Functional metal-organic liquids.

For decades, the study of coordination polymers (CPs) and metal-organic frameworks (MOFs) has been limited primarily to their behavior as crystalline solids. In recent years, there has been increasing evidence that they can undergo reversible crystal-to-liquid transitions. However, their "liquid" states have primarily been considered intermediate states, and their diverse properties and applications of the liquid itself have been overlooked. As we learn from organic polymers, ceramics, and metals, understanding the structures and properties of liquid states is essential for exploring new properties and functions that are not achievable in their crystalline state. This review presents state-of-the-art research on the liquid states of CPs and MOFs while discussing the fundamental concepts involved in controlling them. We consider the different types of crystal-to-liquid transitions found in CPs and MOFs while extending the interpretation toward other functional metal-organic liquids, such as metal-containing ionic liquids and porous liquids, and try to suggest the unique features of CP/MOF liquids. We highlight their potential applications and present an outlook for future opportunities.

Modular Design of Highly Stable Semiconducting Porous Coordination Polymer for Efficient Electrosynthesis of Ammonia.

Developing highly stable porous coordination polymers (PCPs) with integrated electrical conductivity is crucial for advancing our understanding of electrocatalytic mechanisms and the structure-activity relationship of electrocatalysts. However, achieving this goal remains a formidable challenge because of the electrochemical instability observed in most PCPs. Herein, we develop a "modular design" strategy to construct electrochemically stable semiconducting PCP, namely, Fe-pyNDI, which incorporates a chain-type Fe-pyrazole metal cluster and π-stacking column with effective synergistic effects. The three-dimensional electron diffraction (3D ED) technique resolves the precise structure. Both theoretical and experimental investigation confirms that the π-stacking column in Fe-pyNDI can provide an efficient electron transport path and enhance the structural stability of the material. As a result, Fe-pyNDI can serve as an efficient model electrocatalyst for nitrate reduction reaction (NO3RR) to ammonia with a superior ammonia yield of 339.2 µmol h-1 cm-2 (14677 µg h-1 mgcat. -1) and a faradaic efficiency of 87 % at neutral electrolyte, which is comparable to state-of-the-art electrocatalysts. The in-situ X-ray absorption spectroscopy (XAS) reveals that during the reaction, the structure of Fe-pyNDI can be kept, while part of the Fe3+ in Fe-pyNDI was reduced in situ to Fe2+, which serves as the potential active species for NO3RR.

Gold(I)-Thiolate Coordination Polymers as Multifunctional Materials: The Case of Au(I)-p-Fluorothiophenolate.

Gold-sulfur interaction has vital importance in nanotechnologies and material chemistry to design functional nanoparticles, self-assembled monolayers, or molecular complexes. In this paper, a mixture of only two basic precursors, such as the chloroauric acid (HAu(III)Cl4) and a thiol molecule (p-fluorothiophenol (p-HSPhF)), are used for the synthesis of gold(I)-thiolate coordination polymers. Under different conditions of synthesis and external stimuli, five different functional materials with different states of [Au(I)(p-SPhF)]n can be afforded. These gold-thiolate compounds are (i) red emissive, flexible, and crystalline fibers; (ii) composite materials made of these red emissive fibers and gold nanoparticles; (iii) amorphous phase; (iv) transparent glass; and (v) amorphous-to-crystalline phase-change material associated with an ON/OFF switch of luminescence. The different functionalities of these materials highlight the great versatility of the gold(I) thiolate coordination polymers with easy synthesis and diverse shaping that may have great potential as sustainable phosphors, smart textiles, sensors, and phase change memories.

Close Stacking of Antiaromatic Ni(II) Norcorrole Originating from a Four-Electron Multicentered Bonding Interaction.

A π-conjugated molecule with one electronic spin often forms a π-stacked dimer through molecular orbital interactions between two unpaired electrons. The bonding is recognized as a multicentered two-electron interaction between the two π-conjugated molecules. Here, we disclose a multicentered bonding interaction between two antiaromatic molecules involving four electrons. We have synthesized an antiaromatic porphyrin analogue, Ni(II) bis(pentafluorophenyl)norcorrole. Its dimer adopts a face-to-face stacked structure with an extremely short stacking distance of 2.97 Å. The close stacking originates from a multicenter four-electron bonding interaction between the two molecules. The bonding electrons were experimentally observed via synchrotron X-ray diffraction analysis and corroborated by theoretical calculations. The intermolecular interaction of the molecular orbitals imparts the stacked dimer with aromatic character that is distinctly different from that of its monomer.

Creating glassy states of dicarboxylate-bridged coordination polymers.

We report the direct formation of dicarboxylate-based coordination polymer glasses through thermal dehydration. The rearrangement of the coordination networks caused by dehydration was monitored by in situ powder X-ray diffraction, infrared spectroscopy, and synchrotron X-ray characterizations. The microporosity and mechanical properties of these glasses were investigated.

Elucidating the Mechanism of Fe Incorporation in In Situ Synthesized Co-Fe Oxygen-Evolving Nanocatalysts.

Ni- and Co-based catalysts with added Fe demonstrate promising activity in the oxygen evolution reaction (OER) during alkaline water electrolysis, with the presence of Fe in a certain quantity being crucial for their enhanced performance. The mode of incorporation, local placement, and structure of Fe ions in the host catalyst, as well as their direct/indirect contribution to enhancing the OER activity, remain under active investigation. Herein, the mechanism of Fe incorporation into a Co-based host was investigated using an in situ synthesized Co-Fe catalyst in an alkaline electrolyte containing Co2+ and Fe3+. Fe was found to be uniformly incorporated, which occurs solely after the anodic deposition of the Co host structure and results in exceptional OER activity with an overpotential of 319 mV at 10 mA cm-2 and a Tafel slope of 28.3 mV dec-1. Studies on the lattice structure, chemical oxidation states, and mass changes indicated that Fe is incorporated into the Co host structure by replacing the Co3+ sites with Fe3+ from the electrolyte. Operando Raman measurements revealed that the presence of doped Fe in the Co host structure reduces the transition potential of the in situ Co-Fe catalyst to the OER-active phase CoO2. The findings of our facile synthesis of highly active and stable Co-Fe particle catalysts provide a comprehensive understanding of the role of Fe in Co-based electrocatalysts, covering aspects that include the incorporation mode, local structure, placement, and mechanistic role in enhancing the OER activity.

Synthesizing Interpenetrated Triazine-based Covalent Organic Frameworks from CO2.

Crystalline triazine-based covalent organic frameworks (COFs) are aromatic nitrogen-rich porous materials. COFs typically show high thermal/chemical stability, and are promising for energy applications, but often require harsh synthesis conditions and suffer from low crystallinity. In this work, we propose an environmentally friendly route for the synthesis of crystalline COFs from CO2 molecules as a precursor. The mass ratio of CO2 conversion into COFs formula unit reaches 46.3 %. The synthesis consists of two steps; preparation of 1,4-piperazinedicarboxaldehyde from CO2 and piperazine, and condensation of the dicarboxaldehyde and melamine to construct the framework. The CO2 -derived COF has a 3-fold interpenetrated structure of 2D layers determined by powder X-ray diffraction, high-resolution transmission electron microscopy, and select-area electron diffraction. The structure shows a high Brunauer-Emmett-Teller surface area of 945 m2 g-1 and high stability against strong acid (6 M HCl), base (6 M NaOH), and boiling water over 24 hours. Post-modification of the framework with oxone has been demonstrated to modulate hydrophilicity, and it exhibits proton conductivity of 2.5×10-2  S cm-1 at 85 °C, 95 % of relative humidity.

Heterogenous CO2 Reduction Photocatalysis of Transparent Coordination Polymer Glass Membranes Containing Metalloporphyrins.

Transparent and grain boundary-free substrates are essential to immobilize molecular photocatalysts for efficient photoirradiation reactions without unexpected light scattering and absorption by the substrates. Herein, membranes of coordination polymer glass immobilizing metalloporphyrins were examined as a heterogeneous photocatalyst for carbon dioxide (CO2) reduction under visible-light irradiation. [Zn(HPO4)(H2PO4)2](ImH2)2 (Im = imidazolate) liquid containing iron(III) 5,10,15,20-tetraphenyl-21H,23H-porphine chloride (Fe(TPP)Cl, 0.1-0.5 w/w%) was cast on a borosilicate glass substrate, followed by cooling to room temperature, resulting in transparent and grain boundary-free membranes with the thicknesses of 3, 5, and 9 µm. The photocatalytic activity of the membranes was in proportion to the membrane thickness, indicating that Fe(TPP)Cl in the subsurface of membranes effectively absorbed light and contributed to the reactions. The membrane photocatalysts were intact during the photocatalytic reaction and showed no recrystallization or leaching of Fe(TPP)Cl.

Pore-Networked Gels: Permanently Porous Ionic Liquid Gels with Linked Metal-Organic Polyhedra Networks.

Porous liquids (PLs) are attractive materials because of their capability to combine the intrinsic porosity of microporous solids and the processability of liquids. Most of the studies focus on the synthesis of PLs with not only high porosity but also low viscosity by considering their transportation in industrial plants. However, a gap exists between PLs and solid adsorbents for some practical cases, where the liquid characteristics and mechanical stability without leakage are simultaneously required. Here, we fill in this gap by demonstrating a new concept of pore-networked gels, in which the solvent phase is trapped by molecular networks with accessible porosity. To achieve this, we fabricate a linked metal-organic polyhedra (MOPs) gel, followed by exchanging the solvent phase with a bulky liquid such as ionic liquids (ILs); the dimethylformamide solvent trapped inside the as-synthesized gel is replaced by the target IL, 1-butyl-3-methylimidazolium tetrafluoroborate, which in turn cannot enter MOP pores due to their larger molecular size. The remaining volatile solvents in the MOP cavities can then be removed by thermal activation, endowing the obtained IL gel (Gel_IL) with accessible microporosity. The CO2 capacities of the gels are greatly enhanced compared to the neat IL. The exchange with the IL also exerts a positive influence on the final gel performances such as mechanical properties and low volatility. Besides ILs, various functional liquids are shown to be amenable to this strategy to fabricate pore-networked gels with accessible porosity, demonstrating their potential use in the field of gas adsorption or separation.

Hyperpolarization of Biomolecules in Eutectic Crystals at Room Temperature Using Photoexcited Electrons.

The hyperpolarization of biomolecules at room temperature could facilitate highly sensitive magnetic resonance imaging for metabolic studies and nuclear magnetic resonance (NMR)-based screenings for drug discovery. In this study, we demonstrate the hyperpolarization of biomolecules in eutectic crystals using photoexcited triplet electrons at room temperature. Eutectic crystals composed of the domains of benzoic acid doped with the polarization source and analyte domains were prepared using a melting-quenching process. Spin diffusion between the benzoic acid and analyte domain was elucidated using solid-state NMR analysis, indicating that hyperpolarization was transferred from the domain of benzoic acid to the domain of the analyte.

Photoexcited Anhydrous Proton Conductivity in Coordination Polymer Glass.

Optically switchable proton-conductive materials will enable the development of artificial ionic circuits. However, most switchable platforms rely on conformational changes in crystals to alter the connectivity of guest molecules. Guest dependency, low transmittance, and poor processability of polycrystalline materials hinder overall light responsiveness and contrast between on and off states. Here, we optically control anhydrous proton conductivity in a transparent coordination polymer (CP) glass. Photoexcitation of tris(bipyrazine)ruthenium(II) complex in CP glass causes reversible increases in proton conductivity by a factor of 181.9 and a decrease in activation energy barrier from 0.76 eV to 0.30 eV. Modulating light intensity and ambient temperature enables total control of anhydrous protonic conductivity. Spectroscopies and density functional theory studies reveal the relationship between the presence of proton deficiencies and the decreasing activation energy barrier for proton migrations.

Water as a Modifier in a Hybrid Coordination Network Glass.

Chemical diversification of hybrid organic-inorganic glasses remains limited, especially compared to traditional oxide glasses, for which property tuning is possible through addition of weakly bonded modifier cations. In this work, it is shown that water can depolymerize polyhedra with labile metal-ligand bonds in a cobalt-based coordination network, yielding a series of nonstoichiometric glasses. Calorimetric, spectroscopic, and simulation studies demonstrate that the added water molecules promote the breakage of network bonds and coordination number changes, leading to lower melting and glass transition temperatures. These structural changes modify the physical and chemical properties of the melt-quenched glass, with strong parallels to the "modifier" concept in oxides. It is shown that this approach also applies to other transition metal-based coordination networks, and it will thus enable diversification of hybrid glass chemistry, including nonstoichiometric glass compositions, tuning of properties, and a significant rise in the number of glass-forming hybrid systems by allowing them to melt before thermal decomposition.

Fine Pore-Structure Engineering by Ligand Conformational Control of Naphthalene Diimide-Based Semiconducting Porous Coordination Polymers for Efficient Chemiresistive Gas Sensing.

Exploring new porous coordination polymers (PCPs) that have tunable structure and conductivity is attractive but remains challenging. Herein, fine pore structure engineering by ligand conformation control of naphthalene diimide (NDI)-based semiconducting PCPs with π stacking-dependent conductivity tunability is achieved. The π stacking distances and ligand conformation in these isoreticular PCPs were modulated by employing metal centers with different coordination geometries. As a result, three conjugated PCPs (Co-pyNDI, Ni-pyNDI, and Zn-pyNDI) with varying pore structure and conductivity were obtained. Their crystal structures were determined by three-dimensional electron diffraction. The through-space charge transfer and tunable pore structure in these PCPs result in modulated selectivity and sensitivity in gas sensing. Zn-pyNDI can serve as a room-temperature operable chemiresistive sensor selective to acetone.

Coordination polymer-forming liquid Cu(2-isopropylimidazolate).

The structure of the melt state of one-dimensional (1D) coordination polymer crystal Cu(isopropylimidazolate) (melting temperature T m = 143 °C) was characterized by DSC, variable temperature PXRD, solid-state NMR (SSNMR), viscoelastic measurements, XAS, and DFT-AIMD calculations. These analyses suggested "coordination polymer-forming liquid" formation with preserved coordination bonds above T m. Variable chain configurations and moderate cohesive interaction in adjacent chains are the keys to the rarely observed polymer-forming liquid. The melt structure is reminiscent of the common 1D organic polymer melts such as entanglement or random coil structures.

Separating water isotopologues using diffusion-regulatory porous materials.

The discovery of a method to separate isotopologues, molecular entities that differ in only isotopic composition1, is fundamentally and technologically essential but remains challenging2,3. Water isotopologues, which are very important in biological processes, industry, medical care, etc. are among the most difficult isotopologue pairs to separate because of their very similar physicochemical properties and chemical exchange equilibrium. Herein, we report efficient separation of water isotopologues at room temperature by constructing two porous coordination polymers (PCPs, or metal-organic frameworks) in which flip-flop molecular motions within the frameworks provide diffusion-regulatory functionality. Guest traffic is regulated by the local motions of dynamic gates on contracted pore apertures, thereby amplifying the slight differences in the diffusion rates of water isotopologues. Significant temperature-responsive adsorption occurs on both PCPs: H2O vapour is preferentially adsorbed into the PCPs, with substantially increased uptake compared to that of D2O vapour, facilitating kinetics-based vapour separation of H2O/HDO/D2O ternary mixtures with high H2O separation factors of around 210 at room temperature.

Crystallization Kinetics of a Liquid-Forming 2D Coordination Polymer.

We investigated a mechanism of crystal melting and crystallization behavior of a two-dimensional coordination polymer [Ag2(L1)(CF3SO3)2] (1, L1 = 4,4'-biphenyldicarbonitrile) upon heating-cooling processes. The crystal showed melting at 282 °C, and the following gentle cooling induced the abrupt crystallization at 242 °C confirmed by DSC. A temperature-dependent structural change has been discussed through calorimetric, spectroscopic, and mechanical measurements. They indicated that the coordination-bond networks are partially retained in the melt state, but the melt showed a significantly low viscosity of 9.8 × 10-2 Pa·s at Tm which is six orders lower than that of ZIF-62 at Tm (435 °C). Rheological studies provided an understanding of the fast relaxation dynamics for the recrystallization process, along with that the high Tm provides enough thermal energy to crossover the activation energy barrier for the nucleation. The isothermal crystallization kinetics through calorimetric measurements with applying the Avrami equation identified the nature of the nuclei and its crystal growth mechanism.

Reversible Discrete-to-Extended Metal-Organic Polyhedra Transformation by Sulfonic Acid Surface Functionalization.

Metal-organic polyhedra (MOPs) are molecular porous units in which desired functionalities can be installed with precise geometrical and compositional control. By combing two complementary chemical moieties, such as sulfonic acid groups and Rh(II)-carboxylate paddlewheel, we synthesized a robust water-soluble cuboctahedral MOP with excellent features in both solution and solid states. Herein, we demonstrate that the superior chemical stability of the Rh2 unit and the elevated number of functional groups on the surface (24 per cage) result in a porous cage with high solubility and stability in water, including acidic, neutral, and basic pH conditions. We also prove that the sulfonic acid-rich form of the cage can be isolated through postsynthetic acid treatment. This transformation involves an improved gas uptake capacity and the capability to reversibly assemble the cages into a three-dimensional (3D) metal-organic framework (MOF) structure. Likewise, this sulfonic acid functionalization provides both MOP and MOF solids with high proton conductivities (>10-3 S cm-1), comparable to previously reported high conducting metal-organic materials. The influence of the MOP-to-MOF processing in the gas adsorption capacity indicates that this structural transformation can provide materials with higher and more controllable porous properties. These results illustrate the high potential of acidic MOPs as more flexible porous building units in terms of processability, structural complexity, and tunability of the properties.

Eutectic CsHSO4-Coordination Polymer Glasses with Superprotonic Conductivity.

Superprotonic phase transition in CsHSO4 allows fast protonic conduction, but only at temperatures above the transition temperature of 141 °C (Tc). Here, we preserve the superprotonic conductivity of CsHSO4 by forming a binary CsHSO4-coordination polymer glass system, showing eutectic melting. Their anhydrous proton conductivities below Tc are at least 3 orders of magnitude higher than CsHSO4 without compromising conductivity at higher temperatures or the need for humidification, reaching 6.3 mS cm-1 at 180 °C. The glass also introduces processability to the conductor, as its viscosity below 103 Pa·s can be achieved at 65 °C. Solid-state NMR and X-ray pair distribution functions reveal the oxyanion exchanges and the origin of the preserved conductivity. Finally, we demonstrate the preparation of a micrometer-scale thin-film proton conductor showing low resistivity with high transparency (transmittance >85% between 380-800 nm).

Three-Dimensional Metal-Organic Network Glasses from Bridging MF62- Anions and Their Dynamic Insights by Solid-State NMR.

Glassy-state coordination polymers (CPs) are a new class of network-forming glasses. In this work, we constructed glass-forming CPs composed of both anionic and neutral ligands as network formers. With the use of hexafluoro anions (MF62-) and 1,3-bis(4-pyridyl)propane (bpp), two isostructural CP crystals, [Zn(SiF6)(bpp)2] (ZnSi) and [Zn(TiF6)(bpp)2] (ZnTi), were synthesized. Solid-state 19F NMR revealed rotational motion of MF62- with dissociation and re-formation of the Zn-F coordination bonds in both CP crystals, which reflects the thermodynamic parameters related to the glass formability. The mobility of SiF62- is larger than that of TiF62-, suggesting a higher glass formability of ZnSi. When mechanical ball milling was conducted, ZnSi completely changed into a glassy state, whereas ZnTi showed incomplete glass formation. Examination of the amorphous structures elucidated retention and partial destruction of the Zn-F coordination bonds in ball-milled ZnSi and ZnTi, respectively. These results provide the relationship between the ligand dynamics and glass formability of CPs.