Terpyridine Diphosphine Ruthenium Complexes as Efficient Photocatalysts for the Transfer Hydrogenation of Carbonyl Compounds

Over the past decade, visible-light photoredox catalysis or photocatalysis has grown to become a commonly employed powerful tool in organic synthesis leading to new unique and valuable molecular transformations, inaccessible from thermally activated processes.[1] Photocatalysis can be conducted in homogeneous conditions as well as employing heterogeneous transition metal or solid semiconductors.[2] The commonly employed homogeneous visible-light photocatalysts are homoleptic Ru and Ir polypyridyl complexes, like [Ru(bpy)3]2+ and [Ir(ppy)3] (bpy = 2,2’-bipyridine; ppy = 2-phenylpyridine). These compounds, when excited by visible light undergo a metal-to-ligand-charge transfer (MLCT) transitions from HOMO and LUMO orbitals of the ligand.[3] The 2,2’:6’,2”- terpyridine (tpy) is an NNN-type Pincer ligand, which can give tight chelation of various metal cations in a nearly planar geometry. The presence of the electron-deficient pyridine cycles make it a strong σ-donor and also as a very good π-receptor, moreover the presence of low energetic LUMO levels allows it to participate in the redox reactions as a non-innocent ligand.[4] Among the catalytical reactions, the transfer hydrogenation (TH) of carbonyl compounds promoted by Ru complexes is a core process for the synthesis of alcohols in an environmentally friendly and widely accepted method in industry.[5] We report herein a practical and innovative procedure for the synthesis of a new class of ruthenium cationic [RuX(PP)(tpy)]Y (PP = diphosphine; X = Cl, OAc; Y = Cl, OAc, PF6) complexes containing tpy and a suitable diphosphine (Figure 1). These cationic complexes are active visible-light photocatalysts for the TH of carbonyl compounds at 30 °C in 2- propanol.