The role of electronic structure in the hydrogen evolution reaction dynamics as catalyzed by Ru-based complexes

The development of efficient catalysts for the hydrogen evolution reaction (HER) is of paramount importance in the field of clean energy technology. In this work, we provide a detailed theoretical molecular-level investigation of the catalytic activity of three structurally related ruthenium complexes, by using DFT calculations and molecular dynamics (MD) simulations. The catalytic ability of a di-nuclear complex (C1) and two mononuclear analogues (C2, C3) are examined. Complex C2 shows a low activation barrier (8.6 kcal mol^-1), but H₂ release is kinetically slow compared to C1. C3, despite having favourable energy pattern, fails to release hydrogen. This reveals that factors beyond simple activation barriers, such as ligand coordination flexibility, are crucial for effective catalysis. Analysis of the LUMO distribution shows a significant anti-bonding character for C1 and C2, <i>i.e.</i> H₂ release. On the contrary H₂ is not involved in the C3 LUMO, which is mainly of π nature and localized on the ligands. Thus, this work provides a new paradigm for the rational design of HER catalysts based on the LUMO distribution (anti-bonding) analysis in the catalyst transition state, which is relevant to H₂ release.