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 (1-a and 1-b) and NNC-coordinated hydrazone complexes (2-a and 2-b) via the corresponding hydrido intermediates (IM-1 and IM-2). 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 (1-a, 1-b, 2-a, and 2-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.