Cu[Ni(2,3-pyrazinedithiolate)2] Metal-Organic Framework for Electrocatalytic Hydrogen Evolution

Structurally isomeric ditopic 2-mercaptobenzoxazole and 2-hydroxybenzothiazole as ligands for design of 2D sodium-based luminescent coordination polymers

The novel ditopic centrosymmetric soft-base ligands 3,7-dihydrobenzo[1,2-d:4,5-d']bis(oxazole)-2,6-dithione (H2L1) and 3,7-dihydrobenzo[1,2-d:4,5-d']bis(thiazole)-2,6-dione (H2L2) were obtained via a one-pot method. Both compounds form 2D coordination polymers (CPs) through the reaction of free ligands with sodium bis(trismethylsilyl)amide in various solvent media. H2L1 containing products were [Na2L1(DME)2]n (1) and [Na2L1(DMSO)4]n (2), and for H2L2, [Na6L23(DMSO)12]n (3) was formed. X-ray analysis revealed that compound 1 is a 2D CP, in which deprotonated H2L1 acts as a tetratopic linker, linking four sodium atoms via nitrogen and bridging sulfur. Unlike 1, in CP 2, deprotonated H2L1 acts as a ditopic linker coordinating sodium atoms in an unprecedented Na4O4 metallocenter formed by the oxygens of DMSO molecules. Compound 3 is a 2D CP featuring rare Na3O6 secondary building unit (SBU), whereas H2L2 exhibits both bridging and mixed chelating/bridging mode, linking four sodium atoms via nitrogen and oxygen atoms. Despite the fact that free ligands H2L1 and H2L2 are non-luminescent in solution and in the solid state, their deprotonated forms at 298 K in DME media demonstrate moderate photoluminescence (PL) in DME solutions with broad bands in the range of 360-500 nm. At 77 K, the same solutions show broadened and intense PL consisting of bands in the 360-670 nm region corresponding to ligand fluorescence and phosphorescence. CPs 1-3 in the solid state exhibit PL at 298 and 77 K. At 298 K, CPs 1 and 3 exhibit intense fluorescence in the range of 370-450 nm and moderate microsecond timescale phosphorescence in the range of 500-670 nm, while CP 2 exhibits only fluorescence. Upon cooling to 77 K, all three CPs demonstrate both fluorescence and phosphorescence. Based on the low-temperature phosphorescence spectra of 1 and 3 in DME solutions, the triplet energy levels of H2L1 and H2L2 were established at 21 000 and 20 800 cm-1, respectively.

Discovery of Dual Ion-Electron Conductivity of Metal-Organic Frameworks via Machine Learning-Guided Experimentation

Identifying conductive metal-organic frameworks (MOFs) with a coupled ion-electron behavior from a vast array of existing MOFs offers a cost-effective strategy to tap into their potential in energy storage applications. This study employs classification and regression machine learning (ML) to rapidly screen the CoREMOF database and experimental methodologies to validate ML predictions. This process revealed the structure-property relationships contributing to MOFs' bulk ion-electron conductivity. Among the 60 conductive compounds predicted, only two p-type conductive MOFs, [Cu3(μ3-OH) (μ3-C4H2N2O2)3(H3O)]·2C2H5OH·4H2O (1) and NH4[Cu3(μ3-OH)(μ3-C4H2N2O2)3]·8H2O or (2) (C4H2N2O = 1H-pyrazole-4-carboxylic acid), were validated for their coupled electron-ion behavior. MOFs utilize earth-abundant copper and pyrazoles as ligands, demonstrating significant potential following thorough electrochemical characterization. Further analysis confirmed the critical role of strong σ-donating, π-accepting, and redox-active ligands in promoting electron mobility. In-depth structural investigations revealed that the presence of the O-Cu-N chain significantly influences conductivity, outperforming MOFs with only Cu-N or Cu-O bonds. Additionally, this study highlights how higher ionic conductivity is correlated with the ion mobility through linkers in 1 or the presence of ammonium ions in 2. These structure-property relationships offer valuable insights for future research in using ML coupled with experimentation to design MOFs containing earth-abundant reagents for ion-electron conductivity without employing a host-guest MOF strategy.

Decrypting the hydrogen evolution in alkaline water with novel magnetoactive cobalt(II) complex-driven cobalt oxide electrocatalysts

Under the gravity of future socio-economic development, the viability of water electrolysis still hinges on the accessibility of stable earth-abundant electrocatalysts and net energy efficiency. This work emphasizes the design and synthesis of two newly developed cobalt(II) complexes, [Co(HL)2(NCS)2] (Comono) and [Co2(L)3(CH3OH)]ClO4 (Codi), with a (N,O)-donor ligand, HL (2-methoxy-6-(((2-methoxyphenyl)imino)methyl)phenol). The study delves into understanding their structural, morphological, magnetic, and charge transport characteristics. Moreover, the study explores the potential of these complexes in catalyzing hydrogen production through heterogeneous electrocatalysis. The X-ray crystal structure of Comono reveals the octahedral geometry of the Co(II) ion, adopting two HL units and two NCS- units. The Codi complex exhibits a doubly-phenoxo-O-bridged (μ1,1) dinuclear complex, forming a typical octahedral geometry for both the Co(II) centres in coupling with three units of L-. Temperature-dependent magnetic susceptibility measurements showed that all of the Co(II) ion in Comono shows a typical paramagnetic behaviour for high spin octahedral Co(II) ions while the Co(II) centres in Codi are coupled with doubly-phenoxo-bridges bearing weak ferromagnetic characteristics at low temperature. Electron transport properties of the Co(II) complex-mediated Schottky device address the superior carrier mobility (μ) for Codi (9.21 × 10-5) over Comono (2.02 × 10-5 m2 v-1 s-1) with respective transit times of 1.70 × 10-9 and 7.77 × 10-9 s. Additionally, electron impedance spectral analysis supports the lower electrical transport resistance of Codi relative to Comono. The heterogeneous electrocatalytic HER activity of Codi and Comono in 0.1 M KOH shows excellent electrocatalytic efficiency in terms of the various electrochemical parameters. Constant potential electrolysis, multi-cycle CVs, and post-HER analysis reveal the pre-catalytic nature of the complexes, which in turn delivers Co3O4 nanoparticles as the active catalysts for efficient hydrogen evolution.

Electrochemical Anion Sensing Using Conductive Metal-Organic Framework Nanocrystals with Confined Pores

Anion sensing technology is motivated by the widespread and critical roles played by anions in biological systems and the environment. Electrochemical approaches comprise a major portion of this field but so far have relied on redox-active molecules appended to electrodes that often lack the ability to produce mixtures of distinct signatures from mixtures of different anions. Here, nanocrystalline films of the conductive metal-organic framework (MOF) Cr(1,2,3-triazolate)2 are used to differentiate anions based on size, which consequently affect the reversible oxidation of the MOF. During framework oxidation, the intercalation of larger charge-balancing anions (e.g., ClO4-, PF6-, and OTf-) gives rise to redox potentials shifted anodically by hundreds of mV due to the additional work of solvent reorganization and anion desolvation. Smaller anions (e.g., BF4-) may enter partially solvated, while larger ansions (e.g., OTf-) intercalate with complete desolvation. As a proof-of-concept, we leverage this "nanoconfinement" approach to report an electrochemical ClO4- sensor in aqueous media that is recyclable, reusable, and sensitive to sub-100-nM concentrations. Taken together, these results exemplify an unusual combination of distinct external versus internal surface chemistry in MOF nanocrystals and the interfacial chemistry they enable as a novel supramolecular approach for redox voltammetric anion sensing.

Molecular Catalysis of Energy Relevance in Metal-Organic Frameworks: From Higher Coordination Sphere to System Effects

The modularity and synthetic flexibility of metal-organic frameworks (MOFs) have provoked analogies with enzymes, and even the term MOFzymes has been coined. In this review, we focus on molecular catalysis of energy relevance in MOFs, more specifically water oxidation, oxygen and carbon dioxide reduction, as well as hydrogen evolution in context of the MOF-enzyme analogy. Similar to enzymes, catalyst encapsulation in MOFs leads to structural stabilization under turnover conditions, while catalyst motifs that are synthetically out of reach in a homogeneous solution phase may be attainable as secondary building units in MOFs. Exploring the unique synthetic possibilities in MOFs, specific groups in the second and third coordination sphere around the catalytic active site have been incorporated to facilitate catalysis. A key difference between enzymes and MOFs is the fact that active site concentrations in the latter are often considerably higher, leading to charge and mass transport limitations in MOFs that are more severe than those in enzymes. High catalyst concentrations also put a limit on the distance between catalysts, and thus the available space for higher coordination sphere engineering. As transport is important for MOF-borne catalysis, a system perspective is chosen to highlight concepts that address the issue. A detailed section on transport and light-driven reactivity sets the stage for a concise review of the currently available literature on utilizing principles from Nature and system design for the preparation of catalytic MOF-based materials.

Highly enhanced electrocatalytic OER activity of water-coordinated copper complexes: effect of lattice water and bridging ligand

The use of metal-organic compounds as electrocatalysts for water splitting reactions has gained increased attention; however, a fundamental understanding of the structural requirement for effective catalytic activity is still limited. Herein, we synthesized water-coordinated mono and bimetallic copper complexes (CuPz-H2O·H2O, CuPz-H2O, CuBipy-H2O·H2O, and CuMorph-H2O) with varied intermetallic spacing (pyrazine/4,4'-bipyridine) and explored the structure-dependent oxygen evolution reaction (OER) activity in alkaline medium. Single crystal structural studies revealed water-coordinated monometallic complexes (CuMorph-H2O) and bimetallic complexes (CuPz-H2O·H2O, CuPz-H2O, CuBipy-H2O·H2O). Further, CuPz-H2O·H2O and CuBipy-H2O·H2O contained lattice water along with coordinated water. Interestingly, the bimetallic copper complex with lattice water and shorter interspacing between the metal centres (CuPz-H2O·H2O) showed strong OER activity and required an overpotential of 228 mV to produce a benchmark current density of 10 mA cm-2. Bimetallic copper complex (CuPz-H2O) without lattice water but the same intermetallic spacing and bimetallic complex with increased interspacing but with lattice water (CuBipy-H2O·H2O) exhibited relatively lower OER activity. CuPz-H2O and CuBipy-H2O·H2O required an overpotential of 236 and 256 mA cm-2, respectively. Monometallic CuMorph-H2O showed the lowest OER activity (overpotential 271 mV) compared to bimetallic complexes. The low Tafel slope and charge transfer resistance of CuPz-H2O·H2O facilitated faster charge transfer kinetics at the electrode surface and supported the enhanced OER activity. The chronoamperometric studies indicated good stability of the catalyst. Overall, the present structure-electrocatalytic activity studies of copper complexes might provide structural insight for designing new efficient electrocatalysts based on metal coordination compounds.

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.

Phosphine Oxide Porous Organic Polymers Incorporating Cobalt(II) Ions: Synthesis, Characterization, and Investigation of H2 Production

Suitably functionalized porous matrices represent versatile platforms to support well-dispersed catalytic centers. In the present study, porous organic polymers (POPs) containing phosphine oxide groups were fabricated to bind transition metals and to be investigated for potential electrocatalytic applications. Cross-linking of mono- and di-phosphine monomers with multiple phenyl substituents was subject to the Friedel-Crafts (F-C) reaction and the oxidation process, which generated phosphine oxide porous polymers with pore capacity up to 0.92 cm³/g and a surface area of about 990 m²/g. The formation of the R₃P·BH₃ borohydride adduct during synthesis allows to extend the library of phosphine-based monomeric entities when using FeCl₃. The porous polymers were loaded with 0.8-4.2 w/w % of cobalt(II) and behaved as hydrogen evolution reaction (HER) catalysts with a Faradaic efficiency of up to 95% (5.81 × 10^-5 mol H₂ per 11.76 C) and a stable current density during repeated controlled potential experiments (CPE), even though with high overpotentials (0.53-0.68 V to reach a current density of 1 mA·cm^-2). These studies open the way to the effectiveness of tailored phosphine oxide POPs produced through an inexpensive and ecofriendly iron-based catalyst and for the insertion of transition metals in a porous architecture, enabling electrochemically driven activation of small molecules.

Linker micro-regulation of a Hofmann-based metalorganic framework for efficient propylene/propane separation

Fabricating easily available adsorbents with highly selective adsorption separation performance for propylene/propane mixtures is a desirable and energy-efficient way to produce high-purity propylene in the petrochemical industry. In this study,...

Novel ditopic 2-mercaptothiazoles and their sodium salts: synthesis, structural diversity and luminescence

A set of novel ditopic 2-mercaptothiazoles has been synthesized. Depending on the nature of the bisthiazoles their sodium salts can form coordination compounds or 2D polymers.