SILSESQUIOXANES AS LIGANDS FOR GOLD CLUSTERS

A new type of gold cluster has been synthesized using the silsesquioxa ne derivative (cyclopentyl)(7)Si8O12(CH2)(3)SH (T-8-OSS-SH) to exchang e the PPh3 ligands in (PPh3)(12)Au55Cl6 by T-8-OSS-SH quantitatively. This exchange causes several important changes in the physical and che mical behavior of the compound. Owing to the presence of cyclopentyl s ubstituents, the cluster becomes soluble in non-polar solvents such as pentane. The stability of this compound compared with that of the PPh 3-protected cluster is considerably higher meaning that decomposition in solution, and even under the high-energetic irradiation in the micr oscope, is not observed over longer periods of time. The increase in t he total diameter from 2.1 to ca. 4.4 nm is probably responsible for t hese advantageous properties, supported by strong S-Au bonds. The incr ease in size is also expressed as an increase in the activation energy for electron-tunneling processes through the ligand shells of neighbo ring clusters in a densely packed pellet. This increase is from 0.16 e V for (PPh3)(12)Au55Cl6 to 0.26 eV for the T-8-OSS-SH-protected cluste r.