A central challenge in the development of inorganic hydrogen evolution catalysts is to avoid deleterious coupling between the energetics of metal site reduction and the kinetics of metal hydride formation. In this work, we combine theoretical and experimental methods to investigate cobalt diimine-dioxime catalysts that show promise for achieving this aim by introducing an intramolecular proton shuttle via a pyridyl pendant group. Using over 200 coupled-cluster-level electronic structure calculations of the Co-based catalyst with a variety of pyridyl substituents, the energetic and kinetic barriers to hydrogen formation are investigated, revealing nearly complete decoupling of the energetics of Co reduction and the kinetics of intramolecular Co hydride formation. These calculations employ recently developed quantum embedding methods that allow for local regions of a molecule to be described using high-accuracy wavefunction methods (such as CCSD(T)), thus overcoming significant errors in the DFT-level description of transition-metal complexes. Experimental synthesis and cyclic voltammetry of the methyl-substituted form of the catalyst indicate that protonation of the pendant group leaves the Co reduction potential unchanged, which is consistent with the theoretical prediction that these catalysts can successfully decouple the electronic structures of the transition-metal and ligand-protonation sites. Additional computational analysis indicates that introduction of the pyridyl pendant group enhances the favorability of intramolecular proton shuttling in these catalysts by significantly reducing the energetic barrier for metal hydride formation relative to previously studied cobalt diimine-dioxime catalysts. These results demonstrate a promising proof of principle for achieving uncoupled and locally tunable intramolecular charge-transfer events in the context of homogeneous transition-metal catalysts.
Bibliographical noteFunding Information:
This work was supported by the Air Force Office of Scientific Research (USAFOSR) under Grant No. FA9550-11-1-0288, the (U.S.) Department of Energy (DOE) under Grant No. DESC0006598, and by the NSF Center for Chemical Innovation Solar Fuels Grant CHE-1305124. P.H. thanks Kara Bren and Richard Eisenberg for helpful discussions. Computing resources were provided by the National Energy Research Scientific Computing Center (NERSC) (DE-AC02-05CH11231) and XSEDE (TG-CHE130108).
© 2016 American Chemical Society.
- cobalt diimine-dioxime catalysts
- coupled-cluster theory
- density functional theory
- hydrogen evolution
- proton shuttle