Cation exchange at the secondary building units of metal–organic frameworks

Cation exchange is an emerging synthetic route for modifying the secondary building units (SBUs) of metal–organic frameworks (MOFs). This technique has been used extensively to enhance the properties of nanocrystals and molecules, but the extent of its applications for MOFs is still expanding. To harness cation exchange as a rational tool, we need to elucidate its governing factors. Not nearly enough experimental observations exist for drawing these conclusions, so we provide a conceptual framework for approaching this task. We address which SBUs undergo exchange, why certain ions replace others, how the framework influences the process, the role of the solvent, and current applications. Using these guidelines, certain trends emerge from the available data and missing experiments become obvious. If future studies follow this framework, then a more comprehensive body of observations will furnish a deeper understanding of cation exchange and inspire future applications.United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-SC0006937)3M CompanyAlfred P. Sloan FoundationResearch Corporation for Science AdvancementNational Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374)MISTI (Hayashi Seed Fund

Dynamic Structural Flexibility of Fe-MOF-5 Evidenced by ⁵⁷Fe Mössbauer Spectroscopy

Temperature-dependent ⁵⁷Fe Mössbauer spectra were collected on Fe[subscript x]Zn[subscript 4−x](1,4-benzenedicarboxylate)₃ (Fe-MOF-5). When measured under an Ar atmosphere, the data at higher temperatures reveal thermal population of the lowest-lying electronic excited state, as expected for low symmetry tetrahedral ferrous ions. In the presence of N₂, however, the temperature dependence becomes exaggerated and the spectra cannot be fitted to a single species. A fluctuating electric field gradient at the Fe nuclei best explains these data and suggests dynamic structural distortions induced by weak interactions with N₂. This direct evidence of dynamic behaviour at MOF open metal sites is relevant for the use of MOF SBUs in catalysis, gas separation, and other applications that invoke similar phenomena

Selective Dimerization of Ethylene to 1-Butene with a Porous Catalyst

Current heterogeneous catalysts lack the fine steric and electronic tuning required for catalyzing the selective dimerization of ethylene to 1-butene, which remains one of the largest industrial processes still catalyzed by homogeneous catalysts. Here, we report that a metal–organic framework catalyzes ethylene dimerization with a combination of activity and selectivity for 1-butene that is premier among heterogeneous catalysts. The capacity for mild cation exchange in the material MFU-4l (MFU-4l = Zn[subscript 5]Cl[subscript 4](BTDD)[subscript 3], H[subscript 2]BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin) was leveraged to create a well-defined and site-isolated Ni(II) active site bearing close structural homology to molecular tris-pyrazolylborate complexes. In the presence of ethylene and methylaluminoxane, the material consumes ethylene at a rate of 41,500 mol per mole of Ni per hour with a selectivity for 1-butene of up to 96.2%, exceeding the selectivity reported for the current industrial dimerization process.Saudi AramcoAmerican Society for Engineering Education. National Defense Science and Engineering Graduate Fellowship3M CompanyResearch Corporation for Science Advancement. Cottrell Scholars ProgramAlfred P. Sloan Foundatio

The reactivity and cation exchange of MOF-5

Thesis: Ph. D. in Inorganic Chemistry, Massachusetts Institute of Technology, Department of Chemistry, 2015.Vita. Cataloged from PDF version of thesis.Includes bibliographical references (pages 245-258).The aim of this thesis is to demonstrate that the inorganic clusters within MOF-5 can be derivatized with redox-active cations for subsequent use in coordination chemistry and small-molecule activation. Rather than reproduce known metal species, this work harnesses the intrinsic properties of the MOF-5 lattice to isolate species that are difficult or impossible to achieve with solution-phase molecules or even other materials. Most of these MOF-5 variants are synthesized through a technique known as cation exchange-a process not well understood. Part 1, chapters 1-6, is devoted to studying the parameters that govern this phenomenon and how they may be manipulated. The first chapter surveys the known examples of cation exchange at the secondary building units of MOFs and poses questions to guide future studies. Chapter 2 reports the isolation of a Ni2+-exchanged variant of this material and demonstrates that this unusual species and changes to its coordination environment can be monitored by conventional methods. Chapter 3 establishes that the original Zn 2+ in MOF-5 also interact with coordinating ligands, but with no more than one Zn 2+ in each cluster interacting at a time. Chapter 4 proposes that this observation explains why only one Zn2+ is replaceable by 0h cations and provides a strategy for replacing the remaining Zn2+ ions. In chapter 5, methods for analyzing crystallographic data are presented for locating and quantifying the occupancy of cations inserted into a MOF. Finally, Chapter 6 describes the solvent dependence of the kinetics and thermodynamics of cation exchange. Part 2, chapters 7-9, describes the reactivity of MOF-5 after replacement with redox-active cations. Chapter 7 provides evidence that the Fe2+ sites in Fe-MOF-5 possess the flexibility and reactivity to interact with N2 . In Chapter 8, Ti3+, V3+, V2+, Cr3+, Cr2+, Mn2+, and Fe2+ variants are shown to undergo electron transfer. Finally, we conclude with evidence that Fe-MOF-5 promotes the disproportionation of NO. Together, these chapters lay the foundation for the eventual goal of heterogeneous catalysis at well-defined metal sites in MOFs.by Carl Kavanaugh Brozek.Ph. D. in Inorganic Chemistr

Dynamic DMF Binding in MOF-5 Enables the Formation of Metastable Cobalt-Substituted MOF-5 Analogues

Multinuclear solid-state nuclear magnetic resonance, mass spectrometry, first-principles molecular dynamics simulations, and other complementary evidence reveal that the coordination environment around the Zn2+ ions in MOF-5, one of the most iconic materials among metal–organic frameworks (MOFs), is not rigid. The Zn2+ ions bind solvent molecules, thereby increasing their coordination number, and dynamically dissociate from the framework itself. On average, one ion in each cluster has at least one coordinated N,N-dimethylformamide (DMF) molecule, such that the formula of as-synthesized MOF-5 is defined as Zn4O(BDC)3(DMF)x (x = 1–2). Understanding the dynamic behavior of MOF-5 leads to a rational low-temperature cation exchange approach for the synthesis of metastable Zn4–xCoxO(terephthalate)3 (x > 1) materials, which have not been accessible through typical high-temperature solvothermal routes thus far.National Science Foundation (U.S.) (NSF CAREER Award (DMR-1452612))National Science Foundation (U.S.) (NSF Graduate Research Fellowship Grant 1122374)National Institutes of Health (U.S.) (grant EB002026)Natural Sciences and Engineering Research Council of CanadaGovernment of Canada (Banting postdoctoral fellowship