A global search of highly stable gold-covered bimetallic clusters M@Aun (n=8-17):: Endohedral gold clusters

Gold nanoclusters have been proven to be a new class of active catalysts.([1-4]) Much experimental and theoretical efforts hitherto have been devoted to structures, stability and properties of bare gold clusters.([2-8]) Recently, increasing attention has been paid towards gold-based bimetallic clusters ([1,9-13]) since the additional degree of freedom in the stoichiometry can offer chemical versatility for tuning properties of gold-based clusters, as well as enhancing their potential applications. Pyykko and Runeberg([9a]) were the first to predict the existence of highly stable gold-covered heteroatomic clusters W@Au-12 and M@Au-12(-) (M=V, Nb, and Ta). The unusual chemical stability of these clusters can be attributed to two physical/chemical reasons: 1) strong relativistic effects, and 2) the perfect 18-electron bonding to the center hetero-metal atom.([9]) These gold-covered clusters have been isolated experimentally by Wang and co-workers.([10,11]) Neukeremans et al. performed photofragmentation experiments and studied the stability of bimetallic gold clusters containing a 3d transition-metal atom (Sc, Ti, V, Cr, Mn, Fe, Co, or N)([12]). They also found that the closed-shell clusters with 18 valence electrons are particularly stable. We recently reported a highly stable gold-covered bimetallic neutral cluster, Zr@Au-14, which has a very large energy gap (BP86: 2.23 eV) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).([13]) This HOMO-LUMO gap is larger than that of the magic-number Au-20 cluster with a calculated/measured HOMO-LUMO/optical gap of 1.85/1.77 eV. 141 Furthermore, we reported that the electron affinity (EA) of the Sc@Au-14 cluster (BP86: 4.13 eV) is even higher than that of the chlorine atom which has the highest elemental EA of 3.61 eV. Common to these previous studies is the compliance to the 18-electron rule;([15]) a key factor to the high stability of the gold-based bimetallic clusters.