Unusual Attractive Au‐π Interactions in Diacetylene‐modified Small Gold Clusters

It is well known that alkynes serve as π‐acid to form metal complexes through strong π‐coordination bonds. In this work, we highlight unprecedented "attractive" Au‐π interactions found in diacetylene‐modified [core+exo]‐type [Au8]4+ clusters (2). The 4‐phenyl‐1,3‐butadiynyl‐modified cluster (2a) showed unusually short Au‐Cα distances in the crystal structure, revealing the presence of attractive interactions of the coordinating C≡C moieties with the neighboring bitetrahedral Au6 core, which was further supported by IR and NMR spectra. To note is the fact that such weak interactions were not found in mono‐acetylene‐modified clusters (1), indicating they are specific for diacetylenic ligands. The Au‐π attractive interactions are likely associated with the low π* energy level of the diacetylenic moieties, to which the valence electrons in the gold core may be donated in a back‐donation‐like manner. 2 showed clear red shifts of > 10 nm with respect to the corresponding mono‐acetylenic clusters (1) for the visible absorption bands, indicating the substantial electronic perturbation effects of the Au‐π interactions. This work implies potential utility of diacetylenic ligand for the emergence of unique binding motif and cluster structures.

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