Alkylammonium Cations Research Articles

Understanding the Role of Protic Ionic Liquids (PILs) in Reactive Systems: Rational Selection of PILs for the Design of Green Synthesis Strategies for Allylic Amines and β-Amino Esters.

The reactive behaviour of protic ionic liquids (PILs) has been shown to be governed not only by their chemical structures but also by their global compositions, which include the presence of free acids and bases at equilibrium with ionic pairs. Six PILs composed of primary, secondary, or tertiary alkyl ammonium cations with two couterions, nitrate or acetate, were tested in model reactions with unsaturated substrates. The free species that were naturally present in these liquids were identified by cyclic voltammetry. Only tributylammonium nitrate was found to be mostly composed just of the ionic pair; the other five PILs also contain variable amounts of free acid and amine. In reactive systems, these free species determine the products of the reaction. In particular, allylic amines and β-amino esters were obtained in good yields (91 and 75 %, respectively) by reaction of conjugated dienes and acrylates in the presence of PILs. By taking into account the actual composition of each PIL, it was possible to direct the reaction path towards a specific product with good yields, to ensure acid catalysis, to avoid polymerization of the substrate, and to promote phase transfer of products. These results establish some useful guidelines for the rational design of new PIL-based one-step synthetic strategies.

Dodecan-1-aminium sulfate trihydrate

The asymmetric unit of the title salt, 2C12H28N+·SO4 2−·3H2O, contains two n-dodecylammonium cations, one sulfate anion and three water molecules. In the crystal, N—H...O hydrogen bonds link the cations and anions into layers parallel to (100). These layers are further connected through O—H...O hydrogen-bonding interactions involving the sulfate ions and the isolated water molecules. The three-dimensional structure can also be considered as the superposition of thin inorganic layers of SO4 2− anions and thick layers of alkylammonium cations perpendicular to the c axis.

Activation of Oxygen Reduction Reaction on Well-Defined Pt Electrocatalysts in Alkaline Media Containing Hydrophobic Organic Cations

The activity of the oxygen reduction reaction (ORR) is significantly affected by the nonspecifically adsorbed ions at the electrode/electrolyte interface as well as the surface structures of the catalytic substrate. In this study, we found that an organic cation activates the ORR on Pt single-crystal electrodes in alkaline media, and we evaluated ORR activity using shape-controlled Pt nanoparticles. The ORR activity on Pt(111) was significantly enhanced by a tetramethylammonium cation (TMA+), which correlates with its hydrophobicity as is the case for alkali metal cations. However, an alkylammonium cation with a long chain deactivated electrochemical reactions because these cations assemble at high concentrations on the surface through their hydrophobic interionic interactions with alkyl chains. The ORR activity on Pt nanoparticles was also enhanced by TMA+; cuboctahedral Pt nanoparticles with a (111) plane exhibit a markedly high rate of increase in ORR activity.

Probing the Impact of Interfacial Electric Fields and Hydrogen-Bonding Interactions on the Electroreduction of CO to Ethylene

The electrochemical reduction of carbon dioxide to hydrocarbons on Cu electrodes could potentially provide sustainable pathways to renewable fuels and valuable commodity chemicals, such as ethylene. However, the poor product selectivity of this process hinders the adoption of this promising technology. It is well-known that the electrolyte can profoundly impact the selectivity of electrocatalytic carbon dioxide reduction. However, the molecular-level origins of this dependence are still poorly understood. A better understanding of how surface intermediates interact with their liquid reaction environment is essential for improving the product selectivity of this process. Herein, we employed differential electrochemical mass spectrometry (DEMS) and surface-enhanced infrared absorption spectroscopy (SEIRAS) to dissect how the interfacial properties of the Cu/electrolyte contact impact the catalytic selectivity of the reduction of surface-adsorbed CO, a key intermediate in the reduction of carbon dioxide to ethylene. Specifically, we employed a series of four quaternary alkyl ammonium cations (methyl4N+, ethyl4N+, propyl4N+, and butyl4N+) to systematically tune the properties of the electrochemical double layer. Using DEMS, we found that ethylene is only produced in the presence of methyl4N+ and ethyl4N+. However, ethylene is not formed in the presence of propyl4N+ and butyl4N+. Using SEIRAS, we characterized the Cu/electrolyte interface in the presence of the different cations. We found that, irrespective of the cation of the electrolyte, approximately the same CO surface-coverage is reached, suggesting that the cations do not block CO-adsorption sites. Further, we extracted the electrochemical Stark tuning rates from the dependence of the CO stretch frequency on applied potential. Based on these rates, we have derived the interfacial electric field strengths in the presence of the four different cations. The electric fields are on the order of 0.1 V Å-1. A simple electrostatic model reveals that these electric fields make a negligible electrostatic contribution to the CO adsorption energy. Analysis of the O-H stretch spectrum of interfacial water reveals that the hydrogen bond between surface-adsorbed CO and interfacial water is disrupted in the presence of propyl4N+ and butyl4N+. Our observation suggests that this hydrogen bond is essential for the formation of ethylene during the reduction of CO on Cu. These insights are expected to guide the design of electrocatalytic interfaces with high selectivity for ethylene.

Structural characterization of organo-montmorillonites prepared from a series of primary alkylamines salts: Mid-IR and near-IR study

Primary alkylammonium cations with progressively increasing chain length from methylammonium (CH3-NH3+) to nonadecylammonium (CH3-(CH2)18-NH3+) were used to prepare unique series of organo-montmorillonites (O-Mts). Infrared spectroscopy in the mid-IR and near-IR regions and XRD were used to characterize samples and to investigate the effect of the chain length on the arrangement of cations confined in montmorillonite (Mt) interlayers. For the first time detailed assignment of the vibration bands in the near-IR region (8000–4000 cm−1) was provided not only for O-Mts but also for selected primary alkylamines and alkylammonium salts, used for their preparation. The IR spectra showed that both, type of head-group, i.e. NH2 vs NH3+, and nature of the anionic counterpart, i.e. chloride vs Mt layer, significantly affected the positions the NH vibration modes as a result of different H-bonds in O-Mts. The alkyl chain length had significant effect on the arrangement of the intercalated cations. With increasing chain length the stretching (νCH2) and first overtone (2νCH2) bands were shifted to lower wavenumbers indicating decreasing amount of disordered gauche in favor of ordered all-trans conformers. Significantly higher shift of the 2νasCH2 band than νasCH2 benefits the NIR region for conformational studies more than MIR.

Hydrogen bonding steers the product selectivity of electrocatalytic CO reduction

The product selectivity of many heterogeneous electrocatalytic processes is profoundly affected by the liquid side of the electrocatalytic interface. The electrocatalytic reduction of CO to hydrocarbons on Cu electrodes is a prototypical example of such a process. However, probing the interactions of surface-bound intermediates with their liquid reaction environment poses a formidable experimental challenge. As a result, the molecular origins of the dependence of the product selectivity on the characteristics of the electrolyte are still poorly understood. Herein, we examined the chemical and electrostatic interactions of surface-adsorbed CO with its liquid reaction environment. Using a series of quaternary alkyl ammonium cations ([Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]), we systematically tuned the properties of this environment. With differential electrochemical mass spectrometry (DEMS), we show that ethylene is produced in the presence of [Formula: see text] and [Formula: see text] cations, whereas this product is not synthesized in [Formula: see text]- and [Formula: see text]-containing electrolytes. Surface-enhanced infrared absorption spectroscopy (SEIRAS) reveals that the cations do not block CO adsorption sites and that the cation-dependent interfacial electric field is too small to account for the observed changes in selectivity. However, SEIRAS shows that an intermolecular interaction between surface-adsorbed CO and interfacial water is disrupted in the presence of the two larger cations. This observation suggests that this interaction promotes the hydrogenation of surface-bound CO to ethylene. Our study provides a critical molecular-level insight into how interactions of surface species with the liquid reaction environment control the selectivity of this complex electrocatalytic process.

Host-Guest Complexations of Amine Boranes and Isoelectronic/Isostructural Quaternary Alkylammonium Cations by Cucurbit[7]uril in Aqueous Solution

The host-guest complexation of six amine boranes (R3NBH3) by the macrocyclic host molecule cucurbit[7]uril (CB[7]) in aqueous solution has been investigated using 1H and 11B NMR spectroscopy. The limiting complexation-induced 1H and 11B chemical shift changes indicate that the amine boranes are included in the hydrophobic cavity of the host molecule. The host-guest stability constants for neutral R3NBH3∙CB[7] complexes (in the range of 105-107M-1) have been determined by 1H NMR competition experiments and are compared with the corresponding values for the isoelectronic/isostructural R3NCH3∙CB[7] + complexes. Ammonia borane (H3NBH3) does not form a host-guest complex with CB[7]. The trends in the host-guest stability constant with the guest molar volume are examined, and the stability is ascribed to the hydrophobic effect (packing coefficient) and quadrupole-dipole interactions.

Polymer electrolytes based on PVdF-HFP doped with protic ionic liquids containing different cations

Several protic ionic liquids based on trifluoroacetate anion and different alkylammonium (diethyl-, dimethylethyl-, tributyl-, diisopropylethyl-) cations have been synthesized and investigated by different methods. Based on the results obtained earlier for triethylammonium and triethanolammonium trifluoroacetates, the influence of the cations structure on the thermal (phase transition and decomposition temperatures) and electrochemical (conductivity, electrochemical window, ionicity) properties of the PILs has been studied. For the investigated salts, the specific conductivity within the cation series DMEA > DEA > TEA > TBA decreases. These PILs have been used as dopants to prepare membranes based on (poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP)) whose electrochemical and thermal properties have also been investigated. The electric conductivity of these membranes was lower than that of the PILs themselves because of the confined mobility of the ions. However, the conductivity decrease was not too significant and did not exceed one order of magnitude. Doping PILs into the PVdF-HFP copolymer lowered the polymer crystallinity degree. The values of the electrochemical window of the studied PILs and PVdF-HFP–PILs membranes ranged from 0.9 to 1.3 V and from 6 to 7 V, respectively.

Faujasites exchanged with alkylammonium cations applied to basic catalysis

Abstract NaX and NaY faujasites (with Si/Al ratios of 1.3 and 2.5, respectively) were subjected to different degrees of ionic exchange with linear monoalkylammonium cations containing between 1 and 6 carbon atoms. The products were characterized using X-ray diffraction (XRD), nitrogen physisorption, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The Knoevenagel condensation was used as a model reaction to characterize the activity and accessibility of the catalytic sites. Ionic exchange isotherms showed that only the sodium cations present in the supercavities were exchangeable for the alkylammonium ions. The XPS results showed that the monoalkylammonium cations increased the basicity of the sites of both faujasites, with the basicity increasing with the length of the carbon chain. The catalytic activity in the Knoevenagel condensation was governed by three factors: the degree of exchange, the aluminum content, and the water content of the zeolites.

Strain coupling and dynamic relaxation in multiferroic metal-organic framework [(CH3)2NH2][Mn(HCOO)3] with perovskite structure

Abstract Strain coupling with ferromagnetism and ferroelectricity plays an important role in the development of multiferroic metal-organic frameworks (MOFs) with strong magnetoelectric coupling, but the underlying mechanisms have not been well understood. Strain coupling and dynamic relaxation in multiferroic MOF with perovskite structure [(CH3)2NH2][Mn(HCOO)3] were investigated using X-ray diffraction (XRD), Raman spectroscopy, Infra-red (IR) spectroscopy, differential scanning calorimetry (DSC), magnetic measurements and dynamic mechanical analysis (DMA). DSC results showed peaks at 183 K and 190 K at the rate of 5 K/min during cooling and heating processes, respectively. Magnetic measurements showed magnetic transition at ∼8.5 K at the heating rate of 2 K/min. Temperature and frequency dependences of elastic properties studied by DMA at frequencies of 0.5 Hz–10 Hz between 140 K and 300 K at heating rate of 2 K/min indicated that the minimum in storage modulus and the maximum in loss modulus and loss factor occurred near 190 K. The peak height of loss modulus and loss factor decreased at higher frequency, and the peak temperature was independent of frequency, showing the features of first-order phase transition. Near 190 K, paraelectric to ferroelectric phase transition triggered by disorder–order transition of alkylammonium cations located in the framework cavities occurred accompanied by the structural phase transition from rhombohedral space group R 3 - c to monoclinic space group Cc. The elastic anomalies and large energy loss near 190 K were associated with the coupling of the local strain with the freezing of dimethylammonium cation and the freezing of twin walls.

Introduction of Hydrophobic Ammonium Salts with Halogen Functional Groups for High‐Efficiency and Stable 2D/3D Perovskite Solar Cells

Abstract2D perovskites have attracted extensive attention due to their excellent stability compared with 3D perovskites. However, the intrinsic hydrophilicity of introduced alkylammonium salts effects the humidity stability of 2D/3D perovskites. Devices based on longer chain alkylammonium salts show improvement in hydrophobicity but lower efficiency due to the poorer charge transport among various layers. To solve this issue, two hydrophobic short‐chain alkylammonium salts with halogen functional groups (2‐chloroethylamine, CEA+ and 2‐bromoethylamine, BEA+) are introduced into (Cs0.1FA0.9)Pb(I0.9Br0.1)3 3D perovskites to form 2D/3D perovskite structure, which achieve high‐quality perovskite films with better crystallization and morphology. The optimal 2D/3D perovskite solar cells (PSCs) with 5% CEA+ display a power conversion efficiency (PCE) as high as 20.08% under 1 sun irradiation. Because of the notable hydrophobicity of alkylammonium cations with halogen functional groups and the formed 2D/3D perovskite structure, the optimal PSCs exhibit superior moisture resistance and retain 92% initial PCE after aging at 50 ± 5% relative humidity for 2400 h. This work opens up a new direction for the design of new‐type 2D/3D PSCs with improved performance by employing proper alkylammonium salts with different functional groups.

Switching of ionic conductivities in columnar liquid-crystalline anilinium salts: effects of alkyl chains, ammonium cations and counter anions on thermal properties and switching temperatures

Effects of molecular structures on the thermal switching behavior of ion conductivities for columnar anilinium liquid crystals have been examined.

Influence of the head group on dynamics in surfactant intercalated graphite oxide.

The influence of the headgroup on the dynamics of three different alkylammonium cations confined in graphite oxide (GO) was studied by temperature-dependent impedance spectroscopy. X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, thermogravimetry and elemental analysis served to characterize the composites. The alkylammonium cations were connected to the C–O− group of GO via ionic interactions, and the backbone of the confined molecule was distributed as a flat monolayer with the long axis parallel to the GO layer. Ngai's correlated-state model was used, with asymmetry at low temperature in the loss peaks. The calculated true activation energy of 114 meV ± 6% meV is almost the same as the internal rotation barrier of the alkyl macromolecule. We conclude that the relaxation process is definitely attributed to the wobbling around the long molecular axes of the confined ions, an intrinsic motion, not the reorientation of C–H at the headgroup, and it is also due to geometric structural symmetry at the headgroup of the alkylammonium molecules with the interaction of their backbones and the skeletons of GO resulting in the difference in the apparent activation energy.

Self-assembly and adsorption of cetyltrimethylammonium bromide and didodecyldimethylammonium bromide surfactants at the mica-water interface.

The self-assembly and adsorption of the surfactants cetyltrimethylammonium bromide (CTAB) and didodecyldimethylammonium bromide (DDAB) at the muscovite mica-water interface are studied using molecular-dynamics simulations. Adsorption takes place by an ion-exchange mechanism, in which K+ ions are replaced by the organic alkylammonium cations from the solution. Simulations are performed with and without the surface K+ ions, with pure water, and with the surfactants in aqueous solution. CTAB and DDAB form micellar structures in bulk solution, and in the absence of the surface K+ ions, they quickly adsorb and form bilayer structures. The bilayer ordering of CTAB is not perfect, and there is a competition with the formation of cylindrical micelles. DDAB, on the other hand, forms a well-ordered bilayer structure, with the innermost layer showing strong orientational ordering, and the outermost layer being more disordered. The simulations with pure water highlight the molecular ordering and strong electrostatic interactions with the mica-surface atoms. Using simulated scattering length density profiles, the results are compared directly and critically with existing neutron reflectivity measurements. The simulation results are generally consistent with experiments, and yield new insights on the molecular-scale ordering at the mica-water interface.

Volumetric Properties of Protic Ionic Liquids Based on Alkylammonium Cations at T = (293.15–353.15) K and Atmospheric Pressure

In this work, we measured the densities of 20 protic ionic liquids (PILs) based on four different cations: diethylammonium, triethylammonium, dimethylethylammonium, and triethanolammonium, and 14 anions of the following acids: phosphoric, acetic, trifluoroacetic, methanesulfonic, trifluoromethanesulfonic, benzoic, salicylic, propanoic, sulfuric, nitrobenzenesulfonic, sulfobenzoic, salicylic, sulfosalicylic, and bis(trifluoromethylsulfonyl)imide ones. The values of density were measured over the temperature range from 293.15 K to 353.15 K by densimeter based on the vibrating tube principle and they were approximated by a polynomial as a function of temperature. The thermal expansion coefficients were calculated; the fractional free volume in the molar volume of the studied PILs was estimated, and the influence of the ion structure on their volumetric properties was analyzed. Two different group contribution methods were analyzed to determine their applicability to predicting the density of the studied PILs.

Zeolite hydrothermal conversion in the presence of various cyclic alkylammonium cations and synthesis of nanosized BEA and MFI zeolites

Zeolite synthesis using faujasite-type (FAU) zeolites as the starting silica/alumina source in the presence of various organic structure-directing agents (OSDAs) with cyclic alkylammonium structures was investigated. The obtained zeolite phases strongly depended on the structure of the OSDA and the Si/Al ratio of the starting FAU. Zeolites with different framework structures, such as BEA, MFI, CHA, and LEV, were obtained. Among the used OSDAs, methylpropylpyrrolidinium, butylmethylpyrrolidinium, and methylpropylpiperidinium effectively produced nanosized BEA and MFI zeolites, whereas dimethylpiperidinium and cyclohexyltrimethylammonium produced CHA and LEV zeolites, respectively. The nanosized MFI and BEA zeolites obtained through zeolite hydrothermal conversion exhibited remarkably smaller particle sizes and higher external surface area as compared to those of the conventional MFI and BEA zeolite samples synthesized using tetrapropylammonium and tetraethylammonium, respectively. We also found that the highly crystalline nanosized BEA and MFI zeolites exhibited better catalytic performance in the dehydration of benzyl alcohol to benzyl ether as compared to the conventional zeolites with large particle sizes. In addition, nanosized MFI zeolites, synthesized using FAU as a starting material and cyclic ammonium compounds, showed catalytic activities superior to those of the nanosized MFI zeolites synthesized from amorphous starting materials and cyclic ammonium compounds.

Switching from Negative-Cooperativity to No-Cooperativity in the Binding of Ion-Pair Dimers by a Bis(calix[4]pyrrole) Macrocycle.

We report the synthesis of a macrocyclic receptor containing two di- meso-phenylcalix[4]pyrrole units linked by two triazole spacers. The 1,4-substitution of the 1,2,3-triazole spacers conveys different binding affinities to the two heteroditopic binding sites. These features make the receptor an ideal candidate to investigate allosteric cooperativity in the binding of ion-pair dimers. We probed the interaction of tetraalkylammonium salts (TBA·Cl, TBA·OCN, and MTOA·Cl) with the tetra-heterotopic macrocyclic receptor in chloroform solution using 1H NMR spectroscopic titration experiments. The results obtained show that, at millimolar concentration, the addition of 2 equiv of the salt to the receptor's solution induced the quantitative pairwise binding of the ion-pairs. The 2:1 (ion-pair:receptor) complexes feature different binding geometries and binding cooperativities depending on the nature of the alkylammonium cation. The binding geometries assigned to the complexes of the ion-pair dimers in solution are fully supported by X-ray diffraction analyses of single crystals. The thermodynamic features of the binding processes (separate or concomitant formation of 1:1 and 2:1 complexes), derived from isothermal titration calorimetry (ITC) experiments, are rationalized by combining the different ion-pair binding modes of the salt dimers with the dissimilar electronic properties of the two nearby heteroditopic binding sites of the receptor.

Multivalent Crown Ether Receptors Enable Allosteric Regulation of Anion Exchange in an Fe4L6 Tetrahedron

AbstractWe report a strategy for regulating the rate of internally bound anion exchange within an Fe4L6 metal–organic tetrahedron through external coordination of tripodal tris(alkylammonium) cations. The cage features three flexible 18‐crown‐6 receptors at each of its FeII vertices, facilitating strong tritopic interactions with tris(ammonium) cations to “cap” the vertices of the tetrahedron. This capping mechanism restricts the flexibility of the cage framework, thereby reducing the rate of anion exchange within its central cavity by 20‐fold. Thus, we demonstrate the first use of an externally bound multivalent effector to allosterically control internal guest binding in a molecular cage.

Multivalent Crown Ether Receptors Enable Allosteric Regulation of Anion Exchange in an Fe4 L6 Tetrahedron.

We report a strategy for regulating the rate of internally bound anion exchange within an Fe4 L6 metal-organic tetrahedron through external coordination of tripodal tris(alkylammonium) cations. The cage features three flexible 18-crown-6 receptors at each of its FeII vertices, facilitating strong tritopic interactions with tris(ammonium) cations to "cap" the vertices of the tetrahedron. This capping mechanism restricts the flexibility of the cage framework, thereby reducing the rate of anion exchange within its central cavity by 20-fold. Thus, we demonstrate the first use of an externally bound multivalent effector to allosterically control internal guest binding in a molecular cage.

FTIR observation of cleavage state of a mica–organic hybrid solid lubricant under reciprocating sliding conditions

The lubrication performance of a mica–alkyl ammonium cation hybrid solid lubricant fabricated via an intercalation reaction was investigated by using a cylinder-on-disk tribotester and FTIR spectroscopy, and a cleavage mechanism was proposed. In micas with different contents of the intercalated organic cations, the results indicated a high correlation between the friction coefficient of residual mica and the absorption wavenumber of in-plane Si–O bonds. When the friction coefficient was lower, the absorption band shifted to a higher wavenumber as the sliding progressed. The IR shift was caused by a change in an interaction between mica and the alkyl ammonium cation derived from its cleavage. • Lubrication performance of mica is improved by an intercalated alkyl ammonium cation. • The improved lubricity is due to the promotion of the mica's cleavability. • The absorption wavenumbers for in-plane Si–O bonds were higher as sliding progressed. • The IR shift is caused by mica's cleavage.