Polymorphism and Its Influence on Catalytic Activities of Lanthanide-Glutamate-Oxalate Coordination Polymers

To study the relationship between polymorphism and catalytic activities of lanthanide coordination polymers in the cycloaddition reactions of CO2 with epoxides, the monoclinic and triclinic polymorphs of [LnIII(NH3-Glu)(ox)]·2H2O, where LnIII = LaIII (I), PrIII (II), NdIII (III), SmIII (IV), EuIII (V), GdIII (VI), TbIII (VII), and DyIII (VIII), NH3-Glu- = NH3+ containing glutamate, and ox2- = oxalate, were synthesized and characterized. Factors determining polymorphic preference, the discrepancy between the two polymorphic framework structures, potential acidic and basic sites, thermal and chemical stabilities, active surface areas, void volumes, CO2 sorption/desorption isotherms, and temperature-programmed desorption of NH3 and CO2 are comparatively presented. Based on the cycloaddition of CO2 with epichlorohydrin in the presence of tetrabutylammonium bromide under solvent-free conditions and ambient pressure, catalytic activities of the two polymorphs were evaluated, and the relationship between polymorphism and catalytic performances has been established. Better performances of the monoclinic catalysts have been revealed and rationalized. In addition, the scope of monosubstituted epoxides was experimented and the outstanding performance of the monoclinic catalyst in the cycloaddition reaction of CO2 with allyl glycidyl ether under ambient pressure has been disclosed.

Two novel Ln8 clusters bridged by CO32- effectively convert CO2 into oxazolidinones and cyclic carbonates

It is difficult and challenging to design and construct high-nuclearity Ln(III)-based clusters due to the high coordination numbers and versatile coordination geometries of Ln(III) ions. Herein, two novel octanuclear Ln(III)-based clusters [Ln₈(H₂L^-)₄(HL^2-)₄(NO₃)₆ (CO₃)₂](NO₃)₂·2CH₃CN (Ln = Nd (1) and Sm (2)) have been synthesized under solvothermal conditions. The X-ray single analysis reveals that both 1 and 2 are octanuclear structures and the eight central Ln(III) ions are bridged by two CO₃^2- anions. Catalytic study revealed that 1 and 2 can effectively catalyze the cycloaddition reaction of CO₂ and aziridines or epoxides simultaneously under mild conditions. What is more, cluster 1, as a heterogeneous catalyst, can be reused at least three times without obvious loss in catalytic activity for coupling of CO₂ and epoxides. To our knowledge, cluster 1 is the first Ln(III)-based cluster catalyst used for the conversion of CO₂ with aziridines or epoxides simultaneously. This work provides a successful strategy to integrate high-nuclear Ln(III)-based clusters for CO₂ conversion, which may open a new space for the construction of multifunctional high-nuclear Ln(III)-based clusters as efficient catalysts for CO₂ conversion.

Direct coupling of CO2 with epoxides catalyzed by lanthanum(III) supported on magnetic mesoporous organosilica nanoparticles

Lanthanum(III) supported on the magnetic mesoporous organosilica nanoparticle (La@MON) has been described as an efficient, simple, and durable heterogeneous catalyst for the synthesis of 5-membered cyclic carbonates from carbon dioxide (CO₂) and epoxides. Under optimized reaction conditions, various terminal epoxides have been converted to the corresponding carbonates in the presence of 0.3 mol% La@MON and 0.5 mol% tetrabutylammonium iodide (TBAI) as co-catalyst at relatively mild reaction conditions. It was also found that La@MON catalysts had significantly higher catalytic activity than some selected reference catalysts, which can be explained by the abundance of lanthanum(III) species acting as Lewis acidic sites for activating both carbon dioxide and epoxide molecules, along with the fact that the catalyst channels are short and provided facile mass transfer. The catalyst showed good reusability for at least five reaction cycles while the magnetic core of the catalyst helps the easy separation of the catalyst by just using an external magnet.

Functionalized β-Cyclodextrins Catalyzed Environment-Friendly Cycloaddition of Carbon Dioxide and Epoxides

Ammonium, imidazole, or pyridinium functionalized β-cyclodextrins (β-CDs) were used as efficient one-component bifunctional catalysts for the coupling reaction of carbon dioxide (CO₂) and epoxide without the addition of solvent and metal. The influence of different catalysts and reaction parameters on the catalytic performance were examined in detail. Under optimal conditions, Im-CD1-I catalysts functionalized with imidazole groups were able to convert various epoxides into target products with high selectivity and good conversion rates. The one-component bifunctional catalysts can also be recovered easily by filtration and reused at least for five times with only slight decrease in catalytic performance. Finally, a possible process for hydroxyl group-assisted ring-opening of epoxide and functionalized group- induced activation of CO₂ was presented.

Self-Assembly Bifunctional Tetranuclear Ln2Ni2 Clusters: Magnetic Behaviors and Highly Efficient Conversion of CO2 under Mild Conditions

A series of heterometallic tetranuclear clusters, Ln₂Ni₂(NO₃)₄L₄(μ₃-OCH₃)₂·2(CH₃CN) (Ln = Gd(1), Tb(2), Dy(3), Ho(4), Er(5); HL = methyl 3-methoxysalicylate), were synthesized solvothermally. The intramolecular synergistic effect of two metal centers of Ln(III) and Ni(II) and the exposed multimetallic sites serving as Lewis acid activators greatly increase the efficiency of the CO₂ conversion, and the yield for cluster 3 can be achieved at 96% at atmospheric pressure and low temperature. In particular, the self-assembly multimetal center with polydentate ligand shows good generality and enhanced recyclability. The design of such 3d-4f heterometallic clusters provides an effective strategy for the conversion of CO₂ under greener conditions. Meanwhile, magnetic investigations indicate that cluster 1 is a good candidate for magnetic refrigerant materials with a relatively large magnetocaloric effect (MCE) (-ΔS_m = 28.5 J kg^-1 K^-1 at 3.0 K and 7.0 T), and cluster 3 shows single-molecular magnet behavior under zero dc field. Heterometallic clusters with special magnetic properties and good catalytic behavior for the conversion of CO₂ are rare. Thus, they are potential bifunctional materials applied in practice.

Reversible Structural Transformation and Catalytic Potential of Lanthanide-Azobenzenetetracarboxylates

Inspired by the catalytic potential of lanthanide coordination polymers of 3,3',5,5'-azobenzenetetracarboxylic acid (H₄abtc), two new isostructural [Ln₂^III(Habtc)₂(DMSO)₄]·DMSO·H₂O (Ln^III = Sm^III (I), Eu^III = (II), DMSO = dimethyl sulfoxide) were synthesized and characterized. Their single-crystal structures were elucidated and described. Structural transformations of II in the solid state prompted by ligand substitution and thermal treatment were studied, from which genuine reversible transformation of II to [Eu^III(Habtc)(H₂O)₄]·3H₂O (II') and [Eu^III(Habtc)(H₂O)₂]·2H₂O (II″) was revealed. This illustrates the rare case of reversible transformation in lanthanide coordination polymers. The transformation between II' and II″ was also investigated. Structural transformations among these frameworks are discussed with regard to the coordination environment of Eu^III, coordination modes of Habtc^3-, and similarities and disparities in framework architecture and registration. In addition, the catalytic performance of II with and without the prior activation in CO₂ cycloaddition reaction with epichlorohydrin was studied in comparison with II' and II″. The excellent performance of II disregarding the activation process has been demonstrated with the maximum turnover number and turnover frequency of 7682 and 1921 h^-1, respectively, for the activated II and 7142 and 1786 h^-1, respectively, for the nonactivated II. The maintenance of the catalytic efficiency over 10 cycles of the catalysis and the regeneration process is illustrated and discussed with respect to structural transformation.

Efficient Synthesis of Cyclic Carbonates from Unsaturated Acids and Carbon Dioxide and their Application in the Synthesis of Biobased Polyurethanes

Bio-derived furan- and diacid-derived cyclic carbonates have been synthesized in high yields from terminal epoxides and CO₂ . Furthermore, four highly substituted terpene-derived cyclic carbonates were isolated in good yields with excellent diastereoselectivity in some cases. Eleven new cyclic carbonates derived from 10-undecenoic acid under mild reaction conditions were prepared, providing the corresponding carbonate products in excellent yields. The catalyst system also performed the conversion of an epoxidized fatty acid n-pentyl ester into a cyclic carbonate under relatively mild reaction conditions (80 °C, 20 bar, 24 h). This bis(cyclic carbonate) was obtained in high yields and with different cis/trans ratios depending on the co-catalyst used. An allyl alcohol by-product was only observed as a minor product when bis(triphenylphosphine)iminium chloride was used as co-catalyst. Finally, two cyclic carbonates were used as building blocks for the preparation of non-isocyanate poly(hydroxy)urethanes by reaction with 1,4-diaminobutane.

The chemistry of Ce-based metal-organic frameworks

Metal-organic frameworks (MOFs) have gained widespread attention due to their modular construction that allows the tuning of their properties. Within this vast class of compounds, metal carboxylates containing tri- and tetravalent metal ions have been in the focus of many studies due to their often high thermal and chemical stabilities. Cerium has a rich chemistry, which depends strongly on its oxidation state. Ce(iii) exhibits properties typically observed for rare earth elements, while Ce(iv) is mostly known for its oxidation behaviour. In MOF chemistry this is reflected in their unique optical and catalytic properties. The synthetic parameters for Ce(iii)- and Ce(iv)-MOFs also differ substantially and conditions must be chosen to prevent reduction of Ce(iv) for the formation of the latter. Ce(iii)-MOFs are usually reported in comprehensive studies together with those constructed with other RE elements and normally they are isostructural. They exhibit a greater structural diversity, which is reflected in the larger variety of inorganic building units. In contrast, the synthesis conditions of Ce(iv)-MOFs were only recently (2015) established. These lead selectively to hexanuclear Ce-O clusters that are well-known for Zr-MOFs and therefore very similar structural and isoreticluar chemistry is found. Hence Ce(iv)-MOFs exhibit often high porosity, while only a few porous Ce(iii)-MOFs have been described. Some of these show structural flexibility which makes them interesting for separation processes. For Ce(iv)-MOFs the redox properties are most relevant. Thus, they are intensively discussed for catalytic, photocatalytic and sensing applications. In this perspective, the synthesis, structural chemistry and properties of Ce-MOFs are summarized.

Highly efficient 3d/4d-4f coordination polymer catalysts for carbon dioxide fixation into cyclic carbonates

Two novel highly efficient 3d/4d-4f one-dimensional (1D) double-chain coordination polymer catalysts with unique structures were synthesized for the first time. An X-ray single crystal structure analysis revealed that the two compounds are isomorphous and have a 1D metal-organic network coordination polymer structure. Both compounds also showed significant thermal stability and their structures remained stable up to 325 °C. The reaction conditions, type of substrate, amount of catalyst and its catalytic mechanism were investigated. The catalysts ([Dy2M2L4 (OAc)2 (MeOH)5 (H2O)]) (M = Zn, Cd) exhibited excellent catalytic activity in the cycloaddition of CO2 and styrene oxide (C8H8O, SO). High product yields, high selectivity, and the highest turnover frequency (TOF) of 28 400 h-1 were achieved. Additionally, the catalysts can significantly enhance the application of the present types of 3d/4d-4f catalysts in catalysis for transformations involving the fixation of CO2.

Lanthanide(iii) coordination polymers for luminescence detection of Fe(iii) and picric acid

Two 2D coordination polymers exhibit a sensitive and fast luminescence quenching response to Fe³⁺and picric acid (PA) in water with a high quenching effect constant (K_sv) for PA (>17 000 M⁻¹) and Fe³⁺(>3950 M⁻¹).

A multifunctional Eu-CP as a recyclable luminescent probe for the highly sensitive detection of Fe3+/Fe2+, Cr2O72−, and nitroaromatic explosives

A multifunctional Eu-CP (1) as a recyclable luminescent probe for the highly sensitive detection of Fe³⁺/Fe²⁺, Cr₂O₇²⁻, and nitroaromatic explosives.