Magnesium and zinc complexes bearing NNO-tridentate ketiminate ligands: synthesis, structures and catalysis in the ring-opening polymerization of lactides

Abstract

A series of magnesium and zinc complexes, [(L1-3)2M2(μ-OBn)2] (M = Mg (1-3), Zn (4-6)), have been synthesized by the reaction of NNO-tridentate ketiminate ligands (L1-3-H) and a stoichiometric amount of MgnBu2 (or ZnEt2) and BnOH. In addition, the reaction of these ketiminate ligands (L1-4-H) with a half equivalent of MgnBu2 (or ZnEt2) in toluene provides [M(L1-4)2] (M = Mg (7-10), Zn (11-14)) in good yields. All of these complexes have been fully characterized by NMR spectroscopy and elemental analysis, and the molecular structures of [(L1)2Mg2(μ-OBn)2] (1), [(L3)2Mg2(μ-OBn)2] (3), [(L1)2Zn2(μ-OBn)2] (4), [Mg(L1)2] (7), [Zn(L1)2] (11) and [Zn(L4)2] (14) have been further confirmed by X-ray crystallographic studies. X-ray diffraction studies revealed that complexes 1, 3 and 4 were dimeric in the solid state, bridging through the benzyloxy groups, while the solid-state structures of 7, 11 and 14 revealed a mononuclear species, six-coordinated by N, N, and O atoms of two ketiminate ligands, forming a distorted octahedron around the metal centre. Complexes 1-6 acted as efficient initiators for the ring-opening polymerization of lactides, producing PLAs with controlled molecular weights and narrow molecular weight distributions. Complex 6 [(L3)2Zn2(μ-OBn)2] exhibited the highest activity towards the ROP of lactides. And complexes 1-6 initiated rac-lactide polymerization to afford heterotactic enriched polymers (Pr up to 0.82). In addition, complexes 7-14 were also shown to efficiently catalyze the ring-opening polymerization of lactides in the presence of BnOH.

Heterolepic β‐Ketoiminate Zinc Phenoxide Complexes as Efficient Catalysts for the Ring Opening Polymerization of Lactide

“…The TOF values as summarized in Table demonstrate the strong influence of the substitution pattern of the phenoxide substituents on the catalytic activity of the L 1 ‐substituted ( 1 > 2 > 3 > 4 ) and L 2 ‐substituted complexes ( 5 > 6 > 7 > 8 ). The catalytic activity in both groups follows the same trend and was found to increase upon introduction of an electron donating group R at the phenyl ring as was previously reported . Moreover, complexes 1 and 5 containing two methyl substituents in ortho ‐position of the phenoxide ligand are more active than complexes 2 and 6 , which contain two methyl groups in meta ‐position.…”

“…reported on magnesium and zinc complexes containing tridentate NNO ‐chelating pyrazoline‐ β ‐ketoiminate ligands with different electron‐donating substituents. These complexes showed high activities towards ROP of LA and produced controlled molecular weight polymers with narrow molecular weight distribution (MWD) . They also found that the metal substituents as well as the electron‐donating substituents of the pyrrolidine‐ β ‐ketoiminate ligands play an important role in the activity and stereoselectivity of the polymerization process.…”

Synthesis and Catalytic Activity of Gallium Schiff‐base Complexes in the Ring‐Opening Homo‐ and Copolymerization of Cyclic Esters

“…Each catalyst polymerizes ϵ ‐CL faster (turn over frequencies (TOF) 1090 h −1 1 , 570 h −1 2 , 780 h −1 3 ) than rac ‐LA (TOF 780 h −1 1 , 450 h −1 2 , 570 h −1 3 ) and the catalytic activity in both homopolymerization reactions follow the order 1 > 3 > 2 . The results show that the presence of electron donating t ‐Bu groups (+ I effect) at the ortho and para positions of the imino(phenolate) substituent increases the catalytic activity, whereas electron withdrawing Cl substituents (− I effect) diminish the catalytic activity, confirming the trends recently reported for analogous Me‐substituted complexes L 1–3 2 Ga 4 (Me) 8 [21] and others [22,25,26] . More importantly, the t ‐Bu‐substituted complexes L 1–3 2 Ga 4 ( t ‐Bu) 8 1 – 3 are catalytically more active than the corresponding Me‐substituted derivatives L 1–3 2 Ga 4 Me 8 , [21] most likely due to the higher basicity of t ‐butyl compared to the methyl substituent, which enforces the protonation reaction with BnOH and faster formation of the active (BnO‐substituted) catalyst.…”

Alternating Copolymerization of Epoxides and Anhydrides Catalyzed by Aluminum Complexes

“…Biodegradable polymeric materials derived from renewable and bioderived resources are a potential alternative to petroleum-based plastics. − Amongst them, biodegradable polyesters such as polylactide (PLA) and polycaprolactone (PCL) have already found many applications. − These polymers are often obtained by ring-opening polymerization (ROP) of cyclic esters catalyzed by organometallic complexes − and organocatalysts. − However, the design of new biodegradable polyester architectures with improved properties by ROP is seriously restricted by the limited number of commercially available cyclic esters. The development of other catalytic processes for the preparation of a broader range of biodegradable polyesters is necessary.…”