Electrochemical Reaction of Bis(acetylacetonato)Palladium in an Amide-Type Ionic Liquid

Abstract

Palladium is well known as a useful metal for such as catalytic reaction, hydrogen absorption and improvement of corrosion resistance. Although the electrochemical reaction and deposition of palladium has been researched in some ionic liquids so far, the palladium sources have been limited to halogeno complexes such as [PdBr4]2− or [PdCl4]2−.1,2 We have already reported electrodeposition of Pd from 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing bis(acetylacetonato)palladium (Pd(acac)2).3 In the present study, the electrochemical reaction of Pd(acac)2in BMPTFSA was investigated using a rotating ring-disk electrode. BMPTFSA was prepared by metathesis reaction of LiTFSA and BMPBr, which was synthesized by quaternization reaction of 1-methylpyrrolidinium with n-butyl bromide in acetonitrile, followed by recrystallization with ethyl acetate. Pd(acac)2 was used as received. Electrochemical measurements were conducted in a glovebox of dry Ar atmosphere. A ring-disk electrode (Pt ring: 1.2 cm2; Pt disk: 0.79 cm2) was used as a working electrode. A Pt wire was used for a counter electrode. A silver wire immersed in BMPTFSA containing 0.1 M AgCF3SO3was used as a reference electrode. The counter electrode was separately set from the working and reference electrode by a glass filter. The UV-vis spectrum of BMPTFSA containing 0.25 mM Pd(acac)2 was well accorded with that of ethanol solution, suggesting that Pd(acac)2 exists as a tetra-coordinated Pd complex coordinated by the oxygen atoms of acetylacetonate in BMPTFSA. The deposition of metallic palladium was confirmed on the electrode after the potentiostatic cathodic redction at −1.9 V. An anodic current was observed at the ring electrode kept at −1.25 V when the cathodic current corresponding to reduction of Pd(acac)2 was observed on the disk electrode at the potentials more positive than –1.9 V. It has been reported that some intermediate species were detected by polarogram in N, N-dimethylformamide solution containing Pd(acac)2.4 The obtained result indicated that the intermediate species, [Pd(acac)2]−, also formed by the reduction of Pd(acac)2 in this study. Thus, it is considered that the metallic palladium may be formed by the disproportionation reaction of [Pd(acac)2]−. Acknowledgments Part of this study was financially supported by MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2012–2016.