Synthesis and development of framework materials: MOFs, COFs and more
One of the most prominent research areas of our group are framework materials, including MOFs and COFs. Our focus has been equally distributed towards developing new, cost-effective and environmentally-benign syntheses of MOFs and COFs, as well as developing a better understanding of their stability and opportunities for design. Using Zeolitic Imidazolate Frameworks (ZIFs) as model systems, we have developed mechanochemical, accelerated ageing, as well as RAM-based synthesis techniques to make MOFs directly from oxides or carbonates, and demonstrated their applicability to a range of other materials, including novel HKUST-1, UiO, MOF-74 and other classes of frameworks. The use of real-time, in situ monitoring techniques has enabled both an unprecedented mechanistic understanding of such reactions, as well as the discovery of new frameworks and intermediates indicating templating effects. Our current focus is increasingly turning to COFs and scalable synthesis.
We are currently developing large scale sustainable syntheses of MOFs with interesting properties such as MOF-74 and UiO-66. Our interest in MOFs is not limited to synthetic methodologies and we are synthesizing new MOFs, including the rare organic minerals stepanovite and zhemchuzhnikovite which are porous magnetic and proton conducting metal oxalate MOFs. We continue to synthesize a wide variety of MOF materials and explore their properties for gas storage, catalysis, and fluorescence.
Thermodynamic Stability of MOFs
In collaboration with Professor Alexandra Navrotsky's group at Arizona State University, we have combined solution calorimetry and periodic DFT to analyze the thermodynamics of two families of topologically distinct polymorphs of zinc zeolitic imidazolate frameworks (ZIFs) based on 2-methyl- and 2-ethylimidazolate linkers. These results demonstrate a correlation between density and thermodynamic stability, and allow us to probe the effects of ligand substitution. These results suggest that mechanochemical synthesis of ZIFs proceeds through thermodynamically increasingly stable and more dense phases.
One of our main goals includes the development of mechanochemistry for organometallic synthesis, to conduct oxidative addition, one of the fundamental reaction types of organometallic chemistry. Mechanochemical reactions have recently demonstrated impressive potential for improving the speed, energy- and materials-efficiency of chemical reactions, and especially so in organic chemistry, metal-organic synthesis and syntheses of molecular solids. However, the potential of mechanochemistry in organometallic synthesis has remained almost completely unexplored. In addition to the rhenium complexes below we are investigating both ruthenium based olefin metathesis. Recently we describe the first application of mechanochemistry to conduct a fundamental transformation of organometallic chemistry: oxidative addition.1 By using simple organometallic rhenium(I) precursors as model compounds, we have demonstrated for the direct oxidative addition of halogens (Cl, Br and I) onto a metal centre via a mechanochemical procedure that, unlike all relevant solution procedures reported in almost 50 years, does not require solvent, elementary halogens, or photochemical and high-pressure treatment. Instead, the mechanochemical procedure utilises a conventional oxidation agent in combination with readily available metal halides.
Solvent-free metal separation and recycling
Metals, such as gold, platinum, palladium, etc, are omnipresent in our everyday life and technology. Most of these metals are not phase pure in nature, but occur as mixed ores; mostly oxides, silicates, and sulfides. Their separation, purification, as well as recycling is not only very expensive but also requires the use of hot, concentrated acids, bulk solvent, and a tremendous amount of water. This puts an immense strain on our environment. As green chemist, it is our believe that we can make a difference by developing solvent-free, solid-state separation and recycling of metals from ore-like matrices and metal-waste using cheap, ecologically friendly organic ligands. These reactions are facilitated by mechanochemical grinding and accelerated ageing at room, or slightly elevated temperatures. Therefore, we not only make the process more eco-friendly by avoiding corrosive and hazardous chemicals, generate neglectable solvent waste, and using less energy, but also through recycling of noble metals. In addition, we also have the advantage to directly functionalize the metals during recycling and separation process. We are currently applying our methods towards more efficient lanthanide separations.
Our group has recently started working on hypergolic materials, i.e. materials that are capable of spontaneous ignition upon contact with an external oxidizer. Hypergolic materials are of critical importance as fuels and propellants in aerospace applications (e.g., rockets and spacecraft). While typical propellants are energetic hydrazine-based molecules that (extremely toxic and cancerogenic!), we synthesized hypergolic MOFs and cocrystals with high combustion energies using solution, and mechanochemical based syntheses. Our approached is based on introducing alkene or alkyne “trigger” groups onto the azolate linkers in popular zeolitic imidazolate framework (ZIF) class of MOFs. Through crystal engineering and material optimizations enable access to high combustion energies and ultrafast ignition delays as low as a 1-2 ms.
Mechanistic Studies of Mechanochemical Reactions
Our group is interested in a better fundamental understanding of underlying reaction mechanism of mechanochemical reactions. In collaboration with international research teams, we pioneered real-time in situ monitoring first by X-ray powder diffraction. This work has originally been performed at the European Synchrotron Radiation Facility (ESRF), nowadays spread over to other European synchrotron, such as BESSY, SLS, and DESY. Currently, we utilize time-resolved X-ray powder diffraction and Raman spectroscopy to not only monitor mechanochemical reactions of MOFs, ZIFs, cocrystals, and other materials in order to elucidate their reaction mechanism and kinetics, but also to obtain a better understanding of the individual reaction parameters, such as milling frequency, grinding additives, milling assembly, etc. These techniques have enabled us to make ground-breaking discoveries, such as observation of unknown metastable phases and ZIF frameworks with a previously unknown topologies.
Medicinal Mechanochemistry
Our research interest surrounds the synthesis of biologically active and pharmaceutically relevant organic compounds via the use of mechanochemistry and sonochemistry, as an alternative route that is more environmentally benign when compared to traditional methodologies.
Thioureas
We have recently developed a greener and more highly efficient protocol for the direct synthesis of unsymmetrical thioureas and sulfonyl-(thio)urea compounds via a click mechanochemical coupling of the respective amines, sulfonamides and (thio)isocyanates without the use of bulk organic solvents. While inherently poor nucleophilicity of the sulfonamide nitrogen atom reduces its reactivity in addition reactions, we reported the use of potassium carbonate as a mild, environmentally benign base for the deprotonation of the sulphonamide group as well as a catalytic coupling between the sulphonamide and (thio)isocyanate via the use of substoichiometric amounts of CuCl. Furthermore, a second generation anti-diabetic sulfonyl-urea drug, Glibenclamide, has also been successfully synthesized. We are expanding this methodology to other synthetic reactions.
Our lab has pioneered the development of mechanochemical olefin metathesis reactions using commercially available ruthenium complexes. This rapid, room-temperature approach enables us to minimize the amount of solvents needed in these high-yielding reactions.
Our group is strongly committed to improving the quality of chemistry education at the undergraduate level by designing new experiments for our teaching labs. Currently we are focused on generating modern experiments involving mechanochemistry for introductory undergraduate courses. We are developing new green chemistry experiments that enable students to evaluate the cost and sustainability of various synthetic methods. In addition, we are focusing efforts on generating new conceptual frameworks for designing upper year teaching labs in order to better integrate important concepts in chemistry into experiments that highlight current challenges in research and teach fundamental lab techniques.
