Ruthenium(0)-Mediated Successive N–H and S–H/C–H Bond Activation in Selected Imine Ligands: Elucidation of the Activation Mechanism and Application in Borrowing Hydrogen Catalysis

The reported study demonstrates the remarkable ability of the ruthenium(0) center in [Ru(CO)₃(PPh₃)₂] to bring about σ-bond activation in organic molecules. This has been exemplified through reactions of [Ru(CO)₃(PPh₃)₂] with two types of imine ligands, namely, thiosemicarbazone and 2,4-dinitrophenylhydrazone of 2-formylpyrrole and 2-acetylpyrrole, which afforded NNS-coordinated thiosemicarbazone complexes (<b>1-a</b> and <b>1-b</b>) and NNC-coordinated hydrazone complexes (<b>2-a</b> and <b>2-b</b>) via the corresponding hydrido intermediates (<b>IM-1</b> and <b>IM-2</b>). Mechanistic studies revealed the initial coordination of the ligands to ruthenium(0) through the imine-nitrogen, followed by N-H activation via oxidative addition of the metal center, generating the hydrido-Ru(II) intermediates, and Ru-H-assisted activation of the proximal S-H or C-H bond, yielding the final products via elimination of H₂. Isolation of the hydrido intermediates and confirmation of H₂ evolution supported the formation mechanism traced out through density functional theory (DFT) calculations. Crystal structure determination of the four complexes and two intermediates by the X-ray diffraction method helped in establishing their identity. The NNS- and NNC-coordinated (<b>1-a</b>, <b>1-b</b>, <b>2-a</b>, and <b>2-b</b>) complexes are found to serve as efficient catalyst precursors for the reduction of carbonyl compounds to corresponding alcohols via the borrowing hydrogen (from 2-propanol) protocol, where the catalytically active dihydrido-Ru(II) species is generated in situ via solvent-assisted cleavage of the Ru-S/Ru-C bond.