Areas of Research
Investigation of Electrocatalytic PCET Shuttles
The efficient transfer of protons and electrons to substrates in a selective, coupled fashion is central to modern research efforts in electrocatalysis for solar fuels and organic synthesis. Proton-coupled electron-transfer (PCET) pathways are often the most efficient means of mediating challenging reductive (or oxidative) transformations. We are developing new classes of PCET reagents and catalysts, for example using metallocenes that can be protonated to install remarkably weak and hence reactive C–H bonds that deliver H-atoms, to enable new (electro)catalytic approaches in chemical synthesis and small-molecule reduction catalysis.
Iron-Catalyzed Nitrogen/Ammonia Cycling
Nitrogen fixation to ammonia (N2RR) and its reverse, ammonia oxidation (AO), are key challenges in catalysis that offer the promise of low- or zero-carbon fuels and fertilizers. Well-defined coordination complexes offer a means to constrain mechanistic hypotheses in enzymatic catalysis (e.g., nitrogen fixation by nitrogenase enzymes), and to develop new catalysts well suited to detailed mechanistic studies in their own right. Our group continues to discover fascinating iron and other metal catalyst systems in this vein with sample rich mechanistic landscapes that we continue to explore.
Photoinduced Copper-Catalyzed C–N Coupling
A wide array of nitrogen-containing compounds exhibit bioactivity, and new and versatile methods for the synthesis of Calkyl–N bonds, especially using secondary and tertiary alkyl electrophile coupling partners, are an important area for synthetic advances. In collaboration with the Fu group at Caltech we have reported the first examples of photoinduced, Cu-catalyzed Ullmann-type C–N couplings, including examples of enantioconvergent couplings. A common thread in this chemistry appears to be the intermediacy of an organic radical R•, generated by photoinduced single-electron transfer (SET), and a persistent copper(II) metalloradical, LnCu(II)-Nu. These radicals combine to form an R-Nu bond (Nu = nucleophile). We continue to explore methodological opportunities and fundamental mechanistic questions for these systems.
Towards Solar Fuels via Electrocatalysis
A central challenge towards the design and implementation of reductive fuel-forming electrocatalysts is substrate selectivity. The hydrogen evolution reaction (HER), of interest for H2 fuel, is often kinetically dominant and limits electrocatalytic reductions of other substrates (e.g., CO2, N2, nitrates). Our lab is exploring well-defined coordination complexes and heterogeneous surfaces as electrocatalysts for nitrogen fixation, key to enabling ammonia as a renewable fuel, as well as electrocatalytic CO2 fixation. For example, as part of the Joint Center for Artificial Photosynthesis (JCAP, supported by DOE) and the Liquid Sunlight Alliance (LiSA, also supported by DOE), our group, in collaboration with the Agapie lab at Caltech and other groups, has studied the efficacy of molecular additives on copper and other electrodes as a means of tuning their electrocatalytic CO2 conversion properties.