Proton Coupled Electron Transfer (PCET) reactions are at the heart of many fundamental reactions in chemistry and biology, including water oxidation, nitrogen fixation and carbon dioxide reduction. In PCET reactions, electron transfer events are accompanied by a change in proton content. For example, the reduction of carbon dioxide, a known greenhouse gas to useful fuel sources such as methanol or methane, requires multiple electrons and multiple protons. Due to the significance and desire to carry out such reactions, there is a great need for catalysts that can facilitate PCET. Research conducted in my laboratory contributes to developing a better understanding of electron transfer reactions involving proton transfer, which will ultimately lead to the development of superior catalysts for these types of reactions. Research projects in my laboratory involve three significant aims.
- Fundamental studies of metal complexes with protonatable ligands
The primary goal of fundamental studies is to learn about the different properties of the metal complexes we make. Most notably, the metal complexes’ properties change based on the protonation state of the ligand and the orientation of the protonatable groups on the ligand. My laboratory studies how the structural and electronic properties of these complexes change with different ligands and in their varying protonation states. We use a variety of experimental techniques, including electrochemistry, UV/visible absorbance spectroscopy, luminescence spectroscopy, NMR spectroscopy, IR spectroscopy and others. In addition, we work with Dr. Timothy Dudley (University of Minnesota – Crookston) to carry out computational studies on the molecules we make. Dr. W. Scott Kassel has solved crystal structures of the molecules we have built to learn more about their structure in different protonation states. By gaining a thorough understanding of these properties and how different ligand protonation states affect the metal, we can begin to design tunable metal systems, which lead to more applied areas of chemistry described below.
- Water oxidation with metal complexes
Water as a fuel source is a process realized by plants in photosynthesis. This process is called water oxidation, where water is broken down into its principle components: oxygen, electrons and protons. The electrons and protons can be later recombined into a useable fuel, such as hydrogen. Most importantly, the byproduct of water oxidation is oxygen, avoiding the harmful byproducts that are commonly observed in the use of fossil fuels. The development of catalysts and an understanding of how they work are critical in realizing a new, alternative energy future. In fact, several of the complexes that we synthesize in my laboratory are water oxidation catalysts. While we carry out fundamental studies on the complexes, we have also partnered with two laboratories, Dr. Elizabeth Papish (University of Alabama) and Dr. Douglas Grotjahn (San Diego State University), who have the capabilities to carry out water oxidation studies. Together, we continue to develop new catalysts for water oxidation that can be tuned based on their protonation state.
- Metal complex prodrugs for anti-cancer activity
While carrying out fundamental studies on a metal complex synthesized in my laboratory, we discovered that it would photo-dissociate a ligand when exposed to blue light. In collaboration with Dr. Elizabeth Papish (University of Alabama) and Dr. Edward Merino (University of Cincinnati), we study the photo-dissociation events in ruthenium complexes that lead to cancer cell death. The metal complex acts as a prodrug that is activated to the cell-killing agent by exposure to light. What makes our complexes truly unique is that the ligand will only photo-dissociate (become a cell-killing agent) when in the protonated form. The ligand will remain on the metal when deprotonated. Cancer cells are inherently more acidic than normal cells due to their higher rates of metabolism, leading to the hypothesis that we can selectively target the more acidic cancer cells that would favor a protonated ligand. My laboratory continues to synthesize new metal complexes where we are learning to control the specific pH at which the ligand will photo-dissociate, alter the wavelength of light where the complex becomes active, and change the rates at which photo-dissociation occurs. This work is an exciting example of how the fundamental studies we carry out in my laboratory can lead to new discoveries and new directions.