[2]Pseudorotaxanes based on the cryptand/monopyridinium recognition motif

The first cryptand/monopyridinium [2]pseudorotaxanes were prepared from five bis(m-phenylene)-32-crown-10- and one bis(m-phenylene)-26-crown-8-based cryptand hosts and three monopyridinium guests. These pseudorotaxanes were studied by proton NMR spectroscopy, mass spectrometry, and X-ray crystallography. Association constants ranged from 141 M⁻¹ to 1.86×10⁴ M⁻¹ in 1:1 acetone: chloroform at 22 °C.     

[3]Pseudorotaxanes based on the cryptand/monopyridinium salt recognition motif

The first cryptand/monopyridinium salt [3]pseudorotaxanes were prepared from two cryptand hosts and two bispyridinium guests as confirmed by proton NMR characterization, electrospray ionization mass spectrometry, and X-ray analysis. It was found that the two monopyridinium binding sites are independent of each other for the formation of one [3]pseudorotaxane.     

Electrochemically controlled formation/dissociation of phosphonate-cavitand/methylpyridinium complexes

The phosphorus-bridged cavitand 1 self-assembles very efficiently in CH2Cl2 with either the monopyridinium guest 2+ or the bispyridinium guest 3(2+). In the first case a 1:1 complex is obtained, whereas in the second case both 1:1 and 2:1 host-guest complexes are observed. The association between 1 and either one of the guests causes the quenching of the cavitand fluorescence; in the case of the adduct between 1 and 3(2+), the fluorescence of the latter is also quenched. Cavitand complexation is found to affect the reduction potential values of the electroactive guests. Voltammetric and spectroelectrochemical measurements show that upon one-electron reduction both guests are released from the cavity of 1. Owing to the chemical reversibility of such redox processes, the supramolecular complexes can be re-assembled upon removal of the extra electron from the guest. Systems of this kind are promising for the construction of switchable nanoscale devices and self-assembling supramolecular materials, the structure and properties of which can be reversibly controlled by electrochemical stimuli.     

Dependency of the Dimethyldihydropyrene Photochromic Properties on the Number of Pyridinium Electron-Withdrawing Groups

Photochromic dimethyldihydropyrenes substituted with electron-withdrawing pyridinium groups have shown an increase of photo-induced ring-opening efficiency and a light sensitivity that is red shifted relative to the unsubstituted compound. However, a recently synthesized tetrapyridinium derivative showed a considerable decrease of the photo-isomerization quantum yield relative to the monopyridinium and bispyridinium derivatives. We provide a rationale for this unexpected photochemical behavior based on the comparative theoretical investigations of the relevant excited states of these systems. In particular, we found that the nature and order of the lowest two excited states depend on the number of pyridinium groups and on the symmetry of the system. While the lowest S₁ excited state is photo-active in the monopyridinium and bispyridinium derivatives, the photo-isomerizing state is S₂ in the reference unsubstituted compound and both S₁ and S₂ lead to isomerization in the tetrapyridinium derivative, albeit with a low efficiency. In the latter derivative, the photo-isomerization is hindered by the particular S₁/S₂ conical intersection topology.     

Parking and restarting a molecular shuttle in situ

Herein we report an easy-to-synthesize [2]rotaxane, which incorporates two ionic monopyridinium stations and one 2,2'-bipyridine station as the shaft of the dumbbell-shaped component and a bis-p-xylyl[26]crown-6 (BPX26C6) unit as the macrocyclic component. In this molecular shuttle, the BPX26C6 unit can be docked selectively on either the central 2,2'-bipyridine station or one of the two terminal pyridinium stations, and subsequently, returned to its shuttling molecular motion through the in situ addition of simple reagents (acid/base or metal ion/metal-ion-complexing ligand pairs).     

Salts of 2,5-dimercapto-1,3,4-thiadiazole

Reaction of 2,5-dimercapto-1,3,4-thiadiazole with ammonia or pyridine gives monoammonium or monopyridinium slats, and the reaction with hydrazine hydrate gives both mono- and dihydrazine salts, which was confirmed by alkylation of the salts obtained. Difference in the chemical shifts of the SCH₂R groups was found in the¹H NMR spectra of the mono- and dialkyl-substituted 2,5-dimercapto-1,3,4-thiadiazoles.Institute of Chemistry, Vilnius LT-2600, LithuaniaTranslated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 682–687, May, 2000.     

Long-range ordering in the Bi1−xAexFeO3−x/2 perovskites: Bi1/3Sr2/3FeO2.67 and Bi1/2Ca1/2FeO2.75

Two-ordered perovskites, Bi_1/3Sr_2/3FeO_2.67 and Bi_1/2Ca_1/2FeO_2.75, have been stabilized and characterized by transmission electron microscopy, Mössbauer spectroscopy and X-ray powder diffraction techniques. They both exhibit orthorhombic superstructures, one with a≈b≈2a_p and c≈3a_p (S.G.: Pb2n or Pbmn) for the Sr-based compound and one with a≈b≈2a_p and c≈8a_p (S.G.: B222, Bmm2, B2mm or Bmmm) for the Ca-based one. The high-resolution transmission electron microscopy (HRTEM) images evidence the existence of one deficient [FeO_x]_∞ layer, suggesting that Bi_1/3Sr_2/3FeO_2.67 and Bi_1/2Ca_1/2FeO_2.75 behave differently compared to their Ln-based homolog. The HAADF-STEM images allow to propose a model of cation ordering on the A sites of the perovskite. The Mössbauer analyses confirm the trivalent state of iron and its complex environment with three types of coordination. Both compounds exhibit a high value of resistivity and the inverse molar susceptibility versus temperature curves evidence a magnetic transition at about 730 K for the Bi_1/3Sr_2/3FeO_2.67 and a smooth reversible transition between 590 and 650 K for Bi_1/2Ca_1/2FeO_2.75.     

Ordered perovskites in the A(Li1/4Nb3/4)O3-A(Li2/5W3/5)O3 (A=Sr, Ca) systems

Single-phase 1:2 B-site ordered perovskites are formed in the (1−x)A²⁺(Li_1/4Nb_3/4)O₃-(x)A²⁺(Li_2/5W_3/5)O₃ systems, A²⁺=Sr and Ca, within the range 0.238?x?0.333. The X-ray and electron diffraction patterns are consistent with a P2₁/c monoclinic supercell, , , , β≈125°, where the 1:2 order is combined with b⁻b⁻c⁺ octahedral tilting. Rietveld refinements of the ordered A(B^I_1/3B^II_2/3)O₃ structures give a good fit to a model with B^I occupied by Li and Nb, B^II by W and Nb, and a general stoichiometry (Sr,Ca)(Li_3/4+y/2Nb_1/4−y/2)_1/3(Nb_1−yW_y)_2/3O₃, y=0.9x=0.21-0.30. The Sr system also includes regions of stability of a 1:3 ordered phase for 0.0?x?0.111, and a 1:1 ordered double perovskite for 0.833?x?1.0. The formation of the non-stoichiometric 1:2 ordered phases is associated with the large site charge/size differences that can be accessed in these systems, and restricted by local charge imbalances at the A-sites for W-rich compositions. These concepts are used to generate stability maps to rationalize the formation of the known 1:2 ordered oxide perovskites.     

Structural characterisation of the perovskite series SrxCa1−x−yNdyMnO3: Influence of the Jahn-Teller effect

The crystal structures of the perovskite manganites Sr_xCa_1−x−yNd_yMnO₃ with y=0.1 or 0.2 have been investigated using synchrotron X-ray powder diffraction. At room temperature the structures change from depending on the cation distribution, the different structures exhibiting different tilts of the MnO₆ octahedra. High temperature diffraction measurements demonstrate the presence of, an apparently continuous, isosymmetric I4/mcm to I4/mcm phase transition associated with the removal of long range orbital ordering. Heating the manganites to still higher temperatures results in a continuous transition to the cubic structure. A feature of such transitions is the continuous evolution of the octahedral tilt angle through the I4/mcm to I4/mcm phase transition. The orthorhombic structures do not exhibit orbital ordering and although a first order transition to the tetragonal structure is observed in Sr_0.4Ca_0.5Nd_0.1MnO₃, this high temperature tetragonal structure does not exhibit orbital ordering.     

High Lithium Capacity MxV2O5Ay·nH2O for Rechargeable Batteries

The aqueous synthesis and electrochemical properties of nanocrystalline M_xV₂O₅A_y·nH₂O are described. It is easily and quickly prepared by precipitation from acidified vanadate solutions. M_xV₂O₅A_y·nH₂O has been characterized by X-ray powder diffraction, electron microscopy, TGA, chemical analyses, and electrochemical studies. The atomic structure is related to that of xerogel-derived V₂O₅·nH₂O. In M_xV₂O₅A_y·nH₂O, M is a cation from the starting vanadate salt and A is an anion from the mineral acid. This material exhibits high, reversible Li capacity and may be considered for use in a cathode in primary and secondary batteries. The lithium capacity of an electrode composed of M_xV₂O₅A_y·nH₂O/EPDM/carbon (88/4/8) is ∼380(mA h)/g (C/80 rate) and the energy density is ∼1000(W h)/kg (120-μm-thick cathode, 4-1.5 V, versus Li metal anode). Critical parameters identified in the synthesis of M_xV₂O₅A_y·nH₂O, with respect to achieving high Li-ion insertion capacity, are acid/vanadium ratio, starting vanadate salt, and temperature. Inclusion of carbon black in the synthesis yields a composite that maintains the high Li capacity, lowers the electrochemical-cell polarization, and preserves the lithium capacity at higher discharge rates. Li-ion coin cells, using pre-lithiated graphite anodes, exhibit electrochemical performance comparable to that of Li-metal coin cells.     

Superstructure of hollandite KxMg(8+x)/3Sb(16−x)/3O16 (x≈1.76)

Single crystals of K_xMg_(8+x)/3Sb_(16−x)/3O₁₆ (x≈1.76) with a hollandite superstructure were grown. Ordering schemes for guest ions (K) and the host structure were confirmed by the structure refinement using X-ray diffraction intensities. The space group is I4/m and cell parameters are a=10.3256(6), c=9.2526(17)Å with Z=3. Superlattice formation is primarily attributed to the Mg/Sb occupational modulation in the host structure. Mg/Sb ratios at two nonequivalent metal sites are 0.8977/0.1023 and 0.1612/0.8388. Two types of the cavity are seen in the tunnel, where parts of K ions deviate from the cavity center along the tunnel direction. Probability densities for K ions in the two cavities are different from each other, which seems to have arisen from the Mg/Sb modulation.     

Visible light-sensitive yellow TiO2−xNx and Fe-N co-doped Ti1−yFeyO2−xNx anatase photocatalysts

Nitrogen substituted yellow colored anatase TiO_2−xN_x and Fe-N co-doped Ti_1−yFe_yO_2−xN_x have been easily synthesized by novel hydrazine method. White anatase TiO_2−δ and N/Fe-N-doped samples are semiconducting and the presence of ESR signals at g ∼1.994-2.0025 supports the oxygen vacancy and g∼4.3 indicates Fe³⁺ in the lattice. TiO_2−xN_x has higher conductivity than TiO_2−x and Fe/Fe-N-doped anatase and the UV absorption edge of white TiO_2−x extends in the visible region in N, Fe and Fe-N co-doped TiO₂, which show, respectively, two band gaps at ∼3.25/2.63, ∼3.31/2.44 and 2.8/2.44 eV. An activation energy of ∼1.8 eV is observed in Arrhenius log resistivity vs. 1/T plots for all samples. All TiO₂ and Fe-doped TiO₂ show low 2-propanol photodegradation activity but have significant NO photodestruction capability, both in UV and visible regions, while standard Degussa P-25 is incapable in destroying NO in the visible region The mid-gap levels that these N and Fe-N-doped TiO₂ consist may cause this discrepancy in their photocatalytic activities.     

Ag1/8Pr5/8MoO4: An incommensurately modulated scheelite-type structure

The crystal structure of a complex molybdenum oxide Ag_1/8Pr_5/8MoO₄ is reported. The Ag_1/8Pr_5/8MoO₄ sub-structure can be described on the base of the scheelite (CaWO₄) structure. Transmission electron microscopy reveals that the real structure is better described in a (3+1)D formalism. According to electron diffraction study the new scheelite-type complex oxide Ag_1/8Pr_5/8MoO₄ crystallizes in the B2/b(αβ0)00 (3+1)D superspace group with unit cell parameters , , and γ≈135° (Z=4) and modulation vector q=0.56a^*+0.59b^*. The structure of Ag_1/8Pr_5/8MoO₄ is refined from X-ray powder data in the scheelite setting I2/b(αβ0) with , , q=1.14690(14)a^*+0.58921(12)b^* with fixed γ=90° angle (R_p=0.033, R=0.033, R_m=0.029, R₁=0.047, S=1.36). Displacement modulations apply for all atoms. The occupancy modulation shows that one-fourth of the Ag/Pr atoms are absent. The structure can be considered as a crystallographic shear structure with incommensurate ordering of vacancies and displacement modulations for all atoms. The arrangement of Ag/Pr atoms and vacancies is at the origin of the incommensurate modulation in the cation-deficient Ag_1/8Pr_5/8MoO₄ phase.     

REAuAl4Ge2 and REAuAl4(AuxGe1−x)2 (RE=rare earth element): Quaternary intermetallics grown in liquid aluminum

The two families of intermetallic phases REAuAl₄Ge₂ (1) (RE=Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Tm and Yb) and REAuAl₄(Au_xGe_1−x)₂ (2) (x=0.4) (RE=Ce and Eu) were obtained by the reactive combination of RE, Au and Ge in liquid aluminum. The structure of (1) adopts the space group R-3m (CeAuAl₄Ge₂, , ; NdAuAl₄Ge₂, , ; GdAuAl₄Ge₂, , ; ErAuAl₄Ge₂, , ). The structure of (2) adopts the tetragonal space group P4/mmm with lattice parameters: , for EuAuAl₄(Au_xGe_1−x)₂ (x=0.4). Both structure types present slabs of “AuAl₄Ge₂” or “AuAl₄(Au_xGe_1−x)₂” stacking along the c-axis with layers of RE atoms in between. Magnetic susceptibility measurements indicate that the RE atoms (except for Ce and Eu) possess magnetic moments consistent with +3 species. The Ce atoms in CeAuAl₄Ge₂ and CeAuAl₄(Au_xGe_1−x)₂ (x=0.4) appear to be in a mixed +3/+4 valence state; DyAuAl₄Ge₂ undergoes an antiferromagnetic transition at 11 K and below this temperature exhibits metamagnetic behavior. The Eu atoms in EuAuAl₄(Au_xGe_1−x)₂ (x=0.4) appear to be in a 2+ oxidation state.     

Crystal Structure of Pseudorhombohedral InFe1−xTixO3+x/2 (x=2/3)

The structure of pseudorhombohedral-type InFe_1−xTi_xO_3−x/2 (x=2/3) was refined by Rietveld profile fitting. The crystal is a commensurate member of a series in a solution range on InFeO₃-In₂Ti₂O₇ including incommensurate structures. The structure with the unit cell of a=5.9188(1), b=10.1112(2), and c=6.3896(1) Å, β=108.018(2)°, and a space group P2₁/a is the alternate stacking of an edge-shared InO₆ octahedral layer and an Fe/Ti-O plane along c^*. Metal sites on the Fe/Ti-O plane are surrounded by four oxygen atoms on the Fe/Ti-O plane and two axial ones. Electric conductivities of the order 10⁻⁴ S/cm were observed for the samples at 1000 K, while the oxide ion transport number is almost zero as no electromotive force was detected by an oxygen concentration cell.     

Non-stoichiometric 1:2 ordered perovskites in the Ba(Li1/4Nb3/4)O3-Ba(Li2/5W3/5)O3 system

Although both end members in the (1−x)Ba(Li_1/4Nb_3/4)O₃-xBa(Li_2/5W_3/5)O₃ (BLNW) system adopt a hexagonal perovskite structure, B-site ordered cubic perovskites are formed for the majority of their solid solutions (0.238?x?0.833). Within this range, single-phase 1:2 order (, , ) is stabilized for 0.238?x?0.385. In contrast to all known A(B_1/3^IB_2/3^II)O₃ perovskites, the 1:2 ordered BLNW solid solutions do not include any composition with a 1:2 cation distribution and the structure exhibits extensive non-stoichiometry. Structure refinements support a model where Li and W occupy different positions and Nb is distributed on both sites, i.e. Ba[(Li_3/4+y/2Nb_1/4−y/2)_1/3(Nb_1−yW_y)_2/3]O₃ (y=0.21-0.35, where y=0.9x). The stabilization of the non-stoichiometric order arises from the large charge/size site differences; the loss of 1:2 order for W-rich compositions is related to local charge imbalances on the A-site sub-lattice. The range of single-phase 1:1 order is confined to x=0.833, (Ba(Li_3/4Nb_1/4)_1/2(W)_1/2)O₃), where the site charge/size difference is maximized and the on-site mismatches are minimized. The microwave dielectric loss properties of the ordered BLNW solid solutions are significantly inferior as compared to their stoichiometric counterparts.     

Phase transitions in A-site substituted perovskite compounds: The (Ca1-2xNaxLax)TiO3 (0?x?0.5) solid solution

The (Ca_1-2xNa_xLa_x)TiO₃ (0?x?0.5) A-site substituted perovskite compounds have been synthesized and characterized by XRD and Raman spectroscopy at room temperature. The XRD powder diffraction study suggests that the end-member Na_1/2La_1/2TiO₃ crystallizes in the tetragonal space group I4/mcm. The phase transition from Pbnm to I4/mcm is located between x=0.34 and 0.39 and is driven by the variation of ionic radii at the A-site. The observed Raman modes are in agreement with group theory analysis, and the relationships between the behavior of structural parameters (e.g. Ti-O-Ti bond angle), indicated by long-range order, and the corresponding Raman frequency shifts and intensity evolution, indicated by short-range order, are established and discussed in terms of the radius effect and the mass effect.     

Synthesis of LiNi1/3Co1/3Mn1/3O2−zFz cathode material from oxalate precursors for lithium ion battery

The layered LiNi_1/3Co_1/3Mn_1/3O_2−zF_z (0 ≤ z ≤ 0.12) cathode materials were synthesized from oxalate precursors by a simple self-propagating solid-state metathesis method with the help of the ball milling and the following calcination. Li(Ac)·2H₂O, Ni(Ac)₂·4H₂O, Co(Ac)₂·4H₂O, Mn(Ac)₂·4H₂O(Ac = acetate), LiF and excess H₂C₂O₄·2H₂O were used as starting materials without any solvent. The structural and electrochemical properties of the prepared LiNi_1/3Co_1/3Mn_1/3O_2−zF_z were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and electrochemical measurements, respectively. The XRD patterns indicate that all samples have a typical hexagonal structure with a space group of . The FESEM images show that the primary particle size of LiNi_1/3Co_1/3Mn_1/3O_2−zF_z gradually increases with increasing fluorine content. Though the fluorine-substituted LiNi_1/3Co_1/3Mn_1/3O_2−zF_z have lower initial discharge capacities, a small amount of fluorine-substituted LiNi_1/3Co_1/3Mn_1/3O_2−zF_z (z = 0.04 and 0.08) exhibit excellent cycling stability and rate capability compared to fluorine-free LiNi_1/3Co_1/3Mn_1/3O₂.     

Syntheses and structures of three new scandium selenites: Sc2(SeO3)3·H2O, Sc2 (SeO3)3·3H2O and CsSc3(SeO3)4 (HSeO3)2·2H2O

Three new hydrated scandium selenites have been hydrothermally synthesized as single crystals and structurally and physically characterized. Sc₂(SeO₃)₃·H₂O crystallizes as a new structure type containing novel ScO₇ pentagonal bipyramidal and ScO₆₊₁ capped octahedral coordination polyhedra. Sc₂(SeO₃)₃·3H₂O contains typical ScO₆ octahedra and is isostructural with its M₂(SeO₃)₃·3H₂O (M=Al, Cr, Fe, Ga) congeners. CsSc₃(SeO₃)₄(HSeO₃)₂·2H₂O contains near-regular ScO₆ octahedra and has essentially the same structure as its indium-containing analogue. All three phases contain the expected pyramidal [SeO₃]^2- selenite groups. Crystal data: Sc₂(SeO₃)₃·3H₂O, M_r=524.85, trigonal, R3c (No. 161), , , , Z=6, R(F)=0.018, wR(F²)=0.036; Sc₂(SeO₃)₃·H₂O, M_r=488.82, orthorhombic, P2₁2₁2₁ (No. 19), , , , , Z=4, R(F)=0.051, wR(F²)=0.086; CsSc₃(SeO₃)₄(HSeO₃)₂·2H₂O, M_r=1067.60, orthorhombic, Pnma (No. 62), , , , , Z=4, R(F)=0.035, wR(F²)=0.070.