Highly Active and Iso-selective Catalysts for ROP of Cyclic Esters Using Group 2 Metal Initiators

Panda, Tarun K and Bhattacharjee, J and Harinath, A and Nayek, H P and Sarkar, A (2017) Highly Active and Iso-selective Catalysts for ROP of Cyclic Esters Using Group 2 Metal Initiators. Chemistry - A European Journal. ISSN 0947-6539

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Abstract

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

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