Thermally Stable Heteroleptic Trans-Bis(Chelate) Ruthenium(II) Complex Bearing 2,2’-Bipyridine and Acetylacetonato: Synthesis, Isomerization, and Crystal Structure

Thermally stable trans-bis(chelate)-type Ru(II) complexes are challenging to prepare owing to steric hindrance between the two chelate ligands. Herein, we investigated the isomerization of a heteroleptic cis-bis(chelate) complex to obtain its trans form. cis-[Ru(acac)(bpy)(dmso-S)₂](OTf)⋅0.5H₂O (1⋅(OTf)⋅0.5H₂O; acac⁻=acetylacetonato; bpy=2,2’-bipyridine; dmso=dimethyl sulfoxide; OTf⁻=CF₃SO₃⁻) was prepared by reacting trans(O,S)-[Ru(bpy)(dmso-S)₂(dmso-O)₂](OTf)₂ (P0) with Li(acac) in acetone at 298 K. In 1⋅(OTf)⋅0.5H₂O, the two labile dmso-O ligands of P0 were replaced by an acac⁻ ligand. The dihedral angle between the chelate rings of bpy and acac⁻ in 1⁺ was 76.7°, suggesting steric hindrance between ligands owing to the two bulky dmso-S ligands at the cis position. 1⋅(OTf)⋅0.5H₂O was thermally stable in DMSO and acetone; however, in methanol and water, a dmso-S ligand was dissociated from 1⁺, and the complex isomerized to trans-[Ru(acac)(bpy)(dmso-S)(solvent)]⁺. Refluxing of 1⋅(OTf)⋅0.5H₂O in methanol for 18 h, evaporation to dryness under vacuum, and treatment with water yielded trans-[Ru(acac)(bpy)(dmso-S)(OH₂)](OTf) (2⋅(OTf)) with excellent purity. 2⋅(OTf) was characterized by X-ray crystallography, elemental analysis, and ¹H NMR spectroscopy. As expected, steric hindrance was not observed between the trans-arranged bpy and acac⁻, and the two chelates laid flat on the equatorial plane in 2⁺.