Letter pubs.acs.org/JPCL
Fully Cationized Gold Clusters: Synthesis of Au25(SR+)18 Yohei Ishida,*,† Kunihiro Narita,† Tetsu Yonezawa,*,† and Robert L. Whetten‡ †
Division of Material Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan ‡ Department of Physics & Astronomy, The University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-0697, United States S Supporting Information *
ABSTRACT: Although many thiolate-protected Au clusters with different numbers of Au atoms and a variety of thiolate ligands have been synthesized, to date there has been no report of a fully cationized Au cluster protected with cationic thiolates. Herein, we report the synthesis of the first member of a new series of thiolate-protected Au cluster molecules: a fully cationized Au25(SR+)18 cluster.
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the solubility of the Au(I)−SR species in a reaction solvent, the homogeneous reduction and growth of the Au clusters are highly restricted. Atomically pure cationic Au clusters are thus difficult to produce. With respect to issue (ii), cationic groups such as trimethylammonium or methylpyridinium groups have positive charges under any pH conditions. Because the cationic groups would be densely populated on the surface of a very small Au cluster (depending on the ligand’s chain length and cluster size), the Coulombic repulsion between the cationic ligands on the cluster’s surface would be strong and able to decompose the Au25 structure, as reported for anionic charged Au25(mercaptocarboxylic acid)18 clusters with different chain lengths.17 This rapid decomposition of the cluster structures due to the surface charge makes the synthesis difficult. Au clusters capped with thiolate ligands bearing amino groups (−NH2) such as cysteamine, which can be cationized via protonation under acidic conditions, have, on the contrary, been stably obtained.16,18,19 Predictably, because of the aforementioned mechanistic considerations, cationic thiolate-protected molecular Au clusters have not yet been synthesized. Several reports have described the use of cationic thiols for the synthesis of gold or other metal nanoparticles with diameters exceeding ca. 2 nm.20−23 A cationized Au cluster has only been reported for a mixture of Au144 and Au146 clusters through the partial ligand exchange of the initial hexanethiolate with a cationic thiolate ligand with an uncontrollable exchange ratio.24 A very small, fluorescent, cationic Au cluster has also been reported; however, its composition could not be ascertained and remains
hiolate-protected Au clusters consist of ultrasmall Au particles, stabilized by a number of thiolate ligands, with core diameters of
