Metal-ligand core-shell nanocomposite catalysts for the selective semihydrogenation of alkynes.

In recent years, hybrid nanocomposites with core–shell structures have increasingly attracted enormous attention in many important research areas such as quantum dots, optical, magnetic, and electronic devices, and catalysts. In the catalytic applications of core–shell materials, core-metals having magnetic properties enable easy separation of the catalysts from the reaction mixtures by a magnet. The core-metals can also affect the active shell-metals, delivering significant improvements in their activities and selectivities. However, it is difficult for core-metals to act directly as the catalytic active species because they are entirely covered by the shell. Thus, few successful designs of core–shell nanocomposite catalysts having active metal species in the core have appeared to date. Recently, we have demonstrated the design of a core–shell catalyst consisting of active metal nanoparticles (NPs) in the core and closely assembled oxides with nano-gaps in the shell, allowing the access of substrates to the core-metal. The shell acted as a macro ligand (shell ligand) for the core-metal and the core–shell structure maximized the metal–ligand interaction (ligand effect), promoting highly selective reactions. The design concept of core–shell catalysts having core-metal NPs with a shell ligand is highly useful for selective organic transformations owing to the ideal structure of these catalysts for maximizing the ligand effect, leading to superior catalytic performances compared to those of conventional supported metal NPs. Semihydrogenation of alkynes is a powerful tool to synthesize (Z)-alkenes which are important building blocks for fine chemicals, such as bioactive molecules, flavors, and natural products. In this context, the Lindlar catalyst (Pd/ CaCO3 treated with Pb(OAc)2) has been widely used. [13] Unfortunately, the Lindlar catalyst has serious drawbacks including the requirement of a toxic lead salt and the addition of large amounts of quinoline to suppress the over-hydrogenation of the product alkenes. Furthermore, the Lindlar catalyst has a limited substrate scope; terminal alkynes cannot be converted selectively into terminal alkenes because of the rapid over-hydrogenation of the resulting alkenes to alkanes. Aiming at the development of environmentally benign catalyst systems, a number of alternative lead-free catalysts have been reported. 15] Recently, we also developed a leadfree catalytic system for the selective semihydrogenation consisting of SiO2-supported Pd nanoparticles (PdNPs) and dimethylsulfoxide (DMSO), in which the addition of DMSO drastically suppressed the over-hydrogenation and isomerization of the alkene products even after complete consumption of the alkynes. This effect is due to the coordination of DMSO to the PdNPs. DMSO adsorbed on the surface of PdNPs inhibits the coordination of alkenes to the PdNPs, while alkynes can adsorb onto the PdNPs surface because they have a higher coordination ability than DMSO. This phenomenon inspired us to design PdNPs coordinated with a DMSO-like species in a solid matrix. If a core–shell structured nanocomposite involving PdNPs encapsulated by a shell having a DMSO-like species could be constructed, it would act as an efficient and functional solid catalyst for the selective semihydrogenation of alkynes. Herein, we successfully synthesized core–shell nanocomposites of PdNPs covered with a DMSO-like matrix on the surface of SiO2 (Pd@MPSO/SiO2). The shell, consisting of an alkyl sulfoxide network, acted as a macroligand and allowed the selective access of alkynes to the active center of the PdNPs, promoting the selective semihydrogenation of not only internal but also terminal alkynes without any additives. Moreover, these catalysts were reusable while maintaining high activity and selectivity. Pd@MPSO/SiO2 catalysts were synthesized as follows. Pd/ SiO2 prepared according to our procedure [16] was stirred in n-heptane with small amounts of 3,5-di-tert-butyl-4-hydroxytoluene (BHT) and water at room temperature. Next, methyl3-trimethoxysilylpropylsulfoxide (MPSO) was added to the mixture and the mixture was heated. The slurry obtained was collected by filtration, washed, and dried in vacuo, affording Pd@MPSO/SiO2 as a gray powder. Altering the molar ratios of MPSO to Pd gave two kinds of catalysts: Pd@MPSO/SiO21 (MPSO:Pd = 7:1), and Pd@MPSO/SiO2-2 (MPSO:Pd = 100:1). [*] Dr. T. Mitsudome, Y. Takahashi, Dr. T. Mizugaki, Prof. Dr. K. Jitsukawa, Prof. Dr. K. Kaneda Department of Materials Engineering Science Graduate School of Engineering Science, Osaka University 1–3, Machikaneyama, Toyonaka, Osaka 560-8531 (Japan) E-mail: kaneda@cheng.es. osaka-u.ac. jp