Observation of proton transfer between bridging ligands on a catalyst by 2D-IR spectroscopy

Currently accepted hydrogenation mechanisms of Shvo's catalyst include an activation step in which the inactive ruthenium dimer undergoes scission to form two different catalytic species. In this study, two-dimensional infrared spectroscopy (2D-IR) of the metal carbonyl vibrations of Shvo's catalyst was used to monitor early reaction dynamics for the inactive and activated catalyst. Kinetic analysis of exchange peaks in the 2D-IR spectra demonstrate that thermally activated intramolecular proton exchange occurs on the ultrafast time scale. The results indicate an activation barrier for proton transfer of 2.1 kcal/mol and an upper limit for the dimer scission rate constant of 1.3 × 10¹¹ s^-1, which is well above the previously reported value. Deprotonation of the dimer leads to a pseudo stable species that remains dimeric at the ruthenium-hydride bridge for several hours. 2D-IR spectroscopy of this species shows that proton transfer is turned off, as expected. The data reveal new mechanistic details of the dynamic behavior of Shvo's catalyst leading up to activation and introduce the feasibility of substrate binding to the dimeric form of the catalyst prior to scission.