ESI-MS Identification of Abundant Copper–Gold Clusters Exhibiting High Plasmonic Character

Abstract

The protected noble-metal structures comprising 145 metal-atom sites and 60 ligands are among the frequently identified larger metal-cluster systems exploited in many avenues of research. Herein we report a comparative electrospray ionizationmass spectrometry (ESI-MS) investigation of the 60-fold thiolated Au 144 and CuAu 144 clusters, in various positive charge-states, in conjunction with a density-functional theoretical (DFT) analysis based upon the icosahedral Pd 145 -structure-type applicable to these systems. Samples rich in the hexanethiolate-protected CuAu 144 clusters are obtained via a single-phase reduction process. The predicted electronic structure of the vacancy-centered Au 144 (SR) 60 system provided a simple rationale for the limiting [4+] charge-state observed of Au 144 , whereas the maximal [3+] charge detected on the CuAu 144 (SR) 60 cluster can be explained if the 145th atom occupies the central site. Occupancy of the center-site stabilizes the superatomic 3S-orbital, and thereby shifts the shell-closing count from 82 to 84 free electrons. The DFT-calculated energetics also predicts a strong (0.65 eV) preference for placing the smaller Cu ion in this central site. Remarkably, the optical absorption spectra of dilute tetrahydrofuran (THF) solutions feature a broad band centered near 2.3 eV, in contrast to the previously reported "nonplasmonic" response of sub-2.0-nm all-gold or -copper clusters. Other methods (matrix-assisted laser desorption ionization mass spectrometry and high-resolution electron microscopy) were used to investigate whether aggregation phenomena might account for this observed plasmon emergence. This unusual result points to the need to obtain highly purified samples of copper-doped gold clusters of ca. 145 atoms total. ■ INTRODUCTIONIn the physical chemistry of metal clusters, one prime concern has been to elucidate the rules, or criteria, that can explain the selection of exceptionally stable cluster sizes, or compositional "magic numbers", and predict the existence of other related special clusters. Typically, such special clusters are first identified by mass-spectrometric analysis, well before definitive structural characterization can be achieved. In the simpler ligand-free metal clusters, the main rules select for (i) the number of free electrons, n e , available to fill the set of angularmomentum shells, according to the universal aufbau of the spherical electron-shell model (or superatom analogy); and (ii) the number of metal atoms, n, required to complete concentric shells of atomic sites, as relating to crystallography and surface structure. 1,2 A simple metal cluster of formula A n[z] is said to be "doubly magic", if both counts, n and n e = v A n − z, satisfy the respective shell-closing criteria, where v A represents the metalatom valence and ze is the electrical charge. 3 Selected monolayer-protected clusters (MPCs), 4 denoted compositionally by A n X p [z] , feature an additional, external shell of p anionic ligand-groups, X − , that coordinate specifically t...

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