Li 4 C 60 : A Polymeric Fulleride with a Two-Dimensional Architecture and Mixed Interfullerene Bonding Motifs Serena Margadonna,* ,² Daniele Pontiroli, # Matteo Belli, # Toni Shiroka, # Mauro Ricco ` , # and Michela Brunelli & Department of Chemistry, UniVersity of Cambridge, Cambridge CB2 1EW, U.K., Dipartimento di Fisica, UniVersita ` degli Studi di Parma, 43100 Parma, Italy, and European Synchrotron Radiation Facility, 38042 Grenoble, France Received August 26, 2004; E-mail: serena.margadonna@ed.ac.uk An unexpected discovery in fullerene chemistry has been the ease with which C 60 units can covalently bond together to give rise to polymerized fullerene networks with a variety of structural architectures. 1 Such fullerene-bridged arrays display varying di- mensionality and interesting electronic (metallic behavior) and magnetic (ferromagnetism above room temperature) properties. The predominant C-C bridging structural motif, encountered in photo- and pressure-polymerized neutral C 60 and in the AC 60 (A ) K, Rb, Cs) solids arises from [2 + 2] cycloaddition reactions, which result in the formation of four-membered carbon rings (Figure 1a), fusing together adjacent molecules and propagating in one (1D chains) or two (2D layers) dimensions. 1 An alternative bridging mechanism involves the formation of single interfullerene C-C covalent bonds (Figure 1b), as encountered in 1D C 60 3- (Na 2 RbC 60 ) 2 and 2D C 60 4- (Na 4 C 60 ) 3 fulleride polymers. Recent work on the related fulleride salt, Li 4 C 60 , described its structure as 2D tetragonal with interfullerene bonds formed by [2 + 2] cycloaddition. 4 Here we report that when we probed the structural properties of Li 4 C 60 by high-resolution synchrotron X-ray diffraction, we found that it indeed adopts a layered polymeric structure. However, contrary to all other known fullerene polymers, each C 60 unit now bonds to its four nearest neighbors in the layers using both the [2 + 2] cycloaddition and the single C-C bridging motifs (Figure 1c), thereby giving rise to two types of differently bonded chains running perpendicular to each other. The resulting 2D fulleride network has neither been observed before experimentally nor been antici- pated theoretically. The synchrotron X-ray powder diffraction profile of Li 4 C 60 5 obtained at ambient temperature (Figure 2) revealed that its structure was body-centered monoclinic. Analysis with the LeBail pattern decomposition technique resulted in lattice parameters of a ) 9.3267(3) Å, b ) 9.0499(3) Å, c ) 15.03289(1) Å, and  ) 90.949(3)° (space group I2/m; R wp ) 4.24%, R exp ) 1.53%). A notable feature of these results is that the structure of Li 4 C 60 is strongly anisotropic with the closest center-to-center contacts between the C 60 units of ∼9.33, ∼9.05, and ∼9.95 Å along the a and b axis and the body diagonal, respectively. While the latter contact is comparable to those encountered in monomeric fullerenes and fullerides (∼10.0 Å), the one along b is reminiscent of that in monoclinic polymerized AC 60 (∼9.11 Å) 6 in which there are two bridging C-C bonds between C 60 - ions (Figure 1a). Therefore, the starting structural model used in the Rietveld refinement of the Li 4 C 60 diffraction data was based on that of RbC 60 with linear fulleride chains running along b (Figure 1a). However, the Rietveld refinement did not proceed smoothly within this model with the diffraction profile described poorly (R wp ) 18.6%), especially in the high 2θ region. At this stage, we note that short C-C contacts are also implied along the a axis, with the interfullerene distances comparing well with earlier observations in fullerides with single C-C interfullerene connections, like the 1D Na 2 RbC 60 and the 2D Na 4 C 60 polymers, 2,3 in which the shortest center-to-center distances are 9.38 and 9.28 Å, respectively. Thus, starting from the previous model, we performed a search of possible alternative C 60 orientations in the ab plane by allowing the two C 60 units present in the monoclinic unit cell to rotate about the [010] direction anticlockwise and monitoring the resulting quality-of-fit factors (R wp ) of the Rietveld refinements. Figure 1S (Supporting Information) presents the evolution of R wp with the rotation angle, φ, which was varied between 0 and 180° in increments of δφ ) 5°. The refinements were stable throughout the φ-range with a single deep minimum in R wp (10.02%) evident at an angle, φ ) 100°. 7 At this angle, it is remarkable that the relative orientation of the molecules is such that pairs of carbon ² University of Cambridge. # Universita ` degli Studi di Parma. & European Synchroton Radiation Facility. Figure 1. Schematic drawing of the interfullerene C-C bridging structural motifs in polymeric fullerides. (a) [2 + 2] cycloaddition in RbC60, (b) single C-C covalent bonds in Na2RbC60, and (c) mixed bonding in Li4C60. Figure 2. Final observed (O) and calculated (s) synchrotron X-ray diffraction profile for Li4C60 at 295 K (a ) 9.3264(4) Å, b ) 9.0478(4) Å, c ) 15.03294(2) Å,  ) 90.967(3)°, space group I2/m, agreement factors of the Rietveld refinement: R wp ) 5.12%, Rexp ) 1.53%, RF2 ) 3.46%). The lower solid line shows the difference profile, and the tick marks show the reflection positions. Inset: geometry of the frontier (C(5) and C(16)) and bridging (C(15)) carbon atoms on adjacent fullerenes along the direction (a axis) of single C-C polymerization. Published on Web 10/30/2004 15032 9 J. AM. CHEM. SOC. 2004, 126, 15032-15033 10.1021/ja044838o CCC: $27.50 © 2004 American Chemical Society