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

• Published in: Organometallics 2024-05, Vol.43 (9), p.1005-1020
• Main Authors: Jones, Rebecca L., Turner, Zoë R., Collins Rice, Clement G., O’Hare, Dermot
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/acs.organomet.4c00028

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