Philip Pattison

A two-circle powder diffractometer for synchrotron radiation with a closed loop encoder feedback system

A high-angular-resolution two-circle powder diffractometer equipped with long diffracted-beam collimators has been built at Daresbury Laboratory. The diffractometer has encoders mounted directly on the 2θ and ω axes. These give a nominal angular resolution of 0.1 and 1.0 mdeg respectively. Repeated scans of single powder peaks have demonstrated a reproducibility of 0.1 mdeg 2θ. Measurements on five independent peaks of tungsten give a self consistency of 1(1) × 10−5 A. An example data set from synthetic olivine Mg2SiO4 has been refined using the Rietveld method and the results compare very well with single-crystal structure refinements.

A new multipurpose diffractometer PILATUS@SNBL.

The diffraction beamline BM01A at the European Synchrotron Radiation Facility (CRG Swiss-Norwegian beamlines) has been successfully operational for 20 years. Recently, a new multifunctional diffractometer based on the Dectris Pilatus 2M detector has been constructed, commissioned and offered to users. The diffractometer combines a fast and low-noise area detector, which can be tilted and moved horizontally and vertically, together with flexible goniometry for sample positioning and orientation. The diffractometer is controlled by a user-friendly and GUI-based software Pylatus which is also used to control various auxiliary equipment. The latter includes several heating and cooling devices, in situ cells and complimentary spectroscopic tools.

The charge density of urea from synchrotron diffraction data.

The charge density of urea is studied using very high precision single-crystal synchrotron-radiation diffraction data collected at the Swiss-Norwegian Beam Lines at ESRF. An unprecedented resolution of 1.44 A(-1) in sin theta;/lambda is obtained at 123 K. The optimization of the experiment for charge-density studies is discussed. The high precision of the data allowed the refinement of a multipole model extending to hexadecapoles and quadrupoles on the heavy and H atoms, respectively, as well as a liberal treatment of radial functions. The topological properties of the resulting electron density are analysed and compared with earlier experimental results as well as with periodic Hartree-Fock calculations. The properties of the strongly polarized C-O bond agree with trends derived from previous experimental results while the ab initio calculations differ significantly. The results indicate that the description of the C-O bond requires more flexible basis sets in the theoretical calculations. The calculated integrated atomic charges are much larger than the observed ones. It is suggested that the present experimental results provide new target values for validation of future ab initio calculations. The molecular dipole moment derived from the integrated atomic properties is the same as the one obtained from the multipole model even though the individual atomic contributions differ. Comparison with literature data for urea in solution and the gas phase yields a dipole enhancement in the solid of about 1.5 D. The thermal expansion of urea is determined using synchrotron powder diffraction data. With decreasing temperature, an increasing anisotropic strain is observed.

Homologous critical behavior in the molecular frameworks Zn(CN)2 and Cd(imidazolate)2.

Using a combination of single-crystal and powder X-ray diffraction measurements, we study temperature- and pressure-driven structural distortions in zinc(II) cyanide (Zn(CN)2) and cadmium(II) imidazolate (Cd(im)2), two molecular frameworks with the anticuprite topology. Under a hydrostatic pressure of 1.52 GPa, Zn(CN)2 undergoes a first-order displacive phase transition to an orthorhombic phase, with the corresponding atomic displacements characterized by correlated collective tilts of pairs of Zn-centered tetrahedra. This displacement pattern sheds light on the mechanism of negative thermal expansion in ambient-pressure Zn(CN)2. We find that the fundamental mechanical response exhibited by Zn(CN)2 is mirrored in the temperature-dependent behavior of Cd(im)2. Our results suggest that the thermodynamics of molecular frameworks may be governed by considerations of packing efficiency while also depending on dynamic instabilities of the underlying framework topology.

Ligand Aspect Ratio as a Decisive Factor for the Self-Assembly of Coordination Cages

It is possible to control the geometry and the composition of metallasupramolecular assemblies via the aspect ratio of their ligands. This point is demonstrated for a series of iron- and palladium-based coordination cages. Functionalized clathrochelate complexes with variable aspect ratios were used as rod-like metalloligands. A cubic Fe(II)8L12 cage was obtained from a metalloligand with an intermediate aspect ratio. By increasing the length or by decreasing the width of the ligand, the self-assembly process resulted in the clean formation of tetrahedral Fe(II)4L6 cages instead of cubic cages. In a related fashion, it was possible to control the geometry of Pd(II)-based coordination cages. A metalloligand with a large aspect ratio gave an entropically favored tetrahedral Pd(II)4L8 assembly, whereas an octahedral Pd(II)6L12 cage was formed with a ligand of the same length but with an increased width. The aspect ratio can also be used to control the composition of dynamic mixtures of Pd(II) cages. Out of two metalloligands with only marginally different aspect ratios, one gave rise to a self-sorted collection of Pd(II)4L8 and Pd(II)6L12 cages, whereas the other did not.

The luminescence of NaxEu3+(2−x)/3MoO4 scheelites depends on the number of Eu-clusters occurring in their incommensurately modulated structure

Scheelite related compounds with general formula Mn(XO4)m are the subject of hefty interest owing to their optical properties, stability and relatively simple preparation. Eu3+-containing scheelites are considered as red-emitting phosphors and the main factors affecting their luminescence are thought to be chemical composition and particle size while the influence of their structure is generally ignored. Here we report eight compounds from the NaxEu(2−x)/3MoO4 series prepared by conventional solid-state reaction and present a detailed analysis of their crystal structures. Six of them have modulated structures, a common feature of SRCs, in which dopant Eu3+ ions are orderly distributed. Moreover, different amounts of Eu3+ dimers are detected in the modulated structures, characterized by weak satellite reflections appearing in the lower angle part of the XRD patterns. These reflections are indexed and incorporated into Rietvelds refinement using superspace (3 + 1)-dimension symmetry. The remarkable feature of the compounds is that the characteristic luminescence parameters, overall (QEuL) and intrinsic (QEuEu) quantum yields, Eu(5D0) lifetimes, and sensitization efficiencies (ηsens), correlate with the number of Eu3+ aggregates, but not directly with the composition x of the materials. This provides an efficient tool for understanding and controlling the luminescence properties of scheelite related compounds.

Crystal structure and solution species of Ce(III) and Ce(IV) formates: from mononuclear to hexanuclear complexes.

Cerium(III) and cerium(IV) both form formate complexes. However, their species in aqueous solution and the solid-state structures are surprisingly different. The species in aqueous solutions were investigated with Ce K-edge EXAFS spectroscopy. Ce(III) formate shows only mononuclear complexes, which is in agreement with the predicted mononuclear species of Ce(HCOO)(2+) and Ce(HCOO)2(+). In contrast, Ce(IV) formate forms in aqueous solution a stable hexanuclear complex of [Ce6(μ3-O)4(μ3-OH)4(HCOO)x(NO3)y](12-x-y). The structural differences reflect the different influence of hydrolysis, which is weak for Ce(III) and strong for Ce(IV). Hydrolysis of Ce(IV) ions causes initial polymerization while complexation through HCOO(-) results in 12 chelate rings stabilizing the hexanuclear Ce(IV) complex. Crystals were grown from the above-mentioned solutions. Two crystal structures of Ce(IV) formate were determined. Both form a hexanuclear complex with a [Ce6(μ3-O)4(μ3-OH)4](12+) core in aqueous HNO3/HCOOH solution. The pH titration with NaOH resulted in a structure with the composition [Ce6(μ3-O)4(μ3-OH)4(HCOO)10(NO3)2(H2O)3]·(H2O)9.5, while the pH adjustment with NH3 resulted in [Ce6(μ3-O)4(μ3-OH)4(HCOO)10(NO3)4]·(NO3)3(NH4)5(H2O)5. Furthermore, the crystal structure of Ce(III) formate, Ce(HCOO)3, was determined. The coordination polyhedron is a tricapped trigonal prism which is formed exclusively by nine HCOO(-) ligands. The hexanuclear Ce(IV) formate species from aqueous solution is widely preserved in the crystal structure, whereas the mononuclear solution species of Ce(III) formate undergoes a polymerization during the crystallization process.

Highly selective additions of hydride and organolithium nucleophiles to helical carbenium ions

Unsymmetrical cationic [4]helicenes react with hydride or organolithium reagents and the two diastereotopic faces can be discriminated with high efficiency (dr up to and higher than 49 : 1)

Neutron diffraction study of YVO3, NdVO3, and TbVO3

The structural and magnetic properties of YVO3, NdVO3 and TbVO3 were investigated by single-crystal and powder neutron diffraction. YVO3 shows a structural phase transition at 200 K from an orthorhombic structure with the space group Pbnm to a monoclinic one with the space group P21 /b. But supplementary highresolution synchrotron diffraction experiments showed that the monoclinic distortion is extremely small. A group theoretical analysis shows that this magnetic state in the monoclinic phase is incompatible with the lattice structure, unless terms of higher than bilinear order in the spin operators are incorporated in the spin Hamiltonian. This observation is discussed in the light of recent theories invoking unusual many-body correlations between the vanadium t2g orbitals. A structural phase transition back to the orthorhombic space group Pbnm is observed upon cooling below 77 K. This transition is accompanied by a rearrangement of the magnetic structure into a mode compatible with the lattice structure. The crystal structures of NdVO3 and TbVO3 are closely similar to that of YVO3. However, only a single magnetic phase transition was found in the vanadium sublattice down to 9.5 K. Below 60 K the magnetic moments of the Nd 3+ - and Tb 3+ -ions are gradually polarized by the ordered vanadium moments. Below 11 K, we found a noncollinear order of the terbium moments.

Carboxylic Acid Functionalized Clathrochelate Complexes:Large, Robust, and Easy-to-Access Metalloligands

Polycarboxylate ligands are among the most important building blocks for the synthesis of metal-organic frameworks (MOFs). The ability to access these ligands in an efficient way is of key importance for future applications of MOFs. Here, we demonstrate that mono- and dinuclear clathrochelate complexes are versatile scaffolds for the preparation of polytopic carboxylate ligands. The largely inert clathrochelate complexes have a trigonal-bipyramidal shape. The synthesis of functionalized clathrochelates with two, three, four, or five carboxylic acid groups in the ligand periphery can be achieved in a few steps from simple starting materials. Apart from being easily accessible, the metalloligands display interesting characteristics for applications in metallasupramolecular chemistry and materials science: they are rigid, large (up to 2.2 nm), and robust and they can show additional functions (e.g., fluorescence or extra charge) depending on the metal ion that is present in the clathrochelate core. The utility of these new metalloligands in MOF chemistry is demonstrated by the synthesis of zinc- and zirconium-based coordination polymers. The combination of Zn(NO3)2 with clathrochelates having two or three carboxylic acid groups gives MOFs in which the clathrochelate ligands are connected by Zn4O clusters or zinc paddlewheel links. The structures of the resulting two- and three-dimensional networks could be established by single-crystal X-ray crystallography. The reaction of carboxylic acid functionalized clathrochelates with ZrCl4 gives amorphous powders that display permanent porosity after solvent removal.