Catalysts are substances that cause a chemical reaction to proceed more quickly without being consumed or permanently altered. Because catalysts can be used repeatedly for the same reaction without being consumed, they are used extensively in laboratory and industrial settings to make a variety of plastics, fuels, and medicine. Many different molecules can act as catalysts, but some of the best contain metal atoms. High activity and stability has been achieved in metal-containing catalysts containing hemilabile ligands. Hemilabile ligands are molecules that can form a complex with the metal, preventing catalysis from occurring, or interact with other substances in solution to allow catalysis to occur. These ligands are composed of several parts bound together: the weakly donating part can disassociate to allow access to the metal center of a catalyst, while an anchoring component keeps the complex from decomposing.
The cartoon shows how hemilabile ligands can be thought of as a gated system. The closed “off” state features the hemilabile ligand bound to the metal center, preventing catalysis; the open “on” state features the hemilabile ligand dissociated from the metal center, allowing catalysis to occur. Controlling whether the “gate” is open or closed is a major challenge in developing hemilabile ligands. Our group has developed a way to manipulate hemilability using external stimuli, which enables extensive control over the rates of reactions.
Methods and Findings
We devised a series of hemilabile, pincer-crown ether catalysts to allow us to tune the ratio between the “off” and “on” state. An iridium catalytic center is supported by a stable pincer backbone bound to a hemilabile crown ether ligand. Crown ethers act as the gate and positively charged metal ions (cations) act as the “key” that opens the gate, allowing organic molecules to reach the iridium catalyst. We hoped that using cations to dial the ratio between the open and closed conformations would allow us to tune the rates of reactions using salt additives.
We explored the influence of lithium and sodium salts on our iridium pincer-crown ether complexes. Cations were found to interact with our catalyst and increase the rate of allylbenzene isomerization – a reaction where the molecule is rearranged, but no new atoms are added – by a factor of 1000 relative to the rate of a reaction without cationic additives! The reaction rate is sensitive to both the concentration and identity of the cation, with lithium ions accelerating the reaction the most.
These results illustrate that cationic additives can be used to tune the rates of reactions, with both cation identity (lithium or sodium) and concentration in solution acting as knobs with which we can tune reaction rates.
Current work in our lab includes exploring how the size of the crown ether will affect the selectivity for cations and reaction rates. Larger crowns are expected to have a higher affinity for larger cations, allowing us to test a hypothesis based on known cation binding affinity trends. We are also exploring new double bond containing molecules to probe key questions in cation-modulated catalysis. Can we vary the cations added to a solution to make different products? Can we utilize interactions between the crown, cation, and substrate to shape catalysis? We are very excited to apply our tunable catalysts to new reactions.
Research Article: Controlling ligand binding for tunable and switchable catalysis: cation-modulated hemilability in pincer-crown ether ligands. Dalton Trans. 2017.