Pillared Derivatives of γ-Zirconium Phosphate Containing Nonrigid Alkyl Chain 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.

Stimuli-responsive structural changes in metal–organic frameworks

This feature article mainly summarizes how the structure of MOFs changes under external stimuli.

The Anisotropic Responses of a Flexible Metal-Organic Framework Constructed from Asymmetric Flexible Linkers and Heptanuclear Zinc Carboxylate Secondary Building Units

A new porous and flexible metal-organic framework (MOF) has been synthesized from the flexible asymmetric linker N-(4-carboxyphenyl)succinamate (CSA) and heptanuclear zinc oxo-clusters of formula [Zn₇O₂(carboxylate)₁₀DMF₂] involving two coordinated terminal DMF ligands. The structural response of this MOF to the removal or exchange of its guest molecules has been probed using a combination of experimental and computational approaches. The topology of the material, involving double linker connections in the a and b directions and single linker connections along the c axis, is shown to be key in the material's anisotropic response. The a and b directions remain locked during guest removal, whereas the c axis linker undergoes large changes significantly reducing the material's void space. The changes to the c axis linker involve a combination of a hinge motion on the linker's rigid side and conformational rearrangements on its flexible end, which were probed in detail during this process despite the presence of crystallographic disorder along this axis, which prevented accurate characterization by experimental methods alone. Although inactive during guest removal, the flexible ends of the a and b axis linkers are observed to play a prominent role during DMF to DMSO solvent exchange, facilitating the exchange reaction arising in the cluster.

Structural flexibility in crystallized matter: from history to applications

The large reversible flexibility of hybrid crystallized matter is relatively new. After briefly recalling the history of this discovery, the article will analyze the different parameters influencing this phenomenon. They relate first to the various structural characteristics of the framework, in both its inorganic and organic parts. The influence of the energies of the guest-guest and host-guest interactions is then analyzed. Once the reasons are explained, a third section will be devoted to the various physical properties of these flexible solids. The last section concerns recent industrial applications of this family of solids.

Giant flexibility of crystallized organic–inorganic porous solids: facts, reasons, effects and applications

Giant structural flexibility is a characteristic of organic–inorganic frameworks. This perspective describes its history, its behaviours, the analysis of its structural reasons at its consequences in terms of properties and applications.

Conformation-controlled sorption properties and breathing of the aliphatic Al-MOF [Al(OH)(CDC)]

The Al-MOF CAU-13 ([Al(OH)(trans-CDC)]; trans-H2CDC = trans-1,4-cyclohexanedicarboxylic acid) is structurally related to the MIL-53 compounds that are well-known for their "breathing" behavior, i.e., the framework flexibility upon external stimuli such as the presence of adsorbate molecules. The adsorption properties of CAU-13 were investigated in detail. The sorption isotherms of N2, H2, CH4, CO, CO2, and water were recorded, and the adsorption enthalpies for the gases were determined by microcalorimetry. The structural changes upon adsorption of CO2 were followed with in situ synchrotron powder X-ray diffraction (PXRD). The patterns were analyzed by parametric unit cell refinement, and the preferential arrangement of the CO2 molecules was modeled by density functional theory calculations. The adsorption and separation of mixtures of o-, m-, and p-xylene from mesitylene showed a preferred adsorption of o-xylene. The structures of o/m/p-xylene-loaded CAU-13 were determined from PXRD data. The adsorption of xylene isomers induces a larger pore opening than that in the thermal activation of CAU-13. In the crystal structure of the activated sample CAU-13(empty pore), half of the linkers adopt the a,a confirmation and the other half the e,e conformation, and the presence of a,a-CDC(2-) ions hampers the structural flexibility of CAU-13. However, after the adsorption of xylene, all linkers are present in the e,e conformation, allowing for a wider pore opening by this new type of "breathing".

Metal–organic frameworks based on flexible ligands (FL-MOFs): structures and applications

This review presents the recent developments on FL-MOFs, including their structures and applications in gas adsorption, catalysis and proton conduction.

Molecular motions in functional self-assembled nanostructures

The construction of "smart" materials able to perform specific functions at the molecular scale through the application of various stimuli is highly attractive but still challenging. The most recent applications indicate that the outstanding flexibility of self-assembled architectures can be employed as a powerful tool for the development of innovative molecular devices, functional surfaces and smart nanomaterials. Structural flexibility of these materials is known to be conferred by weak intermolecular forces involved in self-assembly strategies. However, some fundamental mechanisms responsible for conformational lability remain unexplored. Furthermore, the role played by stronger bonds, such as coordination, ionic and covalent bonding, is sometimes neglected while they can be employed readily to produce mechanically robust but also chemically reversible structures. In this review, recent applications of structural flexibility and molecular motions in self-assembled nanostructures are discussed. Special focus is given to advanced materials exhibiting significant performance changes after an external stimulus is applied, such as light exposure, pH variation, heat treatment or electromagnetic field. The crucial role played by strong intra- and weak intermolecular interactions on structural lability and responsiveness is highlighted.

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.

Soft porous crystals

The field of host-guest complexation is intensely attractive from diverse perspectives, including materials science, chemistry and biology. The uptake and encapsulation of guest species by host frameworks are being investigated for a wide variety of purposes, including separation and storage using zeolites, and recognition and sensing by enzymes in solution. Here we focus on the concept of the cooperative integration of 'softness' and 'regularity'. Recent developments on porous coordination polymers (or metal-organic frameworks) have provided the inherent properties that combine these features. Such soft porous crystals exhibit dynamic frameworks that are able to respond to external stimuli such as light, electric fields or the presence of particular species, but they are also crystalline and can change their channels reversibly while retaining high regularity. We discuss the relationship between the structures and properties of these materials in view of their practical applications.

Large breathing effects in three-dimensional porous hybrid matter: facts, analyses, rules and consequences

This critical review focuses on a strange behaviour of crystallized solid matter: its reversible swelling with large magnitude. This will be of interest for experts in porous solids but also for solid state chemists and physicists. Some examples, classified according to the dimensionality of the inorganic subnetwork, present the general requirements and the structural rules which govern the existence of this phenomenon. Its consequences concern specific applications related to sensors, energy savings, sustainable development and health (100 references).

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.

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.

Metal−Organotetraphosphonate Inorganic−Organic Hybrids: Crystal Structure and Anticorrosion Effects of Zinc Hexamethylenediaminetetrakis(methylenephosphonate) on Carbon Steels

The synthesis and structural characterization of the first polymeric M-HDTMP organic-inorganic hybrids are described [M = Zn2+, Ca2+; HDTMP = hexamethylenediaminetetrakis(methylenephosphonate)]. The 3D crystal structure of the Zn2+ analogue [Zn(HDTMP) x H2O] is described. The Zn center is found in a distorted octahedral environment of phosphonate oxygens. There is a long Zn...O interaction (2.622 A) originating from a protonated -P-OH group. Synergistic combinations of Zn2+ and the tetraphosphonate are found to form films that protect against the corrosion of carbon steels.

Silver(I) Compounds Consisting of [2.2]Paracyclophane: Reversible Guest-Driven Solid-State Transformation and Incorporation Behavior

Reaction of [2.2]paracyclophane with silver(I) heptafluorobutyrate (AgC3F7CO2) has isolated three novel networks: [Ag4(pcp)(C3F7CO2)4] x pyrene (1), [Ag4(pcp)(C3F7CO2)4] x phen (phen = phenanthrene) (2), and [Ag4(pcp)(C3F7CO2)4] x fluorene (3), and an intercalation compound [Ag4(pcp)(C3F7CO2)4] x 2benzene (4). All the four complexes exhibit two-dimensional (2D) sheet structures in which AgC3F7CO2 form an infinite chain and pcp acts as linkage. 1, 2, and 3 show 2D flat sheets with cavities in which guest molecules are situated, whereas 4 exhibits 2D zigzag layers between which guest benzene molecules are intercalated. Pcp shows mu-di-eta1-eta2 coordination mode in 1, mu-tetra-eta1 coordination mode in 2 and 3, and mu-tetra-eta2 coordination mode in 4. The reversible guest exchanges were observed between complex 1, 2, or 3 and intercalation compound 4. It is unprecedented for metal-organic inclusion complexes that the guest exchange occurs where the guest is the solute molecule. Furthermore, 4 can release the guest, and the original framework was completely recovered after reincorporation of benzene. It should be noted that 4 can incorporate pyrene, phen, and fluorene to give 1, 2, and 3, respectively, after desorption.

Dynamic porous properties of coordination polymers inspired by hydrogen bonds

In a decade, many porous coordination polymers have been synthesized, providing a variety of properties ranging from storage, separation, exchange of guests in their cavities, magnetism, conductivity and catalysis by their frameworks. In this tutorial review, we focus on the hydrogen bonding type arrangements for dynamic porous coordination polymers exhibiting elastic guest accommodations, in contrast to rigid three-dimensional (3-D) frameworks. Such dynamic porous properties induce highly-selective guest accommodation and magnetic modulation, and could now be considered a new class of practical materials.

Rational Design and Crystal Structure Determination of a 3-D Metal−Organic Jungle-Gym-like Open Framework

A new three-dimensional (3-D) jungle-gym-like open metal-organic framework has been synthesized from a two-dimensional (2-D) layer compound using a heterogeneous pillar insertion reaction. Both the starting 2-D layer and the resulting 3-D open compounds have been characterized using X-ray crystallography.

Vibrational Study of Some Layered Structures Based on Titanium and Zirconium Phosphates

A Raman and infrared study was carried out on layered zirconium and titanium acid phosphates of alpha- and gamma-type, alpha-M[O(3)POH](2).H(2)O and gamma-M[PO(4)][O(2)P(OH)(2)].2H(2)O, respectively. The spectra were initially approached by means of the classical correlation method in the solid state, which accounts for the complexity of the infrared spectra of both species. However, the number of bands and their relative intensity in the Raman spectra suggest a quite total absence of quadrupolar coupling between the vibrating units. So, if interunit coupling is neglected, a molecular approach considering the vibrations of isolated tetrahedral [PO(4)] and octahedral [MO(6)] building blocks can allow an affordable spectroscopic description of the title compounds. Interesting insights on the relationships between spectral properties and structure can be drawn by comparison with the spectra of alkali phosphates and of MO(6) oxoanions. A significant high-energy shift of the nu(P-O) modes is observed in the layered phosphates with respect to the corresponding salts, which parallels the low-energy shift of the nu(M-O) modes. Surprisingly, an increase of the M-OP interaction can reinforce the P-O bond. A simple theoretical model, based on the interaction between the [PO(4)] unit and four Li(+) in similar geometrical arrangement found in the structures of the layered phosphates, offers a reasonable explanation of this phenomenon.