Trigonal Prismatic Arrangement Research Articles

Nitrogenase beyond the Resting State: A Structural Perspective.

Nitrogenases have the remarkable ability to catalyze the reduction of dinitrogen to ammonia under physiological conditions. How does this happen? The current view of the nitrogenase mechanism focuses on the role of hydrides, the binding of dinitrogen in a reductive elimination process coupled to loss of dihydrogen, and the binding of substrates to a binuclear site on the active site cofactor. This review focuses on recent experimental characterizations of turnover relevant forms of the enzyme determined by cryo-electron microscopy and other approaches, and comparison of these forms to the resting state enzyme and the broader family of iron sulfur clusters. Emerging themes include the following: (i) The obligatory coupling of protein and electron transfers does not occur in synthetic and small-molecule iron-sulfur clusters. The coupling of these processes in nitrogenase suggests that they may involve unique features of the cofactor, such as hydride formation on the trigonal prismatic arrangement of irons, protonation of belt sulfurs, and/or protonation of the interstitial carbon. (ii) Both the active site cofactor and protein are dynamic under turnover conditions; the changes are such that more highly reduced forms may differ in key ways from the resting-state structure. Homocitrate appears to play a key role in coupling cofactor and protein dynamics. (iii) Structural asymmetries are observed in nitrogenase under turnover-relevant conditions by cryo-electron microscopy, although the mechanistic relevance of these states (such as half-of-sites reactivity) remains to be established.

A Series of Non-Oxido VIV Complexes of Dibasic ONS Donor Ligands: Solution Stability, Chemical Transformations, Protein Interactions, and Antiproliferative Activity.

A series of mononuclear non-oxido vanadium(IV) complexes, [VIV(L1-4)2] (1-4), featuring tridentate bi-negative ONS chelating S-alkyl/aryl-substituted dithiocarbazate ligands H2L1-4, are reported. All the synthesized non-oxido VIV compounds are characterized by elemental analysis, spectroscopy (IR, UV-vis, and EPR), ESI-MS, as well as electrochemical techniques (cyclic voltammetry). Single-crystal X-ray diffraction studies of 1-3 reveal that the mononuclear non-oxido VIV complexes show distorted octahedral (1 and 2) or trigonal prismatic (3) arrangement around the non-oxido VIV center. EPR and DFT data indicate the coexistence of mer and fac isomers in solution, and ESI-MS results suggest a partial oxidation of [VIV(L1-4)2] to [VV(L1-4)2]+ and [VVO2(L1-4)]-; therefore, all these three complexes are plausible active species. Complexes 1-4 interact with bovine serum albumin (BSA) with a moderate binding affinity, and docking calculations reveal non-covalent interactions with different regions of BSA, particularly with Tyr, Lys, Arg, and Thr residues. In vitro cytotoxic activity of all complexes is assayed against the HT-29 (colon cancer) and HeLa (cervical cancer) cells and compared with the NIH-3T3 (mouse embryonic fibroblast) normal cell line by MTT assay and DAPI staining. The results suggest that complexes 1-4 are cytotoxic in nature and induce cell death in the cancer cell lines by apoptosis and that a mixture of VIV, VV, and VVO2 species could be responsible for the biological activity.

Hafnium(IV) Chemistry with Imide-Dioxime and Catecholate-Oxime Ligands: Unique {Hf5} and Metalloaromatic {Hf6}-Oxo Clusters Exhibiting Fluorescence.

Hafnium(IV) molecular species have gained increasing attention due to their numerous applications ranging from high-resolution nanolithography, heterogeneous catalysis, and electronics to the design of molecule-based building blocks in metal-organic frameworks (MOFs), with applications in gas separation, sorption, luminescence sensing, and interim storage of radioactive waste. Despite great potential, their chemistry is relatively underdeveloped. Here, we use strong chelators (2Z-6Z)-piperidine-2,6-dione (H3pidiox) and 2,3-dihydroxybenzaldehyde oxime (H3dihybo) to synthesize the first ever reported pentanuclear {Hf5/H3pidiox} and hexanuclear {Hf6/H3dihybo} clusters (HfOCs). The {Hf6} clusters adopt unique core structures [Hf6IV(μ3-O)2(μ-O)3] with a trigonal-prismatic arrangement of the six hafnium atoms and have been characterized via single-crystal X-ray diffraction analysis, UV-vis spectroscopy in the solid state, NMR, fluorescence spectroscopy, and high-resolution mass spectrometry in solution. One-dimensional (1D) and two-dimensional (2D) 1H NMR and mass spectroscopies reveal the exceptional thermodynamic stability of the HfOCs in solution. Interestingly, the conjunction of the oxime group with the catechol resulted in the remarkable reduction of the clusters' band gap, below 2.51 eV. Another prominent feature is the occurrence of pronounced metalloaromaticity of the triangular {Hf3} metallic component revealed by its NICSzz scan curve calculated by means of density functional theory (DFT). The NICSzz(1) value of -44.6 ppm is considerably higher than the -29.7 ppm found at the same level of theory for the benzene ring. Finally, we investigated the luminescence properties of the clusters where 1 emits light in the violet region despite the lack of fluorescence of the free H3pidiox ligand, whereas the {Hf6} 3 shifts the violet-emitting light of the H3dihybo to lower energy. DFT calculations show that this fluorescence behavior stems from ligand-centered molecular orbital transitions and that HfIV coordination has a modulating effect on the photophysics of these HfOCs. This work not only represents a significant milestone in the construction of stable low-band-gap multinuclear HfIV clusters with unique structural features and metal-centered aromaticity but also reveals the potential of Hf(IV) molecule-based materials with applications in sensing, catalysis, and electronic devices.

Blue-Emitting Ligand-Mediated Assembly of Rare-Earth MOFs toward White-Light Emission, Sensing, Magnetic, and Catalytic Studies.

New lanthanide carboxylate compounds with two- (2D) and three-dimensional (3D) structures have been prepared by employing 2,5-bis(prop-2-yn-1-yloxy)terephthalic acid (2,5-BPTA) as an organic linker. The compounds, [Ln(C14H8O6)(C7O3H4)·2H2O]·4(H2O), Ln = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy and [Ln(C7O3H4)3·(C3H7ON)·(H2O)]·2(H2O)(C3H7NO), Ln = La, Ce, Pr, have two- and three-dimensional structures, respectively. In all compounds, lanthanide ions are connected together, forming a dimer, which is connected by the 2,5-BPTA ligand. In the two-dimensional structure, there are two 2,5-BPTA moieties present, and in the three-dimensional structure, there are three 2,5-BPTA moieties present. The lanthanide centers are nine-coordinated, the 2D structure has a tricapped trigonal prismatic arrangement, and the 3D structure has a monocapped distorted square antiprismatic arrangement. The Pr compound forms in both 2D and 3D structures, whose formation depends on the time of the reaction (2 days─2D and 5-6 days─3D). The ligand emits in the blue region, and using the characteristic emission of Eu3+ (red) and Tb3+ (green) ions, we achieve white light emission in the (Y0.96Tb0.02Eu0.02) compound. The overall quantum yield for the white light emission is 28%. The strong green luminescence of the Tb3+-containing compound was employed to selectively sense the Cr3+ and Fe3+ ions in aqueous solution with limits of detection (LODs) at 0.41 and 8.6 ppm, respectively. The Tb compound was found to be a good heterogeneous catalyst for the Ullman-type O-arylation reaction between phenol and bromoarene with yields of 95%. Magnetic studies on the Gd-, Tb-, and Dy-containing compounds showed weak exchange interactions within the dimeric Ln2 units. The present work demonstrates the many utilities of the rare-earth-containing MOFs, especially toward white-light emission, metal-ion sensing, and heterogeneous catalysis.

Crystal structure and investigation of Bi2TeO6·nH2O (0 ≤ n ≤ {raise0.5exhbox{scriptstyle 2} kern-0.1em/kern-0.15em lower0.25exhbox{scriptstyle 3}}): natural and synthetic montanite

The crystal structure of montanite has been determined using single-crystal X-ray diffraction on a synthetic sample, supported by powder X-ray diffraction (PXRD), electron microprobe analysis (EPMA) and thermogravimetric analyses (TGA). Montanite was first described in 1868 as Bi2TeO6·nH2O (n = 1 or 2). The determination of the crystal structure of synthetic montanite (refined composition Bi2TeO6·0.22H2O) has led to the reassignment of the formula to Bi2TeO6·nH2O where 0 ≤ n ≤ {raise0.5exhbox{scriptstyle 2} kern-0.1em/kern-0.15em lower0.25exhbox{scriptstyle 3}} rather than the commonly reported Bi2TeO6·2H2O. This change has been accepted by the IMA–CNMNC, Proposal 22-A. The PXRD pattern simulated from the crystal structure of synthetic montanite is a satisfactory match for PXRD scans collected on both historical and recent natural samples, showing their equivalence. Two specimens attributed to the original discoverer of montanite (Frederick A. Genth) from the cotype localities (Highland Mining District, Montana and David Beck’s mine, North Carolina, USA) have been designated as neotypes. Montanite crystallises in space group Poverline{6 }, with the unit-cell parameters a = 9.1195(14) Å, c = 5.5694(8) Å, V = 401.13(14) Å3, and three formula units in the unit cell. The crystal structure of montanite is formed from a framework of BiOn and TeO6 polyhedra. Half of the Bi3+ and all of the Te6+ cations are coordinated by six oxygen atoms in trigonal-prismatic arrangements (the first example of this configuration reported for Te6+), while the remaining Bi3+ cations are coordinated by seven O sites. The H2O groups in montanite are structurally incorporated into the network of cavities formed by the three-dimensional framework, with other cavity space occupied by the stereoactive 6s2 lone pair of Bi3+ cations. While evidence for a supercell was observed in synthetic montanite, the subcell refinement of montanite adequately indexes all reflections in the PXRD patterns observed in all natural montanite samples analysed in this study, verifying the identity of montanite as a mineral.

Synthesis, Structural, and Physicochemical Characterization of a Ti6 and a Unique Type of Zr6 Oxo Clusters Bearing an Electron-Rich Unsymmetrical {OON} Catecholate/Oxime Ligand and Exhibiting Metalloaromaticity.

The chelating catechol/oxime ligand 2,3-dihydroxybenzaldehyde oxime (H3dihybo) has been used to synthesize one titanium(IV) and two zirconium(IV) compounds that have been characterized by single-crystal X-ray diffraction and 1H and 13C NMR, solid-state UV-vis, and ESI-MS spectroscopy. The reaction of TiCl4 with H3dihybo and KOH in methanol, at ambient temperature, yielded the hexanuclear titanium(IV) compound K2[TiIV6(μ3-O)2(μ-O)3(OCH3)4(CH3OH)2(μ-Hdihybo)6]·CH3OH (1), while the reaction of ZrCl4 with H3dihybo and either nBu4NOH or KOH also gave the hexanuclear zirconium(IV) compounds 2 and 3, respectively. Compounds 1-3 have the same structural motif [MIV6(μ3-Ο)2(μ-Ο)3] (M = Ti, Zr), which constitutes a unique example with a trigonal-prismatic arrangement of the six zirconium atoms, in marked contrast to the octahedral arrangement of the six zirconium atoms in all the Zr6 clusters reported thus far, and a unique Zr6 core structure. Multinuclear NMR solution measurements in methanol and water proved that the hexanuclear clusters 1 and 3 retain their integrity. The marriage of the catechol moiety with the oxime group in the ligand H3dihybo proved to be quite efficient in substantially reducing the band gaps of TiO2 and ZrO2 to 1.48 and 2.34 eV for the titanium and zirconium compounds 1 and 3, respectively. The application of 1 and 3 in photocurrent responses was investigated. ESI-MS measurements of the clusters 1 and 3 revealed the existence of the hexanuclear metal core and also the initial formation of trinuclear M3 (M = Ti, Zr) building blocks prior to their self-assembly into the hexanuclear M6 (M = Ti, Zr) species. Density functional theory (DFT) calculations of the NICSzz scan curves of these systems revealed that the triangular M3 (M = Ti, Zr) metallic ring cores exhibit pronounced metalloaromaticity. The latter depends upon the nature of the metallic center with NICSzz(1) values equal to -30 and -42 ppm for the Ti (compound 1) and Zr (compound 2) systems, respectively, comparable to the NICSzz(1) value of the benzene ring of -29.7 ppm calculated at the same level of theory.

New VIV, VIVO, VVO, and VVO2 Systems: Exploring their Interconversion in Solution, Protein Interactions, and Cytotoxicity.

The synthesis and characterization of one oxidoethoxidovanadium(V) [VVO(L1)(OEt)] (1) and two nonoxidovanadium(IV) complexes, [VIV(L2-3)2] (2 and 3), with aroylhydrazone ligands incorporating naphthalene moieties, are reported. The synthesized oxido and nonoxido vanadium complexes are characterized by various physicochemical techniques, and their molecular structures are solved by single crystal X-ray diffraction (SC-XRD). This revealed that in 1 the geometry around the vanadium atom corresponds to a distorted square pyramid, with a O4N coordination sphere, whereas that of the two nonoxido VIV complexes 2 and 3 corresponds to a distorted trigonal prismatic arrangement with a O4N2 coordination sphere around each "bare" vanadium center. In aqueous solution, the VVO moiety of 1 undergoes a change to VVO2 species, yielding [VVO2(L1)]- (1'), while the nonoxido VIV-compounds 2 and 3 are partly converted into their corresponding VIVO complexes, [VIVO(L2-3)(H2O)] (2' and 3'). Interaction of these VVO2, VIVO, and VIV systems with two model proteins, ubiquitin (Ub) and lysozyme (Lyz), is investigated through docking approaches, which suggest the potential binding sites: the interaction is covalent for species 2' and 3', with the binding to Glu16, Glu18, and Asp21 for Ub, and His15 for Lyz, and it is noncovalent for species 1', 2, and 3, with the surface residues of the proteins. The ligand precursors and complexes are also evaluated for their in vitro antiproliferative activity against ovarian (A2780) and prostate (PC3) human cancer cells and in normal fibroblasts (V79) to check the selectivity of the compounds for cancer cells.

Structural Enzymology of Nitrogenase Enzymes.

The reduction of dinitrogen to ammonia by nitrogenase reflects a complex choreography involving two component proteins, MgATP and reductant. At center stage of this process resides the active site cofactor, a complex metallocluster organized around a trigonal prismatic arrangement of iron sites surrounding an interstitial carbon. As a consequence of the choreography, electrons and protons are delivered to the active site for transfer to the bound N2. While the detailed mechanism for the substrate reduction remains enigmatic, recent developments highlight the role of hydrides and the privileged role for two irons of the trigonal prism in the binding of exogenous ligands. Outstanding questions concern the precise nature of the intermediates between N2 and NH3, and whether the cofactor undergoes significant rearrangement during turnover; resolution of these issues will require the convergence of biochemistry, structure, spectroscopy, computation, and model chemistry.

Chiral Discrete and Polymeric Uranyl Ion Complexes with (1 R,3 S)-(+)-Camphorate Ligands: Counterion-Dependent Formation of a Hexanuclear Cage.

Reaction of (1 R,3 S)-(+)-camphoric acid (H2cam) with uranyl ions under solvo-hydrothermal conditions and in the presence of bulky countercations gave five chiral complexes of varying dimensionality. [Cu( R,S-Me6cyclam)][UO2(Hcam)2(HCOO)2] (1) and [Ni( R,S-Me6cyclam)][UO2(cam)(HCOO)2] (2), in which the formate coligand is formed in situ, involve very similar countercations, but 1 is a discrete, mononuclear complex, whereas 2 crystallizes as a one-dimensional (1D) coordination polymer, and NH-bond donation by the macrocyclic ligand of the countercation complexes is present in both. [Co(en)3][(UO2)4(cam)( R,R-tart)2(OH)]·3H2O (3), in which en is ethylenediamine and H4 R,R-tart is R,R-tartaric acid, contains three enantiomerically pure chiral species, and it displays a two-dimensional (2D) arrangement, with the countercation again involved in NH-bond donation. While [PPh4][UO2(cam)(NO3)] (4) is a 1D polymer, [PPh3Me]3[NH4]3[(UO2)6(cam)9] (5) is a discrete, homochiral, and homoleptic hexanuclear cage with C3 point symmetry and a trigonal prismatic arrangement of the uranium atoms. This cage differs from the octanuclear, pseudocubic uranyl camphorate species previously described, thus providing an example of modulation of the cage size through variation of the structure-directing counterions. The cage in 5 is closely associated with three PPh3Me+ cations, two of them outside and with their methyl group directed toward the prism basis center, and one inside the cage cavity. While complex 5 is nonluminescent, complexes 1 and 4 have emission spectra in the solid state typical of equatorially hexacoordinated uranyl complexes. Solid-state photoluminescence quantum yields of 2 and 23% have been measured for complexes 1 and 4, respectively.

Two new POM-based compounds modified by lanthanide-Schiff base complexes with interesting NLO properties.

Two POM-based lanthanide derivatives, namely {triaqua[2,6-diacetylpyridine bis(semicarbazone)-κ5O,N,N',N'',O']terbium(III)}-μ-oxido-[tricosa-μ2-oxido-dodecaoxido-μ12-phosphato-dodecamolybdenum(VI)] pentahydrate (1), [Tb(C11H15N7O2)(H2O)3][PMo12O40]·5H2O, and the dysprosium(III) analogue (2), have been isolated successfully by the reaction of Keggin-POM [PMo12O40]3- (abbreviated as PMo12), the Ln3+ ion and the Schiff base 2,6-diacetylpyridine bis(semicarbazone) (DAPSC) ligand under hydrothermal conditions. [Ln(DAPSC)(H2O)3][PMo12O40]·5H2O is a PMo12-supported cluster featuring a lanthanide-Schiff base complex [denoted Ln-L(Schiff base)]. Single-crystal X-ray diffraction analysis reveals that the LnIII ion is in a distorted tricapped trigonal prismatic arrangement, coordinated by six O atoms and three N atoms. Two O atoms and three N atoms are provided by one DAPSC ligand, while the additional O atoms originate from a PMo12 cluster and three water molecules. Hydrogen-bonding interactions between adjacent clusters form an interesting three-dimensional supramolecular structure. The identities of 1 and 2 were characterized by IR spectroscopy, thermogravimetric analysis and powder X-ray diffraction. Interestingly, both compounds possess excellent two-photon absorption (TPA) responses to the third-order nonlinear optics (NLO) (2264 GM for 1 and 941 GM for 2), suggesting that they have potential applications in the field of nonlinear optics (NLO). To our knowledge, 1 and 2 are the first POM-based Ln-L(Schiff base) complexes showing excellent two-photon responses. Meanwhile, the electrochemical properties of both compounds were studied in detail.

Thorium copper phosphides: more diverse metal-phosphorus and phosphorus-phosphorus interactions than U analogues.

To explore the chemical analogy between thorium and heavier actinides in soft anionic environments, three new thorium phosphides (ThCuP2, β-ThCu2P2, and ThCu5P3) have been prepared through solid-state reactions using CuI as a reaction promoter. The structure of ThCuP2 can be described as a filled UTe2-type with both dimeric P24- and monomeric P3- anions, in which Th is coordinated by eight P atoms in a bicapped trigonal prismatic arrangement and Cu is tetrahedrally coordinated by four P atoms. β-ThCu2P2 contains only P3- anions and is isostructural with BaCu2S2. In this structure, Th is coordinated by seven P atoms in monocapped trigonal prismatic geometry and Cu is tetrahedrally coordinated by four P atoms. ThCu5P3 adopts the YCo5P3-type structure consisting of P3- anions. This structure contains Th atoms coordinated by six P atoms in a trigonal prismatic arrangement and Cu atoms that are either tetrahedrally coordinated by four P atoms or square pyramidally coordinated by five P atoms. Electric resistivity measurements and electronic structure calculations on β-ThCu2P2 indicate a metal. These new compounds may be charge-balanced and formulated as Th4+Cu+(P24-)1/2P3-, Th4+(Cu+)2(P3-)2, and Th4+(Cu+)5(P3-)3, respectively. The structural, bonding, and property relationships between these Th compounds and related actinide and rare-earth phases are discussed. Titled compounds display more diverse ion-ion interactions and different electronic structures from those in UCuP2 and UCu2P2 that were synthesized under similar experimental conditions, suggesting divergence of thorium-phosphide chemistry from uranium-phosphide chemistry.

Syntheses, structures and luminescent properties of two 3D pillared open frameworks with unique 6-connected structures based on 4,5-dihydroxy-1,3-benzenedisulfonate

Two coordination polymers, [Ba(H2L)(H2O)]n·nH2O (1) and [La(HL)(H2O)]n·nH2O (2) (Na2H2L = 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt), have been synthesized under hydrothermal conditions. The central metal ions are nine-coordinate with distorted tricapped trigonal prismatic arrangements. Compounds 1 and 2 have 3-D metal–organic framework (MOF) structures which are created by 2-D inorganic layers [Ba2S2O5]n and [La2S2O5]n through organic phenyl moieties of HL3− linkages. The inorganic layers and organic pillars are alternately arranged to generate the 3-D pillared-layered open frameworks with (411, 64) topologies. Results of fluorescence measurements reveal that two decayed emission bands centered at 435 and 408 nm may be caused by interactions of the ligands and the metal ions. The respective luminescence emission peaks appear at different wavelengths and intensities, which can be affected by the metal ions.

Crystal structures of dioxonium lanthanoid(III) pentakis(tetrafluoridoborates) of lanthanum and cerium

The fluoropolymers as FEP and PFA are widely used materials for making reaction vessels applied to experiments involving liquid anyhdrous HF, elemental F2 and various inorganic fluorides and fluoro-organic compounds. However, due to their permeability to gases and liquids, crystals of unexpected compounds can be sometimes grown during extended durations of the crystallizations. One of such examples is the growth of single crystals of (H3O)2Ce(BF4)5, which were grown unintentionally from the CeF3/BF3/aHF solution after five months of crystallization. Reaction between the La2O3 and excess of BF3 in anhydrous HF yielded a similar compound, i.e., (H3O)2La(BF4)5 upon crystallization. The attempts to prepare single crystals of analogous compounds of Nd and Sm failed. (H3O)2Ce(BF4)5 crystallizes in the monoclinic P21/n space group (No. 14) with a=11.9160(14)Å, b=9.1551(12)Å, c=14.5290(14)Å, β=94.929(5)°, V=1579.1(3)Å3, and Z=4 at 200K. The crystal structure of (H3O)2La(BF4)5 is isotypic with cerium compound. Corresponding unit cell is monoclinic, P21/n space group (No.14), with a=12.004(10)Å, b=9.197(2)Å, c=14.577(7)Å, β=94.459(9)°, V=1604.5(17)Å3 and Z=4 at 100K. The crystal structures of (H3O)2Ln(BF4)5 (Ln=La, Ce) consist of slabs formed from the Ln3+ cations interconnected with the [BF4]−anions. The oxonium cations are located in the space between the slabs. Ln3+ cations are nine-fold coordinated with a tricapped trigonal prismatic arrangement of F ligands.

Two second-order non-linear optical coordination polymers with noninterpenetrating honeycomb-like 2-D (6,3)-type networks

Two isostructural metal–organic frameworks, [ML(H2O)4]n (M = Y(II) (1) and Nd(II) (2)), NaH2L = 5-sulfoisophthalic monosodium salt), have been synthesized by hydrothermal reactions. Each metal is nine-coordinate with a distorted tricapped trigonal prismatic arrangement. L3− is a μ3 bridge to generate a homometallic 2-D noninterpenetrating (6,3) honeycomb-like topological network structure. The luminescence spectra indicate emission maxima at 406 and 402 nm, respectively. Compounds 1 and 2 show second-harmonic generation responses that are 0.5 and 0.8 times that of urea, respectively.

ɛ-TiO, a Novel Stable Polymorph of Titanium Monoxide.

For the Ti/O system, three titanium monoxide (TiO) phases (α, β, and γ) with defective NaCl-type structures and a high-temperature hexagonal phase (H) have been known for decades. In this work, single crystals of a novel polymorph, ɛ-TiO, were synthesized by using a bismuth flux. X-ray diffraction (XRD) revealed a hexagonal crystal structure (a=4.9936(3) Å, c=2.8773(2) Å, P6‾ 2m) that is isotypic with ɛ-TaN. While the Ti atoms are surrounded by trigonal prismatic (sixfold coordination) and trigonal planar (threefold coordination) arrangements of O atoms, the O atoms are found in a pseudo-square-pyramidal arrangement of Ti atoms. First-principles calculations of the formation enthalpy and the electron and phonon density of states and crystal orbital Hamilton population (COHP) analysis revealed that ɛ-TiO is more stable than α-TiO, which had previously been regarded as the most stable phase at low temperatures.

ɛ‐TiO, a Novel Stable Polymorph of Titanium Monoxide

AbstractFor the Ti/O system, three titanium monoxide (TiO) phases (α, β, and γ) with defective NaCl‐type structures and a high‐temperature hexagonal phase (H) have been known for decades. In this work, single crystals of a novel polymorph, ɛ‐TiO, were synthesized by using a bismuth flux. X‐ray diffraction (XRD) revealed a hexagonal crystal structure (a=4.9936(3) Å, c=2.8773(2) Å, P 2m) that is isotypic with ɛ‐TaN. While the Ti atoms are surrounded by trigonal prismatic (sixfold coordination) and trigonal planar (threefold coordination) arrangements of O atoms, the O atoms are found in a pseudo‐square‐pyramidal arrangement of Ti atoms. First‐principles calculations of the formation enthalpy and the electron and phonon density of states and crystal orbital Hamilton population (COHP) analysis revealed that ɛ‐TiO is more stable than α‐TiO, which had previously been regarded as the most stable phase at low temperatures.

Synthesis, structure, and magnetic characterization of Cr4US8

The compound Cr4US8 has been synthesized at 1073K and its crystal structure has been determined at 100K. The structure is modulated with a two-fold commensurate supercell. The subcell may be indexed in an orthorhombic cell but weak supercell reflections lead to the monoclinic superspace group P21/c(α0γ)0s with two Cr sites, one U site, and four S sites. The structure comprises a three-dimensional framework of CrS6 octahedra with channels that are partially occupied by U atoms. Each U atom in these channels is coordinated by eight S atoms in a bicapped trigonal-prismatic arrangement. The magnetic behavior of Cr4US8 is complex. At temperatures above ~120K at all measured fields, there is little difference between field-cooled and zero field-cooled data and χ(T) decreases monotonously with temperature, which is reminiscent of the Curie–Weiss law. At lower temperatures, the temperature dependence of χ(T) is complex and strongly dependent on the magnetic field strength.

A novel second-order non-linear optical coordination polymer with three-fold interpenetrated CdSO4-type network constructed by carboxylate–sulfonate ligands and strontium ions

A novel strontium carboxylate–sulfonate coordination polymer, [Sr(HSIP)(H2O)3]n·nH2O (1) (NaH2SIP=5-sulfoisophthalic monosodium salt) has been synthesized by hydrothermal reaction. It was characterized by X-ray single crystal diffraction, infrared spectroscopy, elemental and thermogravimetric analysis. Each strontium atom is eight-coordinate with a distorted bicapped trigonal prismatic arrangement. The whole HSIP2− ligand acts as a η5μ4 bridge to generate three-fold interpenetrated CdSO4-type network structure, which is constructed from the left- and right-handed helixes paralleled to each other bridged by the HSIP2− ligands. The luminescence spectrum indicates an emission maximum at 459nm. Compound 1 shows a second harmonic generation (SHG) response that is 4 times that of KH2PO4.

Nonoxido Vanadium(IV) Compounds Involving Dithiocarbazate-Based Tridentate ONS Ligands: Synthesis, Electronic and Molecular Structure, Spectroscopic and Redox Properties.

A new series of nonoxido vanadium(IV) compounds [VL2] (L = L(1)-L(3)) (1-3) have been synthesized using dithiocarbazate-based tridentate Schiff-base ligands H2L(1)-H2L(3), containing an appended phenol ring with a tert-butyl substitution at the 2-position. The compounds are characterized by X-ray diffraction analysis (1, 3), IR, UV-vis, EPR spectroscopy, and electrochemical methods. These are nonoxido V(IV) complexes that reveal a rare distorted trigonal prismatic arrangement around the "bare" vanadium centers. Concerning the ligand isomerism, the structure of 1 and 3 can be described as intermediate between mer and sym-fac isomers. DFT methods were used to predict the geometry, g and (51)V A tensors, electronic structure, and electronic absorption spectrum of compounds 1-3. Hyperfine coupling constants measured in the EPR spectra can be reproduced satisfactorily at the level of theory PBE0/VTZ, whereas the wavelength and intensity of the absorptions in the UV-vis spectra at the level CAM-B3LYP/gen, where "gen" is a general basis set obtained using 6-31+g(d) for S and 6-31g for all the other elements. The results suggest that the electronic structure of 1-3 can be described in terms of a mixing among V-dxy, V-dxz, and V-dyz orbitals in the singly occupied molecular orbital (SOMO), which causes a significant lowering of the absolute value of the (51)V hyperfine coupling constant along the x-axis. The cyclic voltammograms of these compounds in dichloroethane solution display three one-electron processes, two in the cathodic and one in the anodic potential range. Process A (E1/2 = +1.06 V) is due to the quasi-reversible V(IV/V) oxidation while process B at E1/2 = -0.085 V is due to the quasi-reversible V(IV/III) reduction, and the third one (process C) at a more negative potential E1/2 = -1.66 V is due to a ligand centered reduction, all potentials being measured vs Ag/AgCl reference.

Syntheses and characterization of the cubic uranium chalcogenides Rh2U6S15, Cs2Ti2U6Se15, Cs2Cr2U6Se15, and Cs2Ti2U6Te15

The compounds Rh2U6S15, Cs2Ti2U6Se15, Cs2Cr2U6Se15, and Cs2Ti2U6Te15 have been synthesized at 1173K. All crystallize in space group Oh9−Im3¯m of the cubic system. Rh2U6S15 has a framework structure with three-dimensional channels. The compounds Cs2Ti2U6Se15, Cs2Cr2U6Se15, and Cs2Ti2U6Te15 have structures similar to that of Rh2U6S15, but with Cs cations variably filling the channels. In all four structures the transition element is octahedrally coordinated by chalcogens and the uranium atom is in a bicapped trigonal-prismatic arrangement. The temperature dependence of the magnetic susceptibility of Cs2Cr2U6Se15 implies both Cr and U magnetic contributions. From these data the compound is not antiferromagnetic, but it could have either a ferrimagnetic or a ferromagnetic ground state.