Multifunctional Zn-N4 Catalysts for the Coupling of CO₂ with Epoxides into Cyclic Carbonates

Abstract

The catalytic conversion of greenhouse gas CO2 into valuable chemicals is a vital goal toward carbon balance and sustainability. In recent decades, the chemical fixation of CO2 into cyclic carbonates has gained much attention. In this work, a series of zinc complexes bearing tetradentate aminopyridine (N4) ligands have been synthesized and characterized. These zinc complexes were applied to the coupling of CO2 with epoxides in excellent yields and with a broad substrate scope under cocatalyst- and solvent-free conditions. Moreover, the zinc catalysts could be readily recovered and reused five times without an obvious loss in catalytic activity. Based on spectroscopic characterizations and experimental results, catalyst Zn-3 (DAP-ZnBr2, DAP = 1,4-bis(2-pyridymethyl)-1,4-diazepane) has been found to be a multifunctional catalyst because of the presence of a Lewis acidic zinc center and a nuclephilic halide anion, and one pyridine is released for the activation of CO2 during the reaction.

Co(II)–N4 Catalysts for the Coupling of CO2 with Epoxides into Cyclic Carbonates: Catalytic Activity, Computational and Kinetic Studies

“…The linear aminopyridine (MEP = N , N ′-dimethyl- N , N ′-bis­(2-pyridylmethyl) −1,2-ethanediamine; DAP = 1,4-bis­(2-pyridymethyl)-1,4-diazepane; NMe 2 MEP = N , N ′-bis­((4-(dimethylamino)­pyridin-2-yl)­methyl)- N , N ′-dimethylethane-1,2-diamine) and aminobenzimidazole N4 (EEB = N , N ′-bis­((1-ethyl-1H-benzo­[d]­imidazol-2-yl)­methyl)- N , N ′-dimethylethane-1,2-diamine) ligands were easily synthesized according to the reported methods. ,− The corresponding Co­(II)–N4 complexes 1 – 4 were prepared by the reaction of CoBr 2 with the N4 ligands under an Ar atmosphere in high yields, as shown in Figure . Subsequently, single crystals suitable for X-ray crystallography were grown for Co-1 and Co-4 by slowly diffusing diethyl ether into solutions of methanol and acetonitrile, respectively.…”

“…Recently, we also developed a series of Zn−N4 complexes for the CO 2 fixation with terminal epoxides under solvent-and cocatalyst-free conditions. 45 However, the catalyst efficiency for this type of catalyst supported by neutral nitrogen ligands was still low, and it may depend on the dissociation rate of a nucleophilic halide anion from the metal center. With this in mind, we envisioned that the dissociation rate of a nucleophilic halide anion coordinated to the metal may be regulated by changing the metal and the ligand scaffold.…”

“…Inspired by the structural motifs of nonheme enzymes, in recent years, the Mn and Fe complexes with tetradentate aminopyridine (N4) ligands have proven to be efficient catalysts for the oxidation of the CC and C–H bonds. − We are very interested in expanding the applications of these ligands in catalysis. Recently, we also developed a series of Zn–N4 complexes for the CO 2 fixation with terminal epoxides under solvent- and cocatalyst-free conditions . However, the catalyst efficiency for this type of catalyst supported by neutral nitrogen ligands was still low, and it may depend on the dissociation rate of a nucleophilic halide anion from the metal center.…”

Ionic Porous Organic Polymer Loaded with Zn(II) Ions for Efficient Cycloaddition of CO2 and Epoxides to Cyclic Carbonate

“…Among them, the coupling reaction of CO 2 with epoxides into CCs has received considerable attention owing to the advantages of the high uptake of CO 2 and the targeted five-membered CCs with wide applications such as polar aprotic solvents, electrolytes in lithium-ion batteries, and chemical intermediates in multiple fine chemical synthesis . For the purpose of meeting the needs of green chemistry, researchers are interested in exploring catalysts with low toxicity, high activity, and sustainability for the cycloaddition reaction between CO 2 and epoxides toward CCs. − …”

Efficient Selective Catalytic Fixation of CO2 into Epoxide to Form Cyclic Carbonates Using Sodium Aluminate Engineered Gamma Alumina Catalyst

“…Till today, for cyclic carbonates synthesis, few efficient non‐recyclable and recyclable catalysts were reported, such as DBU, 15 ILs, 16 ligand‐based catalysts, 17 organocatalysts, 18 metal complexes, 19,20 metal/mixed metal oxides, 21,22 covalent organic frameworks (COFs), 23 metal organic frameworks (MOFs), 1 porous organic polymers (POPs), 8,24 silica‐based oxides, 25 carbon‐supported oxides, 26,27 and so on. However, earlier reported catalysts are suffering from a few drawbacks, such as a tedious catalyst synthesis procedure, use of hazardous chemicals, a long reaction time, less stability, less catalytic activity, harsh reaction conditions, recovery, and recyclability 8 .…”