DFT study of an unusual proton-relay role for Cp* in hydrogen evolution catalysis

Abstract

Understanding mechanisms of the hydrogen evolution reaction (HER) is crucial to designing efficient catalysts for the prodn. of solar fuels. Cp*Rh(bpy) (Cp* = η^5- pentamethylcyclopentadienyl; bpy = κ^2-2,2'- bipyridyl) generates hydrogen in the presence of acid. However, the nature of the elementary steps leading to H-H formation has not been clear, as chem. characterization of intermediates in the catalytic reaction has been difficult to obtain. Here, we present a joint exptl.- computational study that addresses this challenge. D. functional theory (DFT) calcns. demonstrate that the catalyst first undergoes a 2e- redn. to form a Rh^I complex. Subsequently, in presence of acid, the Rh complex undergoes protonation at the Cp* ligand to form a complex bearing an [η^4-Cp*H] ligand, preserving the RhI center. DFT calcns. show that this complex is 6.8 kcal /mol more stable than the analogous Rh^(III) hydride. Following the formation of this intermediate, a second protonation can be carried out which results in evolution of hydrogen and restoration of η^5-Cp*. To the best of our knowledge, these results are among the first to show Cp* can serve as a proton relay in HER. New DFT results on the full mechanism for this compd. will be presented, and predictions of possible improvements to the catalyst will be discussed in light of the newly characterized intermediate