UC San Diego

It is imperative that we improve clean and renewable energy technology in the face of growing energy demand. Solar energy is potentially abundant, but we need to capture, convert and store solar energy in usable forms. Hydrogen could be ideal for energy storage, if made using sunlight energy to split water by oxidizing water to dioxygen and protons and using the released electrons to reduce protons to hydrogen in a net transfer of electrons. In Chapter 2, we explored water oxidation which is believed to be the more challenging of the two steps. We found evidence for catalyst degradation while using iridium complexes with ceric ammonium nitrate (CAN) as a sacrificial oxidant, possibly forming iridium oxide nanoparticles. Chapter 3 describes studies of analogs of well-known ruthenium water oxidation catalysts with bipyridine ligands that were designed to be bifunctional, by adding potential pendant bases such as hydroxy, alkoxy, amino, and heterocyclic amines, and some combinations of these groups. One successful catalyst was the methoxy complex which proved to be more robust than the parent unsubstituted complex while using CAN. Extended π systems, e.g. naphthyridine were found to lead to less active catalysts; therefore, we attempted to slow degradation and increase duration of catalysis by protecting the complex with sterically hindered groups, but we suspect that ligand loss interfered. Chapter 4 focused on ligands that would support formation of binuclear complexes which could improve catalysis, taking into account one of the proposed mechanisms for water oxidation involving two metal complexes

Work presented in Chapter 1 involves synthesis (by direct metalation of C-H bonds) and characterization of bis protic N-heterocylic carbene (PNHC) pincer complexes of platinum, palladium, and even nickel, in addition to their reactivity with alkynes and under basic conditions. Alkynes were converted to aldehydes as the major anti-Markovnikov addition products. The complexes were treated with different bases to form proposed trifunctional complexes containing one PNHC and one basic imidazolyl ligand. Addition of sodium alkoxide yielded dimeric species and lithium bases gave LiCl-imidazolyl adducts, which proved to be moisture sensitive but did not lose LiCl and undergo ligand exchange with substrates.