The stability enhancement factor beyond eight-electron shell closure in thiacalix[4]arene-protected silver clusters

Abstract

We report the synthesis and structures of two 34-atom metal nanoclusters, namely [Ag₃₄(BTCA)₃(CCBu^t)₉(tfa)₄(CH₃OH)₃]SbF₆ and [AuAg₃₃(BTCA)₃(CCBu^t)₉(tfa)₄(CH₃OH)₃]SbF₆, where H₄BTCA is p-tert-butylthiacalix[4]arene and tfa is trifluoroacetate. Their compositions and structures have been determined by single-crystal X-ray structural analysis and ESI-MS. The cationic cluster consists of a centered icosahedron M@Ag₁₂ (M = Ag or Au) core that is surrounded by 21 peripheral silver atoms. Surrounding protection is provided by four kinds of ligands, including three BTCA, nine ^tBuCC, four tfa, and three methanol solvent ligands. It was found that the Ag₅@BTCA μ₅-coordination motif of thiacalixarene is critical for high stability of the title clusters, and extra stability enhancement can be achieved by doping a gold atom at the center of the silver cluster. This work suggests that coordination saturation should be taken into account in addition to electronic and geometric factors for analyzing metal nanocluster stabilities.