Isomerization in Alkynyl-Protected Gold Nanoclusters

We report the controlled synthesis and structures of two isomeric gold nanoclusters, whose compositions are determined to be Au-23(C CBut)(15) (denoted as Au-23-1 and Au-23-2) by single-crystal X-ray diffraction and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. This is the first time isomerism is discovered in alkynyl-protected gold nanoclusters. The metal-to-ligand ratios in these two clusters are different from known Au-n(SR)(m) systems and have not been observed in the Au-x(C CPh)(y) family. This pair of isomers exhibits different optical properties, although they have similar structures and identical components. For both Au-23 clusters, time-dependent density functional theory calculations revealed the frontier orbitals highest occupied molecular orbital (HOMO)-1, HOMO, and lowest unoccupied molecular orbital (LUMO) are mainly constructed from the Au-15 kernel and V-shaped alkynyl-gold motifs. The HOMO -> LUMO transition of Au-23 -1 is optically forbidden, whereas it is allowed in Au-23-2. It is also found that Au-23-2 cluster can be transformed to Au-23-1 spontaneously under ambient conditions. This work offers further insight into the synthesis and isomerism of all-alkynyl-protected gold nanoclusters and will stimulate more investigation of isomeric metal nanoclusters.