UC Berkeley

Metal–organic frameworks have emerged as a class of porous materials with structures that can be defined by combining synthetic chemistry with precise characterization through crystallography. The work compiled in this dissertation pursues the study of these materials for many different applications, ranging from gas storage and molecular separations to cluster synthesis and catalysis. These investigations have all relied on single-crystal X-ray diffraction to provide insight into the influence of framework structure on adsorption properties and reactivity, ultimately leading to the discovery of new chemical species and behavior.

Chapter 1 serves as a brief introduction to the role of single-crystal X-ray diffraction in advancing research on metal–organic frameworks. In particular, the reports discussed in this chapter exemplify experiments that have been critical in establishing that these materials can be designed to both retain porosity and bind guests through specific interactions. A few examples were chosen to feature relevant collaborative work conducted alongside the studies presented in the succeeding chapters.

Chapter 2 describes the development of in situ single-crystal X-ray diffraction techniques that have enabled the direct observation of CO, CH4, N2, O2, Ar, and P4 adsorption in the metal–organic framework Co2(dobdc) (dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate), which features a high density of coordinatively unsaturated cobalt(II) centers. These molecules exhibit such weak interactions with the high-spin cobalt(II) sites that no analogous molecular structures exist. Several of these structures have also led to the location of secondary and tertiary binding sites in the framework. Analysis of gas adsorption isotherms confirms that these gases bind to the cobalt(II) sites through mainly physisorptive interactions and that secondary binding sites become more relevant at elevated pressures.

While gas storage and gas separations have been the two most prominent applications for metal–organic frameworks, these materials have also shown promise as adsorbents for liquid-phase separations. Chapter 3 reports the evaluation of Co2(dobdc) and its structural isomer Co2(m-dobdc) (m-dobdc4− = 4,6-dioxido-1,3-benzenedicarboxylate) for the separation of xylene isomers using single-component adsorption isotherms and multi-component breakthrough measurements. The framework Co2(dobdc) distinguishes among all four molecules, with binding affinities that follow the trend o-xylene > ethylbenzene > m-xylene > p-xylene. Structural characterization by single-crystal X-ray diffraction reveals that both frameworks facilitate the separation of these isomers through the extent of interaction between each C8 guest molecule and two adjacent cobalt(II) centers, as well as the ability of each isomer to pack within the framework pores. Moreover, in the presence of either o-xylene or ethylbenzene, Co2(dobdc) exhibits an unexpected structural distortion that increases its adsorption capacity for these guest molecules.

Metal–organic frameworks featuring ligands with open chelating sites have proven to be versatile platforms for the preparation of heterogeneous catalysts through post-synthetic metalation. Chapter 4 details initial efforts toward the application of these frameworks as heterogeneous catalysts with crystallographically-defined active sites. In particular, a highly porous and thermally robust metal–organic framework, Zr6O4(OH)4(bpydc)6 (bpydc2– = 2,2-bipyridne-5,5-dicarboxylate), bears open bipyridine sites that readily react with a variety of solution- and gas-phase metal sources. Upon metalation, this framework undergoes a single-crystal-to-single-crystal transformation that enables precise structural determination of the resulting metal–linker complexes. Furthermore, the framework yields an active heterogeneous catalyst for arene C–H borylation when metalated with [Ir(COD)2]BF4 (COD = 1,5-cyclooctadiene).

Chapter 5 builds upon the work in Chapter 4 and leverages structural insight afforded by crystallography to investigate pore environment effects on ethylene oligomerization in the metal–organic frameworks Zr6O4(OH)4(bpydc)6 and Zr6O4(OH)4(bpydc)0.84(bpdc)5.16 (bpdc2– = bpdc2– = biphenyl-4,4-dicarboxylate). In these systems, the pore structure around the active nickel sites significantly influences their selectivity for formation of oligomers over polymer. Specifically, the single-crystal structure of Zr6O4(OH)4(bpydc)6(NiBr2)5.64 indicates that neighboring metal–linker complexes enforce a steric environment on each nickel site that causes polymer formation to become favorable. Minimizing this steric congestion by isolating the nickel(II)–bipyridine complexes in the mixed-linker framework Zr6O4(OH)4(bpydc)0.84(bpdc)5.16 markedly improves both catalytic activity and selectivity for oligomers. Furthermore, both frameworks give product mixtures that are enriched in shorter olefins (C4–10), leading to deviations from the expected Schulz-Flory distribution of oligomers. Although these deviations indicate possible pore confinement effects on selectivity, control experiments reveal that they likely arise at least in part from the presence of nickel species that are not ligated by the bipyridine within both frameworks.

Finally, Chapter 6 demonstrates that a metal–organic framework can act as a multidentate ligand scaffold to template the formation of discrete inorganic clusters, enabling their stabilization within a porous crystalline support. The framework Zr6O4(OH)4(bpydc)6 confines the growth of atomically-defined nickel(II) bromide, nickel(II) chloride, cobalt(II) chloride, and iron(II) chloride sheets through coordination of six chelating bipyridine linkers. Characterization by single-crystal X-ray diffraction reveals that each metal(II) halide sheet represents a fragment excised from a single layer in the bulk solid. Moreover, structures obtained at different precursor loadings allow for the observation of successive stages of cluster assembly. Magnetic susceptibility measurements demonstrate that the isolated clusters exhibit behavior distinct from that of their corresponding bulk materials.