Self-Assembly of a Chiral Cubic Three-Connected Net from the High Symmetry Molecules C60 and SnI4

Abstract

The design of new chiral materials usually requires stereoselective organic synthesis to create molecules with chiral centers. Less commonly, achiral molecules can self-assemble into chiral materials, despite the absence of intrinsic molecular chirality. Here, we demonstrate the assembly of high-symmetry molecules into a chiral van der Waals structure by synthesizing crystals of C<sub>60</sub>(SnI<sub>4</sub>)<sub>2</sub> from icosahedral buckminsterfullerene (C<sub>60</sub>) and tetrahedral SnI4 molecules through spontaneous self-assembly. The SnI<sub>4</sub> tetrahedra template the Sn atoms into a chiral cubic three-connected net of the SrSi<sub>2</sub> type that is held together by van der Waals forces. Our results represent the remarkable emergence of a self-assembled chiral material from two of the most highly symmetric molecules, demonstrating that almost any molecular, nanocrystalline, or engineered precursor can be considered when designing chiral assemblies.

Generalizing the Chiral Self-Assembly of Spheres and Tetrahedra to Non-Spherical and Polydisperse Molecules in (C70)x(C60)(1-x)(SnI4)2

“…22 Single crystals of C70(SnI4)2 are synthesized similarly to C60(SnI4)2 (Supporting Information). 12 Like C60(SnI4)2, C70(SnI4)2 crystallizes in the chiral enantiomorphic space group P4132 (#213) (Table S1), and the Flack parameter of -0.04(3) confirms the single crystal is a single chiral domain and that the absolute configuration (handedness) of the crystal has been successfully resolved. 23 Because neither C60 nor SnI4 contain a stereocenter, it is likely that both left-and right-handed single crystals of C70(SnI4)2 will form, so it may also crystallize in the P4332 (#212) space group, which is identical to P4132 except for its handedness.…”

“…Like in C60(SnI4)2, the location of individual carbon atoms cannot be resolved. 12 We are not able to determine whether C70 freely rotates or whether it there is some degree of dynamic or static orientational order such that C70 preferentially rotates about its long axis. Compared to C60, C70 is identical in width but is elongated owing to the addition of five hexagons around its girth.…”

“…The structure of C60(SnI4)2 is shown in Figure 1B for comparison. 12 Details of the crystal structure refinement are given in Supporting Information. At room temperature, C70 is rotating within the crystal, which we observe through a lack of resolved atomicity in the structural model derived from the single crystal diffraction data.…”

“…20 In the FCC phase where the C70 molecules exhibit full orientational disorder, the lattice constant is 14.96 A, which indicates that the effective van der Waals radius of C70 is 5.29 Å, 5.6% larger than the 5.01 Å radius of C60. 19 Based on the effective radius of SnI4 of 2.95 Å, 12 the radius ratio γ of C70(SnI4)2 is 0.56. This ratio is similar to the ratio for SrSi2 (γ = 0.55) as well as for C60(SnI4)2 (γ = 0.59), consistent with these three materials being isostructural and indicating that that the larger size of C70 compared to C60 does not significantly change γ.…”

“…The fact that C70(SnI4)2 and C60(SnI4)2 are both isostructural to SrSi2 despite the difference in size and shape of the fullerene supports our previous conclusion that the chirality is templated by SnI4 and not by the fullerene. 12 By mixing C70 and C60 in the precursor solution and varying the fraction x of C70, we synthesize single crystals of the mixed fullerene variant (C 70 ) (C 60 ) 1− (SnI 4 ) 2 (x = 0-1), which is isostructural to both C60(SnI4)2 and C70(SnI4)2. Asymmetric units of the single crystal structures are shown in Figures S3-5, and refinement statistics are given in Tables S2-4.…”