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(η6‐arene){κ1‐(P)‐PPh2py}] and [RuCl2(η6‐arene){κ1‐(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 κ2‐(P,N)‐chelate rings, as demonstrated by NMR spectroscopy and catalytic experiments performed with the isolated cationic derivatives [RuCl(η6‐arene){κ2‐(P,N)‐P N}][SbF6] [P N = PPh2py, PPh2(py‐4‐NMe2)]. Despite its reluctance to adopt a chelating κ2‐(P,N) coordination mode, cooperative effects of the bulky 2‐(diphenylphosphanyl)pyridine ligand PPh2(py‐6‐tert‐amyl) were not observed in complexes [RuCl2(η6‐arene){κ1‐(P)‐PPh2(py‐6‐tert‐amyl)}] (arene = benzene, p‐cymene). The novel bis(allyl)‐ruthenium(IV) derivatives [RuCl(η3:η3‐C10H16){κ2‐(P,N)‐P N}][SbF6] [P N = PPh2py, PPh2(py‐4‐NMe2)] and [RuCl2(η3:η3‐C10H16){κ1‐(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(η6‐p‐cymene){κ1‐(P)‐PPh2py}], [RuCl(η6‐p‐cymene){κ2‐(P,N)‐PPh2py}][SbF6] and [RuCl(η3:η3‐C10H16){κ2‐(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{κ2‐(P,N)‐PPh2py}3]2+.