Chang Group - Renewable Energy Chemistry

A grand challenge facing our global future is energy. Within 50 years, 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 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. With the goal of developing principles of fundamental reactions pertinent to sustainable, carbon-neutral energy cycles, we are synthesizing new first-row transition metal complexes and evaluating their stoichiometric and catalytic reactivity towards water, oxygen, hydrogen, and other small molecules of energy consequence. Synthetic targets of particular interest are complexes with metal-ligand multiple bonds and species that can be activated by controlled proton and electron transport. As an example, the cycle for water/oxygen interconversion shown below highlights potentially important chemical intermediates and transformations. Our approach to synthetic design is guided by principles of Nature, which uses first- and second-sphere coordination chemistry with cheap and abundant metals for energy catalysis. 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.

First-Row Metal Catalysis. 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.

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. 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.