Structural studies of TcO2 by neutron powder diffraction and first-principles calculations

Crystalline technetium dioxide was prepared and for the first time its crystal structure determined by neutron powder diffraction. In addition, electronic structure calculations using density functional theory were performed to further elucidate the bonding mechanisms in TcO2. The crystal structure determined by Rietveld analysis with the NPD data is of a distorted rutile type, similar to that of MoO2; space group P21/c, a = 5.6891(1), b = 4.7546(1), c = 5.5195(1) A, and beta = 121.453(1) degrees . The NPD analysis also establishes a new neutron scattering length of 6.00(3) fm for 99Tc. Our results clearly show metal-metal bonding between Tc pairs along the edge-sharing chains of TcO6 octahedra. The Tc-Tc bond was found to be 2.622(1) A from NPD profile analysis and 2.59 A from first-principles DFT calculations. The bond is somewhat longer than expected from earlier predictions, suggesting that the nature of the Tc-Tc interaction is weaker than anticipated for the Tc(IV) cation with three outer electrons. The NPD results supported by the DFT calculations suggest that the filling of antibonding orbitals and the influence of the crystal field stabilization of the d3 Tc cations lead to more regular TcO6 octahedra and diminish the metal-metal bond strength compared with closely related oxides such MoO2 and WO2.     

A 2D homochiral inorganic-organic framework exhibiting a spin-flop transition

A 2D homochiral inorganic-organic framework {[Mn(NPTA)(4,4'-bpy)(H(2)O)]·(H(2)O)(2)}(n) was prepared by assembling achiral polar 4-nitrophthalic acid, manganese ions, and ancillary 4,4'-bipyridine ligands (NPTA = 4-nitrophthalate) (4,4'-bpy = 4,4'-bipyridine). The isomorphous Zn(ii) compound was also prepared as a diamagnetic analogue. Adjacent manganese spin centres are linked by the syn-anti carboxylate to form a helical chain, and chains of the same chirality are connected by 4,4'-bpy ligands to generate a homochiral layered framework. Edge-to-face aromatic interactions between neighboring layers lead to a 3D homochiral supramolecular structure. Magnetization and heat capacity measurements indicate that the framework is a weak antiferromagnet at low applied field. The magnetic interactions between adjacent manganese ions in the helical chain can be fitted using the 1D Fisher model, with 2J/k = -0.68 K and g = 2.00. Moreover, the compound displays a unique field-dependent spin-flop transition in high magnetic fields, with a critical field of 23.6 kOe at 1.9 K.     

Atroposelective formation of dibenz[c,e]azepines via intramolecular direct arylation with centre-axis chirality transfer

5-Substituted 6,7-dihydrodibenz[c,e]azepines, a class of secondary amine incorporating a centre-axis chirality relay, are accessible from 1-substituted N-(2-bromobenzyl)-1-phenylmethanamines via N-acylation and ring-closing intramolecular direct arylation. The ring closure proceeds with high atropodiastereoselectivity due to strain effects that are induced by trigonalisation of the nitrogen atom, as predicted using molecular mechanics calculations.     

Magnetic and mechanical anisotropy in a manganese 2-methylsuccinate framework structure

Hybrid inorganic-organic framework materials exhibit unique properties that can be advantageously tuned through choice of the inorganic and organic components and by control of the crystal structure. We present a new hydrothermally prepared 3D hybrid framework, [Mn(2-methylsuccinate)](n) (1), comprising alternating 2D manganese oxide sheets and isolated MnO(6) octahedra, pillared via syn, anti-syn carboxylates. Powder magnetic characterization shows that the compound is a homospin Mn(II) ferrimagnet below 2.4 K. The easy-axis is revealed by single-crystal magnetic susceptibility studies and a magnetic structure is proposed. Anisotropic elastic moduli and hardness, observed through nanoindentation on differing crystal facets, were correlated with specific structural features. Such measurements of anisotropy are not commonly undertaken, yet allow for a more comprehensive understanding of structure-property relationships.     

关于K个复相关矩阵相等的检验

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Guest-mediated phase transitions in a flexible pillared-layered metal–organic framework under high-pressure

The guest-dependent flexibility of the pillared-layered metal–organic framework (MOF), Zn₂bdc₂dabco·X(guest), where guest = EtOH, DMF or benzene, has been examined by high-pressure single crystal X-ray diffraction. A pressure-induced structural phase transition is found for the EtOH- and DMF-included frameworks during compression in a hydrostatic medium of the guest species, which is dependent upon the nature and quantity of the guest in the channels. The EtOH-included material undergoes a phase transition from P4/mmm to C2/m at 0.69 GPa, which is accompanied by a change in the pore shape from square to rhombus via super-filling of the pores. The DMF-included material undergoes a guest-mediated phase transition from I4/mcm to P4/mmm at 0.33 GPa via disordering of the DMF guest. In contrast, the benzene-included framework features a structure with rhombus-shaped channels at ambient pressure and shows direct compression under hydrostatic pressure. These results demonstrate the large influence of guest molecules on the high-pressure phase behavior of flexible MOFs. Guest-mediated framework flexibility is useful for engineering MOFs with bespoke pore shapes and compressibility.

The guest-dependent flexibility of the pillared-layered metal–organic framework (MOF), Zn₂bdc₂dabco·X(guest), where guest = EtOH, DMF or benzene, has been examined by high-pressure single crystal X-ray diffraction.