Synthesis and characterisation of inorganic framework materials

Inorganic framework materials, with structures based on arsenate As04 tetrahedra and a variety of additional trigonal, tetrahedral or octahedral units have been synthesised by hydrothermal methods. The framework topologies of these materials have been characterised by single crystal X-ray diffraction; additional techniques, including thermogravimetric analysis and SQUID magnetoinetry, have been used to assess properties ofsome materials. Iron arsenates with 2D and 3D framework topologies are reported; these include structures templated by piperazine; 1,4-diaminobutane, DABCO, Fe2As207·2H20 and LiFeAs040H. The boron arsenic system has been investigated using a molten salt technique and a variety of alkali metal templates. A total of ten boroarsenate materials have been characterised, including [Cs2CBAs030H)s(As04)2] [(CsCI4)]CI, which is templated on both anions and cations and a family of materials XBAs04F ex = Cs, Rb, NHt). Hydrothermal techniques have been used to produce arsenate frameworks in combination with molybdenum, zinc and scandium. Two 'zinc arsenate materials with new framework topologies have been synthesised; a chiral framework with 16-ring channels and a 2D layer framework, both with caesium templates. Five scandium arsenate frameworks have been synthesised and are characterised; three have the same basic framework structure but with differing amine templates. Further new compounds have been synthesised by evaporating solutions of ainines with metal chloride or arsenate covnter-ions; these include three arsenate DABCO structures and a related phosphate DABCO material. Zinc chloride, cobalt chloride and hydrated manganese chloride salts with piperazinium counter-ions were also synthesised. Hydrothermal syntheses with vanadium reagents resulted in the formation of five new structures - a vanadium arsenate; a vanadium oxalate; a nickel vanadate cluster cap'ped by 2,2bipyridyl and isostructural nickel and cobalt metavanadates, NiV20 6·H20 and CoV206·H20. Powder neutron diffraction was used to show the isostructural,materials possessed differing magnetic cells. During investigations into copper arsenate framework materials, four new copper halide coordination polymers were discovered. Two materials with tetramethylammonium (TMA) templates - CU2Br3(TMA) and CU2I3(TMA) - and a DABCO templated structure CU2Ch(DABCO) share a common stoichiometry but were shown to possess different structural motifs. A fourth material discovered, CU7I7(DABCOhs, forms a· complex 3D organic-inorganic hybrid framework.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Synthesis and structural study of stoichiometric BiTi2O7 pyrochlore

Bi2Ti2O7 has been synthesized using a co-precipitation route from H2O2/NH3(aq) solutions of titanium with aqueous bismuth nitrate. The stoichiometric material crystallizes into a pale yellow cubic pyrochlore phase. A powder X-ray diffraction study showed this crystallization to be very temperature sensitive, the pure phase can only be obtained within a few degrees of 470degreesC. Time-of-flight powder neutron diffraction studies of Bi2Ti2O7 (Space group Fd (3) over barm, a = 10.37949(4) Angstrom at ambient temperature, Z = 8, R-p = 3.95%, R-wp = 4.75%) revealed positional disorder in the bismuth site and in the O' oxide site both at ambient temperature and at 2K

Boroarsenates: a framework motif and family templated on cations and anions

Molten salt reactions of NH4H2AsO4, H3BO3, and MX (M = Li, Na, K, Rb Cs, NH4 and X = F, Cl, Br) yield numerous new alkali metal and alkali metal salt templated three-dimensional boroarsenate and fluoroboroarsenate frameworks. The structures of these materials are formed from BO4 (BO3F) and As(O,OH)4 tetrahedra defining channels and interlayer regions containing either simple alkali metal cations or both cations and halide anions. These boroarsenate-based frameworks are unusual in comparison with other oxotetrahedral-based materials in that terminal OH, on As, may be present, decorating the inner surfaces of the channels, as in the 12-membered rings of K2[B(AsO3O)2H]. This unit also permits coordination to nonframework anions as well as cations, so that (Cs2[BAsO3OH]8[AsO4]2[CsCl4]Cl)2 (and its Br analogue) contains layers of [CsCl4]3- and Cl- ions separated and coordinated by the protonated boroarsenate framework

A chiral, 16-ring channel framework and a layered caesium zincoarsenate

Caesium zinc arsenate frameworks with a large, chiral pore and an expansive interlayer region have been synthesised

Fast energy minimization of the CCDC drug subset structures by molecule-in-cluster computations allows independent structure validation and model completion

Optimizing structures with computations on clusters of molecules permits generation of structure-specific restraints for refinement. Equally importantly, retrospective structure validation and addition of hydrogen atoms consistent with quantum chemistry is possible for experimental structures or the solvent molecules in them, should they be missing in earlier CIF depositions. Revisiting the drug subset structures of the CCDC demonstrates that structure validation through ab initio cluster computations is a tremendous validation tool. The time required for optimization can be similar to the time required to carry out least squares refinement for small-molecule structures, and becomes feasible for large structures. Several questions arise: is it valid to augment experimental structures with structure-specific restraints, ideally through accompanying refinement with computation? Do energy minimized structures (using the experimental determinations as a starting point) still constitute an experimental result? When re-refinement is impossible in retrospect, like for most of the drug-subset molecules, then additional value lies in completion and validation of existing structures so that they are chemically and crystallographically correct, and contain missing water or solvent hydrogen atoms. Our results suggest that retroactive validation and addition of hydrogen atoms becomes possible for the entire Cambridge Structural Database. Generation of database entries of optimized alongside existing structures will provide the flexibility needed to make full use of the information gained by computation

Iron arsenate frameworks

Six new iron arsenate framework structures, Fe2As2O7·2H2O, [Fe6As8O32H4]2–(1,4-butanediamininium2+)·2H2O, [Fe4As6O22H2]2–(piperazinium2+), [Fe5As5O24H4]2–(piperazinium2+)·2H2O, [Fe6As7O31H5]2–(dabco2+) and LiFeAsO4OH have been synthesised under hydrothermal conditions. Incorporation of the amine cation templates leads to more open framework geometries and, in contrast to iron phosphates which have topologies based on PO4 tetrahedra, the iron arsenate structures typically contain protonated As(O,OH)4 units. The magnetic properties of the iron arsenates studied show Curie–Weiss behaviours with maxima in the (T) vs.T plots in the range 10–50 K

Development of a Cambridge Structural Database Subset: A Collection of Metal–Organic Frameworks for Past, Present, and Future

We report the generation and characterization of the most complete collection of metal–organic frameworks (MOFs) maintained and updated, for the first time, by the Cambridge Crystallographic Data Centre (CCDC). To set up this subset, we asked the question “what is a MOF?” and implemented a number of “look-for-MOF” criteria embedded within a bespoke Cambridge Structural Database (CSD) Python API workflow to identify and extract information on 69 666 MOF materials. The CSD MOF subset is updated regularly with subsequent MOF additions to the CSD, bringing a unique record for all researchers working in the area of porous materials around the world, whether to perform high-throughput computational screening for materials discovery or to have a global view over the existing structures in a single resource. Using this resource, we then developed and used an array of computational tools to remove residual solvent molecules from the framework pores of all the MOFs identified and went on to analyze geometrical and physical properties of nondisordered structures

Deconstruction of Crystalline Networks into Underlying Nets: Relevance for Terminology Guidelines and Crystallographic Databases

This communication briefly reviews why network topology is an important tool (for understanding, comparing, communicating, designing, and solving crystal structures from powder diffraction data) and then discusses the terms of an IUPAC project dealing with various aspects of network topology. One is the ambiguity in node assignment, and this question is addressed in more detail. First, we define the most important approaches: the "all node" deconstruction considering all branch points of the linkers, the "single node" deconstruction considering only components mixed, and the ToposPro "standard representation" also considering linkers as one node but, if present, takes each metal atom as a separate node. These methods are applied to a number of metal organic framework structures (MOFs, although this is just one example of materials this method is applicable on), and it is concluded that the "all node" method potentially yields more information on the structure in question but cannot be recommended as the only way of reporting the network topology. In addition, several terms needing definitions are discussed