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.     

Novel copolymers of styrene. 3. Some ring-substituted propyl 2-cyano-3-phenyl-2-propenoates

Novel trisubstituted ethylenes, ring-substituted propyl 2-cyano-3-phenyl-2-propenoates, RPhCH?C(CN)CO₂C₃H₇ (where R is 2-C₆H₅CH₂O, 3-C₆H₅CH₂O, 4-C₆H₅CH₂O, 4-CH₃COO, 3-CH₃CO, 4-CH₃CONH, 2-CN, 3-CN, 4-CN, 4-(CH₃)₂N, 4-(C₂H₅)₂N) were prepared and copolymerized with styrene. The monomers were synthesized by the piperidine catalyzed Knoevenagel condensation of ring-substituted benzaldehydes and propyl cyanoacetate, and characterized by CHN analysis, IR, ¹H and ¹³C-NMR. All the ethylenes were copolymerized with styrene (M₁) in solution with radical initiation (ABCN) at 70°C. The compositions of the copolymers were calculated from nitrogen analysis and the structures were analyzed by IR, ¹H and ¹³C-NMR. Decomposition of the copolymers in nitrogen occurred in two steps, first in the 200–500°C range with residue (2.7–8.6% wt.), which then decomposed in the 500–800°C range.     

Precipitation of silver/palladium alloy platelets from homogeneous solutions

Dispersed silver/palladium (Ag/Pd) nanoplatelets were prepared by delivering in parallel solutions of mixed metal nitrates and L-ascorbic acid into a nitric acid solution containing Arabic gum. The shape and size of bimetallic nanoparticles varied with the silver/palladium weight ratio and the concentration of nitric acid. The optimum conditions for platelets formation were a palladium content of ~2.0 wt.% and nitric acid concentrations above 1.0 mol dm(-3). The data presented show that both parameters play a critical role in the nucleation and growth of AgPd particles. A mechanism explaining the formation of the bimetallic nanoplatelets is proposed.     

Ring-borylated 15-electron and 17-electron ansa-chromocene complexes, their physical properties and molecular structures

A detailed study of the thermal decomposition of the zwitterionic, ring-borylated ansa-chromocene hydrido carbonyl complex [Cr(CO)H{Me(4)C(2)(C(5)H(4))[C(5)H(3)B(C(6)F(5))(3)]}] (2) is described. This complex is formed in the reaction between [Cr(CO){Me(4)C(2)(C(5)H(4))(2)}] (1) and B(C(6)F(5))(3) in toluene at -78 degrees C. Above -25 degrees C, 2 decomposes to a 50:50 mixture of the low-spin, 17e Cr(III) complexes [Cr(CO){Me(4)C(2)(C(5)H(4))[C(5)H(3)B(C(6)F(5))(3)]}] (3b) and [Cr(CO){Me(4)C(2)(C(5)H(4))(2)}][HB(C(6)F(5))(3)] (4). Carbon monoxide elimination from 3 b generates high-spin, 15 e [Cr{Me(4)C(2)(C(5)H(4))[C(5)H(3)B(C(6)F(5))(3)]}] (3a), which coordinates two other electron-donating ligands, such as xylyl isocyanide, PMe3, and PPh(2)Me to form the low-spin, 17 e electron complexes 3c, 3d, and 3e, respectively. High-spin, 15 e [Cr{Me(4)C(2)(C(5)H(4))(2)}][HB(C(6)F(5))(3)] (5) is generated by heating 3 b in toluene at 100 degrees C and periodically removing the evolved CO. Efforts to isolate more than a few X-ray quality crystals of 5 were thwarted by its tendency to form an insoluble precipitate (6) with the same molecular formula. Heating the solution of 5 at 120 degrees C results in its partial conversion (ca. 28 %) to 3a, thereby allowing the formation of 3a in yields as high as 74 % from the reaction between 1 and B(C(6)F(5))(3). The X-ray crystal structures of 3 b-e and 5 are described. Cyclic voltammetry measurements on 3 a-e reveal a dramatic reduction in the redox potentials of the complexes relative to their non-borylated analogues. DFT calculations show that this is due primarily to electrostatic stabilization of the oxidized species by the negatively charged borylate group. EPR and 19F NMR spectroscopy allow 3a to be distinguished from its Lewis base adducts 3 b-e and reveal the relative affinities of different Lewis bases for the chromium.     

The nature of halogen...halide synthons: theoretical and crystallographic studies

Two types of halogen...halide synthons are investigated on the basis of theoretical and crystallographic studies; the simple halogen...halide synthons and the charge assisted halogen...halide synthons. The former interactions were investigated theoretically (ab initio) by studying the energy of interaction of a halide anion with a halocarbon species as a function of Y...X- separation distance and the C-Y...X- angle in a series of complexes (R-Y...X-, R=methyl, phenyl, acetyl or pyridyl; Y=F, Cl, Br, or I; X-=F-, Cl-, Br-, or I-). The theoretical study of the latter interaction type was investigated in only one system, the [(4BP)Cl]2 dimer, (4BP=4-bromopyrdinium cation). Crystal structure determinations, to complement the latter theoretical calculations, were performed on 13 n-chloropyridinium and n-bromopyridinium halide salts (n=2-4). The theoretical and crystallographic studies indicate that these interactions are controlled by electrostatics and are characterized by linear C-Y...X- angles and separation distances less than the sum of van der Waals radius (rvdW) of the halogen atom and the ionic radii of the halide anion. The strength of these contacts from calculations varies from weak or absent, e.g., H3C-Cl...I-, to very strong, e.g., HCC-I...F- (energy of interaction ca. -153 kJ/mol). The strengths of these contacts are influenced by four factors: (a) the type of the halide anion; (b) the type of the halogen atom; (c) the hybridization of the ipso carbon; (d) the nature of the functional groups. The calculations also show that charge assisted halogen...halide synthons have a comparable strength to simple halogen...halide synthons. The nature of these contacts is explained on the basis of an electrostatic model.     

Identification of diastereomeric chlorophyll allomers by atmospheric pressure chemical ionisation liquid chromatography/tandem mass spectrometry

Atmospheric pressure chemical ionisation liquid chromatography/mass spectrometry (APCI-LC/MS) has been used for identification of the epimers of hydroxy, methoxy and methoxylactone allomers of chlorophyll a (13(2)-HO-chl a, 13(2)-MeO-chl a and 15(1)-MeO-lact-chl a), the hydroxy allomer of bacteriochlorophyll a (13(2)-HO-bchl a) and the hydroxy and methoxylactone allomers of bacterioviridin a (13(2)-HO-bvir a and 15(1)-MeO-lact-bvir a). The APCI mass spectra show that facile fragmentations involve the methoxyl or hydroxyl groups at the C-13(2) or C-15(1) chiral centres. Losses involving the C-13(2) or C-15(1) hydroxyl or methoxyl groups occur more easily from the S-epimer than from the R-epimer due to the greater relief of the steric strain associated with interaction with the bulky C-17 substituent. The differences in mass spectrometric fragmentation can be used as a diagnostic tool for the assignment of the stereochemical configuration at the C-13(2) or C-15(1) chiral centres.     

Ionic conductivity and thermal expansion of anion-deficient Sr11Mo4O23 perovskite

Transport properties of perovskite-type Sr₁₁Mo₄O₂₃ and composite Sr₁₁Mo₄O₂₃ - 1 wt% Al₂O₃ were studied at 400–1300 K in the oxygen partial pressure range from 0.21 down to 10⁻¹⁹ atm. The electromotive force and faradaic efficiency measurements, in combination with the energy-dispersive spectroscopy of the fractured electrochemical cells, unambiguously showed prevailing role of the oxygen ionic conductivity under oxidizing conditions. At temperatures above 600 K, protonic and cationic transport can be neglected. The oxygen ion transference numbers vary in the range of 0.95–1.00 at 973–1223 K. At temperatures lower than 550 K, the total conductivity of Sr₁₁Mo₄O₂₃ - 1 wt% Al₂O₃ composite measured by impedance spectroscopy tends to increase in wet atmospheres, thus indicating that hydration and protonic transport become significant. Reducing oxygen partial pressure below 10⁻¹⁰–10⁻⁹ atm leads to a significant increase in the n-type electronic conduction. The average thermal expansion coefficients in oxidizing atmospheres are (14.3–15.0) × 10⁻⁶ K⁻¹ at 340–740 K and (18.3–19.2) × 10⁻⁶ K⁻¹ at 870–1370 K.

     

Acetic acid induced self-assembly of supramolecular compounds [Et4N]3[(WS4Cu2)2(mu-CN)3].2MeCN and [PPh4][WS4Cu3(mu-CN)2].MeCN from preformed clusters [A]2[WS4(CuCN)2] (A = Et4N, PPh4)

Reactions of two preformed trinuclear W/Cu/S clusters, [A](2)[WS(4)(CuCN)(2)] (1: A = Et(4)N; 2: A = PPh(4)), with different concentrations of acetic acid in MeCN generate two interesting 2D polymeric clusters [Et(4)N](3)[(WS(4)Cu(2))(2)(mu-CN)(3)].2MeCN (3), and [PPh(4)][WS(4)Cu(3)(mu-CN)(2)].MeCN (4), respectively. Compound 4 can also be readily obtained in a high yield from the reaction of 2 with equimolar [Cu(MeCN)(4)]PF(6) in MeCN. These compounds have been characterized by elemental analysis, IR spectra, thermal analysis, and single-crystal X-ray diffraction. An X-ray analysis reveals that compound 3 retains the WS(4)Cu(2) cluster core, which serves as a 3-connecting node to link equivalent nodes via single cyanide bridges, forming an anionic 2D (6,3) net. Compound 4 consists of a T-shaped WS(4)Cu(3) core, which also acts as a 3-connecting node, with links to 3 equivalent clusters either through single or double cyanide bridges, affording a different anionic 2D (6,3) network. The acetic acid induced aggregation of 3 and 4 from the two cluster precursors 1 and 2 suggests that this simple synthetic strategy is likely to be applicable to many related systems.     

Iridium(III) Complexes Bearing Pyrene‐Functionalized 1,10‐Phenanthroline Ligands as Highly Efficient Sensitizers for Triplet–Triplet Annihilation Upconversion

“Chemistry‐on‐the‐complex” synthetic methods have allowed the selective addition of 1‐ethynylpyrene appendages to the 3‐, 5‐, 3,8‐ and 5,6‐positions of Ir^III‐coordinated 1,10‐phenanthroline via Sonogashira cross‐coupling. The resulting suite of complexes has given rise to the first rationalization of their absorption and emission properties as a function of the number and position of the pyrene moieties. Strong absorption in the visible region (e.g. 3,8‐substituted Ir‐3 : λ_abs=481 nm, ?=52 400 m ^?1 cm^?1) and long‐lived triplet excited states (e.g. 5‐substituted Ir‐2 : τ_T=367.7 μs) were observed for the complexes in deaerated CH₂Cl₂. On testing the series as triplet sensitizers for triplet–triplet annihilation upconversion, those Ir^III complexes bearing pyrenyl appendages at the 3‐ and 3,8‐positions ( Ir‐1 , Ir‐3 ) were found to give optimal upconversion quantum yields (30.2 % and 31.6 % respectively).