Metal/Main Group Cooperative Chemistry
Part of our research program involves studying metal/main group cooperative reactivity for new bond activation chemistry. This chemistry explores typically redox-innocent main group centers which are rendered "non-innocent" through electronic delocalization to a metal. This reactivity was initially proposed by Goddard who performed DFT studies on the V/P Oxide (VPO) commercial catalyst for the partial oxidation of butane to maleic anhydride. For the first time, it was proposed that initial C–H activation was initiated by a terminal P(+5)=O bond, rather than the terminal vanadyl (V(+5)=O) bonds. For the first time, we provided experimental support for this so-called ROA mechanism using molecular model compounds, H-atom donors, and H-atom surrogates. Since then, we continue to investigate this cooperative reactivity with the goal of accessing multi-electron transformations.
Emerging from this work, we explored the effect of substituting V with carboranes, which are known to accept up to 2 e-. Using a bis(phosphineoxide)-ortho-carborane, we discovered that reduction led to cage opening and P=O bite angle increase. Harnessing this redox-switchable chelation, we have recently demonstrated how we can chemically or electrochemically capture and release a metal, such as U. We have applied this methodology to the mono- or bi-phasic capture and release of the uranyl (UO2(2+)) cation, and are continuing to extend this chemistry to other metals of energy importance.
Energy Storage: Redox-Flow Batteries
Redox flow batteries (RFBs) are widely applicable energy storage devices that operate using soluble redox-active molecules, known as charge carriers, and which have modular design, fast response times, and are easily scalable to meet the demands of grid-scale energy storage. RFBs contain two electrolyte tanks (catholyte and anolyte) containing these charge carrier molecules capable of accepting or delivering multiple electron equivalents. We are investigating new, symmetric charge carriers using metal-based redox events with tunable redox innocent or non-innocent ligand frameworks. These molecules are being designed to increase solubility and electrochemical stability, while using cheap and readily available starting materials. Along with standard RFB designs, we are also investigating mixed capacitor RFB systems that use conductive carbon in conjunction with charge carriers to increase the performance of the system.
Energy Storage: Probing Ammonia Oxidation
Our group is interested in exploring the electrocatalytic oxidation of ammonia (NH3) in order to use this potentially clean fuel as an effective renewable energy (RE) vector for fuel cells. Central to using NH3 as a RE vector is understanding and developing effective electrocatalysts for this 6 electron oxidation chemistry. We are currently investigating first-row transition metal complexes as potential electrocatalysts for NH3 oxidation. Metal complexes based on ligand platforms such as salen, pthalocyanines, and novel pacman motifs are used to study the mechanism of NH3 oxidation, either through stepwise H+/e- elimination, or through H-atom abstraction routes to generate N2. Compounds currently being investigated include reactive Fe phthalocyanines, high-valent nickel centers, and manganese-salen nitride complexes. Our strategy is to use this mechanistic understanding to guide our electrocatalyst design.