Unusual Attractive Au-π Interactions in Small Diacetylene-Modified Gold Clusters.

It is well known that alkynes act as π-acids in the formation of complexes with metals. We found unprecedented attractive Au-π interactions in diacetylene-modified [core+exo]-type [Au8 ]4+ clusters. The 4-phenyl-1,3-butadiynyl-modified cluster has unusually short Au-Cα distances in the crystal structure, revealing the presence of attractive interactions between the coordinating C≡C moieties and the neighboring bitetrahedral Au6 core, which is further supported by IR and NMR spectra. Such weak interactions are not found in mono-acetylene-modified clusters, which indicates that they are specific for diacetylenic ligands. The attractive Au-π interactions are likely associated with the low energy of the π* orbital in the diacetylenic moieties, into which the valence electrons of the gold core may be back donated. The [Au8 ]4+ clusters show clear red-shifts of >10 nm with respect to the corresponding mono-acetylenic clusters in UV/Vis absorption bands, which indicates substantial electronic perturbation effects of the Au-π interactions.