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 (HL)-L-R,Dipp [1-OH-2,6-(HC=NDipp)-4-R-C6H2, where R = H, Me, or Bu-t] were deprotonated using KH to afford the corresponding potassium salts (LK)-L-R,Dipp center dot thf(x) [R = H (1 center dot thf(x)), Me (2 center dot thf(x)), and Bu-t (3 center dot 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 (LCaI)-L-R,Dipp(thf)(3) [R = H (4), Me (5), and Bu-t (6)] were prepared by the salt metathesis reaction of 1-3 center dot thf(x) 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., similar to 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.