Reports Conventional binary solid-state compounds, A x B y , are infinite, crystal- line arrays of atoms A and B. Here we describe analogous binary solids in which the “atomic” building blocks are pseudo-spherical molecular clusters rather than simply atoms [for reviews on molecular clusters, see (1–3)]. We prepare these new solids by simply combining independently synthesized molecular clusters (4–6). The internal structures of the con- stituent clusters remain unchanged, but charge is transferred between them, forming ionic solids analogous to NaCl. We report three new sol- ids: [Co 6 Se 8 (PEt 3 ) 6 ][C 60 ] 2 , [Cr 6 Te 8 (PEt 3 ) 6 ][C 60 ] 2 , and [Ni 9 Te 6 (PEt 3 ) 8 ][C 60 ]. The former two assemble into a superatomic rela- tive of the CdI 2 structure type, and the latter forms a simple rock-salt crystal. Despite their ready availability, molecular clusters have been used infrequently as electronic materials. Noteworthy examples of success in this area are the organic-inorganic hybrid materials reported by Batail and Mitzi (7–11). Nanocrystals have been assembled into striking superlattices (12–14), but they do not have discrete structural, electronic and magnetic properties and cannot be regarded as genuine artificial atoms. Here, we combine independently prepared electronically and structurally complementary molecular cluster building blocks to form atomically precise binary solid-state compounds. When the building blocks are atoms (ions), binary solids assemble into simple crystalline arrays such as the rock-salt and CdI 2 lattices [for an authoritative text on solid-state inorganic chemistry, see (15)]. We show that when similarly- sized clusters combine the same lattice results, albeit at the dramatically increased length scale of nanometers rather than Angstroms. The con- stituent clusters interact to produce collective properties such as electri- cally conducting networks and magnetic ordering. Our strategy was to use constituent molecular clusters that have the same, roughly spherical, shape but very different electronic properties in order to encourage reaction and subsequent structural association. By analogy to “atomic” solid-state chemistry, we reasoned that the in situ transfer of charge would produce ions (or the equivalent) that could then form an ordered solid. Thus, we sought cluster pairs in which one cluster is relatively electron-poor and the other is relatively electron-rich. C 60 carbon clusters are good electron acceptors (16). The electrically neu- tral metal chalcogenide clusters Co 6 Se 8 (PEt 3 ) 6 (1), Cr 6 Te 8 (PEt 3 ) 6 (2), and Ni 9 Te 6 (PEt 3 ) 8 (3) are all electron- rich. Importantly, these clusters (Fig. 1) are similar in size and shape to the fullerene. We combined 1 and two equiva- lents of C 60 in toluene and obtained black crystals after ~12 hours. Sin- gle-crystal x-ray diffraction (SCXRD) revealed that this solid is a 1:2 stoichiometric combination of 1 and C 60 (12C 60 ) (Fig. 2, A and B) composed of hexagonal arrays of C 60 s in a chair-like arrangement sepa- rated by layers of the clusters. The C 60 layers are 12.5 Å apart. The cen- troid-to-centroid distance and the shortest non-bonded C-C spacing between two adjacent C 60 ’s are 9.9 Å and 3.4 Å, respectively. These dis- tances are comparable to crystalline C 60 (17). We obtain the exact same structure when we combine the Cr 6 Te 8 (PEt 3 ) 6 cluster 2 and two equivalents of C 60 in toluene (figs. S2 and S3) (18). Nanoscale Atoms in Solid-State Chemistry Xavier Roy, 1 Chul-Ho Lee, 1,2 Andrew C. Crowther, 3 Christine L. Schenck, 1 Tiglet Besara, 4 Roger A. Lalancette, 6 Theo Siegrist, 4,5 Peter W. Stephens, 7 Louis E. Brus, 1 Philip Kim, 2 Michael L. Steigerwald, 1 * Colin Nuckolls 1 * 1 Department of Chemistry, Columbia University, New York, NY 10027, USA. 2 Department of Physics, Columbia University, New York, NY 10027, USA. 3 Department of Chemistry, Barnard College, New York, NY 10027, USA. 4 National High Magnetic Field Laboratory, FSU, Tallahassee, FL 32310, USA. 5 Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, Tallahassee, FL 32310, USA. 6 Department of Chemistry, Rutgers State University, Newark, NJ 07102, USA. 7 Department of Physics and Astronomy, SUNY Stony Brook, Stony Brook, NY 11794, USA. *Corresponding author. E-mail: cn37@columbia.edu (C.N.); mls2064@columbia.edu (M.L.S.) We describe a solid-state material formed from binary assembly of atomically precise molecular clusters. [Co 6 Se 8 (PEt 3 ) 6 ][C 60 ] 2 and [Cr 6 Te 8 (PEt 3 ) 6 ][C 60 ] 2 assembled into a superatomic relative of the CdI 2 structure type. These solid-state materials showed activated electronic transport with activation energies of 100 to 150 millielectron volts. The more reducing cluster Ni 9 Te 6 (PEt 3 ) 8 transferred more charge to the fullerene and formed a rock-salt related structure. In this material, the constituent clusters are able to interact electronically to produce a magnetically ordered phase at low temperature, akin to atoms in a solid-state compound. We measured how much charge was transferred between the com- ponents in the solid-state material using Raman spectroscopy. The A 2 g pentagonal pinch mode of C 60 (1468 cm –1 for pristine C 60 ) shifts to lower energy by 6 cm –1 per electron transferred to C 60 independent of the do- pant or the crystal structure [see, for example, (19); for a review on dis- crete fulleride anions, see (20)]. The solid-state Raman spectra of 12C 60 and 22C 60 (fig. S4) (18) were taken using a 514.5 nm excitation laser at 4.6–7.8 kW/cm 2 power densities. The A 2 g modes of C 60 were centered at 1463 cm –1 and 1462 cm –1 in 12C 60 and 22C 60 , respectively. The differ- ence between the A 2 g peak position of 12C 60 and 22C 60 is small and within experimental error. We estimate that clusters 1 and 2 transfer two electrons, and each C 60 receives one electron. The solid-state electronic absorption spectra of 12C 60 and 22C 60 pro- vide additional confirmation for the formation of charge transfer com- plexes in the materials. The electronic spectra of both materials dispersed in KBr pellets show a series of transitions between 900 and 1150 nm with the strongest band centered at 1100 nm (figs. S6 and S7) (18). These features are transitions for the radical anion of fullerene, C 60 − (20). Cluster 1 has four weak transitions between 350 and 700 nm that were observed in 12C 60 but not in 22C 60 . We can compare these solids to traditional simple M 2+ X 1- 2 solids. The CdI 2 structure type (21) is formed by a hexagonally close-packed array of monoanions with half of the octahedral interstitial sites occupied by dications. The cations are ordered such that along the crystallographic c-direction the cation layers are alternatively empty and fully occupied, and the layers are held together by van der Waals bonding between ani- ons of neighboring layers. The structures of compounds 12C 60 and 22C 60 can be appreciated in these same terms. Wireframe representation of 12C 60 are shown in Fig. 3; in Fig. 3A we compare one C 60 -cluster- C 60 layer to the corresponding layer in CdI 2 . In panels (B), (C) and (D) we show edge-on and packing views of these same layers; the similarity between our cluster-solid and the “atomic” solid is evident. Although atomic solid CdI 2 appears in many different polytypes, which are related / http://www.sciencemag.org/content/early/recent / 6 June 2013 / Page 1/ 10.1126/science.1236259 on June 11, 2013 www.sciencemag.org Downloaded from