Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement of gamma-Zirconium Phosphate Containing Polyethylenoxide Pillars

Spiers Memorial Lecture: Coordination networks that switch between nonporous and porous structures: an emerging class of soft porous crystals

Coordination networks (CNs) are a class of (usually) crystalline solids typically comprised of metal ions or cluster nodes linked into 2 or 3 dimensions by organic and/or inorganic linker ligands. Whereas CNs tend to exhibit rigid structures and permanent porosity as exemplified by most metal-organic frameworks, MOFs, there exists a small but growing class of CNs that can undergo extreme, reversible structural transformation(s) when exposed to gases, vapours or liquids. These "soft" or "stimuli-responsive" CNs were introduced two decades ago and are attracting increasing attention thanks to two features: the amenability of CNs to design from first principles, thereby enabling crystal engineering of families of related CNs; and the potential utility of soft CNs for adsorptive storage and separation. A small but growing subset of soft CNs exhibit reversible phase transformations between nonporous (closed) and porous (open) structures. These "switching CNs" are distinguished by stepped sorption isotherms coincident with phase transformation and, perhaps counterintuitively, they can exhibit benchmark properties with respect to working capacity (storage) and selectivity (separation). This review addresses fundamental and applied aspects of switching CNs through surveying their sorption properties, analysing the structural transformations that enable switching, discussing structure-function relationships and presenting design principles for crystal engineering of the next generation of switching CNs.

Formation of Layered Proton-Conducting Zirconium and Titanium Organophosphonates by Topotactic Reaction: Physicochemical Properties, Proton Dynamics, and Atomic-Resolution Structure

A series of new zirconium and titanium phosphates-organophosphonates, in which the organophosphonate moiety is functionalized with a sulfo group, was prepared by a topotactic reaction involving the gamma modification of zirconium or titanium hydrogen phosphate with 2-bis(phosphonomethyl)amino-ethan-1-sulfonic acid (H₄TDP). H₄TDP represents a new type of functionalizing agent, which can be easily prepared by a Moedritzer-Irani reaction from taurine (2-aminoethanesulfonic acid). The gamma modification of zirconium hydrogen phosphate (γ-ZrP) with H₄TDP provides mixed phosphate-organophosphonate compounds with the formula Zr(PO₄)(H₂PO₄)_1-2x(H₂TDP)_x·yH₂O, where x = 0.15, 0.34, 0.45, and is controlled by the γ-ZrP/H₄TDP ratio in the starting mixture. On the contrary, by the topotactic gamma modification of titanium hydrogen phosphate (γ-TiP) with H₄TDP, only one product with the formula Ti(PO₄)(H₂PO₄)_1-2z(H₂TDP)_z·yH₂O, where z = 0.41 ± 0.01, was obtained regardless of the composition of the starting mixture. The synthesized compounds were characterized by elemental analysis, thermogravimetric analysis, energy-dispersive X-ray analysis, and infrared spectroscopy. The way the topotactic reaction proceeds and how the grafted organophosphonate groups are bonded to the layers of the host structure were suggested on the basis of the solid-state NMR data. It was found that the grafted moieties are spread evenly in the host layers among the hydrogen phosphate groups. The obtained solids are able to intercalate basic molecules, as was proved by the intercalation reactions of the zirconium series with butylamine. The amount of intercalated butylamine increases with increasing x. It is known that both host compounds, γ-ZrP and γ-TiP, are protonic conductors. It was found that the incorporation of H₂TDP increases conductivity of the zirconium compound when x = 0.15, but further incorporation of H₂TDP into the γ-ZrP host structure leads to a decrease of conductivity. This behavior is explained on the basis of the ¹H MAS and the ¹H-¹H EXSY NMR data.

Mechanochemistry-Based Synthesis of Highly Crystalline γ-Zirconium Phosphate for Selective Ion Exchange

Highly crystalline γ-zirconium phosphate has been synthesized by a novel minimalistic approach and investigated as a selective ion exchanger for cesium, ammonium and potassium. In contrast to current solution-based preparations, the mechanochemistry-based synthesis provides easy access to γ-zirconium phosphate with short synthesis times and low crystallization temperature. The addition of NaF as a mineralizer increases the crystallinity of γ-zirconium phosphate, which forms micrometer-sized uniformly shaped rectangular platelets. The crystalline material has extremely high selectivity to cesium even in the presence of 1000- or 500-fold excess Na⁺ or Ca²⁺, respectively. The removal efficiency was >98% in the pH range of 2-5.5. As an ion exchanger for purification of dialysate, crystalline γ-zirconium phosphate shows higher uptake of ammonium and potassium ions than the amorphous gel compound currently used in sorbent cartridges. This sustainable protocol opens up opportunities for many practical applications of γ-zirconium phosphate.

Molecular Structures of the Products of a Diphosphonate Ester Building Block with Lewis Bases

By treating a suitable Wittig reagent under acid conditions, the phosphonate ester 1,4-bimethylenebenzene phosphonate ethyl ester (H₂[BBPE], 1) was obtained. As a building block, compound 1 has been reacted with the Lewis-base N,N-dimethylpiperazine, ammonia and NaOH yielded compounds 2–4. The crystal structures show that a 1D chain forming a tubular channel is constructed through hydrogen bonds in 1; hydrogen bonds form two 1D chains with left-hand and right-hand helixes and form 3D networks in compound 2; 1D hydrogen-bond chains are further connected together to afford a 3D network architecture in compound 3; the phosphonate is coordinated by two Na atoms which present different coordination environments in compound 4. Additionally, the relationships between the structure and fluorescence of the four compounds in the solid state and in different solvents have also been studied at room temperature.

Electrochemical and density functional theory investigation on high selectivity and sensitivity of exfoliated nano-zirconium phosphate toward lead(II)

A new strategy on the understanding of selective and sensitive identification of Pb(II) using combined experimental and theoretical efforts is described. Amorphous phase formation of exfoliated nano-zirconium phosphate (ZrP) has been prepared via a hydrothermal process and subsequent intercalation reaction. Exfoliated ZrP was used as coating on the electrode surface, and it was found to be selective and sensitive for Pb(II) detection due to its selective adsorption ability. To better and scientifically understand the microscopic adsorption mechanism, density functional theory (DFT) calculations about the details of chemical interactions between heavy metal ions and exfoliated ZrP were carried out at an atomistic level. It is verified that the exfoliated ZrP shows the strongest adsorption capability toward Pb(II) among all heavy metal ions, thereby resulting in selective detection consequently. With our combined experimental and theoretical efforts, we are able to provide a new route to realize the improved selectivity in electrochemical sensing of toxic metal ions.

Flexible two-dimensional square-grid coordination polymers: structures and functions

Coordination polymers (CPs) or metal-organic frameworks (MOFs) have attracted considerable attention because of the tunable diversity of structures and functions. A 4,4'-bipyridine molecule, which is a simple, linear, exobidentate, and rigid ligand molecule, can construct two-dimensional (2D) square grid type CPs. Only the 2D-CPs with appropriate metal cations and counter anions exhibit flexibility and adsorb gas with a gate mechanism and these 2D-CPs are called elastic layer-structured metal-organic frameworks (ELMs). Such a unique property can make it possible to overcome the dilemma of strong adsorption and easy desorption, which is one of the ideal properties for practical adsorbents.

Usual Molecules in Unusual Environments Displaying Unusual Properties

The design of porous solids of controlled molecular geometry for various applications is a challenge of enormous technological and scientific importance. The placing of organic molecules between the layers of certain inorganic salts leads to enduring, solid materials where the confinement induces the organic molecules to change their properties or even display new ones at the supramolecular level. Many years ago, Feynman asked the question ‘What would the properties of materials be if we could really arrange the atoms (molecules) the way we want them?’. Chemistry, Physics, Engineering and even Biology are nowadays highly intertwined in the pursuing of Feynman’s ambition. However, despite all the huge efforts and smart developments, the finding of new materials is as yet, for the time being, quite open a field that allows for many serendipity-driven discoveries. In this context, we have a relatively large experience in the building of supramolecular scaffolds based in laminar zirconium phosphate (ZrP) containing covalently bonded phosphonates. In this paper, a short review will be presented concerning our past achievements and recent progress in the synthesis of organic-inorganic materials based in layered ZrP.

Intercalation and stitching of graphite oxide with diaminoalkanes

The intercalation reaction of graphite oxide with diaminoalkanes, with the general formula H2N(CH2)nNH2 (n = 4-10), was studied as a method for synthesizing pillared graphite with tailored interlayer spacing. Interlayer spacings from 0.8 to 1.0 nm were tailored by varying the size of the intercalant from (CH2)4 to (CH2)10. X-ray diffraction and infrared spectroscopy were used to confirm intercalation, and the frequency of the CH2 stretch confirmed that the intercalants are in a disordered state, with an important contribution from the gauche conformer. Sequential intercalation of diaminoalkanes followed by dodecylamine demonstrated the inability of these "stitched" systems to undergo expansion along the c-direction, indicative of cross-linking. Finally, the reaction of graphite oxide with diaminoalkanes under reflux and for extended periods (>72 h) resulted in the chemical reduction of the graphite oxide to a disordered graphitic structure.

Transformation from a low-dimensional framework to a high-dimensional architecture based on different metal ions: syntheses, structures, and photoluminescences

Five novel metal pamidronates (3-ammonium-1-hydroxypropylidene-1,1-bisphosphonate, APD) formulated as Ni2(C3NH10P2O7)4.4H2O (1), M(C3NH9P2O7).H2O (M = Co (2), Mn (3), Zn (4)) and Cu3(C3NH8P2O7)2.2H2O (5) have been hydrothermally synthesized under similar conditions. Compound 1 is a molecular binuclear nickel cluster. Compounds 2 and 3 exhibit similar one-dimensional ladderlike structures. Compound 4 possesses a novel two-dimensional gridlike framework. Compound 5 shows an unusual three-dimensional architecture, in which two-dimensional layers with a parquet motif are pillared by {CuO4} planar squares. Different dinuclear secondary building units (SBUs) are observed in compounds 1-5, depending on the different coordination modes of APD. They also exhibit different photoluminescence properties.

Chemistry of porous coordination polymers

Remarkable advances in the recent development of porous compounds based upon coordination polymers have paved the way toward functional chemistry having potential applications such as gas storage, separation, and catalysis. From the synthetic point of view, the advantage is a designable framework, which can readily be constructed from building blocks, the so-called bottom-up assembly. Compared with conventional porous materials such as zeolites and activated carbons, porous inorganic-organic hybrid frameworks have higher potential for adsorption of small molecules because of their designability with respect to the coordination geometry around the central metal ion as well as size and probable multifunctionality of bridging organic ligands. Although rigidity and robustness of porous framework with different degree of adsorption are the most studied properties of metal-organic coordination frameworks, there are few studies on dynamic porous frameworks, which could open up a new dimension in materials chemistry.

Novel expansion/shrinkage modulation of 2D layered MOF triggered by clathrate formation with CO(2) molecules

Crystal-to-crystal transformation from a 3D interpenetrated-type MOF {[Cu(BF(4))(2)(bpy)(H(2)O)(2)] (bpy)} (1) to a 2D square-grid-type [Cu(BF(4))(2)(bpy)(2)] (2) (bpy = 4,4'-bipyridine) was observed. It was derived from dehydration and confirmed by in situ FT-IR, TG, and elemental analysis. Moreover, we elucidate the novel expansion/shrinkage dynamic modulation of 2 triggered by clathrate formation with gas molecules.

2D Hydrogen-Bonded Square-Grid Coordination Networks with a Substitution-Active Metal Site

Reported here is the preparation and property of 2D coordination networks composed of rodlike ligands with ethylene glycol side chains (1). Two 2D coordination networks, [[Co(1)2(H2O)2](NO3)2.1.5H2O]n and [[Ni(1)2(H(2)O)2](NO3)2.1.5H2O]n, have been synthesized and characterized by single-crystal X-ray diffraction, TG, DSC, UV-vis spectroscopy, and magnetic measurements. The structural analyses clarified that infinite 1D hydrogen-bond arrays composed of ethylene glycol chains contribute to the stabilization of 2D coordination frameworks, keeping the environment of substitution-active metal sites unchanged. They are more stable than a similar square-grid coordination network that does not possess an ethylene glycol chain on the ligand. We also succeeded in the direct observation of a reversible apical-ligand-exchange reaction at the cobalt(II) and nickel(II) ions in a single-crystal-to-single-crystal fashion because of the considerable stability as well as moderate flexibility of the framework. The cobalt-containing coordination network crystal showed chromic behavior depending on temperatures. Crystallographic and spectroscopic studies revealed that the color change of the crystal was attributed to the ligand-exchange process between H2O and a NO3 anion on the cobalt metal. Magnetic measurements indicated weak antiferromagnetic nearest-neighbor spin coupling between cobalt(II) ions.

Conformational behavior of dodecyldiamine inside the confined space of montmorillonites

We have investigated the change of the intercalated amount and conformational behavior of dodecyldiamine (C(12)H(28)N(2), di-C(12)amine) inside the confined space of the montmorillonite (MMT) intergallery at different pHs of the intercalation solution (H(2)O/ethanol/Na(+)-MMT/di-C(12)amine), and these results were compared with those of dodecylmonoamine (C(12)H(27)N, mono-C(12)amine). The mono-C(12)amine with one end-functional amine (-NH(2)) has a constant intercalated amount independent of the pH of the intercalation solution, confirming the tail conformation in the MMT intergallery. On the other hand, the intercalated amount of di-C(12)amines remains quite low at low pH due to the long-range Coulombic repulsion among protonated amines. The di-C(12)amines dominantly take the bridge conformation inside the MMT intergallery up to pH 9.5 and adopt a fraction of the tail conformation at pH 11.5, which is verified by Fourier transform infrared spectra with an abrupt frequency shift of the CH(2) stretching band and the emergence of a new NH(2) scissoring band at 1598 cm(-)(1) related to unanchored free amines. The d-spacing and the square of the half-width at half-maximum of the MMT based on X-ray diffraction measurements are also constant up to pH 9.5, while these two variables are suddenly increased at pH 11.5. The bridge conformation of di-C(12)amines in the confined space of the MMT intergallery prevents poly(ethylene oxide) chains from intercalation into the MMT intergallery, called the gluing effect.

A Contrivance for a Dynamic Porous Framework: Cooperative Guest Adsorption Based on Square Grids Connected by Amide−Amide Hydrogen Bonds

Flexible porous coordination polymers containing amide groups as a function origin have been synthesized and categorized as "Coordination Polymer with Amide Groups". Bispyridyl ligands with a spacer of amide group afford two-dimensional (2-D) motifs with a deformed square grid, resulting in three-dimensional (3-D) frameworks of [Co(NO(3))(2)(3-pna)(2)](n)(1), [Co(Br)(2)(3-pna)(2)](n)(2), and [[Co(NCS)(2)(4-peia)(2)].4Me(2)CO](n)(3 subset 4Me(2)CO) (3-pna = N-3-pyridylnicotinamide, 4-peia = N-(2-pyridin-4-yl-ethyl)-isonicotinamide), where the 2-D motifs are bound by complementary hydrogen bond between the amide groups. In the case of the 3 subset 4Me(2)CO, the amide groups form a contrivance for a dynamic porous framework because of their relevant position and orientation in the mutual nearest neighboring motifs. Consequently, 3 subset 4Me(2)CO shows amorphous (nonporous)-to-crystal (porous) structural rearrangement in the Me(2)CO adsorption and desorption process, where the framework of the 2-D motif is maintained. The adsorption isotherm has threshold pressure (P(th)), a sort of gate pressure. The heat of Me(2)CO adsorption (DeltaH(ad) = -25 kJ/mol) is obtained from the temperature dependence of threshold pressure (P(th)), which is close to acetone vaporization enthalpy (DeltaH(vap) = 30.99 kJ/mol).

Asymmetric induction under confinement

Confinement may efficiently condition the stereochemical outcome of a reaction through space constriction and molecular close contact. This article briefly reviews recent approaches of supramolecular chemistry to achieve chiral confinement. Crystallization is not always possible and the use of chiral crystals or clathrates lacks generality. The construction of solid supramolecular assemblies circumvents some of the problems of the crystal chemistry. In this regard, molecular imprinting of polymeric matrices with orifices mimicking the transition state of an enantioselective process is a very young, promising technique. Zeolites provide porous, rigid environments to host molecules without the need of lucky crystallizations, yet zeolites are not chiral per se and must be chirally modified. Besides, the limited dimension of their pores restricts the size of the guest molecules. Despite these problems, useful asymmetric photochemical reactions have been performed on zeolites. Finally, the formation of pillared lamellar structures, from inorganic salts of tetravalent transition metals covalently grafted with organic chains, is considered. The adequate selection of functionality and chirality of the organic pillars would afford custom-made, highly porous, 3D hybrid organo-inorganic scaffolds. However, the production of asymmetric processes within these layered materials still remains to be seen.