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
