Fixation of CO2 with Epoxides Catalyzed by Pincer-Type Azo-Aromatic Complexes of Cobalt as Catalysts

Employing a series of azo-aromatic pincer-type cobalt(II) complexes, 1-5, and an imine-based cobalt complex, 6, a highly efficient catalytic protocol for the cycloaddition of CO2 with epoxides at low pressure of CO2 is reported. The electron-withdrawing group-substituted ligands containing complexes 2 and 4 were most efficient. The catalytic protocol with 2 involved a synergistic participation of an azo-aromatic catalyst (0.1 mol %) and tetra-butyl ammonium iodide (TBAI), a cocatalyst (0.2 mol %) at 90 °C temperature, and 1 bar CO2 pressure. A very good conversion, high turnover number, and reusability were observed. Complex 4 worked directly in the reaction, and its efficiency was similar to the efficiency of 2 and TBAI. As 2 was synthesized from a cheaper CoCl2, 2 showed to be more stable than 4; the combination of 2 and TBAI was used for a detailed study. The imine-based complex 6 was less efficient than the corresponding azo-aromatic complex 5. The catalytic protocol was versatile. It was also very effective for the full conversion of bis-epoxides to bis-carbonates at only 2 bar of CO2 pressure in 24 h. The reaction mechanism was investigated using various spectroscopic and computational studies.

Polymeric bis(triphenylphosphine)iminium chloride as a recyclable catalyst

Metal-free catalysts have garnered considerable interest as an environmental and economical alternative to precious metal catalysts. Bis(triphenylphosphine)iminium chloride (PPNCl) has emerged as a prominent choice due to its air and thermal stability and broad reactivity, especially in applications where a bulky cation is needed. The high phosphorus content and synthetic effort required for catalyst synthesis increase environmental impact; the recyclability of PPNCl in catalytic processes remains largely unexplored. The potential development of a polymer-supported PPNCl catalysts therefore desirable to enable this recyclability. In this work, we synthesise polymeric PPNCl (poly(PPNCl)) for the first time. Poly(PPNCl) demonstrates a comparative catalytic reactivity to its small molecule variant when employed as a catalyst in halogen-exchange reactions and CO2/epoxide coupling. For the latter the effect of catalyst loading, CO2 pressure, reaction time and addition of co-catalyst on conversion and selectivity was investigated. Poly(PPNCl) was easily recovered from the crude product by simple precipitation and its catalytic reactivity was well-maintained over three reaction cycles, providing environmental and economic advantages for sustainable reaction development.

Epoxide/CO2 Cycloaddition Reaction Catalyzed by Indium Chloride Complexes Supported by Constrained Inden Schiff-Base Ligands

Cyclic carbonates have received significant interests for uses as reagents, solvents, and monomers. The coupling reaction of epoxides with carbon dioxide (CO2 ) to produce cyclic carbonate is an attractive route which can significantly reduce greenhouse gas emissions and environmental hazards. Herein, a series of five indium chloride complexes supported by inden Schiff-base ligands were reported along with four X-ray crystal structures. The constrained five-membered rings were added to the ligands to enhance the coordination of epoxides to the In metal. From the catalyst screening, In inden complex having tert-butyl substituents and propylene backbone in combination with tetrabutylammonium bromide (TBAB) exhibited the highest catalytic activity (TON up to 1017) for propylene oxide/CO2 coupling reaction with >99 % selectivity for cyclic carbonate under solvent-free conditions. In addition, the catalyst was shown to be active at atmospheric pressure of CO2 at room temperature. The catalyst system can be applied to various internal and terminal epoxide substrates to exclusively produce the corresponding cyclic carbonates.

New Co and Mn Catalysts Bearing ONO Ligands Containing Nucleophile for the Coupling of CO2 and Propylene Oxide

A series of novel ONO ligands bearing an ionic pendant-armed (hereinafter indicated as ONONu, where Nu corresponds to an anionic nucleophile) were synthesized, characterized, and successfully coordinated to cobalt and manganese precursors. New air-stable cobalt (III) complexes (1–6) and manganese (II) complexes (7 and 8) were obtained and characterized. Single crystal X-ray diffraction analysis of the Co(III) compound 5 confirmed the presence of two quaternized ligands coordinated to the metal and iodide as counterion. These novel complexes were revealed to be active catalysts in the coupling reaction of carbon dioxide and propylene oxide (PO) in different degrees of success. Among these, the manganese complex 8 afforded the best results towards the formation of propylene carbonate (PC) with a productivity of 256 kg PC/(kg cat·h), achieving a TON of 4860.

Perfluoroaryl Zinc Catalysts Active in Cyclohexene Oxide Homopolymerization and Alternating Copolymerization with Carbon Dioxide

The zinc complex Zn(C6F5)2(toluene) (1) behaves as a very active and selective catalyst in cyclohexene oxide (CHO) polymerization to produce poly(cyclohexene oxide) (PCHO) by the trans-ring-opening of CHO with remarkable TOF values at room temperature. The ring-opening copolymerization (ROCOP) of CO2 with CHO catalysed by 1 yields poly(cyclohexene carbonate) (PCHC) when using benzyl alcohol (BnOH) as an initiator at 120°C. The 1H NMR monitoring of the in situ reaction of 1 with BnOH highlighted the formation of the dinuclear species [(C6F5)2Zn2(BnO)2 (2) that was isolated and found an active catalyst in the ROCOP of CO2 with CHO in the absence of initiators. Interestingly, PCHCs by 2 in solventless conditions show polydispersity index (Mw/Mn) values close to 2, corresponding to those expected for a single-site catalyst; on the contrary, a broader polydispersity index of the polymer products was found in toluene solution, suggesting the formation of new zinc catalysts during the polymerization reaction.

Self-Assembled Bimetallic Aluminum-Salen Catalyst for the Cyclic Carbonates Synthesis

Bimetallic bis-urea functionalized salen-aluminum catalysts have been developed for cyclic carbonate synthesis from epoxides and CO₂. The urea moiety provides a bimetallic scaffold through hydrogen bonding, which expedites the cyclic carbonate formation reaction under mild reaction conditions. The turnover frequency (TOF) of the bis-urea salen Al catalyst is three times higher than that of a μ-oxo-bridged catalyst, and 13 times higher than that of a monomeric salen aluminum catalyst. The bimetallic reaction pathway is suggested based on urea additive studies and kinetic studies. Additionally, the X-ray crystal structure of a bis-urea salen Ni complex supports the self-assembly of the bis-urea salen metal complex through hydrogen bonding.

Syntheses, Characterization, and Application of Tridentate Phenoxyimino-Phenoxy Aluminum Complexes for the Coupling of Terminal Epoxide with CO2: From Binary System to Single Component Catalyst

A series of binuclear aluminum complexes 1–3 supported by tridentate phenoxyimino-phenoxy ligands was synthesized and used as catalysts for the coupling reaction of terminal epoxide with carbon dioxide. The aluminum complex 1, which is catalytically inactive toward the coupling of epoxide with CO2 by itself, shows moderate activity in the presence of excess nucleophiles or organic bases at high temperature. In sharp contrast to complex 1, bifunctional complexes 2 and 3, which incorporate tertiary amine groups as the built-in nucleophile, are able to efficiently transform terminal epoxide with CO2 to corresponding cyclic carbonates as a sole product by themselves at 100 °C. The number of amine groups on the ligand skeleton and the reaction temperature exert a great influence on the catalytic activity. The bifunctional complexes 2 and 3 are also active at low carbon dioxide pressure such as 2 atm or atmospheric CO2 pressure. Kinetic studies of the coupling reactions of chloropropylene oxide/CO2 and styrene oxide/CO2 using bifunctional catalysts under atmospheric pressure of CO2 demonstrate that the coupling reaction has a first-order dependence on the concentration of the epoxide.

Dinuclear [OSSO]-Fe complexes for the reaction of CO2 with epoxides

Synthesis and characterization of new dinuclear iron complexes for the coupling of CO₂ with epoxides. A bimetallic cooperative reaction mechanism is proposed on the basis of kinetic analyses.

Metal Complexes Bearing Sulfur-Containing Ligands as Catalysts in the Reaction of CO2 with Epoxides

Coupling of CO2 with epoxides is a green emerging alternative for the synthesis of cyclic organic carbonates (COC) and aliphatic polycarbonates (APC). The scope of this work is to provide a comprehensive overview of metal complexes having sulfur-containing ligands as homogeneous catalytic systems able to efficiently promote this transformation with a concise discussion of the most significant results. The crucial role of sulfur as the hemilabile ligand and its influence on the catalytic activity are highlighted as well.

Exploring a new dinuclear Fe(iii) complex for the fixation of atmospheric CO2 and optical recognition of nano-molar levels of Zn2+ ions

A dinuclear Fe(iii) complex (F1) of an imine derivative (L1) derived from 3-ethoxy-2-hydroxy-benzaldehyde and hydrazine, structurally characterised via single crystal X-ray studies, is employed for the catalytic conversion of epoxides to cyclic carbonates utilizing carbon dioxide. In addition, F1 is employed for the selective optical recognition of nano-molar levels of Zn²⁺ (42.23 nM) via a metal displacement approach. The Job plot reveals interactions between F1 and Zn²⁺ at a 1 : 3 molar ratio with an association constant of 7.71 × 10⁴ M⁻¹. Studies on the catecholase-like activity of F1 reveal a k_cat value of 4.42 × 10³ h⁻¹.

A new Fe(iii) complex (F1), structurally characterised using single crystal X-ray studies, was explored for CO₂ fixation, Zn²⁺ recognition and catecholase activity.

A Novel [OSSO]-Type Chromium(III) Complex as a Versatile Catalyst for Copolymerization of Carbon Dioxide with Epoxides

A new chromium(III) complex, bearing a bis-thioether-diphenolate [OSSO]-type ligand, was found to be an efficient catalyst in the copolymerization of CO₂ and epoxides to achieve poly(propylene carbonate), poly(cyclohexene carbonate), poly(hexene carbonate) and poly(styrene carbonate), as well as poly(propylene carbonate)(cyclohexene carbonate) and poly(propylene carbonate)(hexene carbonate) terpolymers.

Mechanistic guidelines in nonreductive conversion of CO2: the case of cyclic carbonates

This perspective provides general mechanistic guidelines for the catalytic formation of cyclic organic carbonates from CO₂ and cyclic ethers.

Salalen vs. thiolen: in the ring(-opening of epoxide and cyclic carbonate formation)

A range of Fe(iii)-salalen and -thiolen–chloride complexes have been prepared and are shown to be active catalysts for the selective coupling of CO₂ and cyclohexene oxide (CHO).

Photo-generation of cyclic carbonates using hyper-branched Ru-TiO2

Anthropogenic CO2 is the main contributor to the increased concentration of greenhouse gases in the atmosphere, and thus utilising waste CO2 for the production of valuable chemicals is a very appealing strategy for reducing CO2 emissions. The catalytic fixation of CO2 with epoxides for the production of cyclic carbonates has gained increasing attention from the research community in search of an alternative to the homogeneous catalytic routes, which are currently being used in industry. A novel photocatalytic heterogeneous approach to generate cyclic carbonates is demonstrated in this work. Hyper-branched microstructured Ru modified TiO2 nanorods decorated with RuO2 nanoparticles, supported on fluorine-doped tin oxide (FTO) glass were fabricated for the first time and were used to catalyse the photo-generation of propylene carbonates from propylene oxides. Propylene carbonate was used as a reference for cyclic carbonates. The photo-generation of cyclic carbonates from epoxides and CO2 was carried out at a maximum temperature of 55 °C at 200 kPa in a stainless steel photoreactor with a quartz window, under solar irradiation for 6 h. The best performing photocatalyst exhibited an estimated selectivity of 83% towards propylene carbonates under the irradiation of a solar simulator.

Multifunctional and Sustainable Fe-Iminopyridine Complexes for the Synthesis of Cyclic Carbonates

The use of multifunctional and sustainable Fe catalysts for the formation of cyclic carbonates from epoxides and carbon dioxide at 80 °C and 3 bar pressure is presented. The optimal catalyst possesses a halide counteranion and a hydrogen bond donor to activate the epoxide for ring opening, affording a single-component, cocatalyst-free catalytic system.

Experimental and theoretical study for CO2 activation and chemical fixation with epoxides

The synthesis of five-membered cyclic carbonates via catalytic cycloaddition reaction of CO₂ with epoxides is considered to be an effective technology for alleviation of the energy crisis and global warming. Various commercial organic bases and ionic salts were used as catalysts, while the relationship of catalytic activity and compound structure has been seldom explored. Herein, a facilely obtained binary catalytic system based on triethylamine/NBu₄Br was developed for CO₂ activation and chemical fixation. The highly efficient catalytic system showed outstanding conversion and above 99% selectivity under metal-free mild reaction conditions (100 °C, 1 atm) in one hour. The detailed process of CO₂ activation and chemical fixation was investigated at the molecular level by a series of experiments and theoretical calculation, which provided a mode for the design and synthesis of a highly efficient catalytic system for conversion of CO₂ under mild conditions.

NEt₃/NBu₄Br works as an excellent metal-free catalyst for CO₂ cycloaddition with epoxides and the detailed process of CO₂ activation by NEt₃ is first studied by theoretical calculation.

Sustainable fixation of CO2 into epoxides to form cyclic carbonates using hollow marigold CuCo2O4 spinel microspheres as a robust catalyst

The present work demonstrates the chemical fixation of CO₂ for the synthesis of organic carbonates using mesoporous hollow marigold CuCo₂O₄ spinel microspheres as a catalyst prepared using the solvothermal method.

The synthesis, characterisation and application of iron(iii)–acetate complexes for cyclic carbonate formation and the polymerisation of lactide

Cracking the whip: Simple iron(iii) acetate complexes have been applied to the catalytic cyclic carbonate formation and lactide ROP.

Highly active dinuclear cobalt complexes for solvent-free cycloaddition of CO2 to epoxides at ambient pressure

Dinuclear Co-based catalysts are used for the coupling reaction of epoxides and CO₂ in the presence of a cocatalyst.

Cobalt amino-bis(phenolate) complexes for coupling and copolymerization of epoxides with carbon dioxide

Electron-withdrawing groups on phenolate donors enhance polycarbonate production from CO₂ and cyclohexene oxide by Co(ii) amino-bis(phenolate) complexes.