Hydroamination of alkenes with dinitrogen and titanium polyhydrides.

An ideal synthesis of alkyl amines would involve the direct use of abundant and easily accessible molecules such as dinitrogen (N2) and feedstock alkenes1-4. However, this ambition remains a great challenge as it is usually difficult to simultaneously activate both N2 and a simple alkene and combine them together through carbon-nitrogen (C-N) bond formation. Currently, the synthesis of alkyl amines relies on the use of ammonia produced through the Haber-Bosch process and prefunctionalized electrophilic carbon sources. Here we report the hydroamination of simple alkenes with N2 in a trititanium hydride framework, which activates both alkenes and N2, leading to selective C-N bond formation and providing the corresponding alkyl amines on further hydrogenation and protonation. Computational studies reveal key mechanistic details of N2 activation and selective C-N bond formation. This work demonstrates a strategy for the transformation of N2 and simple hydrocarbons into nitrogen-containing organic compounds mediated by a multinuclear hydride framework.

Mechanistic Insights into Rare-Earth-Catalyzed Alternating Copolymerization through C-H Polyaddition of Functionalized Organic Compounds to Unconjugated Dienes.

Density functional theory (DFT) calculations have been conducted to elucidate the detailed mechanisms of yttrium-catalyzed C-H polyaddition of 1,4-dimethoxybenzene (DMB) to 1,4-divinylbenzene (DVB). It was computationally determined that DMB not only serves as a substrate but also performs a crucial role as a ligand, stabilizing the catalytically active species and promoting alkene insertion. Side pathways involving Cß-H activation and C═C continuous insertion were excluded due to steric and electronic factors, respectively, explaining why the reaction occurred efficiently and selectively to give perfectly alternating DMB-DVB polymers. Interestingly, the theoretical prediction of the reactivity of N,N-dimethyl-1,4-phenylenediamine and 2,2'-biethyl-4,4'-bipyridine reveals significant differences in the coordination effects of these substrates, leading to distinct mechanisms, primarily influenced by their steric effects. These findings shed new light on the previously overlooked role of substrate ligand effects in rare-earth-catalyzed step-growth copolymerization reactions.

Origins of Ligand-Controlled Stereoselective Polymerization of ortho-Methoxystyrene by Rare-Earth Catalysts: A Theoretical Perspective.

The stereoselective polymerization of polar vinyl monomers has recently received much attention due to their excellent physicochemical properties. Over the past decade, breakthroughs have been achieved in this field by rare-earth catalysts. However, the mechanistic origins of those stereoselective polymerizations still remain unclear. Herein, stereoselective polymerization of ortho-methoxystyrene (oMOS) by several representative rare-earth catalysts bearing different ligands (i.e., η5-C5Me5, pyridinyl-methylene-fluorenyl, quinolyl-anilido, ß-diketiminato) were systematically investigated by density functional theory (DFT) calculations. After achieving agreement between the calculations and experiments, we focused on discussing the role of ligands in controlling stereoselectivity. Our results reveal that the stereoregularity of oMOS polymerization is mainly controlled by the steric effect of the catalyst-monomer structures. Specifically, the type of ligand influences the orientation and configuration of the inserting monomer, thereby affecting the tacticity of the polymers. In the cases of η5-C5Me5-, pyridinyl-methylene-fluorenyl, and quinolyl-anilido-ligated yttrium catalysts, we observe consistent insertion directions and alternating insertion sides of oMOS monomers, leading to syndiotactic selectivity. The opposite insertion directions and the alternating insertion sides of oMOS monomers were observed in the case of the ß-diketiminato yttrium catalyst, leading to isotactic selectivity. These findings reported here offer valuable insights into the role of ligands in controlling stereoselectivity in rare-earth catalyzed coordination polymerization of polar vinyl monomers, thus providing guidance for the rational design of new ligands for stereospecific polymerization of polar monomers in the future.

Unveiling the Detailed Mechanism and Origins of Chemo-, Regio-, and Stereoselectivity of Rare-Earth Catalyzed Alternating Copolymerization of Polar and Nonpolar Olefins.

The direct copolymerization of polar and nonpolar olefins is of great interest and significance, as it is the most atom-economical and straightforward strategy for the synthesis of functional polyolefin materials. Despite considerable efforts, the precise control of monomer-sequence and their regio- and stereochemistry is full of challenges, and the related mechanistic origins are still in their infancy to date. Herein, the mechanistic studies on the model reaction of Sc-catalyzed co-syndiospecific alternating copolymerization of anisylpropylene (AP) and styrene were performed by DFT calculations. The results suggest that the subtle balance between electronic and steric factors plays an important role during monomer insertions, and a new amino-dissociated mechanism was proposed for AP insertion at chain initiation. AP insertion follows the 2,1-si-insertion pattern, which is mainly controlled by steric factors caused by the restricted MeO···Sc interaction. As for styrene insertion, it prefers the 2,1-re-insertion manner and its regio- and stereoselectivities are influenced by steric repulsions between the inserting styrene and the polymer chain or the ligand. More interestingly, it is found that the alternating monomer-sequence is mainly determined by the "steric matching" principle, which is quantitatively expressed by the buried volume of the metal center of the preinserted species. The concept of steric pocket has been successfully applied to explain the different performances of several catalysts and other alternating copolymerization reactions. The insightful mechanistic findings and the quantitative steric pocket model present here are expected to promote rational design of new rare-earth catalysts for developing regio-, stereo-, and sequence-controlled copolymerization of specific polar and nonpolar olefins.

Nickel-Catalyzed Enantioconvergent Transformation of Anisole Derivatives via Cleavage of C-OMe Bond.

Herein, a protocol for enantioconvergent transformation of anisole derivatives is disclosed via nickel-catalyzed dynamic kinetic asymmetric cross-coupling of the C(Ar)-OMe bond. Versatile axially chiral heterobiaryls are successfully assembled. Synthetic transformations demonstrate the application potential of this method. Mechanistic studies indicate that the enantioconvergence of this transformation might be accessed through a chiral ligand-controlled epimerization of diastereomeric 5-membered aza-nickelacycle species rather than a conventional dynamic kinetic resolution.

Mechanism and Origin of Site Selectivity and Regioselectivity of Scandium-Catalyzed Benzylic C-H Alkylation of Tertiary Anilines with Alkenes.

Benzylic C(sp3)-H alkylation of tertiary anilines with alkenes by an anilido-oxazoline-ligated scandium alkyl catalyst was recently reported with C-H site selectivity and alkene-dependent regioselectivity. Revealing the mechanism and origin of selectivity is undoubtedly of great importance for understanding experimental observations and developing new reactions. Herein, density functional theory (DFT) calculations have been carried out on the model reaction of Sc-catalyzed benzylic C(sp3)-H alkylation of N,N-dimethyl-o-toluidine with allylbenzene. The reaction generally undergoes the generation of active species, alkene insertion, and protonation steps. The difference of the distortion energy of the aniline moiety in transition states, which is related to the ring size of the forming metallacycles, accounts for the site selectivity of C-H activation. Benzylic C(sp3)-H activation possessing less strained five-membered metallacycle compared to the ortho-C(sp2)-H and α-methyl C(sp3)-H activation results in benzylic C(sp3)-H alkylation observed experimentally. Both steric and electronic factors are responsible for the 1,2-insertion regioselectivity for alkyl-substituted alkenes, while electronic factors control the 2,1-insertion manner for vinylsilanes. The analysis of original alkene substrates further strengthens the understanding of the alkene-dependent regioselectivity. These results help us to obtain the mechanistic understanding and are expected to be conducive to the development of new C-H functionalization reactions.

Substrate Facilitating Roles in Rare-Earth-Catalyzed C-H Alkenylation of Pyridines with Allenes: Mechanism and Origins of Regio- and Stereoselectivity.

Although considerable progress has been achieved in C-H functionalization by cationic rare-earth alkyl complexes, the potential facilitating roles of heteroatom-containing substrates during the catalytic cycle remain highly underestimated. Herein, theoretical studies on the model reaction of C(sp2)-H addition of pyridines to allenes by scandium catalyst were carefully carried out to reveal the detailed mechanism. A coordinating pyridine substrate as a ligand can effectively stabilize some key structures. An obvious facilitating role delivered by the coordinating pyridine was found for allene insertion, while the pyridine-free mechanism prefers to occur for C(sp2)-H activation processes. Importantly, the elusive role of heteroatom-containing substrates was systematically revealed for the C-H activation event by designing a metal/ligand combination of catalysts and substrates. We found that the pyridyl C(sp2)-H activation would be switched to the pyridine-coordinated mechanism in the cases of the designed Y and La catalysts. To date, this is the first time to realize the potential substrate-facilitating role in cationic rare-earth-catalyzed C-H activation processes. Moreover, theoretical predictions show that similar switchable mechanisms also work for other types of C-H bonds and other heteroatom-involved substrates by fine-adjusting the steric surroundings of catalysts. The two C-H activation mechanisms are mainly the result of the delicate balance between electronic and steric factors. In general, the catalytic system with less steric hindrance prefers to undergo the substrate-coordinated mechanism. In contrast, the substrate-free mechanism is favorable due to steric repulsion. These results are helpful for us to better understand the variant mechanisms in rare-earth-catalyzed C-H functionalization at the atomistic level and may help guide the rational design of new catalytic reactions. In addition, the origins of the regio- and stereoselectivity were discussed through geometric parameters and distortion/interaction analysis.

Cantilevered adaptive fiber-optics collimator based on piezoelectric bimorph actuators.

A novel, to the best of our knowledge, cantilever construction design of an adaptive fiber-optics collimator (AFOC) based on piezoelectric bimorph actuators for tip/tilt control is introduced. With this new cantilever structure, an AFOC with a diameter of only 6 mm was developed, and the output laser beam deviation angle and resonance frequency of the device were measured. The experimental results show that this new AFOC can provide more than 1 mrad deflection angle at a 20 V driving voltage, and the first resonance frequency is about 500 Hz. Further, in order to verify whether the cantilever structure can be used in a high-power fiber collimator, a high-power X-Y positioner with an 8 mm diameter fiber end cap was developed. The experimental results show that the high-power X-Y positioner can output more than 2 kW laser power and provide about 330 µm displacement of the fiber end cap in the X direction and about 770 µm in the Y direction at a 150 V driving voltage.

Modular Access to Spiro-dihydroquinolines via Scandium-Catalyzed Dearomative Annulation of Quinolines with Alkynes.

The catalytic enantioselective construction of three-dimensional molecular architectures from planar aromatics such as quinolines is of great interest and importance from the viewpoint of both organic synthesis and drug discovery, but there still exist many challenges. Here, we report the scandium-catalyzed asymmetric dearomative spiro-annulation of quinolines with alkynes. This protocol offers an efficient and selective route for the synthesis of spiro-dihydroquinoline derivatives containing a quaternary carbon stereocenter with an unprotected N-H group from readily accessible quinolines and diverse alkynes, featuring high yields, high enantioselectivity, 100% atom-efficiency, and broad substrate scope. Experimental and density functional theory studies revealed that the reaction proceeded through the C-H activation of the 2-aryl substituent in a quinoline substrate by a scandium alkyl (or amido) species followed by alkyne insertion into the Sc-aryl bond and the subsequent dearomative 1,2-addition of the resulting scandium alkenyl species to the C═N unit in the quinoline moiety. This work opens a new avenue for the dearomatization of quinolines, leading to efficient and selective construction of spiro molecular architectures that were previously difficult to access by other means.

Enantioselective C-H Alkenylation of Ferrocenes with Alkynes by Half-Sandwich Scandium Catalyst.

The enantioselective C-H alkenylation of ferrocenes with alkynes is, in principle, a straightforward and atom-efficient route for the construction of planar-chiral ferrocene scaffolds bearing alkene functionality but has remained scarcely explored to date. Here we report for the first time the highly enantioselective C-H alkenylation of quinoline- and pyridine-substituted ferrocenes with alkynes by a half-sandwich scandium catalyst. This protocol features broad substrate scope, high enantioselectivity, and 100% atom efficiency, selectively affording a new family of planar-chiral ferrocenes bearing N/alkene functionalities. The mechanistic details have been clarified by DFT analyses. The use of a quinoline/alkene-functionalized ferrocene product as a chiral ligand for asymmetric catalysis is also demonstrated.

Theoretical Studies of Rare-Earth-Catalyzed [3 + 2] Annulation of Aromatic Aldimine with Styrene: Mechanism and Origin of Diastereoselectivity.

The synthesis of multisubstituted 1-aminoindanes through catalyst-controlled diastereodivergent [3 + 2] annulation of aromatic imines with alkenes is of great interest and importance. An understanding of the exact reaction mechanism, especially for the origin of diastereoselectivity, is an essential aspect for further development of such reactions. In this study, density functional theory calculations have been carried out on the rare-earth-catalyzed diastereodivergent [3 + 2] annulation of benzaldimine with styrene. The results show that the reaction mainly involves generation of active species, olefin insertion, cyclization, and protonation steps. The noncovalent interactions, such as C-H···π and metal···π interactions, play an important role in stabilizing the key transition state or intermediate. Both steric and electronic factors account for the diastereoselectivity. The preferred cis-diastereoselectivity could be ascribed to more efficient orbital interaction, while the crowded space will induce the formation of a C-H···π interaction between the NtBu group and benzene ring in a trans-diastereoselectivity manner, thus stabilizing the trans-selective transition state. Therefore, the stereospecific product could be obtained by fine-tuning the ligand/metal combination of the catalysts.

Three-Coordinate Pd(0) with Rare-Earth Metalloligands: Synergetic CO Activation and Double P-C Bond Cleavage-Formation Reactions.

Metalation of ß-diketiminato rare-earth metal complexes LnacnacLn(PhNCH2PPh2)2 (Ln = Y, Yb, Lu) with (COD)Pd(CH2SiMe3)2 afforded three-coordinate Pd(0) complexes supported by two sterically less bulky phosphines and a Pd → Ln dative interaction. The Pd(0) center is prone to ligation with isonitrile and CO; in the latter case, the insertion of a second CO with the Y-N bond was assisted via a precoordination of CO on the Pd(0) center, which led to the formation of an anionic Pd(0) carbamoyl. The reaction of the Pd-Y complex with iodobenzene showed a remarkable double P-C bond cleavage-formation pathway within the heterobimetallic Pd-Y core to afford (Ph3P)2PdI(Ph), imine PhNCH2, and a ß-diketiminato yttrium diiodide. In the related reaction of LnacnacY(PhNCH2PPh2)2 with (Ph3P)2PdI(Ph), the P-C bond cleavage following with a N-C bond formation was observed. Computational studies revealed a synergetic bimetallic mechanism for these reactions.

An oncogenic lncRNA, GLCC1, promotes tumorigenesis in gastric carcinoma by enhancing the c-Myc/IGF2BP1 interaction.

Accumulating evidence has shown that long non-coding RNAs (lncRNAs) are vital regulators of the expression of various genes in multiple human diseases. The aim of this study was to investigate the role of glycolysis-associated lncRNA of colorectal cancer (GLCC1) in the progression of gastric carcinoma as well as the underlying mechanism. The expression levels of GLCC1 and c-Myc were determined in 47 pairs of gastric carcinoma tissues and cell lines using quantitative real-time polymerase chain reaction (qRT-PCR). Next, the functional roles of GLCC1 and c-Myc in the proliferation, apoptosis, migration, and invasion of gastric carcinoma cells (BGC823 and SGC7901 cells) were determined by siRNA-mediated knockdown of these molecules, and the cells were evaluated by Cell Counting Kit-8 (CCK-8), flow cytometry, and Transwell assays. In addition, RIP and RNA pull-down assays were used to examine the interaction between GLCC1 and c-Myc/IGF2BP1. Further mechanistic studies were conducted using western blotting. lncRNA GLCC1 and c-Myc were observed to be significantly increased in both gastric carcinoma tissues and cell lines. Knockdown of GLCC1 or c-Myc suppressed cell proliferation, migration, and invasion but promoted apoptosis in both the BGC823 and SGC7901 cell lines. Mechanistically, c-Myc was identified as a downstream regulator involved in the GLCC1-mediated biological effects in gastric carcinoma. The RNA pull-down and RIP assays further showed that the upregulation of lncRNA GLCC1 enhanced the interaction of the IGF2BP1 protein with c-Myc mRNA, thus promoting the stabilization of c-Myc mRNA. Altogether, we demonstrated that lncRNA GLCC1 modulates gastric cancer cell migration and invasion by enhancing the c-Myc/IGF2BP1 interaction, and lncRNA GLCC1 may serve as a potential therapeutic target for preventing the development and progression of human gastric carcinoma.

Amino-Supported Palladium Catalyst for Chemo- and Stereoselective Domino Reactions.

A solid amino-supported palladium catalyst is used in an oxidative domino reaction for the diastereoselective construction of alkyne-substituted cyclopentenol compounds. This heterogeneous catalyst exhibits high efficiency and excellent chemoselectivity, as well as good recyclability. The chemoselectivity of the domino reactions was readily controlled by switching the solvent and catalyst. Asymmetric syntheses and an oxidative carbocyclization-borylation reaction have also been developed based on the heterogeneous palladium catalyst.

Dinitrogen Activation and Hydrogenation by C5Me4SiMe3-Ligated Di- and Trinuclear Chromium Hydride Complexes.

Activation of dinitrogen (N2) by well-defined metal hydrides is of much interest and importance, but studies in this area have remained limited to date. We report here N2 activation and hydrogenation by C5Me4SiMe3-ligated di- and trinuclear chromium polyhydride complexes. Hydrogenolysis of [Cp'Cr(µ-Me)2CrCp'] (Cp' = C5Me4SiMe3) (1) with H2 in a dilute hexane solution under N2-free conditions affords the dichromium dihydride complex [Cp'Cr(µ-H)2CrCp'] (2), while hydrogenolysis of 1 in a concentrated solution or without solvent provides the trinuclear chromium tetrahydride complex [(Cp'Cr)3(µ3-H)(µ-H)3] (3). When the reaction is carried out in the presence of N2 in a dilute hexane solution, the tetranuclear diimide/dihydride complex [(Cp'Cr)4(µ3-NH)2(µ3-H)2] (4) is formed via N-N bond cleavage and N-H bond formation. The reaction of 2 with N2 at room temperature gives the tetranuclear imide/nitride/dihydride complex [(Cp'Cr)3(C5Me3(CH2)SiMe3)Cr(µ3-NH)(µ3-N)(µ-H)2] (5) via N2 cleavage and hydrogenation and C-H bond activation of a Cp methyl group. At -30 °C, the reaction of 2 with N2 affords the dinitride intermediate [(Cp'Cr)4(µ3-N)2(µ3-H)2] (6), which is quantitatively transformed to 5 at room temperature. Complex 5 reversibly converts to the stereoisomer 5'. The hydrogenation of a mixture of 5 and 5' with H2 affords 4. The reaction of 3 with N2 proceeds at 100 °C to afford [(Cp'Cr)3(µ3-NH)2] (7). This transformation has also been investigated by DFT calculations. Both experimental and computational studies suggest that N2 incorporation into the chromium hydride cluster is involved in the rate-determining step. This work represents the first example of N2 cleavage and hydrogenation by well-defined chromium hydride complexes.

CO2 Activation by Lewis Pairs Generated Under Copper Catalysis Enables Difunctionalization of Imines.

Integration of distinct substrate activation modes in a catalytic circle is critical for the development of new, powerful synthetic methodologies toward complex and value-added chemicals from simple and readily available feedstocks. Here, we describe a highly selective difunctionalization of imines through incorporation of activation of CO2 by intramolecular N/B Lewis pairs into a copper catalytic cycle. Experimental and computational studies on the mechanistic aspect revealed an α-borylalkylamido intermediate, a metal amide-based Lewis pair formed by borylation of a C-N double bond, and enabled an unprecedented CO2 fixation pattern that is in sharp contrast to the traditional CO2 insertion into transition-metal-element bonds. The unique lithium cyclic boracarbamate products could be easily transformed into multifunctional N-carboxylated α-amino boronates. The highly diastereoselective reactions of chiral N-tert-butanesulfinyl aldimines were also achieved. We hope that our findings may inspire further development of selective multicomponent reactions by incorporation of Lewis pair chemistry into transition-metal catalysis.

Regiodivergent C-H Alkylation of Quinolines with Alkenes by Half-Sandwich Rare-Earth Catalysts.

The regiodivergent catalysis of C-H alkylation with alkenes is of great interest and importance but has remained hardly explored to date. We report herein the first regiodivergent C-H alkylation of quinolines with alkenes by half-sandwich rare-earth catalysts. The regiodivergence was achieved by fine-tuning the metal/ligand combination or steric and electronic properties of the catalysts. The use of the C5Me5-ligated scandium catalyst Sc-3 for the reaction of quinolines with styrenes and that of the C5Me4H-ligated yttrium catalyst Y-2 for the reaction with aliphatic olefins exclusively afforded the corresponding C8-H alkylation products, thus constituting the first example of direct C8-H alkylation of neutral quinolines. In contrast, the Sc-3-catalyzed reaction of 2-arylquinolines with aliphatic olefins and the Y-2-catalyzed reaction with styrenes selectively gave the 2-aryl o-C-H alkylation products. On the basis of the catalyst/substrate-controlled regiodivergence, the sequential regiospecific dialkylation of quinolines with two different alkenes has also been achieved. DFT studies revealed that the C-H activation of 2-phenylquinoline at both the C8 position and an ortho position of the 2-phenyl substituent was possible, and these two types of initially formed C-H activation products were interconvertible through the coordination and C-H activation of another molecule of quinoline. The regioselectivity for the C-H alkylation reactions was governed not only by the ease of the initial formation of the C-H activation products but also by the energy barriers for their interconversions, as well as by the energy barriers or steric and electronic influences in the subsequent alkene insertion processes. This work has not only constituted an efficient protocol for the selective synthesis of diversified quinoline derivatives but also offered unprecedented insights into the C-H activation and transformation of quinolines and may help in the design of more efficient, selective, or complementary catalysts.

Fragmentation Mechanism of White Phosphorus: A Theoretical Insight into Multiple Cleavage/Formation of P-P and P-C Bonds.

Molecular-level understanding of metal-mediated white phosphorus (P4 ) activation is meaningful but challenging because of its direct relevance to the conversion of P4 into useful organophosphorus compounds as well as the complicated and unforeseeable cleavage process of P-P bonds. The related study, however, has still rarely been achieved to date. Here, a theoretical insight into the step-by-step process of three P-P bond cleavage/four P-C bond formation for [P3 +P1 ]-fragmentation of P4 mediated by lutetacyclopentadienes is reported. The unique charge-separated intermediate and the intermolecular cooperation between two lutetacyclopentadienes play a vital role in the subsequent P-P/P-C bond breaking/forming. It is found that, although the first P-C formation is involved in the assembly of the cyclo-P3 [R4 C4 P3 ]- unit, the construction of the aromatic five-membered P1 heterocycle [R4 C4 P]- is completed prior to the cyclo-P3 formation. The reaction mechanism has been carefully elucidated by analyses of the geometric structure, frontier molecular orbitals, bond index, and natural charge, which greatly broaden and enrich the general knowledge of the direct functionalization of P4 .

Chiral and Regenerable NAD(P)H Models Enabled Biomimetic Asymmetric Reduction: Design, Synthesis, Scope, and Mechanistic Studies.

The coenzyme NAD(P)H plays an important role in electron as well as proton transmission in the cell. Thus, a variety of NAD(P)H models have been involved in biomimetic reduction, such as stoichiometric Hantzsch esters and achiral regenerable dihydrophenantheridine. However, the development of a general and new-generation biomimetic asymmetric reduction is still a long-term challenge. Herein, a series of chiral and regenerable NAD(P)H models with central, axial, and planar chiralities have been designed and applied in biomimetic asymmetric reduction using hydrogen gas as a terminal reductant. Combining chiral NAD(P)H models with achiral transfer catalysts such as Brønsted acids and Lewis acids, the substrate scope could be also expanded to imines, heteroaromatics, and electron-deficient tetrasubstituted alkenes with up to 99% yield and 99% enantiomeric excess (ee). The mechanism of chiral regenerable NAD(P)H models was investigated as well. Isotope-labeling reactions indicated that chiral NAD(P)H models were regenerated by the ruthenium complex under hydrogen gas first, and then the hydride of NAD(P)H models was transferred to unsaturated bonds in the presence of transfer catalysts. In addition, density functional theory calculations were also carried out to give further insight into the transition states for the corresponding transfer catalysts.

Theoretical Mechanistic Insights into Dinitrogen Activation by a Diniobium Tetrahydride: Two-State Reactivity and the Role of Potassium Cation Promoter.

A "two-state reactivity" scenario has been computationally disclosed for dinitrogen activation by a diniobium tetrahydride. Remarkably, it is revealed that alkali metal cations play a crucial role via alkali···N coordination interaction, which is capable of decreasing the activation barriers of N-H formation and subsequent H2 elimination in particular. Furthermore, the effect of various alkali metal cations has also been investigated systematically.