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

Phenoxy-imine NON pro-ligands HR,DippL [1-OH-2,6-(HC═NDipp)-4-R-C6H2, where R = H, Me, or tBu] were deprotonated using KH to afford the corresponding potassium salts R,DippLK·thfx [R = H (1·thfx), Me (2·thfx), and tBu (3·thfx)]. 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,DippLCaI(thf)3 [R = H (4), Me (5), and tBu (6)] were prepared by the salt metathesis reaction of 1–3·thfx with CaI2. 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.