Post-Synthetic Cyano-ferrate(II) Functionalization of a Metal-Organic Framework, NU-1000.

Starting with ferrocyanide ions in acidic aqueous solution, cyano-ferrate(II) species are post-synthetically grafted to the nodes of a mesoporous zirconium-based MOF, NU-1000. As indicated by single-crystal X-ray crystallography, grafting occurs by substitution of cyanide ligands by node-based hydroxo and oxo ligands rather than by substitution of node aqua ligands by cyanide ligands as bridges between Fe(II) and Zr(IV). The installed moieties yield a broad absorption band that is tentatively ascribed to iron-to-zirconium charge transfer. Consistent with Fe(III/II) redox activity, a modest fraction of the installed iron complexes are directly electrochemically addressable.

Stabilization of an enzyme cytochrome c in a metal-organic framework against denaturing organic solvents.

Enzymes are promising catalysts with high selectivity and activity under mild reaction conditions. However, their practical application has largely been hindered by their high cost and poor stability. Metal-organic frameworks (MOFs) as host materials show potential in protecting proteins against denaturing conditions, but a systematic study investigating the stabilizing mechanism is still lacking. In this study, we stabilized enzyme cytochrome c (cyt c) by encapsulating it in a hierarchical mesoporous zirconium-based MOF, NU-1000 against denaturing organic solvents. Cyt c@NU-1000 showed a significantly enhanced activity compared to the native enzyme, and the composite retained this enhanced activity after treatment with five denaturing organic solvents. Moreover, the composite was recyclable without activity loss for at least three cycles. Our cyt c@NU-1000 model system demonstrates that enzyme@MOF composites prepared via post-synthetic encapsulation offer a promising route to overcome the challenges of enzyme stability and recyclability that impede the widespread adoption of biocatalysis.

Thermochemical Investigation of Oxyanion Coordination in a Zirconium-Based Metal-Organic Framework.

Porous materials possess high internal surface areas and void fractions that make them valuable in several applications, including gas storage, heterogeneous catalysis, and water purification. Despite the plentiful effort allocated to porous materials research annually, few methods exist to directly monitor and characterize chemical events occurring within a pore's confines. The crystalline nature of zeolites, covalent organic frameworks (COFs), and metal-organic frameworks (MOFs) permit structural characterization by X-ray diffraction; yet, quantifying the thermodynamics of chemical processes and transformations remains tedious and error ridden. Herein, we employ isothermal titration calorimetry (ITC) to determine the full thermodynamic profile of oxyanion adsorption in a zirconium-based MOF, NU-1000. To further validate this method, which we recently introduced to the field, we replicated ITC experiments as bulk adsorption measurements to demonstrate the correlation between the extracted stoichiometric parameter from ITC thermograms and the MOF uptake capacity. Moreover, based on the calculated association constants, we accurately predicted which analytes might be able to displace others. For example, dihydrogen phosphate can displace selenate and sulfate because of its higher association constant (ΔGphosphate = -5.41 kcal/mol; ΔGselenate = -4.98 kcal/mol; ΔGsulfate = -4.77 kcal/mol). We monitored the exchange processes by titrating oxyanion-functionalized MOF samples with a more strongly binding analyte.

Insights into the Structure-Activity Relationship in Aerobic Alcohol Oxidation over a Metal-Organic-Framework-Supported Molybdenum(VI) Catalyst.

The understanding of structure-activity relationships at the atomic level has played a profound role in heterogeneous catalysis, providing valuable insights into designing suitable heterogeneous catalysts. However, uncovering the detailed roles of how such active species' structures affect their catalytic performance remains a challenge owing to the lack of direct structural information on a specific active species. Herein, we deposited molybdenum(VI), an active species in oxidation reactions, on the Zr6 node of a mesoporous zirconium-based metal-organic framework (MOF) NU-1200, using solvothermal deposition in MOFs (SIM). Due to the high crystallinity of the NU-1200 support, the precise structure of the resulting molybdenum catalyst, Mo-NU-1200, was characterized through single-crystal X-ray diffraction (SCXRD). Two distinct anchoring modes of the molybdenum species were observed: one mode (Mo1), displaying an octahedral geometry, coordinated to the node through one terminal oxygen atom and the other mode (Mo2) coordinated to two adjacent Zr6 node oxygen atoms in a tetrahedral geometry. To investigate the role of base in the catalytic activity of these Mo centers, we assessed the activity of Mo-NU-1200 for the aerobic oxidation of 4-methoxybenzyl alcohol as a model reaction. The results revealed that Mo-NU-1200 exhibited remarkably higher catalytic reactivity under base-free conditions, while the presence of base inhibited the catalytic reactivity of this species. SCXRD studies revealed that the molybdenum binding motifs (structures of the supported metal on the Zr6 node in the MOF) changed over the course of the reactions. Following the oxidation without base, both pristine coordination modes (Mo1 and Mo2) evolved into a new coordination mode (Mo3), in which the molybdenum atom coordinated to two adjacent oxygen atoms from the Zr6 node in an octahedral geometry, while in the presence of base, the pristine Mo1 coordination mode evolved entirely into the pristine Mo2. This study demonstrates the direct observation of an active species' structural evolution from metal installation to subsequent catalytic reaction. As a result, these subtle structural changes in catalyst binding motifs led to distinct differences in catalytic activities, providing a compelling strategy for elucidating structure-activity relationships.

Proton Conductivity via Trapped Water in Phosphonate-Based Metal-Organic Frameworks Synthesized in Aqueous Media.

Metal-organic frameworks (MOFs) are promising candidates for proton-conducting applications. Herein, we report the aqueous synthesis of two new phosphonate-based MOFs comprising glyphosate linkers, [Mg(dpmp)]·2H2O (Mg-NU-225) and [Fe(dpmp)]·2H2O (Fe-NU-225), (dpmp = N,N'-diphosphonomethyl-2,5-piperazinedione), and explore their proton conductivities. Single crystal X-ray diffraction measurements revealed that both frameworks display a two-dimensional layered structure with a cyclic ring ligand which forms in situ from the condensation of two glyphosate molecules. Under humid conditions and over a wide temperature range, water molecules are trapped between adjacent layers and facilitate rapid proton conduction. Mg-NU-225 and Fe-NU-225 recorded proton conductivities of 1.5 × 10-5 and 1.7 × 10-5 S cm-1, respectively, along the plane direction and 1.6 × 10-3 and 9.1 × 10-5 S cm-1 perpendicular to the plane direction at 55 °C and 95% relative humidity, as confirmed by two-contact probe impedance methods. The mechanism of proton transport was found to be that of the Grotthuss model from the low activation energy for proton hopping.

Spherical Nucleic Acids: Integrating Nanotechnology Concepts into General Chemistry Curricula.

Nanoscience and technology research offer exciting avenues to modernize undergraduate-level General Chemistry curricula. In particular, spherical nucleic acid (SNA) nanoconjugates, which behave as "programmable atom equivalents" (PAEs) in the context of colloidal crystals, are one system that one can use to reinforce foundational concepts in chemistry including matter and atoms, the Periodic Table, Lewis dot structures and the octet rule, valency and valence-shell electron-pair repulsion (VSEPR) theory, and Pauling's rules, ultimately leading to enriching discussions centered on materials chemistry and biochemistry with key implications in medicine, optics, catalysis, and other areas. These lessons connect historical and modern concepts in chemistry, relate course content to current professional and popular science topics, inspire critical and creative thinking, and spur some students to continue their science, technology, engineering, and mathematics (STEM) education and attain careers in STEM fields. Ultimately, and perhaps most importantly, these lessons may expand the pool of young students interested in chemistry by making connections to a broader group of contemporary concepts and technologies that impact their lives and enhance their view of the field. Herein, a way of teaching aspects of General Chemistry in the context of modern nanoscience concepts is introduced to instructors and curricula developers at research institutions, primarily undergraduate institutions, and community colleges worldwide.

The state of the field: from inception to commercialization of metal-organic frameworks.

As chemists and materials scientists, it is our duty to synthesize and utilize materials for a multitude of applications that promote the development of society and the well-being of its citizens. Since the inception of metal-organic frameworks (MOFs), researchers have proposed a variety of design strategies to rationally synthesize new MOF materials, studied their porosity and gas sorption performances, and integrated MOFs onto supports and into devices. Efforts have explored the relevance of MOFs for applications including, but not limited to, heterogeneous catalysis, guest delivery, water capture, destruction of nerve agents, gas storage, and separation. Recently, several start-up companies have undertaken MOF commercialization within industrial sectors. Herein, we provide a brief overview of the state of the MOF field from their design and synthesis to their potential applications, and finally, to their commercialization.

Tuning the Atrazine Binding Sites in an Indium-Based Flexible Metal-Organic Framework.

Constructing flexible metal-organic frameworks (MOFs) with targeted properties is of high interest given their demonstrated potential as smart materials that undergo structural transformations in response to external stimuli. Herein, we report a flexible and interpenetrated indium-based MOF, NU-50, comprising four-connected [In(CO2)4]- nodes and tetracarboxylate pyrene-based ligands assembled in the pts topology. The flexible framework of NU-50 exhibits intricate structural transformations upon exposure to external stimuli, namely, guest solvent molecules and elevated temperatures. The high density of pyrene moieties throughout the interpenetrated framework offers numerous sites for the adsorption of highly conjugated guest molecules such as atrazine via π-π interactions. As a result, NU-50 efficiently removes atrazine from water, achieving a maximum atrazine uptake capacity of 74 mg of atrazine per gram of NU-50. Molecular simulations reveal that the dynamic behavior of NU-50 involves the distortion of metal-ligand bonds, resulting in a narrow pore structure that affords effective adsorption of atrazine molecules in a sandwich-like geometry. Moreover, washing in acetone quickly regenerates the sorbent.

Maximizing Magnetic Resonance Contrast in Gd(III) Nanoconjugates: Investigation of Proton Relaxation in Zirconium Metal-Organic Frameworks.

Gadolinium(III) nanoconjugate contrast agents (CAs) provide significant advantages over small-molecule complexes for magnetic resonance imaging (MRI), namely increased Gd(III) payload and enhanced proton relaxation efficiency (relaxivity, r1). Previous research has demonstrated that both the structure and surface chemistry of the nanomaterial substantially influence contrast. We hypothesized that inserting Gd(III) complexes in the pores of a metal-organic framework (MOF) might offer a unique strategy to further explore the parameters of nanomaterial structure and composition, which influence relaxivity. Herein, we postsynthetically incorporate Gd(III) complexes into Zr-MOFs using solvent-assisted ligand incorporation (SALI). Through the study of Zr-based MOFs, NU-1000 (nano and micronsize particles) and NU-901, we investigated the impact of particle size and pore shape on proton relaxivity. The SALI-functionalized Gd nano NU-1000 hybrid material displayed the highest loading of the Gd(III) complex (1.9 ± 0.1 complexes per node) and exhibited the most enhanced proton relaxivity (r1 of 26 ± 1 mM-1 s-1 at 1.4 T). Based on nuclear magnetic relaxation dispersion (NMRD) analysis, we can attribute the performance of Gd nano NU-1000 to the nanoscale size of the MOF particles and larger pore size that allows for rapid water exchange. We have demonstrated that SALI is a promising method for incorporating Gd(III) complexes into MOF materials and identified crucial design parameters for the preparation of next generation Gd(III)-functionalized MOF MRI contrast agents.

Isothermal Titration Calorimetry to Explore the Parameter Space of Organophosphorus Agrochemical Adsorption in MOFs.

The expansion of manufacturing and commercial agriculture alongside rapid globalization have resulted in the widespread contamination of freshwater supplies with chemical toxins including persistent organic pollutants. Effective mitigation of such pollution is paramount to the safeguarding of human health, animal and aquatic life, and the environment. Currently, adsorption is the most economically viable water purification strategy. Owing to their crystallinity and modular nature, metal-organic frameworks (MOFs) are an excellent platform material for systematically investigating the physical and chemical properties which govern adsorption processes. X-ray diffraction techniques provide atomically precise descriptions of toxin-MOF interactions, while liquid-phase adsorption isotherms readily allow for the determination of uptake capacity and kinetics; however, determination of the thermodynamics of toxin-MOF interactions in aqueous media remains tedious. Herein, we add isothermal titration calorimetry (ITC) to our arsenal of techniques for characterizing adsorption mechanisms in MOFs. With this method, we are able to directly quantify the full thermodynamic profile of a chemical process (Ka, ΔG, ΔH, TΔS), providing critical details to support the rational design of next-generation sorbents. We demonstrate the suitability of ITC through our exploration of the parameter space of organophosphorus agrochemical adsorption in zirconium-based MOFs.

Exploiting π-π Interactions to Design an Efficient Sorbent for Atrazine Removal from Water.

The United States Environmental Protection Agency (EPA) recognizes atrazine, a commonly used herbicide, as an endocrine disrupting compound. Excessive use of this agrochemical results in contamination of surface and ground water supplies via agricultural runoff. Efficient removal of atrazine from contaminated water supplies is paramount. Here, the mechanism governing atrazine adsorption in Zr6-based metal-organic frameworks (MOFs) has been thoroughly investigated by studying the effects of MOF linkers and topology on atrazine uptake capacity and uptake kinetics. We found that the mesopores of NU-1000 facilitated rapid atrazine uptake saturating in <5 min and that the pyrene-based linkers offered sufficient sites for π-π interactions with atrazine as demonstrated by the near 100% uptake. Without the presence of a pyrene-based linker, NU-1008, a MOF similar to NU-1000 with respect to surface area and pore size, removed <20% of the exposed atrazine. These results suggest that the atrazine uptake capacity demonstrated by NU-1000 stems from the presence of a pyrene core in the MOF linker, affirming that π-π stacking is responsible for driving atrazine adsorption. Furthermore, NU-1000 displays an exceptional atrazine removal capacity through three cycles of adsorption-desorption. Powder X-ray diffraction and Brunauer-Emmett-Teller surface area analysis confirmed the retention of MOF crystallinity and porosity throughout the adsorption-desorption cycles.

Direct Imaging of Isolated Single-Molecule Magnets in Metal-Organic Frameworks.

Practical applications involving the magnetic bistability of single-molecule magnets (SMMs) for next-generation computer technologies require nanostructuring, organization, and protection of nanoscale materials in two- or three-dimensional networks, to enable read-and-write processes. Owing to their porous nature and structural long-range order, metal-organic frameworks (MOFs) have been proposed as hosts to facilitate these efforts. Although probing the channels of MOF composites using indirect methods is well established, the use of direct methods to elucidate fundamental structural information is still lacking. Herein we report the direct imaging of SMMs encapsulated in a mesoporous MOF matrix using high-resolution transmission electron microscopy. These images deliver, for the first time, direct and unambiguous evidence to support the adsorption of molecular guests within the porous host. Bulk magnetic measurements further support the successful nanostructuring of SMMs. The preparation of the first magnetic composite thin films of this kind furthers the development of molecular spintronics.

A Bismuth Metal-Organic Framework as a Contrast Agent for X-ray Computed Tomography.

A new bismuth metal-organic framework (MOF), bismuth-NU-901 (Bi-NU-901), featuring the scu topology and a pore with a diameter of ∼11 Å, was solvothermally synthesized, and its use as an X-ray computed tomography (CT) contrast agent was tested. X-ray CT is a common diagnostic method used in the medical field. Inside the body, contrast media enhance the distinction between tissues and organs of similar density. Bi-NU-901 consists of eight connected Bi6 nodes and tetratopic 1,3,5,8-(p-benzoate)pyrene linkers (TBAPy). Numerous material characterization studies including powder X-ray diffraction (PXRD), scanning transmission electron microscopy (STEM), and DFT pore size distribution support the scu structure. Additionally, given the framework's high density of nontoxic heavy atoms, Bi-NU-901 was evaluated as an X-ray computed tomography (CT) agent. Importantly, in vitro studies revealed this new bismuth MOF demonstrates ∼7 times better contrast intensity compared to a zirconium MOF featuring the same topology and ∼14 times better contrast than a commercially available CT contrast agent. These results suggest bismuth MOFs may be promising CT contrast agents.

Inorganic "Conductive Glass" Approach to Rendering Mesoporous Metal-Organic Frameworks Electronically Conductive and Chemically Responsive.

A representative mesoporous metal-organic-framework (MOF) material, NU-1000, has been rendered electronically conductive via a robust inorganic approach that permits retention of MOF crystallinity and porosity. The approach is based on condensed-phase grafting of molecular tin species onto the MOF nodes via irreversible reaction with hydroxyl and aqua ligands presented at the node surface, a self-limiting process termed solvothermal installation (of metal ions) in MOFs (SIM, a solution-phase analog of atomic layer deposition in MOFs). Treatment of the modified MOF with aerated steam at 120 °C converts the grafted tin molecules to tetratin(IV)oxy clusters, with the clusters being sited between insulating pairs of zirconia-like nodes (the zirconium component being key to endowing the parent material with requisite chemical and thermal stability). By introducing new O-H presenting ligands on the modified-MOF node, the high-temperature steam step additionally serves to reset the material to reactive form, thus enabling a second self-limiting tin-grafting step to be run (and after further steam treatment, enabling a third). Difference-envelop-density (DED) analyses of synchrotron-derived X-ray scattering data, with and without installed tin species, show that the clusters formed after one cycle are spatially isolated, but that repetitive SIM cycling adds metal and oxygen ions in a way that enshrouds nodes, links clusters, and yields continuous one-dimensional strands of oxy-tin(IV), oriented exclusively along the c axis of the MOF. Two-probe conductivity measurements show that the parent MOF and the version containing isolated oxy-tin(IV) clusters are electrically insulating, but that the versions featuring continuous strands show an electrical conductivity of 1.8 × 10-7 S/cm after three Sn-SIM cycles. When combined with interdigitated microelectrodes, the solvent-free and conductive-glass-modified material (three Sn-SIM cycles) displays a substantial and persistent increase in electrical conductivity during exposure to 5% H2, indicating a role for dissociated H2 as an electronic dopant. The increase can be repetitively reversed by alternating H2 with air, illustrating the ability of the conductive MOF to function as a resistive sensor for H2 and suggesting further potential applications that may capitalize on the combination of high volumetric surface area, high mesoporosity, high chemical and thermal stability, and significant electrical conductivity.

Acid-Resistant Mesoporous Metal-Organic Framework toward Oral Insulin Delivery: Protein Encapsulation, Protection, and Release.

Diabetes affects millions of people worldwide and the number of diagnoses continues to climb annually. Though several effective medications and therapeutic methods have been developed to treat type 1 (T1DM) and type 2 (T2DM) diabetes mellitus, direct insulin injection remains the only effective treatment for insulin resistant (IR) diabetes patients. Here, we immobilize insulin in a crystalline mesoporous metal-organic framework (MOF), NU-1000, and obtain a high loading of ∼40 wt % in only 30 min. The acid-stable MOF capsules are found to effectively prevent insulin from degrading in the presence of stomach acid and the digestive enzyme, pepsin. Furthermore, the encapsulated insulin can be released from NU-1000 under simulated physiological conditions.