Optical Resolution, Determination of Absolute Configuration, and Photoracemization of cis-RuL2(CN)2 (L = 2,2′-Bipyridine and Its Analogues)

We synthesized neutral Ru­(II) complexes cis-Ru­(bpy)2(CN)2 (bpy = 2,2′-bipyridine), cis-Ru­(dmb)2(CN)2 (dmb = 4,4′-dimethyl-2,2′-bipyridine), cis-Ru­(dbb)2(CN)2 (dbb = 4,4′-di-tert-butyl-2,2′-bipyridine), and cis-Ru­(phen)2(CN)2 (phen = 1,10-phenanthroline) and optically resolved them into respective enantiomers using high-performance liquid chromatography with a chiral column. The absolute configuration of enantiomer of cis-Ru­(dbb)2(CN)2 was determined by an X-ray crystallography. Upon photoirradiation, the entire enantiomers of the complexes underwent the racemization with considerably slow rates (k = 1 × 10–6 to 1 × 10–5 s–1) and small quantum yields (ϕ = 1 × 10–6 to 1 × 10–5). The photoracemization was concluded to proceed via a five-coordinate pyramidal intermediate with the base plane composed of Ru, bidentate polypyridine, and two cyanides and the axial ligand of monodentate polypyridine. We derived the equations for photoracemization rate and quantum yield by a kinetics analysis of the photoracemization reaction that depended on polypyridine ligand, solvent, temperature, wavelength and intensity of irradiation light, and emission lifetime. From the temperature-dependent photoracemization reaction, the energy gap between 3MLCT (metal-to-ligand charge transfer) and 3d–d* states was estimated as ΔE = 4000–5000 cm–1, and the energy of invisible 3d–d* state was estimated to be ca. 20 500 cm–1, which was in good agreement with that of [Ru­(bpy)3]2+.