Recent Advances in Photocatalytic CO2 Reduction Using Earth‐Abundant Metal Complexes‐Derived Photocatalysts

Photochemical reduction of CO₂ with H₂O into energy‐rich chemicals using inexhaustible solar energy is an appealing strategy to simultaneously address the global energy and environmental issues. Earth‐abundant metal complexes show promising application in this field due to their easy availability, rich redox valence and tunable property. Great progress has been seen on catalytic reduction of CO₂ under visible light illumination employing earth‐abundant metal complexes and their hybrids as key contributors, especially for producing CO and HCOOH via the two‐electron reduction process. In this minireview, we will summarize and update advances on earth‐abundant metal complex‐derived photocatalytic system for visible‐light driven CO₂ photoreduction over the last 5 years. Homogeneous earth‐abundant metal complex photocatalysts and earth‐abundant metal complex derived hybrid photocatalysts were both presented with focus on efficient improvement strategy.     

The Reduction of Rare‐Earth Metal Halides with Unlike Metals – Wöhler's Metallothermic Reduction

Rare‐earth halides may be reduced by rare‐earth metals (conproportionation) and, as an alternative, by unlike metals such as alkali or alkaline‐earth metals, a route first established for the production of rare‐earth metals. It has great power for exploratory research subject to enhanced reactivity at lower temperatures and the formation of alkali halide flux for crystal growth. A large number of new compounds, ternary and higher, salt‐like and (semi‐)metallic including interstitially stabilized cluster compounds has been synthesized and characterized during the last decades.     

When VSEPR Fails: Experimental and Theoretical Investigations of the Behavior of Alkaline‐Earth‐Metal Acetylides

The synthesis, structural, and spectral characterization as well as a theoretical study of a family of alkaline‐earth‐metal acetylides provides insights into synthetic access and the structural and bonding characteristics of this group of highly reactive compounds. Based on our earlier communication that reported unusual geometry for a family of triphenylsilyl‐substituted alkaline‐earth‐metal acetylides, we herein present our studies on an expanded family of target derivatives, providing experimental and theoretical data to offer new insights into the intensively debated theme of structural chemistry in heavy alkaline‐earth‐metal chemistry.     

Molecular Rare‐Earth‐Metal Hydrides in Non‐Cyclopentadienyl Environments

Molecular hydrides of the rare‐earth metals play an important role as homogeneous catalysts and as counterparts of solid‐state interstitial hydrides. Structurally well‐characterized non‐metallocene‐type hydride complexes allow the study of elementary reactions that occur at rare‐earth‐metal centers and of catalytic reactions involving bonds between rare‐earth metals and hydrides. In addition to neutral hydrides, cationic derivatives have now become available.     

Rare‐Earth Oxide Nanostructures: Rules of Rare‐Earth Nitrate Thermolysis in Octadecylamine

The decomposed regularity of rare‐earth nitrates in octadecylamine (ODA) is discussed. The experimental results show that these nitrates can be divided into four types. For rare‐earth nitrates with larger RE³⁺ ions (RE=rare earth, La, Pr, Nd, Sm, Eu, Gd), the decomposed products exhibited platelike nanostructures. For those with smaller RE³⁺ ions (RE=Y, Dy, Ho, Er, Tm, Yb), the decomposed products exhibited beltlike nanostructures. For terbium nitrate with a middle RE³⁺ ion, the decomposed product exhibited a rodlike nanostructure. The corresponding rare‐earth oxides, with the same morphologies as their precursors, could be obtained when these decomposed products were calcined. For cerium nitrate, which showed the greatest differences, flowerlike cerium oxide could be obtained directly from decomposition of the nitrate without further calcination. This regularity is explained on the basis of the lanthanide contraction. Owing to their differences in electron configuration, ionic radius, and crystal structure, such a nitrate family therefore shows different thermolysis properties. In addition, the potential application of these as‐obtained rare‐earth oxides as catalysts and luminescent materials was investigated. The advantages of this method for rare‐earth oxides includes simplicity, high yield, low cost, and ease of scale‐up, which are of great importance for their industrial applications.     

Reversing Conventional Reactivity of Mixed Oxo/Alkyl Rare‐Earth Complexes: Non‐Redox Oxygen Atom Transfer

The preferential substitution of oxo ligands over alkyl ones of rare‐earth complexes is commonly considered as “impossible” due to the high oxophilicity of metal centers. Now, it has been shown that simply assembling mixed methyl/oxo rare‐earth complexes to a rigid trinuclear cluster framework cannot only enhance the activity of the Ln‐oxo bond, but also protect the highly reactive Ln‐alkyl bond, thus providing a previously unrecognized opportunity to selectively manipulate the oxo ligand in the presence of numerous reactive functionalities. Such trimetallic cluster has proved to be a suitable platform for developing the unprecedented non‐redox rare‐earth‐mediated oxygen atom transfer from ketones to CS₂ and PhNCS. Controlled experiments and computational studies shed light on the driving force for these reactions, emphasizing the importance of the sterical accessibility and multimetallic effect of the cluster framework in promoting reversal of reactivity of rare‐earth oxo complexes.     

Electro‐kinetic Separation of Rare Earth Elements Using a Redox‐Active Ligand

Purification of rare earth elements is challenging due to their chemical similarities. All of the deployed separation methods rely on thermodynamic properties, such as distribution equilibria in solvent extraction. Rare‐earth‐metal separations based on kinetic differences have not been examined. Herein, we demonstrate a new approach for rare‐earth‐element separations by exploiting differences in the oxidation rates within a series of rare earth compounds containing the redox‐active ligand [{2‐(t BuN(O))C₆H₄CH₂}₃N]³⁻. Using this method, a single‐step separation factor up to 261 was obtained for the separation of a 50:50 yttrium–lutetium mixture.     

Rare-Earth Metal Chlorides Catalyzed One-pot Syntheses of Quinolines under Solvent-free Microwave Irradiation Conditions

Under microwave irradiation and solvent‐free conditions, rare‐earth metal chlorides (RECl₃) have been efficient catalysts for one‐pot synthesis of quinoline derivatives to give products in good to excellent yields through the multi‐component reactions of aldehydes, amines, and alkynes. The rare‐earth metal chlorides can be recycled for six times without notable loss of catalytic activities. This new synthetic approach has prominent features of a short reaction time, high yields of products, operational simplicity, broad substrate scopes, environmentally friendly property and commercially available catalysts. It extends the applications of rare‐earth metal compounds as catalysts in organic synthesis.     

Dual Functionalization of White Phosphorus: Formation,Characterization, and Reactivity of Rare‐Earth‐Metal Cyclo‐P3 Complexes

The [3+1] fragmentation reaction of rare‐earth metallacyclopentadienes 1 a – c with 0.5 equivalents of P₄ affords a series of rare‐earth metal cyclo‐P₃ complexes 2 a – c and a phospholyl anion 3. 2 a – c demonstrate an unusual η³ coordination mode with one P−P bond featuring partial π‐bonding character. 2 a – c are the first cyclo‐P₃ complexes of rare‐earth metals, and also the first organo‐substituted polyphosphides in the category of Group 3 and f‐block elements. Rare‐earth metallacyclopentadienes play a dual role in the combination of aromatization and Diels–Alder reaction. Compounds 2 a – c can coordinate to one or two [W(CO)₅] units, yielding 4 a – c or 5 c , respectively. Furthermore, oxidation of 2 a with p ‐benzoquinone produces its corresponding phospholyllithium and regenerated P₄.     

Dual Functionalization of White Phosphorus: Formation,Characterization, and Reactivity of Rare‐Earth‐Metal Cyclo‐P3 Complexes

The [3+1] fragmentation reaction of rare‐earth metallacyclopentadienes 1 a – c with 0.5 equivalents of P₄ affords a series of rare‐earth metal cyclo‐P₃ complexes 2 a – c and a phospholyl anion 3. 2 a – c demonstrate an unusual η³ coordination mode with one P−P bond featuring partial π‐bonding character. 2 a – c are the first cyclo‐P₃ complexes of rare‐earth metals, and also the first organo‐substituted polyphosphides in the category of Group 3 and f‐block elements. Rare‐earth metallacyclopentadienes play a dual role in the combination of aromatization and Diels–Alder reaction. Compounds 2 a – c can coordinate to one or two [W(CO)₅] units, yielding 4 a – c or 5 c , respectively. Furthermore, oxidation of 2 a with p ‐benzoquinone produces its corresponding phospholyllithium and regenerated P₄.     

Investigation of the Hydrolysis Stability of Triazine Tricarboxylate in the Presence of Transition Metal(II) Ions and Synthesis and Crystal Structure of the Alkaline Earth Triazine Tricarboxylates M3[C3N3(CO2)3]2·12H2O (M = Sr,Ba)

The stability against hydrolysis of triazine tricarboxylate (TTC) in the presence of divalent transition metal and alkaline earth ions was investigated by means of X‐ray diffraction and FTIR spectroscopy. Depending on the size of the cation either formation of the respective triazine tricarboxylate salts or hydrolysis of TTC yielding oxalate was observed. The hydrolysis of TTC induced by transition metal ions could be explained in analogy to the hydrolysis of triazine tris(2‐pyrimidyl) as a result of ring tension caused by the coordination of these ions. By the reaction of potassium triazine tricarboxylate with alkaline earth salts in aqueous solution the alkaline earth triazine tricarboxylates M₃[C₃N₃(CO₂)₃]₂ · 12H₂O (M = Sr, Ba) were obtained and analyzed by single‐crystal X‐ray diffraction. The isotypic salts represent the first examples of alkaline earth triazine tricarboxylates and the first TTC salts comprising solely divalent cations.     

Synthesis,characterization, thermal,electrical and magnetic properties of polyaniline composites with rare earth gadolinium coordination complex

Novel polyaniline/gadolinium (PANI/Gd) composites were successfully synthesized by “in‐situ” polymerization at the presence of rare earth Gd coordination complex and D‐tartaric acid (an a dopant). It is rarely to find the studies on related field to add rare earth Gd coordination complex as fillers. Fourier transform infrared (FTIR) spectra, X‐ray diffraction (XRD) and scanning electron microscope (SEM) were used to examine the structure and surface appearance characterization of materials. The thermal stability performance of composites was investigated by thermogravimetry and derivative thermogravimetry (TG‐DTG). Electrochemical performance was measured by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge–discharge test. The magnetic property was investigated by physical property measurement system (PPMS). The structure and surface appearance characterization and the magnetic properties jointly demonstrate the polymerization of rare earth Gd coordination complex and PANI–D‐tartrate (DTA) not only simple physical mixing but also chemical mixing. TG‐DTG analysis suggests that thermal stability of PANI/Gd composites is higher than that of PANI–DTA. Electrochemical performance tests and SEM indicate that the composite (PANI/Gd = 3.3:1,mass ratio) has the most regular morphology and best specific capacitance. The magnetization of the composite (PANI/Gd = 3.3:1,mass ratio)is evidently smaller compared with PANI–DTA and rare earth Gd coordination complex. Copyright © 2016 John Wiley & Sons, Ltd.     

Crosslinking of chlorine‐containing polymers by dicyclopentadiene dicarboxylic salts

Alkali‐ and alkali‐earth‐metal salts of dicyclopentadiene dicarboxylic acid (DCPDCA) were prepared and employed as crosslinkers for chlorine‐containing polymers such as polychloromethylstyrene (PCMS), chlorinated polypropylene (CPP), polyepichlorohydrin (PECH), and poly(vinyl chloride) (PVC). Thermally reversible covalent crosslinks (i.e.,  DCPD bridges) between polymer chains were generated through esterification between the chlorine–carbon bonds of the polymer and the carboxylic salt groups of the crosslinker. The crosslinking reactivity decreased in the following sequence: K > Na > LiDCPDCA > alkali‐earth‐metal salts of DCPDCA. In addition, PCMS and CPP had higher gelation rates than PECH and PVC. Good flowability at about 195 °C and solubility in maleimide‐containing dichlorobenzene on heating indicated that the crosslinked PCMS and CPP exhibited thermally reversible crosslinking because of dimer/monomer (cyclopentadiene) conversion of  DCPD moieties via reversible Diels–Alder cycloaddition. Samples of PECH and PVC crosslinked by the alkali salts of DCPDCA were insoluble even when heated in maleimide‐containing dichlorobenzene. However, these crosslinked polymers could be dissolved partially after the same treatment when the crosslinker was an alkali‐earth‐metal salt of DCPDCA. Thermal degradation such as dehydrochlorination of the PECH and PVC might have been responsible for uncontrolled crosslinking because these two polymers are known to be thermally unstable. The unreacted COOK, COONa, or COOLi of the crosslinkers might have initiated base‐induced dehydrochlorination when PECH and PVC were heated at high temperatures. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 818–825, 2000     

The Utilization of Solid State Chemistry Reaction Routes as New Syntheses Strategies for the Coordination Chemistry of Rare Earth Amides

Solvent free high‐temperature reactions in melts are well known procedures in Solid‐State Chemistry. Although the reaction conditions are extreme considering the properties of organic ligands they can also be utilized for Coordination Chemistry and offer a fruitful alternative to usual solvent treatments. This includes the chemistry of organic amides of the rare earth elements. The avoidance of any solvent renders novel homoleptic complexes accessible but also implies difficulties bound to the solid state of the reaction mixtures. The high chemical affinity of the rare earth elements towards halides and especially oxygen limits known homoleptic amides obtained via solvent treatments mostly to multi‐chelating ligands like porphyrines, calix‐pyrroles etc. With no special conditions met like a high steric demand, solvent molecules as co‐coordinating partners enforce the formation of heteroleptic species. This influence can be avoided by the use of completely solvent free reactions, such as melt reactions in which a solid is reacted directly with a melt or with a substance under solvothermal conditions. The high reactivity of the rare earth metals allows the direct oxidation with amines and thus to use high‐temperature reactions for the formation of rare earth amides. This includes homoleptic compounds from simple ligands. Crystallization under reaction conditions is possible; no re‐crystallization step is necessary preventing the risk of a change of the chemical character of the products. Additionally, the solubility of rare earth elements in liquid ammonia under formation of an electride solution enlarges the temperature range of these oxidation reactions down to the melting point of ammonia. It further enhances the reactivity of the metals and less N‐H acidic and thermally less stable amines can be introduced into these syntheses enabling the formation of meta stable products. The crystal structures and hence the properties of the products of both high‐ and low‐temperature oxidation of rare earth metals with amines strongly differ from reactions carried out in classic solvents. Thus reaction routes frequently used in Solid State Chemistry can well be utilized for Coordination Chemistry and offer alternatives to classic solvent based synthesis, particularly if certain properties like homoleptic character or the coordination of elements with a low chemical affinity are aimed for.     

Fabrication of fluorescent holographic micropatterns based on the rare earth complexes using azobenzene‐containing poly(aryl ether)s as macromolecular ligands

In this work, on the basis of photoinduced surface relief gratings (SRGs) with the rare earth complexes using azo‐polymers as macromolecular ligands, a series of novel materials for fabricating rewritable fluorescent two‐dimensional micropatterns, whose color can be easily adjusted by changing the species of the rare earth ions, are demonstrated. The rare earth complexes are prepared using a series of poly(aryl ether)s containing azobenzene chromophores and carboxyl group as macromolecular ligands and 1,10‐phenanthroline as co‐ligands. The fluorescence properties of the rare earth complexes and the influence of the contents of azobenzene chromophores on the fluorescent intensity are investigated by means of fluorescence excitation and emission spectroscopy. By exposing the films of the rare earth complexes to an interference pattern laser beam, SRGs can be formed on the films. Under the excitation, fluorescent patterns of the SRGs can be observed by the measurement of fluorescence microscopy. © 2015 Wiley Periodicals, Inc. J. Polym. Sci. Part A: Polym. Chem. 2015 , 53, 936–943     

Earth‐Abundant Nanomaterials for Oxygen Reduction

Replacing the rare and precious platinum (Pt) electrocatalysts with earth‐abundant materials for promoting the oxygen reduction reaction (ORR) at the cathode of fuel cells is of great interest in developing high‐performance sustainable energy devices. However, the challenging issues associated with non‐Pt materials are still their low intrinsic catalytic activity, limited active sites, and the poor mass transport properties. Recent advances in material sciences and nanotechnology enable rational design of new earth‐abundant materials with optimized composition and fine nanostructure, providing new opportunities for enhancing ORR performance at the molecular level. This Review highlights recent breakthroughs in engineering nanocatalysts based on the earth‐abundant materials for boosting ORR.     

Microemulsion synthesis, characterization of highly visible light responsive rare Earth-doped bi(2) o(3)

In this paper, Bi₂O₃ and rare earth (La, Ce)‐doped Bi₂O₃ visible‐light‐driven photocatalysts were prepared in a Triton X‐100/n‐hexanol/cyclohexane/water reverse microemulsion. The resulting materials were characterized by X‐ray powder diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) surface area, photoluminescence spectra (PLS) and UV–Vis diffuse reflectance spectroscopy. The XRD patterns of the as‐prepared catalysts calcined at 500°C exhibited only the characteristic peaks of monoclinic α‐Bi₂O₃. PLS analysis implied that the separation efficiency for electron‐hole has been enhanced when Bi₂O₃ was doped with rare earth. UV–Vis diffuse reflectance spectroscopy measurements presented an extension of light absorption into the visible region. The photocatalytic activity of the samples was evaluated by degradation of methyl orange (MO) and 2,4‐dichlorophenol (2,4‐DCP). The results displayed that the photocatalytic activity of rare earth‐doped Bi₂O₃ was higher than that of dopant‐free Bi₂O₃. The optimal dopant amount of La or Ce was 1.0 mol%. And the mechanisms of influence on the photocatalytic activity of the catalysts were discussed.     

Structural Transformation Pathways of Alkaline Earth Family Coordination Polymers Containing 3,3′,5,5′‐Biphenyl Tetracarboxylic Acid

The understanding of crystal stepwise transformation is very important to enclose the “black box” in the preparation of crystal materials. In this work, different structural intermediates were isolated prior to the formation of the final alkali earth coordination polymers (CPs) during the preparation of three pairs of alkali earth CPs through solvothermal method and convenient oil‐bath reactions. Single crystal X‐ray diffraction analysis demonstrated the structural transformation from a 0 D to 1 D inorganic connectivity for the Ca‐CPs and Sr‐CPs, but a 1 D to 0 D inorganic connectivity for Ba‐CPs, involving the breakage/formation of chemical bonds in the reaction solutions. Further analyses indicated that these two different structural transformation pathways are determined by the deprotonation of organic acid, competitive balance between the inorganic and organic connectivity, and the twist of the linker. FT‐IR spectra, thermogravimetric and luminescence behaviors agree with their structural characteristics.     

Alkaline Earth Metallocenes Coordinated with Ester Pendants: Synthesis,Structural Characterization,and Application in Metathesis Reactions

A variety of ester‐substituted cyclopentadiene derivatives have been synthesized by one‐pot reactions of 1,4‐dilithio‐1,3‐butadienes, CO, and acid chlorides. Direct deprotonation of the ester‐substituted cyclopentadienes with Ae[N(SiMe₃)₂]₂ (Ae=Ca, Sr, Ba) efficiently generated members of a new class of heavier alkaline earth (Ca, Sr, Ba) metallocenes in good to excellent yields. Single‐crystal X‐ray structural analysis demonstrated that these heavier alkaline earth metallocenes incorporated two intramolecularly coordinated ester pendants and multiply‐substituted cyclopentadienyl ligands. The corresponding transition metal metallocenes, such as ferrocene derivatives and half‐sandwich cyclopentadienyl tricarbonylrhenium complexes, could be generated highly efficiently by metathesis reactions. The multiply‐substituted cyclopentadiene ligands bearing an ester pendant, and the corresponding heavier alkaline earth and transition‐metal metallocenes, may have further applications in coordination chemistry, organometallic chemistry, and organic synthesis.