CO2 adducts of N-heterocyclic carbenes: thermal stability and catalytic activity toward the coupling of CO2 with epoxides

Thermal stability of CO2 adducts of N-heterocyclic carbenes (NHCs) was studied by means of in situ FTIR method with monitoring of the nu(CO2) region of the infrared spectra under various conditions. 1,3-Bis(2,6-diisopropylphenyl)imidazolinium-2-carboxylate (SIPr-CO2) shows higher thermal stability compared with 1,3-bis(2,6-diisopropylphenyl)imidazolium-2-carboxylate (IPr-CO2). The presence of free CO2 can significantly inhibit the decomposition of NHC-CO2 adducts, while the addition of an epoxide such as propylene oxide has a negative effect on stabilizing these adducts. As zwitterionic compounds, NHC-CO2 adducts were also proved to be effective organic catalysts for the coupling reaction of CO2 and epoxides to afford cyclic carbonates, for which a possible mechanism was proposed. Among these NHC-CO2 adducts, the relatively unstable IPr-CO2 exhibits the highest catalytic activity. The presence of an electrophile such as SalenAlEt could greatly improve the catalytic activity of IPr-CO2 due to intermolecular cooperative catalysis of the binary components.     

On the Oxidation State of Cu2O upon Electrochemical CO2 Reduction: An XPS Study

The encouraging selectivity of copper oxides for the electroreduction of CO₂ into ethylene and alcohols has led to a vivid debate on the possible relation between their operando (sub-)surface oxidation state (i. e. fully reduced or partially oxidized) and this distinct reactivity. The high roughness of the Cu oxides used in previous studies on this matter adds complexity to this controversy and motivated us to prepare quasi-planar Cu₂O thin films that displayed a CO₂ reduction selectivity similar to that of oxide-derived copper catalysts reported in previous studies. Most importantly, when the post-mortem thin films were transferred for characterization in an air-free environment, X-ray photoelectron spectroscopy measurements confirmed their complete reduction in the course of the CO₂ reduction reaction. Thus, our results indicate that the selectivity of the Cu oxides featured in previous studies stems from their enhanced roughness, highlighting the importance of controlled sample transfer upon post-mortem characterization with ex situ techniques.     

Interaction of the Fungal Metabolite Harzianic Acid with Rare-Earth Cations (Pr3+, Eu3+, Ho3+, Tm3+)

Rare-earth elements (REEs) are in all respect a class of new contaminants that may have toxic effects on organisms and microorganisms and information on their interactions with natural ligands should be of value to predict and control their diffusion in natural environments. In the current study, we investigate interactions of tripositive cations of praseodymium, europium, holmium, and thulium with harzianic acid (H₂L), a secondary metabolite produced by selected strains of fungi belonging to the Trichoderma genus. We applied the same techniques and workflow previously employed in an analogous study concerning lanthanum, neodymium, samarium, and gadolinium tripositive cations. Therefore, in the current study, HPLC-ESI-HRMS experiments, circular dichroism (CD), and UV-Vis spectrophotometric absorption data, as well as accurate pH measurements, were applied to characterize bonding interactions between harzianic acid and Pr³⁺, Eu³⁺, Ho³⁺, and Tm³⁺ cations. Problems connected to the low solubility of harzianic acid in water were overcome by employing a 0.1 M NaClO₄/(CH₃OH + H₂O 50/50 w/w) mixed solvent. For Pr³⁺, Ho³⁺, and Tm³⁺, only the mono complexes PrL⁺, HoL⁺, and TmL⁺ were detected and their formation constant determined. Eu³⁺ forms almost exclusively the bis complex EuL2− for which the corresponding formation constant is reported; under our experimental conditions, the mono complex EuL⁺ is irrelevant. Combining the results of the present and previous studies, a picture of interactions of harzianic acid with rare-earth cations extending over 8 of the 17 REEs can be composed. In order to complement chemical information with toxicological information, a battery of bioassays was applied to evaluate the effects of praseodymium, europium, holmium, and thulium tripositive cations on a suite of bioindicators including Aliivibrio fischeri (Gram-negative bacterium), Raphidocelis subcapitata (green alga), and Daphnia magna (microcrustacean), and median effective concentration (EC50) values of Pr³⁺, Eu³⁺, Ho³⁺, and Tm³⁺ for the tested species were assessed.     

The catalytic action of trimethylgallium on the reaction of alkynyllithium with epoxides

Trimethylgallium catalyzes the reaction of 1-lithio-1-alkynes with epoxides affording 3-alkyn-1-ols in good to excellent yields.     

A catalytic approach to the base-promoted reaction of epoxides with activated methylenes

This contribution reports the results obtained in the definition of a catalytic method for the nucleophilic ring opening of epoxides by activated methylenes promoted by a polymer-supported base. The attention has been focused on the use of polymer supported bases and the best results have been obtained by using 4-(dimethylamino)pyridine (PS-DMAP) and 2-tert-butylimino-2-diethylamino-1,3-dimethyl-perhydro-1,3,2-diazaphosphorine both on polystyrene (PS-BEMP). Solvent-free conditions has been essential for reaching a sufficient reactivity to realize this process, in fact when a reaction medium is used, the processes are almost unfeasible.     

8-Hydroxyquinolates of trivalent metals as new catalysts for the reaction of CO2 with epoxides

Tris-(8-hydroxyquinolates) of aluminum(iii), chromium(iii), and iron(iii) in the presence of triphenylphosphine oxide efficiently catalyze the formation of alkylenecarbonates by the reaction of CO₂ with oxiranes, viz., propylene oxide, but-2-ene oxides, and isobutylene oxide. The catalytic system is active at 140—170 °C and a CO₂ pressure of 15—100 atm.     

Postmetallocene lanthanide-hydrido chemistry: A new family of complexes [{Ln{(Me3Si)2NC(NiPr)2}2(mu-H)}2] (Ln = Y, Nd, Sm, Gd, Yb) supported by guanidinate ligands-synthesis, structure, and catalytic activity in olefin polymerization

The new family of Lewis base free hydrido complexes of rare-earth metals supported by guanidinate ligands [{Ln{(Me3Si)2NC(NiPr)2}2(mu-H)}2] (Ln = Y, Nd, Sm, Gd, Yb) was synthesized and structurally characterized. Single-crystal X-ray and solution NMR studies revealed that these complexes are dimeric in both solid state and in [D6]benzene. The dimeric hydrido complexes can adopt eclipsed (Nd, Sm, Gd) or staggered (Y, Yb, Lu) conformations depending on the metal-atom size. Catalytic activity of these [{Ln{(Me3Si)2NC(NiPr)2}2(mu-H)}2] complexes in the polymerization of ethylene, propylene, and styrene has been investigated. Complexes of Sm and Y have high catalytic activity in ethylene polymerization (1268 and 442 g mmol(-1) atm(-1) h(-1), respectively).     

Influence of rare-earth metal doping on the catalytic performance of CuO-CeO2 for the preferential oxidation of CO in excess hydrogen

Doping of different rare-earth metals （Pr, Nd, Y and La） had an evident influence on the catalytic performance of CuO-CeO2 for the preferential oxidation （PROX） of CO in excess hydrogen. As for Pr, the doping enhanced the catalytic activity of CuO-CeO2 for PROX. For example, the CO conversion over the above catalyst for PROX was higher than 99% at 120 °C. Especially, the doping of Pr widened the temperature window by 20 °C over CuO-CeO2 with 99% CO conversion. For Nd, Y, and La, the doping depressed the catalytic activity of CuO-CeO2 for PROX. However, the doping of transition metals markedly improved the selectivity of CuO-CeO2 for PROX.     

New mechanistic insight into the coupling reactions of CO2 and epoxides in the presence of zinc complexes

Coupling reactions of CO(2) and epoxide to produce cyclic carbonates were performed in the presence of a catalyst [L(2)ZnX(2)] (L=pyridine or substituted pyridine; X=Cl, Br, I), and the effects of pyridine and halide ligands on the catalytic activity were investigated. The catalysts with electron-donating substituents on pyridine ligands exhibit higher activity than those with unsubstituted pyridine ligands. On the other hand, the catalysts with electron-withdrawing substituents at the 2-position of the pyridine ligands show no activity; this demonstrates the importance of the basicity of the pyridine ligands. The catalytic activity of [L(2)ZnX(2)] was found to decrease with increasing electronegativity of the halide ligands. A series of highly active zinc complexes bridged by pyridinium alkoxy ions of the general formula [((mu-OCHRCH(2)L)ZnBr(2))(n)] (n=2 for R=CH(3); n=3 for R=H; L=pyridine or substituted pyridine) were synthesized and characterized by X-ray crystallography. The dinuclear zinc complexes obtained from propylene oxide adopt a square-planar geometry for the Zn(2)O(2) core with two bridging pyridinium propoxy ion ligands. Trinuclear zinc complexes prepared from ethylene oxide adopt a boat geometry for the Zn(3)O(3) core, in which three zinc and three oxygen atoms are arranged in an alternate fashion. These zinc complexes bridged by pyridinium alkoxy ions were also isolated from the coupling reactions of CO(2) and epoxides performed in the presence of [L(2)ZnBr(2)]. Rapid CO(2) insertion into the zincbond;oxygen bond of the zinc complexes bridged by pyridinium alkoxy ions leads to the formation of zinc carbonate species; these which yield cyclic carbonates and zinc complexes bridged by pyridinium alkoxy ions upon interaction with epoxides. The mechanistic pathways for the formation of active species and cyclic carbonates are discussed on the basis of results from structural and spectroscopic analyses.     

2-Amino homopropargyl alcohols from the highly regioselective Ag2CO3-catalyzed nucleophilic openings of alkynyl epoxides by amines

The nucleophilic ring opening of propargyl epoxides by amines based on a silver catalyst is presented. The reaction takes place under mild conditions and features a high regioselectivity to provide an effective method for the synthesis of 2-amino homopropargyl alcohols in moderate to high yields.     

均相催化体系对CO2与环氧化物耦合反应的研究进展

二氧化碳(CO₂)是造成温室效应的主要因素,同时也是集储量丰富、无毒和可再生等优点的C1资源。其与环氧化物的耦合反应具有高的原子经济性,产物环状碳酸酯作为高极性、低挥发性和良好生物降解性溶剂,可广泛应用于萃取剂、医药中间体和聚合物单体等研究方向。本文首先对CO₂与环氧化物耦合反应发生的机理进行了分析,然后鉴于均相催化体系易制备、高活性等优点,重点从设计思路、催化参数、催化效率等方面,系统综述了催化该耦合反应的不同均相催化体系。最后,总结了为实现均相催化体系可长久发展所必须逾越的阻碍和后续研究方向,希望能够为探索新型、高催化性能的催化体系提供一定的参考价值。     

Tackling the activity and selectivity challenges of electrocatalysts toward the CO2RR via biatom catalysts on the 2D extended phthalocyanines

The development of efficient CO₂ electrocatalytic reduction catalysts has become increasingly important. However, addressing issues related to activity and selectivity remains challenging due to the lack of robust design criteria. Organometallic sheets, characterized by their high surface area and well-dispersed metal sites, offer a unique platform for catalysis. In this study, we employed large-scale density functional theory calculations to investigate the transition metal dimers doped two-dimensional extended phthalocyanines (TM₂-Pcs) as the biatom catalysts for the electrocatalysis of the CO₂ reduction reaction (CO₂RR). After systematical studies, the four catalysts of Mo₂-Pc, W₂-Pc, Ti₂-Pc, and Re₂-Pc were determined from 26 different TM₂-Pcs. Among them, Mo₂-Pc, Ti₂-Pc, and W₂-Pc have not only high faradaic efficiency (FE) but also small limiting potentials of reducing CO₂ to CH₄. Besides, the limiting potentials for the reduction of CO₂ to CH₂CH₂ on Mo₂-Pc and Re₂-Pc fall within the range of −1.4 to −.8 V versus reversible hydrogen electrode. They demonstrate higher FE than the experimental results obtained on Cu(111). This work not only expands the possibilities for discovering more effective CO₂ reduction catalysts but also provides a feasible strategy for the rational design of electrocatalysts CO₂RR.     

Stereoregular polycarbonate synthesis: Alternating copolymerization of CO2 with aliphatic terminal epoxides catalyzed by multichiral cobalt(III) complexes

Completely stereoregular polycarbonate synthesis was achieved with the use of unsymmetric multichiral cobalt‐based complexes bearing a derived chiral BINOL and an appended 1,5,7‐triabicyclo[4.4.0] dec‐5‐ene as catalyst for the copolymerization of CO₂ and aliphatic terminal epoxides at mild conditions. The (S,S,S)‐Co(III) complex 1c with sterically hindered substituent group is more stereoregular catalyst for the copolymerization of CO₂ and racemic propylene oxide to afford a perfectly regioregular poly(propylene carbonate) (PPC), with >99% head‐to‐tail linkages, >99% carbonate linkages, and a K_rel of 24.4 for the enchainment of (R)‐epoxide over (S)‐epoxide. The isotactic PPC exhibits an enhanced glass transition temperature of 47 °C, which is 10–12 °C higher than that of the corresponding irregular polycarbonate. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011.     

Co(II) and Fe(II) phthalocyanines: synthesis,investigation of their catalytic activity towards phenolic compounds and electrochemical behaviour

4‐[(1‐Benzylpiperidin‐4‐yl)oxy]‐substituted cobalt(II) and iron(II) phthalocyanine complexes were synthesized and their catalytic activity towards various phenolic compounds was investigated. Converting from environmentally harmful phenolic compounds into less harmful oxidation products using phthalocyanines makes this study attractive. This catalysis is feasible and time‐saving in terms of procedure and the best oxidation conditions determined. Electrochemical studies were also carried out using cyclic voltammetry and square wave voltammetry techniques. Voltammetric analyses of the synthesized phthalocyanine complexes supported their proposed structures. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis of Ag-SiO2 composite nanospheres and their catalytic activity

A simple,mild,and time-saving method is employed to synthesize Ag-SiO2 composite nanospheres with Ag nanoparticles uniformly distributed on the surface of SiO2 nanoparticles.The chemical elements and the morphology of Ag-SiO2 composite nanospheres were analyzed with transmission electron microscopy(TEM),X-ray power diffraction(XRD),and X-ray photoelectron spectroscopy(XPS).On the surface of Ag-SiO2 composite nanospheres,silane coupling agent(KH-550)is introduced as an intermediary to connect the surfaces of SiO2 nanospheres and Ag nanoparticles,which is also helpful for avoiding the aggregation of Ag nanoparticles.It is found that Ag-SiO2 composite nanospheres have very good catalytic properties for the reduction of organic dyes,which may have potential application in wastewater treatment.