Heteroleptic Calcium Complexes Supported by a Phenoxy-Imine NON Ligand: Polymerization of Cyclic Esters by a Ligand-Assisted, Activated-Monomer Mechanism

Abstract

Phenoxy-imine NON pro-ligands H R,Dipp L [1-OH-2,6-(HC�NDipp)-4-R-C 6 H 2 , where R = H, Me, or t Bu] were deprotonated using KH to afford the corresponding potassium salts R,Dipp LK•thf x [R = H (1•thf x ), Me (2•thf x ), and t Bu (3•thf x )]. The addition of crown ether (18-c-6) to these salts allowed for the structures of the resulting adducts to be elucidated in the solid state: [1(18-c-6)] n , [2(18-c-6)] n , and [3(18-c-6)(thf)].The derivatives with the smaller para-substituents were found to be 1D coordination polymers stabilized by unusual non-covalent interactions between the diisopropyl-methyl groups and the potassium center. Heteroleptic calcium complexes R,Dipp LCaI(thf) 3 [R = H (4), Me (5), and t Bu (6)] were prepared by the salt metathesis reaction of 1−3•thf x with CaI 2 . Complexes 4−6 were evaluated as initiators for the ring-opening polymerization of lactide monomers and were all found to be active; the addition of benzyl alcohol resulted in large rate increases, e.g., ∼12-fold difference for 6 (0.70 vs 0.06 h −1 ). The propagation rate constants were found to lie within the range 88−135 M −1 h −1 . Variation of co-initiator concentration revealed only a fractional dependence; this agrees with the other experimental observations, which suggest that the heteroleptic catalysts work via a "ligand-assisted, activated monomer" mechanism.