Ligand design for functional metal-organic frameworks.

Metal-organic frameworks (MOFs), also known as coordination polymers, are formed by the self-assembly of metallic centres and bridging organic linkers. In this critical review, we review the key advances in the field and discuss the relationship between the nature and structure of specifically designed organic linkers and the properties of the products. Practical examples demonstrate that the physical and chemical properties of the linkers play a decisive role in the properties of novel functional MOFs. We focus on target materials suitable for the storage of hydrogen and methane, sequestration of carbon dioxide, gas separation, heterogeneous catalysis and as magnetic and photoluminescent materials capable of both metal- and ligand-centred emission, ion exchangers and molecular sieves. The advantages of highly active discrete complexes as metal-bearing ligands in the construction of MOFs are also briefly reviewed (128 references).

Microwave-assisted synthesis of metal-organic frameworks.

Microwave heating, used in organic chemistry for several decades, has only recently been applied to the preparation of multi-dimensional coordination polymers, more commonly known as metal-organic frameworks (MOFs). Microwave heating allows short reaction times, fast kinetics of crystal nucleation and growth, and high yields of desirable products which can be isolated with few or no secondary products. The most significant developments in the use of microwave heating for the preparation of MOFs are briefly reviewed from this perspective, emphasizing systematic studies of well-characterised materials, which lead to their isolation in large quantities over economically-viable periods of time. Emphasis is given to the growth of nano-sized crystallites which may find direct applications in functional devices.

Chemistry and catalytic activity of molybdenum(VI)-pyrazolylpyridine complexes in olefin epoxidation. Crystal structures of monomeric dioxo, dioxo-µ-oxo, and oxodiperoxo derivatives.

The dioxomolybdenum(VI) complexes [MoO2Cl2(PzPy)] (1) and [MoO2(OSiPh3)2(PzPy)] (5) (PzPy = 2-[3(5)-pyrazolyl]pyridine) were synthesized and characterized by vibrational spectroscopy, with assignments being supported by DFT calculations. Complex 5 was additionally characterized by single crystal X-ray diffraction. Recrystallization of 1 under different conditions originated crystal structures containing either the mononuclear [MoO2Cl2(PzPy)] complex co-crystallized with 2-[3(5)-pyrazolyl]pyridinium chloride, binuclear [Mo2O4(µ2-O)Cl2(PzPy)2] complexes, or the oxodiperoxomolybdenum(VI) complex [MoO(O2)2Cl(PzPyH)], in which a 2-[3(5)-pyrazolyl]pyridinium cation weakly interacts with the Mo(VI) center via a pyrazolyl N-atom. The crystal packing in the different structures is mediated by a variety of supramolecular interactions: hydrogen bonding involving the pyridinium and/or pyrazolyl N-H groups, weak CH · · · O and CH · · · π contacts, and strong π-π stacking. Complexes 1 and 5 are moderately active catalysts for the epoxidation of cis-cyclooctene at 55 °C using tert-butylhydroperoxide as oxidant, giving 1,2-epoxycyclooctane as the only reaction product. Insoluble materials were recovered at the end of the first catalytic runs and characterized by IR spectroscopy, elemental analysis, scanning electron microscopy (SEM)-energy dispersive spectroscopy (EDS), and powder X-ray diffraction. For complex 5 the loss of the triphenylsiloxy ligands during the catalytic run resulted in the formation of a tetranuclear complex, [Mo4O8(µ2-O)4(PzPy)4]. The recovered solids could be used as efficient heterogeneous catalysts for the epoxidation of cyclooctene, showing no loss of catalytic performance between successive catalytic runs.

Oxidation of volatile organic compounds on SBA-15 mesoporous molecular sieves modified with manganese.

Catalytic combustion of volatile organic compounds, such as propene, has been studied on manganese modified mesoporous molecular sieves. Powder X-ray diffraction, (29)Si nuclear magnetic resonance, nitrogen sorption and transmission electron microscopy show that the SBA-15 mesoporous silica molecular sieve can be modified with manganese using Mn(2)(CO)(10) or Mn(O(2)CMe)(2) without significant distortion of the host structure. The two products were catalytically active in propene oxidation, with SBA-15 modified with Mn(2)(CO)(10) showing significantly higher activity, possibly due to higher Mn content, than SBA-15 modified with Mn(O(2)CMe)(2).

Poly[[µ(2)-aqua-tetraaquahexakis(µ(4)-naph-thalene-2,6-dicarboxylato)tetra-holmium(III)] 1.75-hydrate].

In the title compound, {[Ho(4)(C(12)H(6)O(4))(6)(H(2)O)(5)]·1.75H(2)O}(n), which is isostructural with its Tb(3+)- and Eu(3+)-containing analogues, there are four crystallographically independent Ho(3+) centres, each exhibiting a highly distorted HoO(8) bicapped trigonal-prismatic coordination environment. Adjacent polyhedra are inter-connected via the carboxyl-ate groups and one µ(2)-bridging water mol-ecule, forming one-dimensional chains propagating along [100]. The naphthalene-2,6-dicarboxylate ligands further inter-connect these chains into a three-dimensional framework, which has zigzag channels housing the water mol-ecules. Two naphthalene-2,6-dicarboxylate bridging ligands have their centroids located on crystallographic centres of inversion. One water O atom has a fixed site occupancy factor of 0.75.

Poly[[aqua-µ(3)-picolinato-µ(2)-picolinato-dipicolinatopotassium(I)terbium(III)] 2.5-hydrate].

In the title compound, [KTb(C(6)H(4)NO(2))(4)(H(2)O)]·2.5H(2)O, each Tb(3+) centre is coordinated by four N and five O atoms from five distinct picolinate ligands in a geometry resembling a highly distorted tricapped trigonal prism. One of the ligands establishes a skew bridge between neighbouring Tb(3+) centres, leading to the formation of one-dimensional anionic polymeric chains, {[(C(6)H(4)NO(2))(4)Tb](-)}(n), running along the direction [010]. Each K(+) cation is seven-coordinated by six O atoms from one anionic polymeric chain and one water mol-ecule [K⋯O 2.676 (3)-3.208 (4) Å]. The uncoordinated water mol-ecules are involved in O-H⋯O hydrogen bonding.

A highly efficient dioxo(mu-oxo)molybdenum(VI) dimer catalyst for olefin epoxidation.

The oxo-bridged dimer [Mo(2)O(4)(mu(2)-O)Cl(2)(pzH)(4)] (1; pzH = pyrazole) exhibits unusually high activity in the liquid-phase catalytic epoxidation of the cyclic olefins cyclooctene and (R)-(+)-limonene under mild conditions and in the absence of additional organic solvents, using tert-butyl hydroperoxide as the oxidant. The complex is stable under the reaction conditions and can be used in further catalytic runs without significant loss of performance. An X-ray crystallographic investigation reveals that 1 has an unprecedented and extremely rare all-cis configuration at each of the MoO(2)-(mu(2)-O)Cl(pzH)(2) cores, which can be understood by considering supramolecular contacts and geometric factors.

Poly[hexa-aqua-bis(µ(3)-naphthalene-2,6-dicarboxyl-ato)(µ(2)-naphthalene-2,6-dicarboxyl-ato)diholmium(III)].

The crystal structure of the title compound, [Ho(2)(C(12)H(6)O(4))(3)(H(2)O)(6)](n), contains binuclear centrosymmetric {Ho(2)O(2)(CO(2))(4)(H(2)O)(6)} cores inter-connected via the naphthalene-2,6-dicarboxyl-ate (NDC(2-)) bridging ligands into a two-dimensional neutral plane net, ∞(2)[Ho(2)(NDC)(3)(H(2)O)(6)], exhibiting a typical (4,4)-topology. Inter-actions between adjacent layers are assured by a series of C-H⋯π contacts and a number of strong and highly directional O-H⋯O hydrogen bonds involving the coordinated water mol-ecules and neighbouring coordinated carboxyl-ate groups. One NDC(2-) bridging ligand has its centroid located at a crystallographic centre of inversion.

Decaaqua-dioxidobis[µ(3)-N-(phospho-n-atometh-yl)imino-diacetato]-dizinc(II)-divanadium(IV) dihydrate.

The title compound, [Zn(2)V(2)(C(5)H(6)NO(7)P)(2)O(2)(H(2)O)(10)]·2H(2)O, contains a [V(2)O(2)(pmida)(2)](4-) dimeric anionic unit [where H(4)pmida is N-(phosphono-meth-yl)imino-diacetic acid] lying on a centre of symmetry which is exo-coordinated via the two deprotonated phospho-nate groups to two Zn(2+) cations, with the coordination environment of Zn completed by five water mol-ecules. The crystal packing is mediated by an extensive network of strong and highly directional O-H⋯O hydrogen bonds involving the water mol-ecules (coordinated and uncoordinated) and the functional groups of pmida(4-), leading to a three-dimensional supra-molecular network.

1-Hydr-oxy-1,1,3,3,3-penta-phenyl-disiloxane, [Si(2)O(OH)(Ph)(5)], at 100 K.

In the crystal structure of the title compound, C(30)H(26)O(2)Si(2), one Si(Ph)(3) residue is bound to another Si(OH)(Ph)(2) residue via a non-linear Si-O-Si bridge. The asymmetric unit is composed of two such molecules which inter-act, on the one hand, via a strong and highly directional O-H⋯O hydrogen bond involving the two neighbouring Si-OH units and, on the other, via an O-H⋯π contact connecting the second hydroxyl group with an adjacent phenyl group.

1-Hydr-oxy-1,1,3,3,3-penta-phenyl-disiloxane, [Si(2)O(OH)(Ph)(5)], at 150 K.

In the crystal structure of the title compound, C(30)H(26)O(2)Si(2), one Si(Ph)(3) residue is bound to another Si(OH)(Ph)(2) residue via a nonlinear Si-O-Si bridge. The asymmetric unit is composed of four [Si(2)O(OH)(Ph)(5)] molecules. Each pair of adjacent molecules inter-acts via strong and highly directional O-H⋯O hydrogen bonds connecting neighbouring Si-OH units, and via inter-unit O-H⋯π contacts connecting the second hydroxyl groups with adjacent phenyl groups.

Bis(tetra-methyl-amonium) bis-(2,4,5-carboxy-benzoate)-benzene-1,2,4,5-tetra-carboxylic acid (1/1).

The asymmetric unit of the title compound, 2C(4)H(12)N(+)·2C(10)H(5)O(8) (-)·C(10)H(6)O(8), consists of a tetra-methyl-amonium cation, an anion derived from the singly deprotonated pyromellitic acid anion, 2,4,5-carboxy-benzoate (H(3)bta(-)), and one-half of a benzene-1,2,4,5-tetra-carboxylic acid (H(4)bta) mol-ecule, which has the centroid of the aromatic ring positioned at a crystallographic centre of inversion. The H(4)bta and H(3)bta(-) residues are involved in an extensive inter-molecular O-H⋯O hydrogen-bonding network, which leads to a three-dimensional supra-molecular structure containing one-dimensional channels running parallel to the [001] crystallographic direction. These channels house the tetra-methyl-amonium cations.

Simulating self-assembly of ZnS nanoparticles into mesoporous materials.

Characterization of materials is crucial for the quantification and prediction of their physical, chemical, and mechanical properties. However, as the complexity of a system increases, so do the challenges involved in elucidating its structure. While molecular simulation and modeling have proved invaluable as complements to experiment, such simulations now face serious challenges: new materials are being synthesized with ever increasing structural complexity, and it may soon prove impossible to generate models that are sufficiently realistic to describe them adequately. Perhaps, ultimately, it will only be possible to generate such models by simulating the synthetic process itself. Here, we attempt such a strategy to generate full atomistic models for mesoporous molecular sieves. As in experiment, this is done by allowing nanoparticles to self-assemble at high temperature to form an amorphous mesoporous framework. The temperature is then reduced, and the system is allowed to crystallize. Animations of atomic trajectories, available as Supporting Information, reveal the evolution of multiple seeds which propagate to form a complex framework. The products are polycrystalline mesoporous framework structures containing cavities connected by channels running along "zero", one, two, and three perpendicular directions. We suggest that it is easier to generate these model structures by attempting to simulate the synthetic process rather than by using more conventional techniques. The strategy is illustrated using ZnS as a model system. Further development of the mathematics of minimal surfaces will advance our understanding of these structures.

One-dimensional silver(I) chain of lacunary alpha-Keggin anions.

A novel chain-like silver polyoxotungstophosphate is formed when Ag(I) metal centres, exhibiting an unusual eight-coordination fashion, bridge a monolacunary [PW11O39]7- anion to four bridging mu2-oxygen atoms of a neighbouring lacunary alpha-Keggin anion.

Synthesis and structure of a sodium complex of an aromatic beta-diketone and pyrazolylpyridine.

Reaction of NaH with a THF solution of Eu(BTA)3(pypzH) [BTA = 1-benzoyl-3,3,3-trifluoroacetonate, pypzH = 2-(3-pyrazolyl)pyridine] leads to the formation of the europium-free tetrasodium complex [Na(4)(pypzH)(2)(micro4-BTA)(2)(micro2-BTA)(2)]. Single-crystal X-ray diffraction studies revealed the presence of a centrosymmetric Na+ hybrid tetramer,which fully occupies the contents of the triclinic unit cell. The crystal structure contains two individual Na+ cations, Na(1) and Na(2), which have highly irregular [NaN(2)O(3)] and[NaO(6)] local coordination environments, respectively. One of the key features is the presence of a central [Na(4)O(6)] core, which is unprecedented for Na+ . Externally to this [Na(4)O(6)] cluster pyrazolylpyridine organic molecules are N,N-chelated to Na(1). Even though all of the organic residues contain aromatic rings, the crystal packing of individual centrosymmetric tetrasodium [Na(4)(pypzH)(2)(micro4-BTA)(2)(micro2-BTA)(2)] molecular moieties is essentially driven through geometrical aspects combined with weak C-H...pi interactions, rather than the expected a priori pi-pi interactions. The material also contains classical strong hydrogen bonds, even though these do not directly contribute to the packing driving forces.

Microwave assisted synthesis of molybdenum and tungsten tetracarbonyl complexes with a pyrazolylpyridine ligand. Crystal structure of cis-[Mo(CO)4{ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate}].

We report the one-step syntheses in good yields of the complexes cis-[M(CO)4(pzpy)] {M = Mo, W; pzpy = ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate} directly from the corresponding M(CO)6 starting materials by using microwave-assisted heating and reaction times of either 30 s (M = Mo) or 15 min (M = W). The structure of the molybdenum tetracarbonyl complex was determined by single crystal X-ray diffraction. The compound is monomeric and the molybdenum atom has a highly distorted octahedral geometry. The close packing of the individual cis-[Mo(CO)4(pzpy)] species is essentially driven by the need to fill the space effectively, closely mediated by weak C-H-O and pi-pi interactions.

X-ray diffraction and solid-state NMR studies of a germanium binuclear complex.

A compound formulated as (C4H12N2)[Ge2(pmida)2(OH)2] x 4 H2O (where pmida(4-) = N-(phosphonomethyl)iminodiacetate and C4H12N2(2+) = piperazinedium cation), containing the anionic [Ge2(pmida)2(OH)2]2- complex, has been synthesised by the hydrothermal approach and its structure determined by single-crystal X-ray diffraction analysis. Several high-resolution solid-state magic-angle spinning (MAS) NMR techniques, in particular two-dimensional 1H-X(13C,31P) heteronuclear correlation (HETCOR) and 1H-1H homonuclear correlation (HOMCOR) experiments incorporating a frequency-switched Lee-Goldburg (FS-LG) decoupling scheme, have been employed for the first time in such a material. Using these tools in tandem affords an excellent general approach to study the structure of other inorganic-organic hybrids. We assigned the NMR resonances with the help of C...H and P...H internuclear distances obtained through systematic statistical analyses of the crystallographic data. The compound was further characterised by powder X-ray diffraction techniques, IR and Raman spectroscopy, and by elemental and thermal analyses (thermogravimetric analysis and differential scanning calorimetry).

Hypothetical binodal zeolitic frameworks.

Hypothetical binodal zeolitic structures (structures containing two kinds of tetrahedral sites) were systematically enumerated using tiling theory and characterized by computational chemistry methods. Each of the 109 refineable topologies based on "simple tilings" was converted into a silica polymorph and its energy minimized using the GULP program with the Sanders-Catlow silica potential. Optimized structural parameters, framework energies relative to alpha-quartz and volumes accessible to sorption have been calculated. Eleven of the 30 known binodal topologies listed in the Atlas of Zeolite Framework Types were found, leaving 98 topologies that were unknown previously. The chemical feasibility of each structure as a zeolite was evaluated by means of a feasibility factor derived from the correlation between lattice energy and framework density. Structures are divided into 15 families, based on common structural features. Many "feasible" structures contain only small pores. Several very open structures were also enumerated, although they contain three-membered rings which are thermodynamically disfavoured and not found in conventional zeolites. We believe that such topologies may be realizable as framework materials, but with different elemental compositions to those normally associated with zeolites.