exhibits a distorted pentagonal bipyramidal geometry around it. All Ae metal complexes 5 a,b–7 a,b were tested for the ring-opening polymerization (ROP) of racemic lactide (rac-LA) and ?-caprolactone (?-CL) at room temperature. Calcium complexes 5 a and 5 b show excellent iso-selectivity, with P_i values of 0.78–0.87 at 298 K and with a high degree of polymerization control, whereas the corresponding strontium complexes 6 a and 6 b exhibit moderate iso-selectivity, and barium complexes 7 a and 7 b yield only atactic polylactides (PLAs). In all cases, the catalyst initiates the ROP catalytic cycle in the absence of any external initiator. Kinetic studies of the polymerization reactions indicate the relative order of polymerization rate increases with increase in the size of the metal ion: Ba>Sr>Ca.