We are inspired by the use of hydrogen bonds and proton transfer events to control catalysis. This inspiration comes from both natural and man-made catalysts. The types of reactions we catalyze are those that are relevant to energy sources and energy storage; those that impact the environment; and those that are relevant to human health. Furthermore, catalysts that resemble natural catalysts (enzymes) are called enzyme mimics, and these mimics frequently teach us lessons about enzyme structure and function. These projects include those that closely resemble enzymes (bioinorganic projects) and those that are bio-inspired but utilize non-biologic metals (organometallic projects) are described further below:
Bioinorganic project 1 – use of Ttz ligands to model copper nitrite reductase. The global nitrogen cycle includes denitrification, which converts nitrogen from excess fertilizers (nitrates and nitrites) to less harmful forms. The products of denitrification include nitric oxide (NO); NO is relevant to human health and how bacteria evade our immune response. The Papish group has examined Ttz copper complexes as functional and structural mimics for copper nitrite reductase. We are interested in how protonation of Ttz ligands enhances nitrite reduction to NO (g). See Kumar, Dixon, et al. Inorg. Chem. 2012.
Bioinorganic project 2 – use of Ttz ligands for C-H activation. C-H activation is one of the grand challenges in organic chemistry, as the C-H bond is quite strong and hard to break selectively. We recently reported copper complexes that activate C-H bonds under mild conditions. These catalysts are activated by protonation events, and this tuning of function with pH is biomimetic. See Dixon et al. Chemical Communications, 2013.
Organometallic project 1 – hydrogenation chemistry. Hydrogenation of organic substrates can be achieved via the transfer of H+ and H- (a.k.a. ionic hydrogenation). We have an interest in using ligands that can transfer H+ to improve hydrogenation. Furthermore, some of these ligands impart water solubility to the complexes, and have led to greener chemical reactions. See Nieto et al. Organometallics, 2011 and DePasquale et al. Inorganic Chemistry 2013 and other papers below. Hydrogenation reactions can be also used to store hydrogen; this hydrogen storage amounts to an energy storage solution.
Organometallic project 2 – water oxidation chemistry. Water oxidation is one of the most promising means of storing the sun’s energy. The products of water oxidation are O2, electrons and protons (H+), and ideally the electrons and protons can be combined to give H2 to store as a fuel. The Papish group has pioneered the use of dihydroxybipyridine ligands for water oxidation. Changing the pH of water can activate these homogeneous water oxidation catalysts by ligand deprotonation. Dianionic ligands are known to enhance water oxidation by facilitating catalyst oxidation, but the use of ligands that change from neutral to dianionic in situ is novel and leads to switchable catalyst properties. For more details see DePasquale et al. Inorganic Chemistry 2013.
Design of new ligands with hydrogen bonds on novel scaffolds – talk to Dr. Papish about this in person, as this work is in progress.
