Larger Alkali Metal Research Articles

Structural Properties of Some Vacancy-Ordered Platinum Halide Perovskites.

The structural changes that accompany the dehydration of Na2PtX6·6H2O (X = Cl, Br) were studied using in situ variable temperature synchrotron X-ray diffraction. The two hexahydrates are isostructural, containing isolated PtX6 octahedra separated by Na cations. Removal of the water results in the formation of the anhydrous vacancy ordered double perovskites Na2PtX6. The Na cation is too small for the cuboctahedron site of the parent cubic structure, resulting in cooperative tilting of the PtX6 octahedra and lowering of the symmetry. Replacing Na with a larger alkali metal (K, Rb, or Cs) invariably enabled the isolation of the anhydrous hexahalide, and we found no evidence that these readily hydrated. For all cations, other than Na, it was possible to observe the archetypical cubic structure, although for the two potassium salts K2PtBr6 and K2PtI6, this was only observed above a critical temperature of 175 and 460 K, respectively. As these two samples were cooled, symmetry lowering was observed, yielding a tetragonal structure initially and ultimately a monoclinic structure: Fm3̅m → P4/mnc → P21/n. These phase transitions are associated with the onset of long-range cooperative tilting of the PtX6 octahedra described using the Glazer tilt notation as a0a0a0 → a0a0c+ → a-a-c+.

Noncovalent Host-Guest Complexes of Artemisinin with α-, β-, and γ- Cyclodextrin Examined by Structural Mass Spectrometry Strategies.

Cyclodextrins (CDs) are a family of macrocyclic oligosaccharides with amphiphilic properties, which can improve the stability, solubility, and bioavailability of therapeutic compounds. There has been growing interest in the advancement of efficient and reliable analytical methods that assist with elucidating CD host-guest drug complexation. In this study, we investigate the noncovalent ion complexes formed between naturally occurring dextrins (αCD, βCD, γCD, and maltohexaose) with the poorly water-soluble antimalarial drug, artemisinin, using a combination of ion mobility-mass spectrometry (IM-MS), tandem MS/MS, and theoretical modeling approaches. This study aims to determine if the drug can complex within the core dextrin cavity forming an inclusion complex or nonspecifically bind to the periphery of the dextrins. We explore the use of group I alkali earth metal additives to promote the formation of various noncovalent gas-phase ion complexes with different drug/dextrin stoichiometries (1:1, 1:2, 1:3, 1:4, and 2:1). Broad IM-MS collision cross section (CCS) mapping (n > 300) and power-law regression analysis were used to confirm the stoichiometric assignments. The 1:1 drug:αCD and drug:βCD complexes exhibited strong preferences for Li+ and Na+ charge carriers, whereas drug:γCD complexes preferred forming adducts with the larger alkali metals, K+, Rb+, and Cs+. Although the ion-measured CCS increased with cation size for the unbound artemisinin and CDs, the 1:1 drug:dextrin complexes exhibit near-identical CCS values regardless of the cation, suggesting these are inclusion complexes. Tandem MS/MS survival yield curves of the [artemisinin:βCD + X]+ ion (X = H, Li, Na, K) showed a decreased stability of the ion complex with increasing cation size. Empirical CCS measurements of the [artemisinin:βCD + Li]+ ion correlated with predicted CCS values from the low-energy theoretical structures of the drug incorporated within the βCD cavity, providing further evidence that gas-phase inclusion complexes are formed in these experiments. Taken together, this work demonstrates the utility of combining analytical information from IM-MS, MS/MS, and computational approaches in interpreting the presence of gas-phase inclusion phenomena.

Microporous framework polar silicate-germanates with a wide isomorphic substitution: (K2.9Cs0.1)(Sc0.7In0.3)[(Si2.95Ge0.05)O9]·H2O and (K2.16Cs0.84)Bi[(Si1.5Ge1.5)O9]·H2O

Abstract New silicate-germanates (K2.9Cs0.1)(Sc0.7In0.3)[(Si2.95Ge0.05)O9]·H2O and (K2.16Cs0.84)Bi[(Si0.5Ge0.5)3O9]·H2O have been synthesized in multi-component systems under mild hydrothermal conditions. The new compounds are classified as new representatives of close related K3ScSi3O9·H2O parent structure, sp. gr. Pmn21. Their structural and isomorphic peculiarities are compared with it as well as with earlier investigated K1.46Pb1.54Сa[(Ge0.23Si0.77)3O9](ОН)0.54·0.46Н2О. Together with other known compounds, silicate-germanates form the extensive family A3M[T3O9]·H2O, A = K, Cs, Ca, Pb; M = Ho, Sc, Lu, Tb, Er, Y, Bi, Pb, In; T = Si, Ge, with a mixed microporous framework combined of M-octahedra and T-tetrahedra. Large alkali metal or/and Ca, Pb cations fill broad framework channels with cross-section up to 7.3 Å. Because of wide isomorphic substitution in the channels, and in tetrahedra and octahedra, ion exchange properties in the family are expected. Due to polar symmetry, all the crystals possess second-order nonlinearity which was confirmed with positive SHG tests for four compositions. Powder SHG experiments demonstrated moderate second harmonic intensities of order of α-quartz standard signals.

Effects of alkali metal cations and fluoride anions on viscosity and structure of CaO–SiO2–Al2O3 system

Alkali metal oxides (R2O) in calcium-aluminosilicate-based melts were substituted with alkali metal fluorides (RF) while maintaining a constant concentration of the alkali metal cation to investigate the effects of the alkali metal cations and fluoride anions on the viscosities and structures of high-temperature melts. Substitution with larger alkali metal cations, e.g., Li+, Na+, and K+, increased the viscosity of the melts and enhanced the degree of polymerization of the Q3 structural units. Substitution of oxides with alkali metal fluorides in the range of 0–10 mol% reduced the viscosity, but enhanced the polymerization of the structural units. F1s binding energies estimated through X-ray photoelectron spectroscopy indicated that the F− anions coordinated with the Ca2+ cations. The occupation of the network-modifying Ca2+ cations by the F− anions increased the overall degree of polymerization of the melt structures, but the melt viscosities were slightly reduced because NBO-Ca2+-NBO links transformed to NBO-Ca2+-F-, where NBO corresponds to the non-bridging oxygen. Isothermal and non-isothermal viscosity measurements indicated the viscosities were lower under non-isothermal conditions because of the high excess thermal energy of the melts and the presence of large alkali metal cations with low mobility. These factors were found to increase the difference between the isothermal and non-isothermal viscosity measurements.

Potential replacement to lead: Alkali metal potassium and transition metal zinc in organo-metal halide perovskite materials

Following the plenty of research work on solving the issue of the popular and promising perovskite solar cells, this article is focused on methylammonium lead iodide (MAPbI3) and the alternative element which can perform as efficiently as lead does. Here, not only the different concentrations of the substitute element are investigated, but also higher stable perovskite is introduced as well. Several characterization techniques are used to assure the validity of the results. The large alkali metal potassium (K) and the transition metal zinc (Zn) are studied side by side. The results are interesting when the 5% concentration of the KI alloyed MAPbI3 shows considerable transport properties like 54 nm diffusion length, 245 × 10−9 Sm−1 photoconductivity, and significant majority and minority carrier mobility–lifetime product as compared to MAPbI3 but it becomes more interesting when after 60 days the 5% concentration of the aged ZnCl2 alloyed MAPbI3 turns into a leading element of this research by showing high stability among of all. The morphology investigations show both perovskite materials achieve large islands which are desired morphology in perovskite materials as well.

Complementarity in Cyclotricatechylene Assemblies: Symmetric Cages Linked within 3D Cubic Hydrogen Bonded Networks

A serendipitous discovery has led to the generation of a family of four compounds in which six components combine to form symmetric metal-cyclotricatechylene (H6ctc) cages. The four compounds, which have the compositions, [Cs((CH3)2CO)6][K4(H6ctc)4(H2O)8][Cs4(H2O)6](PO4)3, [Rb((CH3)2CO)6][Rb2K2(H6ctc)4(H2O)6][Rb4(H2O)6](PO4)3, [Cs((CH3)2CO)6][K4(H6ctc)4(H2O)8]-[Cs(H2O)9](SO4)3 and [Rb((CH3)2CO)6][Rb2K2(H6ctc)4(H2O)6][Rb(H2O)9](SO4)3 possess cubic symmetry that arises from the complementary interactions that govern the assembly of the components. The cage cavities contain water molecules and either one or four large alkali metal ions (either Rb+ or Cs+) which interact with the internal aromatic surfaces of the cage. Each cage is linked to six tetrahedral anions (PO43− or SO42−) through 24 equivalent hydrogen bonds and each anion bridges a pair of cages through eight such hydrogen bonds. An unusual octahedral complex M((CH3)2CO)6+ (M = Rb or Cs), in which the M-C=O link is linear, appears to be a key structural component. A feature of this family of crystalline compounds is the presence of a range of complementary interactions which combine to generate materials that exhibit high crystallographic symmetry.

A -site cation size effect on oxygen octahedral rotations in acentric Ruddlesden-Popper alkali rare-earth titanates

We demonstrate inversion symmetry breaking induced by oxygen octahedral rotations in layered perovskite oxides $\mathrm{K}{A}_{R}{\mathrm{TiO}}_{4}$ (${A}_{R}$ = rare earth) using a combined experimental and theoretical approach including synchrotron x-ray diffraction, optical second harmonic generation, and first-principles lattice dynamics calculations. We experimentally find an interesting but counterintuitive phenomenon, i.e., the acentric-to-centric phase transition temperatures for K family are higher than those for previously reported Na family, in contrast to expectations based on the Goldschmidt tolerance factor, where the octahedral rotation instability toward the acentric phases would reduce with an increase in the radius of $A$-site alkali metal ions. Our detailed analysis of first-principles calculations for ${A}_{A}{A}_{R}{\mathrm{TiO}}_{4}$ (${A}_{A}=\mathrm{Na}$, K, Rb) reveals that the alkali metal and rare-earth ions play quite different roles in driving the octahedral rotations. Since rare-earth ions attract oxide ions more strongly than alkali metal ions due to the higher valence of the former in comparison with the latter (${A}_{R}^{3+}$ vs ${A}_{A}^{+}$), the optimization of coordination environment of rare-earth ions is the primary driving force of the octahedral rotations. Alkali metal ions serve to impose ``bond strains'' parallel to the layers, playing a secondary role in the octahedral rotations. Incorporation of large alkali metal ions generates a significant in-plane biaxial bond strain in ${A}_{R}\mathrm{O}$ and ${\mathrm{TiO}}_{2}$ layers through the expanded ${A}_{A}\mathrm{O}$ layers, and thereby facilitates the octahedral rotations because of the otherwise highly underbonding of rare-earth ions. Thus, the effect of $A$-site alkali metal size on the octahedral rotation instability can be explained in terms of the interlayer lattice mismatch. This understanding allows us to propose a geometric descriptor governing the structural instability in ${A}_{A}{A}_{R}{\mathrm{TiO}}_{4}$ layered perovskites. We believe that control over the interlayer lattice mismatch could be a useful strategy to tune the octahedral rotations in layered compounds.

Intensifying the CO2 Electrolysis Process

Every year we emit approximately 26 billion tons of CO2 to the atmosphere.1 As a result, the atmospheric CO2 concentration has increased from 316 to 408 ppm (NOAA, Mauna Loa HI) over the past 50 years. Global temperature anomalies have increased by 1°C leading to adverse environmental impact. The atmospheric CO2 concentration needs to be reduced in order to curb such effects. Electrochemical conversion of CO2 into value added chemicals such as methanol, formic acid, and carbon monoxide is a promising method for reducing the excess atmospheric CO2 content.2-5 If the end product obtained from the process is CO, the system could be coupled to a Fischer Tropsch (FT) reactor to form liquid fuels.6 Many research efforts in academia and industry seek to identify and optimize suitable catalysts, electrodes, electrolytes, and associated operation conditions that allow for the energy efficient and selective electrolysis of CO2 into one of the aforementioned value-added intermediates.6 By using a gas-diffusion electrode (GDE) based flow electrolyzer, CO2 can be fed as a gas, thereby circumventing any solubility and mass transfer issues, resulting in high current densities (>400 mA cm-2).7-8 Despite progress in catalyst activity and selectivity, as well as in electrode and electrolyzer performance, still significant further improvements are needed to reach techno-economic feasibility at scale.9 This presentation will cover multiple directions that can be explored (for an alkaline GDE-based flow electrolyzer) to achieve the needed improvement in performance: The first part of this presentation will cover process optimization where we systematically vary various process parameters like cathode catalyst loading, electrolyte flow rate, and electrolyte concentration to operate at peak performance. In the second part, we will cover how electrolyte composition can be tailored to achieve higher current densities and better Faradaic efficiencies by using high pH and using large alkali metal cations such as Cs+. The third part demonstrates the use of a Ni & Fe based bimetallic anode catalyst which gives a 200 mV reduction in overpotential compared to the typically used IrO2 anode catalyst. And lastly, in the fourth part, we present some results regarding further process intensification where we studied the effect of operating temperature. References J. Oerlemans, Science, 2005, 308, 675-677K. W. Frese, S. Leach, Journal of the Electrochemical Society, 1985, 132, 259-260Y. Hori, H. Wakebe, T. Tsukamoto, O. Koga, Electrochimica Acta, 1994, 39, 1833-1839Y. Hori, K. Kikuchi, S. Suzuki, Chemistry Letters, 1985, 11, 1695-1698B. A. Rosen, A. Salehi-Khojin, M.R. Thorson, W. Zhu, D.T. Whipple, P.J.A. Kenis, R.I. Masel, Science, 2011, 334, 643-644B. Endrodi, G. Bencsik, F. Darvas, R. Jones, K. Rajeshwar, C. Janaky, Progress in Energy and Combustion Science, 2017, 62, 133-154D. T. Whipple, E.C. Finke, P.J.A. Kenis, Electrochemical and Solid-State Letters, 2010, 13, B109-B111S. Verma, X. Lu, S. Ma, R. I. Masel, P. J. A. Kenis, Physical Chemistry Chemical Physics, 2016, 18, 7075-7084S. Verma, B. Kim, H. R. M. Jhong, S. Ma, P. J. A. Kenis, ChemSusChem, 2016, 9, 1972-1979

Understanding the complex structural features and phase changes in Na2Mg2(SO4)3: A combined single crystal and variable temperature powder diffraction and Raman spectroscopy study

Sodium mixed metal sulphates have attracted considerable attention, both in terms of mineralogy and more recently due to interest in Na ion containing materials for battery applications. The phase, Na2Mg2(SO4)3, has been previously reported to undergo a phase change to langbeinite at high temperatures, which is interesting given that usually the langbeinite structure is only adopted when large alkali metal ions, e.g. K, Cs, are present. Nevertheless the room temperature structure of this phase has remained elusive, and so in this work, we report a detailed structural study of this system. We show that room temperature Na2Mg2(SO4)3 can only be prepared by quenching from high temperature, with slow cooling leading to phase separation to give the previously unreported systems, Na2Mg(SO4)2 and Na2Mg3(SO4)4. We report the structures of quenched Na2Mg2(SO4)3 (monoclinic, P21), as well as Na2Mg(SO4)2 (triclinic, P1̅) and Na2Mg3(SO4)4 (orthorhombic, Pbca), detailing their complex structural features. Furthermore, we report a study of the thermal evolution of quenched Na2Mg2(SO4)3 with temperature through variable temperature XRD and Raman studies, which shows a complex series of phase transitions, highlighting why this phase has proven so elusive to characterise previously, and illustrating the need for detailed characterisation of such sulphate systems.

Intercalation of barium into monoclinic tungsten oxide nanoplates for enhanced photoelectrochemical water splitting

Intercalating alkali earth metals into WO3 lattice is a promising strategy to tune its optical and electrical properties. However, hollow cubic tunnels present in monoclinic WO3 do not provide sufficient space for the diffusion and intercalation of large ions into its interstitial cavities. Therefore, a unique synthesis strategy is required to intercalate large ions into the monoclinic WO3 lattice without causing structural distortion or damage. In this study, a single-step hydrothermal method is proposed for the fabrication of barium-intercalated monoclinic WO3 thin films for efficient photoelectrochemical water splitting applications. The intercalation of barium into WO3 is achieved by the hydrothermal condensation of peroxopolytungstic acid solution. The proposed method yields hydrated orthorhombic WO3, which provides enough space in the hexagonal interstitial cavities between its [001] planes for the stable intercalation of barium. Subsequently, stable barium-intercalated monoclinic WO3 is obtained via reconstructive transformation without causing any significant structural distortion or damage. The intercalation of barium affects the preferential direction of crystal growth and the morphology of WO3. Furthermore, barium intercalation reduces the bandgap of WO3 and slightly increases its carrier density. As a result, higher photocurrent and incident photon-to-current efficiency values are achieved. Based on spectroscopic and electrochemical results, a probable band diagram of barium-intercalated WO3 is proposed. Importantly, the proposed method can be efficiently extended to fabricate monoclinic WO3 thin films intercalated with large alkali metal and alkali earth metal ions for various applications.

Cation\u2013\u03c0 interactions in competition with cation microhydration: a theoretical study of alkali metal cation\u2013pyrene complexes

Cation–π interactions were systematically investigated for the adsorption of H+ and alkali metal cations M+ to pyrene by means of Møller–Plesset perturbation theory (MP2) and density functional theory (DFT). The main aims were to determine the preferred adsorption sites and how the microhydration shell influences the adsorption process. The preferred adsorption sites were characterized in terms of structural parameters and energetic stability. Stability analysis of the M+–pyrene complexes revealed that the binding strength and the barrier to transitions between neighboring sites generally decreased with increasing cation size from Li+ to Cs+. Such transitions were practically barrierless (<<1 kcal/mol) for the large Rb+ and Cs+ ions. Further, the influence of the first hydration shell on the adsorption behavior was investigated for Li+ and K+ as representatives of small and large (alkali metal) cations, respectively. While the isolated complexes possessed only one minimum, two minima—corresponding to an inner and an outer complex—were observed for microhydrated complexes. The small Li+ ion formed a stable hydration shell and preferentially interacted with water rather than pyrene. In contrast, K+ favored cation–π over cation–water interactions. It was found that the mechanism for complex formation depends on the balance between cation–π interactions, cation–water complexation, and the hydrogen bonding of water to the π-system.

Oxygen Anion Solubility as a Factor in Molten Flux Crystal Growth, Synthesis, and Characterization of Four New Reduced Lanthanide Molybdenum Oxides: Ce4.918(3)Mo3O16, Pr4.880(3)Mo3O16, Nd4.910(3)Mo3O16, and Sm4.952(3)Mo3O16

Four new reduced lanthanide molybdenum oxides containing mixed valent Mo(V/VI)O4 tetrahedra were prepared in single crystal form by utilizing a high temperature molten salt flux synthesis involving an in situ reduction step. Calculations support the experimentally observed result that large alkali metal cations such as cesium are superior compared to the smaller alkali metal cations such as sodium in solvating O2– to facilitate oxide crystal growth in halide melts. All four compounds were structurally characterized by single crystal and powder X-ray diffraction methods and were found to crystallize in the cubic space group Pn3n. The temperature dependence of the magnetic susceptibility of these compounds was measured, and all were found to exhibit simple paramagnetism.

Near infrared photoluminescence of the univalent bismuth impurity center in leucite and pollucite crystal hosts

The bismuth doped aluminosilicate phases leucite (KAlSi2O6), gallium leucite (KGaSi2O6) and pollucite (CsAlSi2O6) display broadband NIR photoluminescence.

Perovskites and their potential use in solar energy applications.

1. Introduction A material may be described as having a perovskite structure1 if it has same type of crystal structure as perovskite--calcium titanium oxide (CaTi[O.sub.3])--does (Figure 1). Perovskite was first discovered in 1839 by Gustav Rose, in the Ural mountains in Russia, and is named after the Russian mineralogist L. A. Perovski (1792-1856), who first characterised the material. The general formula for perovskites is [ABX.sub.3], with 'A' and 'B' being two cations of significantly different sizes ('A'>'B'), while X is an that binds with both cations. The perovskite structure is adopted by many oxides that have an elemental composition: AB[O.sub.3]. The ideal cubic-structure has the 'B' cation in a 6-fold coordination, surrounded by an octahedron of anions, with the 'A' cation in a 12-fold cuboctahedral coordination. Cations 'A' occupy the cube corner positions (0, 0, 0), while cations 'B' occupy the body centred positions (1/2, 1/2, 1/2) with oxygen anions 'O' being located at face centred positions (1/2, 1/2, 0). Figure 1 shows edges for an equivalent unit cell with 'A' in body centre, 'B' at the comers, and 'O' in mid-edge. The requirements of relative ionic radii are quite exacting to maintain a stable cubic structure, meaning that even relatively minor degrees of buckling and distortion can result in a number of alternative versions with lower symmetry, in which the coordination numbers of either the 'A' cations, 'B' cations, or both, are reduced. Tilting of the B06 octahedron reduces the coordination of a too-small 'A' cation from 12 down to as low as 8. Conversely, when a small 'B' cation is brought off-centre, within its octahedral coordination, a stable bonding arrangement can be obtained. Such distortions can create an electric dipole and it is for this reason that perovskites such as BaTi03, which distort in this manner, exhibit the property of ferroelectricity. The most usual non-cubic forms of perovskites are the orthorhombic and tetragonal variants. There are also some more complex perovskite structures which contain two different 'B'-site cations, with the result that ordered and disordered variants are possible. [FIGURE 1 OMITTED] Under the high pressure conditions of the Earth's lower mantle, the pyroxene enstatite, MgSi[O.sub.3], is converted to a more dense perovskite-type polymorph, and indeed it is speculated that this particular phase of the material might be the most common mineral in the Earth (2). It has a perovskite structure, with an orthorhombic distortion, and is stable at pressures from ~24 GPa to ~110 GPa. [For comparison, the pressure at the centre of the Earth is ca 300 GPa.] However, it is stable only at depths of several hundred kilometres and could not be transported back to the Earth's surface without reforming into less dense materials. At yet greater pressures, MgSi[O.sub.3] perovskite undergoes a transformation to form post-perovskite. Although the most common perovskite compounds contain oxygen, perovskites containing fluoride anions are known, e.g. NaMg[F.sub.3]. Metallic perovskite compounds also exist (1), with the general formula [RT.sub.3]M, where R represents a rare-earth or other relatively large cation, T is a transition metal ion and M represents light metalloids (anions) which occupy the octahedrally coordinated 'B' sites, e.g. [RPd.sub.3]B, [RRh.sub.3]B and Ce[Ru.sub.3]C. MgC[Ni.sub.3] is a metallic perovskite compound, and is of particular interest on account of its superconducting properties. A further category has mixed oxide-aurides of Cs and Rb, such as [Cs.sub.3]AuO, which contain large alkali metal cations in the traditional anion sites, bonded to [O.sup.2-] and [Au.sup.-] anions. 2. Properties of perovskites As noted, the perovskite structure is imparted with an appreciable element of structural pliancy, and the ideal cubic structure (Figure 1) can be distorted in many different ways. Thus, the octahedra may become tilted, the cations can be displaced from the centres of their coordination polyhedra, and the octahedra might be distorted at the behest of electronic factors (e. …

Alkali-Metal-Induced C–C Bond Cleavage and CH4 Elimination in the Amido → Aza-Allyl Transformation of the (S)-N-α-(Methylbenzyl)benzylamido Anion

In this systematic study six new chiral alkali-metal complexes derived from (S)-α-(methylbenzyl)benzylamine (α-mmba) have been synthesized and characterized in the solution state, with five successfully analyzed by single-crystal X-ray diffraction. The reaction products observed are highly dependent on the choice of alkali metal and Lewis donor used, affording either the amido deprotonation complex [Li{PhCH(Me)NCH2Ph}·HMPA]2 (3) when using nBuLi or the aza-allyl complexes [Na{PhC(Me)NCHPh}·TMEDA]∞ (4), [Na{PhC(Me)NCHPh}·3THF] (5), [Na{PhC(Me)NCHPh}·THF·TMEDA] (6), and [K{PhC(Me)NCHPh}·PMDETA]∞ (8) via hydrogen elimination reactions. Unexpectedly a co- secondary product is also obtained when tridentate or pseudo-tridentate donor ligands are used in the reaction of α-mbba with either nBuNa or nBuK to give the unexpected 1, 3-diphenyl-2-aza-allyl complexes[M{(PhCH)2N}·L] [M = Na, L = 3 THF (5a) or TMEDA/THF (6a); M = K, L = PMDETA (8a)] in various ratios. Reinforcing the experimental findings, formation of the methane elimination complex was confirmed via GC-headspace analysis experiments, while theoretical DTF calculations on model systems reinforce the experimental findings showing that thermodynamic gas elimination pathways trend toward the larger alkali metals with H2 elimination kinetically favored over CH4.

An Imine‐Based Molecular Cage with Distinct Binding Sites for Small and Large Alkali Metal Cations

The synthesis of a cylindrical, imine-based cage composed of two trimeric metallamacrocycles is described. The cage acts as a heterotopic receptor for alkali metal cations. The small cations Li(+), Na(+), and K(+) bind to the outside of the cage with good selectivity for Li(+), whereas the larger cations Rb(+) and Cs(+) are bound inside the cage to form unusual π complexes with a good selectivity for Cs(+). Negative heterotopic cooperativity between the two binding sites is observed. The complexation of Cs(+) is associated with a color change, which enables the cage to be used as a specific sensor for Cs(+).

Alkali Metal/Lanthanoid Heterobimetallic Complexes of 8‐Hydroxyquinolines Accessed by Pseudo‐Solid‐State Reactions

AbstractThe heterobimetallic 1D‐polymeric alkali metal/lanthanoid [AMLn(Q)4]n [Q = OQ (8‐quinolinolate): AM = Li, Ln = Tb, Ho, Er; AM = K, Ln = Er; Q = MQ (2‐methyl‐8‐quinolinolate): AM = Rb; Ln = Tb, Er] complexes were obtained after rearrangement reactions between AM(Q) and Ln(Q)3 in a 1,2,4,5‐tetramethylbenzene flux at elevated temperatures. In these compounds, the eight‐coordinate lanthanoid ion is surrounded by four (O, N) chelating Q ligands, which bridge to the adjacent alkali metal through their oxygen atoms to form the linear polymers. Dimeric [Cs2(MQ)2(HMQ)2] was obtained from an attempt to prepare [CsEr(MQ)4]n, and the caesium atoms are each ligated by one terminal HMQ and one chelating‐bridging MQ ligand. In addition, the chelating‐bridging ligands in [Cs2(MQ)2(HMQ)2] and in the rubidium derivatives [RbLn(MQ)4]n (Ln = Tb, Er) exhibit bonding of the π‐electron system to coordination saturate the large ionic alkali metal ion. The reaction of Yb(MQ)3 in the presence of K2C2O4·H2O afforded the monomeric heterobimetallic [KYb2(MQ)7] complex in which the three metal atoms are arranged in a triangular fashion. They are surrounded by seven MQ ligands, which display the previously encountered chelating (O, N) and the chelating‐bridging μ‐κ2(N,O):κ1(O) and μ3‐κ2(N,O):κ1(O):κ1(O) ligation modes. Heating K(OQ) in 1,2,3,4‐tetrachlorobenzene gave crystals of 8‐hydroxyquinolinium chloride. The structure of [Na4(OQ)4(H2O)8]n, obtained by neutralisation, has sodium ions coordinated by one chelating OQ and four aqua ligands, the latter bridge to adjacent sodium ions to give a double stranded 2D‐polymer.

Spectral properties of rhodamine 6G in smectite dispersions: Effect of the monovalent cations

Montmorillonite monoionic forms with alkali metal and NH4+-cations were prepared by ion exchange. The hydration properties and binding of the ions to montmorillonite surface and the swelling properties of the mineral specimens were analyzed. Whereas Na+- and Li+-ions were fully hydrated over a large range of conditions, large size K+, NH4+, and mainly Rb+ and Cs+ ions were apt to bind directly to the oxygen atoms on the mineral surface. The forms with large ions exhibited reduced hydration and swelling and the absence of macroscopic swelling of the respective aqueous colloids. The interaction of laser dye rhodamine 6G (R6G) in montmorillonite colloids was investigated by absorption and steady-state fluorescence spectroscopies. Significant effects of the properties of both the inorganic ions and swelling properties of colloidal dispersions on R6G molecular aggregation were observed. Large amounts of the molecular aggregates were formed in the colloids of Na+- and Li+-montmorillonites. The aggregates absorbed light at significantly lower wavelengths (∼460nm) with respect to the light absorption by monomers (535nm). Fluorescence spectroscopy provided a key evidence for the assignment of the type of the aggregates: The emission of the aggregates at relatively low energies proved these assemblies are rather a mixed H-/J-type than ideal H-aggregates. The presence of parent inorganic cations of larger size led to a significant lowering of the amount of the R6G aggregates in favor of the monomers. Investigations of the evolution of the dye aggregation with time indicated basic features of dye aggregation reaction: The size of parent inorganic ions did not affect the reaction mechanism, but rather limited the extent of the reaction. Probably the forms with large inorganic ions, such as Rb+ and Cs+, did not provide sufficient surface for the formation of the large size assemblies of the dye. This property can be explained in terms of strong association of the large alkali metal ions to clay mineral surface, as well as to reduced swelling in the colloidal systems of respective forms.

Zeolite synthesis: an energetic perspective

Taking |D(H(2)O)(x)|[AlSiO(4)] based materials (where D is Li, Na, K, Rb or Cs) as an archetypal aluminosilicate system, we use accurate density functional theory calculations to demonstrate how the substitution of silicon cations in silica, with pairs of aluminium and (alkali metal) cations, changes the energetic ordering of different competing structure-types. For large alkali metal cations we further show that the formation of porous aluminosilicate structures, the so-called zeolites, is energetically favored. These findings unequivocally demonstrate that zeolites can be energetic preferred reaction products, rather than being kinetically determined, and that the size of the (hydrated) cations in the pore, be it inorganic or organic, is critical for directing zeolite synthesis.

Novel highly luminescent europium dinitrosalicylates

A series of lanthanide dinitrosalicylates M 3Ln(3,5-NO 2-Sal) 3 · nH 2O (Ln = Eu, Gd; M = Li, Na, K, Cs) was synthesized. It was found that the luminescence efficiency of some M 3Eu(3,5-NO 2-Sal) 3 · nH 2O compounds was near to the high efficiency of europium dibenzoylmethanate with 1,10-phenanthroline, Eu(DBM) 3 · Phen. The luminescence excitation spectra, electron-vibrational luminescence spectra, and vibrational IR spectra were investigated. The energy of the lowest excited triplet state of the ligand was obtained from phosphorescence spectra of M 3Gd(3,5-NO 2-Sal) 3 · nH 2O, M(3,5-NO 2-HSal) · nH 2O, and M 2(3,5-NO 2-Sal) · nH 2O. The details of the structure of compounds were discussed. The influence of different M-cations on the Eu 3+ luminescence efficiency and on the processes of excitation energy transfer to a Eu 3+ ion was analyzed. The presence of large alkali metal cations in lanthanide dinitrosalicylates and an increase in the temperature weaken the network of hydrogen bonds and, to some extent, the “ligand–metal” bonds. This is a cause of a long-wavelength shift of the intraligand charge transfer (ILCT) band in Eu 3+ excitation spectra arising at inclusion of Cs + instead of Li + cations in the crystal lattice and at the heating of compounds. A change of the energies of ligand electronic states at substitution of Li + and Na + for Cs + can give a tenfold enhancement of the Eu 3+ luminescence efficiency at 300 K.