Relating catalytic activity and electrochemical properties: The case of arene–ruthenium phenanthroline complexes catalytically active in transfer hydrogenation

The electrochemical properties of cationic complexes [(η⁶-arene)Ru(N ∩ N)Cl]Cl (arene/N ∩ N = C₆H₆/1,10-phenanthroline (<b>1</b>), <i>p</i>-MeC₆H₄Pri/1,10-phenanthroline (<b>2</b>), C₆Me₆/1,10-phenanthroline (<b>3</b>), C₆Me₆/5-NO₂-1,10-phenanthroline (<b>4</b>), and C₆Me₆/5-NH₂-1,10-phenanthroline (<b>5</b>)) were studied by cyclic voltammetry in order to rationalize catalytic activity in transfer hydrogenation of the respective aqua complexes [(η⁶-arene)Ru(N ∩ N)(OH₂)](BF₄)₂ (<b>6–10</b>). Complexes <b>1–5</b> were chosen because the ‘true’ catalysts <b>6–10</b> are unstable under the conditions of the measurement. The electrochemical behaviour of <b>1–5</b> in acetonitrile solution is rather complicated due to consecutive and parallel chemical reactions that accompany electron transfer processes. Nonetheless, interpretation of the electrochemical data allowed to assess the influence of the structure and substitution on the redox and catalytic properties: the catalytic ability correlates with the reduction potentials, indicating the decisive role of the η6-arene ring directly bonded to the catalytic centre (Ru).