Isomeric Complexes of [Ru^II(trpy)(L)Cl] (trpy = 2,2′:6′,2′′-Terpyridine and HL = Quinaldic Acid): Preference of Isomeric Structural Form in Catalytic Chemoselective Epoxidation Process

The present work deals with the isomeric complexes of the molecular composition [Ru^II(trpy)(L)Cl] in 1 and 2 (trpy = 2,2′:6′,2′′-terpyridine, L = deprotonated form of quinaldic acid, HL). Isomeric identities of 1 and 2 have been established by their single-crystal X-ray structures, which reveal that under the meridional configuration of trpy, O⁻ and N donors of the unsymmetrical L are in trans, cis and cis, trans configurations, respectively, with respect to the Ru−Cl bond. Compounds 1 and 2 exhibit appreciable differences in bond distances involving Ru−Cl and Ru−O1/Ru−N1 associated with L on the basis of their isomeric structural features. In relation to isomer 2, the isomeric complex 1 exhibits a slightly lower Ru(II)−Ru(III) oxidation potential [0.35 (1), 0.38 (2) V versus SCE in CH₃CN] as well as lower energy MLCT transitions [559 nm and 417 nm (1) and 533 nm and 378 nm (2)]. This has also been reflected in the DFT calculation where a lower HOMO−LUMO gap of 2.59 eV in 1 compared to 2.71 eV in 2 is found. The isomeric structural effect in 1 and 2 has also been prominent in their ¹H NMR spectral profiles. The relatively longer Ru−Cl bond in 1 (2.408(2) Å) as compared to 2 (2.3813(9) Å) due to the trans effect of the anionic O⁻ of coordinated L makes it labile, which in turn facilitates the transformation of [Ru^II(trpy)(L)(Cl)] (1) to the solvate species, [Ru^II(trpy)(L)(CH₃CN)](Cl) (1a) while crystallizing 1 from the coordinating CH₃CN solvent. The formation of 1a has been authenticated by its single-crystal X-ray structure. However, no such exchange of “Cl⁻” by the solvent molecule occurs in 2 during the crystallization process from the coordinating CH₃CN solvent. The labile Ru−Cl bond in 1 makes it a much superior precatalyst for the epoxidation of alkene functionalities. Compound 1 is found to function as an excellent precatalyst for the epoxidation of a wide variety of alkene functionalities under environmentally benign conditions using H₂O₂ as an oxidant and EtOH as a solvent, while isomer 2 remains almost ineffective under identical reaction conditions. The remarkable differences in catalytic performances of 1 and 2 based on their isomeric structural aspects have been addressed.