Arene-Ruthenium(II) and Bis(allyl)-Ruthenium(IV) Complexes Containing 2-(Diphenylphosphanyl)pyridine Ligands: Potential Catalysts for Nitrile Hydration Reactions?

Neutral arene-ruthenium(II) complexes [RuCl2(?6-arene){?1-(P)-PPh2py}] and [RuCl2(?6-arene){?1-(P)-PPh2(py-4-NMe2)}] (arene = benzene, p-cymene, mesitylene, hexamethylbenzene) have been synthesized and studied as potential catalysts for the selective hydration of nitriles to amides using benzonitrile as a model substrate. The effectiveness of these complexes was low due to the high tendency of the 2-(diphenylphosphanyl)pyridine ligands to form stable ?2-(P,N)-chelate rings, as demonstrated by NMR spectroscopy and catalytic experiments performed with the isolated cationic derivatives [RuCl(?6-arene){?2-(P,N)-P?N}][SbF6] [P?N = PPh2py, PPh2(py-4-NMe2)]. Despite its reluctance to adopt a chelating ?2-(P,N) coordination mode, cooperative effects of the bulky 2-(diphenylphosphanyl)pyridine ligand PPh2(py-6-tert-amyl) were not observed in complexes [RuCl2(?6-arene){?1-(P)-PPh2(py-6-tert-amyl)}] (arene = benzene, p-cymene). The novel bis(allyl)-ruthenium(IV) derivatives [RuCl(?3:?3-C10H16){?2-(P,N)-P?N}][SbF6] [P?N = PPh2py, PPh2(py-4-NMe2)] and [RuCl2(?3:?3-C10H16){?1-(P)-PPh2(py-6-tert-amyl)}] (C10H16 = 2,7-dimethylocta-2,6-diene-1,8-diyl) were also synthesized and fully characterized, and again led to modest conversions in the benzonitrile hydration reaction. Improvements in the catalytic activities of complexes [RuCl2(?6-p-cymene){?1-(P)-PPh2py}], [RuCl(?6-p-cymene){?2-(P,N)-PPh2py}][SbF6] and [RuCl(?3:?3-C10H16){?2-(P,N)-PPh2py}][SbF6] were observed in the presence of excess PPh2py due to the in situ formation of the catalytically more active dication [Ru{?2-(P,N)-PPh2py}3]2+.