Metal pyrazolate frameworks: crystal engineering access to stable functional materials

As the focus evolves from structure discovery/characterization (what it is) to property/performance exploration (what it is for), the pursuit of stable functional metal-organic frameworks (MOFs) has been ongoing in terms of both fundamental research and industrial implementation. Under the guidance of crystal engineering principles, a plethora of research has developed pyrazolate MOFs (metal pyrazaolate frameworks, MPFs) featuring strong coordination M-N bonding. This attribution helps them retain their structures and functions under the alkaline conditions required for practical use. Based on poly-topic pyrazolate ligands, various classic MOFs, such as Co(bdp), Fe2(BDP)3, Ni8L6, PCN-601, and BUT-55, to name a few, have revealed fascinating architectures, intriguing properties, and record-breaking performances in applications during the past decade. This review will present the full scope of MPFs to date: (1) the superiority and significance of constructing MPFs through the crystal engineering approach, (2) synthetic strategies adopted in building and/or modifying MPFs, (3) structural features and stability of the MPF community, and (4) potential applications in energy and environmental related fields. The future opportunities of MPFs are also discussed for designing the next-generation of smart materials. Overall, this review attempts to provide insights and guidelines for the customization of pyrazolate-based MOFs for specific purposes, which would also promote the development of stable functional porous materials for addressing societal challenges.

A coordination polymer with a silylene-supported Pd6 core as an efficient heterogeneous hydrogenation catalyst

A hexanuclear palladium cluster supported by two silylene units was readily linked by molecules of a linear ditopic isocyanide to afford a coordination polymer that retained the core Pd6(SiPh2)2Cl2 framework. The obtained coordination polymer exhibited good performance as a heterogeneous catalyst in the hydrogenation of various alkenes in common organic solvents and in protic solvents such as H2O. Furthermore, the obtained coordination polymer showed sufficient stability during the hydrogenation in order for it to be recycled and reused.

Recent advances in heterogeneous porous Metal-Organic Framework catalysis for Suzuki-Miyaura cross-couplings

Suzuki-Miyaura coupling (SMC), a crucial C-C cross-coupling reaction, is still associated with challenges such as high synthetic costs, intricate work-ups, and contamination with homogeneous metal catalysts. Research intensely focuses on strategies to convert homogeneous soluble metal catalysts into insoluble powder solids, promoting heterogeneous catalysis for easy recovery and reuse as well as for exploring greener reaction protocols. Metal-Organic Frameworks (MOFs), recognized for their high surface area, porosity, and presence of transition metals, are increasingly studied for developing heterogeneous SMC. The molecular fence effect, attributed to MOF surface functionalization, helps preventing catalyst deactivation by aggregation, migration, and leaching during catalysis. Recent reports demonstrate the enhanced catalytic activity, selectivity, stability, application scopes, and potential of MOFs in developing greener heterogeneous synthetic methodologies. This review focuses on the catalytic applications of MOFs in SMC reactions, emphasizing developments after 2016. It critically examines the synthesis and incorporation of active metal species into MOFs, focusing on morphology, crystallinity, and dimensionality for catalytic activity induction. MOF catalysts are categorized based on their metal nodes in subsections, with comprehensive discussion on Pd incorporation strategies, catalyst structures, optimal SMC conditions, and application scopes, concluding with insights into challenges and future research directions in this important emerging area of MOF applications.

CO2-actuated spin transition tuning in an interdigitated Hofmann-type coordination polymer

The increased anthropogenic emission level of CO2 urges the development of CO2-responsive materials, but is it possible to regulate the inherent electronic properties through weak physisorption of a ubiquitous gas such as CO2? Herein, we intended to answer this imperative question by the first case of CO2-actuated variable spin-state stabilisation in an interdigitated Hofmann-type coordination polymer [FeIIPd(CN)4L2] (1, L = methyl isonicotinate), showing a wide shift in transition temperature (T eq) from 178 K at P CO2 = 0 kPa to 229 K at P CO2 = 100 kPa. Interestingly, the emergence of a stepped behaviour in the heating process below P CO2 = 10 kPa and overlapping magnetic susceptibility values above P CO2 = 10 kPa elucidate the selective LS state stabilisation solely correlated with the extent of CO2 accommodation. Based on the magnetic response and phase transition diagrams obtained under respective P CO2 , a plausible scenario of the spin-state switching can be interpreted as (1ls + ) → (1hs + ) → 1hs at P CO2 ≤ 10 kPa, → 1hs at 100 kPa < P CO2 ≥ 32 kPa and → → 1hs at 100 kPa, where 1 and 1' represent CO2-free and CO2-encapsulated states, respectively. The cooperative CO2 sorption with SCO based on the varied CO2 pressure corroborates a novel case for developing CO2-responsive magnetic materials henceforth.

N, N'-bidentate ligand anchored palladium catalysts on MOFs for efficient Heck reaction

Metal-organic frameworks (MOFs), recognized as advanced catalyst carriers due to their adjustable porous, diverse structure and highly exposed active sites, have earned increasing attention for their potential to address the longevity of catalytic centers. In this manuscript, we have devised and synthesized a multifunctional amino-pyridine benzoic acid (APBA) ligand to replace the modulator ligand of the MOF-808 and disperse the palladium catalytic centers atomically on the MOF-APBA. The resulting single-site catalytic system, Pd@MOF-APBA, demonstrates preeminent efficiency and stability, as evidenced by a high average turnover number (95000) and a low metal residue (4.8 ppm) in the Heck reaction. This catalyst has exhibited recyclability for multiple runs without significant loss of reactivity for gram-scale reactions. The catalyst's high activity and efficiency can be attributed to the suitable electrical properties and structures of the N, N'-bidentate ligand for the catalytic palladium ions, postponing their deactivations, including leaching and agglomeration.

A new PdMOF-loaded molecularly imprinted polyaniline nanocatalytic probe for ultratrace oxytetracycline with SERS technique

In this paper, a new Pd metal organic framework (PdMOF) surface molecularly imprinted polyaniline nanocatalytic probe (PdMOF@MIP) with dual functions of recognition and catalysis was synthesized. It is found that the PdMOF@MIP nanoprobe can not only identify OTC but also catalyze the new nanoreaction of NaH2PO2-HAuCl4 to generate gold nanoparticles (AuNPs), and the generated AuNPs could be traced by surface-enhanced Raman scattering (SERS). When OTC specifically binds to PdMOF@MIP to generate PdMOF@MIP-OTC conjugate, its catalytic effect is weakened and the analytical signal is reduced linaerly. Accordingly, a new, highly sensitive, selective and simple SERS/RRS/Abs trimode detection platform for OTC was constructed. The linear range of SERS was 0.0625 ng/mL ∼ 1.75 ng/mL and the limit of detection was 0.015 ng/mL. This new nanocatalytic probe detection strategy can also be used for the selective detection of other antibiotics such as tetracycline and doxycycline, respectively. In addition, the nanocatalytic mechanism has been investigated.

Generic and facile mechanochemical access to versatile lattice-confined Pd(ii)-based heterometallic sites

Metal-organic frameworks (MOFs) show remarkable potential in a broad array of applications given their physical and chemical versatility. Classical synthesis of MOFs is performed using solution chemistry at elevated temperatures to achieve reversible metal-ligand bond formation. These harsh conditions may not be suitable for chemical species sensitive to high temperature or prone to deleterious reactions with solvents. For instance, Pd(ii) is susceptible to reduction under solvothermal conditions and is not a common metal node of MOFs. We report a generic and facile mechanochemical strategy that directly incorporates a series of Pd(ii)-based heterobimetallic clusters into MOFs as metal nodes without Pd(ii) being reduced to Pd(0). Mechanochemistry features advantages of short reaction time, minimum solvent, high reaction yield, and high degree of synthetic control. Catalytic performances of lattice-confined heterobimetallic sites are examined for nitrene transfer reactions and we demonstrate that the chemoselectivity for allylic amination versus olefin aziridination is readily tuned by the identity of the first-row metal ion in Pd(ii)-based heterobimetallic clusters.

Principles of Design and Synthesis of Metal Derivatives from MOFs

Materials derived from metal-organic frameworks (MOFs) have demonstrated exceptional structural variety and complexity and can be synthesized using low-cost scalable methods. Although the inherent instability and low electrical conductivity of MOFs are largely responsible for their low uptake for catalysis and energy storage, a superior alternative is MOF-derived metal-based derivatives (MDs) as these can retain the complex nanostructures of MOFs while exhibiting stability and electrical conductivities of several orders of magnitude higher. The present work comprehensively reviews MDs in terms of synthesis and their nanostructural design, including oxides, sulfides, phosphides, nitrides, carbides, transition metals, and other minor species. The focal point of the approach is the identification and rationalization of the design parameters that lead to the generation of optimal compositions, structures, nanostructures, and resultant performance parameters. The aim of this approach is to provide an inclusive platform for the strategies to design and process these materials for specific applications. This work is complemented by detailed figures that both summarize the design and processing approaches that have been reported and indicate potential trajectories for development. The work is also supported by comprehensive and up-to-date tabular coverage of the reported studies.

A Palladium-Based MOF for the Preferential Sorption of Benzene

Selective sorption of volatile aromatic compounds is a challenging issue for their total abatement. Despite the well-known affinity of palladium toward rich π systems, studies dedicated to volatile organic compound (VOC) capture with Pd(II)-based metal-organic frameworks (MOFs) are still very scarce. Intending to shed more light on this complex topic, this work compares the adsorption properties of two isostructural MOFs [Cu(2-pymo)₂]_n and [Pd(2-pymo)₂]_n and their selectivity for the sorption of linear, cyclic, or aromatic VOCs. The combination of both experimental and computational investigations highlights an increasing aromatic affinity over saturated hydrocarbons when palladium is chosen as a metal center (n_Benzene/n_n-hexane = 1.8 at 0.5 p/p₀) in the MOF instead of copper (n_Benzene/n_n-hexane = 0.7 at 0.5 p/p₀). Furthermore, [Pd(2-pymo)₂]_n clearly exhibits preferential adsorption of benzene over toluene (n_Benzene/n_Toluene = 1.7 at 0.5 p/p₀), due to the steric hindrance effects of the latter. The present results clearly underline the attractiveness of Pd-based MOFs for the design of selective aromatic adsorbents. Moreover, they also highlight the [Pd(2-pymo)₂]_n MOF as a relevant candidate for the selective capture of benzene, by a synergistic combination of both charge interactions and steric hindrance effects.

Construction of a MOF-Supported Palladium Catalyst via Metal Metathesis

A new MOF-supported heterogeneous palladium catalyst Pd/NBB-1 has been synthesized successfully through the effective metal metathesis between Pd(CF₃ COO)₂ and NBB-1. NBB-1 is a two-dimensional zinc metal-organic framework constructed from 2-aminoterephthalate (NH₂ -H₂ BDC) and 2,2'-bipyridine-5-carboxylate (HBPC) by solvothermal method. The replacement efficiency of Pd(II) to Zn(II) is up to 72% after only 24 hours, which is beneficial to the catalytic application. Pd/NBB-1 with a low loading of 2 mol% works efficiently in the 1,4-addition reaction of arylboronic acids with α,β-unsaturated ketones in air, and its catalytic activity keeps unchanged after 3 reaction cycles. This work provides a new strategy to effectively prepare supported noble metal/MOF catalysts, which would further increase the practical applications of metal-organic frameworks.

Catalytic activity and mechanistic investigation of 1D 2-Picolinic acid based Cu(II) coordination polymer in the selective construction of 1,4-disubstituted triazoles

The catalytic activity of 1D 2-Picolinic acid based Cu (II) coordination polymer (CP1) in click reaction was evaluated to generate 1,4-disubstituted 1,2,3-triazoles selectively. The CP1 catalyst loading of 2 mol% was applied successfully in the reaction for primary azides with diverse functionalities of terminal alkynes in green solvent (EG/H2O). Moreover, the one-pot, multicomponent click reaction involving benzyl bromide, sodium azide, and phenylacetylene was also catalyzed by CP1. The findings show that 1D 2-Picolinic acid based Cu (II) coordination polymer catalytic systems are highly efficient for green click triazoles synthesis. DFT calculation supported the plausible mechanism involved in the CP1 catalyzed click reaction.

A Practice of Reticular Chemistry: Construction of a Robust Mesoporous Palladium Metal-Organic Framework via Metal Metathesis

Constructing stable palladium(II)-based metal-organic frameworks (MOFs) would unlock more opportunities for MOF chemistry, particularly toward applications in catalysis. However, their availability is limited by synthetic challenges due to the inertness of the Pd-ligand coordination bond, as well as the strong tendency of the Pd(II) source to be reduced under typical solvothermal conditions. Under the guidance of reticular chemistry, herein, we present the first example of an azolate Pd-MOF, BUT-33(Pd), obtained via a deuterated solvent-assisted metal metathesis. BUT-33(Pd) retains the underlying sodalite network and mesoporosity of the template BUT-33(Ni) and shows excellent chemical stability (resistance to an 8 M NaOH aqueous solution). With rich Pd(II) sites in the atomically precise distribution, it also demonstrates good performances as a heterogeneous Pd(II) catalyst in a wide application scope, including Suzuki/Heck coupling reactions and photocatalytic CO₂ reduction to CH₄. This work highlights a feasible approach to reticularly construct noble metal based MOFs via metal metathesis, in which various merits, including high chemical stability, large pores, and tunable functions, have been integrated for addressing challenging tasks.