Construction of Polycarbonates with Pendant Multifunctional Groups via a One-Step CO2/ Diepoxide Copolymerization Approach

A "one-step" strategy has been demonstrated for the tunable synthesis of multifunctional aliphatic polycarbonates (APCs) with ethylene oxide (EO), ethylene carbonate (EC), and cyclohexene oxide (CHO) side groups by the copolymerization of 4-vinyl-1-cyclohexene diepoxide with carbon dioxide under an aminotriphenolate iron/PPNBz (PPN = bis(triphenylphosphine)-iminium, Bz = benzoate) binary catalyst. By adjusting the PPNBz-to-iron complex ratio and incorporating auxiliary solvents, the content of functional side groups can be tuned within the ranges of 53-75% for EO, 18-47% for EC, and <1-7% for CHO. The yield and molecular weight distribution of the resulting multifunctional APCs are affected by the viscosity of the polymerization system. The use of tetrahydrofuran as an auxiliary solvent enables the preparation of narrow-distribution polycarbonates at high conversion. This work presents a novel perspective for the preparation of tailorable multifunctional APCs.

Novel salenCo(iii) photoinitiators and their application for cycloaddition of carbon dioxide

Carbon dioxide (CO₂) is a renewable carbon resource that can be effectively used in the production of polycarbonate (PPC) and cyclic carbonate (CPC) through open-loop copolymerization with epoxides and CO₂. SalenCo(iii) can successfully break the carbon-oxygen link between propylene oxide (PO) and CO₂. On this basis, we prepared four different types of photosensitive salenCo(iii) complexes and investigated their catalytic copolymerization of CO₂ and PO. The results show that the catalytic performance of 1,2-cyclohexamediamine complexes is better than that of 1,2-o-phenylenediamine complexes. The catalytic efficiency of salenCo(iii) catalyst increases with the expansion of the photosensitive conjugate system. In addition, the introduction of light can improve the catalytic efficiency. When we increased the power of the external light source from 100 W to 200 W, the TON of the catalyst [C4] increased by nearly 50%.

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.

Cyclic Carbonate Synthesis from Epoxides and CO2 Catalyzed by Aluminum-Salen Complexes Bearing a nido-C2B9 Carborane Ligand

The active and well-designed Schiff base ligands are considered "privileged ligands". The so-called salen ligands, i.e., the tetradentate [O, N, N, O] bis-Schiff base ligands, have also found broad applications in many homogeneous catalytic reactions. Modification of the salen ligands has concentrated on altering the substituents in the phenolate rings and variations in the diamine backbones. Herein, o-carborane-supported salen ligands (2) were designed and prepared. A series of aluminum-salen complexes (3·(sol)₂), which were supported by the nido-C₂B₉ carborane anions, were synthesized. These Al(III) complexes showed high activities (TOF up to 1500 h^-1) in catalyzing the cycloaddition of epoxides and CO₂ at atmospheric pressure and near room temperature. Complexes 3·(sol)₂ are one of the rare examples of Al-based catalysts capable of promoting cycloaddition at 1 bar pressure of CO₂. Density functional theory (DFT) studies combined with the catalytic results reveal that the catalytic cycles occur on two axial sites of the Al(III) center.

Catalyst Engineering Empowers the Creation of Biomass-Derived Polyesters and Polycarbonates

ConspectusThe introduction of circular principles in chemical manufacturing will drastically change the way everyday plastics are produced, thereby affecting several aspects of the respective value chains in terms of raw feedstock, recyclability, and cost. The ultimate aim is to ensure a paradigm shift toward plastic-based (consumer) materials that overall can offer a more attractive and sustainable carbon footprint, which is an important requisite from a societal, political, and eventually economical point of view. To realize this important milestone, it is vitally important to control the polymerization processes associated with the creation of novel sustainable materials. In this respect, we realized that expanding the portfolio of biomass-derived monomers may indeed create an impetus for atom circularity; however, the often sterically congested nature of biomass-derived monomers minimizes the ability of previously developed catalysts to activate and transform these precursors. Our motivation was thus spurred by an apparent lack of catalysts suitable for addressing the conversion of such biomonomers, as we realized the potential that new catalytic processes could have to advance and contribute to the development of sustainable materials produced from polycarbonates and polyesters. These two classes of polymers represent crucial ingredients of important and large-scale consumer products and are therefore ideal fits for implementing new catalytic protocols that enable a gradual transition to plastic materials with an improved carbon footprint.When we started our research expedition, the field was dominated by metal catalysts that incorporated preferred, and in some cases even privileged, ligand backbones (such as salens) able to mediate both ring-opening and ring-opening copolymerization manifolds. One major drawback of these aforementioned catalysts is their rather rigid nature, a feature that reduces their ability to act as adaptive systems, especially in cases where bulky monomers are involved. While our initial focus was on the utilization of sustainable metal salen complexes (M = Zn, Fe) for the activation of small cyclic ethers, which are privileged monomers for polyester and polycarbonate production, we were rapidly confronted with severe limitations related to their inability to activate a wider range of complex epoxides and oxetanes, which was imparted by the planar coordination geometry of the salen ligand in most of its applied metal complexes. In our quest to find a catalytically more effective metal complex with the ability to electronically and sterically tune its substrate-binding and substrate-activation potential, we identified aminotriphenolates as structurally versatile, easily accessible, and scalable ligands for various earth-abundant metal cations. Moreover, the ligand backbone allows for switchable coordination environments around the metal centers, thus offering the necessary adaptation in substrate activation events.This Account describes how Al(III)- and Fe(III)-centered aminotriphenolates have conquered a prominent position as catalyst components in the synthesis of new biobased polyester and polycarbonate architectures, thereby changing the landscape of previously difficult to convert biomonomers, and expanding the chemical space of biobased functional polymers. With the ever-increasing influence of legislation and the restrictions placed on the use of fossil-fuel-based feedstock, the polymer industry needs viable alternatives to design materials that are greener, cost-effective, and allow for the exploration and optimization of their recycling and properties.

Rationalizing the Mechanism of Peroxyformate Decomposition: Computational Insights To Understand Solvent Influence

The heterolytic decomposition of tert‐butyl peroxyformate to tert‐butanol and carbon dioxide, catalyzed by pyridine, is a long‐known example of a reaction whose kinetics are strongly affected by solvent polarity. From DFT and ab initio methods together with the SMD implicit solvation model, an extension on the formerly accepted mechanism is proposed. This novel proposal involves the formation of a carbonic acid ester intermediate and its further decomposition, through an unreported pyridine‐mediated stepwise route. Computed barriers for this mechanism at DLPNO/CCSD(T)‐def2‐TZVP are in excellent agreement with experimental kinetic data across different solvents. Furthermore, the strong relationships between activation energies, geometric parameters in the transition state and the characteristics of the different solvents are also analyzed in depth.

Metal-Cocatalyst Interaction Governs the Catalytic Activity of MII-Porphyrazines for Chemical Fixation of CO2

Chemical fixation of CO₂ to produce cyclic carbonates can be a green and atomic efficient process. In this work, a series of porphyrazines (Pzs) containing electron-withdrawing groups and central M^II ions (where M = Mg, Zn, Cu, and Co) were synthesized and investigated as catalysts for the cycloaddition of CO₂ to epoxides. Then, the efficiency of the Pzs was tested by varying cocatalyst type and concentration, epoxide, temperature, and pressure. Mg^IIPz bearing trifluoromethyl groups (1) showed the best conversion, producing, selectively, 78% of propylene cyclic carbonate (PCC), indicating that a harder and stronger Lewis acid is more effective for epoxide activation. Moreover, cocatalyst variation showed a notable effect on the reaction yields. Spectrophotometric titrations, MALDI-TOF mass spectra, and theoretical calculations suggest poisoning of the catalyst when tetrabutylammonium chloride (TBAC) and large amounts of tetrabutylammonium bromide (TBAB) were used in the system. The same was not observed for tetrabutylammonium iodide (TBAI), indicating that the metal-cocatalyst interaction may govern the reaction rate. In addition, two rare examples of crystalline structures were obtained, proving the distorted square pyramidal geometry with water molecule as axial ligand. This is one of the first studies reporting Pzs as catalysts for the chemical fixation of CO₂, and we believe that the intricate balance between cocatalyst concentration and conversion efficiency shown here may aid future studies in the area.

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.

Catalytic, Kinetic, and Mechanistic Insights into the Fixation of CO2 with Epoxides Catalyzed by Phenol-Functionalized Phosphonium Salts

A series of hydroxy-functionalized phosphonium salts were studied as bifunctional catalysts for the conversion of CO2 with epoxides under mild and solvent-free conditions. The reaction in the presence of a phenol-based phosphonium iodide proceeded via a first order rection kinetic with respect to the substrate. Notably, in contrast to the aliphatic analogue, the phenol-based catalyst showed no product inhibition. The temperature dependence of the reaction rate was investigated, and the activation energy for the model reaction was determined from an Arrhenius-plot (Ea =39.6 kJ mol-1 ). The substrate scope was also evaluated. Under the optimized reaction conditions, 20 terminal epoxides were converted at room temperature to the corresponding cyclic carbonates, which were isolated in yields up to 99 %. The reaction is easily scalable and was performed on a scale up to 50 g substrate. Moreover, this method was applied in the synthesis of the antitussive agent dropropizine starting from epichlorohydrin and phenylpiperazine. Furthermore, DFT calculations were performed to rationalize the mechanism and the high efficiency of the phenol-based phosphonium iodide catalyst. The calculation confirmed the activation of the epoxide via hydrogen bonding for the iodide salt, which facilitates the ring-opening step. Notably, the effective Gibbs energy barrier regarding this step is 97 kJ mol-1 for the bromide and 72 kJ mol-1 for the iodide salt, which explains the difference in activity.

Cycloaddition of carbon dioxide to epoxides by highly active constrained aluminum chloride complexes

Novel constrained aluminum inden complexes were developed revealing high activities and excellent selectivity for cyclic carbonates in epoxides/CO2 coupling reactions.

Mn(iii)–porphyrin catalysts for the cycloaddition of CO2 with epoxides at atmospheric pressure: effects of Lewis acidity and ligand structure

Mn(iii)–porphyrin catalysts with electron-withdrawing substituents were designed to uncover electronic and structural aspects in the cycloaddition of CO₂ with epoxides.

Organocatalytic Trapping of Elusive Carbon Dioxide Based Heterocycles by a Kinetically Controlled Cascade Process

A conceptually novel approach is described for the synthesis of six-membered cyclic carbonates derived from carbon dioxide. The approach utilizes homoallylic precursors that are converted into five-membered cyclic carbonates having a β-positioned alcohol group in one of the ring substituents. The activation of the pendent alcohol group through an N-heterocyclic base allows equilibration towards a thermodynamically disfavored six-membered carbonate analogue that can be trapped by an acylating agent. Various control experiments and computational analysis of this manifold are in line with a process that is primarily dictated by a kinetically controlled acylation step. This cascade process delivers an ample diversity of six-membered cyclic carbonates in excellent yields and chemoselectivities under mild reaction conditions.

Cyclic Carbonate Formation from Epoxides and CO2 Catalyzed by Sustainable Alkali Halide-Glycol Complexes: A DFT Study to Elucidate Reaction Mechanism and Catalytic Activity

We provide a comprehensive DFT investigation of the mechanistic details of CO2 fixation into styrene oxide to form styrene carbonate, catalyzed by potassium iodide-tetraethylene glycol complex. A detailed view on the intermediate steps of the overall reaction clarifies the role of hydroxyl substances as co-catalysts for the alkali halide-catalyzed cycloaddition. The increase of iodide nucleophilicity in presence of tetraethylene glycol is examined and rationalized by NBO and Hirshfeld charge analysis, and bond distances. We explore how different alkali metal salts and glycols affect the catalytic performance. Our results provide important hints on the synthesis of cyclic carbonates from CO2 and epoxides promoted by alkali halides and glycol complexes, allowing the development of more efficient catalysts.

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.

Copolymerization of CHO/CO2 catalyzed by a series of aluminum amino-phenolate complexes and insights into structure-activity relationships

Two series of monometallic aluminum complexes were prepared and characterized by elemental analyses, ¹H and ¹³C{¹H} NMR spectroscopy, and X-ray crystallography: Al[L]X, where [L] = dimethylaminoethylamino-N,N-bis(2-methylene-4,6-tert-butylphenolate) and X = Cl, OEt, and Al[L]₂Cl, where [L] = 6-{[(2R,6R)-2,6-dimethyl-4-morpholino]methylene}-2,4-bis(tert-butyl)phenolate or 6-(piperidinomethylene)-2-(tert-butyl)-4-(methyl)phenolate. All the complexes, including the previously reported morpholinyl complex Al[L]Cl, where [L] = 4-(2-aminoethyl)morpholinylamino-N,N-bis(2-methylene-4,6-tert-butylphenolate), were tested as catalysts for copolymerization of cyclohexene oxide and CO₂ in the presence and absence of PPNCl. When coupled with 1 equiv. PPNCl, the complexes exhibit similar activities and the best selectivity for poly(cyclohexenecarbonate) vs. the cyclic product, cyclohexene carbonate, was obtained with the morpholinyl complex (ca. 90%) whereas significantly lower selectivities (<1-63%) were obtained with the other complexes. Preliminary DFT calculations investigating this difference in selectivity were carried out by analyzing the aluminum partial atomic charges in the Al-carbonate intermediates.

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.

Highly Selective Single-Component Formazanate Ferrate(II) Catalysts for the Conversion of CO2 into Cyclic Carbonates

The development of new families of active and selective single-component catalysts based on earth-abundant metal is of interest from a sustainable chemistry perspective. In this context, anionic mono(formazanate) iron(II) complexes bearing labile halide ligands, which possess both Lewis acidic and nucleophilic functionalities, have been developed as novel single-component homogeneous catalysts for the reaction of CO₂ with epoxides to produce cyclic carbonates. The influence of the halide ligand and the electronic properties of the formazanate ligand backbone on the catalytic activity are investigated by employing the iron(II) complexes with and without an additional nucleophile. Very high selectivity is achieved towards the formation of the cyclic carbonate products from various terminal and internal epoxides without the need of a cocatalyst.

Entropic corrections for the evaluation of the catalytic activity in the Al(iii) catalysed formation of cyclic carbonates from CO2 and epoxides

Entropic corrections are found to be crucial for evaluating the catalytic performance in solution for the reaction of CO₂ with epoxides.

Synthesis of well-defined yttrium-based Lewis acids by capturing a reaction intermediate and catalytic application for cycloaddition of CO2 to epoxides under atmospheric pressure

Single-site yttrium complexes were prepared by immobilization of an intermediate of cycloaddition of CO₂ to epoxides and applied in catalysis.

Advances in the use of CO2 as a renewable feedstock for the synthesis of polymers

Carbon dioxide offers an accessible, cheap and renewable carbon feedstock for synthesis. Current interest in the area of carbon dioxide valorisation aims at new, emerging technologies that are able to provide new opportunities to turn a waste into value. Polymers are among the most widely produced chemicals in the world greatly affecting the quality of life. However, there are growing concerns about the lack of reuse of the majority of the consumer plastics and their after-life disposal resulting in an increasing demand for sustainable alternatives. New monomers and polymers that can address these issues are therefore warranted, and merging polymer synthesis with the recycling of carbon dioxide offers a tangible route to transition towards a circular economy. Here, an overview of the most relevant and recent approaches to CO2-based monomers and polymers are highlighted with particular emphasis on the transformation routes used and their involved manifolds.

Sustainable Heterogeneous Catalysts for CO2 Utilization by Using Dual Ligand ZnII /CdII Metal-Organic Frameworks

Two-dimensional Zn^II /Cd^II -based dual ligand metal-organic frameworks (MOFs) {[M(CHDC)(L)]⋅H₂ O}_n involving 4-pyridyl carboxaldehyde isonicotinoylhydrazone (L) in combination with flexible 1,4-cyclohexanedicarboxylic acid (H₂ CHDC) as linkers have been synthesized by adaptable synthetic protocols including a green mechanochemical (grinding) method. Characterization, chemical/thermal stability, phase purity, and solid-state luminescent properties of both MOFs have been established by various analytical methods. Structural analysis revealed dimeric metal clusters composed of [M₂ (CHDC)₂ ]_n loops doubly pillared with L, generating a 2D framework. Both MOFs can be used as highly active solvent-free binary catalysts for CO₂ cycloaddition with epoxides in the presence of the co-catalyst tetrabutylammonium bromide (TBAB) with good catalytic conversion in up to six catalytic cycles without significant loss of activity. The present investigation demonstrates the application of MOFs as efficient heterogeneous catalysts for CO₂ utilization under moderate reaction conditions. Based on the single-crystal X-ray data, a probable mechanism for the cycloaddition reaction has also been proposed.

Synthesis and characterization of trimetallic cobalt, zinc and nickel complexes containing amine-bis(benzotriazole phenolate) ligands: efficient catalysts for coupling of carbon dioxide with epoxides

New trimetallic cobalt, nickel and zinc complexes 1-3 coordinated by amine-bis(benzotriazole phenolate) ligands and ancillary acetate groups have been developed for the use of CO₂/epoxide coupling. All complexes were structurally characterized by single crystal X-ray crystallography; tri-Co complex 1 is the first solid-state example in which three different geometrical configurations exist in the same benzotriazole phenoxide metal complex. Tri-nuclear complexes 1 and 2 with cobalt and zinc metal centers were demonstrated to be very active catalysts for cycloaddition of cyclohexene oxide with CO₂ in the presence of ammonium salt co-catalysts to give cis-cyclohexene carbonate under the conditions of 80 °C and 300 psi initial CO₂ pressure. Particularly, tri-cobalt complex 1 was found to efficiently couple CO₂ with epoxides showing broad substrate scope, producing the corresponding cyclic organic carbonates with good activities and high selectivities. This is a successful example of catalysis for cyclic carbonate synthesis using one cobalt(ii) complex as a homogeneous catalyst.