A New Binding Motif of Sterically Demanding Thiolates on a Gold Cluster Jun-ichi Nishigaki, † Risako Tsunoyama, ‡ Hironori Tsunoyama, ‡ Nobuyuki Ichikuni, § Seiji Yamazoe, † Yuichi Negishi, ∥ Mikinao Ito, ⊥ Tsukasa Matsuo, ⊥ Kohei Tamao, ⊥ and Tatsuya Tsukuda* ,†,‡ † Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan ‡ Catalysis Research Center, Hokkaido University, Nishi 10, Kita 21, Sapporo 001-0021, Japan § Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, Inage-ku, Chiba 263-8522, Japan ∥ Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ⊥ Functional Elemento-Organic Chemistry Unit, RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan * S Supporting Information ABSTRACT: A gold cluster, Au 41 (S-Eind) 12 , was synthe- sized by ligating the bulky arenethiol 1,1,3,3,5,5,7,7- octaethyl-s-hydrindacene-4-thiol (Eind-SH) to preformed Au clusters. Extended X-ray absorption fine structure, X- ray photoelectron spectroscopy, and the fragmentation pattern in the mass spectrometry analysis indicated that formation of gold−thiolate oligomers at the interface was suppressed, in sharp contrast to conventional thiolate- protected Au clusters. C rystallographic structure determination of Au 102 (p- MBA) 44 (MBA = mercaptobenzoic acid) by Kornberg’s group 1 has made a breakthrough in our understanding of the structure of thiolate-protected Au clusters (Au:SR). It was found that gold−thiolate oligomers, −SR−[Au−SR−] (named “staples”) and −SR−[Au−SR−] 2 , are completely coordinated to a decahedral Au 79 core. 2 Subsequently, similar interfacial structures have been theoretically predicted and experimentally observed in Au 25 (SR) 18 (ref 3) as well as Au 38 (SR) 24 . 4 The formation of such oligomers has also been theoretically proposed for other isolated clusters, including Au 18 (SR) 14 , 5 Au 20 (SR) 16 , 5−7 Au 24 (SR) 20 , 8 Au 44 (SR) 28 , 9 and Au 144 (SR) 60 . 10 Gold−thiolate oligomeric interfacial structures have a big impact on the physicochemical properties of small Au:SR, such as magnetism, 11 photoluminescence, 12 chiroptical activity, 13 and catalysis. 14 New properties can emerge from the construction of new interfacial structures in Au:SR. In a previous study, we intended to ligate the thiolates directly on the surface of the Au core by reacting n-alkanethiol with preformed Au clusters weakly stabilized by poly(N-vinyl-2-pyrrolidone) (PVP). 15 However, the structure of the Au core was substantially reconstructed in the ligation process, and Au−SR oligomers were formed at the interface. In this study, the bulky, rigid arenethiols 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacene-4-thiol (Eind-SH) and 3,3,5,5-tetraethyl-1,1,7,7-tetramethyl-s-hydrindacene-4-thiol (MEind-SH) (Figure 1) were employed as protecting ligands. 16,17 Two modes of the steric effect are expected for these bulky arenethiols. One is the steric repulsion between adjacent ligands. This should reduce the coverage of the ligand and/or the size of the Au clusters; Tracy and co-workers demonstrated that Au clusters protected by bulky thiolate (1- adamantanethiolate and cyclohexanethiolate) have slightly smaller coverage of the ligands and that bulkier ligands afforded smaller sizes of Au−thiolate clusters. 18 The other effect is the steric constraint around the sulfur group. This should suppress the formation of Au−SR oligomers on the Au cluster surface. Here we report the formation of a new binding motif of the bulky Eind thiolate on a Au 41 cluster. Details of the synthesis of Au clusters protected by the Eind- and MEind-thiolates (Au:S-Eind and Au:S-MEind) are described in the Supporting Information (SI); 17 only the general synthetic method for Au:S-Eind is described below. We initially attempted to prepare Au:S-Eind by the Brust method. 19 However, this method yielded precipitates and nanoparticles larger than ∼2 nm as major products. Next, the ligand exchange approach was employed; Au n clusters stabilized by PVP (Au n :PVP) were allowed to react with Eind-SH in toluene (Scheme 1). 17 An aqueous dispersion of Au n :PVP (n ≈ 38; Figure S1 in the SI) was mixed with a toluene solution of Eind- SH by either batch or microfluidic mixing at 90 °C. The intense dark-brown color of the water phase was transferred to the Received: June 6, 2012 Published: August 17, 2012 Figure 1. (a) Molecular structures of Eind-SH and MEind-SH. (b) Space-filling model of Eind-SH. The green ball represents the sulfur atom. Communication pubs.acs.org/JACS © 2012 American Chemical Society 14295 dx.doi.org/10.1021/ja305477a | J. Am. Chem. Soc. 2012, 134, 14295−14297