Ligand-protected gold clusters with defined compositions and structures have attracted special attention because of diverse geometric structures and unique optical/electronic properties associated with their molecule-like features. During our recent studies on phosphine-ligated gold clusters in the subnanometer regime (nuclearity ~10), some examples of peculiar interactions at the metal-ligand interface, such as weak attractive forces involving heteroatoms and π-functionalities in the ligand moieties, have been found . Herein we report unusual Au…H-C hydrogen bonds, which were characterized by X-ray crystallography and solution NMR .
The cluster we used here is core+exo type Au6 cluster carrying m-phenylene-bridged diphosphines. The crystal structure showed close contacts of the gold framework to the bridging m-phenylene units. The hydrogen atoms at the 2-position of the bridges (H-2) are located in proximity to the tetrahedral core with Au-H distances of 2.60 – 2.65 Å, which are shorter than the sum of the van der Waals radii (2.86 Å). Accordingly, the distances to the C-2 atoms (3.641 – 3.699 Å) were shorter than that estimated with the assumption that C, H and Au atoms are aligned with van der Waals Au-H contact (3.95 Å). These observations imply the presence of hydrogen-bond-type interactions between the H-2 atoms and the Au cluster unit. It should be noted that the Au…H-C interactions were also observed in solution NMR, which showed 1H and 13C NMR signals due to the C-H units at considerably downfield regions (δH = 11.6, δC = 147.3); The downfield shifts from normal aromatic protons were more than 4 and 10 ppm for 1H and13C, respectively. We also show similar metal-H hydrogen bonds in related cluster systems. These results not only demonstrate the unique capability of small gold cluster to interact with unfunctionalized C-H groups but also shed light on the elucidation of recently emerging gold cluster catalysis.
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