Fe and Cr K‐edges EXAFS Study of Double Perovskite (Sr2‐xCax)FeMoO6 (0 ≤ x ≤ 2.0) and Sr2CrMO6 (M = Mo,W) Systems

The local structure of the double perovskite (Sr_2‐xCa_x)FeMoO₆ (0 ≤ × ≤ 2.0) and Sr₂CrMO₆ (M = Mo, W) systems have been probed by extended X‐ray absorption fine structure (EXAFS) spectroscopy at the Fe and Cr K‐edges. We found Fe‐O (ave) distance apparently decreases from 1.999 Å (x = 0) to 1.991 Å (x = 1.0) in (Sr_2‐xCa_x)FeMoO₆ (tetragonal structure). When x is increased further from 1.5 to 2.0, the Fe‐O bond distance decreased from 2.034 Å to 2.012 Å (monoclinic structure). In addition, Cr‐O, Sr‐Cr, and Cr‐Mo bond distances in Sr₂CrWO₆ are all slightly larger than the bond distances of Sr₂CrMoO₆, which is due to the ionic radius of the W⁵⁺ (0.62 Å) which is larger than the ionic radius of Mo⁵⁺ (0.61 Å). The results are consistent with our XRD refinements data.     

Molecular Structure of Triphenylphosphine by Gas-Phase Electron Diffraction and ab initio Calculations

The molecular structure of triphenylphosphine was determined by gas-phase electron diffraction and quantum-chemical calculations. The symmetry of the molecule is C ₃ with the L_pPCC torsion angle of 32.2(3.0)°. The main geometrical parameters are as follows: bond distances, Å: P-Cl 1.839(2), C-Cav 1.400(1), and C-H 1.098(3); bond angles, deg: CPC 102.2(3), PCC 115.3(5), and CC _ipso C 119.4(4).     

The reaction of 2′ 2′ 3′ 3′-tetrafluoropropyl-5-iodo-3-oxaoctafluoropentane sulfonate with dimethylsulfoxide and determination of the crystal structure of one of the products

The title reaction gave three known compounds (2, 3 and 4) and two new compounds, CH₃SCH₂(CF₂)₂H (5) and I(CF₂)₂O(CF₂)₂SO₃⁻S+(CH₃)₃ (6). The structure of 6 was confirmed by X-ray diffraction analysis. The crystals of 6 belong to monoclinic space group P2₁/C with a = 9.399, b = 15.651, c=10.934Å, β = 94.80° and z = 4. The structure was solved by heavy-atom method and refined by block-diagonal matrix least-squares procedure to a final R of 0.054 for 1999 independent observed reflexions. The S C bonds around the sulphur atom in trimethylsulphonium are pyramidal with the bond lengths of 1.814 Å, 1.800Å and 1.818 Å and the bond angles C-S-C of 101.06°, 101.52° and 102.53°. The distances of the sulphur atom in trimethylsulphonium to three oxygen atoms in the sulphonate radical are 3.79 Å, 3.64 Å and 3.34 Å respectively. These distances are out of the range of the normal S-O bond length. The structure consists of trimethylsulphonium cations and 5-iodo-3-oxaoctafluoropentane-sulphonate anions.     

Structures of molecular complexes of SbCl5 with pyridine and acetonitrile: equal bond lengths,different stability

Molecular structure of crystalline complex SbCl₅ ? Py was determined for the first time and the structure of the complex SbCl₅ ? AN was refined by means of single crystal X-ray diffraction analysis. It is shown that donor-acceptor Sb–N bond lengths for these complexes are equal within the experimental error. Quantum chemical calculations of the complexes in the gas phase reveal that the energies of the donor-acceptor bond Sb–N are 88 and 180 kJ mol^?1 for SbCl₅ ? AN and SbCl₅ ? Py, respectively. It is established that the length of the donor-acceptor bond Sb–N in the crystal cannot serve as a measure of its strength.

     

The nature of the chemical bond in UO2

The nature of the chemical bond in UO₂ was analyzed taking into account the X-ray photoelectron spectroscopy (XPS) structure parameters of the valence and core electrons, as well as the relativistic discrete variation electronic structure calculation results for this oxide. The ionic/covalent nature of the chemical bond was determined for the UO₈ (D_4h) cluster, reflecting uranium's close environment in UO₂, and the U₁₃O₅₆ and U₆₃O₂₁₆ clusters, reflecting the bulk of solid uranium dioxide. The bar graph of the theoretical valence band (from 0 to ~35 eV) of XPS spectrum was built such that it was in satisfactory agreement with the experimental spectrum of a UO₂ single crystalline thin film. It was shown that unlike the crystal field theory results, the covalence effects in UO₂ are significant due to the strong overlap of the U 6p and U 5f atomic orbitals with the ligand orbitals, in addition to the U 6d atomic orbital (AO). A quantitative molecular orbital (MO) scheme for UO₂ was built. The contribution of the MO electrons to the chemical bond covalence component was evaluated on the basis of the bond population values. It was found that the electrons of inner valence molecular orbitals (IVMO) weaken the chemical bond formed by the electrons of outer valence molecular orbitals (OVMO) by 32% in UO₈ and by 25% in U₆₃O₂₁₆.     

Semi-synthesis of disulfide-linked branched tri-ubiquitin mimics

Ubiquitin (Ub) chain isopeptide bond mimics are useful molecules for biochemical and biophysical studies. Herein, we report the semi-synthesis of the disulfide-linked K11/K48-branched tri-Ub (Ub₃^11/48(S–S)), the first example of an isopeptide mimic for the branched Ub chains, which have recently emerged as an interesting category of Ub modifications. Our strategy comprised the E1-dependent synthesis of the Ub conjugate of aminoethanethiol, followed by disulfide formation with Ub(K11C, K48C). The structure of the synthetic isopeptide bond mimics was verified by the crystal structure of Ub₃^11/48(S–S). Deubiquitination and pulldown assays indicated that the synthetic Ub₃^11/48(S–S) could be hydrolyzed by linkage-specific deubiquitinases (K11-specific Cezanne and K48-specific OTUB1), and recognized by proteasomal ubiquitin receptor S5a.     

1,2,4-Triazincs XIII. The bond lengths and bond angles of a 1,2,4-triazine

The bond distances and bond angles of 5-(p-chlorophenyl)-1,2,4-triazine determined by three dimensional X-ray crystallographic analysis are reported. The pertinent bond lengths are N₁? N₂, 1.335Å, N₂-C₃, 1.314Å, C₃? N₄, 1.339; N₄-C₅, 1.317; C₅-C₆, 1.401; C₆? N₁, 1.317Å. A comparison of these bond distances with those of similar polyazabenzenes shows that the canonical structure of 1,2,4-triazine with a N₁? N₂ single bond more closely represents the ground state of this ring system, than the one with a N₁? N₂ double bond.     

Theoretical studies of C60/C70 fullerene derivatives: C60O and C70O

A semiempirical (AM1) calculation on the structures and stabilities of isomers of the fullerene derivatives C₆₀O and C₇₀O is carried out. The ozonolysis reaction mechanism and the thermodynamics of the compounds are studied. The two isomers of C₆₀O (56 bond and 66 bond) formed by an oxygen atom bridging across a C-C bond have an epoxide-like or an annulene-like structure. According to the ozonolysis reaction mechanism and kinetic factor analysis, the possible products of this ozonolysis reaction are C₆₀O with oxygen bridging over the 66 bond (C_2v) as an epoxide-like isomer and that with oxygen bridging over the 56 bond (C_s) as an annulene-like isomer. Further, the sixteen isomers of C₇₀O (both epoxide-like and annulene-like structures) have been studied with respect to the same reaction mechanism. The most possible product in this ozonolysis reaction contains oxygen bridging across in the upper part (66 bond in C₇₀O-2 or C₇₀O-4) as an epoxide-like structure. The other possible product is C₇₀O-8 (annulene-like structure), in which oxygen bridges across an broken equatorial CC bond in C₇₀ (D_5h). The vibrational frequency analysis and the electronic structure of the selected C₆₀O and C₇₀O isomers are generated for experimental characterisation. The experimental results indicate that C₆₀O and C₇₀O may decompose into the odd number fullerenes C₅₉ and C₆₉. We therefore studied the structures of C₅₉ and C₆₉ also.     

Unusual Five-Center,Four-Electron Bonding in a Rhodium–Bismuth Complex with Pentagonal-Bipyramidal Geometry

A better understanding of the chemical bond in general is gained from the electronic structure of the molecular complex [{RhBi₇}Br₈]. The interactions in the central Bi₅ ring can be interpreted as an unusual five-center, four-electron bond based on ab initio calculations and group theory. Of the linear combinations of five Bi p orbitals two of the molecular orbitals are binding (depicted in the sketch).     

Cleavage of the Fe—Fe Bond of （Me2SiSiMe2） [η^5－C5H4Fe（CO）2]2 with Na／Hg and Molecular Structure of the Ring－Opened Complex

The reaction of the rifle cyclic complex (1) with sodium amalgam in THF resulted in the expected cleavage of the Fe-Fe bond to afford his-sodium salt ( Me2SiSiMe2 ) [η^5-C5H4Fe(CO)2]2 (4). The latter was not isolated and was used directly to react with MeI, PhCH2Cl, CH3C(O)Cl, PhC(O)Cl,Cy3SnCl (Cy= cyclohexyl) or Ph3SnCl to afford corresponding ring-opened derivatives (Me2SiSiMe2) [η^5-C5H4Fe(CO)2]2 [5, R=Me; 6, R=PhCH2; 7, R=CH3C(O); 8, R=PhC(O); 9, R = Cy3Sn or 10, R = Ph3Sn ]. The crystal and molecular structures of 10 were determined by X-ray diffraction analysis. The molecule took the desired ant/ conformation around the Si-Si bond. The length of the Si--Si bond is 0.2343(3)nm, which is essentially identical to that in the cyclic structure of 1[0.2346(4) tun]. This result unambiguously demonstrates that the Si--Si bond in the cyclic structure of 1 is not subject to obvious strain.     

Gas-phase molecular structure of 1,1,1,2-tetrabromo-2,2-dimethyldisilane: theoretical and experimental investigation of a super-halogenated disilane and computational investigation of the F, Cl and I analogues

The molecular structure of 1,1,1,2-tetrabromo-2,2-dimethyldisilane (Br₃SiSiBrMe₂) has been determined in the gas phase by electron diffraction and ab initio molecular-orbital calculations. The computational investigation was used to augment the experimental investigation using the Structure Analysis Restrained by Ab initio Calculations for Electron diffractioN (SARACEN) method. The structure was found to adopt a staggered structure with C _s symmetry by both theory and experiment. Important structural parameters (r _h1) include: rSi–Si 235.6(5) pm, rSi–C 185.4(3) pm, rSi–Br_av 220.3(1) pm, ∠Si–Si–Br(14) 106.1(4)°, ∠Si–Si–C 109.2(8)° and ?Br–Si–Si–Br 180.0°(fixed). These experimental observations are supported by theoretical predictions obtained at the MP2/6-311+G* level. An analogous theoretical investigation was also performed for the series X₃SiSiXMe₂ (X = F, Cl and I) and structural trends identified. The Si–X bond was observed to lengthen as a function of the halogen substituent, with corresponding changes to the Si–Si–X bond angles in the SiX₃ groups. The Si–Si–X bond angle in the SiXMe₂ groups displayed rather different behaviour, and was relatively stable to substitution until X = I. The flexible nature of bond angles about silicon atoms was observed, even in this relatively sterically unhindered system.     

Structure and dynamic features of an intramolecular frustrated Lewis pair

Di(mesityl)cyclohexenylphosphine undergoes hydroboration with Piers' borane [HB(C₆F₅)₂] to yield the cyclohexylene‐anellated frustrated Lewis pair 5 . This P/B pair splits H₂ with the formation of the product 4 and adds to the C?O double bond of phenyl isocyanate to yield 6 . In the crystal, compound 5 features a puckered four‐membered heterocyclic core structure with a long P? B bond (av. 2.197(5) Å). The activation energy of the P? B cleavage of the frustrated Lewis pair 5 was determined by dynamic ¹⁹F NMR spectroscopy at ΔG^≠(298 K)=12.1±0.3 kcal mol^?1.     

Preparation and properties of stable protonation products of permethylmetallocenecarbaldehydes (M = Fe,Ru, Os)

The products ofO-protonation of metallocenecarbaldehydes C₅Me₅MC₅Me₄CHO (M = Fe, Ru, Os) with HBF₄ and CF₃COOH were obtained for the first time, the adducts [C₅Me₅MC₅Me₄CH(OH)]⁺BF^– being isolated in a pure state. The structures of the adducts depend on the nature of the metal and the anion and are governed by the correlation between their basicities. For example, the Fe-containing adducts have an open fulvenoid structure with an interionic hydrogen bond irrespective of the nature of the anion X (X = BF₄ ^–, CF₃COO^–). In the case of the Ru- and Os-containing adducts in which the metal atoms are more basic than iron, the competition between the metal atom and the anion for the formation of a hydrogen bond with the OH group is clearly manifested in the IR and¹H NMR spectra. In the presence of the weakly basic BF₄ ^– anion, a cyclic structure with an intramolecular M...HO hydrogen bond is formed, while in the presence of a more basic F^– or CF₃COO^– anion. it transforms to the open fulvenoid structure with an interionic hydrogen bond. The structures of the compounds obtained were determined by IR and¹H NMR spectra, and the structure of the (C₅Me₅RuC₅Me₄CHOH)⁺...F^– salt was proved by X-ray diffraction analysis.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No.12, pp. 2486–2493, December, 1995.The work was carried out with financial support of the International Science Foundation (Grant Nos. JFN-100 and NSAW 000), the Russian Foundation for Basic Research (Grant No. 93-03-4610), and the Ministry of Science and Technical Policy of RF, whom the authors wish to thank.     

Ein neues Syntheseverfahren für die Wolframbronze Cs0,29WO3

A Novel Synthetic Access to the Tungsten Bronze Cs_0.29WO₃ and its Crystal Structure The hexagonal tungsten bronze Cs_0.29WO₃ was obtained in form of black, prismatic crystal by the reduction of WO₃ with molten cesium iodide at 700°C. Its crystal structure was determined by X-ray diffraction (399 unique observed reflexions, R = 0.058). Crystal data: a = 741.2(3), c = 760.0(5) pm, space group P6₃22, Z = 6. It corresponds to the known structure of hexagonal tungsten bronzes, having tungsten atoms displaced from the octahedra centres by 11.9 pm and with three different W? O bond lengths (198, 191, 187 pm). The WO₆ octahedra are slightly titled mutually.     

Synthese und Kristallstruktur des μ-Dinitridosulfato(II)-Komplexes [Na-15-Krone-5]2[μ-(NSN)(MoF5)2]

Synthesis and Crystal Structure of the μ-Dinitridosulfate(II) Complex [Na-15-crown-5]₂[μ-(NSN)(MoF₅)₂] The title compound is formed at room temperature by the reaction of [MoCl₄(NSCl)]₂ with the equivalent amount NaF in acetonitrile in presence of the crown ether 15-crown-5. It forms black, moisture-sensitive crystals that were characterized by an X-ray structure determination (1 756 unique observed reflexions, R = 0.073). Crystal data (19°C): a = 965.5, b = 3 219, c = 1 161.1 pm, β = 95.42°; space group P2₁/n, Z = 4. The structure consists of ion triples in which the [Na-15-crown-5]⁺ ions are associated with the [μ-(NSN)(MoF₅)₂]^2? ions via Na…?F contacts of 215 to 265 pm length. Each of the two MoF₅ units of the anion is linked with one of the N atoms of the dinitridosulfate(II) group; bond angle NSN 103°, MoN bond lengths 172 pm.     

The molecular structure of a three-coordinate palladium(II)-styrene complex [Pd(η5-C5H5)(PEt3)(styrene)]BF4

The molecular structure of a three-coordinate palladium(II)-styrene complex, [Pd(η⁵-C₅H₅)(PEt₃)(styrene)]BF₄ has been determined by means of X-ray diffraction. The crystal belongs to the monoclinic system, space group P2₁/c, with four formula units in a cell of dimensions: a 10.229(3), b 11.262(3), c 18.760(5) Å and β 103.77(2)°. The structure was solved by the heavy atom method, and refined by the least-squares procedure to R = 0.050 for 3635 observed reflections. The palladium atom is surrounded by the cyclopentadienyl group, the triethylphosphine ligand and the olefinic bond of styrene in the cationic complex. In the palladiumstyrene bonding, the olefinic bond is inclined by 77.3° to the coordination plane defined by the Pd and P atoms and the center of the cyclopentadienyl ring (PdC(1) 2.176(6), PdC(2) 2.234(5) and C(1)C(2) 1.369(8) Å).     

Synthesis,spectroscopic investigation and molecular structure of pentacarbonyl-5-t-butyl-5-aza-2,8-dithia-1-stannobicyclo[3.3.01,5]octanechromium(0)

The compound pentacarbonyl-5-t-butyl-5-aza-2,8-dithia-1-stannobicyclo-[3.3.0^1,5]octanechromium(0) was prepared in good yield by the reaction of 5-t-butyl-5-aza-2,8-dithia-1-stannobicyclo[3.3.0^1,5]octane with Cr(CO)₆ in THF under UV-irradiation. The crystal and molecular structure was determined from three-dimensional X-ray data. The crystals are monoclinic, space group P2₁/c. The unit cell, with dimensions a 14.963(9), b 10.026(5), c 13.565(5)Å, β 114.68°(5), contains 4 molecules. The structure was solved by the Patterson-method. The full-matrix refinement with the 2734 independent reflexions gave a final R value of 0.034. The eight-membered ring adopts an asymmetric boat-boat conformation with a tin-nitrogen bond lenght of 2.40 Å; the tin-chromium bond length is 2.62 Å.     

Bis[(tert-butylnitren)tetrachlorowolfram] Synthese,IR-Spektrum und Kristallstruktur

Bis[(tert-butylnitrene)tetrachlorotungsten]: Synthesis, I.R. Spectrum, and Crystal Structure The title compound, forming red crystals, is prepared by the reaction of tungsten hexachloride with the iminoborane Me₃CB?NCMe₃. With PPh₄Cl it forms the chloro complex PPh₄[WCl₅(NCMe₃)]. Both complexes are characterized by their i.r. spectra. The crystal structure of [WCl₄(NCMe₃)]₂ was determined by the aid of X-ray diffraction (space group P2₁/c, Z = 2 dimeric units, 1938 independent, observed reflexions, R = 0.028, a = 636, b = 1537, c = 1124 pm; β 104.74°). The compound forms centrosymmetric molecules in which the tungsten atoms are bridged via chloro atoms with W? Cl bond lengths of 241 and 275 pm. In position trans to the longer WCl bond the nitrene ligand is attached with a WN distance of 170 pm which corresponds to a triple bond; the W?N? C bond angle is 173°.     

The molecular structure of fumaric acid and the deformation of carboxyl group geometry on crystallization

The molecular structure of fumaric acid was studied by means of gas electron diffraction at 260° C. The molecular parameters and their standard deviations obtained for a C_2h model are (r_a distances in Å, angles in degrees): CO: 1.202(0.002), C-O: 1.341(0.006), C-C: 1.486(0.004), CC: 1.356(0.016). C-C-O: 112.1(1.0), C-CO: 124.4(1.1), C-CC: 121.8(1.2). From the available data on carboxylic acids the weighted average deformation of the structure of a carboxylic group on crystallization was determined; a significant expansion of the O-H bond (0.040 Å ), the CO bond (0.010 Å ) and the C-C-O bond angle (1.5° ) and a shrinkage of the C-O bond (0.041 Å ), the C_α-C bond (0.012 Å ) and the C-CO bond angle (2.0° ) was found. The energy for these deformations was estimated to be 1.8 kcal mol^?1.     

Synthesis and crystal and molecular structure of μ-oxo-bis[trifluoroacetato(p-tolyl)iodine]

Crystal and molecular structure of μ-oxo-bis[trifluoroacetato(p-tolyl)iodine] (I) synthesized by a new procedure was determined by X-ray diffraction analysis. Crystals I are orthorhombic, unstable, space group Pbcn, a=17.684(3), b=8.453(3), c=30.560(4) Å, Z=8. The structure of I was solved by direct and Fourier methods and refined by the full-matrix least-squares procedure in an anisotropic-isotropic approximation to R=0.098 (CAD-4 automatic diffractometer, λCuK_α, 1200 observed reflections with I≥2σ). In molecule I, two iodine atoms have T-configuration of valence bonds with the average bond angles O?I?O 169(1) and O?I?C 86(2)°, average bond lengths I?O_μ 2.009(9), I?O_acet 2.269(9), and I?C_aryl 2.11(1) Å, and the bond angle I?O?I 118.1(5)°. In molecule I, two p-Tol substituents are directed to approximately the same side of the medium plane of the central O?I?O?I?O fragment. Crystal structure I has I...O type intra-and intermolecular nonvalent interactions (secondary bonds).