Beyond Metal-Arenes: Monocarbonyl Ruthenium(II) Catalysts for Transfer Hydrogenation Reactions in Water and in Cells

With the aim to design water-soluble organometallic Ru(II) complexes acting as anticancer agents catalyzing transfer hydrogenation (TH) reactions with biomolecules, we have synthesized four Ru(II) monocarbonyl complexes (1–4), featuring the 1,4-bis(diphenylphosphino)butane (dppb) ligand and different bidentate nitrogen (N∧N) ligands, of general formula [Ru(OAc)CO(dppb)(N∧N)]n (n = +1, 0; OAc = acetate). The compounds have been characterized by different methods, including 1H and 31P NMR spectroscopies, electrochemistry, as well as single-crystal X-ray diffraction in the case of 1 and 4. The compounds have also been studied for their hydrolysis in an aqueous environment and for the catalytic regioselective reduction of nicotinamide adenine dinucleotide (NAD+) to 1,4-dihydronicotinamide adenine dinucleotide (1,4-NADH) in aqueous solution with sodium formate as a hydride source. Moreover, the stoichiometric and catalytic oxidation of 1,4-NADH have also been investigated by UV–visible spectrophotometry and NMR spectroscopy. The results suggest that the catalytic cycle can start directly from the intact Ru(II) compound or from its aquo/hydroxo species (in the case of 1–3) to afford the hydride ruthenium complex. Overall, initial structure–activity relationships could be inferred which point toward the influence of the extension of the aromatic N∧N ligand in the cationic complexes 1–3 on TH in both reduction/oxidation processes. While complex 3 is the most active in TH from NADH to O2, the neutral complex 4, featuring a picolinamidate N∧N ligand, stands out as the most active catalyst for the reduction of NAD+, while being completely inactive toward NADH oxidation. The compound can also convert pyruvate into lactate in the presence of formate, albeit with scarce efficiency. In any case, for all compounds, Ru(II) hydride intermediates could be observed and even isolated in the case of complexes 1–3. Together, insights from the kinetic and electrochemical characterization suggest that, in the case of Ru(II) complexes 1–3, catalytic NADH oxidation sees the H-transfer from 1,4-NADH as the rate-limiting step, whereas for NAD+ hydrogenation with formate as the H-donor, the rate-limiting step is the transfer of the ruthenium hydride to the NAD+ substrate, as also suggested by density functional theory (DFT) calculations. Compound 4, stable with respect to hydrolysis in aqueous solution, appears to operate via a different mechanism with respect to the other derivatives. Finally, the anticancer activity and ability to form reactive oxygen species (ROS) of complexes 1–3 have been studied in cancerous and nontumorigenic cells in vitro. Noteworthy, the conversion of aldehydes to alcohols could be achieved by the three Ru(II) catalysts in living cells, as assessed by fluorescence microscopy. Furthermore, the formation of Ru(II) hydride intermediate upon treatment of cancer cell extracts with complex 3 has been detected by 1H NMR spectroscopy. Overall, this study paves the way to the application of non-arene-based organometallic complexes as TH catalysts in a biological environment.