Amido rare-earth complexes supported by an ansa bis(amidinate) ligand with a rigid 1,8-naphthalene linker: synthesis, structures and catalytic activity in rac-lactide polymerization and hydrophosphonylation of carbonyl compounds

Abstract

A synthetic approach to rare-earth amido complexes coordinated by an ansa bis(amidinate) ligand with a 1,8-naphthalene linker was developed and allowed for the synthesis of a series of complexes [1,8-C₁₀H₆{NC(tBu)N-2,6-Me₂C₆H₃}₂]LnN(SiMe₃)₂(THF)_n (Ln = Y, n = 0 (2); Ln = Sm, n = 1 (3); Ln = Nd, n = 1 (4)) in reasonable yields. Complexes 2–4 initiate ring-opening polymerization (ROP) of rac-lactide and enable complete conversion of 100–250 equiv. of monomer within 60–90 min at 25 °C. The obtained polylactides feature atactic structures and moderate molecular-weight distributions (M_w/M_n = 1.30–2.12). The experimental M_n values of the obtained polymers are found to be significantly higher than the calculated ones due to a slow initiation stage. Effective immortal ROP of lactide with 3–5 equiv. of isopropanol per metal center was performed using complexes 2–4 as the catalysts. The systems 2–4/iPrOH exhibit higher activities in ROP and allow for complete conversion of 100–300 equiv. of rac-lactide to polymer within 30–60 min at 25 °C and provide a living polymerization mode and very narrow polydispersities (M_w/M_n = 1.13–1.27). Complexes 2–4 as well as related borohydrides [1,8-C₁₀H₆{NC(tBu)N-2,6-Me₂C₆H₃}₂]Ln(BH₄)(μ-BH₄)Li(THF)₂ (Ln = Sm, Nd) catalyze hydrophosphonylation of aldehydes at room temperature with good reaction rates and hydrophosphonylation of benzylideneacetone at 65 °C.