An X-ray diffraction and MAS NMR study of the thermal expansion properties of calcined siliceous ferrierite

Powder and single-crystal X-ray diffraction, combined with MAS NMR measurements, has been used to study the thermal expansion of siliceous zeolite ferrierite as it approaches a second-order displacive phase transition from a low-symmetry (Pnnm) to a high-symmetry (Immm) structure. Below the transition temperature, ferrierite exhibits positive thermal expansivity. However, above the transition temperature a significant change in thermal behavior is seen, and ferrierite becomes a negative thermal expansion material. Accurate variable-temperature single-crystal X-ray diffraction measurements confirm the transition temperature and allow the changes in average atomic position to be followed with temperature. The results from the single-crystal X-ray diffraction study can be correlated with (29)Si MAS NMR chemical shifts for the low-temperature phase. At low temperatures the results show that the positive thermal expansivity is driven by an overall increase in Si[bond]Si distances related to an increase in Si[bond]O[bond]Si bond angles. However, in the high-temperature phase the Si[bond]O[bond]Si angles are approximately invariant with temperature, and the negative thermal expansion in this case is caused by transverse vibrations of the Si[bond]O[bond]Si units.     

The pH-controlled hydrothermal synthesis and crystal structures of two zinc N,N'-piperazinebis(methylenephosphonate) frameworks

An exploration of the reactions of N,N'-piperazinebis(methylenephosphonic acid), H4L, with zinc salts has led to the isolation of two new framework zinc phosphonates. ZnLH2.H2O (I) is isostructural with previously reported manganese(II) and cobalt(II) analogues, and consists of infinite 'zinc phosphate' chains bridged into three dimensions via the organic moieties. The resulting framework encloses large channels in which the loosely bound H2O resides. The H2O is lost reversibly at around 160 degree C, without framework collapse. Zn2L (II) has a novel framework structure, prepared at an initial pH > 7, which consists of two-dimensional 'zinc phosphate' sheets, comprising both four- and eight-membered -Zn-O-P- rings, which are also linked into three dimensions via the organic groups. In both cases, the zinc centre is tetrahedral; in I coordination is by oxygen atoms from four different phosphonate groups, whereas in II the additional deprotonation of the ligand allows coordination via three oxygen atoms plus the amine nitrogen atom.     

The crystal structure of [Ni(dmf)6][NiCl4] and comments on the hydrolysis of coordinated amides and peptides in metal complexes

Solutions of nickel(II) chloride in N,N-dimethylformamide (DMF) in the presence of Et₂O slowly give bluegreen crystals of [Ni(dmf)₆][NiCl₄] whose structure has been confirmed by X-ray crystallography. The complex crystallises with three crystallographically distinct [Ni(dmf)₆]²⁺ cations in the asymmetric unit, Ni(1) on a general position and Ni(2) and Ni(3) on centres of symmetry. There are also two unique [NiCl₄]²⁻ anions, both on general positions. N,N-Dimethylformamide is O-bonded to nickel as coordination via nitrogen would cause the loss of the resonance energy of the amide group. All Ni—O bond lengths are within the expected range for such complexes [2.029(4)–2.084(4) \rA]. The C=O bond lengths, which range from 1.219(7) to 1.259(7) \rA, are not significantly different from those for uncomplexed dmf (1.232 ± 0.004 \rA), suggesting that there is very little polarisation of the C=O bond by nickel(II). Oxygen-bonded dmf undergoes rapid hydrolysis in metal complexes, for example the base hydrolysis of [Co(NH₃)₅dmf]³⁺ to give the formato complex is some 10⁴-fold faster than that of the free ligand. Copper(II)-catalysed amide and peptide bond hydrolysis is subject to ca. 10⁶-fold rate enhancements. The origin of these rate enhancements, which appear to be due to transition state effects rather than ground state effects, is discussed in detail. This revised version was published online in June 2006 with corrections to the Cover Date.     

Enantioselective Hydrogenation of Olefins with Iridium–Phosphanodihydrooxazole Catalysts

High turnover numbers and up to 98% ee were obtained in the catalytic hydrogenation of unfunctionalized aryl-substituted olefins with iridium–phosphanyldihydrooxazole complexes 1 (see reaction scheme). The anion of the complex—for example, hexafluorophosphate or tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF⁻)—has a remarkable effect on the reactivity and longevity of the catalyst.     

Magnetic Properties and Structure of Polynuclear Compounds Based on Macrocyclic Azacyclam Complexes of Nickel(II) and Bridging Oxalate Ions

By X-ray diffraction analysis, we have established the structure of a binuclear complex of nickel(II) with an azacyclam ligand and a bridging oxalate ion. Study of the magnetic properties of a series of complexes of this type with related macrocyclic ligands has allowed us to observe antiferromagnetic coupling between the metal ions and a magnitude of the exchange integral J that varies in the range from –11.8 cm^–1 to –25.6 cm^–1. We have shown that, despite the close structural parameters of the macrocyclic ligands, the complexes are characterized by different magnetochemical behavior.     

Crystal structure and Mössbauer spectroscopy of Y2SrFeCuO6.5, a double layer perovskite intergrowth phase

The crystal structure of Y₂SrFeCuO_6.5 was determined from single-crystal X-ray and neutron powder diffraction studies. M_r = 488.81, orthorhombic, Ibam, a = 5.4036(8)[5.4149(1)] Å, b = 10.702(1)[10.7244(1)] Å, c = 20.250(2)[20.2799(2)] Å; values in square brackets are neutron data. V = 1171.0(4), Z = 8, D_x = 5.544 g cm⁻³, λ = 0.71069 Å, μ = 345.1 cm⁻¹, R = 0.048 for 567 observed reflections. The Fe/Cu atoms occupy randomly the approximate center of oxygen pyramids. The pyramids share the apical oxygen and articulate laterally by corner sharing of oxygen to form a double pyramidal layer perpendicular to c. The pyramidal slabs are separated by double layers of Y that are in 7-fold coordination to oxygen, forming a defect fluorite unit. Mössbauer spectra indicate a unique iron environment and magnetic ordering at about 265 K. The paramagnetic phase coexists with the magnetic phase over an approximate temperature range 300-263 K, characteristic of magnetic ordering in 2-D magnetic structures. The isomer shift, 0.26, and quadrupole splitting, 0.56 mm sec⁻¹, are consistent with Fe³⁺ in 5-fold coordination and H_int values also indicate classic high spin Fe³⁺. The average Y---O bond length is 2.331(6) Å and Sr is in a dodecahedral environment in which, however, two oxygen atoms at the corners of the cube are missing. The average Sr---O bond length is 2.793(10) Å. The structure is derived from the Ruddlesden-Popper phase Sr_n+1Ti_nO_3n+1 with n = 2.     

Ionic liquids and deep eutectic mixtures as new solvents for the synthesis of vanadium fluorides and oxyfluorides

An exploratory study of the synthesis of vanadium (oxy)fluorides (VOFs) using ionic liquids (ILs) and deep eutectic mixtures (DESs) as a solvent yielded 10 different materials. The previously reported chain type: (NH(4))(2)VF(5) (1), (NH(4))(2)VOF(4) (2), NH(4)VO(3) (3) and (H(2)NH(2)(CH(2))(2)NH(2))VF(5) (9) have been successfully produced for the first time using ILs as the reaction media. The monomeric (HNH(2)CH(3))(2)VOF(4)(H(2)O) (4), the dimer (HNH(2)CH(3))(4)V(2)O(2)F(8) (5) and the 1D chains (HNH(2)CH(3))(2)VF(5) (6), (H(2)O)(2)VF(3) (7), α-(H(2)NH(2)(CH(2))(2)NH(2))VOF(4) (8) and β-(H(2)NH(2)(CH(2))(2)NH(2))VOF(4) (10) are novel materials. Template control has also been achieved by the selective choice of ILs or the appropriate deep eutectic mixture, where the expected template is delivered to the reaction by the partial breakdown of the urea derivative portion of the DES.     

Lead-strontium borate halides with hilgardite-type structure and their SHG properties

Lead-containing members of the hilgardite family of borate halides, M₂B₅O₉X (M=Ca, Sr, Ba, Pb; X=Cl, Br) exhibit an abnormally large nonlinear optical response. In order to establish any underlying crystal-chemical rationale for this we have carried out detailed crystallographic characterisations of the representative members of this series, Sr₂B₅O₉Cl and Pb₂B₅O₉Cl, using powder neutron diffraction. Both adopt the orthorhombic hilgardite structure type, in space group Pnn2. The full solid solution range Pb_2−xSr_xB₅O₉Cl has also been prepared, and studied by X-ray Rietveld and second harmonic generation (SHG) measurements. The SHG response shows a linear decrease with increasing x, whereas the unit cell shows an increasing orthorhombic distortion. However, the crystallographic results suggest no significant or systematic changes in the nature of the borate sublattice between Sr₂B₅O₉Cl and Pb₂B₅O₉Cl. We conclude that the enhanced SHG response in Pb-containing hilgardites is due predominantly to the presence of the polarizable nature of Pb²⁺, in particular the presence of a stereochemically active lone pair.