Comparative study of lactide polymerization by zinc alkoxide complexes with a β-diketiminato ligand bearing different substituents

Tunable zinc benzamidinate complexes: coordination modes and catalytic activity in the ring-opening polymerization of L-lactide

Seven asymmetric zinc benzamidinate complexes featuring or lacking side-arm functionalities were synthesized. Using equimolar zinc reagent produced distinct dinuclear motifs [(C6H5-C = NC6H5)ZnEt]2 (R = tBu, 1; (CH2)2OMe, 2; (CH2)2NMe2, 3). Half the zinc reagent yielded dinuclear [(C6H5-C = NC6H5)2Zn]2 (R = tBu, 4) or mononuclear zinc bis(chelate) complexes (R = (CH2)2OMe, 5; (CH2)2NMe2, 6; CH2Py, 7). Molecular structures of 1-4 and 7 were determined via single-crystal X-ray diffraction. Altering benzamidinate substituents modifies both coordination modes and catalytic activities in ring-opening polymerization of L-lactide. Specifically, complex 7 exhibits enhanced catalytic activity at 25 °C using 100 equivalents of L-lactide with a turnover frequency of 1820 h-1.

Synthesis and crystal structure of [Zn6Br4(C9H18NO)4(OH)4]·2C3H6O2

The complete mol-ecule of the hexa-metallic title complex, namely, tetra-bromido-tetra-μ-hydroxido-hexa-kis-[μ-2-methyl-3-(pyrrolidin-1-yl)propan-2-olato]hexa-zinc(II) acetone disolvate, [Zn6Br4(C9H18NO)4(OH)4]·2C3H6O2, is generated by a crystallographic centre of symmetry. Two of the unique zinc atoms adopt distorted ZnO2NBr tetra-hedral coordination geometries and the other adopts a ZnO3N tetra-hedral arrangement. Both unique alkoxide ligands are N,O-chelating and both hydroxide ions are μ2 bridging. The crystal structure displays an O-H⋯O hydrogen bond between a μ2-OH group and an acetone solvent mol-ecule. The Hirshfeld surface has been calculated and is described.

Synthesis and Characterization of N,N,O-Tridentate Aminophenolate Zinc Complexes and Their Catalysis in the Ring-Opening Polymerization of Lactides

A series of aminophenolate ligands with various pendant groups and associated ethyl Zn complexes were synthesized and studied as catalysts for the ring-opening polymerization (ROP) of lactides (LAs). The thiophenylmethyl group (L⁴ZnEt) increased the catalytic activity more than the benzyl group (L¹ZnEt) did, and 2-fluorobenzyl (L³ZnEt) and 2-methoxybenzyl (L²ZnEt) groups had the opposite effect. In addition, the LA polymerization mechanism proved by Nuclear Magnetic Resonance and Density Function Theory was that LA was attracted by H···O bond of an α-hydrogen of the LA molecule and the phenoxyl oxygen of the catalyst. After the dissociation of amino group from the Zn atom, the benzyl alcohol initiated LA without replacing the ethyl group of Zn complex. It is the first case where the ethyl group is regarded as a ligand and cannot be replaced by benzyl alcohol, and this information is very important for the mechanism study of ROP.

Bimetallic and trimetallic zinc amino-bis(phenolate) complexes for ring-opening polymerization of rac-lactide

Di- and trinuclear zinc complexes of an amino-bis(phenolate) ligand show good lactide polymerization activity in both solution and melt.

Fluorinated triazapentadienyl ligand supported ethyl zinc(ii) complexes: reaction with dioxygen and catalytic applications in the Tishchenko reaction

Ethyl zinc complexes [N{(C3F7)C(Dipp)N}2]ZnEt, [N{(C3F7)C(Cy)N}2]ZnEt, [N{(CF3)C(2,4,6-Br3C6H2)N}2]ZnEt and [N{(C3F7)C(2,6-Cl2C6H3)N}2]ZnEt have been synthesized from the corresponding 1,3,5-triazapentadiene and diethyl zinc. X-ray data show that [N{(C3F7)C(Dipp)N}2]ZnEt has a distorted trigonal planar geometry at the zinc center. The triazapentadienyl ligand binds to zinc in a κ(2)-mode. The zinc-ethyl bonds of [N{(C3F7)C(Dipp)N}2]ZnEt, [N{(C3F7)C(Cy)N}2]ZnEt, [N{(CF3)C(2,4,6-Br3C6H2)N}2]ZnEt and [N{(C3F7)C(2,6-Cl2C6H3)N}2]ZnEt readily undergo oxygen insertion upon exposure to dry air to produce the corresponding zinc-ethoxy or zinc-ethylperoxy compounds. The ethoxy zinc adducts {[N{(CF3)C(2,4,6-Br3C6H2)N}2]ZnOEt}2 and {[N{(C3F7)C(2,6-Cl2C6H3)N}2]ZnOEt}2 as well as the ethylperoxy zinc adduct {[N{(C3F7)C(Cy)N}2]ZnOOEt}2 have been isolated and fully characterized by several methods including X-ray crystallography. They feature dinuclear structures with four-coordinate zinc sites and bridging-ethoxy or -ethylperoxy groups. The ethyl zinc complexes catalyze the Tishchenko reaction of benzaldehyde under solventless conditions affording benzyl benzoate. The reaction of ethyl zinc complexes with dioxygen and their catalytic behaviour in the Tishchenko reaction are affected by the electronic and steric factors of the triazapentadienyl ligand. {[N{(C3F7)C(Cy)N}2]ZnOOEt}2 is an excellent reagent for the epoxidation of trans-chalcone.

Steric and chelating ring concerns on the l-lactide polymerization by asymmetric β-diketiminato zinc complexes

Asymmetric β-diketiminato zinc complexes were investigated in l-lactide polymerization. The catalytic rate depended heavily on the N-aryl substituents.

A closer look at ε-caprolactone polymerization catalyzed by alkyl aluminum complexes: the effect of induction period on overall catalytic activity

Previous studies on the ring-opening polymerization of ε-caprolactone using structurally related aluminum complexes as pre-catalysts showed inconsistent trends in the total conversion time.

Lactide polymerization catalyzed by Mg and Zn diketiminate complexes with flexible ligand frameworks

Diketimine ligands bearing N-benzyl, N-9-anthrylmethyl and N-mesitylmethyl substituents (nacnac(Bn)H, nacnac(An)H, and nacnac(Mes)H) were prepared from condensation of the amine with either acetyl acetone or its ethylene glycol monoketal. Chlorination with N-chlorosuccinimide in the 3-position yielded Clnacnac(Bn)H and Clnacnac(An)H. The ligands were reacted with Zn(TMSA)2 (TMSA = N(SiMe3)2) to yield nacnac(An)Zn(TMSA) and Clnacnac(Bn)Zn(TMSA). Protonation with isopropanol afforded nacnac(An)ZnOiPr and Clnacnac(Bn)ZnOiPr. Reaction of the diketimines with Mg(TMSA)2 afforded nacnac(An)Mg(TMSA), nacnac(Mes)Mg(TMSA), Clnacnac(Bn)Mg(TMSA) and Clnacnac(An)Mg(TMSA). Subsequent protonation with tert-butanol produced nacnac(Mes)MgOtBu and Clnacnac(Bn)MgOtBu, but only decomposition was observed with N-anthrylmethyl substituents. Most complexes were characterized by X-ray diffraction studies. TMSA complexes were monomeric and alkoxide complexes dimeric in the solid state. All alkoxide complexes, as well as nacnac(An)Mg(TMSA)/BnOH and Clnacnac(An)Mg(TMSA)/BnOH were moderately to highly active in rac-lactide polymerization (90% conversion in 30 s to 3 h). nacnac(An)ZnOiPr produced highly heterotactic polymers (P(r) = 0.90), Clnacnac(Bn)MgOtBu/BnOH produced slightly isotactic polymers at -30 °C (P(r) = 0.43), and all other catalysts produced atactic polymers with a slight heterotactic bias.

Mechanism of living lactide polymerization by dinuclear indium catalysts and its impact on isoselectivity

A family of racemic and enantiopure indium complexes 1-11 bearing bulky chiral diaminoaryloxy ligands, H(NNO(R)), were synthesized and fully characterized. Investigation of both the mono- and the bis-alkoxy-bridged complexes [(NNO(R))InX](2)[μ-Y][μ-OEt] (5, R = (t)Bu, X = Y = Cl; 8, R = Me, X = I, Y = OEt) by variable temperature, 2D NOESY, and PGSE NMR spectroscopy confirmed dinuclear structures in solution analogous to those obtained by single-crystal X-ray crystallography. The dinuclear complexes in the family were highly active catalysts for the ring-opening polymerization (ROP) of lactide (LA) to form poly(lactic acid) (PLA) at room temperature. In particular, complex 5 showed living polymerization behavior over a large molecular weight range. A detailed investigation of catalyst stereoselectivity showed that, although (R,R/R,R)-5 is highly selective for l-LA, only atactic PLA is obtained in the polymerization of racemic LA. No such selectivity was observed for complex 8. Importantly, the selectivities obtained for the ROP of racemic LA with (R,R/R,R)-5 and (R,R/R,R)-8 are different and, along with kinetics investigations, suggest a dinuclear propagating species for these complexes.