Event

Kinetic Barriers to Metal Hydride Complex Formation in Fuel-forming Catalysis

Proton-coupled electron transfer (PCET) processes underpin the catalytic transformations that take energy-poor substrates to energy-rich fuels. Many of these catalytic transformations proceed through metal hydride intermediates. Intriguingly, mechanistic studies undertaken by our lab reveal the PCET reactions that form metal hydride intermediates almost exclusively follow a stepwise electron transfer–proton transfer pathways. Moreover, in several examples, ligands are initially protonated, followed by a tautomerization to yield the stable metal hydride product. The preference for a stepwise reaction over a concerted proton-coupled electron transfer (CPET) route that circumvents the high energy, charged intermediates associated with stepwise pathways, as well as a kinetic preference for protonating the ligand over direct metal protonation, suggests multiple factors dictate the PCET reactions that form metal hydride species. Research in our lab aims to understand the factors underpinning reaction pathways that yield metal hydride complexes.

We hypothesized that an intrinsic barrier to metal protonation both inhibits access to the CPET  pathway and kinetically favors ligand protonation over metal protonation. High inner-sphere reorganization energies arising from the geometric changes that occur upon metal protonation are suspected to give rise to this barrier. In support of this hypothesis, we show a strong correlation between the isolated proton transfer reaction and associated reorganization energy. This trend holds for a range of metal and ligand architectures. Our ongoing work is focused on examining the mechanisms by which PCET reactions proceed to test the hypothesis that a CPET process is accessible for coordination complexes that have low reorganization energies for proton transfer.

In parallel, we are examining how ligand acid-base functionality can be leveraged for kinetically accessible protonation sites. Using two model complexes which react to form cobalt hydride species, we are examining the thermodynamic and kinetic factors that dictate ligand-based vs. metal-based PCET reactivity. Combining electrochemistry and spectroscopy, we are mapping the PCET pathways by which the cobalt hydride species form. Our data show that acid strength, acid concentration, pendant amine basicity, structural rigidity, timescale between electron transfer steps dictate the mechanism of hydride formation.

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Jillian L. Dempsey is a professor at the University of North Carolina at Chapel Hill where she holds the Bowman and Gordon Gray Distinguished Term Professorship. She is currently the Deputy Director of the Center for Hybrid Approaches in Solar Energy to Liquid Fuels (CHASE), and serves as the Director of Undergrad Studies for the Dept of Chemistry.

Jillian received her S.B. from the Massachusetts Institute of Technology in 2005 where she worked in the laboratory of Prof. Daniel G. Nocera. As an NSF Graduate Research Fellow, she carried out research with Prof. Harry B. Gray and Dr. Jay R. Winkler at the California Institute of Technology, receiving her PhD in 2011. From 2011–2012 she was an NSF ACC Postdoctoral Fellow with Daniel R. Gamelin at the University of Washington.

In 2012, Jillian joined the faculty at the University of North Carolina at Chapel Hill. Her research group explores charge transfer processes associated with solar fuel production, including proton-coupled electron transfer reactions and electron transfer across interfaces. Her research bridges molecular and materials chemistry and relies heavily on methods of physical inorganic chemistry, including transient absorption spectroscopy and electrochemistry. She has received numerous awards including the Harry B. Gray Award for Creative Work in Inorganic Chemistry by a Young Investigator (2019), the J. Carlyle Sitterson Award for Teaching First-Year Students (2017), a Sloan Research Fellowship (2016), a Packard Fellowship for Science and Engineering (2015), the Agnes Fay Morgan Research Award (2020), and the University Award for Advancement of Women (2021).