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

Abstract

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.