Stabilization of oxidized ruthenium complexes by adsorption on clay minerals

Abstract Adsorption of in situ oxidized ruthenium complexes on sodium montmorillonite (Na-Mt) was examined using two approaches (methods A and B) in order to establish a versatile method for stabilizing oxidized compounds. [Ru(acac)3]+ (1-Ru+) and [Ru(acac)2(hino)]+ (2-Ru+, Hhino = hinokitiol), which were each prepared by in situ oxidation of 1-Ru and 2-Ru with ammonium cerium(IV) nitrate (CAN), were used as models of oxidized compounds. In method A, an acetonitrile solution containing oxidized ruthenium complexes was added to an aqueous colloidal solution of Na-Mt, followed by filtering and air drying. In method B, an acetonitrile solution containing the oxidized complexes was drop-casted on powders of Na-Mt, followed by grinding and air drying. 1-Ru+ was hardly adsorbed on Mt in both methods, probably owing to the short lifetime of 1-Ru+ in solution. In contrast, 2-Ru+ was adsorbed on Mt when method B was applied. X-ray diffraction analysis indicated that 2-Ru+ existed not only in the interlayer space but also on the surfaces or edges of the Mt particles. The 2-Ru+ complexes in the cast sample, including non-intercalated ones, were stable at least for a week according to Raman spectroscopy, whereas 2-Ru+ without Mt returned to 2-Ru within a day in the solid state. The formation of a “house-of-cards”-like structure of Mt particles is proposed to be one reason why non-intercalated 2-Ru+ complexes are stabilized in the cast sample.