Tunable Anisotropic Thermal Expansion of a Porous Zinc(II) Metal–Organic Framework

Switchable colossal anisotropic thermal expansion in a spin crossover framework

Advanced materials with tunable thermal expansion properties have garnered significant attention due to their potential applications in thermomechanical sensing and resistance to thermal stress. Here, switchable colossal anisotropic thermal expansion (ATE) behaviors are realized in a Hofmann-type framework [Fe(bpy-NH2){Au(CN)2}2]·iPrOH (Fe·iPrOH, bpy-NH2 = [4,4'-bipyridin]-3-amine) through a three-in-one strategy: a vibrational mechanism, an electronic mechanism and molecular motion. Spin crossover (SCO) centers coordinate with dicyanoaurate linkers to form flexible wine-rack frameworks, which exhibit structural deformations driven by host-guest interactions with iPrOH molecules. By means of the vibrational mechanism, a scissor-like motion driven by the rotation of dicyanoaurate is observed within the rhombic grids, resulting in the emergence of colossal ATE in the high temperature region. When the spin transition comes into play, the electronic mechanism is predominant to form reverse ATE behavior, which is associated with host-guest cooperation involving significant molecular motion of the iPrOH guest and adaptive deformation of the host clathrate. A remarkably high negative thermal expansion coefficient up to -7.49 × 105 M K-1 accompanied by abrupt SCO behavior is observed. As a proof of concept, this study provides a novel perspective for designing dynamic crystal materials with tunable thermomechanical properties by integrating various ATE-related elements into a unified platform.

Flexible silver-metal-organic framework probe for highly sensitive and visual detection of tetracycline hydrochloride in freshwater fish

Tetracycline hydrochloride (TCH) is commonly used in aquaculture to prevent bacterial infections, renders the detection of its residues highly crucial for ensuring the safety of aquatic products. In this study, an Ag-based metal-organic framework, Ag-dcp (H3dcp = 3-(3,5-dicarboxyphenyl)-6-carboxypyridine), was synthesized through hydrothermal method. The Ag-dcp framework offers environmental advantages, including a green synthesis process and potential for sustainable, eco-friendly applications. The uncoordinated carboxyl groups in Ag-dcp provide potential interaction sites for TCH, while the rhombic channels in the flexible structure of Ag-dcp that match the size of TCH effectively facilitate host-guest interactions. The Ag-dcp enables ratiometric fluorescence detection with high sensitivity via intramolecular charge transfer (ICT) triggered by TCH-induced ether bond rotation. The probe demonstrates a fluorescence color changes from blue to orange, achieving a detection limit as low as 0.14 μM and offering an ultrafast response time of just 10 s. Principal component analysis (PCA) was employed to effectively distinguish TCH from other tetracycline analogs (minocycline hydrochloride (MH) and doxycycline hydrochloride (DOX)), thereby enhancing the selectivity of the Ag-dcp probe. The fluorescence probe demonstrated performance comparable to HPLC in detecting TCH in freshwater fish. The detection strategy proposed here, based on the MOF structure, offers valuable insights for designing MOF materials and their specialized applications.

Thermally activated structural phase transitions and processes in metal-organic frameworks

The structural knowledge of metal-organic frameworks is crucial to the understanding and development of new efficient materials for industrial implementation. This review classifies and discusses recent advanced literature reports on phase transitions that occur during thermal treatments on metal-organic frameworks and their characterisation. Thermally activated phase transitions and procceses are classified according to the temperaturatures at which they occur: high temperature (reversible and non-reversible) and low temperature. In addition, theoretical calculations and modelling approaches employed to better understand these structural phase transitions are also reviewed.

Self-Assembly of Rhenium(I) Double-Stranded Helicate and Mesocate from Flexible Ditopic Benzimidazolyl/Naphthanoimidazolyl N-Donor and Rigid Bis-Chelating Hydroxyphenylbenzimidazolyl N∩OH-Donor Ligands: Synthesis, Characterization, and Photophysical and B-DNA Docking Studies

The self-assembly of three rheniumtricarbonyl core-based supramolecular coordination complexes (SCCs), fac-[Re(CO)3(μ-L)(μ-L')Re(CO)3] (1-3) was carried out using Re2(CO)10, rigid bis-chelating ligand (HO∩N-Ph-N∩OH (L1) (where HO∩N = 2-hydroxyphenylbenzimidazolyl), and flexible ditopic N-donor ligands (L2 = bis(3-((1H-benzoimidazol-1-yl)methyl)-2,4,6-trimethylphenyl)methane, L3 = bis(3-((1H-naphtho[2,3-d]imidazol-1-yl)methyl)-2,4,6-trimethylphenyl)methane, L4 = bis(4-(naphtho[2,3-d]imidazol-1-yl-methyl)phenyl)methane) via a one-pot solvothermal approach. In the solid state, the dinuclear SCCs adopt heteroleptic double-stranded helicate and meso-helicate architectures. The supramolecular structures of the complexes are retained in the solution based on the 1H NMR and electrospray ionization (ESI)-mass analysis. The spectral and photophysical properties of the complexes were studied both experimentally and using time-dependent density functional theory (TDDFT) calculations. All of the supramolecules exhibited emission in both solution and solid states. Theoretical studies were conducted to determine the chemical reactivity parameters, molecular electrostatic potential surface plots, natural population, and Hirshfeld analysis for complexes 1-3. Additionally, molecular docking studies were carried out for complexes 1-3 with B-DNA.

Introduction of pseudo-base benzimidazole derivatives into nucleosides via base exchange by a nucleoside metabolic enzyme

In alternate organic synthesis, biocatalysis using enzymes provides a more stereoselective and cost-effective approach. Synthesis of unnatural nucleosides by nucleoside base exchange reactions using nucleoside-metabolizing enzymes has previously shown that the 5-position recognition of pyrimidine bases on nucleoside substrates is loose and can be used to introduce functional molecules into pyrimidine nucleosides. Here we explored the incorporation of purine pseudo bases into nucleosides by the base exchange reaction of pyrimidine nucleoside phosphorylase (PyNP), demonstrating that an imidazole five-membered ring is an essential structure for the reaction. In the case of benzimidazole, the base exchange proceeded to give the deoxyribose form in 96 % yield, and the ribose form in 23 % yield. The reaction also proceeded with 1H-imidazo[4,5-b]phenazine, a benzimidazole analogue with an additional ring, although the yield of nucleoside was only 31 %. Docking simulations between 1H and imidazo[4,5-b]phenazine nucleoside and the active site of PyNP (PDB 1BRW) supported our observation that 1H-imidazo[4,5-b]phenazine can be used as a substrate by PyNP. Thus, the enzymatic substitution reaction using PyNP can be used to incorporate many purine pseudo bases and benzimidazole derivatives with various functional groups into nucleoside structures, which have potential utility as diagnostic or therapeutic agents.

Stress-Induced Inversion of Linear Dichroism by 4,4'-Bipyridine Rotation in a Superelastic Organic Single Crystal

The molecular crystals that manifest unusual mechanical properties have attracted growing attention. Herein, we prepared an organic single crystal that shows bidirectional superelastic transformation in response to shear stress. Single-crystal X-ray diffractions revealed this crystal-twinning related shape change is owed to a stress-controlled 90° rotation of 4,4'-bipyridine around the hydrogen bonds of a chiral organic trimer. As a consequence of the 90° shift in the aromatic plane, an interconversion of crystallographic a-, b-axes (a→b' and b→a') was detected. The molecular rotations changed the anisotropic absorption of linearly polarized light. Therefore, a stress-induced inversion of linear dichroism spectra was demonstrated for the first time. This study reveals the superior mechanical flexibilities of single crystals can be realized by harnessing the molecular rotations and this superelastic crystal may find applications in optical switching and communications.

A porous cobalt(II)-organic framework exhibiting high room temperature proton conductivity and field-induced slow magnetic relaxation

A two-dimensional (2D) cobalt(II) metal-organic framework (MOF) constructed by a ditopic organic ligand, formulated as {[Co(Hbic)(H₂O)]·4H₂O}_n (1) (H₂bic = 1H-benzimidazole-5-carboxylic acid), was hydrothermally synthesized and structurally characterized. Single-crystal X-ray diffraction shows that the distorted octahedral Co²⁺ ions, as coordination nodes, are bridged to form 2D honeycomb networks, which are further organized into a 3D supramolecular porous framework through multiple hydrogen bonds and interlayer π-π interactions. Dynamic crystallography experiments reveal the anisotropic thermal expansion behavior of the lattice, suggesting a flexible hydrogen-bonded 3D framework. Interestingly, hydrogen-bonded (H₂O)₄ tetramers were found to be located in porous channels, yielding 1D proton transport pathways. As a result, the compound exhibited a high room-temperature proton conductivity of 1.6 × 10^-4 S cm^-1 under a relative humidity of 95% through a Grotthuss mechanism. Magnetic investigations combined with theoretical calculations reveal giant easy-plane magnetic anisotropy of the distorted octahedral Co²⁺ ions with the experimental and computed D values being 87.1 and 109.3 cm^-1, respectively. In addition, the compound exhibits field-induced slow magnetic relaxation behavior at low temperatures with an effective energy barrier of U_eff = 45.2 cm^-1. Thus, the observed electrical and magnetic properties indicate a rare proton conducting SIM-MOF. The foregoing results provide a unique bifunctional cobalt(II) framework material and suggest a promising way to achieve magnetic and electrical properties using a supramolecular framework platform.

Reduced thermal expansion by surface-mounted nanoparticles in a pillared-layered metal-organic framework

Control of thermal expansion (TE) is important to improve material longevity in applications with repeated temperature changes or fluctuations. The TE behavior of metal-organic frameworks (MOFs) is increasingly well understood, while the impact of surface-mounted nanoparticles (NPs) on the TE properties of MOFs remains unexplored despite large promises of NP@MOF composites in catalysis and adsorbate diffusion control. Here we study the influence of surface-mounted platinum nanoparticles on the TE properties of Pt@MOF (Pt@Zn₂(DP-bdc)₂dabco; DP-bdc^2-=2,5-dipropoxy-1,4-benzenedicarboxylate, dabco=1,4-diazabicyclo[2.2.2]octane). We show that TE is largely retained at low platinum loadings, while high loading results in significantly reduced TE at higher temperatures compared to the pure MOF. These findings support the chemical intuition that surface-mounted particles restrict deformation of the MOF support and suggest that composite materials exhibit superior TE properties thereby excluding thermal stress as limiting factor for their potential application in temperature swing processes or catalysis.

Tunable uniaxial, area, and volume negative thermal expansion in quartz-like and diamond-like metal-organic frameworks

This paper proposes that it will be an effective way to discover and explore organic negative thermal expansion (NTE) materials based on the specific topologies in inorganic NTE materials. Various NTE behaviors from the uniaxial, area, and volume-NTE can be achieved by adjusting the topology, for instance, quartz-like and diamond-like. Zn(ISN)₂ and InH(BDC) metal-organic frameworks (MOFs) with quartz-like topology have been studied by first principles calculations. The calculated area-NTE of Zn(ISN)₂ and uniaxial-NTE of InH(BDC) within quasi-harmonic approximation (QHA) agree well with the experimental data. Through the calculation of Grüneisen parameters, it is shown that low-frequency optical phonons appear dominant resulting in their NTE, but the coupling to high-frequency phonons is of greater ultimate importance. The lattice vibrational modes of great contribution to area-NTE of Zn(ISN)₂ and uniaxial-NTE of InH(BDC) are analyzed in detail. Also, four MOFs with diamond-like topology are predicted to exhibit volume-NTE behavior. Moreover, it is found that there is a bulk modulus anomaly in some studied MOFs with the quartz-like and diamond-like framework, where the temperature dependence of bulk modulus does not follow the inverse dependence on that of volume. These specific topologies provide key geometric frameworks for various NTE behaviors of MOFs, and meanwhile, the local structure and bond environment in MOFs can lead to abnormal interatomic force, i.e., bulk modulus anomaly. This abnormal elastic property also deserves more attention.

A neural network potential for the IRMOF series and its application for thermal and mechanical behaviors

Metal-organic frameworks (MOFs) with their exceptional porous and organized structures have been the subject of numerous applications. Predicting the bulk properties from atomistic simulations requires the most accurate force fields, which is still a major problem due to MOFs' hybrid structures governed by covalent, ionic and dispersion forces. Application of ab initio molecular dynamics to such large periodic systems is thus beyond the current computational power. Therefore, alternative strategies must be developed to reduce computational cost without losing reliability. In this work, we construct a generic neural network potential (NNP) for the isoreticular metal-organic framework (IRMOF) series trained by PBE-D4/def2-TZVP reference data of MOF fragments. We confirmed the success of the resulting NNP on both fragments and bulk MOF structures by prediction of properties such as equilibrium lattice constants, phonon density of states and linker orientation. The RMSE values of energy and force for the fragments are only 0.0017 eV atom^-1 and 0.15 eV Å^-1, respectively. The NNP predicted equilibrium lattice constants of bulk structures, even though not included in training, are off by only 0.2-2.4% from experimental results. Moreover, our fragment based NNP successfully predicts the phenylene ring torsional energy barrier, equilibrium bond distances and vibrational density of states of bulk MOFs. Furthermore, the NNP enables revealing the odd behaviors of selected MOFs such as the dual thermal expansion properties and the effect of mechanical strain on the adsorption of hydrogen and methane molecules. The NNP based molecular dynamics (MD) simulations suggest IRMOF-4 and IRMOF-7 to have positive-to-negative thermal expansion coefficients while the rest to have only negative thermal expansion at the studied temperatures of 200 K to 400 K. The deformation of the bulk structure by reduction of the unit cell volume has been shown to increase the volumetric methane uptake in IRMOF-1 but decrease the volumetric methane uptake in IRMOF-7 due to the steric hindrance. To the best of our knowledge, this study presents the first pre-trained model publicly available giving the opportunity for the researchers in the field to investigate different aspects of IRMOFs by performing large-scale simulation at the first-principles level of accuracy.

Near-room-temperature dielectric switch and thermal expansion anomaly in a new hybrid crystal: (Me2NH2)[CsFe(CN)5(NO)]

A new hybrid crystal, (Me2NH2)[CsFe(CN)5(NO)], featuring a double-layered nitroprusside-based inorganic framework with cubic-like cages encapsulating organic cations, undergoes a near-room-temperature phase transition accompanying with dielectric switch and thermal expansion anomaly....

Molecular tiltation and supramolecular interactions induced uniaxial NTE and biaxial PTE in bis-imidazole-based co-crystals

Uniaxial NTE and biaxial PTE has been observed in bis-imidazole-based co-crystals induced by molecular tiltation and supramolecular interactions.

Size and crystal symmetry breaking effects on negative thermal expansion in ScF3 nanostructures

Nowadays, one of the most typical and important potential applications of negative thermal expansion (NTE) materials is to prepare zero thermal expansion or controllable coefficient thermal expansion materials by compounding them with positive thermal expansion materials. The research on NTE properties at the nanoscales is the basis and premise for the realization of high-quality composites. Here, using first-principles calculations, we take a typical open framework material ScF₃ as an example to study a new NTE mechanism at the nanoscale, which involves edge and size effects, as well as crystal symmetry breaking. By analyzing the vibrational modes in ultrathin ScF₃ films, three effects contributing to the NTE properties are identified, namely, the acoustic mode (ZA mode) induced by surface truncation, the enhanced rotations of ScF₆ octahedra in the surface layer and the suppressed rotations of ScF₆ octahedra in the inner layer due to crystal symmetry breaking. With increasing thickness, the effect of the ZA mode vibration gradually weakens, while the rotations of the ScF₆ octahedra in the surface and inner layers are enhanced. Ultimately, the approximately mutual compensation of these three effects makes the NTE coefficients of different thicknesses almost unchanged. Finally, we simply generalize our conclusions to zero dimensional nanoparticles. This work reveals a new NTE mechanism in low-dimensional open framework materials, which serves as a guide in designing NTE materials at the nanoscale.

Tuning extreme anisotropic thermal expansion in 1D coordination polymers through metal selection and solid solutions

The thermal expansion behaviour of a series of 1D coordination polymers has been investigated. Variation of the metal centre allows tuning of the thermal expansion behaviour from colossal positive volumetric to extreme anomalous thermal expansion. Preparation of solid solutions increased the magnitude of the anomalous thermal expansion further, producing two species displaying supercolossal anisotropic thermal expansion (ZnCoCPHTαY2 = -712 MK-1, αY3 = 1632 MK-1 and ZnCdCPHTαY2 = -711 MK-1, αY3 = 1216 MK-1).

A Porous Coordination Polymer Showing Guest-Amplified Positive and Negative Thermal Expansion

A solvothermal reaction of Zn(NO₃)₂ and 4-(1H-pyrazol-4-yl)benzoic acid (H₂pba) with toluene (Tol) as the template yielded a porous coordination polymer, [Zn(pba)]·0.5Tol, possessing a three-dimensional (3D) fence-like coordination framework based on inclined two-dimensional (2D) fence-like coordination layers. By virtue of the classic deformation mode of the 2D/3D fence structures, the guest-free structure exhibits very large positive thermal expansion of 347 MK^-1 and moderate negative thermal expansion of -63/-83 MK^-1, which are remarkably enhanced to new records of 689 and -171/-249 MK^-1, respectively, by inclusion of Tol.

Precise regulating synergistic effect in metal–organic framework for stepwise-controlled adsorption

MAC-20 shows a unique two-step pore-shape change and executes a stepwise-controlled adsorption of dyes mixture in order of their sizes.

Anisotropic Thermal Expansion in an Anionic Framework Showing Guest-Dependent Phases

Crystalline materials generally show small positive thermal expansion along all three crystallographic axes because of increasing anharmonic vibrational amplitudes between bonded atoms or ions pairs on heating. In very rare cases, structural peculiarities may give rise to negative, anomalously large or zero thermal expansion behaviors, which remain poorly understood. Host-guest composites may exhibit such anomalous behavior if guest motions controllable. Here we report an anionic framework of helical nanotubes comprising three parallel helical chains. The anisotropic interaction between the guest and the framework, results in anisotropic thermal expansion in this framework. A series of detailed structural determination at 50 K intervals enable process visualization at the molecular level and the observed guest-dependent phases of the framework strongly support our proposed mechanism.

Flexibility control in alkyl ether-functionalized pillared-layered MOFs by a Cu/Zn mixed metal approach

Flexible metal-organic frameworks (MOFs) exhibit large potential as next-generation materials in areas such as gas sensing, gas separation and mechanical damping. By using a mixed metal approach, we report how the stimuli reponsive phase transition of flexible pillared-layered MOFs can be tuned over a wide range. Different Cu2+ to Zn2+ metal ratios are incorporated into the materials by using a simple solvothermal approach. The properties of the obtained materials are probed by differential scanning calorimetry and CO2 sorption measurements, revealing stimuli responsive behaviour as a function of metal ratio. Pair distribution functions derived from X-ray total scattering experiments suggest a distortion of the M2 paddlewheel as a function of the Cu content. We rationalize these phenomena by the different distortion energies of Cu2+ and Zn2+ ions to deviate from the square pyramidal structure of the relaxed paddlewheel node. Our work follows on from the large interest in tuning and understanding the materials properties of flexible MOFs, highlighting the large number of parameters that can be used for the targeted manipulation and design of properties of these fascinating materials.

Metal-organic framework based on hinged cube tessellation as transformable mechanical metamaterial

Mechanical metamaterials exhibit unusual properties, such as negative Poisson's ratio, which are difficult to achieve in conventional materials. Rational design of mechanical metamaterials at the microscale is becoming popular partly because of the advance in three-dimensional printing technologies. However, incorporating movable building blocks inside solids, thereby enabling us to manipulate mechanical movement at the molecular scale, has been a difficult task. Here, we report a metal-organic framework, self-assembled from a porphyrin linker and a new type of Zn-based secondary building unit, serving as a joint in a hinged cube tessellation. Detailed structural analysis and theoretical calculation show that this material is a mechanical metamaterial exhibiting auxetic behavior. This work demonstrates that the topology of the framework and flexible hinges inside the structure are intimately related to the mechanical properties of the material, providing a guideline for the rational design of mechanically responsive metal-organic frameworks.

Desolvation process in the flexible metal–organic framework [Cu(Me-4py-trz-ia)], adsorption of dihydrogen and related structure responses

Structural changes and the unusual H₂ adsorption behaviour of a Cu²⁺-based MOF were studied by X-ray diffraction in combination with DFT modelling and by inelastic neutron scattering.

Guest-dependent negative thermal expansion in a lanthanide-based metal–organic framework

A lanthanide-based metal–organic framework (MOF) named SION-2, displays strong and tuneable uniaxial negative thermal expansion (NTE).

Continuous negative-to-positive tuning of thermal expansion achieved by controlled gas sorption in porous coordination frameworks

Control of the thermomechanical properties of functional materials is of great fundamental and technological significance, with the achievement of zero or negative thermal expansion behavior being a key goal for various applications. A dynamic, reversible mode of control is demonstrated for the first time in two Prussian blue derivative frameworks whose coefficients of thermal expansion are tuned continuously from negative to positive values by varying the concentration of adsorbed CO₂. A simple empirical model that captures site-specific guest contributions to the framework expansion is derived, and displays excellent agreement with the observed lattice behaviour.

Achieving control over the thermomechanical properties of functional materials is desirable, yet remains highly challenging. Here, the authors demonstrate continuous negative-to-positive tuning of thermal expansion in two Prussian blue analogues, by varying the concentration of adsorbed CO₂.

Switching from positive to negative axial thermal expansion in two organic crystalline compounds with similar packing

Switching from positive to negative axial thermal expansion in pure organic materials is reported for the first time. This rare phenomenon has been rationalized based on the packing of molecules in crystal structures and transverse thermal vibrations of atoms in the molecule. Unique packing of the molecules in the crystal structure contributes to the restricted movement of molecules along the c axis. Subsequently, contraction of molecular dimensions with increasing temperature, due to transverse vibrations of some atoms, assists with the switch from Positive Thermal Expansion (PTE) to Negative Thermal Expansion (NTE).

Positive and Negative Two-Dimensional Thermal Expansion via Relaxation of Node Distortions

The ability to tune physical properties is attractive for the development of new materials for myriad applications. Understanding and controlling the structural dynamics in complicated network structures like coordination polymers (CPs) is particularly challenging. We report a series of two-dimensional CPs [Mn(salen)]₂[M(CN)₄]· xH₂O (M = Pt (1), PtI₂ (2), and MnN (3)) incorporating zigzag cyano-network layers that display composition-dependent anisotropic thermal expansion properties. Variable-temperature single-crystal X-ray structural analyses demonstrated that the thermal expansion behavior is caused by double structural distortions involving [Mn(salen)]⁺ units incorporated into the zigzag layers. Thermal relaxations produce structural transformations resulting in positive thermal expansion for 2·H₂O and negative thermal expansion for 3. In the case of 1·H₂O, the relaxation does not occur and zero thermal expansion results in the plane between 200 to 380 K. The present study proposes a new strategy based on structural distortions in coordination networks to control thermal responsivities of frameworks.

Low-Frequency Phonon Driven Negative Thermal Expansion in Cubic GaFe(CN)6 Prussian Blue Analogues

The understanding of the negative thermal expansion (NTE) mechanism is vital not only for the development of new NTE compounds but also for effectively controlling thermal expansion. Here, we report an interesting isotropic NTE property in cubic GaFe(CN)₆ Prussian blue analogues (α _l = -3.95 × 10^-6 K^-1, 100-475 K), which is a new example to understand the complex NTE mechanism. A combined study of synchrotron X-ray diffraction, X-ray total scattering, X-ray absorption fine structure, neutron powder diffraction, and density functional theory calculations shows that the NTE of GaFe(CN)₆ originates from the low-frequency phonons (< ∼100 cm^-1), which are directly related to the transverse vibrations of the atomic -Ga-N≡C-Fe- chains. Both the Ga-N and Fe-C chemical bonds are much softer to bend than to stretch. The direct evidence that transverse vibrational contribution to the NTE of GaFe(CN)₆ is dominated by N, instead of C atoms, is illustrated. It is interesting to find that the polyhedra of GaFe(CN)₆ are not rigid, which is a starting assumption in some models describing the NTE properties of other systems. The NTE mechanism can be vividly described by the "guitar-string" effect, which would be the common feature for the NTE property of many open-framework functional materials, such as Prussian blue analogues, oxides, cyanides, metal-organic frameworks, and zeolites.

Sulfate Donor Based Dinuclear Heteroleptic Triple-Stranded Helicates from Sulfite and Ditopic Nitrogen Donor Ligands and Their Transformation to Dinuclear Homoleptic Double-Stranded Mesocates

Sulfate donor based supramolecular coordination complexes [{ fac-Re(CO)₃}(μ-SO₄)(L ⁿ)₂{ fac-Re(CO)₃}] (1-3) were obtained using ditopic N donors (L ⁿ; n = 1-3), NaHSO₃, and Re₂(CO)₁₀ in a one-pot, multicomponent, coordination-driven self-assembly approach, in which SO₃^2- becomes oxidized to SO₄^2- during the reaction and acts as a building framework. Complexes 1-3 were characterized using IR, ESI-TOF-MS, and ¹H NMR spectroscopy. The structures of complexes 1-3 were confirmed using single-crystal X-ray diffraction analysis. The transformation of the dinuclear heteroleptic triple-stranded helicate to the dinuclear homoleptic double-stranded mesocate [{Re(CO)₃Cl}₂(L ⁿ)₂] (L ⁿ = L¹, L², L³; 4a-6a) was achieved by the addition of BaCl₂. The direct treatment of Re(CO)₅X (X = Cl, Br) with L¹/L²/L³ yielded the dinuclear homoleptic double-stranded helicates [{Re(CO)₃X}₂ (L ⁿ)₂] (4b-6b and 7-9).

Controlling Thermal Expansion Behaviors of Fence-Like Metal-Organic Frameworks by Varying/Mixing Metal Ions

Solvothermal reactions of 3-(4-pyridyl)-benzoic acid (Hpba) with a series of transition metal ions yielded isostructral metal-organic frameworks [M(pba)2]·2DMA (MCF-52; M = Ni2+, Co2+, Zn2+, Cd2+, or mixed Zn2+/Cd2+; DMA = N,N-dimethylacetamide) possessing two-dimensional fence-like coordination networks based on mononuclear 4-connected metal nodes and 2-connected organic ligands. Variable-temperature single-crystal X-ray diffraction studies of these materials revealed huge positive and negative thermal expansions with |α| > 150 × 10-6 K-1, in which the larger metal ions give the larger thermal expansion coefficients, because the increased space not only enhance the ligand vibrational motion and hinged-fence effect, but also allow larger changes of steric hindrance between the layers. In addition, the solid-solution crystal with mixed metal ions further validates the abundant thermal expansion mechanisms of these metal-organic layers.

A thermo-responsive structural switch and colossal anisotropic thermal expansion in a chiral organic solid

Trianglimine, a chiral triangular-shaped hexaimine, exists in at least three apohost polymorphic forms. Form I had been previously obtained by crystallisation from a mixture of dichloromethane and acetonitrile and we have now crystallised Form II from acetone. Both forms possess similar packing arrangements, but Form II undergoes a reversible phase transition to Form III, as well as colossal anisotropic positive thermal expansion. Form I does not exhibit any remarkable thermal properties.

Rhenium(i) based irregular pentagonal-shaped metallacavitands

Neutral ditopic flexible N-donor ligands (Ln = bis(4-(naphtho[2,3-d]imidazol-1-ylmethyl)phenyl)methane, L1 bis(4-(benzimidazol-1-ylmethyl)phenyl)methane, L2 or bis(4-(2-nonylbenzimidazol-1-ylmethyl)phenyl)methane, L3) possessing a bis(4-methylphenyl)methane spacer with two imidazolyl donor units were designed and synthesized. The ligands were utilized to develop metallacavitands analogous to irregular pentagonal-shaped metallacavitands with larger cavities. The metallacavitands 1-4 were assembled from Re2(CO)10, a rigid bis-chelating donor (1,4-dihydroxy-9,10-anthraquinone or chloranilic acid) and Lnvia a solvothermal approach. The ligands and the metallacavitands were characterized by analytical and spectroscopic methods. The molecular structures of 1 and 4 were further confirmed by single crystal X-ray diffraction analysis which revealed that a toluene molecule resides in the hydrophobic cavity. Ln and 1-4 are emissive in DMSO at room temperature. The internal cavity of the metallacavitand acts as a host for aromatic guest molecules. The host-guest interaction properties of 1 with anthracene, naphthalene, nitrobenzene, 2-nitrotoluene, 4-nitrotoluene and 2,4-dinitrotoluene were studied by an emission spectroscopic method.

Joint Experimental and Computational Investigation of the Flexibility of a Diacetylene-Based Mixed-Linker MOF: Revealing the Existence of Two Low-Temperature Phase Transitions and the Presence of Colossal Positive and Giant Negative Thermal Expansions

Solvothermal reaction in N,N-dimethylformamide (DMF) between 1,6-bis(1-imidazolyl)-2,4-hexadiyne monohydrate (L1⋅H₂ O), isophthalic acid (H₂ L2), and Zn(NO₃ )₂ ⋅6 H₂ O gives the diacetylene-based mixed-ligand coordination polymer {[Zn(L1)(L2)](DMF)₂ }_n (UMON-44) in 38 % yield. Combination of DSC with variable-temperature single-crystal X-ray diffraction revealed the occurrence of two phase transitions spanning the ranges 129-144 K and 158-188 K. Furthermore, the three structurally similar phases of UMON-44 show giant negative and/or colossal positive thermal expansions. These unusual phenomena exist without any change in the contents of the unit cell. DFT calculations using the PBE+D3 dispersion scheme were able to distinguish between these polymorphs by accurately reproducing their salient structural features, although corrections in the size of the unit cell turned out to be necessary for the high-temperature phase to account for its large thermal expansion. In addition, the infrared spectra (vibration frequencies and peak intensities) of these theoretical models were calculated, allowing for univocal identification of the corresponding polymorphs. Last, the limits of our computational method were tested by calculating the phase transition temperatures and their associated enthalpies, and the derived figures compare favorably with the values determined experimentally.

Open and closed forms of the interpenetrated [Cu2(Tae)(Bpa)2](NO3)2·nH2O: magnetic properties and high pressure CO2/CH4 gas sorption

Two closed and one open structural forms of the interpenetrated [Cu₂(Tae)(Bpa)₂](NO₃)₂·nH₂O (H₂Tae = 1,1,2,2-tetraacetylethane, Bpa = 1,2-bis(4-pyridyl)ethane) cationic coordination polymer have been synthesized. Three crystallographically related interpenetrated "ths" cationic nets encapsulate water molecules and nitrate anions giving rise to the closed structural forms of [Cu₂(Tae)(Bpa)₂](NO₃)₂·nH₂O. Depending on the location of water molecules and nitrate groups, two different closed forms with 5.5 and 3.6 crystallization water molecules have been obtained. The thermal activation of the closed structures gives rise to a 29% expansion of the unit cell. This closed to open transformation is reversible, and is triggered by the loss or uptake of solvent. The high pressure gas adsorption experiments show similar selectivity values towards CO₂ for CO₂/CH₄ mixtures to that showed by some metal organic frameworks without unsaturated metal sites, and isosteric heats for CO₂ adsorption similar to that for the HKUST-1 compound.

Negative thermal expansion in molecular materials

Some mechanisms resulting in negative thermal expansion in molecular materials are summarized.

Area negative thermal expansion in a mixed metal mixed organic MOF: “elevator-platform” mechanism induced by O–H⋯O hydrogen bonding

Rare area negative thermal expansion of a new mixed metal mixed organic MOF has been described using an “elevator-platform” analogy induced by O–H⋯O hydrogen bonding.

3D negative thermal expansion in orthorhombic MIL-68(In)

Rare three-dimensional negative thermal expansion (NTE) has been found in a low symmetry compound, orthorhombic MIL-68(In).

Uniaxial negative thermal expansion induced by moiety twisting in an organic crystal

Anomalous thermal expansion of a new diyn-diol molecule was studied by means of variable-temperature single-crystal X-ray diffraction. Analysis of the unit cell axes as a function of temperature shows that the material experiences uniaxial negative thermal expansion. Packing analysis of the crystal structures reveals twisting of the cyclopentyl moiety relative to the diyne spine with increasing temperature.

Reversible thermosalience of 4-aminobenzonitrile

Crystals of 4-aminobenzonitrile grown by sublimation undergo a reversible thermosalient phase change during cooling and subsequent heating. Single-crystal diffraction studies have been carried out at 20 K intervals during cooling from 300 to 100 K in order to explain the structural change that occurs.