Reaction Mechanisms of Water Splitting and H2 Evolution by a Ru(II)- Pincer Complex Identified with Ab Initio Metadynamics Simulations

Water splitting is at the basis of artificial photosynthesis for solar energy conversion into chemical fuels. While the oxidation of water to molecular oxygen and the reduction of protons to molecular hydrogen are typically promoted by different catalysts, the Ru(II)-pincer complex recently synthesized by Kohl et al. [Science 2009, 324, 74] has been shown to promote both the thermal driven formation of H2 and the UV−vis driven evolution of O2. Here, we investigate, through density functional theory calculations, a portion of the catalytic cycle, focusing on the formation of hydrogen. We adopt an explicit description of the solvent and employ metadynamics coupled with the Car−Parrinello method to study the reaction mechanism and determine the activation free energies. Our simulations predict a novel catalytic cycle, which has considerably lower activation energies than earlier proposals and which does not involve the sequential aromatization− dearomatization of the PNN ligand of the complex. This work clearly demonstrates the general importance of an explicit description of the solvent for a predictive modeling of chemical reactions that involve the active participation of the solvent.