Ruthenium(II) Complexes of 4′‐(Aryl)‐2,2′:6′,2′′‐terpyridyl Ligands as Simple Catalysts for the Transfer Hydrogenation of Ketones

Abstract

A series of cationic [Ru(L)(PPh3)2Cl]+ (1–3) and neutral [Ru(L)(PPh3)Cl2] (4–6) RuII complexes were synthesized by reacting [RuCl2(PPh3)2] with 4′‐(aryl)‐2,2′:6′,2′′‐terpyridyl‐based ligands (L1–L3) with various aryl groups (tolyl, phenyl and 4‐fluorophenyl). The synthesized RuII complexes were unambiguously characterized by various spectroscopic techniques such as FTIR and multinuclear NMR spectroscopy as well as HRMS. The neutral complexes (4–6) were also structurally characterized by single‐crystal X‐ray diffraction studies. Photophysical and electrochemical studies of the RuII complexes were performed to elucidate the effects of the 4′‐aryl substituents of L1–L3. These RuII complexes show good catalytic activities in the transfer hydrogenation (TH) of ketones with a wide substrates scope in 2‐propanol under reflux. An optimization study revealed that the neutral RuII complexes are better catalysts than the cationic RuII complexes for TH reactions. Finally, [Ru(L1)(PPh3)2H]+ (7) with a [RuII–H] functionality was successfully synthesized and isolated and is proposed as the catalytically active species. A control experiment with the [RuII–H] complex in the absence of base was performed to establish the mechanism for the catalytic TH of ketones.