Ligand accommodation causes the anti-centrosymmetric structure of Au13Cu4 clusters with near-infrared emission.

Abstract

We synthesized an [Au13Cu4(PPh3)4(SPy)8]+ nanocluster co-capped by phosphine and thiolate ligands. Interestingly, this Au13Cu4 cluster corresponds to an anti-centrosymmetric structure with the four copper atoms coordinated to the mixed ligands on the same side of the Au13 icosahedron, which is in sharp contrast to the [Au13Cu4(PPh2Py)4(SPhtBu)8]+ and [Au13Cu2(PPh3)6(SPy)6]+ clusters which possess highly symmetric structures with well-separated Cu adatoms. Both [Au13Cu4(PPh3)4(SPy)8]+ and [Au13Cu2(PPh3)6(SPy)6]+ clusters correspond to 8 valence electron superatoms with large HOMO-LUMO gaps, respectively. The difference in structure is rooted in the nature of the mixed ligands, with the bidentate SPy binding strongly to Cu on both binding sites (-N-Cu and Au-SR-Cu) leading to the co-linking of adjacent Cu atoms, while the bidentate PPh2Py binds Cu on one site and Au on the other giving rise to a separation of the Cu atoms even in the presence of relatively higher monomer concentration. Both [Au13Cu4(PPh3)4(SPy)8]+ and [Au13Cu2(PPh3)6(SPy)6]+ display emissions in the near-IR regions. TD-DFT calculations reproduce the spectroscopic results with specified excited states, shedding light on the geometric and electronic behaviors of the ligand-protected Au13Mx clusters.