Isoreticular Contraction of Metal-Organic Frameworks Induced by Cleavage of Covalent Bonds.

Isoreticular chemistry, in which the organic or inorganic moieties of reticular materials can be replaced without destroying their underlying nets, is a key concept for synthesizing new porous molecular materials and for tuning or functionalization of their pores. Here, we report that the rational cleavage of covalent bonds in a metal-organic framework (MOF) can trigger their isoreticular contraction, without the need for any additional organic linkers. We began by synthesizing two novel MOFs based on the MIL-142 family, (In)BCN-20B and (Sc)BCN-20C, which include cleavable as well as noncleavable organic linkers. Next, we selectively and quantitatively broke their cleavable linkers, demonstrating that various dynamic chemical and structural processes occur within these structures to drive the formation of isoreticular contracted MOFs. Thus, the contraction involves breaking of a covalent bond, subsequent breaking of a coordination bond, and finally, formation of a new coordination bond supported by structural behavior. Remarkably, given that the single-crystal character of the parent MOF is retained throughout the entire transformation, we were able to monitor the contraction by single-crystal X-ray diffraction.

Stepwise assembly of heterometallic, heteroleptic "triblock Janus-type" metal-organic polyhedra.

Increasing the chemical complexity of metal-organic cages (MOCs) or polyhedra (MOPs) demands control over the simultaneous organization of diverse organic linkers and metal ions into discrete caged structures. Herein, we show that a pre-assembled complex of the archetypical cuboctahedral MOP can be used as a template to replicate such caged structure, one having a "triblock Janus-type" configuration that is both heterometallic and heteroleptic.

Guest Exchange through Facilitated Transport in a Seemingly Impenetrable Hydrogen-Bonded Framework.

A new inclusion compound consisting of a guanidinium 1,3,5-tri(4-sulfophenyl)benzene (G3TSPHB) host framework containing isophorone guests that surround isolated and seemingly inaccessible pockets was amenable to guest exchange with hexafluorobenzene (HFB) through a single crystal-single crystal transformation (SCSCT). Single-crystal X-ray diffraction of intermediate transformation states, from the parent compound G3TSPHB·(isophorone)3.7·(methanol)5.4 to the final state G3TSPHB·(isophorone)3.1·(HFB)2·(methanol)2, indicated a crystal symmetry change from monoclinic to hexagonal prior to full incorporation of HFB. Optical microscopy during the SCSCT revealed the formation of lamellae, which expanded and then coalesced into a single crystal when the phase transformation was complete. In situ Raman microscopy revealed changes in the orientation of isophorone guests during the transformation that suggested a pathway for HFB entry into the host cavities. The SCSCT occurs more rapidly than expected on the basis of simple diffusion, consistent with facilitated transport along the lamellae interfaces and a reduction in the length scale for guest exchange.

Encapsulation of Isolated Luminophores within Supramolecular Cages.

The sequestration of luminophores within supramolecular polyhedral compartments of a crystalline zeolite-like hydrogen-bonded framework illustrates a unique approach to limiting the self-quenching ordinarily exhibited at the high concentrations achievable in this framework. A range of differently sized luminescent guests, namely coumarin 1, coumarin 4, fluorescein, [Ru(bpy)3 ]Cl2 , and rhodamine B, can be encapsulated in amounts of up to one molecule per cage, equivalent to a concentration of 0.175 m, which is significantly higher than the concentration at which aggregation-induced quenching occurs in other media. The luminescence spectra of the encapsulated guests are consistent with the presence of isolated monomers and the absence of self-quenching. The emission color of the single crystals can be tuned readily from blue to red through the choice of guest molecules. These observations promise an approach to organic solid-state lasing compounds if crystals of sufficient size and quality can be prepared.

Zn2+ -Ion Sensing by Fluorescent Schiff Base Calix[4]arene Macrocycles.

A macrocyclic ligand (H2 L) containing two o,o'-bis(iminomethyl)phenol and two calix[4]arene head units has been synthesized and its coordination chemistry towards divalent Ni and Zn investigated. The new macrocycle forms complexes of composition [ML] (M=Zn, M=Ni) and [ZnL(py)2 ], which were characterized by elemental analysis; IR, UV/Vis, and NMR spectroscopy; electrospray ionization mass spectrometry (ESI-MS); and X-ray crystallography (for [ZnL(py)2 ] and [NiL]). H2 L allows the sensitive optical detection of Zn2+ among a series of biologically relevant metal ions by a dual fluorescence enhancement/quenching effect in solution. The fluorescence intensity of the macrocycle increases by a factor of ten in the presence of Zn2+ with a detection limit in the lower nanomolar region.

Composition Space Analysis in the Development of Copper Molybdate Hybrids Decorated by a Bifunctional Pyrazolyl/1,2,4-Triazole Ligand.

A bitopic ligand, 4-(3,5-dimethylpyrazol-4-yl)-1,2,4-triazole (Hpz-tr) (1), containing two different heterocyclic moieties was employed for the design of copper(II)-molybdate solids under hydrothermal conditions. In the multicomponent Cu(II)/Hpz-tr/Mo(VI) system, a diverse set of coordination hybrids, [Cu(Hpz-tr)2SO4]·3H2O (2), [Cu(Hpz-tr)Mo3O10] (3), [Cu4(OH)4(Hpz-tr)4Mo8O26]·6H2O (4), [Cu(Hpz-tr)2Mo4O13] (5), and [Mo2O6(Hpz-tr)]·H2O (6), was prepared and characterized. A systematic investigation of these systems in the form of a ternary crystallization diagram approach was utilized to show the influence of the molar ratios of starting reagents, the metal (Cu(II) and Mo(VI)) sources, the temperature, etc., on the reaction products outcome. Complexes 2-4 dominate throughout a wide crystallization range of the composition triangle, while the other two compounds 5 and 6 crystallize as minor phases in a narrow concentration range. In the crystal structures of 2-6, the organic ligand behaves as a short [N-N]-triazole linker between metal centers Cu···Cu in 2-4, Cu···Mo in 5, and Mo···Mo in 6, while the pyrazolyl function remains uncoordinated. This is the reason for the exceptional formation of low-dimensional coordination motifs: 1D for 2, 4, and 6 and 2D for 3 and 5. In all cases, the pyrazolyl group is involved in H bonding (H-donor/H-acceptor) and is responsible for π-π stacking, thus connecting the chain and layer structures in more complicated H-bonding architectures. These compounds possess moderate thermal stability up to 250-300 °C. The magnetic measurements were performed for 2-4, revealing in all three cases antiferromagnetic exchange interactions between neighboring Cu(II) centers and long-range order with a net moment below Tc of 13 K for compound 4.

Cavitands incorporating a Lewis acid dinickel chelate function as receptors for halide anions.

The halide binding properties of the cavitand [Ni2(L(Me2H4))](2+) (4) are reported. Cavitand 4 exhibits a chelating N3Ni(µ-S)2NiN3 moiety with two square-pyramidal Ni(II)N3S2 units situated in an anion binding pocket of ∼4 Å diameter formed by the organic backbone of the (L(Me2H4))(2-) macrocycle. The receptor reacts with fluoride, chloride (in MeCN/MeOH), and bromide (in MeCN) ions to afford an isostructural series of halogenido-bridged complexes [Ni2(L(Me2H4))(µ-Hal)](+) (Hal = F(-) (5), Cl(-) (6), and Br(-) (7)) featuring a N3Ni(µ-S)2(µ-Hal)NiN3 core structure. No reaction occurs with iodide or other polyatomic anions (ClO4(-), NO3(-), HCO3(-), H2PO4(-), HSO4(-), SO4(2-)). The binding events are accompanied by discrete UV-vis spectral changes, due to a switch of the coordination geometry from square-pyramidal (N3S2 donor set in 4) to octahedral in the halogenido-bridged complexes (N3S2Hal donor environment in 5-7). In MeCN/MeOH (1/1 v/v) the log K11 values for the 1:1 complexes are 7.77(9) (F(-)), 4.06(7) (Cl(-)), and 2.0(1) (Br(-)). X-ray crystallographic analyses for 4(ClO4)2, 4(I)2, 5(F), 6(ClO4), and 7(Br) and computational studies reveal a significant increase of the intramolecular distance between two propylene groups at the cavity entrance upon going from F(-) to I(-) (for the DFT computed structure). In case of the receptor 4 and fluorido-bridged complex 5, the corresponding distances are nearly identical. This indicates a high degree of preorganization of the [Ni2(L(Me2H4))](2+) receptor and a size fit mismatch of the receptor binding cavity for anions larger than F(-).

Ruthenium complexes with dendritic ferrocenyl phosphanes: synthesis, characterization, and application in the catalytic redox isomerization of allylic alcohols.

An efficient system for the catalytic redox isomerization of the allylic alcohol 1-octen-3-ol to 3-octanone is presented. The homogeneous ruthenium(II) catalyst contains a monodentate phosphane ligand with a ferrocene moiety in the backbone and provides 3-octanone in quantitative yields. The activity is increased by nearly 90 % with respect to the corresponding triphenyl phosphane ruthenium(II) complex. By grafting the catalyst at the surface of a dendrimer, the catalytic activity is further increased. By introducing different spacers between ferrocene and phosphorus, the influence on the electronic properties of the complexes is shown by evaluating the electrochemical behavior of the compounds.

113Cd solid-state NMR for probing the coordination sphere in metal-organic frameworks.

Spectroscopic techniques are a powerful tool for structure determination, especially if single-crystal material is unavailable. (113)Cd solid-state NMR is easy to measure and is a highly sensitive probe because the coordination number, the nature of coordinating groups, and the geometry around the metal ion is reflected by the isotropic chemical shift and the chemical-shift anisotropy. Here, a detailed investigation of a series of 27 cadmium coordination polymers by (113)Cd solid-state NMR is reported. The results obtained demonstrate that (113)Cd NMR is a very sensitive tool to characterize the cadmium environment, also in non-single-crystal materials. Furthermore, this method allows the observation of guest-induced phase transitions supporting understanding of the structural flexibility of coordination frameworks.

Network flexibility: control of gate opening in an isostructural series of Ag-MOFs by linker substitution.

An isostructural series of 15 structurally flexible microporous silver metal-organic frameworks (MOFs) is presented. The compounds with a dinuclear silver core as secondary building unit (Ag2N4) can be obtained under solvothermal conditions from substituted triazolyl benzoate linkers and AgNO3 or Ag2SO4; they exhibit 2-fold network interpenetration with lvt topology. Besides the crystal structures, the calculated pore size distributions of the microporous MOFs are reported. Simultaneous thermal analyses confirm the stability of the compounds up to 250 °C. Interconnected pores result in a three-dimensional pore structure. Although the porosity of the novel coordination polymers is in the range of only 20-36%, this series can be regarded as a model system for investigation of network flexibility, since the pore diameters and volumes can be gradually adjusted by the substituents of the 3-(1,2,4-triazol-4-yl)-5-benzamidobenzoates. The pore volumes of selected materials are experimentally determined by nitrogen adsorption at 77 K and carbon dioxide adsorption at room temperature. On the basis of the flexible behavior of the linkers a reversible framework transformation of the 2-fold interpenetrated network is observed. The resulting adsorption isotherms with one or two hysteresis loops are interpreted by a gate-opening process. Due to external stimuli, namely, the adsorptive pressure, the materials undergo a phase transition confirming the structural flexibility of the porous coordination polymer.

Mixed-ligand hydroxocopper(II)/pyridazine clusters embedded into 3D framework lattices.

Rational combination of pyridazine, hydroxo and carboxylate bridging ligands led to the assembly of three types of mixed-ligand polynuclear Cu(II) clusters (A: [Cu2(µ-OH)(µ-pdz)(µ-COO)]; B: [Cu4(µ3-OH)2(µ-pdz)2]; C: [Cu5(µ-OH)2(µ-pdz)4(µ-COO)2(µ-H2O)2]) and their integration into 3D framework structures. Mixed-ligand complexes [Cu2(µ-OH){TMA}(L)(H2O)] (1), [Cu4(µ3-OH)2{ATC}2(L)2(H2O)2]·H2O (2) [Cu4(µ3-OH)2{TDC}3(L)2(H2O)2]·7H2O (3) (L = 1,3-bis(pyridazin-4-yl)adamantane; TMA(3-) = benzene-1,3,5-tricarboxylate, ATC(3-) = adamantane-1,3,5-tricarboxylate, TDC(2-) = 2,5-thiophenedicarboxylate) and [Cu5(µ-OH)2{X}4(L)2(H2O)2]·nH2O (X = benzene-1,3-dicarboxylate, BDC(2-), n = 5 (4) and 5-hydroxybenzene-1,3-dicarboxylate, HO-BDC(2-), n = 6 (5)) are prepared under hydrothermal conditions. Trigonal bridges TMA(3-) and ATC(3-) generate planar Cu(II)/carboxylate subtopologies further pillared into 3D frameworks (1: binodal 3,5-coordinated, doubly interpenetrated tcj-3,5-Ccc2; 2: binodal 3,8-coordinated tfz-d) by bitopic pyridazine ligands. Unprecedented triple bridges in 1 (cluster of type A) support short CuCu separations of 3.0746(6) Å. The framework in 3 is a primitive cubic net (pcu) with multiple bis-pyridazine and TDC(2-) links between the tetranuclear nodes of type . Compounds 4 and 5 adopt uninodal ten-coordinated framework topologies (bct) embedding unprecedented centrosymmetric open-chain pentanuclear clusters of type C with two kinds of multiple bridges, Cu(µ-OH)(µ-pdz)2Cu and Cu(µ-COO)(µ-H2O)Cu (CuCu distances are 3.175 and 3.324 Å, respectively). Magnetic coupling phenomena were detected for every type of cluster by susceptibility measurements of 1, 3 and 4. For binuclear clusters A in 1, the intracluster antiferromagnetic exchange interactions lead to a diamagnetic ground state (J = -17.5 cm(-1); g = 2.1). Strong antiferromagnetic coupling is relevant also for type B, which consequently results in a diamagnetic ground state (J1 = -110 cm(-1); J2 = -228 cm(-1), g = 2.07). For pentanuclear clusters of type C in 4, the exchange model is based on a strongly antiferromagnetically coupled central linear trinuclear Cu3 group (J1 = -125 cm(-1)) and two outer Cu centers weakly antiferromagnetically coupled to the terminal Cu ions of the triad (J2 = -12.5 cm(-1)).

Unprecedented trapping of difluorooctamolybdate anions within an α-polonium type coordination network.

New fluorinated hybrid solids [Mo2F2O5(tr2pr)] (1), [Co3(tr2pr)2(MoO4)2F2]·7H2O (2), and [Co3(H2O)2(tr2pr)3(Mo8O26F2)]·3H2O (3) (tr2pr = 1,3-bis(1,2,4-triazol-4-yl)propane) were prepared from the reaction systems consisting of Co(OAc)2/CoF2 and MoO3/(NH4)6Mo7O24, as Co(II) and Mo(VI) sources, in water (2) or in aqueous HF (1, 3) employing mild hydrothermal conditions. The tr2pr ligand serves as a conformationally flexible tetradentate donor. In complex 1, the octahedrally coordinated Mo atoms are linked in the discrete corner-sharing {Mo2(µ2-O)F2O4N4} unit in which a pair of tr-heterocycles (tr = 1,2,4-triazole) is arranged in cis-positions opposite to "molybdenyl" oxygen atoms. The anti-anti conformation type of tr2pr facilitates the tight zigzag chain packing motif. The crystal structure of the mixed-anion complex salt 2 consists of trinuclear [Co3(µ3-MoO4)2(µ2-F)2] units self-assembling in Co(II)-undulating chains (Co···Co 3.0709(15) and 3.3596(7) Å), which are cross-linked by tr2pr in layers. In 3, containing condensed oxyfluoromolybdate species, linear centrosymmetric [Co3(µ2-tr)6](6+) SBUs are organized at distances of 10.72-12.45 Å in an α-Po-like network using bitopic tr-linkers. The octahedral {N6} and {N3O3} environments of the central and peripheral cobalt atoms, respectively, are filled by triazole N atoms, water molecules, and coordinating [Mo8O26F2](6-) anions. Acting as a tetradentate O-donor, each difluorooctamolybdate anion anchors four [Co3(µ2-tr)6](6+) units through their peripheral Co-sites, which consequently leads to a novel type of a two-nodal 4,10-c net with the Schläfli symbol {3(2).4(3).5}{3(4).4(20).5(16).6(5)}. The 2D and 3D coordination networks of 2 and 3, respectively, are characterized by significant overall antiferromagnetic exchange interactions (J/k) between the Co(II) spin centers on the order of -8 and -4 K. The [Mo8O26F2](6-) anion is investigated in detail by quantum chemical calculations.

An isomorphous series of cubic, copper-based triazolyl isophthalate MOFs: linker substitution and adsorption properties.

An isomorphous series of 10 microporous copper-based metal-organic frameworks (MOFs) with the general formulas (∞)(3)[{Cu(3)(µ(3)-OH)(X)}(4){Cu(2)(H(2)O)(2)}(3)(H-R-trz-ia)(12)] (R = H, CH(3), Ph; X(2-) = SO(4)(2-), SeO(4)(2-), 2 NO(3)(2-) (1-8)) and (∞)(3)[{Cu(3)(µ(3)-OH)(X)}(8){Cu(2)(H(2)O)(2)}(6)(H-3py-trz-ia)(24)Cu(6)]X(3) (R = 3py; X(2-) = SO(4)(2-), SeO(4)(2-) (9, 10)) is presented together with the closely related compounds (∞)(3)[Cu(6)(µ(4)-O)(µ(3)-OH)(2)(H-Metrz-ia)(4)][Cu(H(2)O)(6)](NO(3))(2)·10H(2)O (11) and (∞)(3)[Cu(2)(H-3py-trz-ia)(2)(H(2)O)(3)] (12(Cu)), which are obtained under similar reaction conditions. The porosity of the series of cubic MOFs with twf-d topology reaches up to 66%. While the diameters of the spherical pores remain unaffected, adsorption measurements show that the pore volume can be fine-tuned by the substituents of the triazolyl isophthalate ligand and choice of the respective copper salt, that is, copper sulfate, selenate, or nitrate.