Synthesis of dipyrrolo[2,3-a:1′,2′,3′-fg]acridin-12(1H)-ones

Acylation reactions of 4,6-dimethoxyindoles with glyoxyloyl chlorides were achieved by the use of graphite powder in 1,2-dichloroethane at reflux. The products were monoketones as a result of decarbonylation, rather than the expected 1,2-diketones. Treatment of these monoketones with base led to their cyclisation and elimination of methanol to afford the novel dipyrrolo[2.3-a:1′,2′,3′-fg]acridin-12(1H)-ones.     

Synthesis of new 10,11-dihydrodibenzo[a,d]cycloheptene S-thiocarbamate derivatives via a benzylic Newman–Kwart rearrangement

New S-thiocarbamates possessing a 10,11-dihydro-5H-dibenzo[a,d]cycloheptene moiety were obtained unexpectedly in the process of preparing new O-thiocarbamates starting from 5-dibenzosuberol (10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-ol) and aroyl-isothiocyanates. These compounds were obtained via a benzylic Newman–Kwart rearrangement mechanism. The structures of the new thiocarbamates were confirmed by elemental analysis, IR, and NMR spectroscopy and by X-ray crystallography.     

Improved synthesis of (S)-7-amino-5H,7H-dibenzo[b,d]azepin-6-one, a building block for γ-secretase inhibitors

An improved synthesis of (S)-7-amino-5H,7H-dibenzo[b,d]azepin-6-one (1), involving a selective crystallization of epimeric menthylcarbamates for the resolution step followed by simultaneous cleavage of the carbamate and the lactam protecting group is described. Epimerization conditions of the undesired epimer have also been determined.     

Ru-catalyzed direct C–H amidation of 2-arylbenzo[d]thiazoles with sulfonyl azides

Ru-catalyzed direct C–H amidation of 2-arylbenzo[d]thiazoles was developed using sulfonyl azides as the amino source under no external oxidants and free-base conditions to release N₂ gas as the single byproduct. The present protocol shows good functional group tolerance and high regioselectivity, providing various structurally versatile amidated 2-arylbenzothiazoles with potential biological and therapeutic activities in moderate to good yields.     

Coordination of α,β-unsaturated aldehydes on d Ru and Rh complexes: A DFT study

The coordination modes of various α,β-unsaturated aldehydes on d⁶ ML₄ and ML₅ complexes of rhodium and ruthenium have been compared by means of density functional theory (DFT) calculations. The studied aldehydes were acrolein, crotonaldehyde, prenal and cinnamaldehyde and the metallic fragments RuH₂(PH₃)₂, RuH₂(PH₃)₃ and RhH₂Cl(PH₃)₂. On the d⁶ ML₄ Ru fragment, the best coordination geometry is η⁴, where both double bonds are involved. On the d⁶ ML₅ Ru fragment, the η²CC geometry is preferred to the η¹O and to the η²CO ones. Finally on the d⁶ ML₅ Rh fragment, the η²CO mode is not stable and only η¹O and η²CC exist, the former being favored. In conclusion, the η²CO coordination geometry is never favored even if the energy gap between η²CC and η²CO is reduced by the use of bulky ligands (PPh₃) or alkyl substituents on the CC bond. The Ru and Rh complexes behave differently: in the case of Ru, η²CC and η²CO can be in competition and in the case of Rh, the best form is η¹O. This different behavior can explain the results obtained in the hydrogenation reaction of α,β-unsaturated aldehydes.     

Sol-gel synthesis, structural and superconducting properties of (Hg1−ySey)Sr2(Y1−xCax)Cu2O6+δ

A new series of Sr-based Hg-1212 superconducting cuprate (Hg_1−ySe_y)Sr₂(Y_1−xCa_x)Cu₂O_6+δ (y=0.25; 0.0?x?0.7) have been successfully synthesized using a highly homogenous and reactive precursor Sr₂(Y_1−xCa_x)Cu₂O_z prepared by the citrate sol-gel process. This chemical method is fast, cheap, reproducible and more efficient than the traditional solid-state reaction method. X-ray diffraction (XRD) and Energy dispersive X-ray analyses (EDX) studies have shown that Se is required for stabilization of the Sr-based Hg-1212 phase (Hg_1−ySe_y)Sr₂YCu₂O_6+δ; y≈0.25. On the other hand, electrical resistivity and magnetic susceptibility measurements indicated that substitution of Y by Ca is necessary to induce superconductivity in the 1212 (Hg_0.75Se_0.25)Sr₂(Y_1−xCa_x)Cu₂O_6+δ samples. Superconductivity is observed only for samples with x?0.3 and T_c increases with increasing Ca content as well as O₂-annealing. A maximum T_c(onset) of 85 K is found in the (Hg_0.75Se_0.25)Sr₂(Y_0.3Ca_0.7)Cu₂O_6.84 sample annealed in an oxygen atmosphere. The structure of O₂-annealed samples was investigated by the Rietveld refinement. For all samples, it was found that Se substitutes at the Hg site. Each Se atom is surrounded by four oxygen atoms O(3), but these are not at the ideal site. Rather, these oxygen atoms are shifted along the [110] direction ((0.3989, 0.3989, 0) in the case of x=0.5), implying a four-fold split site with an occupancy of 0.22(2) for each of them.     

Synthesis and characterization of La0.8Sr1.2Co0.5M0.5O4−δ (M=Fe, Mn)

The M⁴⁺-containing K₂NiF₄-type phases La_0.8Sr_1.2Co_0.5Fe_0.5O₄ and La_0.8Sr_1.2Co_0.5Mn_0.5O₄ have been synthesized by a sol-gel procedure and characterized by X-ray powder diffraction, thermal analysis, neutron powder diffraction and Mössbauer spectroscopy. Oxide ion vacancies are created in these materials via reduction of M⁴⁺ to M³⁺ and of Co³⁺ to Co²⁺. The vacancies are confined to the equatorial planes of the K₂NiF₄-type structure. A partial reduction of Mn³⁺ to Mn²⁺ also occurs to achieve the oxygen stoichiometry in La_0.8Sr_1.2Co_0.5Mn_0.5O_3.6. La_0.8Sr_1.2Co_0.5Fe_0.5O_3.65 contains Co²⁺ and Fe³⁺ ions which interact antiferromagnetically and result in noncollinear magnetic order consistent with the tetragonal symmetry. Competing ferromagnetic and antiferromagnetic interactions in La_0.8Sr_1.2Co_0.5Fe_0.5O₄, La_0.8Sr_1.2Co_0.5Mn_0.5O₄ and La_0.8Sr_1.2Co_0.5Mn_0.5O_3.6 induce spin glass properties in these phases.     

Novel synthesis of (d,l)-cis-chrysanthemic acid involving α,α′-dibromination of 2,2,5,5-tetramethylcyclohexane-1,3-dione: application to the enantioselective synthesis of (1R)-cis-chrysanthemic acid

cis-Chrysanthemic acid has been prepared in a few steps from dimethyldimedone via dibromination at alpha positions of each carbonyl carbons. The trans-dibromide which is almost exclusively formed has been isomerized to its cis-stereoisomer by highly chemoselective tandem H/K-K/H exchanges involving potassium bases at low temperature (<−40 °C). Carbocyclization of the potassium enolate intermediate takes place at around −30 °C and provide the bicyclo[3.1.0]-hexane skeleton. Lithiated bases behave differently and mainly lead to Br/M rather than to H/M exchange. We have been unsuccessful in using state of the art enantioselective metallation reactions to achieve the enantioselective synthesis of (1R)-cis-chrysanthemic acid using the disclosed strategy. This therefore still remains challenge.     

Energetics of La1−xAxCrO3−δ perovskites (A=Ca or Sr)

A series of perovskites with the general formula La_1−xA_xCrO_3−δ (A=Ca or Sr) have been synthesized in the solid solution range 0.0<x?0.3 and 0.0?δ?0.5x with a variety of heat treatments. High-temperature drop solution calorimetry in molten 2PbO·B₂O₃ at 1080 K was performed to determine their enthalpies of formation from oxides at room temperature. The enthalpy of oxidation involved in the reaction is roughly independent of oxygen nonstoichiometry (δ) in each series with a given dopant composition, but varies with composition (x). The values change from −620±260 to −280±80 kJ/mol O₂ when x=0.1-0.3 for Ca-doped samples, and from −440±150 to −290±50 kJ/mol O₂ for Sr-doped ones. This dependence of enthalpy of oxidation on composition suggests oxygen vacancies are increasingly short-range ordered in reduced samples. The higher oxidation state of chromium is stabilized by the substitution of alkaline earth ions, but with increasing doping, the enthalpy of formation of the fully oxidized sample in both Ca and Sr-doped systems becomes more endothermic. This destabilization effect is attributed to the large endothermic enthalpy of oxygen vacancy formation (395±30 kJ/mol of ) for the reaction (A=Ca or Sr) that over-rides the exothermic enthalpies of oxidation. At a given composition, Sr-doped LaCrO₃ is more stable than its Ca-doped counterpart, which is consistent with basicity arguments.     

The clathrate Ba8CuxGe46−x−y□y: Phase equilibria and crystal structure

Phase relations at 700 °C, 800 °C and solidus temperatures have been derived for the clathrate system Ba₈Cu_xGe_46−x−y□_y via X-ray single crystal and powder diffractometry combined with electron probe micro analysis and differential thermal analysis. The ternary clathrate phase derives from binary Ba₈Ge₄₃□₃ and extends up to x=6. Structure investigations define cubic primitive symmetry with the space group type consistent with a clathrate type I structure throughout the entire homogeneity region 0<x?6 but defect-free Ba₈Cu_xGe_46−x exists for x?5.5.     

Conditions for superconductivity in the electron-doped copper-oxide system, (Nd1−xCex)2CuO4+δ

We report systematic studies on the relations among the Ce^IV-for-Nd^III substitution level (x), oxygen-partial pressure (P_O2), oxygen content (4+δ), lattice parameters (a, c) and superconductivity characteristics (T_c, volume fraction) in the (Nd_1−xCe_x)₂Cu_1−yO_4+δ system which includes electron-doped superconductors. Independent of the Ce-doping level x, samples synthesized in air are found oxygen deficient, i.e. δ<0. Nevertheless, reductive annealing is needed to induce superconductivity in the air-synthesized samples. At the same time, the amount of oxygen removed upon the annealing is found very small (e.g. 0.004 oxygen atoms per formula unit at x=0.075), and consequently the effect of the annealing on the valence of copper (and thereby also on the electron doping level) is insignificant. Rather, the main function of the reductive annealing is likely to repair the Cu vacancies believed to exist in tiny concentrations (y) in the air-synthesized samples.     

Stability and oxygen-storage characteristics of Al-substituted YBaCo4O7+δ

A series of Al-substituted YBa(Co_1−xAl_x)₄O_7+δ samples was synthesized and characterized with respect to the capability to store large amounts of oxygen at low temperatures (at 200-400 °C) and the phase decomposition upon heating under oxidizing conditions at higher temperatures (above 550 °C). It was revealed that increasing the Al-substitution level up to x≈0.10 boosts up the phase-decomposition temperature from ∼550 to ∼700 °C, while the unique oxygen absorption/desorption characteristics remain nearly the same as those of the pristine YBaCo₄O_7+δ phase. The maximum amount of excess oxygen absorbed by the Al-substituted YBa(Co_1−xAl_x)₄O_7+δ samples was determined to be as large as δ≈1.45 for x=0.10 (in 100 atm O₂ at 320 °C). Isothermal annealing experiments carried out for the same x=0.10 phase at 300 °C revealed that it could be reversibly charged and discharged with 1.2 oxygen atoms per formula unit by switching the gas flow from N₂ to O₂ and vice versa.     

Orthoamides by selective borohydride reduction of N,N′,N″-triacyl- and N,N′,N″-tri(alkoxycarbonyl)-guanidines

N,N′,N″-Triacylguanidines and N,N′,N″-tri(alkoxycarbonyl)guanidines were prepared and reduced with borohydride salts in a mixture of tetrahydrofuran and acetic acid to give triacyl and tri(alkoxycarbonyl) orthoamides in yields of 40–85%. However, similar reduction of N,N′,N″-tri(t-butoxycarbonyl)guanidine did not give orthoamide but the aminal di(t-butyl) methylenedicarbamate.     

Phase and valence transitions in Ba2LnSnxSb1−xO6−δ (Ln=Pr and Tb)

Compounds in the double perovskites series Ba₂LnSn_xSb_1−xO_6−δ (Ln=Pr and Tb) have been synthesised and structurally characterised using synchrotron X-ray and neutron powder diffraction. It was found that the two end-members of the Ba₂PrSn_xSb_1−xO_6−δ series both adopt rhombohedral symmetry but the antimonate is a fully ordered double perovskite while the stannate has no B-site cation ordering. X-ray absorption near-edge structure (XANES) and near-infrared spectroscopy indicate that the Pr cations gradually change oxidation state from Pr³⁺ to Pr⁴⁺ with increasing x and that this is likely to be the cause of the loss of B-site ordering. Similarly, both Ba₂TbSbO₆ and Ba₂TbSnO_6−δ are cubic with B-site ordering present in the former but absent in the latter due to the oxidation state change of the Tb from Tb³⁺ to Tb⁴⁺. Multiple linear regression analysis of the Pr and Tb L_III-edge XANES indicates that the rate of Ln³⁺ transforming to Ln⁴⁺ is such that there are no oxygen vacancies in Ba₂PrSn_xSb_1−xO_6−δ but in Ba₂TbSn_xSb_1−xO_6−δ there is a small amount of oxygen vacancies, with a maximum of δ≈0.05 present.     

Structural characterisation of the Ce1−xLaxNbO4+δ solid solution series: In-situ high-temperature powder diffraction studies

High-resolution neutron powder diffraction has been utilised to investigate the effect of lanthanum substitution on the structure of cerium niobate, CeNbO_4+δ, as a function of temperature. Two members of the Ce_1−xLa_xNbO_4+δ solid solution series, Ce_0.8La_0.2NbO_4+δ and Ce_0.2La_0.8NbO_4+δ, were examined over a temperature range of 293-923 K under a positive pressure of O₂ (500 mbar). From this data it was found that on increasing lanthanum substitution there was an associated reduction in the temperature of the monoclinic-to-tetragonal phase transition. The data also suggested that increasing lanthanum substitution caused an associated decrease in the excess oxygen content. In addition, high-temperature X-ray powder diffraction data recorded in static air were also examined for four compositions of the Ce_1−xLa_xNbO_4+δ series (x=0.2, 0.4, 0.6 and 0.8). These data corroborated the results of the neutron diffraction experiments and also suggested that there was formation of an intermediate phase, analogous to the CeNbO_4.08 phase of the parent material, during the phase transitions of the x=0.8 and 0.6 compositions.     

High-Pressure Synthesis and Crystal Structures of β-MNX (M=Zr, Hf; X=Br, I)

The single crystals of four kinds of metal nitride halides, β-MNX (M=Zr, Hf; X=Br, I), were prepared in Pt (or Au) capsules by the reaction of MN or α-MNX powders with NH₄X as fluxes under a high pressure of 3-5 GPa at 1000-1100°C. Their crystal structures were determined by single-crystal X-ray diffraction techniques. All four compounds crystallize in a rhombohedral space group R-3m, Z=6 with a=3.6403(6) Å, c=29.270(5) Å for β-ZrNBr, a=3.718(2) Å, c=31.381(9) Å for β-ZrNI, a=3.610(1) Å, c=29.294(6) Å for β-HfNBr, and a=3.689(1) Å, c=31.329(6) Å for β-HfNI. β-ZrNBr is isotypic with SmSI and the others are isotypic with YOF. Both structure variants are composed of structural slabs [X-M-N-N-M-X] (M=Zr, Hf; X=Br, I) stacked together by X…X van der Waals forces, but the ways of the layer stacking sequence are different: X_AM_cN_BN_CM_bX_A∣X_CM_bN_AN_BM_aX_C∣X_BM_aN_CN_AM_cX_B∣ in the SmSI-type and X_AM_bN_CN_BM_cX_A∣X_CM_aN_BN_AM_bX_C∣X_BM_cN_AN_CM_aX_B∣ in the YOF-type polymorphs.     

Why is cis/trans stereoinversion with Li+(THF)4 migration across the phenyl ring of α-lithiostyrene accelerated by two ortho-methyl groups?

Common wisdom might anticipate that two methyl groups placed on a molecular migration route should act as an impediment. However, the “conducted tour” migration of Li⁺(THF)₄ across the aryl ring (“π-route”) during the cis/trans stereoinversion of α-arylvinyllithiums had been found to occur with practically equal velocities in the presence of either one or two ortho-alkyl substituents. We now report that the omission of both ortho-methyl groups retards the stereoinversion process. In order to arrive at an answer to the title question, we investigate the aggregation equilibria and microsolvation states of ortho, ortho′-unsubstituted α-lithiostyrenes by means of approved secondary NMR criteria. Beyond such necessary knowledge about the ground-state properties, we provide kinetic evidence showing that the retarded cis/trans stereoinversion of α-lithiostyrene proceeds by the pseudomonomolecular, ionic mechanism with Li⁺(THF)₄ migration.     

Lithium insertion into In2S3 studied by perturbed γ-γ angular correlation

Li_xIn₂S₃ was electrochemically prepared with different small amounts x of lithium (0?x?0.13) in order to maintain the initial crystallographic structure of the thiospinel In₂S₃. About 10¹² radioactive ¹¹¹In ions have been implanted into these samples at 400 keV to perform PAC experiments in the temperature range from 10 to 773 K. The results are compared to previous experiments with undoped In₂S₃ samples. According to the structure of In₂S₃ in the β-phase, which belongs to the I4₁/amd space group having a unit cell with 32 In and 48 S atoms and the cell parameters α=7.61 Å and c=32.28 Å, three different electric field gradients were observed. Within two different temperature ranges dynamical EFGs occur, which are clearly influenced by the insertion of Lithium. The strong dependence of one EFG on the Li concentration x can be correlated to the effective charge of the In ions. This correlation is discussed with respect to XRD analyses of the Li_xIn₂S₃ samples and to XANES measurements on a similar sample.     

Synthesis and characterization of the K2NiF4 phases La1+xSr1−xCo0.5Fe0.5O4−δ (x=0, 0.2)

The K₂NiF₄ phases LaSrCo_0.5Fe_0.5O₄ and La_1.2Sr_0.8Co_0.5Fe_0.5O₄, and their reduced forms LaSrCo_0.5Fe_0.5O_3.75 and La_1.2Sr_0.8Co_0.5Fe_0.5O_3.85, have been successfully prepared by solid-state reactions, followed by reduction in 10% H₂/N₂ in order to produce oxygen-deficient materials. All materials crystallize in a tetragonal K₂NiF₄ structure (space group I4/mmm) with Co and Fe randomly distributed over the B-sites of the structure. Mössbauer spectra have confirmed the trivalent state of Fe in these materials. In the reduced materials, oxide ion vacancies are confined to the equatorial planes of the K₂NiF₄ structure and the Co is present almost entirely as Co²⁺ ions; low-temperature neutron powder diffraction data reveal that these reduced phases are antiferromagnetically ordered with a tetragonal noncollinear arrangement of the moments. The Co³⁺ ions, present in stoichiometric LaSrCo_0.5Fe_0.5O₄ and La_1.2Sr_0.8Co_0.5Fe_0.5O₄, inhibit magnetic order and are assumed to be in the low-spin state.     

The Sr2.75Ce0.25Co2O7−δ oxide, n=2 member of the Ruddlesden-Popper series: Structural and magnetic evolution depending on oxygen stoichiometry

The second member of the Ruddlesden-Popper series, n=2 in Sr_n+1Co_nO_3n+1, has been stabilized by substituting cerium for strontium leading to the pure compound Sr_2.75Ce_0.25Co₂O_7−δ. The oxygen vacancies of this phase can be partially filled by a post-annealing oxidizing treatment with δ decreasing from 1.1 to 0.3 for the as-prepared and oxidized phases, respectively. As the samples are oxidized from δ≈1.1 to 0.3, the a and b unit cell parameters decrease from 3.836 to 3.815 Å and from 20.453 to 20.047 Å, respectively. Despite the average value of the cobalt valence state, V_Co≈+3.5, obtained in the oxidized Sr_2.75Ce⁺⁴_0.25Co₂O_6.7 phase, a clear ferromagnetic state wit T_C=175 K and M_S=0.8 μB/Co is reached.