New types of bi- and tri-dentate pyrrole-piperazine ligands and related zinc compounds: Synthesis, characterization, reaction study, and ring-opening polymerization of ε-caprolactone

Anticancer, antioxidant, and antimicrobial studies and molecular docking of a new hexanuclear Zn(II) complex, together with its X-ray crystal analysis

A new hexanuclear Zn(II) complex with the ligand 2,2'-(piperazine-1,4-diyl)bis(ethan-1-amine), [L3Zn6(OH)6][ClO4]6·3MeOH·7H2O, was synthesized. The crystal structure of this complex showed that each Zn atom is in a distorted tetrahedral coordination environment, surrounded by two nitrogen atoms from each ligand and two hydroxide groups, each of which bridges to another Zn atom. The anticancer activities of the ligand and its metal complex against the breast cancer cell line (MCF-7) indicated that the zinc complex had a greater anticancer activity. The free ligand and its metal complex were evaluated for antioxidant activity using the DPPH scavenging method. In addition, the antibacterial activities of both compounds were screened against Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria. The interaction of these compounds with DNA and AChE was also investigated using molecular docking.

Ring-Opening Polymerization of ε-Caprolactone and Styrene Oxide-CO2 Coupling Reactions Catalyzed by Chelated Dehydroacetic Acid-Imine Aluminum Complexes

A series of chelated dehydroacetic acid-imine-based ligands L1H~L4H was synthesized by reacting dehydroacetic acid with 2-t-butylaniline, (S)-1-phenyl-ethylamine, 4-methoxylbenzylamine, and 2-(aminoethyl)pyridine, respectively, in moderate yields. Ligands L1H~L4H reacted with AlMe3 in toluene to afford corresponding compounds AlMe2L1 (1), AlMe2L2 (2), AlMe2L3 (3), and AlMe2L4 (4). All the ligands and aluminum compounds were characterized by IR spectra, 1H and 13C NMR spectroscopy. Additionally, the ligands L1H~L4H and corresponding aluminum derivatives 1, 3, and 4 were characterized by single-crystal X-ray diffractometry. The catalytic activities using these aluminum compounds as catalysts for the ε-caprolactone ring-opening polymerization (ROP) and styrene oxide-CO2 coupling reactions were studied. The results show that increases in the reaction temperature and selective solvent intensify the conversions of ε-caprolactone to polycaprolactone. Regarding the coupling reactions of styrene oxide and CO2, the conversion rate is over 90% for a period of 12 h at 90 °C. This strategy dispenses the origination of cyclic styrene carbonates, which is an appealing concern because of the transformation of CO2 into an inexpensive, renewable and easy excess carbon feedstock.

Nickel(II) and Palladium(II) Complexes Bearing an Unsymmetrical Pyrrole-Based PNN Pincer and Their Norbornene Polymerization Behaviors versus the Symmetrical NNN and PNP Pincers

Unsymmetrical pincers have been shown to be better than the corresponding symmetrical pincers in several catalysis reactions. A new unsymmetrical PNN propincer, 2-(3,5-dimethylpyrazolylmethyl)-5-(diphenylphosphinomethyl)pyrrole (1), was synthesized from pyrrole through Mannich bases in a good yield. In addition, the new byproduct 2-(3,5-dimethylpyrazolylmethyl)-5-(dimethylaminomethyl)- N-(hydroxymethyl)pyrrole was also isolated. The reaction of 1 with [PdCl₂(PhCN)₂] and Et₃N in toluene yielded [PdCl{C₄H₂N-2-(CH₂Me₂pz)-5-(CH₂PPh₂)-κ³ P,N,N}] (2). The analogous reaction between 1 and [NiCl₂(DME)] or NiX₂ (X = Br, I) in the presence of NEt₃ in acetonitrile afforded [NiX{C₄H₂N-2-(CH₂Me₂pz)-5-(CH₂PPh₂)-κ³ P,N,N}] (3; X = Cl, Br, I). All complexes were structurally characterized. The norbornene polymerization behaviors of the unsymmetrical pincer complexes 2 and 3 in the presence of MMAO or EtAlCl₂ were compared with those of the symmetrical pincer complexes chloro[2,5-bis(3,5-dimethylpyrazolylmethyl)pyrrolido]palladium(II) (NNN), chloro[2,5-bis(diphenylphosphinomethyl)pyrrolido]palladium(II), and chloro[2,5-bis(diphenylphosphinomethyl)pyrrolido]nickel(II) (PNP) at different temperatures. The PNN and NNN complexes exhibited far greater activity on the order of 10⁷ g of PNB/mol/h, with quantitative yields in some cases, in comparison to the PNP pincer palladium and nickel complexes. This trend was also supported by the ⁱPr group substituted PNP nickel and palladium pincer complexes. These polymerization behaviors are explained using steric crowding around the metal atom with the support of NMR studies and suggested that the activity increases as the N_pyrazole donor increases. Polymers were characterized by ¹H NMR, IR, TGA, and powder XRD methods.

Structural diversity of alkylzinc complexes with pyrrole-based N,O-ligands: from molecular complexes to coordination polymers

The equimolar reaction of R2Zn (where R = Et or (t)Bu) with pyrrole-based N,O-proligands afforded a series of alkylzinc compounds with a variety of intriguing structures including a hexanuclear macrocyclic complex and 1D coordination polymers with versatile intramolecular or intermolecular bonding modes.

Synthesis, characterization and reactivity study of aluminum compounds incorporating bi- and tri-dentate pyrrole–piperazine ligands

A series of Al derivatives are afforded incorporating substituted bi- and tri-dentate pyrrole–piperazine precursors. The reactivity with small organic molecules and the associated mechanistic details are also reported.