Synthesis and Small Molecule Exchange Studies of a Magnesium Bisformate Metal–Organic Framework: An Experiment in Host–Guest Chemistry for the Undergraduate Laboratory

Abstract

Experiments are described that introduce students to the important concepts of host−guest chemistry and size exclusion in porous metal−organic frameworks (MOFs). The experiment has been successfully carried out in both introductory and advanced-level inorganic chemistry laboratories. Students synthesized the porous MOF, α-Mg 3 (O 2 CH) 6 , and examined its ability to take up a variety of small organic molecules into its pores using 1 H NMR spectroscopy. The MOF exhibited size exclusion and students rationalized this behavior based on the structures of the organic molecules and the pore size of the MOF. R esearch into the design and synthesis of extended materials with specific properties has received extensive interest in recent times. Of the many classes of extended materials, metal−organic frameworks (MOFs) have received tremendous attention. MOFs are hybrid materials, composed of metal ions and organic molecules that are linked into an extended array by what is often referred to as a node-and-linker approach. The most successful synthesis of these materials uses metal−carboxylate systems. 1 In many cases, metal carboxylates form frameworks that enclatherate solvent molecules. In some instances, these materials are robust to solvent removal, generating a porous framework. The porous nature of many MOFs makes them attractive for numerous applications involving the uptake and exchange of small molecules. 2 MOFs are ideal for such purposes as the size and shape of the pores may be controlled through judicious choice of metal ions and organic linker molecules.Despite the high level of interest in MOF materials in the academic setting, undergraduate students rarely receive exposure to such cutting-edge applications of inorganic chemistry. Recently, Sumida and Arnold published a MOF laboratory designed for the undergraduate inorganic chemistry laboratory that gives students excellent examples of synthesis and characterization of such materials. 3 The laboratory experiment described herein provides an 1 H NMR-based study of the uptake properties of the MOF α-magnesium formate, α-Mg 3 (O 2 CH) 6 . 4 While students will gain experience in the synthesis of MOF materials, the novelty of this experiment focuses on the host−guest chemistry of porous materials. Host−guest studies have been the topic of a variety of contributions to this Journal, yet none have focused specifically on such properties in MOFs. 5 As many institutions do not have direct access to some of the instrumentation common to MOF characterization, such as X-ray diffraction and gas sorption equipment, this system uniquely provides a straightforward investigation of small molecule uptake and size selectivity of the MOF using NMR spectroscopy. Guest uptake by the MOF can be determined by dissolving the material in D 2 O and obtaining a 1 H NMR spectrum.

Higher Magnetic Fields, Finer MOF Structural Information: 17O Solid-State NMR at 35.2 T

“…The activation method employed in this work for DMF solvent removal from α-Mg 3 (HCOO) 6 is well established. 60,70 On the basis of our previous experience, we are certain that this activation process completely removes all residual DMF guests from the as-made MOF. 64,66 To confirm, we carried out TGA alongside 1 H− 13 C CP/MAS experiments, and the results unambiguously prove that the activation is complete (see section S2 for details).…”

“…The activation process for this MOF has been proven to be robust. ,,, To verify that the solvent DMF molecules occluded inside the channels are completely removed upon activation, TGA profiles and 1 H– 13 C CP/MAS SSNMR spectra were obtained (Figures S3 and S4, with experimental details being given in section S2). The results unanimously agree that the activation process is very effective and the solvent molecules are completely removed.…”

Exploring Host–Guest Interactions in the α-Zn3(HCOO)6 Metal-Organic Framework

“…400–500 K) 18−21,31,35 and can be synthesized using relatively straightforward procedures and inexpensive reagents. 36−38 M 3 (HCOO) 6 frameworks are also capable of guest adsorption. 19−24,27,33,39−44 For instance, α-Mg 3 (HCOO) 6 has demonstrated remarkable results for carbon dioxide (CO 2 ) capture.…”

Facile Synthesis of a Metal–Organic Framework for Removal of Methyl Blue from Water: First-Year Undergraduate Teaching Lab

“…For example, Sumida et al introduced to undergraduate students not only several useful MOF characterization methods, such as powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), and nuclear magnetic resonance (NMR), but also important concepts about MOFs, including gas adsorption and surface area . Rood et al reported the synthesis of a magnesium bisformate MOF, α-Mg 3 (O 2 CH) 6 , and investigated its host–guest properties by NMR . Crane et al proposed that structural changes of MOFs could be monitored with PXRD, and Wriedt et al presented the CO 2 capture performance of an ultrastable MOF, PCN-200 (PCN = porous coordination network) .…”