Synthesis and characterization of di-nuclear bis(benzotriazole iminophenolate) cobalt complexes: catalysis for the copolymerization of carbon dioxide with epoxides

Well-Defined and Highly Effective Nickel Catalysts Coordinated on Tridentate SNO Schiff-Base Derivatives for Alternating Copolymerization of Epoxides and Anhydrides

A series of Ni complexes supported by SNO Schiff-base derivatives were synthesized in this study. Complex synthesis and characterization data are reported herein. Treatment of the pro-ligands [L¹-H = 2-(((2-(methylthio)ethyl)imino)methyl)phenol, L²-H = 2,4-di-tert-butyl-6-(((2-(methylthio)ethyl)imino)methyl)phenol, L³-H = 2-(((2-(methylthio)ethyl)imino)methyl)-4,6-bis(2-phenylpropan-2-yl)phenol, L⁴-H = 4-bromo-2-(((2-(methylthio)ethyl)imino)methyl)phenol, and L⁵-H = 4-chloro-2-(((2-(methylthio)ethyl)imino)methyl)phenol] with Ni(OAc)₂·4H₂O in refluxing ethanol afforded six-coordinate mono-Ni(II) complexes [L₂ⁿNi] (n = 1-5). Noteworthy, a heptanuclear nickel(II) octacarboxylate species complex 6 and dinuclear nickel complex 6a resulted from treatment of L⁶-H [4-fluoro-2-(((2-(methylthio)ethyl)imino)methyl)phenol] with different metal precursors [Ni(OAc)₂·4H₂O for 6; NiBr₂ for 6a] giving a quantitative yield. The reaction of nickel acetate tetrahydrate and L⁷-H to L⁹-H [L⁷-H = 2-methoxy-6-(((2-(methylthio)ethyl)imino)methyl)phenol, L⁸-H = 5-methoxy-2-(((2-(methylthio)ethyl)imino)methyl)phenol, and L⁹-H = 4-methoxy-2-(((2-(methylthio)ethyl)imino)methyl)phenol] produced the four-coordinate complexes [L₂ⁿNi] (n = 7-9). The highest performing catalyst was complex 3, which was highly efficient for the ring-opening copolymerization of phthalic anhydride (PA) and cyclohexene oxide (CHO) in the presence of a cocatalyst (4-dimethylaminopyridine). In addition, the same copolymerization conditions produced narrowly dispersed polyesters, with high selectivity and polymerization control. In addition to PA-CHO copolymerization, efficient diglycolic anhydride-PA and PA-propene oxide copolymerization was achieved under the same conditions. These catalysts are straightforward to produce and extend the scope of potential substrates.

Heterodinuclear Mg(II)M(II) (M=Cr, Mn, Fe, Co, Ni, Cu and Zn) Complexes for the Ring Opening Copolymerization of Carbon Dioxide/Epoxide and Anhydride/Epoxide

The catalysed ring opening copolymerizations (ROCOP) of carbon dioxide/epoxide or anhydride/epoxide are controlled polymerizations that access useful polycarbonates and polyesters. Here, a systematic investigation of a series of heterodinuclear Mg(II)M(II) complexes reveals which metal combinations are most effective. The complexes combine different first row transition metals (M(II)) from Cr(II) to Zn(II), with Mg(II); all complexes are coordinated by the same macrocyclic ancillary ligand and by two acetate co-ligands. The complex syntheses and characterization data, as well as the polymerization data, for both carbon dioxide/cyclohexene oxide (CHO) and endo-norbornene anhydride (NA)/cyclohexene oxide, are reported. The fastest catalyst for both polymerizations is Mg(II)Co(II) which shows propagation rate constants (k_p ) of 34.7 mM^-1  s^-1 (CO₂ ) and 75.3 mM^-1  s^-1 (NA) (100 °C). The Mg(II)Fe(II) catalyst also shows excellent performances with equivalent rates for CO₂ /CHO ROCOP (k_p =34.7 mM^-1  s^-1 ) and may be preferable in terms of metallic abundance, low cost and low toxicity. Polymerization kinetics analyses reveal that the two lead catalysts show overall second order rate laws, with zeroth order dependencies in CO₂ or anhydride concentrations and first order dependencies in both catalyst and epoxide concentrations. Compared to the homodinuclear Mg(II)Mg(II) complex, nearly all the transition metal heterodinuclear complexes show synergic rate enhancements whilst maintaining high selectivity and polymerization control. These findings are relevant to the future design and optimization of copolymerization catalysts and should stimulate broader investigations of synergic heterodinuclear main group/transition metal catalysts.

Recent Developments in Ring-Opening Copolymerization of Epoxides With CO2 and Cyclic Anhydrides for Biomedical Applications

The biomedical applications of polyesters and polycarbonates are of interest due to their potential biocompatibility and biodegradability. Confined by the narrow scope of monomers and the lack of controlled polymerization routes, the biomedical-related applications of polyesters and polycarbonates remain challenging. To address this challenge, ring-opening copolymerization (ROCOP) has been exploited to prepare new alternating polyesters and polycarbonates, which would be hard to synthesize using other controlled polymerization methods. This review highlights recent advances in catalyst development, including the emerging dinuclear organometallic complexes and metal-free Lewis pair systems. The post-polymerization modification methods involved in tailoring the biomedical functions of resultant polyesters and polycarbonates are summarized. Pioneering attempts for the biomedical applications of ROCOP polyesters and polycarbonates are presented, and the future opportunities and challenges are also highlighted.

Alternating Copolymerization of Carbon Dioxide with Epoxides Using Highly Active Dinuclear Nickel Complexes: Catalysis and Kinetics

A novel series of well-defined dicarboxylate dinuclear nickel complexes containing benzotriazole based 1,3-diamine-bisphenolate (1,3-DiBTP) ligands were readily synthesized through a one-pot procedure, which were highly active single-component catalysts for copolymerization of CO₂ and epoxides. X-ray structural determination of dinickel complexes 1-11 indicates that the DiBTP ligand acted as a N,O,N,N,O,N-hexadentate framework to chelate two nickel atoms, and two carboxylates are nonequivalently coordinated. The best benzoate-bonded dinickel catalyst 6 displayed the effective activity for both high-pressure and 1 atm CO₂-copolymerization of cyclohexene oxide (CHO) in a controllable manner. Noteworthily, a high turnover frequency up to 9600 h^-1 could be reached at 140 °C and a CO₂ pressure of 20.7 bar utilizing a low catalyst loading of 0.01 mol %, and the same copolymerization conditions were capable of producing narrowly dispersed poly(cyclohexene carbonate) (PCHC) having >99% polycarbonate selectivity. In addition to CO₂/CHO copolymerization, 4-vinyl-1,2-cyclohexene oxide or cyclopentene oxide was also applied to efficiently copolymerize CO₂ under conditions of 80 °C and 20.7 bar initial CO₂ pressure. Kinetic studies of CO₂/CHO copolymerization catalyzed by 6 were investigated. Such polymerization revealed first-order dependence for both catalyst 6 and CHO concentrations, and the activation energy for PCHC generation by 6 is 57.69 kJ mol^-1. A possible polymerization mechanism for CO₂-copolymerization of CHO was proposed based on kinetics and structural studies of the obtained polycarbonates.

Bimetallic Zinc Catalysts for Ring-Opening Copolymerization Processes

Novel bimetallic zinc acetate complexes supported by heteroscorpionate ligands have been developed for the ring-opening copolymerization of cyclohexene oxide and CO₂ and the terpolymerization of cyclohexene oxide, phthalic anhydride, and CO₂. Heteroscorpionate ligands precursors L₁-L₃ were reacted with two equivalents of zinc acetate to afford the dinuclear zinc complexes [{Zn(κ³-bpzappe)}(μ-O₂CCH₃)₃-{Zn(HO₂CCH₃)}] (1), [{Zn(κ³-bpzbdmape)}(μ-O₂CCH₃)₃-{Zn(HO₂CCH₃)}] (2), and [{Zn(κ³-bpzbdeape)}(μ-O₂CCH₃)₃{Zn(HO₂CCH₃)}] (3) in excellent yields. The molecular structure of these compounds was determined spectroscopically and confirmed by X-ray diffraction analysis. Zinc acetate complexes 1-3 were screened as catalysts for the copolymerization of cyclohexene oxide and CO₂ to produce poly(cyclohexene)carbonate, and complex 3 was found to be the most active catalyst for this process in the absence of a cocatalyst. Furthermore, the terpolymerization of cyclohexene oxide, phthalic anhydride, and CO₂ was studied using the combination of complex 3 and 4-dimethylaminopyridine as catalyst system yielding the corresponding polyester-polycarbonate materials.

Dinucleating Amino-Phenolate Platform for Zinc Catalysts: Impact on Lactide Polymerization

We report imine- and amine-based dinucleating ligands bearing a bisphenol backbone and explore their coordination chemistry with zinc to form zinc alkyl, alkoxide, acetate, and amide complexes. Full characterization of the complexes shows that this ligand framework can support dinuclear and trinuclear complexes. We explore the reactivity of the zinc alkyl and alkoxide complexes as catalysts for the ring opening polymerization of lactide and compared this reactivity to analogous mononuclear complexes. We show that 1) The amine-based complexes are more reactive than the imine-based analogues; 2) The trinuclear zinc alkyl species show unusual control and reproducibility for lactide polymerization; and 3) The extent of bimetallic cooperation is hampered by the ability of the ligand framework to form trinuclear clusters.

Highly active bimetallic nickel catalysts for alternating copolymerization of carbon dioxide with epoxides

Catalyst 1 was reported for the first time to be effective for nickel-catalyzed CO₂/CHO copolymerization at 1 atm CO₂ pressure.