Alkoxy-Functionalized Schiff-Base Ligation at Aluminum and Zinc: Synthesis, Structures and ROP Capability

Evolution of aluminum aminophenolate complexes in the ring-opening polymerization of ε-caprolactone: electronic and amino-chelating effects

A series of aluminum complexes bearing phenolate (O-Al and O2-Al), biphenolate (OO-Al type), aminophenolate (ON-Al), aminobiphenolate (ONO-Al), bis(phenolato)bis(amine) (NNOO-Al), and Salan (ONNO-Al) type ligands were synthesized. ε-Caprolactone (CL) polymerization using these aluminum complexes as catalysts was investigated. The overall polymerization rates of Al catalysts with different ligands were found to be in the following order (kobs values): ONBr-Al (0.124 min-1) ≥ OBr2-Al (0.121 min-1) > ONNOBr-Al (0.054 min-1) > NNOBr-Al (0.044 min-1) ≥ ONOBr-Al (0.043 min-1) > OBr-Al (0.033 min-1) > NNOOBr-Al (0.015 min-1) ≥ BuONNOBu-Al (0.001 min-1) = OOBr-Al (0.001 min-1). In addition, Al complexes with electron-donating substituents on ligands exhibited higher catalytic activity than those with bromo substituents. Density functional theory (DFT) calculations revealed that a dinuclear Al complex with two bridging methoxides had to rearrange to a phenolate bridged dinuclear Al complex with terminal methoxides. This is due to the low initiating ability of two bridging benzyl alkoxides. Combining the polymerization data and DFT results, it was concluded that the electron-donating substituents on the phenolate ring and chelating amino group enhance the electron density of the Al center. This may prevent the formation of a less active dinuclear Al complex with two bridging alkoxides (initiators) or facilitate its structural rearrangement. OOMe-Al has been established as a powerful candidate with a high polymerization rate and it exhibits well-controlled polymerization for synthesizing the mPEG-b-PCL copolymer.

Intensive Cycloalkyl-Fused Pyridines for Aminopyridyl-Zinc-Heteroimidazoles Achieving High Efficiency toward the Ring-Opening Polymerization of Lactides

The model precatalyst sp3- and sp2-N dinitrogen-coordinated zinc-heteroimidazole has been used as an efficient catalyst for the ring-opening polymerization of cyclic esters. Subsequent to our exceptional active 5,6,7-trihydroquinolin-8-amine-zinc catalysts for the ring-opening polymerization (ROP) of ε-caprolactone, various pyridine-fused cycloalkanones (ring size from five to eight) are developed for the correspondent fused amine-pyridine derivatives and their zinc-heteroimidazole chloride complexes Zn1-Zn8 (LZnCl2) bearing N-diphenylphosphinoethyl pendants. Activated with two equivalents of LiN(SiMe3)2, the title zinc complexes efficiently promote the ROP of L-lactide (L-LA) in situ; among them, Zn4/2Li(NSiMe3)2 catalyzed 500 equivalent L-LA at 80 °C with 92% conversion in 5 min (TOF: 5520 h-1). Under the same conditions, the catalytic efficiency for the ROP of rac-LA by Zn1-Zn8/2Li(NSiMe3)2 was slightly lower than that for L-LA (highest TOF: 4440 h-1). In both cases, cyclooctyl-fused pyridyl-zinc complexes exhibited higher activity than others, while the cycloheptyl-fused zinc complexes showed the lowest activity. The microstructure analysis of the polymers showed they possessed a linear structure capped with CH3O as major and cyclic structure as minor. In this work, all the ligands and zinc complexes were well characterized by 1H/13C/31P NMR, FT-IR spectroscopy as well as elemental analysis.

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.

Improvement of catalytic activity of aluminum complexes for the ring-opening polymerization of ε-caprolactone: aluminum thioamidate and thioureidate systems

In this study, a series of Al complexes bearing amidates, thioamidates, ureidates, and thioureidates were synthesized and their catalytic activity for ε-caprolactone (CL) polymerization was evaluated. SPr-Al exhibited a higher catalytic activity than OPr-Al (3.2 times as high for CL polymerization; [CL] : [SPr-Al] : [BnOH] = 100 : 0.5 : 2; [SPr-Al] = 10 mM, conv. = 93% after 14 min at 25 °C), and USCl-Al exhibited a higher catalytic activity than UCl-Al (4.6 times as high for CL polymerization; [CL] : [USCl-Al] : [BnOH] = 100 : 0.5 : 2; [USCl-Al] = 10 mM, conv. = 90% after 15 min at 25 °C). Regardless of whether aluminum amidates or ureidates were present, thioligands improved the polymerization rate of aluminum catalysts. Density functional theory calculations revealed that the eight-membered ring [SPr-AlOMe2]2 decomposed into the four-membered ring SPr-AlOMe2. However, [OPr-AlOMe2]2 did not decompose because of its strong bridging Al-O bond. The overall activation energy required for CL polymerization was lower when using [SPr-AlOMe2]2 (18.1 kcal mol-1) as a catalyst than when using [OPr-AlOMe2]2 (23.9 kcal mol-1). This is because the TS2a transition state of SPr-AlOMe2 had a more open coordination geometry with a small N-Al-S angle (72.91°) than did TS3c of [OPr-AlOMe2]2, the crowded highest-energy transition state of [OPr-AlOMe2]2 with a larger N-Al-O angle (99.63°).

10th Anniversary of Catalysts: Molecular Catalysis

On the occasion of this Special Issue, I would like to present an editorial message on this good occasion [...]

Aluminium complexes containing indolyl-phenolate ligands as catalysts for ring-opening polymerization of cyclic esters

A family of aluminium complexes supported by mono-anionic indolyl-phenolate ligands are described. Reactions of indolyl-phenolate based ligand precursors, IndHPhROH, with 1.0 or 0.5 equivalents of AlMe2Cl in toluene afforded aluminium indolyl-phenolate complexes 1-4 and aluminium bis-indolyl-phenolate complexes 5-8 respectively. The molecular structure is reported for 5. Based on the NMR spectroscopic and X-ray crystallographic studies, a 1,3-hydrogen shift could happen from nitrogen to carbon on the five-membered ring of the indolyl group upon reacting with aluminium reagents. These novel aluminium complexes demonstrate catalytic activities toward the ring-opening polymerization of cyclic esters in the presence of alcohol.