Gilbertson Research
Highly oxygenated species play a vital role in global biogeochemical cycles such as the carbon cycle (carbon dioxide, CO2) and the nitrogen cycle (nitrate/nitrite, NO3–/NO2–). Buildup of these polyoxygenated species makes them recalcitrant environmental pollutants given their thermodynamic and kinetic stability. For example, NO2– is a pervasive pollutant in groundwater and poses a serious threat to human health. Groundwater treatment is an extremely difficult task, making remediation of the recalcitrant contaminant an area of great interest. In addition, sensible methods of utilization are needed for their mitigation and utilization. Crop overfertilization has resulted in large anthropogenic sinks of nitrous oxide (N2O), which in the global nitrogen cycle is a byproduct of dissimilatory denitrification of nitrate (NO3–) to dinitrogen (N2). N2O is one of the leading causes of ozone depletion and a potent greenhouse gas (∼300 times larger warming potential than CO2). Elucidation of the mechanism of N2O formation is therefore extremely important and vital in our understanding of N–N bond forming reactions that result in N2O. Lastly, The production of CO from CO2 is an attractive route to utilization of CO2 as a C1 source. CO is produced industrially by steam reforming fossil fuels to produce syngas and is a versatile chemical precursor and fuel. We are involved in a research program dedicated to the deoxygenation and conversion of the NOx- and CO2 molecules into useable feedstocks utilizing earth abundant metals.
Redox-inactive metals are often used in combination with redox-active transition metals in synthetic and biological systems to invoke reactivity involving electron transfer. One particularly fascinating example is the role of the redox-inactive Ca2+ in the oxygen-evolving complex of photosystem II. Another is the Haber–Bosch process, which is carried out on potassium-promoted iron surfaces. The addition of redox-inactive Lewis acids shows enhanced electron-transfer rates, more positive reduction potentials, and enhanced rates of dioxygen activation in synthetic chemical systems. They also facilitate O–O, N–N, and H–H bond cleavage, as well as enable O- and H-atom transfer. Our group has been investigating the role of the secondary coordination sphere in complexes containing redox-active ligands of the pyridinediimine (PDI) scaffold. We have shown that pendant Bronsted bases/acids are capable of stabilizing rare intermediates and tuning the reduction potential of the ligand-based redox-active sites. We are also investigating the positioning a Lewis acid binding site in the secondary coordination sphere and how they affect the reduction potentials in the PDI system (similar to redox-responsive pendant crown ferrocene systems).