Impact of the Selenolate Ligand on the Bonding Behavior of Au₂₅ Nanoclusters

Abstract

We report the intriguing bonding behavior of selenolate-protected Au₂₅ nanoclusters (Au₂₅(SeR)₁₈), revealed by temperature-dependent X-ray absorption spectroscopy (XAS) from both the metal (Au L₃-edge) and ligand (Se K-edge) perspectives. The structure of Au₂₅(SeR)₁₈ was first analyzed in great detail using a site-specific, multishell (i.e., Au–Au core/surface/staple, Au–Se/Se-Au and Se–C shells) approach. It was found that the Au₁₃ core of Au₂₅(SeR)₁₈ remains relatively unchanged at low temperature while aurophilic interactions on the surface are significantly longer in distance compared with their thiolate-protected counterpart, Au₂₅(SR)₁₈. Remarkably, temperature-dependent studies showed a significant thermal contraction of the Au–Au framework in Au₂₅(SeR)₁₈, which is absent in the Au₂₅(SR)₁₈ system. This unusual bonding behavior of Au₂₅(SeR)₁₈ is proposed to be induced by the dimeric staple-like motif (−Au–Se–Au–Se–Au−) surface structures, where aurophilic bond distances and Au–Se–Au bond angles are sensitive toward temperature change. Density functional theory and molecular dynamics (DFT-MD) simulations were conducted to confirm this mechanism and provide further insight into the bonding behavior of the Au₂₅(SeR)₁₈ nanocluster. Finally, we use near-edge XAS results to demonstrate that the thermal contraction effect induces a change to the electronic properties of both the Au and Se and consistently accounted for using ab initio simulations of the near-edge and valence band structure