Broad Electronic Modulation of Two-Dimensional Metal-Organic Frameworks over Four Distinct Redox States

Structure and Synthesizability of Iron-Sulfur Metal-Organic Frameworks

Sulfur-based metal-organic frameworks (MOFs) and coordination polymers (CPs) are an emerging class of hybrid materials that have received growing attention due to their magnetic, conductive, and catalytic properties with potential applications in electrocatalysis and energy storage. In this work, we report a high-throughput virtual screening protocol to predict the synthesizability of candidate metal-sulfur MOFs/CPs by computing the thermodynamically stable structures resulting from a particular combination of metal cluster, linker, cation, and synthetic conditions. Free energies are computed by using all-atom classical mechanical thermodynamic integration. Low-free-energy structures are refined using ab initio density functional theory, and pair distribution functions and powder X-ray diffraction patterns are calculated to complement and guide experimental structure determination. We validate the computational approach by retrospective predictions of the stable structure produced by experimental syntheses, and a subsequent screen predicts Fe4S4-BDT-TPP as a new thermodynamically stable one-dimensional (1D) CP comprising a redox-active Fe4S4 cluster, a 1,4-benzenedithiolate (BDT) linker, and a tetraphenylphosphonium (TPP) countercation. This material is experimentally synthesized, and the 1D chain structure of the crystal is confirmed using microcrystal electron diffraction. The computational screening pipeline is generically transferable to neutral and ionic MOFs/CPs comprising arbitrary metal clusters, linkers, cations, and synthetic conditions, and we make it freely available as an open source tool to guide and accelerate the discovery and engineering of novel porous materials.

In Situ Construction of Amide-Functionalized 2D Conjugated Metal-Organic Frameworks with Multiple Active Sites for High-Performance Potassium-Ion Batteries

Two-dimensional conjugated metal-organic frameworks (2D c-MOFs) represent a promising class of electrode materials for potassium-ion batteries (PIBs), attributed to their superior conductivity, large specific surface area, high charge carrier mobility, and tunable active sites. However, most reported 2D c-MOF-based cathode materials for PIBs usually encounter challenges, such as low specific capacity and inadequate cycling stability. In this context, we herein designed and synthesized a new hexahydroxy salicylamide ligand (6OH-HBB) via a straightforward two-step synthesis with a high yield of 93%, which was subsequently utilized to construct a 2D Cu-HBB-MOF with multiple active sites through an in situ metal coordination-induced planarization strategy. Thanks to its abundant active sites and large specific surface area, the Cu-HBB-MOF demonstrated an outstanding high initial capacity of 228.1 mA h g-1 at 0.2 A g-1, surpassing most reported porous material-based PIBs. Furthermore, even at 5.0 A g-1, the Cu-HBB-MOF exhibited a large reversible specific capacity of 103.6 mA h g-1 after 2500 cycles, simultaneously maintaining a low-capacity loss of only 0.011% per cycle and achieving a Coulombic efficiency up to 100%, demonstrating good long-term cycle stability. This work provides fundamental insights into engineering 2D c-MOFs with multisite functionality, charting a new course for developing high-performance MOF-based cathodes in next-generation energy storage systems.

Harnessing Coordination Microenvironment of Metal-bis(dithiolene) Sites for Modulating Electrocatalytic CO2 Reduction by Metal-Organic Framework

Nature's metalloenzymes inspire biomimetic catalysts for the CO2 reduction reaction (CO2RR), particularly using metal-bis(dithiolene) ([MS4]) cores in frameworks. While prior research focused on tuning the chelating atoms of Ni-centered sites or [NiS4] in metal-organic frameworks (MOFs), how different metal centers affect the electronic structure and catalytic activity is often overlooked. Notably, reported [NiS4] molecular analogues exhibits a Faradaic efficiency (FE) of less than 70% for the major carbon product and shows operational stability for only about 4 hours (say falling FE and current density beyond). In this study, MOFs are used to host [MS4] units with varying central metals (M = Ni, Cu, Co, Fe) to assess how the metal center affects electrocatalytic CO2RR. Among the studied MS4-In MOFs, NiS4-In demonstrates the best performance, achieving a FECO of 88.54% and operational stability greater than 6 hours-significantly outlasting the ≈200 seconds of the [NiS4] molecule. This work underscores the importance of frameworks in stabilizing [MS4] units and highlights [MS4] as essential for CO2 binding and reduction, with [NiS4] exhibiting optimal catalytic performance due to its favorable electronic properties. This findings clarify how substituting the metal center within the framework enhances electronic structure and coordination, leading to improved electrocatalytic performance.

A Trimming-π Strategy for Constructing Functional Conductive Metal-Organic Frameworks Using Metalloporphyrazine Units

Developing functional metal-organic frameworks (MOFs) with high electrical conductivity is crucial for their applications as advanced electronic materials. In this work, we for the first time construct a new family of functional and highly conductive MOFs using metalloporphyrazine (MPz) ligands based on a trimming-π concept via cutting the benzene ring from molecular metallopthalocynine (MPc). The deprotonation-after-coordination synthetic method affords crystalline MPz-Cu-NH MOFs with square lattices. Four-point probe conductivity measurements reveal the high room temperature electrical conductivity of MPz-based MOFs ranging from 3.5×10-2 to 1.3×10-1 S cm-1, two orders of magnitude higher than the MPc-based MOF counterparts. Temperature-dependent conductivity measurements and electronic band structure analysis demonstrate ultra-small activation energies with potential metallic conducting behavior for the MPz-Cu-NH MOFs. Encapsulation of the aromatic guest molecules with different electron-donating and -withdrawing features allows the conductivity modulation of the CuPz-Cu-NH in a wide range spanning two orders of magnitude. These conductive MPz-Cu-NH MOFs with built-in MPz functional units exhibit MPz identity-dependent sensing performance, and realize highly sensitive detection of NH3 and NO2 using a low driving voltage of 0.1 V.

Dimensionality Reduction of Metal-Organic Frameworks to Monolayers for Enhanced Electrocatalysis

Metal-organic frameworks (MOFs) are potential candidates for electrocatalysis due to their well-defined, tunable structures, and ability to incorporate diverse active sites. However, their inherent insulating nature restricts electron transfer from electrode to remote active sites, leading to diminished catalytic performance. In this work, we present a novel strategy to overcome this limitation by reducing 3D MOFs (3D_MOFs) into monolayered MOFs (monoMOFs) with a thickness of ∼1.8 nm, maximizing the exposure of catalytic sites to the electrode and enhancing electrocatalytic performance. We designed and synthesized a monoMOF incorporating cobalt(II)-porphyrin sites in the linker (monoMOF-Co) for CO2 electroreduction. After being grafted onto graphene oxide, the monoMOF-Co exhibited a peak faradaic efficiency for CO production (FECO = 93%), surpassing the performance of a 3D_MOF incorporating the same porphyrin-Co-based linker (3D_MOF-Co, FECO = 51%). Additionally, monoMOF-Co achieved a turnover frequency of 10 600 h-1 at -0.8 V versus the reversible hydrogen electrode (RHE) and maintained stability over 47 h in a near-neutral aqueous solution. In situ spectroscopic studies further confirmed the distinct electric field environment in the Stern layer between monoMOF-Co and 3D_MOF-Co. Furthermore, similar enhancement effects of monoMOFs over 3D_MOFs were observed in the nitrate and oxygen electroreduction reactions, highlighting the broader applicability of monoMOFs in electrocatalysis.

A single-crystal 3D Zn-tetrathiolate connected metal-organic framework

Due to thiolate's fast and robust coordination, crystallographically resolved thiolate-ligated metal-organic frameworks (MOFs) are hard to obtain. Following the previous success of the masked synthetic strategy to achieve single-crystal Pb-thiolate connected MOFs, thioester-protected hexathiotriphenylene (HVaTT) was employed in this work to assemble with a Zn(II) centre to yield the first example of a first-row transition-metal based crystallographically resolved metal-tetrathiolate connected MOF, HTT-Zn. HTT-Zn features a 3D anionic framework connected by tetrahedral [ZnS4] nodes with 3-fold interpenetration and windmill-like topology. Thanks to the anionic nature, HTT-Zn demonstrates a fast colorimetric response (from yellow to black) to the positively charged toxic pollutant, paraquat, and potentially serves as a naked-eye sensor for paraquat. Isolation of HTT-Zn proves masked synthesis as a general approach and adds a new dimension to explore metal-thiolate-based MOFs.

Mixed-valence metal-organic frameworks: concepts, opportunities, and prospects

Owing to increasing global demand for the development of multifunctional advanced materials with various practical applications, great attention has been paid to metal-organic frameworks due to their unique properties, such as structural, chemical, and functional diversity. Several strategies have been developed to promote the applicability of these materials in practical fields. The induction of mixed-valency is a promising strategy, contributing to exceptional features in these porous materials such as enhanced charge delocalization, conductivity, magnetism, etc. The current review provides a detailed study of mixed-valence MOFs, including their fundamental properties, synthesis challenges, and characterization methods. The outstanding applicability of these materials in diverse fields such as energy storage, catalysis, sensing, gas sorption, separation, etc. is also discussed, providing a roadmap for future design strategies to exploit mixed valency in advanced materials. Interestingly, mixed-valence MOFs have demonstrated fascinating features in practical fields compared to their homo-valence MOFs, resulting from an enhanced synergy between mixed-valence states within the framework.

Skeletal Nitrogen Functionalization of Isostructural 2D Conjugated MOFs for Enhancement of the Dual-Ion Storage Capacity

Two-dimensional conjugated metal-organic frameworks (2D c-MOFs) are emerging as promising electrode materials for electrochemical energy storage devices. However, a viable path to realize superior dual-ion storage in 2D c-MOFs has remained elusive. Here, we report the synthesis of Cu2(Nx-OHPTP) 2D c-MOFs (x=0,1,2; OHPTP=octahydroxyphenanthrotriphenylene) with precise aromatic carbon-nitrogen arrangements, based on the π-conjugated OHPTP ligand incorporated with one or two nitrogen atoms. The skeletal nitrogen modification in Cu2(Nx-OHPTP) allows the synergistic introduction of additional redox sites, and thus substantially favors the unique dual-ion adsorption capacity. Consequently, the Cu2(N2-OHPTP) cathode exhibits a largely enhanced electrochemical performance for dual-ion storage (i.e., Li+ and Cl-) with a high specific capacity of 53.8 mAh g-1, which is twice that of Cu2(N0-OHPTP) and 1.3 times that of Cu2(N1-OHPTP). Furthermore, the Cu2(N2-OHPTP) electrode displays a favorable rate performance of 52 % and good cycling stability of 96 % after 1000 cycles. We identify N-centered redox sites as additional Li+ adsorption sites by combining ex situ and in situ spectroscopy measurements and theoretical calculations. In addition, calculations underline the synergistic enhancement of the Cl- adsorption energy by about 1.0 eV at the more electron-poor CuO4 linkages after N-incorporation. This work paves the way for the precise design of 2D c-MOFs with superior electrochemical properties, advancing their application in dual-ion storage applications.

A Stable Site-Isolated Mono(phosphine)-Rhodium Catalyst on a Metal-Organic Layer for Highly Efficient Hydrogenation Reactions

Phosphine-ligated transition metal complexes play a pivotal role in modern catalysis, but our understanding of the impact of ligand counts on the catalysis performance of the metal center is limited. Here we report the synthesis of a low-coordinate mono(phosphine)-Rh catalyst on a metal-organic layer (MOL), P-MOL • Rh, and its applications in the hydrogenation of mono-, di-, and tri-substituted alkenes as well as aryl nitriles with turnover numbers (TONs) of up to 390000. Mechanistic investigations and density functional theory calculations revealed the lowering of reaction energy barriers by the low steric hindrance of site-isolated mono(phosphine)-Rh sites on the MOL to provide superior catalytic activity over homogeneous Rh catalysts. The MOL also prevents catalyst deactivation to enable recycle and reuse of P-MOL • Rh in catalytic hydrogenation reactions.

Aliovalent Substitution Tunes Physical Properties in a Conductive Bis(dithiolene) Two-Dimensional Metal-Organic Framework

Two-dimensional conductive metal-organic frameworks have emerged as promising electronic materials for applications in (opto)electronic, thermoelectric, magnetic, electrocatalytic, and energy storage devices. Many bottom-up or postsynthetic protocols have been developed to isolate these materials or further modulate their electronic properties. However, some methodologies commonly used in classic semiconductors, notably, aliovalent substitution, are conspicuously absent. Here, we demonstrate how aliovalent Fe(III) to Ni(II) substitution enables the isolation of a Ni bis(dithiolene) material from a previously reported Fe analogue. Detailed characterization supports the idea that aliovalent substitution of Fe(III) to Ni(II) results in an in situ oxidation of the organic dithiolene linker. This substitution-induced redox tuning modulates the electronic properties in the system, leading to higher electrical conductivity and Hall mobility but slightly lower carrier densities and weaker antiferromagnetic interactions. Moreover, this aliovalent substitution improves the material's electrochemical stability and thus enables pseudocapacitive behavior in the Ni material. These results demonstrate how classic aliovalent substitution strategies in semiconductors can also be leveraged in conductive MOFs and add further support to this class of compounds as emerging electronic materials.

Acid-Dependent Charge Transport in a Solution-Processed 2D Conductive Metal-Organic Framework

The development of conductive metal-organic frameworks (MOFs) presents a unique challenge in materials chemistry because it is unclear how to dope them. Here, we demonstrate that the inclusion of pendant amines on hexahydroxytriphenylene linkages results in two-dimensional (2D) polycrystalline frameworks Cu3(HHTATP)2, isostructural to its Cu3(HHTP)2 parent, and exhibits the highest electrical conductivity of 1.21 S/cm among 2D MOFs featuring CuO4 metal nodes. Moreover, the bulk material can be treated with acid, resulting in a protonation-dependent increase in the conductivity. By spin-coating the acidic solution, we fabricated large-area thin films and collectively demonstrated an intuitive route to solution-processable, dopable, conductive MOFs.