Creating Order in Ultrastable Phosphonate Metal-Organic Frameworks via Isolable Hydrogen-Bonded Intermediates

Lanthanide-Diphosphonate Frameworks Containing Dianthracene: Isolation of Metastable Intermediates and Lanthanide-Dependent UV and X-ray Light-Responsive Properties

Capturing kinetic intermediates is crucial for understanding the formation mechanism of metal-organic frameworks (MOFs), but it is particularly challenging for metal phosphonate frameworks. Here, we employ a modulator-based strategy to control the crystallization of MOFs. By adding the chelating ligand shikimic acid, we obtained three-dimensional (3D) lanthanide-diphosphonate frameworks, [Ln2(amp2H2)3(H2O)x]·4H2O (Ln-3D, x = 6, Ln = Sm, Eu, Gd, Tb, Ho; x = 4, Ln = Er), where amp2H4 is prephotodimerized 9-anthracenemethylphosphonic acid. By reducing the reaction temperature from 90 to 70 °C, we successfully isolated the metastable reaction intermediates, [Ln2(amp2H2)2(H2O)12](amp2H2)·10H2O (Ln-1D), with one-dimensional (1D) chain structures. The dianthracene moiety in the linker amp2H22- confers additional dynamic features to the material due to its potential dissociation into anthracene pairs and affects the photophysical and photochemical properties, which vary with the lanthanide ion. Moreover, the labile coordinated water molecule in Ho-3D is releasable under X-ray irradiation at 300 K, whereas this X-ray responsiveness is not obvious for other Ln-3D (Ln = Sm, Eu, Gd, and Tb). This study not only provides a new case for metal phosphonate intermediate trapping but also demonstrates rare metal-organic materials with ultraviolet (UV) light and X-ray responsiveness.

Dual Free Radical Synergism for Enhancing Proton Conductivity in Photochromism iHOFs

Stimuli-responsive smart materials, as an emerging material, can fulfill reversible transformation of chemical/physical properties under external stimuli such as mechanical stress, light, and electricity, which has the highlights of rapid response, designable structure, and function. Two ionic hydrogen-bonded organic frameworks (iHOFs 36-37) were synthesized by self-assembly of bis(benzene-o/p-sulfonic acid)-naphthalenediimide (o/p-H2BSNDI) and two basic ligands. The naphthalenediimide (NDI) was introduced into the material to equip iHOFs 36-37 with radical-driven photochromic behavior. The proton conductivity of iHOF-37 demonstrated a maximum of 6.50 × 10-4 S·cm-1 at 98% RH and 100 °C, and it increased to 9.10 × 10-3 S·cm-1 due to dual free radical synergism following UV irradiation (NDI and viologen), which represents a significant 14-fold enhancement. Furthermore, the incorporation of iHOF-37 into the chitosan (CS) matrix forms photochromic composite membranes. The proton conductivity of the 5%-iHOF-37/CS composite membrane reached up to 5.70 × 10-2 S·cm-1 at 98% RH and 90 °C, and reached 8.08 × 10-2 S·cm-1 after UV irradiation. This work reveals the dual radicals generated by NDI and viologen derivatives, whose synergistic action plays a significant role in enhancing the proton conductivity in iHOFs and composite membranes, rendering the rational design of stimuli-responsive smart materials feasible.

Hydrolytically Stable Phosphonate-Based Metal-Organic Frameworks for Harvesting Water from Low Humidity Air

Harvesting water from air offers a promising solution to the global water crisis. However, existing sorbents often struggle in arid climates due to limitations such as low sorption capacities, hydrolytic instability, slow mass transport, high desorption enthalpy, and costly operation. Phosphonate-based metal-organic frameworks (MOFs), known for their exceptional water stability, have not been extensively explored for water harvesting. This study systematically investigates the performance of STA-12 (M═Co, Ni, Mg) and STA-16 (M═Co, Ni), a series of stable phosphonate-based MOFs, as water sorbents. STA-12 MOFs demonstrate remarkable adsorption at ultra-low humidity (<10%), while STA-16(Co) exhibits a high water uptake capacity of 0.54 g g-1 at 10-50% relative humidity (RH) and 0.72 g g-1 at 34% RH. Molecular simulations and solid-state NMR identified liquid-like water, critical for harvesting applications, as the key contributor to the superior sorption performance of STA-16(Co). Scalable aqueous synthesis methods are developed, producing tens of grams of MOFs per batch without high-pressure equipment. A prototype device incorporating STA-12(Ni) demonstrated the feasibility of these materials for real-world water harvesting, showcasing their potential to address water scarcity in arid regions.

From Guest-Induced Crystallization to Molecular Imprinting: Calculation-Guided Discovery of Hydrogen-Bonded Tetrazole Frameworks for p-Xylene Separation

Exploring host-guest interactions to regulate hydrogen-bonding assembly offers a promising approach for developing advanced porous crystal materials (PCMs). However, screening compatible guests with appropriate geometries and host-guest interactions that could inhibit the dense packing of building blocks remains a primary challenge. This study presents a novel guest-induced crystallization (GIC) strategy, guided by thermodynamic calculations, to develop porous hydrogen-bonded organic frameworks (HOFs) using structurally challenging tetrazole building units. Thermodynamic principles are established from the crystal structure data and the density functional theory calculation of the formation energy (ΔE). This provides criteria to identify available guests in GIC, enabling the successful discovery of a hidden HOF that is kinetically challenging to crystallize. Furthermore, the potential application for p-xylene (PX) separation is predicted by analyzing the ΔE of guest-induced HOFs. A high PX selectivity (PX/m-xylene=6.1, PX/o-xylene=7.2, and PX/ethylbenzene=4.1) is achieved through selective inclusion of PX from C8 aromatic isomers within the guest-induced HOFs. Significantly, the guest-free HOF (HOF-PX-a), bearing PX-templated cavities derived from molecular imprinting, shows a record-high PX/ethylbenzene selectivity (21.7) in liquid adsorption. This work elucidates the underpinning self-assembly rules of GIC for HOF construction, providing exciting new opportunities for the predictable assembly of PCMs for molecular recognition and target-specific separations.

Vapour phase deposition of phosphonate-containing alumina thin films using dimethyl vinylphosphonate as precursor

Phosphorous-containing materials are used in a wide array of fields, from energy conversion and storage to heterogeneous catalysis and biomaterials. Among these materials, organic-inorganic metal phosphonate solids and thin films present an interesting option, due to their remarkable thermal and chemical stability. Yet, the synthesis of phosphonate hybrids by vapour phase thin film deposition techniques remains largely unexplored. In this work, we present successful deposition of phosphonate-containing films using dimethyl vinylphosphonate (DMVP) as a phosphonate precursor. Two processes have been studied, being a three-step process comprising alternating exposure to trimethylaluminum (TMA), water (H2O) and DMVP (ABC process), and a four-step process with an extra O3 step following the DMVP pulse (ABCD process). The O3 treatment is employed for in situ functionalisation of the adsorbed phosphonate precursor, transforming the vinyl group into a carboxylic acid end group. For both processes, good precursor saturation was found, with the ABCD process exhibiting a more stable growth per cycle (0.54-0.38 Å per cycle) in the investigated temperature range (100-250 °C). Phosphonate features were visible in FTIR spectra for both films, with the ABCD films also exhibiting a carboxylate signal. XPS showed a higher P incorporation in the ABCD films deposited at 250 °C, although still moderate (P/Al = 0.27), consistent with an alumina structure with phosphonate inclusions. The film stability upon immersion in water was tested, showing a slow oxidation over the course of a week. Finally, annealing experiments in air demonstrated stable films up to 400 °C.

Metal-Dilution Effect on Spin Transition Behavior of Solvated/Desolvated Hydrogen-Bonded Cobalt(II)-Organic Frameworks

Terpyridine-based cobalt(II) complex, [CoII(HL)2] (1; H2L = 2,2':6',2″-terpyridine-5,5″-diyl dicarboxylic acid), forms a hydrogen-bonded diamond framework with solvent absorption and desorption capabilities. The desolvated form (1·desolv) exhibits spin transition (ST) behavior accompanied by thermal hysteresis. To investigate the effect of metal-dilution, an Fe2+ center, which has a low-spin state (S = 0) and coordinates to two terpyridine moieties, was introduced. The resulting complexes, [CoII x FeII 1-x (HL)2], where x = 0.88 (2), 0.55 (3), and 0 (4), demonstrated a significant influence of metal-dilution on the desolvated forms, but not on the solvated forms. Namely, the spin state is more strongly affected by the presence of solvent than by metal-dilution. However, in the absence of solvent, the Fe2+ ratio significantly impacts the ST behavior.

Metal Pyrazolyl-Diphosphonate Pillared Materials as Heterogeneous Catalysts in the Mukaiyama-Type Aerobic Olefin Epoxidation

The structural variability and chemical stability of metal phosphonates under harsh conditions are attractive attributes that have drawn considerable attention in recent years. As the need for more sustainable solutions rises, the demand for novel and tolerant materials also increases. Thus, herein we report, for the first time, the synthesis of a novel diphosphonic organic linker named pyrazole diphenyl phosphonate (PZDP), envisioning the fabrication of durable metal phosphonates. In view of this, a series of M-PZDP materials (M=Ca, Sr, Ba, Zn and Co) were synthesized employing either solvothermal or hydrothermal methods. The crystal structures of the Ca, Sr, Zn, and Co derivatives were determined revealing a 2D-pillared architecture. Zn-PZDP is an anionic framework with dimethylammonium cations. The Co-PZDP compound was tested as a heterogeneous catalyst in olefin epoxidation employing molecular oxygen.

Controllable Synthesis of Manganese Organic Phosphate with Different Morphologies and Their Derivatives for Supercapacitors

Morphological control of metal-organic frameworks (MOFs) at the micro/nanoscopic scale is critical for optimizing the electrochemical properties of them and their derivatives. In this study, manganese organic phosphate (Mn-MOP) with three distinct two-dimensional (2D) morphologies was synthesized by varying the molar ratio of Mn2+ to phenyl phosphonic acid, and one of the morphologies is a unique palm leaf shape. In addition, a series of 2D Mn-MOP derivatives were obtained by calcination in air at different temperatures. Electrochemical studies showed that 2D Mn-MOP derivative calcined at 550 °C and exhibited a superior specific capacitance of 230.9 F g-1 at 0.5 A g-1 in 3 M KOH electrolyte. The aqueous asymmetric supercapacitor and the constructed flexible solid-state device demonstrated excellent rate performance. This performance reveals the promising application of 2D Mn-MOP materials for energy storage.

Metal-Phosphonate-Organic Network as Ion Enrichment Layer for Sustainable Zinc Metal Electrode with High Rate Capability

Zinc (Zn) metal batteries could be the technology of choice for sustainable battery chemistries owing to its better safety and cost advantage. However, their cycle life and Coulombic efficiency (CE) are strongly limited by the dendritic growth and side reactions of Zn anodes. Herein, we proposed an in situ construction of a metal-phosphonate-organic network (MPON) with three-dimensional interconnected networks on Zn metal, which can act as an ion enrichment layer for Zn anodes in Zn-metal batteries. This MPON with abundant porous structure and phosphate sites possesses ion enriching properties and high Zn2+ transference number (0.83), which is beneficial for enhancing Zn2+ migration and self-concentrating kinetics. Meanwhile, MPON offers hydrophobicity to effectively inhibit the water-induced Zn anode corrosion. As a result, the Zn electrode exhibits superior Zn/Zn2+ reversibility of over 4 months at 3 mA cm-2 and a high CE of 99.6 %. Moreover, the Zn/NaV3O8 ⋅ 1.5H2O and Zn/MnO2 full cells using ultrathin Zn anodes (10 μm) exhibit high-capacity retention of 81 % and 78 % after 1400 and 1000 cycles, respectively. This work provides a unique promise to design high-performance anode for practical Zn-metal-based batteries.

Enhanced Charge Transfer Process and Photocatalytic Activity over a Phosphonate-based MOF via Amorphization Strategy

Recently, amorphous materials have gained great attention as an emerging kind of functional material, and their characteristics such as isotropy, absence of grain boundaries, and abundant defects are very likely to outrun the disadvantages of crystalline counterparts, such as low conductivity, and ultimately lead to improved charge transfer efficiency. Herein, we investigated the effect of amorphization on the charge transfer process and photocatalytic performance with a phosphonate-based metal-organic framework (FePPA) as the research object. Comprehensive experimental results suggest that compared to crystalline FePPA, amorphous FePPA has more distorted metal nodes, which affects the electron distribution and consequently improves the photogenerated charge separation efficiency. Meanwhile, the distorted metal nodes in amorphous FePPA also greatly promote the adsorption and activation of O2. Hence, amorphous FePPA exhibits a better performance of photocatalytic C(sp3)-H bond activation for selective oxidation of toluene to benzaldehyde. This work illustrates the advantages of amorphous MOFs in the charge transfer process, which is conducive to the further development of high performance MOFs-based photocatalysts.