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+.