AmericanChemicalSociety.com

One of the major objectives of cluster science is to discover stable atomic clusters, which may be used as building blocks for cluster-assembled nanomaterials.  The discovery and bulk synthesis of the fullerenes have sprouted new research disciplines in chemistry and nanoscience and precipitated intense interests to search for other similar stable clusters.  However, despite major research efforts, no other analogous gas-phase clusters have been found and yielded to bulk syntheses.  We have discovered in cluster beam experiments two highly stable and symmetric cage clusters, stannaspherene (Sn₁₂^2–) and plumbaspherene (Pb₁₂^2–).  The names for these two clusters derive from their icosahedral (I_h) symmetry and delocalized spherical ¹-bonding that are characteristics of buckminsterfullerene C₆₀.  Stannaspherene and plumbaspherene have diameters comparable to that of C₆₀ and can be considered as inorganic analogs of the buckyball.  The large internal space in Sn₁₂^2– has been shown to be able to trap any transition metal atom to form new endohedral cage clusters, M@Sn₁₂^–, analogous to endohedral fullerenes.  The doped atom in M@Sn₁₂^– keeps its quasi-atomic nature with large magnetic moments.  These endohedral cages form a rich class of new building blocks for cluster-assembled materials with tunable magnetic, electronic, and chemical properties. 

During our attempt to synthesize endohedral stannaspherenes, we crystallized a new Pd₂@Sn₁₈^4– cluster, which can be viewed as the fusion of two Pd@Sn₁₂^2– clusters.  This result suggests that stannaspherene, plumbaspherene, and a large number of their endohedrally doped species can be synthesized in the bulk.  We have extended this work to germanium clusters.  Although Ge₁₂^2- (germaspherene) is not a stable cage cluster, we have shown in preliminary results that the doping of a transition metal can stabilize it to give a class of M@Ge₁₂^- endohedral germaspherenes. In an attempt to synthesize Pd@Ge₁₂^-, we isolated a new Zintl cluster [Ge₉PdPPh₃]^3– as a (2,2,2-crypt)K⁺ salt through the reaction of K₄Ge₉ and Pd[PPh₃]₄ in ethylenediamine solutions and characterized it via single-crystal X-ray crystallography.  We further found that the as-prepared bimetallic [Ge₉PdPPh₃]^3– cluster could successfully trap a nickel atom to form a trimetallic cluster [Ni@(Ge₉PdPPh₃)]^2–.  The coordination of Ge₉^4– by PdPPh₃ induces a one-electron oxidation and encapsulation of the Ni atom into the Ge₉^3– cage leads to a further one-electron oxidation and a geometry transformation from C_4v (nido) to C_3v (closo).

This grant has allowed the PI to explore a new research direction in nanoscience and, in particular, it has a major impact on the student, Dr. Li-Feng Cui, who made the initial discovery of stannaspherene as a PhD student.  Dr. Cui is currently doing his postdoctoral research on lithium batteries using nanostructured silicon.  His current research has been influenced by his research on the clusters of group-IVA elements, which led to the discovery of the icosahedral cage clusters.