Article pubs.acs.org/crystal
New Insights into the Structural Complexity of C60·2S8: Two Crystal Morphologies, Two Phase Changes, Four Polymorphs Kamran B. Ghiassi, Susanne Y. Chen, Joseph Wescott, Alan L. Balch, and Marilyn M. Olmstead* Department of Chemistry, University of California, One Shields Avenue, Davis, California 95616, United States S Supporting Information *
ABSTRACT: Three new polymorphs of C60·2S8 were discovered. The previously known structure (first reported by Roth and Adelmann in 1993 hereby designated as α) crystallizes in space group C2/c with Z = 4 and changes to a triclinic structure (β) in space group P1̅ with Z = 4 when the temperature is decreased below 260 K. The room-temperature structure was reinvestigated, and the new, ordered, lowtemperature structure is described. A new, concomitant, polymorph (γ) crystallizes in space group P21/c with Z = 4 at room temperature and undergoes a phase change to Pc (δ) with Z = 4 when the temperature is decreased below 180 K. As indicated by geometric and temperature factor changes, it is clear that the low-temperature phases represent an increase in the level of order in the arrangement of C60 molecules. Both of the phase changes are reversible.
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INTRODUCTION Crystals containing both C60-fullerene and S8-sulfur were earlier described as compounds, complexes, and sulfur fullerites, presumably in the hope that a covalent or donor−acceptor intermolecular interaction between the C60 and sulfur could be realized.1 Instead, crystallographic, thermodynamic, and spectroscopic results have shown that the interaction is strictly van der Waals in nature and may be considered as an example of cocrystallization.2−5 In one study, unsuitable, intergrown crystals of C60·2S8 were obtained from the melt, but wellformed single crystals were obtained from slow evaporation of a solution of CCl4 with 10 vol % CS2.6 The structure determination at room temperature yielded a unit cell with the following dimensions in the monoclinic space group C2/c: a = 20.867(4) Å, b = 21.062(4) Å, c = 10.508(2) Å, β = 111.25(7)°, V = 4304.3 Å3, and Z = 4. The asymmetric unit contains one-half of a C60 molecule residing on a 2-fold rotation axis and a molecule of S8. Of interest was the observation of “pancake”-like shapes with the temperature factors (U) of 0.3 Å2 parallel to the C60 surface and slightly less than 0.04 Å2 perpendicular to it. The authors stated, “the real-space resolution of the data at room temperature appears to be just high enough to resolve individual carbon atoms.” They surmised that the smearing out of the carbon atoms could be due to librational motion or local, static deviations from the average orientation of the molecule, and that low-temperature studies could help resolve the issue. They obtained average values of 1.340(8) Å for bonds involving 6:6 ring junctions and 1.448(8) Å for 6:5 ring junctions. In a second room-temperature study, crystals were obtained from trichloroethylene recrystallization.1 Essentially the same structure was obtained. Thermal parameters for the fullerene carbons (U) ranged from 0.13 to 0.32 Å2. Mean values for 6:6 © 2014 American Chemical Society
bonds were 1.32(3) Å and for 6:5 bonds 1.47(3) Å. A third room-temperature study again yielded the same structure, this time from crystals grown from a toluene solution.7 No coordinates were published, but it was stated that “The Rfactors are relatively large because of the large Debye thermal factors of carbon atomic in these compounds. The results of the C60S16 coincide with the previously reported.” Curiously, until now, C60-sulfur cocrystals have not been studied at low temperatures considering that the solvate structures 2C60·3CS28 and C60·4benzene9 undergo phase transitions to ordered phases when their temperatures are decreased. In this report, we demonstrate a C60 ordering transition in C60·2S8 that gives rise to two molecules of C60 and four molecules of S8 in the asymmetric unit. A second, concomitant phase was also found, in which the ordering of the disordered site also occurs at a lower temperature.
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RESULTS Structure of α-C60·2S8 [black lath, room temperature (RT)]. As previously reported, at room temperature the compound crystallizes in the monoclinic space group C2/c with Z = 4 (Table 1). A crystal was grown, and the structure was revisited. The asymmetric unit consists of a C60 cage at half-occupancy and one S8 molecule. The C60 cage resides on a crystallographic 2-fold special position, while the sulfur molecule is on a general position. Upon close inspection, it was determined that the C60 molecule’s 2-fold axis is rotated by ∼6.5° with respect to the crystallographic 2-fold axis. Refinement was continued with suppression of crystallographic Received: October 6, 2014 Revised: November 8, 2014 Published: November 11, 2014 404
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Table 1. Crystallographic Data for C60·2S8 lath form chemical formula formula weight radiation source, λ (Å) crystal system space group T (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z dcalc (g cm−3) μ (mm−1) F(000) crystal size no. of reflections collected data/parameters/restraints R(int) R1 [I > 2σ(I)]a wR2 (all data)b largest difference peak and hole (e Å−3) a
needle form
α
β
γ
δ
C60S16 1233.56 0.71073 monoclinic C2/c 293(2) 20.904(5) 21.120(5) 10.519(2) 90 111.243(3) 90 4328.5(17) 4 1.893 0.850 2464 0.49 × 0.40 × 0.30 34341 6606/439/540 0.0222 0.0754 0.2218 1.367 and −1.329
C60S16 1233.56 0.71073 triclinic P1̅ 90(2) 10.365(2) 14.694(3) 29.000(7) 84.122(3) 84.602(3) 75.664(3) 4245.8(17) 4 1.930 0.867 2464 0.49 × 0.40 × 0.30 134640 26115/1370/0 0.0348 0.0362 0.0938 0.835 and −0.364
C60S16 1233.56 0.71073 monoclinic P21/c 293(2) 16.1733(9) 15.8931(9) 18.5781(10) 90 113.302(2) 90 4385.9(2) 4 1.868 0.839 2464 0.80 × 0.48 × 0.48 60195 10897/1722/20035 0.0276 0.0702 0.2123 1.068 and −0.606
C60S16 1233.56 0.71073 monoclinic Pc 90(2) 16.1355(6) 15.7332(6) 18.3535(7) 90 113.283(2) 90 4279.8(3) 4 1.914 0.860 2464 0.80 × 0.48 × 0.48 143892 28435/1370/2 0.0199 0.0269 0.0732 0.769 and −0.266
R1 = (∑||Fo| − |Fc||)/(∑|Fo|). bwR2 = ({∑[w(Fo2 − Fc2)2]}/{∑[w(Fo2)2]})1/2.
Figure 2, all components of the structure are fully ordered. Anisotropic refinement was conducted with no additional
2-fold symmetry. Free anisotropic refinement of the structure gave some distortion in bond lengths, as well as six “split atoms”. Some atom positions were highly correlated. Consequently, the C60 cage was refined as a 60-carbon rigid group with additional restraints applied to the highly correlated thermal parameters. In spite of the adjustment in atom positions allowed by treatment of the disorder described above, the thermal parameters clearly correspond to a degree of ball-like rotational motion. Equivalent isotropic thermal parameters can be divided into two groups, corresponding to rotation in the sphere (0.12 Å2) and perpendicular to it (0.02 Å2). The asymmetric unit is shown in Figure 1. Structure of β-C60·2S8 [black lath, low temperature (LT)]. Below the phase transition at ∼260 K, the α structure becomes triclinic, in space group P1,̅ with Z = 4. The asymmetric unit consists of two independent C60 cages and four independent S8 molecules. As shown for the 90 K structure in
Figure 2. Asymmetric unit of the structure of β-C60·2S8. Thermal ellipsoids are presented at the 50% probability level.
restraints. The structure was treated as a rotational twin with twin law (−1 0 0 −0.5 0 0.5 −1 2 0) and a refined twin parameter of 0.1874(5). Structure of γ-C60·2S8 (black needle, RT). The second polymorph at room temperature crystallizes in monoclinic space group P21/c with Z = 4. The asymmetric unit consists of one C60 cage and two S8 molecules. The sulfur molecules are
Figure 1. Asymmetric unit of α-C60·2S8 drawn with 50% thermal contours with the crystallographic 2-fold disorder in the fullerene. 405
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Phase Changes. C60·2S8 (α/β, black lath). At room temperature, the α polymorph exists as monoclinic C2/c. Upon cooling, the polymorph transforms to the β polymorph in P1̅. The matrix for transformation10 of the (h k l) indices from α to β is given by
ordered, while the C60 cage is significantly disordered over at least five orientations. All atoms were allowed to refine anisotropically, although the atoms of C60 are highly restrained. Each of the five orientations was modeled and refined as a 60carbon rigid group with additional constraints and restraints. The occupancies are 0.243(2), 0.223(2), 0.220(2), 0.141(2), and 0.173(2). The occupancies were restrained by refinement to add up to 1.00. The asymmetric unit is shown in Figure 3.
⎞ ⎛ 1 −1⎟ ⎜0 2 ⎟ ⎜ ⎟ ⎜ 1 ⎜ 0 − 2 −1⎟ ⎟ ⎜ ⎝−1 0 −1⎠
The inverse transformation is given by ⎞ ⎛ 1 1 −1⎟ ⎜ 2 ⎟ ⎜ 2 ⎜ 1 −1 0 ⎟ ⎟ ⎜ 1 ⎜− − 1 0 ⎟ ⎠ ⎝ 2 2
Because the β polymorph is present for most of the accessible temperature range between 90 and 293 K, we selected several reflections from this phase to be followed during our phase change experiment. From inspection of the β → α transformation matrix, it is seen that (h′ k′ l′) will have noninteger values if h + l ≠ 2n and should therefore be absent. Two reflections with good I/σ(I) values were selected and plotted as a function of temperature. As shown in Figure 5, starting at 250 K the phases are mixed, and the β phase has completely disappeared by 270 K. The phase change is reversible but does display a degree of hysteresis.
Figure 3. Asymmetric unit of the structure of γ-C60·2S8 showing all five modeled orientations of the fullerene. Thermal ellipsoids are presented at the 50% probability level.
Structure of δ-C60·2S8 (black needle, LT). Below the phase change temperature of ∼180 K, the γ form has converted to monoclinic, in space group Pc, with Z = 4. The asymmetric unit consists of two C60 cages and four S8 molecules. All components of the structure are fully ordered, and none reside at special positions. The atoms were allowed to refine anisotropically and as a two-component inversion twin. The twin parameter refined to 0.48(3). A view of the structure is given in Figure 4.
Figure 5. Plot of I/σ(I) vs T for the two reflections (4 1 1) and (−2 −7 −11) in β-C60·2S8.
Because of the disorder inherent in the C60 position, as well as the rigid body assumption, the detailed geometry of the C60 in the α polymorph is not discussed. The general packing motif is depicted in Figure 6a. The shortest possible centroid-tocentroid distance is 9.948(2) Å. The S8 ring has torsion angles and bond distances typical of the crown conformation. The shortest transannular distance is 4.560(16) Å. The shortest S8 ring-to-ring distance is 3.428(17) Å. As shown in Figure 6b, the low-temperature β phase is derived from the α phase by removal of the C-centering, 2-fold axis of symmetry, and c-glide. Only the center of symmetry is
Figure 4. Asymmetric unit of the structure of δ-C60·2S8. Thermal ellipsoids are presented at the 50% probability level. 406
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Figure 6. In panel a, a portion of the packing in α-C60·2S8 is shown, as viewed down the c-axis. In panel b, the same relative orientation in the low-temperature phase, β-C60·2S8 is shown, as viewed down the a-axis. The two independent C60 cages in the asymmetric unit are colored red (cage 1) and blue (cage 2).
retained. These changes in symmetry are clearly visible and due to the rotational ordering of the C60. Below the phase transition, the structure becomes considerably more complicated as the number of intermolecular contacts increases. In the β polymorph, each of the two C60’s is surrounded by six C60’s, four at centroid-to-centroid distances ranging from 9.881(2) to 10.194(2) Å and two at 10.365(2) Å, corresponding to the a-axis translation (see Figure SI7 of the Supporting Information). The shortest centroid-to-centroid distances are summarized in Figure 7. All four S8 rings are in the crown conformation, and the shortest transannular distance is 4.5401(10) Å. The shortest S8 ring-to-ring distance is 3.336(10) Å. Notably, the shortest S···C distances exclusively occur with cage 2 (vide inf ra), and there are 10 of these distances in the range of 3.171(2)−3.385(2) Å. C60·2S8 (γ/δ, black needles). The high- and low-temperature phases are crystallographically similar, with both occurring in the monoclinic crystal system, with similar lattice constants. At room temperature, the γ polymorph exists in space group P21/ c. Upon cooling, the polymorph transforms to the δ phase in monoclinic space group Pc, loses its 21 screw axis, and becomes an inversion twin. Figure 8 shows a temperature profile following the (0 7 0) reflection, which becomes systematically absent at the higher temperature. It is evident that the phase change is somewhat gradual. Figure SI10 of the Supporting Information plots the volume change as a function of temperature. The behavior is linear, and the value of 0.44 Å deg−1 K−1 is the same as that seen in C60·2S8 (α/β, black laths) but does not display the same sharp discontinuity (see Figure SI9 of the Supporting Information). The ordering transition involves loss of disorder, which occurs by removal of some of the rotational disorder of the cagelike molecule. The transition was determined to be fully reversible. As shown in Figures 9 and 10, the single site containing rotationally disordered C60 in γ-C60·2S8 at room temperature evolves into two sites with two different orientations in δ-C60· 2S8 at low temperatures (colored red and blue). In the hightemperature polymorph, the crystallographic screw axis requires that the C60 centroid-to-centroid distance repeat along the screw axis. Thus, at room temperature, the C60 centroid-to-
Figure 7. Shortest centroid-to-centroid distances and their abutting geometric polygons in β-C60·2S8 at 90 K.
Figure 8. Plot of I/σ(I) vs T for reflection (0 7 0).
centroid distance is 9.989 Å, while at 90 K, with the screw axis absent, it alternates between 9.952 and 9.887 Å. Additional complexities accompany the change to lower symmetry. At room temperature (γ), each cage is surrounded by six cages at short centroid-to-centroid distances of 9.968 Å (inversion), 9.989 × 2 Å (screw axis), 10.072 Å (inversion), and 10.136 Å × 2 (glide). At the low temperature (δ), there are six different short distances, ranging from 9.855 to 10.093 Å. These are shown in Figure 11 and Figure SI8 of the Supporting Information. The shortest transannular contact in any S8 is 4.5001(9) Å. As for close contacts involving S8 rings, at room temperature the shortest S···S distance is 3.5135(17) Å and the shortest S···C distance is 3.218(7) Å. At 90 K, the shortest S···S 407
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Figure 9. Portion of the packing of the structure of γ-C60·2S8 with the superposition of all five rotational isomers of C60 at 293 K. Atoms are drawn with an arbitrary size.
Figure 10. Portion of the packing of the structure of δ-C60·2S8 at 90 K. Thermal displacement parameters are drawn at the 50% probability level. The cages colored red correspond to cage 1, while those colored blue correspond to cage 2. The yellow S8 molecules are S1−S16, while the orange-yellow S8 molecules are S17−S32. The cages corresponding to cage 2 have thermal parameters larger than those of cage 1 (vide inf ra).
Figure 11. Shortest centroid-to-centroid distances and their abutting geometric polygons in δ-C60·2S8 at 90 K.
Table 2. Comparison of Isotropic Thermal Displacement Parameters and Two Kinds of C−C Bonds in C60·2S8 Polymorphs
distance between rings is 3.3435(9) Å and the shortest S···C distance is 3.159(3) Å. Thus, in every respect, shorter intermolecular distances occur at low temperatures. Table 2 compares average isotropic thermal displacement parameters and the two types of C−C bond lengths in the two distinct C60 cages in the asymmetric unit of the 90 K structures. Average deviations from the mean are given in parentheses. In each of the two structures, one cage is more rotationally rigid than the other. The bond distances are correspondingly longer in the more rigid cage. Plots showing the cage-by-cage distribution of thermal parameters and bond distances, together with error bars, are given in the Supporting Information (Figures SI1−SI4). Table 3 compares the average S−S bond lengths in each molecule of S8 for the four structures. The distances are compared to those from an earlier report of S8 at 298 and 100 K. The bond distances are consistently longer in the lowertemperature structures than in their room-temperature counterparts.
cage 1 (average) β polymorph (90 K) Ueq (Å2) 6:6 bonds (Å) 5:6 bonds (Å) δ polymorph (90 K) Ueq (Å2) 6:6 bonds (Å) 5:6 bonds (Å)
cage 2 (average)
cage 1 + cage 2
0.019(3) 1.390(5) 1.454(5)
0.0141(13) 1.394(2) 1.453(3)
0.016(3) 1.392(4) 1.454(4)
0.0148(11) 1.391(3) 1.452(4)
0.022(2) 1.387(5) 1.453(8)
0.018(4) 1.389(4) 1.452(6)
Table 3. Comparison of S−S Bond Distances in C60·2S8 Polymorphs average S−S bond distance (Å) α polymorph (293 K) β polymorph (90 K) γ polymorph (293 K) δ polymorph (90 K) S8 (298 K)11 S8 (100 K)11
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DISCUSSION Well-ordered crystals of binary combinations of C60 or C70 and sulfur can be prepared by carefully layering toluene or benzene solutions of both components. We previously reported that iodine may play a role in crystal formation but not incorporate itself into the lattice.12,13 The effect of iodine is observed here. In studying the crystal growth of S8/C60 cocrystals, we have discovered that introducing iodine into solution does not yield
S1−S8
S9−S16
S17−S24
S25−S32
2.042(6) 2.055(4) 2.045(3) 2.052(2) 2.046(3) 2.050(2)
− 2.057(3) 2.045(6) 2.054(3) − −
− 2.057(3) − 2.053(3) − −
− 2.055(2) − 2..052(2) − −
a ternary crystal. Rather, it produces many more nucleation sites. For comparison, a glass tube with only fullerene and sulfur will form a few, very large crystals. However, when iodine is introduced, the tube has many nucleation sites throughout the vessel. Upon inspection, the crystals appear to be significantly 408
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X-ray diffraction at low temperatures probably because of the inherent twinning upon cooling. It is also noteworthy that the crystals may be grown without incorporation of solvent into the lattice using a variety of solvents (toluene, benzene, CCl4, etc.) except for CS2. In the latter case, the C60·S8·CS2 ternary cocrystal is formed.19 In this structure, the fullerene is disordered. Although several fullerenes have been successfully cocrystallized with S8, cocrystallization of others of the many allotropes of sulfur with fullerenes has yet to be experimentally realized.
smaller. The crystals that were grown in the presence of iodine were analyzed via single-crystal X-ray diffraction to determine their composition, which was again simply C60·2S8. It is evident that the phase changes from 293 to 90 K are first-order, structural, disorder-to-order transitions. The phase changes occurring in both habits of crystals are completely reversible. The crystals were brought to 293 and 90 K multiple times for proof of concept. Neither crystal at 293 K has observable twinning. However, upon cooling, the crystals become twinned. Fortunately, although the crystals are twinned, they produce fully ordered structures. When the temperature is returned to 293 K, all evidence of twinning disappears. While it is tempting to assume that specific van der Waals interactions between the cages are driving the ordering transitions, our analysis shows that there is no great similarity among the relative orientations that are depicted in Figures 7 and 11. Of the four structures in this report, the δ form at 90 K exhibits the closest cage-to-cage interactions. However, the β form exhibits the closest C60 cage-to-S8 ring interactions. All of the structures intersperse C60 and S8 throughout the lattice, in contrast to the solvate structure C60·4benzene in which the C60 and benzene regions are segregated and were aptly described as “nanocrystalline domains”.14 Therefore, computational analysis of the crystal packing of the structures in this report would demand a consideration of all of the molecular constituents and their neighbors. In 1961, Kitaigorodskii postulated that the most stable structure is the one with the most efficient packing.15 This corresponds to the structure with the highest density. The computed densities (grams per cubic centimeter) of the four polymorphs decrease in the following order: β (1.930) > δ (1.914) > α (1.893) > γ (1.868). In each of the two crystal forms of C60·2S8, the room-temperature polymorph is less dense than the low-temperature polymorph. Many observations suggest that the α and γ polymorphs are distinct. They undergo different phase changes. The low-temperature phases differ in density, bond length distribution (Figures SI1 and SI2), thermal motion (Figures SI3 and SI4), packing (Figures SI5 and SI6), and centroid-to-centroid distance (Figures SI7 and SI8). It is consistent that the low-temperature polymorphs show a packing efficiency higher than that of their roomtemperature counterparts. The difference in free energy between polymorphs α and β and polymorphs γ and δ can easily be envisioned as an entropy effect. However, the difference between α and γ (or, correspondingly, β and δ) requires sophisticated molecular energetics computations that are beyond the scope of this study. It is interesting that in each of the four polymorphs, the volume occupied by sulfur is ∼38%, and there are no major changes in intermolecular interactions. Those that do occur can be ascribed to loss of orientational disorder of the C60 cage. Higher fullerenes have been successfully cocrystallized with molecular sulfur. Two forms of C70 cocrystallized with S8 have been discovered: C70·6S8 (Amm2)16,17 and C70·2S8 (P21/c).7 In the most recent report of C70·6S8,17 the sulfur positions exhibit partial disorder. There is no disorder in the C70·2S8 form. During the course of investigation, we discovered that the C70· 2S8 form does not undergo a phase change in the range of 293−90 K. The remaining fullerene−sulfur cocrystal, C76·6S8, is disordered and contains enantiomeric pairs of D 2 −C 76 molecules occupying a common site.18 It is interesting to note that C60·2S8 has not been studied prior via single-crystal
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
Materials. C60 was purchased from SES research at 99% purity. Precipitated sulfur was purchased from Mallinckrodt. No further purification was performed. Benzene and toluene were used as received. Physical Properties. Electrical resistivity measurements were taken on polymorphs α and β using a physical properties measurement system (Quantum Design PPMS). A single-crystal sample was studied by the standard four-probe AC method from 2 to 300 K, but the resistivity was too high to measure throughout the temperature range. Previously reported physical property measurements, Fourier transform infrared, and 13C nuclear magnetic resonance also failed to produce evidence of charge transfer or conductivity. Crystal Growth. Crystals were prepared by layering 1 mL aliquot filtered solutions of C60 and sulfur over one another using either toluene or benzene in a glass tube. If the solutions were sonicated, crystals formed in as little as 1 day. The quality of the crystals was found to be dependent on concentration, the best crystals being formed under moderately dilute conditions: 2.0 mM C60 and 33 mM S8. Two crystal types form under the listed conditions: black laths (α and β forms) or needles (γ and δ forms). It is interesting that the report by Takahashi et al. includes a figure with what appears to be both morphologies.20 Crystal Structure Determinations. Crystals were removed from the mother liquor and placed into Paratone oil for screening. The crystals are air stable and do not decompose in Paratone oil. X-ray data were collected using a Bruker ApexII CCD instrument equipped with a CRYO Industries low-temperature apparatus and molybdenum finefocus sealed tube. All data sets were reduced with the use of Bruker SAINT,21 and a multiscan absorption correction was applied with the use of SADABS.22 Structure solution and refinement were conducted with SHELXS-2008 and SHELXL-2014, respectively.22 Two structures are reported per crystal: one at 90 K and one at 293 K (four structures total). In addition to these four data sets, additional data were collected at various temperatures to plot profiles ranging from 90 to 293 K to monitor phase changes. A span of 10 min between temperature changes was allowed. Crystal data for the four polymorphs are listed in Table 1.
S Supporting Information *
Figures depicting void space and distribution plots of thermal parameters and bond distances for β-C60·2S8 and δ-C60·2S8, summaries of the shortest centroid-to-centroid contacts, and plots of unit cell volume versus temperature during the stepwise temperature change and X-ray crystallographic material in cif format. This material is available free of charge via the Internet at http://pubs.acs.org. Full crystallographic information files (CIF+RES+HKL) are also available from the Cambridge Crystallographic Data Center (CCDC) upon request (http:// www.ccdc.cam.ac.uk, CCDC deposition numbers 1027588− 1027591). 409
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AUTHOR INFORMATION
Corresponding Author
*Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616. E-mail: mmolmstead@ ucdavis.edu. Phone: (530) 752-6668. Fax: (530) 752-8995. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by the National Science Foundation (Grant CHE-1305125 to A.L.B. and M.M.O.). We thank J. T. Greenfield and K. Kovnir for performing the physical property measurements and M. M. Aristov for experimental assistance.
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REFERENCES
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