Role of the Chemically Non-Innocent Ligand in the Catalytic Formation of Hydrogen and Carbon Dioxide from Methanol and Water with the Metal as the Spectator

Abstract

The catalytic mechanism for the production of H₂ and CO₂ from CH₃OH and H₂O by [K­(dme)₂]­[Ru­(H) (trop₂dad)] (K­(dme)₂.<b>1_exp</b>) was investigated by density functional theory (DFT) calculations. Since the reaction occurs under mild conditions and at reasonable rates, it could be considered an ideal way to use methanol to store hydrogen. <i>The predicted mechanism begins with the dehydrogenation of methanol to formaldehyde through a new ligand–ligand bifunctional mechanism, where two hydrogen atoms of CH₃OH eliminate to the ligand’s N and C atoms, a mechanism that is more favorable than the previously known mechanisms, β-H elimination, or the metal–ligand bifunctional</i>. The key initiator of this first step is formed by migration of the hydride in <b>1</b> from the ruthenium to the meta-carbon atom, which generates <b>1</b>″ with a frustrated Lewis pair in the ring between N and C. Hydroxide, formed when <b>1</b>″ cleaves H₂O, reacts rapidly with CH₂O to give H₂C­(OH)­O^–, which subsequently donates a hydride to <b>6</b> to generate HCOOH and <b>5</b>. HCOOH then protonates <b>5</b> to give formate and a neutral complex, <b>4</b>, with a fully hydrogenated ligand. The hydride of formate transfers to <b>6</b>, releasing CO₂. The fully hydrogenated complex, <b>4</b>, is first deprotonated by OH^– to form <b>5</b>, which then releases hydrogen to regenerate the catalyst, <b>1</b>″. <i>In this mechanism, which explains the experimental observations, the whole reaction occurs on the chemically non-innocent ligand with the ruthenium atom appearing as a spectator</i>