Synthesis of the Tetrahedral Au20 Cluster in Solution
Tetrahedron of 20 Au atoms with PPh3 ligands attached to the apexes (l), and with eight ligands, the additional four less strongly attached to the center of the faces (r).
One of the longstanding objectives of cluster science is to discover highly stable clusters and use them as building blocks for bulk cluster-assembled materials. The discovery of C60 in the gas phase and its subsequent bulk synthesis provide classical inspiration and a prototypical example. However, it is generally believed that few other stable gaseous clusters maybe assembled into bulk materials because of cluster-cluster interactions leading to agglomeration. Indeed, no cluster-assembled materials have been synthesized on the basis of stable gaseous clusters other than the fullerenes, despite intensive experimental and theoretical efforts. Recently, we reported that a 20-atom gold cluster possesses a tetrahedral (Td) structure with a remarkably large HOMO-LUMO energy gap, suggesting that it would be highly chemically inert and may have novel optical and catalytic properties. Researchers at PNNL have observed the tetrahedral Au20 cluster in solution, ligated with triphenyl phosphine (PPh3) ligands, and its confirmation by theoretical calculations. The current work represents a successful synthetic effort directly guided and motivated by a gas-phase observation, validating the gas-phase-to-condensed-phase approach for the discovery of cluster-assembled nanomaterials.
Because of potential cluster-cluster agglomeration, Au20 must also be protected by ligands in order to use it as a building block for cluster-assembled materials. Preliminary theoretical calculations revealed that the Au20(PR3)4 (R = H, Ph) complexes indeed possess high stability. A high resolution transmission electron microscope image) showed that the soluble samples contained gold nanoparticles with diameters as large as 3 nm, but the majority of the particles have diameters of less than 1 nm. Fourier transform ion cyclotron resonance (FTICR) mass spectrometery revealed doubly charged ions corresponding to Au20 clusters with eight and seven PPh3 ligands. To obtain structural information for the Au20(PPh3)82+ cluster, we conducted collision-induced dissociation (CID) experiments in the FTICR cell. These results suggest that the tetrahedral core of Au20 is intact in the PPh3-coordinated clusters. This is consistent with our initial expectation and calculation that the four apex sites of Au20 are the most reactive sites that bind strongly to the four PPh3 ligands.
The current experimental and theoretical results suggest that Td Au20 clusters coordinated with phosphine ligands may be obtained in bulk quantity. It is expected that by increasing the size of the ligands one can synthesize the Td Au20 clusters with only the four apex sites coordinated. These clusters may be promising catalysts with the highest surface area and wel ldefined surface sites. These exciting results are published in the Journal of Physical Chemistry B and were featured on the cover of that issue. This research was done by HF Zhang, M Stender, R Zhang, C Wang, J Li, and LS Wang:
Reference: Zhang H-F, M Stender, R Zhang, C Wang, J Li, and L-S Wang. 2004. "Toward the Solution Synthesis of the Tetrahedral Au20 Cluster." Journal of Physical Chemistry B 108(33):12259-12628. DOI:10.1021/jp048636q.