CRYSTAL GROWTH & DESIGN
Bilayer Packing of Amphiphilic Cationic Fullerenes in Crystalline Salts: Models for Self-Assembled Nanostructures
2008 VOL. 8, NO. 3 976–980
Christopher J. Chancellor, Adrienne A. Thorn, Christine M. Beavers, Marilyn M. Olmstead, and Alan L. Balch* Department of Chemistry, UniVersity of California, DaVis, California 95616 ReceiVed September 16, 2007
ABSTRACT: The structures of three crystalline saltss[C60(N(CH2CH2)2NH)+][Rh(CO)2Cl2-] · 2CS2, [C60(CH2NH(CH3)CH2)+][Rh(CO)2Cl2-] · CS2, and [C60(CH2N(CH3)(CH2CH3)CH2)+](I-) · CH3CH2Ishave been determined. Each packs in a bilayered arrangement such that the cationic heads of the fullerene units and the corresponding anions form distinct polar regions, whereas the fullerene cage and the solvate molecules (carbon disulfide or ethyl iodide) are located in distinct hydrophobic regions. Introduction
Scheme 1
The nearly spherical shape of fullerenes has given chemists an unusual building block for the assembly of macromolecular and nanostructured materials.1 A number of interesting nanoscale objects, including nanorods, nanotubules, and nanodisks, have been prepared from cationic fullerenes that carry their charge in small appendages.2–5 The external morphologies of these nanoparticles have been identified by transmission electron microscopy, scanning electron microscopy, and atomic force microscopy. However, the internal organizations present in these objects have not been established experimentally. Bilayer structures with the ionic portions facing outward and the nonpolar fullerene portions facing inward have been proposed for several fullerenes that have been externally functionalized so that they can carry a positive charge.2,3 Here, we report the first examples of crystalline salts of cationic fullerenes that show a bilayer organization. In these cations, the charge resides on a small external appendage as seen in Scheme 1. Whereas the solid-state organization of fullerenes is generally dictated by issues of close packing of the nearly spherical molecules with some degree of dipole–dipole alignment for functionalized fullerenes, in the salts examined here, the organization is also determined by the ionic nature of the components. Recently, there also has been an interest in the packing arrangements of cations and anions in crystalline ammine salts.6 Results and Discussion Structural Results for [C60(N(CH2CH2)2NH)+][Rh(CO)2Cl2-] · 2CS2. Crystalline samples of [C60(N(CH2CH2)2NH)+][Rh(CO)2Cl2-] · 2CS2 were obtained by treating a carbon disulfide solution of C60(N(CH2CH2)2N)7,8,9 sequentially with Rh2(µ-Cl)2(CO)4 and hydrogen chloride vapor via eq 1. C60(N(CH2CH2)2N) + HCl + 1/2 Rh2(µ - Cl)2(CO)4 f [C60(N(CH2CH2)2NH)+][Rh(CO)2Cl2-] +
(1) -
Black crystals of [C60(N(CH2CH2)2NH) ][Rh(CO)2Cl2 ] · 2CS2 formed over the course of a few days. The infrared spectrum of the product showed ν(CO) at 2070 and 2001 cm-1. These values of ν(CO) are consistent with the formation of the [RhCl2(CO)2-] anion.10 * Corresponding author. E-mail:
[email protected].
Samples suitable for single-crystal X-ray diffraction were obtained directly from the synthetic process. The crystal data are contained in Table 1. The salt crystallized as black plates with one cation and one anion in the asymmetric unit. In addition, there were two carbon disulfide molecules that were disordered over three or four orientations at two sites. Figure 1 shows a drawing of the structural relationship between the cation and anion in [C60(N(CH2CH2)2NH)+][Rh(CO)2Cl2-] · 2CS2. The N1-H1 bond of the cation points directly toward the Cl2 of the anion. According to the analytical methods of Brammer and co-workers, the distances involved, H1 · · · Cl2 (2.18 Å) and N1 · · · Cl2 (3.1066(19) Å), indicate that there is a hydrogen bond between the cation and the anion.11 Although the anion is a coordinatively unsaturated, 16-electron metal complex, there is no coordination of the fullerene cation to the rhodium anion as there is in the adduct, (η2-C60)RhHCl(PPh3)2.12 Figure 2 shows the packing within the solid with discrete regions for the ionic and nonpolar portions. The polar, aminecontaining ends of the cations (which are colored red) lie in layers with the anions. These regions lie along the ab-plane of the crystal. The arrows in Figure 2 emphasize the location of these regions. The nonpolar carbon cages (which are colored blue) of the cations face away from these layers. The carbon disulfide molecules occupy the spaces between the carbon cages. The width of a bilayer is 16.6519(6) Å, which is the length of the crystallographic c-axis. The layered structure seen in Figure 2 can be compared to the information found earlier for the nonionic counterpart, C60(N(CH2CH2)2N) · 1.5CH2Cl2, the structure of which is shown in Figure 3.8 As Figure 3 shows, the amino addends of the neutral fullerene are surrounded by neighboring carbon cages. Consequently, there is no discrete layer formed
10.1021/cg070643r CCC: $40.75 2008 American Chemical Society Published on Web 02/13/2008
Amphiphilic Cationic Fullerenes Packing in Cyrstalline Salts
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Table 1. Crystal Data and Data Collection Parameters
formula formula weight color and habit crystal system space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 T, K Z dcalcd, g · cm-3 radiation λ, Å µ, mm-1 range of transm. factors no. of unique data no. of restraints no. of params. refined R1a wR2b a
[C60(N(CH2CH2)2NH)+] [Rh(CO)2Cl2-] · 2CS2
[C60(CH2NH(CH3)CH2)+] [Rh(CO)2Cl2-] · CS2
[C60CH2N(CH3)-(CH2CH3) CH2+](I-) · CH3CH2I
C68H9Cl2N2O2RhS4 1187.82 black plates triclinic P1j 10.0021(4) 13.5113(5) 16.6519(6) 76.031(3) 89.573(4) 75.374(3) 2109.61(14) 90(2) 2 1.870 Mo KR (0.71073) 0.793 0.76–0.97 12216 0 713 0.038 0.096
C66H8Cl2NO2RhS2 1084.66 brown plate monoclinic P21/c 23.880(4) 9.9487(16) 17.358(3) 90 108.960(2) 90 3900.0(11) 90(2) 4 1.847 Mo KR (0.71073) 0.744 0.92–0.99 7043 32 385 0.073 0.196
C67H17I2N1 1089.62 brown plate monoclinic P21 9.8929(18) 17.476(3) 21.884(4) 90 94.645(4) 90 3771.2(12) 123(2) 4 1.919 synchrotron (0.77490) 2.145 0.74–0.99 18076 1 1266 0.065 0.188
For data with I > 2σI. R1 ) ∑||Fo| - |Fc||/∑|Fo|. b For all data. wR2 ) [∑(w(Fo2 - Fc2)2)/∑(w(Fo2)2)]1/2.
Figure 1. View of [C60(N(CH2CH2)2NH)+][Rh(CO)2Cl2-] · 2CS2 showing the relationship of the cation and the anion with 50% thermal contours. The carbon disulfide molecules have been omitted.
by the polar addends. Overall, in this case, the fullerene molecules are more closely packed for neutral molecules than they are for cations. In crystalline C60(N(CH2CH2)2N) · 1.5CH2Cl2, the centroid-to-centroid distances between the cages are 10.162, 11.532, and 12.829 Å in the section shown in Figure 3. However, these distances are 13.064, 13.511, and 14.490 Å for the section of the structure of [C60(N(CH2CH2)2NH)+][Rh(CO)2Cl2-] · 2CS2 shown in Figure 2. The closest interfullerene C · · · C contact in [C60(N(CH2CH2)NH+][Rh(CO)2Cl2-] · 2CS2 is 3.069(7) Å between C36 and its inversion-related counterpart. Structural Results for [C60(CH2NH(CH3)CH2)+][Rh(CO)2Cl2-] · CS2. Crystals of [C60(CH2NH(CH3)CH2)+][Rh(CO)2Cl2-] · CS2 were obtained as dark brown plates by a process analogous to that shown in eq 1, starting with C60(CH2N (CH3)CH2).13 The infrared spectrum of the sample showed ν(CO) at 2064 and 1992 cm-1, as expected for the [Rh(CO)2Cl2-] anion.10
Figure 2. View of [C60(N(CH2CH2)2NH)+][Rh(CO)2Cl2-] · 2CS2 showing the packing of the cations, anions, and solvent molecules into layers. Only the major orientations of the carbon disulfide molecules are shown. The arrows emphasize the locations of the layers containing the anions and the cationic portions of the fullerenes.
The crystal contained one cation, one anion, and one carbon disulfide molecule in the asymmetric unit. The carbon disulfide molecule was disordered over three sites with fractional occupancies of 0.45, 0.43, and 0.12. Figure 4 shows the arrangement of the closest cation–anion pair. The dimensions of both the cation and the anion fall within the expected ranges. Again, the N1-H1 group of the cation appears to be involved in hydrogen bonding to the Cl1 of the anion.10 The H1 · · · Cl1 distance is 2.28 Å, and the N1 · · · Cl1 distance is 3.141 (6) Å. Figure 5 shows the packing within the crystal. A bilayertype structure is again present with the anions and cationic appendages of the fullerene in close proximity. These groups are located in regions that are parallel to the crystallographic bc-plane and are denoted by the arrows in Figure 5. The nonpolar fullerene cages associate to form a hydrophobic region,
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Chancellor et al.
Figure 3. View of the structure of C60(N(CH2CH2)2N) · 1.5CH2Cl2 showing the packing of the neutral fullerenes and dichloromethane molecules from the data in ref 9.
Figure 5. View of [C60(CH2NH(CH3)CH2)+][Rh(CO)2Cl2-] · CS2 showing the packing of the cations, anions, and solvent molecules into layers. Only the major position of the carbon disulfide molecule is shown. The arrows point toward the ionic layers.
Structural Results for [C60(CH2N(CH3)(CH2CH3)CH2)+](I-) · CH3CH2I. Brown plates of the salt were obtained by treatment of C60(CH2N(CH3)CH2) with ethyl iodide.
Figure 4. View of [C60(CH2NH(CH3)CH2)+][Rh(CO)2Cl2-] · CS2 showing the orientation of the closest pair of cations and anions with 50% thermal contours. The carbon disulfide molecule has been omitted.
which also contains the nonpolar carbon disulfide molecules. The centroid-to-centroid distances between the cages are 9.936, 13.809, and 14.876 Å in the section shown in Figure 5. Figure 5 shows only the major sites for the carbon disulfide molecules; however, the other locations for the carbon disulfide molecules also reside in the hydrophobic region around the fullerene balls. The packing between the fullerene portions of the cations in the structure of [C60(CH2NH(CH3)CH2)+][Rh(CO)2Cl2-] · CS2 differs from that in the structures of the other two salts reported here. In this case, the closest intermolecular contact between individual carbon atoms is 3.215 Å. Closer contacts exist between pentagon/pentagon and hexagon/hexagon π-π stacking arrangements. The two closest are those between C33/C34/C35/ C50/C51 and C8′/C9′/C24′/C25′/C26′ (′ ) x, y - 1, z), with an average perpendicular distance of 3.130 Å and a lateral shift of 1.89 Å, and between C40/C41/C42/C54/C55/C56 and C40′′/ C41′′/C42′′/C54′′/C55′′/C56′′ (′′ ) -x, 1 - y, -z), with an average perpendicular distance of 3.139 Å and a lateral shift of 1.85 Å.
The crystal contained two cations, two iodide ions, and two ethyl iodide molecules in the asymmetric unit. Figure 6 shows the arrangement of one of the closest cation–anion pairs and the adjacent solvate molecule. The other cation–anion pair has a very similar structure. Figure 7 shows the packing within the crystal. Once more, a bilayer-type structure is present. The iodide ions and the cationic portions of the modified fullerene (which are colored red) occupy the layers in this diagram. The fullerene cages (which are colored blue) lie above and below those layers. The ethyl iodide molecules lie between the layers of fullerenes in the hydrophobic regions. There are some unusually close contacts between the fullerene cages in this structure. These contacts are shown in Figure 8 for one of the two C60 cages in the asymmetric unit. Unlike in the situation for [C60(CH2NH(CH3)CH2)+][Rh(CO)2Cl2-] · CS2, where hexagonal and pentagonal faces of the cages are placed in close proximity or where C-C bonds of one fullerene are positioned over a faceted face of the adjacent fullerene,14 in the present case, the closest contacts involve interactions between individual carbon atoms. These contacts are as short as 2.900(10) Å (between C25 and C34′, where ′ ) 1 + x, y, z) as shown in Figure 8
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Crystal Growth & Design, Vol. 8, No. 3, 2008 979
Figure 8. View of the nearest contacts between fullerene carbon atoms for one of the two fullerenes in the asymmetric unit of [C60(CH2N(CH3)(CH2CH3)CH2)+](I-) · CH3CH2I.
Figure 6. View of one of the pairs of cations and anions and the neighboring ethyl iodide ion in [C60(CH2N(CH3)(CH2CH3)CH2)+](I-) · CH3CH2I with 50% thermal contours. The other pair in the asymmetric unit has a similar structure.
positive charge. Two of these structures were not candidates for bilayer formation of the type found here because of the highly symmetric nature of the cation: [C59N+][Ag(CB11H6Cl6)2-] · 3o-Cl2C6H4,16 which suffered from disorder because of the cation symmetry, and [Fe2(C60Me10)(η5-C5H5)22+](SbCl6-)2.17 Three others, the isostructural salts [(η2-C60)Pd(η5Ph 2 PC 5 H 4 ) 2 Co + ](PF 6 - ) · 0.5CH 3 C 6 H 5 · 0.75ClC 6 H 5 · 0.75oCl2C6H4, [(η2-C60)Pt(η5-Ph2PC5H4)2Co+](PF6-) · 0.5CH3C6H5 · 0.75ClC6H50.75o-Cl2C6H4, and [Fe2(C60Me5Ph5)(η5-C5H5)2+](SbCl6-),17 did not form bilayered arrangements.18 Rather, their packing is dominated by the local antiparallel alignment of the cation. In this study, the first crystalline salts containing cationic fullerenes that do pack into bilayer arrangements have been prepared and crystallographically characterized. In these crystals, the cationic heads of the fullerene units associate with the anions to form distinct polar regions, whereas the major portion of the fullerene cage, along with the solvate molecules (carbon disulfide or ethyl iodide), is located in distinct hydrophobic regions. The resulting structures are quite similar to the model proposed by Tour and co-workers for the nanostructures they prepared using cationic fullerene derviatives.2 Experimental Section
Figure 7. View of [C60(CH2N(CH3)(CH2CH3)CH2)+](I-) · CH3CH2I showing the packing of the cations, anions, and solvent molecules into layers. Only the major position of the carbon disulfide molecule is shown. The arrows point toward the ionic layers.
or 2.922(10) Å (between C125 and C134′′, where ′′ ) x 1, y, z) for the other C60 cage in the asymmetric unit. Conclusions Crystallographic data on materials containing cationic fullerenes are rare. Our search of the Cambridge Crystallographic Data Base uncovered 773 fullerene-containing structures.15 Of these fullerene structures, only five involved a fullerene bearing a
Preparation of Crystalline Samples. [C60(N(CH2CH2)2NH)+][Rh(CO)2Cl2-] · 2CS2. A 1.0 mL portion of a saturated carbon disulfide solution of C60(N(CH2CH2)2N) (0.0060 mmol) was filtered into a 200 mm × 6 mm i.d. glass tube. An equal volume of a filtered, saturated carbon disulfide solution of dicarbonylrhodium(I) chloride dimer was added. The sample was placed in a hydrogen chloride atmosphere and was allowed to stand undisturbed for one week. Black plates of the product started to form almost immediately. The product was collected by decantation: yield, 81%. [C60(CH2NH(CH3)CH2)+][Rh(CO)2Cl2-] · CS2. A 1.0 mL portion of a saturated carbon disulfide solution of N-methyl-3,4-fulleropyrrolidine (0.00077 mmol) was filtered into a 200 mm × 6 mm i.d. glass tube. An equal volume of a filtered, saturated carbon disulfide solution of dicarbonylrhodium(I) chloride dimer was added. The sample was placed in a hydrogen chloride atmosphere and was allowed to stand undisturbed for one week. Brown plates of [C60(CH2NH (CH3)CH2)+][Rh(CO)2Cl2-] · CS2 started to form almost immediately. The product was collected by decantation: yield, 70%. [C60(CH2N(CH3)(CH2CH3)CH2)+](I-) · CH3CH2I. Crystals of [C60(CH2N(CH3)(CH2CH3)CH2)+](I-) · CH3CH2I were obtained by layering
980 Crystal Growth & Design, Vol. 8, No. 3, 2008 a saturated o-dichlorobenzene solution of C60(CH2N(CH3)CH2) over an equal volume of iodoethane. A thin layer of pentane was placed on top. The solutions formed separate layers which slowly mixed together over the course of several days. Pinwheel-shaped crystals comprised of round, reddish-brown plates formed in about 4 weeks. A large, single plate was selected for single-crystal X-ray diffraction. X-ray Data Collection. The crystals were removed from the glass tubes in which they were grown together with a small amount of mother liquor and immediately coated with a hydrocarbon oil on a microscope slide. Suitable crystals of [C60(N(CH2CH2)2NH)+][Rh(CO)2Cl2-] · 2CS2 and of [C60(CH2NH(CH3)CH2)+][Rh(CO)2Cl2-] · CS2 were mounted on glass fibers with silicone grease and placed in the cold stream of a Bruker SMART APEX II diffractometer. Data for a small (0.145 × 0.150 × 0.005 mm) plate of [C60(CH2N(CH3)(CH2CH3)CH2)+](I-) · CH3CH2I were collected on beamline 11.3.1 at the Advanced Light Source Lawrence Berkeley National Laboratory with the use of a synchrotron radiation (l ) 0.7749 Å). The data were collected on a Bruker D8 goniometer with a Platinum200 CCD detector at a temperature of 190 K. All data sets were collected as w scans with 0.3° frame widths and were integrated with the Bruker SAINT (v.7.16) program. Crystal data are reported in Table 1. A semi-empirical absorption correction utilizing equivalents was employed.19 Solution and Structure Refinements. Calculations for the structures were performed by using the SHELXS-97 and SHELXL-97 programs. Tables of neutral atom scattering factors, f ′ and f ′′, and absorption coefficients are from a standard source.20 The structures were all solved via direct methods. All atoms except hydrogen atoms were refined anisotropically. All hydrogen atoms were located in difference Fourier maps and included through the use of a riding model.
Acknowledgment. We thank the National Science Foundation (CHE-0413857) for support. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Supporting Information Available: X-ray crystallographic data, in CIF format, for the three products described in this paper. This material is available free of charge via the Internet at http://pubs.acs.org.
Chancellor et al.
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