Insights into Formation Conditions, Crystal Structures, and Thermal Behavior of Hydrous and Anhydrous Barium Arsenates

During systematic phase formation studies in the system BaO/As₂O₅/H₂O, the phases with composition Ba₃(AsO₄)₂·16.3H₂O, Ba₃(AsO₄)₂·17H₂O, Ba­(H₂AsO₄)₂·H₂O, Ba­(H₂AsO₄)₂, Ba₃(HAs₂O₇)₂, Ba₃As₄O₁₃, Ba₂As₂O₇, and Ba₂As₄O₁₂ were isolated and structurally characterized for the first time, using either X-ray powder diffraction data (Ba₃As₄O₁₃) or single crystal X-ray diffraction data (all other phases). From the eight phases investigated, three (Ba­(H₂AsO₄)₂, Ba₃As₄O₁₃, and Ba₂As₂O₇) crystallize in known structure types and the remaining ones in novel structure types. In the crystal structures, the coordination numbers of the Ba²⁺ cations span a range from 8 to 11, and the different arsenate anions are built up from tetrahedral AsO₄ groups, except for Ba₂As₄O₁₂ that contains a novel type of a <i>catena</i>-metaarsenate anion consisting of condensed AsO₄ tetrahedra and AsO₆ octahedra. Another remarkable structural feature is the hydrogendiarsenate anion present in Ba₃(HAs₂O₇)₂, with the longest As–O_bridging distance of a diarsenate group observed so far. Temperature-dependent X-ray diffraction measurements on selected phases show anhydrous Ba₃(AsO₄)₂ to be the only phase stable above 1000 °C

Perrhenate-Catalyzed Deoxydehydration of a Vicinal Diol: A Comparative Density Functional Theory Study

Oxo–rhenium compounds, such as perrhenate salts, have demonstrated preferable activity in catalyzing the deoxydehydration (DODH) reaction in the presence of reductants. Here, the first computational details of the reported DODH mechanisms are presented using the density functional theory (DFT) (M06/6-311+G­(d,p)/LANL2DZ) to investigate the conversion of a vicinal diol into the corresponding alkene by ReO₄^– as a catalyst. The DFT studies were carried out to evaluate the DODH mechanisms, from the energy point of view, for the conversion of phenyl-1,2-ethanediol to styrene by perrhenate anion in the presence of PPh₃ as a reductant through a detailed comparison of two potential pathways including pathway A and pathway B. Pathway A includes the sequence of condensation of oxo–Re­(VII) with diol before the reduction of Re­(VII) to Re­(V), whereas pathway B involves the reduction of oxo–Re­(VII) to oxo–Re­(V) before the condensation process. In pathway B, two basic routes (B1 and B2) are possible, which can take place through different reaction steps, including the extrusion of alkene from Re­(V)–diolate in route B1, and the second reduction of the Re­(V)–diolate by reductant and then the extrusion of alkene from the Re­(III)–diolate intermediate in route B2. The intermediates and the Gibbs free energy changes, including Δ<i>G</i>°_g and Δ<i>G</i>°_sol, have been calculated for alternative pathways (A and B) in the gas and solvent (chlorobenzene and methanol) phases and compared to each other. In addition, the transition states and the activation energy barriers for two pathways (A and B) in the gas phase and in chlorobenzene have been calculated. The key transition states include the nucleophilic attack of PPh₃ on an ReO bond, the dissociation of OPPh₃ from the rhenium moiety, the transfer of an H atom of diol to the oxo ligand in an oxoRe bond through the condensation step, and the extrusion of styrene from the Re–diolate complexes. The DFT results indicate that the DODH reaction is thermodynamically feasible through both pathways (A and B). However, the calculations reveal that the perrhenate-catalyzed DODH reaction through pathway A has the lowest overall activation barrier energy among the DODH mechanism routes

Synthesis, Structure, and Reactivity of Co(II) and Ni(II) PCP Pincer Borohydride Complexes

The 15e square-planar complexes [Co­(PCP^Me-<i>i</i>Pr)­Cl] (<b>2a</b>) and [Co­(PCP-<i>t</i>Bu)­Cl] (<b>2b</b>), respectively, react readily with NaBH₄ to afford complexes [Co­(PCP^Me-<i>i</i>Pr)­(η²-BH₄)] (<b>4a</b>) and [Co­(PCP-<i>t</i>Bu)­(η²-BH₄)] (<b>4b</b>) in high yields, as confirmed by IR spectroscopy, X-ray crystallography, and elemental analysis. The borohydride ligand is symmetrically bound to the cobalt center in η²-fashion. These compounds are paramagnetic with effective magnetic moments of 2.0(1) and 2.1(1) μ_B consistent with a d⁷ low-spin system corresponding to one unpaired electron. None of these complexes reacted with CO₂ to give formate complexes. For structural and reactivity comparisons, we prepared the analogous Ni­(II) borohydride complex [Ni­(PCP^Me-<i>i</i>Pr)­(η²-BH₄)] (<b>5</b>) via two different synthetic routes. One utilizes [Ni­(PCP^Me-<i>i</i>Pr)­Cl] (<b>3</b>) and NaBH₄, the second one makes use of the hydride complex [Ni­(PCP^Me-<i>i</i>Pr)­H] (<b>6</b>) and BH₃·THF. In both cases, <b>5</b> is obtained in high yields. In contrast to <b>4a</b> and <b>4b</b>, the borohydride ligand is asymmetrically bound to the nickel center but still in an η²-mode. [Ni­(PCP^Me-<i>i</i>Pr)­(η²-BH₄)] (<b>5</b>) loses readily BH₃ at elevated temperatures in the presence of NEt₃ to form <b>6</b>. Complexes <b>5</b> and <b>6</b> are both diamagnetic and were characterized by a combination of ¹H, ¹³C­{¹H}, and ³¹P­{¹H} NMR, IR spectroscopy, and elemental analysis. Additionally, the structure of these compounds was established by X-ray crystallography. Complexes <b>5</b> and <b>6</b> react with CO₂ to give the formate complex [Ni­(PCP^Me-<i>i</i>Pr)­(OC­(CO)­H] (<b>7</b>). The extrusion of BH₃ from [Co­(PCP^Me-<i>i</i>Pr)­(η²-BH₄)] (<b>4a</b>) and [Ni­(PCP^Me-<i>i</i>Pr)­(η²-BH₄)] (<b>5</b>) with the aid of NH₃ to yield the respective hydride complexes [Co­(PCP^Me-<i>i</i>Pr)­H] and [Ni­(PCP^Me-<i>i</i>Pr)­H] (<b>6</b>) and BH₃NH₃ was investigated by DFT calculations showing that formation of the Ni hydride is thermodynamically favorable, whereas the formation of the Co­(II) hydride, in agreement with the experiment, is unfavorable. The electronic structures and the bonding of the borohydride ligand in [Co­(PCP^Me-<i>i</i>Pr)­(η²-BH₄)] (<b>4a</b>) and [Ni­(PCP^Me-<i>i</i>Pr)­(η²-BH₄)] (<b>5</b>) were established by DFT computations

Mechanism of Rare Earth Incorporation and Crystal Growth of Rare Earth Containing Type‑I Clathrates

Type-I clathrates possess extremely low thermal conductivities, a property that makes them promising materials for thermoelectric applications. The incorporation of cerium into one such clathrate has recently been shown to lead to a drastic enhancement of the thermopower, another property determining the thermoelectric efficiency. Here we explore the mechanism of the incorporation of rare earth elements into type-I clathrates. Our investigation of the crystal growth and the composition of the phase Ba_8–<i>x</i>RE_<i>x</i>TM_<i>y</i>Si_{46–<i>y</i>} (RE = rare earth element; TM = Au, Pd, Pt) reveals that the RE content <i>x</i> is mainly governed by two factors, the free cage space and the electron balance

Synthesis and characterization of a family of Fe(II) tetrazole complexes [Fe(C₆mtz)₆]X₂ (X = BF₄ˉ, ClO₄ˉ, PF₆ˉ)

<div><p>Based on 1-(cyclohexylmethyl)-1H-tetrazole (C₆mtz), a series of mononuclear iron(II) spin-crossover complexes with the general formula [Fe(C₆mtz)₆]X₂, where X is the non-coordinating anion (<b>1</b>), (<b>2</b>), or (<b>3</b>), have been synthesized and characterized. Temperature-dependent magnetic susceptibility measurements for <b>1</b>, <b>2,</b> and <b>3</b> show reversible one-step spin crossover (SCO) behavior between high-spin (HS, <i>S</i> = 2) and low-spin (LS, <i>S</i> = 0) states without hysteresis. The compound shows spin transition at <i>T</i>_1/2 = 213 K at a considerably higher temperature than the other compounds, (<i>T</i>_1/2 = 126 K) and (<i>T</i>_1/2 = 119 K). Temperature-dependent Far-IR and Mössbauer spectra of <b>2</b> and <b>3</b> were compared with the measured magnetic susceptibility and confirm the one-step SCO behavior of the compounds.</p></div

Correlation of π-Conjugated Oligomer Structure with Film Morphology and Organic Solar Cell Performance

The novel methyl-substituted dicyanovinyl-capped quinquethiophenes <b>1</b>–<b>3</b> led to highly efficient organic solar cells with power conversion efficiencies of 4.8–6.9%. X-ray analysis of single crystals and evaporated neat and blend films gave insights into the packing and morphological behavior of the novel compounds that rationalized their improved photovoltaic performance