Exploring the Gas-Phase Activation and Reactivity of a Ruthenium Transfer Hydrogenation Catalyst by Experiment and Theory in Concert.

This study elucidates structures, activation barriers, and the gas-phase reactivity of cationic ruthenium transfer hydrogenation catalysts of the structural type [(η6-cym)RuX(pympyr)]+. In these complexes, the central ruthenium(+II) ion is coordinated to an η6-bound p-cymene (η6-cym), a bidentate 2-R-4-(2-pyridinyl)pyrimidine ligand (pympyr) with R = NH2 or N(CH3)2, and an anion X = I-, Br-, Cl-, or CF3SO3-. We present infrared multiple-photon dissociation (IR-MPD) spectra of precursors (before HCl loss) and of activated complexes (after HCl loss), which elucidates C-H activation as the key step in the activation mechanism. A resonant two-color IR-MPD scheme serves to record several otherwise "dark" bands and enhances the validity of spectral assignments. We show that collision-induced dissociation (CID)-derived activation energies of the [(η6-cym)RuX(pympyr)]+ (R = N(CH3)2) complexes depend crucially on the anion X. The obtained activation energies for the HX loss correlate well with quantum chemical activation barriers and are in line with the HSAB concept. We further elucidate the reaction of the activated complexes with D2 under single-collision conditions. Quantum mechanical simulations substantiate that the resulting species represent analogues for hydrido intermediates formed after abstraction of H+ and H- from isopropanol, as postulated for the catalytic cycle of transfer hydrogenation by us before.

Mechanistic Studies on Ruthenium(II)-Catalyzed Base-Free Transfer Hydrogenation Triggered by Roll-Over Cyclometalation.

The synthesis of 2-substituted pyridine-pyrimidine ligands and their complexation with arene ruthenium(II) chloride moieties is reported. Depending on the electronic and steric influences of the ligand, the catalysts undergo CH activation by roll-over cyclometalation. This process opens up the route to the catalytic transfer hydrogenation of ketones with isopropanol as the hydrogen source under base-free and mild conditions. Barriers related to the roll-over cyclometalation process can be determined experimentally by collision-induced dissociation ESI mass spectrometry. They are supported by DFT calculations and allow the classification of the ligands according to their electronic and steric properties, which is also in accordance with critical bond parameters derived from X-ray structure data. DFT calculations furthermore reveal that the formation of a ruthenium(II) hydrido species is plausible through ß-hydride elimination from isopropanol.