Thioether and Thiol-Equipped Porous Frameworks: Synthetic Breakthroughs, Rich Functionalities, and Heavy Metal Uptake


Student thesis: Doctoral Thesis

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Award date18 May 2018


The opening chapter provides a critical review on two topical conductive properties, namely, electronic and ionic conductivities, related to the burgeoning porous materials field of metal-organic frameworks (MOFs). The intense current interest stems from fundamental importance as well as technological relevance for energy conversion and storage technologies (e.g., batteries, photovoltaics, fuel cells and sensing devices), and recent progresses have been extensively reviewed. The raison d'être of the review, consequently, lies in the personal account and reflections weaved into the fabric of current developments of the two areas. For the part of electronic conductivity, large, fused aromatics (e.g., triphenylene or even larger π-electron building blocks) figure prominently because of the intrinsically rich and diverse electronic properties that can be accessed for the synthesis of potentially highly conductive networks in the solid state. For the part of ionic conductivity, we characterize the unifying design scheme as one of establishing strong anionicity on the host frameworks so as to facilitate the incorporation of potentially mobile protons or other cations in the porous domain. Interestingly, the pursuit of the two apparently disparate conductive properties can both be greatly facilitated by the common and abundant element of sulfur, because of its versatile chemical functionalities. For example, whereas thiol-equipped porous MOF grid can readily lock in soft and polarizable metal ions to install metal-thiolate bridges across the organic π-electron systems, and thus to potentially boost electronic transport throughout the solid state framework, similar thiol-equipped net can be treated with oxidants (e.g., H2O2 or O3) to generate post-synthetically the highly ionic sulfonate function to impart strong ionic conductivity.

The above review highlights the unique and importance of versatility of sulfur functions in the development of conductive materials. One major challenge, however, remains in the synthesis of the sulfur-containing building blocks because of the reactivity of sulfur groups such as thiols. The integration of such building blocks into well-defined structure also poses complication. To pay the way for better exploring the sulfur-equipped porous frameworks, this thesis focuses on synthetic methodology not only with regard to the molecular synthesis of building blocks, but also to the supramolecular assembly of frameworks.

In chapter 2, we started with a thioether-equipped system. In particular, we discover that the ligand forms a zirconium-based porous metal-organic framework not only stable in long-term exposure in air, but also in boiling water as well. The Zr(IV)-carboxylate MOF decorated with thioether functions [i.e., Zr6O4(OH)4(L)6; L: 3,3',5,5'-tetrakis(methylthio)biphenyl dicarboxylate] adopts the space group Fm3 ̅m (no. 225), consisting of an fcc (face-centered cubic) array of Zr6O4(OH)4 clusters bridged by the carboxylate units of the L linker to give the fcu topology. The framework (denoated as Zr-L) not only exhibits a high thermal stability, but also an exceptional stability in a broad range of pH and boiling water. The hydrophobicity of the MOF can be tuned by simply oxidizing using H2O2, as illustrated in the water contact angle measurement of the pristine and H2O2-treated crystal samples.

Building on the above thioether-equipped MOF solid, we move on to more reactive and more versatile thiol functions. The versatile reactivity of the thiol (-SH) group offers unique advantages. For example, thiols as strong soft donors readily take up various metal ions, which closely bears on the removal of heavy metal ions, and on the creation of electroactive/semiconducting or catalytic sites (e.g., mimicking the iron-sulfur, copper-sulfur proteins) within the MOF matrices. In chapter 3, we describe the use of thiol-laced Zr-MOFs to anchor metal species (e.g., Hg), which was then oxidized to acidic sulfonate functions for catalyzing acetylene hydration at room temperature. Such conversion has served to liberate the proton and metal centers from the thiol groups, and to create strong acidity and reactivity properties within the MOF pores.

To further explore and highlight the rich chemistry offered by the thiol functions, we proceed to biologically relevant sulfenyl iodide functions. Sulfenyl iodides (RS-I) represent an intriguing class of molecules whose chemical stability and behaviors are peculiarly dependent on the surroundings. For example, In the free-flowing regime of solution chemistry, where molecules can freely approach one another, sulfenyl iodides are highly unstable, and often register a transient existence, as they are prone to disproportionating into disulfide and I2 (2RS-I → RSSR + I2). Incidentally, in the widely used reverse reaction of I2 oxidation of thiols into disulfides, the sulfenyl iodide (SI) is often postulated to be an intermediate. Chapter 4 presents a thiol-appended Zr-MOF which was found to react readily with I2 molecules to form RS-I units. In contrast to its solution chemistry of facile disproportionation into disulfide and I2, the sulfenyl iodide function, being thus anchored onto the rigid MOF grid and prevented from approaching one another to undergo the dismutation reaction, exhibits distinct stability, even at elevated temperatures (e.g., 90 °C). On a conceptual level, this simple and effective solid host also captures the spatial confinement observed of the complex biomacromolecular scaffolds involved in iodine thyroid chemistry, wherein the spatial isolation and consequent stabilization of sulfenyl/selenenyl iodides are exerted by means of the protein scaffolds.

The previous chapters have highlighted the versatle functionality offered by the sulfur groups (e.g., thiols) and other sulfur side chains. However, this field remains relatively slow in its development mainly because of the potentially complicated synthetic protocol. In chapter 5, we present a synthetic breakthrough of sulfur-equipped building blocks for framework construction. This methodology is especially effective in attaching a wide-ranging array of thiol groups onto the building blocks in various sizes and shapes. Specifically, utilizing the nucleophilic substitution between benzyl thiol and aromatic halides, and AlCl3-promoted deprotection of the resultant benzyl thioether, we succeeded in attaching the thiol groups onto the carboxylate linker backbones in a dense array. The widened access of thiol-equipped MOFs facilitates in-depth exploration of open framework sulfur chemistry, achieving, for instance, fast removal of Hg below the drinking limit of 2 ppb.

    Research areas

  • Metal-Organic Frameworks, MOF, Sulfur, Porous materials, Chemistry, Ligand Synthesis, Thioether, Thiol, Coordination networks, Mercury removal, Catalysis