Silicon Nitride Surface Enabled Propane Dehydrogenation Catalyzed by Supported Organozirconium.

Mesoporous silicon nitride (Si3N4) is a nontraditional support for the chemisorption of organometallic complexes with the potential for enhancing catalytic activity through features such as the increased Lewis basicity of nitrogen for heterolytic bond activation, increased ligand donor strength, and metal-ligand orbital overlap. Here, tetrabenzyl zirconium (ZrBn4) was chemisorbed on Si3N4, and the resulting supported organometallic species was characterized by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), Dynamic Nuclear Polarization-enhanced Solid State Nuclear Magnetic Resonance (DNP-SSNMR), and X-ray Absorption Spectroscopy (XAS). Based on the hypothesis that the nitride might enable facile heterolytic C-H bond activation along the Zr-N bond, this material was found to be a highly active (1.53 molpropene molZr-1 h-1 at 450 °C) and selective (99% to propylene) catalyst for propane dehydrogenation. In contrast, the homologous silica supported complex exhibited negligible activity under these conditions.

Metal-Organic Frameworks as a Tunable Platform to Deconvolute Stereoelectronic Effects on the Catalytic Activity of Thioanisole Oxidation.

The local environment of a metal active site plays an important role in affecting the catalytic activity and selectivity. In recent studies, tailoring the behavior of a molybdenum-based active site via modulation of the first coordination sphere has led to improved thioanisole oxidation performance, but disentangling electronic effects from steric influences that arise from these modifications is nontrivial, especially in heterogeneous systems. To this end, the tunability of metal-organic frameworks (MOFs) makes them promising scaffolds for controlling the coordination sphere of a heterogeneous, catalytically active metal site while offering additional attractive features such as crystallinity and high porosity. Herein, we report a variety of MOF-supported Mo species, which were investigated for catalytic thioanisole oxidation to methyl phenyl sulfoxide and/or methyl phenyl sulfone using tert-butyl hydroperoxide (tBHP) as the oxidant. In particular, MOFs of contrasting node architectures were targeted, presenting a unique opportunity to investigate the stereoelectronic control of Mo active sites in a systematic manner. A Zr6-based MOF, NU-1000, was employed along with its sulfated analogue Zr6-based NU-1000-SO4 to anchor a dioxomolybdenum species, which enabled examination of support-mediated active site polarizability on catalytic performance. In addition, a MOF containing a mixed metal node, Mo-MFU-4l, was used to probe the stereoelectronic impact of an N-donor ligand environment on the catalytic activity of the transmetalated Mo center. Characterization techniques, including single crystal X-ray diffraction, were concomitantly used with reaction time course profiles to better comprehend the dynamics of different Mo active sites, thus correlating structural change with activity.

Oxidative Grafting for Catalyst Synthesis in Surface Organometallic Chemistry.

The development of new methods of catalyst synthesis with the potential to generate active site structures orthogonal to those accessible by traditional protocols is of great importance for discovering new materials for addressing challenges in the evolving energy and chemical economy. In this work, the generality of oxidative grafting of organometallic and well-defined molecular metal precursors onto redox-active surfaces such as manganese dioxide (MnO2) and lithium manganese oxide (LiMn2O4) is investigated. Nine molecular metal precursors are explored, spanning groups 4-11 and each of the three periods of the transition metal series. The byproducts of the oxidative grafting reaction, a mixture of protodemetalation and ligand homocoupling for several organometallic precursors, was found to provide insights into the mechanism of the grafting reaction, suggesting oxidation of both the metal d-orbitals, as well as the metal-carbon σ-bonds, resulting in ejection of the ligand radical fragment. Analysis of the supported structures and oxidation state by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) suggests that several of the chemisorbed metal ions are intercalated into interstitial vacancies of the surface structure while other complexes form intact molecular fragments on the surface. Proof of concept for the use of this metalation protocol to generate diverse, metal-dependent catalytic performance is demonstrated by the application of these materials in the conversion of cyclohexane to K/A oil (cyclohexanol and cyclohexanone) with tert-butyl hydroperoxide, as well as in the low-temperature (T ≤ 50 °C) oxidation of carbon monoxide to carbon dioxide.

High-Throughput Experimentation, Theoretical Modeling, and Human Intuition: Lessons Learned in Metal-Organic-Framework-Supported Catalyst Design.

We have screened an array of 23 metals deposited onto the metal-organic framework (MOF) NU-1000 for propyne dimerization to hexadienes. By a first-of-its-kind study utilizing data-driven algorithms and high-throughput experimentation (HTE) in MOF catalysis, yields on Cu-deposited NU-1000 were improved from 0.4 to 24.4%. Characterization of the best-performing catalysts reveal conversion to hexadiene to be due to the formation of large Cu nanoparticles, which is further supported by reaction mechanisms calculated with density functional theory (DFT). Our results demonstrate both the strengths and weaknesses of the HTE approach. As a strength, HTE excels at being able to find interesting and novel catalytic activity; any a priori theoretical approach would be hard-pressed to find success, as high-performing catalysts required highly specific operating conditions difficult to model theoretically, and initial simple single-atom models of the active site did not prove representative of the nanoparticle catalysts responsible for conversion to hexadiene. As a weakness, our results show how the HTE approach must be designed and monitored carefully to find success; in our initial campaign, only minor catalytic performances (up to 4.2% yield) were achieved, which were only improved following a complete overhaul of our HTE approach and questioning our initial assumptions.

Enhanced Catalytic Performance of a Ce/V Oxo Cluster through Confinement in Mesoporous SBA-15.

To increase catalytic efficiency, mesoporous supports have been widely applied to immobilize well-defined metal oxide clusters due to their ability to stabilize highly dispersed clusters. Herein, a redox-active heterometallic Ce12V6-oxo cluster (CeV) was first presynthesized and then incorporated into mesoporous silica, SBA-15, via a straightforward impregnation method. Scanning transmission electron microscopy (STEM) and Fourier transform infrared spectroscopy (FTIR), in concert with scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS), verified the successful introduction of the CeV cluster inside the pore of SBA-15. The 51V magic angle spinning solid-state nuclear magnetic resonance (51V MAS NMR) spectroscopy and differential pair distribution function (dPDF) analysis confirmed the structural integrity of the CeV cluster inside the SBA-15. The composite was then benchmarked for liquid-phase oxidation of 2-chloroethyl ethyl sulfide (CEES) under mild conditions and gas-phase oxidative dehydrogenation (ODH) of propane under high temperatures (up to 550 °C). The catalytic reactivity results demonstrated 8- and 14-fold increase in turnover frequency (TOF) values of the composite (CeV@10SBA-2) than the bulk CeV cluster under the same conditions for CEES oxidation and ODH, respectively. These results highlight the improved reactivity of the catalytically active CeV cluster as attributed to the higher dispersion of the discrete cluster upon immobilization within the SBA-15 support.

Identification of a metastable uranium metal-organic framework isomer through non-equilibrium synthesis.

Since the structure of supramolecular isomers determines their performance, rational synthesis of a specific isomer hinges on understanding the energetic relationships between isomeric possibilities. To this end, we have systematically interrogated a pair of uranium-based metal-organic framework topological isomers both synthetically and through density functional theory (DFT) energetic calculations. Although synthetic and energetic data initially appeared to mismatch, we assigned this phenomenon to the appearance of a metastable isomer, driven by levers defined by Le Châtelier's principle. Identifying the relationship between structure and energetics in this study reveals how non-equilibrium synthetic conditions can be used as a strategy to target metastable MOFs. Additionally, this study demonstrates how defined MOF design rules may enable access to products within the energetic phase space which are more complex than conventional binary (e.g., kinetic vs. thermodynamic) products.

Sulfated Zirconium Metal-Organic Frameworks as Well-Defined Supports for Enhancing Organometallic Catalysis.

Understanding heterogeneous catalysts is a challenging pursuit due to surface site nonuniformity and aperiodicity in traditionally used materials. One example is sulfated metal oxides, which function as highly active catalysts and as supports for organometallic complexes. These applications are due to traits such as acidity, ability to act as a weakly coordinating ligand, and aptitude for promoting transformations via radical cation intermediates. Research is ongoing about the structural features of sulfated metal oxides that imbue the aforementioned properties, such as sulfate geometry and coordination. To better understand these materials, metal-organic frameworks (MOFs) have been targeted as structurally defined analogues. Composed of inorganic nodes and organic linkers, MOFs possess features such as high porosity and crystallinity, which make them attractive for mechanistic studies of heterogeneous catalysts. In this work, Zr6-based MOF NU-1000 is sulfated and characterized using techniques such as single crystal X-ray diffraction in addition to diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The dynamic nature of the sulfate binding motif is found to transition from monodentate, to bidentate, to tridentate depending on the degree of hydration, as supported by density functional theory (DFT) calculations. Heightened Brønsted acidity compared to the parent MOF was observed upon sulfation and probed through trimethylphosphine oxide physisorption, ammonia sorption, in situ ammonia DRIFTS, and DFT studies. With the support structure benchmarked, an organoiridium complex was chemisorbed onto the sulfated MOF node, and the efficacy of this supported catalyst was demonstrated for stoichiometric and catalytic activation of benzene-d6 and toluene with structure-activity relationships derived.

A Catalytically Accessible Polyoxometalate in a Porous Fiber for Degradation of a Mustard Gas Simulant.

Polyoxometalates (POMs) are versatile materials for chemical catalysis due to their tunable acidity and rich redox properties. While POMs have attracted significant attention in homogeneous catalysis, challenges regarding aggregation and instability in solvents often prevent the wide implementation of POMs as heterogeneous catalysts. Therefore, the successful incorporation of a POM into a solid support, such as a polymer, is desirable for practical applications where unique functionalities of the POM combine with the advantages of the polymer. In this work, we showcase how polymers of intrinsic microporosity (PIMs) can serve as matrices for anchoring a pure inorganic Keggin-type POM (H3PW12O40) to fabricate PIM-based composite materials. Specifically, we found that PIMs installed with amidoxime functionalities could successfully attach POMs (PW12@PIM-1-AO) without self-segregation. Furthermore, we fabricated porous fibrous mats via electrospinning of the PIM-POM composites. Comprehensive characterization confirmed the integrity of the POM in the composite material. Following this, we demonstrated that the incorporated POMs in the composite fibers maintained their innate catalytic activity for the oxidative degradation of 2-chloroethyl ethyl sulfide, a sulfur mustard simulant, in the presence of hydrogen peroxide as the oxidant. Ultimately, our work highlights that PIM-based hybrid materials provide a potential route for implementing these reactive fiber mats into protective equipment.

Direct Observation of Modulated Radical Spin States in Metal-Organic Frameworks by Controlled Flexibility.

Owing to their switchable spin states and dynamic electronic character, organic-based radical species have been invoked in phenomena unique to a variety of fields. When incorporated in solid state materials, generation of organic radicals proves challenging due to aggregation. Metal-organic frameworks (MOFs) are promising candidates for immobilization and stabilization of organic radicals because of the tunable spatial arrangement of organic linkers and metal nodes, which sequesters the reactive species. Herein, a flexible, redox-active tetracarboxylic acid linker bearing two imidazole units was chosen to construct a new Zr6-MOF, NU-910, with scu topology. By exploiting the structural flexibility of NU-910, we successfully modulate the dynamics between an isolated organic radical species and an organic radical π-dimer species in the MOF system. Single-crystal X-ray diffraction analysis reveals that through solvent exchange from N,N-diethylformamide to acetone, NU-910 undergoes a structural contraction with interlinker distances decreasing from 8.32 Å to 3.20 Å at 100 K. Organic radical species on the bridging linkers are generated via UV light irradiation. Direct observation of temperature-induced spin switches from an isolated radical species to a magnetically silent radical π-dimer in NU-910 after irradiation in the solid state was achieved via variable-temperature single-crystal X-ray diffraction and variable-temperature electron paramagnetic resonance spectroscopy. Ultraviolet-visible-near infrared spectroscopy and density functional theory calculations further substantiated the formation of a radical cation π-dimer upon irradiation. This work demonstrates the potential of using flexible MOFs as a platform to modulate radical spin states in the solid phase.

MOF-enabled confinement and related effects for chemical catalyst presentation and utilization.

A defining characteristic of nearly all catalytically functional MOFs is uniform, molecular-scale porosity. MOF pores, linkers and nodes that define them, help regulate reactant and product transport, catalyst siting, catalyst accessibility, catalyst stability, catalyst activity, co-catalyst proximity, composition of the chemical environment at and beyond the catalytic active site, chemical intermediate and transition-state conformations, thermodynamic affinity of molecular guests for MOF interior sites, framework charge and density of charge-compensating ions, pore hydrophobicity/hydrophilicity, pore and channel rigidity vs. flexibility, and other features and properties. Collectively and individually, these properties help define overall catalyst functional behaviour. This review focuses on how porous, catalyst-containing MOFs capitalize on molecular-scale confinement, containment, isolation, environment modulation, energy delivery, and mobility to accomplish desired chemical transformations with potentially superior selectivity or other efficacy, especially in comparison to catalysts in homogeneous solution environments.

Transient Catenation in a Zirconium-Based Metal-Organic Framework and Its Effect on Mechanical Stability and Sorption Properties.

Interpenetration of two or more sublattices is common among many metal-organic frameworks (MOFs). Herein, we study the evolution of one zirconium cluster-based, 3,8-connected MOF from its non-interpenetrated (NU-1200) to interpenetrated (STA-26) isomer. We observe this transient catenation process indirectly using ensemble methods, such as nitrogen porosimetry and X-ray diffraction, and directly, using high-resolution transmission electron microscopy. The approach detailed here will serve as a template for other researchers to monitor the interpenetration of their MOF samples at the bulk and single-particle limits. We investigate the mechanical stability of both lattices experimentally by pressurized in situ X-ray diffraction and nanoindentation as well as computationally with density functional theory calculations. Both lines of study reveal that STA-26 is considerably more mechanically stable than NU-1200. We conclude this study by demonstrating the potential of these MOFs and their mixed phases for the capture of gaseous n-hexane, used as a structural mimic for the chemical warfare agent sulfur mustard gas.

Unexpected "Spontaneous" Evolution of Catalytic, MOF-Supported Single Cu(II) Cations to Catalytic, MOF-Supported Cu(0) Nanoparticles.

A desirable feature of metal-organic frameworks (MOFs) is their well-defined structural periodicity and the presence of well-defined catalyst grafting sites (e.g., reactive -OH and -OH2 groups) that can support single-site heterogeneous catalysts. However, one should not overlook the potential role of residual organic moieties, specifically formate ions that can occupy the catalyst anchoring sites during MOF synthesis. Here we show how these residual formate species in a Zr-based MOF, NU-1000, critically alter the structure, redox capability, and catalytic activity of postsynthetically incorporated Cu(II) ions. Single-crystal X-ray diffraction measurements established that there are two structurally distinct types of Cu(II) ions in NU-1000: one type with residual formate and one without. In NU-1000 with formate, Cu(II) solely binds to the node via the formate-unoccupied, bridging µ3-OH, whereas in the formate-free case, it displaces protons from two node hydroxo ligands and resides close to the terminal -OH2. Under an inert atmosphere, node-bound formate facilitates the unanticipated reduction of isolated Cu(II) to nanoparticulate Cu(0)-a behavior which is essentially absent in the formate-free analogue because no other sacrificial reductant is present. When the two MOFs were tested as benzyl alcohol oxidation catalysts, we observed that residual formate boosts the catalytic turnover frequency. Density functional calculations showed that node-bound formate acts as a sacrificial two-electron donor and assists in reducing Cu(II) to Cu(0) by a nonradical pathway. The negative Gibbs free energy of reaction (ΔG) and enthalpy of reaction (ΔH) indicate that the reduction is thermodynamically favorable. The work presented here highlights how the often-neglected residual formate prevalent in nearly all zirconium-based MOFs can significantly modulate the properties of supported catalysts.

Integration of Metal-Organic Frameworks on Protective Layers for Destruction of Nerve Agents under Relevant Conditions.

Metal-organic frameworks (MOFs) are promising candidates for the catalytic hydrolysis of nerve agents and their simulants. Though highly efficient, bulk water and volatile bases are often required for hydrolysis with these MOF catalysts, preventing real-world implementation. Herein we report a generalizable and scalable approach for integrating MOFs and non-volatile polymeric bases onto textile fibers for nerve agent hydrolysis. Notably, the composite material showed similar reactivity under ambient conditions compared to the powder material in aqueous alkaline solution. This represents a critical step toward a unified strategy for nerve agent hydrolysis in practical settings, which can significantly reduce the dimensions of filters and increase the efficiency of protective suits.

Electrophilic Organoiridium(III) Pincer Complexes on Sulfated Zirconia for Hydrocarbon Activation and Functionalization.

Single-site supported organometallic catalysts bring together the favorable aspects of homogeneous and heterogeneous catalysis while offering opportunities to investigate the impact of metal-support interactions on reactivity. We report a ( dmPhebox)Ir(III) ( dmPhebox = 2,6-bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) complex chemisorbed on sulfated zirconia, the molecular precursor for which was previously applied to hydrocarbon functionalization. Spectroscopic methods such as diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS), dynamic nuclear polarization (DNP)-enhanced solid-state nuclear magnetic resonance (SSNMR) spectroscopy, and X-ray absorption spectroscopy (XAS) were used to characterize the supported species. Tetrabutylammonium acetate was found to remove the organometallic species from the surface, enabling solution-phase analytical techniques in conjunction with traditional surface methods. Cationic character was imparted to the iridium center by its grafting onto sulfated zirconia, imbuing high levels of activity in electrophilic C-H bond functionalization reactions such as the stoichiometric dehydrogenation of alkanes, with density functional theory (DFT) calculations showing a lower barrier for ß-H elimination. Catalytic hydrogenation of olefins was also facilitated by the sulfated zirconia-supported ( dmPhebox)Ir(III) complex, while the homologous complex on silica was inactive under comparable conditions.