Guest Water Molecules Research Articles

A comparative study of nickel and cadmium coordination polymers: Framework topology and proton conductivity

Two new coordination polymers (CPs), [Ni2(dip)2(obb)2]⋅2H2O, and [Cd2(dip)2(obb)2(H2O)]⋅5.5H2O, were successfully synthesized via hydrothermal processes utilizing the organic linkers 4,4′-oxybisbenzoate (obb) and 2,6-di(1H-imidazole-1-yl)pyridine (dip). The unique structures of both CPs were confirmed by single-crystal X-ray diffraction studies. Ni-CP presents an intricate three-fold interpenetrated 3D framework, featuring a rare cag topology centered around dinuclear nodes. In contrast, Cd-CP exhibits a distinctive non-interpenetrated 2D framework with an unusual kIa topology. Despite Cd-CP's water coordination and the inclusion of substantial guest water molecules, it unexpectedly exhibited lower proton conductivity than Ni-CP, which is non-porous and contains much less water content.

Prominent Intrinsic Proton Conduction in Two Robust Zr/Hf Metal-Organic Frameworks Assembled by Bithiophene Dicarboxylate.

To date, developing crystalline proton-conductive metal-organic frameworks (MOFs) with an inherent excellent proton-conducting ability and structural stability has been a critical priority in addressing the technologies required for sustainable development and energy storage. Bearing this in mind, a multifunctional organic ligand, 3,4-dimethylthiophene[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), was employed to generate two exceptionally stable three-dimensional porous Zr/Hf MOFs, [Zr6O4(OH)4(DTD)6]·5DMF·H2O (Zr-DTD) and [Hf6O4(OH)4(DTD)6]·4DMF·H2O (Hf-DTD), using solvothermal means. The presence of Zr6 or Hf6 nodes, strong Zr/Hf-O bonds, the electrical influence of the methyl group, and the steric effect of the thiophene unit all contribute to their structural stability throughout a wide pH range as well as in water. Their proton conductivity was fully examined at various relative humidities (RHs) and temperatures. Creating intricate and rich H-bonded networks between the guest water molecules, coordination solvent molecules, thiophene-S, -COOH, and -OH units within the framework assisted proton transfer. As a result, both MOFs manifest the maximum proton conductivity of 0.67 × 10-2 and 4.85 × 10-3 S·cm-1 under 98% RH/100 °C, making them the top-performing proton-conductive Zr/Hf-MOFs. Finally, by combining structural characteristics and activation energies, potential proton conduction pathways for the two MOFs were identified.

Terahertz Spectroscopy Unambiguously Determines the Orientation of Guest Water Molecules in a Structurally Elusive Metal-Organic Framework.

Porous materials, particularly metal-organic frameworks (MOFs), hold great promise for advanced applications. MIL-53(Al) is an exceptionally well-studied MOF that exhibits a phase transition upon guest capture─in this case, water─resulting in a dramatic change in the pore volume. Despite extensive studies, the structure of the water-loaded narrow-pore phase, MIL-53(Al)-np, remains controversial, particularly with respect to the positions of the adsorbed water molecules. We use terahertz spectroscopy, coupled with powder X-ray diffraction and density functional theory simulations, to unambiguously resolve this controversy. We show that the low-frequency (<100 cm-1) vibrational spectrum depends on weak long-range forces that are extremely sensitive to the orientation of the adsorbed water molecules. This enables definitively determining the correct structure of MIL-53(Al)-np while highlighting the extreme sensitivity of terahertz spectroscopy to bulk structure, suggesting its potential as a robust complement to X-ray diffraction for precise characterization of host-guest complexes.

Proton Conductivity in a Metal-Organic Cube-Based Framework and Derived Hydrogel with Tubular Morphology.

The hydrogels, formed by self-assembly of predesigned, discrete metal-organic cubes (MOCs), have emerged as a new type of functional soft material whose diverse properties are yet to be explored. Here, we explore the proton conductivity of a MOC-based supramolecular porous framework {(Me2NH2)12[Ga8(ImDC)12]·DMF·29H2O} (1) (ImDC = 4,5-imidazole dicarboxylate) and derived hydrogel (MOC-G1). The intrinsic charge-assisted H-bonded (between anionic MOC {[Ga8(ImDC)12]12-} and dimethylammonium cations) framework 1 exhibits an ambient condition proton conductivity value of 2.3 × 10-5 S cm-1 (@40% RH) which increases with increasing temperature (8.2 × 10-4 S cm-1 at 120 °C and 40% RH) and follows the Grotthuss type of mechanism of proton conduction. Self-assembly of the MOCs in the presence of ammonium cations, as molecular binders, resulted in a hydrogel (MOC-G1) that shows directional H-bonded 1D nanotubular morphology. While guest water molecules are immensely important in deciding the proton conductivity of both 1 and MOC-G1, the presence of additional proton carriers, such as DMA and ammonium cations, resulted in at least 1 order increment in the proton conductivity of the latter (1.8 × 10-2 S cm-1) than the former (1.4 × 10-3 S cm-1) under 25 °C and 98% RH condition. The values of proton conductivity of 1 and MOC-G1 are comparable with those of the best proton conduction reports in the literature. This work may pave the way for the development of proton conductors with unique architecture and conductivity requisite for the state-of-the-art technologies by selecting appropriate MOC and molecular binders.

Guest water-induced structural transformation and spin-crossover variation of a two-dimensional Hofmann-type compound.

In this paper, we report a two-dimensional (2D) Hofmann-type spin-crossover coordination polymer [FeII(o-NTrz)2PtII(CN)4]·H2O (o-NTrz = 4-(o-nitrobenzyl)imino-1,2,4-triazole). Due to the remarkable configurational flexibility of triazole-based ligand, the porous structure of this compound can be reversibly regulated by the loss of guest water molecules as a consequence of rotation of o-NTrz. The 180° reorientation of the o-nitrobenzyl moiety not only induces a response of gate-closing/opening of the porous framework but also significantly modulates the spin transition temperature. The present investigation highlights the potential of Hofmann-type SCO compounds with flexible ligands in exploring unusual physical and chemical phenomena.

Remarkable water-mediated proton conductivity of two porous zirconium(IV)/hafnium(IV) metal-organic frameworks bearing porphyrinlcarboxylate ligands

Obtaining crystalline materials with high structural stability as well as super proton conductivity is a challenging task in the field of energy and material chemistry. Therefore, two highly stable metal-organic frameworks (MOFs) with macro-ring structures and carboxylate groups, Zr-TCPP (1) and Hf-TCPP (2) assembled from low-toxicity as well as highly coordination-capable Zr(IV)/Hf(IV) cations and the multifunctional linkage, meso-tetra(4-carboxyphenyl)porphine (TCPP) have attracted our strong interest. Note that TCPP as a large-size rigid ligand with high symmetry and multiple coordination sites contributes to the formation of the two stable MOFs. Moreover, the pores with large sizes in the two MOFs favor the entry of more guest water molecules and thus result in high H2O-assisted proton conductivity. First, their distinguished structural stabilities covering water, thermal and chemical stabilities were verified by various determination approaches. Second, the dependence of the proton conductivity of the two MOFs on temperature and relative humidity (RH) is explored in depth. Impressively, MOFs 1 and 2 demonstrated the optimal proton conductivities of 4.5 × 10−4 and 0.78 × 10−3 S·cm−1 at 100 °C/98 % RH, respectively. Logically, based on the structural information, gas adsorption/desorption features, and activation energy values, their proton conduction mechanism was deduced and highlighted.

Dehydration-actuated single-molecule magnet behavior in a cyanide-bridged [Fe2Co2] cluster featuring zigzag structure

A rare zigzag cyanide-bridged tetranuclear complex, {[(Tp*)FeIII(CN)3]2[CoII(tphz)1/2(SCN)(MeOH)]2}·4H2O (1·4H2O) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, tphz = tetrapyridophenazine) was successfully isolated and characterized structurally and magnetically. Single-crystal structural analysis indicates the Fe3+ and Co2+ ions are linked by cyanides while the inverse two Co2+ are bridged by the pyrazine of tphz. Magnetic studies for 1·4H2O revealed the ferromagnetic coupling (JFeCo = +3.2 cm−1) between the Fe3+ and Co2+ ions and antiferromagnetic interaction (JCoCo = −2.7 cm−1) between the Co2+ ions of the cluster. After the release of crystallized water molecules, however, the magnetic behaviors were drastically tuned indicated by the notable changes in the magnetic couplings for FeIIICoII (JFeCo = +3.9 cm−1) and CoIICoII (JCoCo = −3.5 cm−1). Reduced magnetization reveals the uniaxial magnetic anisotropy of the dehydrated molecule with the zero-field splitting parameter D = −10.3 cm−1. Additionally, field-supported single-molecule magnet behavior was observed with the effective energy barrier Ueff = 38.2 K, making complex 1 the first example of a zigzag Fe2Co2 single-molecule magnet. This work provides not only a rare dynamic switching molecular complex but also highlights the important influence of guest water molecules in magnetic behaviors.

"On-off" Switching of Functional Guest Molecules via Exchange of Natural Product Solubilizing Agents.

For the development of delivery systems, the solubilization of hydrophobic guest molecules in water is an important yet challenging task. This can be achieved by preparing stable aqueous solutions with a high concentration of guest molecules using a natural product as a solubilizing agent and a mechanochemical high-speed vibration milling apparatus as a solubilizing method. Various solubilizing agent-guest molecule complexes can be obtained via the exchange between solubilizing agents, which enables the "on-off" switching of the properties of functional guest molecules, such as fluorescence intensity, and photodynamic activity. In the exchange method, guest molecules can transfer into cell membranes such as lysosomes and exosomes. Therefore, the exchange method of the solubilizing agents not only creates novel solubilizing agent-guest molecule complexes but also is applied to drug delivery systems.

Dynamics of Guest Water Molecules in Pillared Mordenite Studied by 1H NMR Relaxation

Dynamics of Guest Water Molecules in Pillared Mordenite Studied by 1H NMR Relaxation

Water Content-Controlled Formation and Transformation of Concomitant Pseudopolymorph Coordination Polymers.

Two concomitant pseudopolymorph coordination polymers {[Cd2L2(OAc)4]·2DMSO} n (1) and {[CdL(OAc)2]·2.75H2O} n (2) were synthesized by self-assembly of 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (L) and cadmium acetate in DMSO. Single-crystal X-ray diffraction confirmed that 1D ladder structural motifs exist for pseudopolymorphs 1 and 2 which contain DMSO and water guest molecules, respectively. Our study illustrated the active role of solvent water content in obtaining compound 2. We find that the presence of water as an impurity in the DMSO solvent creates the possibility of formation of concomitant pseudopolymorph coordination polymers which is a unique event. Furthermore, our analyses showed the effect of environmental humidity on the transformation of unstable compound 1. 1D ladder pseudopolymorphic compound 1 could be transformed to guest-free 1D linear compound [CdL(OAc)2(H2O)] n (3') (the powder form of single crystals of 3) through a scarce case of water absorption from air. Also, the crystalline material of coordination polymer 3 was transformed to coordination polymer 2 through the dissolution-recrystallization structural transformation process in DMF or DMSO. Our study clarified that the amount of water in the reaction container can control the formation of one of the compounds 2 or 3. In the presence of a significant amount of water, compound 3 (coordinated water) will be produced, whereas if a small amount of water is present, compound 2 (uncoordinated water) is prepared as an exclusive product.

Simultaneous presence of mono- and bi-cationic bipyridyls within a metal-organic supramolecular host: crystal structure, spectral and Hirshfeld surface analysis

A new supramolecular metal-organic complex, {[Fe(CN)6]3-(H2bipy)2+(Hbipy)+·3H2O} (where bipy ipywhere biolecular metal-organic complex, {[Fe(CN)ccepted: 30 October 2022rized by single crystal X-ray crystallographic study (SC-XRD), powder X-ray diffraction (PXRD) and other spectroscopic analysis. Structural analysis reveals that the complex crystallizes in monoclinic space group P21/c and is a 3 D supramolecular complex. Cyanide-water hydrogen bonding interactions between {Fe(CN)6}3- and guest water molecules form a wave-like 2 D supramolecular layer structure parallel to the crystallographic ac-plane with the formation of rectangular cavities of dimension 10.6 × 9.8 Å. These 2 D layers are further connected by O3W-H2W3···O1W hydrogen bonding interactions to form a 3 D supramolecular structure with the creation of 1 D supramolecular channels along the crystallographic a-axis. The bi-cationic (H2bipy)2+ form two different N+-H···water hydrogen bonding interactions (N8+-H8···O2W and N7+-H7···O3W) with two different water molecules and reside within the rectangular cavities. Monocationic (Hbipy)+ moieties form a 1 D supramolecular chain structure through repetitive N···H-N+ hydrogen bonding interactions and are present within the 1 D supramolecular channels. In addition, these two different types of bipy moieties are interconnected to each other through π···π interactions. Hirshfeld surface analysis and the corresponding 2 D fingerprint plots clearly identify the presence of different types of supramolecular interactions between the two differently charged bipyridyl moieties and the host framework, and N-H/H-N contacts, corresponding to the hydrogen bonding interactions, have a significant contribution to the total surface. Both the absorption and emission spectra of the complex have also been studied.

Multifunctional Dinuclear Dy-Based Coordination Complex Showing Visible Photoluminescence, Single-Molecule Magnet Behavior, and Proton Conduction.

A new Dy-based complex, [Dy2(phen)4(PAA)4](ClO4)2 (1), was obtained by using 4-hydroxyphenyl acetate (HPAA) as a ligand and 1,10-phenanthroline as an auxiliary ligand. Complex 1 shows a dinuclear structure and a 2D supramolecular layer constructed by π-π stacking interactions. The complex displays a characteristic Dy(III) emission. Moreover, magnetic susceptibility measurements revealed that 1 exhibits a single-molecule magnet (SMM) behavior. In addition, it also shows a proton conductivity of 1.08 × 10-5 S cm-1 under 353 K and 100% relative humidity conditions, which is mainly assigned to H-bonded networks formed by the undeprotonated and uncoordinated phenolic groups of HPAA ligands and guest water molecules. Remarkably, 1 is the first example of a dinuclear complex showing photoluminescence, SMM behavior, and proton conduction.

Synthesis, structure, magnetism and proton conductivity of a cyanide-bridged NiIICoIII framework

A two-dimensional cyanide-bridged NiIICoIII coordination polymer, {[Ni(cyclam)]3[Co(CN)6]2·16H2O}n (1, cyclam = 1,4,8,11-tetraazacyclotetradecane), was constructed from hexacyanidocobaltate(III) anions, cyclam ligands and Ni2+ ions. Single-crystal X-ray structural analysis revealed that the [Ni(cyclam)]2+ species were bridged by [Co(CN)6]3- building units, leading to a 2D honeycomb topological structure. Interestingly, 1D microporous channels within the framework were formed with guest water molecules located. An N2 adsorption experiment supports the porosity of the compound. Magnetic investigations reveal the easy-axis magnetic anisotropy of the distorted octahedral Ni2+ ions in 1. However, no single-ion magnet behavior was observed. Due to the presence of significant hydrogen-bonded water channels, temperature- and humidity-dependent proton conduction behavior was observed in 1. The electrical conductivity reaches 2.9 × 10−4 S cm−1 at 44 °C and 100 % relative humidity. The forgoing results provide a relatively rare porous cyanide-bridged magnetic framework with proton conduction behavior. Additionally, this compound also highlights that cyanide-bridged frameworks are a potential great platform for the development of high-performance proton-conducting materials.

Aqueous Solutions of a Porous Host-Poly-L-lysine Complex with Information on Solids as Different Fluorescence by Guests.

N,N'-Dipyrid-3-yl-1,4,5,8-naphthalenediimide linked to two tris(pentafluorophenyl)borane (1) exhibits strong fluorescence emission in the solid state by the formation of a charge-transfer complex containing small aromatic guest molecules. Hydrophobic 1 was dissolved in water by mixing with poly-L-lysine (PLL) as a solubilizing agent. The 1-PLL complex could include small aromatic guest molecules in water, significantly increasing the fluorescence. The fluorescence maxima of 1 in aqueous solution and solid state were different depending on the guest molecule. Therefore, compound 1 was prepared as aqueous solution with information of fluorescence on solids by complexation with PLL.

Assessing the effect of a liquid water layer on the adsorption of hydrate anti-agglomerants using molecular simulations.

We have performed molecular dynamics simulations to study the adsorption of ten hydrate anti-agglomerants onto a mixed methane-propane sII hydrate surface covered by layers of liquid water of various thickness. As a general trend, we found that the more liquid water that is present on the hydrate surface, the less favorable the adsorption becomes even though there are considerable differences between the individual molecules, indicating that the presence and thickness of this liquid water layer are crucial parameters for anti-agglomerant adsorption studies. Additionally, we found that there exists an optimal thickness of the liquid water layer favoring hydrate growth due to the presence of both liquid water and hydrate-forming guest molecules. For all other cases of liquid water layer thickness, hydrate growth is slower due to the limited availability of hydrate-forming guests close to the hydrate formation front. Finally, we investigated the connection between the thickness of the liquid water layer and the degree of subcooling and found a very good agreement between our molecular dynamics simulations and theoretical predictions.

Grand canonical Monte Carlo simulation study on the effect of crystallinity and structural disorder on water sorption in geopolymers

A comparative study on water sorption by different components of the binder phase formed in geopolymerization was performed using the Grand Canonical Monte Carlo (GCMC) simulation method. Water sorption isotherms for crystalline, defective and amorphous model structures with Na+ exchangeable cations at temperatures of 298 K were obtained. The isosteric heats of adsorption of water in each structure were calculated. In addition, the distribution pattern of H2O molecules in the researched frameworks and the preferred sorption sites were also investigated. The GCMC results showed that the structural changes in the host framework significantly affect water sorption properties, leading to changes in water sorption capacity and nature of interactions with guest water molecules. The GCMC simulation results have provided useful information on the behavior of water confined in geopolymeric binder phase, which is difficult to observe experimentally, contributing to a better understanding of the mechanism of water adsorption in geopolymer-zeolite hybrid materials.

Excited state intramolecular proton transfer of 2-Aryl,3-Hydroxybenzo[g]quinolones in surfactant supramolecular assemblies

The 2-Aryl 3-Hydroxybenzo[g]quinolones (2Ar3HBQ) are a relatively new family of ESIPT fluorophores distinguished by their visible absorption spectra, which makes them potential ratiometric probes for biological media. In the present work, we found that 2Ar3HBQ residing as guest fluorophores inside biomimetic host systems like surfactant supramolecular assemblies like micelles and reverse micelles, exhibit dual emission attributable to ESIPT. Ratiometric analysis of the two emission peaks corresponding to the normal (N*) and tautomer (T*) species indicated that the ESIPT rates depend strongly on the nature of the host supramolecular assembly. For example, the ESIPT rate in reverse micelles containing no water was found to be similar to that in weak H-bonding and low polar solvents like acetonitrile. However, addition of water leads to the formation of water nano-droplets inside the reverse micelles, with concomitant sharp decrease of ESIPT rates. In each of the cases studied, the major factor determining the ESIPT rate appears to be the competition between intramolecular H-bonding within the guest fluorophore on the one hand, and intermolecular H-bonding involving the guest fluorophore and water molecules in its local environment, on the other hand.

Mechanochemical Synthesis of Phosphonate-Based Proton Conducting Metal-Organic Frameworks.

Water-stable metal-organic frameworks (MOFs) with proton-conducting behavior have attracted great attention as promising materials for proton-exchange membrane fuel cells. Herein, we report the mechanochemical gram-scale synthesis of three new mixed-ligand phosphonate-based MOFs, {Co(H2PhDPA)(4,4'-bipy)(H2O)·2H2O}n (BAM-1), {Fe(H2PhDPA)(4,4'-bipy) (H2O)·2H2O}n (BAM-2), and {Cu(H2PhDPA)(dpe)2(H2O)2·2H2O}n (BAM-3) [where H2PhDPA = phenylene diphosphonate, 4,4'-bipy = 4,4'-bipyridine, and dpe = 1,2-di(4-pyridyl)ethylene]. Single-crystal X-ray diffraction measurements revealed that BAM-1 and BAM-2 are isostructural and possess a three-dimensional (3D) network structure comprising one-dimensional (1D) channels filled with guest water molecules. Instead, BAM-3 displays a 1D network structure extended into a 3D supramolecular structure through hydrogen-bonding and π-π interactions. In all three structures, guest water molecules are interconnected with the uncoordinated acidic hydroxyl groups of the phosphonate moieties and coordinated water molecules by means of extended hydrogen-bonding interactions. BAM-1 and BAM-2 showed a gradual increase in proton conductivity with increasing temperature and reached 4.9 × 10-5 and 4.4 × 10-5 S cm-1 at 90 °C and 98% relative humidity (RH). The highest proton conductivity recorded for BAM-3 was 1.4 × 10-5 S cm-1 at 50 °C and 98% RH. Upon further heating, BAM-3 undergoes dehydration followed by a phase transition to another crystalline form which largely affects its performance. All compounds exhibited a proton hopping (Grotthuss model) mechanism, as suggested by their low activation energy.

Terephthalato and succinato bridged Mn(II) and Zn(II) coordination polymers involving structure-guiding H-bonded tetrameric assemblies: Antiproliferative evaluation and theoretical studies

Two new coordination polymers of Mn(II) and Zn(II) viz. {[Mn(µ-tp)(3-Ampy)2(H2O)2]2(3-Ampy)·2(H2O)}n (1) and {[Zn(µ-succ)(3-Ampy)2]·2H2O}n (2) (tp = terephthalate, 3-Ampy = 3-aminopyridine, succ = succinate) have been synthesized and characterized using elemental and spectroscopic analyses, TGA and single crystal X-ray diffraction techniques. Uncoordinated 3-AMpy and water molecules of the polymeric co-crystal hydrate 1 are involved in the formation of cyclic tetrameric {(3-Ampy)2(H2O)2} assembly which provide rigidity to the structure. The presence of guest 3-Ampy and water molecules in the voids of supramolecular Mn(II) host tetramer in 1 provides additional reinforcement to the crystal structure. The lattice water molecules of 2 are aggregated via non-covalent interactions to form discrete cyclic {(H2O)4} tetrameric clusters in the crystal structure. Density functional theory (DFT), non-covalent index (NCI) plot and quantum theory of atoms in molecules (QTAIM) computational analyses reveal that the supramolecular H-bonded cyclic tetrameric structure-guiding synthon in 1 and the (H2O)4 cyclic water clusters in 2 have significant energetic contribution towards the stability of the polymers. In vitro anticancer activities of the compounds considering trypan blue and apoptosis assays revealed that the compounds induce concentration dependent cytotoxicity in Dalton’s lymphoma (DL) cancer cells with nominal effects in normal peripheral blood mononuclear cells (PBMC) cells. Comparative analysis reveals that compound 2 induced higher apoptosis in DL cells; which is well consistent with the trypan blue exclusion assay. Molecular docking and pharmacophore modelling studies of the compounds reveal that the compound 2 significantly interacts with the antiapoptotic proteins to establish structure activity relationship (SAR).

Ultrastable Polyoxometalate-Encapsulated Supramolecular Metal-Organic Nanotubes for Single-Crystal Proton Conduction.

Two unique polyoxometalate (POM)-encapsulated tubular materials with the formula K(H2O)6[M6(btp)6(H2O)22](P2W18O62)3(Hbtp)5(btp)3·52H2O [M = Mn (1) and Co (2); btp = 2,6-bis(1,2,4-triazol-1-yl)pyridine] were designed and synthesized based on the Dawson POM and V-type btp ligand, as confirmed by IR, X-ray diffraction (XRD), and element analysis. Single-crystal XRD analyses of compound 1 show that two kinds of remarkable metal-organic supramolecular nanotubes, including trigonal and hexagonal nanotubes, are constructed along the c-axis direction via π···π-packing interactions between {Mn3(btp)3} rings and the btp ligands, of which [α-P2W18O62]6- anions are confined in channels, making the entire structure extraordinarily stable. Meanwhile, the coordinated [α-P2W18O62]6- anion within the hexagonal channel makes the channel highly hydrophilic and attracts a number of guest water molecules to fill in the free space, conducive to proton transport. Therefore, the single-crystal sample of 1 exhibits a high proton conductivity of 6.39 × 10-3 S cm-1 along one-dimensional channels, 30 times higher than that of a pellet sample at 358 K and 98% relative humidity.