Metal Complexes in the Synthesis of Biodegradable Polymers: Achievements and Prospects

This review describes recent advances in the synthesis of homopolymers of lactide and related cyclic esters via ring-opening polymerization (ROP) in the presence of metal complexes based on group 1, 2, 4, 12, 13 and 14 metals. Particular attention is paid to the influence of the initiator structure on the properties of the obtaining homo- and copolymers. Also, a separate chapter is devoted to the study of metal complexes in the synthesis of copolymers of lactide and lactones. This review highlights the efforts made over the last ten years or so, and shows how main-group metals have received increasing attention in the field of the polymerization of lactide and related cyclic esters.

Antioxidant and Anticancer Potential of the New Cu(II) Complexes Bearing Imine-Phenolate Ligands with Pendant Amine N-Donor Groups

Cu(II) complexes bearing NNO-donor Schiff base ligands (2a, b) have been synthesized and characterized. The single crystal X-ray analysis of the 2a complex revealed that a mononuclear and a dinuclear complex co-crystallize in the solid state. The electronic structures of the complexes are optimized by Density Functional Theory (DFT) calculations. The monomeric nature of 2a and 2b species is maintained in solution. Antioxidant activities of the ligands (1a, b) and Cu(II) complexes (2a, b) were determined by in vitro assays such as 1,1-diphenyl-2-picrylhydrazyl free radicals (DPPH^.) and 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) radicals (ABTS⁺). Our results demonstrated that 2a showed better antioxidant activity. MTT assays were performed to assess the toxicity of ligands and Cu(II) complexes in V79 cells. The antiproliferative activity of compounds was tested against two human tumor cell lines: MCF-7 (breast adenocarcinoma) and SW620 (colorectal carcinoma) and on MRC-5 (normal lung fibroblast). All compounds showed high cytotoxicity in the all-cell lines but showed no selectivity for tumor cell lines. Antiproliferative activity by clonogenic assay 2b showed a more significant inhibitory effect on the MCF-7 cell lines than on MRC-5. DNA damage for the 2b compound at 10 µM concentration was about three times higher in MCF-7 cells than in MRC-5 cells.

Trinuclear zinc calix[4]arenes: synthesis, structure, and ring opening polymerization studies

The trinuclear zinc calix[4]arene complexes [Zn₃(O₂CCH₃)₂(L(O)₂(OMe)₂)₂·xMeCN (x = 7.5, 1; x = 6, 1'), [Zn₃(O₂CCH₃)₂(L(O)₂(OnPr)₂)₂·5MeCN (2·5MeCN), [Zn₃(OEt)₂(L(O)₂(OMe)₂)₂]·4MeCN (3·4MeCN), [Zn₃(OEt)₂(L(Opentyl)₂)₂]·4.5MeCN (4·4.5MeCN) and [Zn₃(OH)₂(L(O)₂(On-pentyl)₂]·8MeCN (5·8MeCN) have been isolated from reaction of [(ZnEt)₂(L(O)₂(OR)₂)₂] (L(OH)₂(OR)₂ = 1,3-dialkoxy-4-tert-butylcalix[4]arene; R = methyl, n-propyl or pentyl) and the reagents acetic acid, ethanol, and presumed adventitious water, respectively. Attempts to make 5via a controlled hydrolysis led only to the isolation of polymorphs of (L(OH)₂(Opentyl)₂·MeCN. Reaction of [Zn(C₆F₅)₂] with L(OH)₂(Opentyl)₂, in the presence of K₂CO₃, led to the isolation of the complex [Zn₆(L(On-pentyl))₂(OH)₃(C₆F₅)₃(NCMe)₃]·3MeCN (6·3MeCN). The molecular structures of 1-6 reveal they all contain a near linear (163 to 179°) Zn₃ motif. In 1-5, a central tetrahedral Zn centre is flanked by trigonal bipyramidal Zn centres, whilst in 6, for the linear Zn₃ unit, a central distorted octahedral zinc centre is flanked by trigonal planar and a tetrahedral zinc centres. Screening for the ring opening polymerization (ROP) of ε-caprolactone at 90 °C revealed that they are active with moderate to good conversion affording low to medium molecular weight products with at least two series of ions. For comparative studies, the trinuclear aminebis(phenolate) complex [Zn₃(Oi-Pr)₂L^/] (L^/ = n-propylamine-N,N-bis(2-methylene-4,6-di-tert-butylphenolate) I was prepared. Kinetics revealed the rate order I > 4 > 6 ≈ 2 ≈ 1 > 3.

New homoleptic bis(pyrrolylpyridiylimino) Mg(ii) and Zn(ii) complexes as catalysts for the ring opening polymerization of cyclic esters via an "activated monomer" mechanism

The reaction of MgBu₂ and ZnEt₂ or Zn{N[Si(CH₃)₃]₂}₂ with a tridentate monoanionic pyrrolylpyridiylimino [N^-,N,N] proligand gave homoleptic species, as exclusive products, in high yields. The complexes were characterized in solution by 1D and 2D NMR analysis and by single crystal X-ray crystallography. The new homoleptic complexes were tested as initiators in the polymerization of ε-caprolactone and lactide in the presence of an exogenous alcohol. For both complexes, the polymerizations proceed via an "activated monomer" mechanism that, in the case of the magnesium complex, was correlated with the coordinative flexibility of the ligands, resulting in extremely high productivities under mild conditions.

Zinc bimetallics supported by a xanthene-bridged dinucleating ligand: synthesis, characterization, and lactide polymerization studies

Di-zinc complexes of a new dinucleating xanthene-based bis(iminophenolate) ligand have been synthesized, and their coordination chemistry and lactide polymerization activity have been investigated.

Calcium, Strontium and Barium Homogeneous Catalysts for Fine Chemicals Synthesis

The large alkaline earths (Ae), calcium, strontium and barium, have in the past 15 years yielded a brand new generation of heteroleptic molecular catalysts for the production of fine chemicals. However, the integrity of these complexes is often plagued by ligand redistribution equilibria in solution. This personal account retraces the paths followed in our research group towards the design of stable heteroleptic alkalino-earth complexes, including the use of intramolecular noncovalent Ae···H-Si and Ae···F-C interactions. Their implementation as homogenous precatalysts for reactions such as the intramolecular and intermolecular hydroamination and hydrophosphination of activated alkenes, the hydrophosphonylation of ketones, and the dehydrogenative coupling of amines and hydrosilanes that enable the efficient and controlled formations of CP, CN, or SiN σ-bonds, is presented in a synthetic perspective that highlights their overall outstanding catalytic performance.

Zinc-Catalyzed Highly Isoselective Ring Opening Polymerization of rac-Lactide

A family of chiral zinc amido-oxazolinate complexes are shown to be highly active and isoselective initiators for the ring-opening polymerization (ROP) of rac-lactide, yielding isotactic stereoblock polylactides (PLA) with P_m up to 0.91. This represents the highest isoselectivity observed with zinc-based catalysts for ROP of rac-lactide.

Zinc calixarene complexes for the ring opening polymerization of cyclic esters

A number of zinc calixarene complexes have been structurally characterized and screened for the ROP of ε-caprolactone and rac-lactide.