Tunable Interlayer Interactions in Exfoliated 2D van der Waals Framework Fe(SCN)2(Pyrazine)2.

2D materials can be isolated as monolayer sheets when interlayer interactions involve weak van der Waals forces. These atomically thin structures enable novel topological physics and open chemical questions of how to tune the structure and properties of the sheets while maintaining them as isolated monolayers. Here, this work investigates 2D electroactive sheets that exfoliate in solution into colloidal nanosheets, but aggregate upon oxidation, giving rise to tunable interlayer charge transfer absorption and photoluminescence. This optical behavior resembles interlayer excitons, now intensely studied due to their long-lived emission, but which remain difficult to tune through synthetic chemistry. Instead, the interlayer excitons of these framework sheets can be modulated through control of solvent, electrolyte, oxidation state, and the composition of the framework building blocks. Compared to other 2D materials, these framework sheets display the largest known interlayer binding strengths, attributable to specific orbital interactions between the sheets, and among the longest interlayer exciton lifetimes. Taken together, this study provides a microscopic basis for manipulating long-range opto-electronic behavior in van der Waals materials through molecular synthetic chemistry.

Alloying One-Dimensional Coordination Polymers To Create Ductile Materials.

The preparation of coordination polymer (CP) alloys is demonstrated by the use of two meltable, one-dimensional crystal structures via melt-kneading. The polymer structures of the alloys are studied by synchrotron X-ray absorption and scattering, solid-state NMR spectroscopy, DSC, and viscoelastic measurements. Crystalline and amorphous domains and thermal properties (melting and glass transition) in the alloys depend on the ratio of the two constituent CPs. The glassy alloy composed of an equivalent amount of two CPs shows high plastic deformation properties, and the fracture point reaches 128% without a filler or compatibilizing agent, hence behaving as ductile materials.

Size-Dependent Spin Crossover and Bond Flexibility in Metal-Organic Framework Nanoparticles.

Size reduction offers a synthetic route to tunable phase change behavior. Preparing materials as nanoparticles causes drastic modulations to critical temperatures (Tc), hysteresis widths, and the "sharpness" of first-order versus second-order phase transitions. A microscopic picture of the chemistry underlying this size dependence in phenomena ranging from melting to superconductivity remains debated. As a case study with broad implications, we report that size-dependent spin crossover (SCO) in nanocrystals of the metal-organic framework (MOF) Fe(1,2,3-triazolate)2 arises from metal-linker bonds becoming more labile in smaller particles. In comparison to the bulk material, differential scanning calorimetry indicates a ∼ 30-40% reduction in Tc and ΔH in the smallest particles. Variable-temperature vibrational spectroscopy reveals a diminished long-range structural cooperativity, while X-ray diffraction evidence an over 3-fold increase in the thermal expansion coefficients. This "phonon softening" provides a molecular mechanism for designing size-dependent behavior in framework materials and for understanding phase changes in general.

Electrically conductive [Fe4S4]-based organometallic polymers.

Tailoring the molecular components of hybrid organic-inorganic materials enables precise control over their electronic properties. Designing electrically conductive coordination materials, e.g. metal-organic frameworks (MOFs), has relied on single-metal nodes because the metal-oxo clusters present in the vast majority of MOFs are not suitable for electrical conduction due to their localized electron orbitals. Therefore, the development of metal-cluster nodes with delocalized bonding would greatly expand the structural and electrochemical tunability of conductive materials. Whereas the cuboidal [Fe4S4] cluster is a ubiquitous cofactor for electron transport in biological systems, few electrically conductive artificial materials employ the [Fe4S4] cluster as a building unit due to the lack of suitable bridging linkers. In this work, we bridge the [Fe4S4] clusters with ditopic N-heterocyclic carbene (NHC) linkers through charge-delocalized Fe-C bonds that enhance electronic communication between the clusters. [Fe4S4Cl2(ditopic NHC)] exhibits a high electrical conductivity of 1 mS cm-1 at 25 °C, surpassing the conductivity of related but less covalent materials. These results highlight that synthetic control over individual bonds is critical to the design of long-range behavior in semiconductors.

Giant Redox Entropy in the Intercalation vs Surface Chemistry of Nanocrystal Frameworks with Confined Pores.

Redox intercalation involves coupled ion-electron motion within host materials, finding extensive application in energy storage, electrocatalysis, sensing, and optoelectronics. Monodisperse MOF nanocrystals, compared to their bulk phases, exhibit accelerated mass transport kinetics that promote redox intercalation inside nanoconfined pores. However, nanosizing MOFs significantly increases their external surface-to-volume ratios, making the intercalation redox chemistry into MOF nanocrystals difficult to understand due to the challenge of differentiating redox sites at the exterior of MOF particles from the internal nanoconfined pores. Here, we report that Fe(1,2,3-triazolate)2 possesses an intercalation-based redox process shifted ca. 1.2 V from redox at the particle surface. Such distinct chemical environments do not appear in idealized MOF crystal structures but become magnified in MOF nanoparticles. Quartz crystal microbalance and time-of-flight secondary ion mass spectrometry combined with electrochemical studies identify the existence of a distinct and highly reversible Fe2+/Fe3+ redox event occurring within the MOF interior. Systematic manipulation of experimental parameters (e.g., film thickness, electrolyte species, solvent, and reaction temperature) reveals that this feature arises from the nanoconfined (4.54 Å) pores gating the entry of charge-compensating anions. Due to the requirement for full desolvation and reorganization of electrolyte outside the MOF particle, the anion-coupled oxidation of internal Fe2+ sites involves a giant redox entropy change (i.e., 164 J K-1 mol-1). Taken together, this study establishes a microscopic picture of ion-intercalation redox chemistry in nanoconfined environments and demonstrates the synthetic possibility of tuning electrode potentials by over a volt, with profound implications for energy capture and storage technologies.

Guest-dependent bond flexibility in UiO-66, a "stable" MOF.

We report "flexibility constants"-a conceptual analog to metal-ligand stability constants-of UiO-66, the prototypical "stable" MOF, across a wide temperature range in both vacuum and in the presence of typical guest solvents. With these data, we extract key thermodynamic parameters governing the reversible bond equilibrium and demonstrate that guest molecules strongly favor the reversible dissociation of MOF metal-linker bonds.

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.

Size-Dependent Properties of Solution-Processable Conductive MOF Nanocrystals.

The diverse optical, magnetic, and electronic behaviors of most colloidal semiconductor nanocrystals emerge from materials with limited structural and elemental compositions. Conductive metal-organic frameworks (MOFs) possess rich compositions with complex architectures but remain unexplored as nanocrystals, hindering their incorporation into scalable devices. Here, we report the controllable synthesis of conductive MOF nanoparticles based on Fe(1,2,3-triazolate)2. Sizes can be tuned to as small as 5.5 nm, ensuring indefinite colloidal stability. These solution-processable MOFs can be analyzed by solution-state spectroscopy and electrochemistry and cast into conductive thin films with excellent uniformity. This unprecedented analysis of MOF materials reveals a strong size dependence in optical and electronic behaviors sensitive to the intrinsic porosity and guest-host interactions of MOFs. These results provide a radical departure from typical MOF characterization, enabling insights into physical properties otherwise impossible with bulk analogues while offering a roadmap for the future of MOF nanoparticle synthesis and device fabrication.

One-Pot, Room-Temperature Conversion of CO2 into Porous Metal-Organic Frameworks.

The conversion of CO2 into functional materials under ambient conditions is a major challenge to realize a carbon-neutral society. Metal-organic frameworks (MOFs) have been extensively studied as designable porous materials. Despite the fact that CO2 is an attractive renewable resource, the synthesis of MOFs from CO2 remains unexplored. Chemical inertness of CO2 has hampered its conversion into typical MOF linkers such as carboxylates without high energy reactants and/or harsh conditions. Here, we present a one-pot conversion of CO2 into highly porous crystalline MOFs at ambient temperature and pressure. Cubic [Zn4O(piperazine dicarbamate)3] is synthesized via in situ formation of bridging dicarbamate linkers from piperazines and CO2 and shows high surface areas (∼2366 m2 g-1) and CO2 contents (>30 wt %). Whereas the dicarbamate linkers are thermodynamically unstable by themselves and readily release CO2, the formation of an extended coordination network in the MOF lattices stabilizes the linker enough to demonstrate stable permanent porosity.

Reactivity of borohydride incorporated in coordination polymers toward carbon dioxide.

Borohydride (BH4-)-containing coordination polymers converted CO2 into HCO2- or [BH3(OCHO)]-, whose reaction routes were affected by the electronegativity of metal ions and the coordination mode of BH4-. The reactions were investigated using thermal gravimetric analysis under CO2 gas flow, infrared spectroscopy, and NMR experiments.

Borohydride-containing coordination polymers: synthesis, air stability and dehydrogenation.

Control of the reactivity of hydride (H-) in crystal structures has been a challenge because of its strong electron-donating ability and reactivity with protic species. For metal borohydrides, the dehydrogenation activity and air stability are in a trade-off, and control of the reactivity of BH4 - has been demanded. For this purpose, we synthesize a series of BH4 --based coordination polymers/metal-organic frameworks. The reactivity of BH4 - in the structures is regulated by coordination geometry and neighboring ligands, and one of the compounds [Zn(BH4)2(dipyridylpropane)] exhibits both high dehydrogenation reactivity (1.4 wt% at 179 °C) and high air stability (50 RH% at 25 °C, 7 days). Single crystal X-ray diffraction analysis reveals that H δ+···H δ- dihydrogen interactions and close packing of hydrophobic ligands are the key for the reactivity and stability. The dehydrogenation mechanism is investigated by temperature-programmed desorption, in situ synchrotron PXRD and solid-state NMR.

Synthesis of porous coordination polymers using carbon dioxide as a direct source.

Porous coordination polymers (PCPs) were synthesized by using CO2 and metal borohydrides as precursors. Borohydrides converted CO2 into bridging ligands such as formate (HCO2-) or formylhydroborate ([BH(OCHO)3]-) which are available to construct porous architectures; one of them shows 380 m2 g-1 surface area.

Construction of a Hierarchical Architecture of Covalent Organic Frameworks via a Postsynthetic Approach.

Covalent organic frameworks (COFs) represent an emerging class of crystalline porous materials that are constructed by the assembly of organic building blocks linked via covalent bonds. Several strategies have been developed for the construction of new COF structures; however, a facile approach to fabricate hierarchical COF architectures with controlled domain structures remains a significant challenge, and has not yet been achieved. In this study, a dynamic covalent chemistry (DCC)-based postsynthetic approach was employed at the solid-liquid interface to construct such structures. Two-dimensional imine-bonded COFs having different aromatic groups were prepared, and a homogeneously mixed-linker structure and a heterogeneously core-shell hollow structure were fabricated by controlling the reactivity of the postsynthetic reactions. Solid-state nuclear magnetic resonance (NMR) spectroscopy and transmission electron microscopy (TEM) confirmed the structures. COFs prepared by a postsynthetic approach exhibit several functional advantages compared with their parent phases. Their Brunauer-Emmett-Teller (BET) surface areas are 2-fold greater than those of their parent phases because of the higher crystallinity. In addition, the hydrophilicity of the material and the stepwise adsorption isotherms of H2O vapor in the hierarchical frameworks were precisely controlled, which was feasible because of the distribution of various domains of the two COFs by controlling the postsynthetic reaction. The approach opens new routes for constructing COF architectures with functionalities that are not possible in a single phase.

Synthesis of Oligodiacetylene Derivatives from Flexible Porous Coordination Frameworks.

Oligodiacetylenes (ODAs) with alternating ene-yne conjugated structure are significant materials for optical and electronic properties. Due to the low solubility of ODAs in common solvents, the synthetic approaches are limited. Here we disclose a new synthetic approach of ODAs without a side alkyl chain using a porous coordination polymer (PCP) as a sacrificial template. 1,2-Bis(4-pyridyl)butadiyne, which works as a monomer, was embedded in the flexible framework of the PCP, and ODAs were synthesized via utilization of the anisotropic thermal expansion of the PCP crystal. The oligomeric state of ODAs depends on the metal ion and coligand of the precursor.

Imidazolium cation transportation in a 1-D coordination polymer.

We synthesized a coordination polymer, [EtMeIm][Cu(bpy)(Me2PO4)3], containing an anionic 1-D chain and an ethyl methyl imidazolium cation. The organic cations show fast transport behavior, and the observed ion conductivity is 1.4 × 10-3 S cm-1 at 110 °C. The stability and structure of the material were confirmed by DSC, X-ray adsorption, and long-term AC impedance spectroscopy.

Synthesis of Manganese ZIF-8 from [Mn(BH4)2·3THF]·NaBH4.

Cubic and highly porous [Mn(2-methylimidazolate)2] (Mn-ZIF-8) was synthesized from [Mn(BH4)2·3THF]·NaBH4 under an Ar atmosphere. The structure contains rare Mn2+-4N tetrahedral geometry and has larger cell parameters, resulting in 20% larger amounts of gas uptake compared with [Zn(2-methylimidazolate)2]. A kinetically favored reaction using a reactive metal borohydride precursor is key for the construction of new metal-organic framework systems.

Mechanical Alloying of Metal-Organic Frameworks.

The solvent-free mechanical milling process for two distinct metal-organic framework (MOF) crystals induced the formation of a solid solution, which is not feasible by conventional solution-based syntheses. X-ray and STEM-EDX studies revealed that performing mechanical milling under an Ar atmosphere promotes the high diffusivity of each metal ion in an amorphous solid matrix; the amorphous state turns into the porous crystalline structure by vapor exposure treatment to form a new phase of a MOF solid solution.