A grand challenge facing our global future is energy. By 2050, the planet's energy needs will almost triple from 12.8 to 35 TW, with carbon dioxide levels at the highest they've been in the last 650,000 years. These issues of climate change and energy demand require the development of new, sustainable energy technologies that are carbon neutral, and basic science is needed to meet this ultimate goal. An important aspect of this effort is catalysis, which will allow the efficient storage and transformation of energy in the form of chemical fuels. We are focused on devising new synthetic inorganic and organometallic systems that utilize cheap and abundant metals, particularly those of the first-row transition metal series, which will have reduced environmental impact. Our goal is to develop stoichiometric and catalytic reactions towards water, oxygen, hydrogen, carbon dioxide, and other small molecules of energy consequence that work in benign solvents such as water. In another avenue of research, we are also targeting fundamental aspects of metal-ligand multiple bonding and small-molecule activation using first- and/or second-sphere coordination chemistry approaches.

Bioinspired First-Row Transition Metal Chemistry. Metals are highly useful cofactors for many catalytic processes, but traditional synthetic systems have largely relied on noble metals (e.g. Pd, Pt, Rh) to increase and/or control reactivity. In contrast, over 1/3 of natural proteins require metal cofactors, but metalloenzymes employ cheap and abundant metals like iron, copper, and zinc for substrate transformations. We are exploring synthetic ligand templates that incorporate a variety of naturally-occurring nitrogen-based donors, including pyrroles, imidazoles, and amides, to house first-row transition metals in unusual geometries. This bioinspired strategy seeks to exploit intellectual principles learned from natural systems but allows freedom to synthesize complexes that are not bound by the structural constraints of specific protein active sites. For example, we have developed hybrid systems that incorporate key attributes of heme and non-heme ancillary ligands for iron, namely the pyrrole donors in heme that can support iron in multiple oxidation states coupled with the flexibility of varying coordination geometries in non-heme frameworks. Recent work on a three-fold symmetric non-heme iron pyrrole shows its ability to bind and activate nitrous oxide (N2O), a thermodynamically potent and benign oxidant that is typically difficult to activate because of its kinetic stability and poor properties as a ligand, for two-electron oxidation chemistry.

Second-Sphere Coordination Chemistry. Nature exploits second-sphere coordination chemistry as an efficient way to carry out many catalytic functions using a limited number of primary metal active sites. In this regard, second-sphere hydrogen bonding is often a key component for controlling reactivity by managing two different types of reagents for a given reaction. An important illustration of this concept is that many important reactions of energy consequence require the timed delivery of proton and electron equivalents; for example, the cycle for water/oxygen interconversion shown below highlights potentially important chemical intermediates and transformations. To achieve this dual control of acid-base and redox chemistry in synthetic systems, we are devising new platforms to test the effects of second-sphere pendants on reaction efficiency and selectivity at conserved primary metal cores. Recent work has focused on oxygen activation reactions mediated by second-sphere hydrogen bonding.