Comparative study between two new phthalate/imidazole Cobalt complexes

The main strategy for preparing novel multifunctional materials is based on self-assembly method which employs polydentate organic ligands containing N- or O-donor as building blocks. In this context, those ligands like imidazole or carboxylate groups are of special interest due to their good coordination ability and diverse coordination modes [1]. As a part of our investigations of extended structures with mixed ligands, new complexes [Co(Hipht)2(Im)2(H2O)2] and [Co(Tpht)(Im)3(H2O)2].H2O were obtained by direct method, then characterized by IR spectroscopy, TG/ATD and X-ray crystallography. In the compound [Co(Hipht)2(Im)2(H2O)2], CoII is located on a symmetry center, surrounded by two aqua ligands, two hydrogeno-isophthalate ligands and two imidazole molecules, where all ligands adopt monodentate mode. The complex's geometry consist of two intermolecular N-H...O bonds (2,157 Å and 2,630 Å) formed by one H atom of imidazole ligand and oxygen atoms of acid molecules evolving along two directions, giving rise to R24 (16) synthons. The interplanar distance of 3,718 Å between two parallel imidazole rings reveals the existence of Π-Π interactions. In the [Co(Tpht)(Im)3(H2O)2].H2O [2], the Co cation exhibits an octahedral coordination sphere, with two aqua, three N-coordinated imidazole ligands and one terephthalato dianion. The three independent imidazole groups and the single terephtalate dianion, all unidentate participate respectively as donor and acceptor in strong to moderate hydrogen bonds, and allow the recognition of supramolecular dimensionality. The backbone of the architecture is the helical hydrogen-bonded ladder running along b axis, composed of alternating R44(10) and R33(8) heterosynthons, which are developed in bicyclic sheets. Non covalent interactions play a significant role in this class of materials [3], in fact, extended hydrogen bonds networks associated to Π-Π interactions lead to 3D supramolecular architecture for the two complexes.

Comparative study of two oxalate isomers with different dimensionalities

We report here two bimetallic oxalate isomers with the same chemical formula [RbCr(C2O4)2(H2O)2], which have been synthesized respectively by a slow evaporation method at room temperature (compound I) [1], and under hydrothermal conditions (compound II) [2] with the same starting salts. Their structures show a several discrepancies, due probably to the synthetic conditions. Indeed, the compound I crystallizes in space group C2/m with the Cr, Rb atoms and one oxygen from water molecule lying on special positions. Moreover, the unique oxalate ligand forms a bridge between metal centers. The Cr atom is coordinated to 2 bidentate-chelating oxalates and 2 aqua ligands in a trans-conformation and any water molecule has been found around the 8-coordinated Rb atom, leading to a layered structure consists of alternating Rb and Cr polyhedra connected via the unique organic ligand. Whereas, the compound II crystallizes in space group P21/n, with all atoms located on general positions. Furthermore, two independent oxalato ligands exhibit different configurations, which one is pentadentate and the other is hexadentate with two different chelating modes. The very slightly distorted Cr octahedra consists of 2 bidentate-chelating oxalato ligands and 2 water molecules in a cis-conformation, while the alkali metal is surrounded by seven O atoms from oxalate groups, completed with two H2O molecules which are bridging the Cr and Rb polyhedra by one common edge. This results in the formation of three different chains of alternating edge- and vertex-shared polyhedra through oxalates groups and aqua ligands, running along the three space directions to build a three dimensional framework. These two compounds can be considered as supramolecular isomers [3].

An unique model for joint refinement of (polarised) neutron and X-ray data

A new charge and spin density model and the corresponding refinement software were recently developed to combine X-ray and polarised neutron diffraction experiments [1,2]. This joint refinement procedure allows for an access to both the charge and spin densities but also to spin up ( ) and spin down ( ) electron distributions. These two quantities ( and ) were thus separately modelled and for the first time it was possible to compare them with theoretical results. The first part of the presentation will introduce the refinement procedure and describe its application to the case of an end-to-end azido double bridged copper(II) complex[3]. The results of this joint refinement of X-ray and polarized neutron diffraction data will be compared to theoretical calculations. The second part will be devoted to recent applications to other materials including a purely organic radical.

Synthesis, magneto-structural features and thermal behavior of a new MOF

By applying soft synthesis conditions, in presence of a mixture that contains malonic acid, (3 mmol), lanthanide salt CeCl3.7H2O (1 mmol), alkaline-earth hydroxide Ca(OH)2 (1 mmol), and at pH below 4, we have obtained single crystals of a novel rare-earth complex which includes the dianion [L2-], {[Ce2(C3H2O4)3(H2O)3].2H2O}.The given formula was deduced from the single crystal structure analysis. The asymmetric unit corresponds to chemical formula (Figure 1). This novel polymeric compound is a three-dimensional MOF, built up from cross-linked infinite chains of one-edge-shared CeO7(H2O)3 and CeO9 polyhedrons embedding solvent molecules. Each crystallographically different metal is connected to two neighbouring ones, through four μ2-oxo bridges, to form repeat four-membered typical Ce/O/Ce/O rings. The three independant ligands show different conformations of their end functional groups: syn-syn, syn-anti, anti-anti, giving explanation for the concomitant magnetic interactions and interesting magneto-structural data. These studies are still in progress. The TG curve shows five successive weight losses. The material begins losing weight directly upon starting the thermal gravimetric experiments. It is one of the more unstable lanthanide malonates known. The correlative thermal behaviour data and X-Ray structural sub-features of the novel Ce(III)-based MOF obtained, brought out the great supramolecular effects: decarboxylation process, crystalline, and thermal stabilities are much more related to the specific infinite hydrogen patterns, than the size, shape, and embedding capability of voids accommodated in the 3D packing.

X-ray and neutron scattering: from ideal zeolites to real guest-host systems

Tetrahedral MOFs, microporous metallophosphates and zeolites share common framework topologies despite very different chemical nature of their crystalline skeleton. In the case of zeolites or metallophosphates, the polymeric [TO2]_ framework is based on single tetrahedral atoms T (T= Si4+/Al3+ or Ga3+/B3+/P5+ respectively) connected by corner oxygen atoms O. In the case of MOFs, the skeleton is built from supertetraedra formed by a central metal atom complexed by 4 organic linkers (terephtalate,...); hence homologous structures expanded by a "scale factor". In "molecular sieves", the static porosity but also the "breathing effect" control, in first approximation, the adsorption and the self-diffusion of guest molecules. These latter are finely tuned by the interaction of these molecules with the framework itself (though H-bonds or electrostatic interactions), with charge compensating cations in basic zeolites (through coulombic interaction), or with other adsorbed molecules. However, the interplay between guest molecule adsorption, framework relaxation, cation redistribution and atomic charge transfer leads to an inextricable problem for the modeling and prediction of adsorption properties in these systems in absence of any polarizable and adaptive force field. We will present experimental charge density studies on activated industrial zeolites FAU, MFI and MOR. The electrostatic properties derived on these systems (electric field, atomics charges) are validated by a comparison with those derived, experimentally and theoretically, on a comprehensive set of simpler quartz-type materials (SiO2, AlPO4, GaPO4,...) representative of typical microporous compositions. Finally, we will combine the informations brought by diffraction, diffuse scattering and Monte-Carlo modelling to analyse the adsorption process or the role of the template in the stabilisation of the framework in these systems and in the famous MIL-53 MOF

The International Year of Crystallography

IYCr2014 aims at improving public awareness of the field, boost access to instrumentation and high-level research, nurture "home-grown" crystallographers in developing nations, and increase international collaborations for the benefit of future generations. The IUCr-UNESCO OpenLab is a network of operational crystallographic laboratories based mainly in Africa, Asia and South America, and implemented in partnership with industry. The OpenLabs will enable students in far-flung lands to have hands-on training in modern techniques and expose them to cutting-edge research in the field. Such project was started based on the strong experience gained through the IUCr Initiative in Africa . The Summit meetings are intended to bring together scientists from countries in three widely separated parts of the world. Karachi (Pakistan), Campinas (Brazil) and Bloemfontein (South Africa). These meetings, attended by scientists from academia and industry and by science administrators, will focus on high-level science, and also highlight the difficulties and problems of conducting competitive scientific research in different parts of the developing world. Moreover, a worldwide crystal-growing competition aims at attracting and inspiring youngsters.