Fluorinated Tetraphenylporphyrins as Cocrystallizing Agents for C60 and C70 Marilyn M. Olmstead* and Daniel J. Nurco Department of Chemistry, One Shields AVenue, UniVersity of California, DaVis, California 95616
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 1 109-113
ReceiVed May 21, 2005; ReVised Manuscript ReceiVed June 28, 2005
W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: Four new cocrystals of fullerenes C60 and C70 have been prepared and characterized by X-ray crystallography, C60‚1.5H2F20TPP‚6benzene, C70‚H2F20TPP‚8benzene, C60‚NiF20TPP‚8benzene, and C70‚ClFe(F20TPP) (F20TPP ) 5,10,15,20pentafluorophenylporphyrin). These structures reveal a marked encapsulation of the fullerene molecule by both the porphyrin and numerous C-F‚‚‚C(fullerene) interactions. Introduction The successful crystallization and X-ray crystal structure determination of numerous fullerenes and endohedral fullerenes has often taken advantage of what has come to be realized as a particularly high propensity of many porphyrin molecules to cocrystallize with fullerenes. Both 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP)1,2 and 5,10,15,20-tetraphenylporphyrin (TPP)3-6 have been highly successful in this regard. Thus, for example, small amounts of rare endohedral fullerenes such as Er2@C82 (two isomers),7a,7b Sc3N@C68,8a Sc3N@C78,8b Sc3N@C80,8c and Ba@C749 have been cocrystallized with Co(OEP) and Ni(OEP), enabling their crystal structures to be determined. The reasons for this porphyrin-fullerene compatibility have been attributed to many factors. Among these are compatible shapes leading to numerous van der Waals contacts,10 π-π interactions between the fullerene surface and the porphyrin plane11, charge-transfer interactions between the metal (such as FeII or CoII) and the fullerene acting as an electron acceptor12 or between the metal (e.g., FeIII) and a fullerene, acting as an electron donor,13 dispersive14 and electrostatic interactions.15 A recent review by Boyd and Reed16 summarizes the complexity of the porphyrin-fullerene interactions and concludes that [it] is “essentially van der Waals in nature but is perturbed by weak electrostatic and possible charge-transfer effects.” This study reports new crystal structures of C60 and C70 associated with porphyrins having reduced electron donating ability conferred by the presence of the meso-substituted pentafluorophenyl groups, namely, 5,10,15,20-pentafluorophenyl porphyrin (H2F20TPP) as well as two metalloporphyrins, Ni(F20TPP) and FeCl(F20TPP). A report of a tetragonal form of H2F20TPP‚C60 was previously described.16 In these instances, new intermolecular forces appear to play a role in the stabilization of these crystal structures. These are of the type C-F‚‚‚C(fullerene), primarily C-F‚‚‚π interactions. Recently, the importance of these intermolecular interactions has been recognized, both in small molecules and in proteins.17,18 While C-F‚‚‚H interactions are much less common than C-X‚‚‚H, X ) Cl, Br, and I, interactions,19 just the reverse is observed for C-X‚‚‚π interactions.20,21 The Cambridge Structural Database study by Prasanna and Guru Row20 of C-F‚‚‚π interactions identified C-F‚‚‚π distances less than a maximum value of 3.33 Å, equal to the sum of the van der Waals radii of the halogen * To whom correspondence should be addressed. E-mail: mmolmstead@ ucdavis.edu.
and half-thickness of the aromatic ring with an added tolerance of 5% to address electrostatic contributions. A broad range of 2.53-3.33 Å was found. Their results clearly demonstrate the prevalence of C-F‚‚‚π contacts in organic crystals and their directing influence in crystal packing. Therefore, the ready formation of cocrystals between porphyrins bearing pentafluorophenyl groups and fullerenes is not surprising, and it opens up a new range of host molecules for the purpose of stabilizing fullerene structures. However, as will be seen, a competing synthon in these structures arises from the solvent, benzene, that is used for crystallization. In all except one of the crystals examined, the prevalence of benzene-perfluorophenyl stacking interactions suggests that these are also important. These are well-known quadrupolar interactions.22,23 Results and Discussion Four cocrystals will be described. Two incorporate the H2F20TPP free base porphyrin, one of these with C60, 1, and one with C70, 2. Two others employ the metalloporphyrins NiF20TPP and ClFeF20TPP, the former with C60, 3, and the latter with C70, 4. All except 4 are benzene solvates. The crystals were obtained by layering a benzene solution of the porphyrin above a benzene solution of the fullerene in a 5-mm tube and allowing the crystals to grow at room temperature. Well-formed crystals grew within a few days. X-ray data collection was carried out using a He cryostat in the case of 3 and 4 and a N2 cryostat for 1 and 2. Some details of the X-ray crystallography are given in Table 1. Further details are available as Supporting Information. C60‚1.5H2F20TPP‚6benzene, 1. The crystals form as dark red blocks in the trigonal space group P3h. The C60 resides on a site of crystallographic 3-fold symmetry. Figure 1 displays how the packing of three H2F20TPP’s around the C60 nearly completely encapsulates the ball. A rotating image of this drawing is available in Windows Media Video (WEO1.wmv) format. The porphyrin has crystallographic inversion symmetry and is close to planar; the mean plane deviation of the porphyrin plane is 0.031 Å. In comparison to the nonsolvated free base porphyrin,10 there is little overall change in porphyrin geometry upon acceptance of the fullerene guest, which occurs both above and below the porphyrin plane. There is rotational disorder in the C60 position, and the two different orientations refined to ratios of two-thirds to one-third by use of rigid C60 groups with free isotropic thermal parameters. Neither orientation uses the available crystallographic 3-fold
10.1021/cg050225r CCC: $33.50 © 2006 American Chemical Society Published on Web 09/01/2005
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Table 1. Crystallographic Data for Compounds 1-4 compound
1
2
3
4
formula fw space group a, Å b, Å c, Å R, ° β, ° γ, ° V, Å3 Z T, °C λ, Å Dcalcd, g cm-3 µ, cm-1 R1 (I > 2σ(I)) wR2 (all data)
C162H51F30N6 2651.09 P3h 20.933(2) 20.933(2) 14.684(2) 90 90 120 5572.0(9) 2 -182 0.71073 1.580 1.28 0.0512 0.1396
C162H58F20N4 2440.12 I41/a 28.7889(13) 28.7889(13) 12.7959(8) 90 90 90 10605.3(9) 4 -182 0.71073 1.528 1.13 0.0848 0.2892
C152H56F20N4Ni 2376.72 I41/a 28.335(2) 28.335(2) 12.494(2) 90 90 90 10031(2) 4 -263(2) 0.71073 1.574 2.98 0.0455 0.1196
C114H8ClF20FeN4 1904.54 Cmc21 24.123(4) 19.831(3) 14.441(3) 90 90 90 6908(2) 4 -255(2) 0.71073 1.831 3.82 0.0608 0.1597
symmetry. Instead, the 3-fold axis passes close to a carbon atom in each case. Both orientations are shown in Figure 2, which also shows several of the closest C-F‚‚‚C(fullerene) contacts. The distance between the center of the porphyrin core and the nearest C atom is 2.689 Å in the major orientation and 2.701 Å in the minor orientation. An examination of the packing without the C60 reveals several close packing interactions. There are examples of C-H(pyrrole)‚‚‚F (three in the range F‚‚‚H 2.462.56 Å), F‚‚‚F (two, F‚‚‚F, 2.86 Å), intermolecular contacts as well as edge-to-face and face-to-face interactions involving the benzene molecules and pentafluorophenyl groups. For comparison to the other structures, the centroid-to-centroid distance between the face-to-face benzene and pentafluorophenyl groups is 3.755 Å. However, the most numerous intermolecular interactions are those involving C-F‚‚‚C(fullerene). For the major orientation there are 14 different interactions of this type ranging between 2.925 and 3.395 Å, while for the minor orientation there are 15 different interactions ranging between 3.054 and 3.364 Å. About two-thirds of these contacts are within the set of three porphyrins depicted in Figure 2 and originate from F atoms in the ortho-position of the ring, while the remainder are to additional fullerenes above and below this view. The directionality of the C-F‚‚‚ball contact is not easy to establish. In some cases, the shortest distance is to the center of a six-membered ring, and in other cases it is to the center of
a five-membered ring. Other instances place the shortest contact directly to C atoms. A striking feature of this structure in comparison to many others not containing F’s is that there are no ball-to-ball contacts and the C60 is fully surrounded by porphyrin planes and F‚‚‚C contacts. C70‚H2F20TPP‚8benzene, 2. Cocrystals of 2 are black blocks and form in the tetragonal crystal system, space group I41/a. A drawing of the host-guest arrangement of the porphyrin and C70 is shown in Figure 3. The porphyrin and the C70 are centered at sites a and b, respectively, with 4h symmetry. The C70 is disordered with respect to this symmetry operation and thus exists in four different orientations with respect to a given porphyrin molecule. It was successfully refined as a single rigid group. The centers of the C70 and the porphyrin alternate with a translation by 1/2 along the z direction. There are two molecules of ordered benzene in the asymmetric unit. Since this is the same porphyrin as in the C60
Figure 2. The major (67%) and minor (33%) orientations of the C60 are shown with a number of the C-F‚‚‚C(fullerene) interactions indicated.
Figure 1. A drawing of the major component of C60 surrounded by three H2F20TPP’s in compound 1. W A rotating image in Windows Media Video format is available.
Figure 3. A view of the stacking following the 41 screw axis parallel to the c-axis (horizontal in the figure) in the cocrystal of C70 with H2F20TPP in compound 2.
Fluorinated Tetraphenylporphyrins
Crystal Growth & Design, Vol. 6, No. 1, 2006 111
Figure 5. A view of C-F contacts directed toward the C70 in 2 and their variation in directionality. Figure 4. A portion of the molecular packing of the components of C70‚H2F20TPP‚8benzene, 2.
structure, 1, it is possible that the change in ratio of porphyrin to fullerene from 1:1.5 in the C60 case to 1:1 in this case results from the larger size of C70, or it may be related to the poorer fit that would result from the packing of C70 around a 3-fold axis. The shortest distance to the center of the porphyrin core is 2.903 Å, longer than that seen in the C60 structure. Figure 4 illustrates that the C70’s are well-surrounded by their cocrystallization components: the benzene molecules fill in all the areas not covered by fluorines. The fluorine interactions are from the porphyrins above and below along the c-axis but also from the next column to the side. In addition, the benzene forms edge-to-fullerene face arrangements. Between the two columns is a face-to-face stacking of benzene with pentafluorophenyl groups with a centroid-to-centroid distance of 3.914 Å between the carbon rings. There are no close C70-C70 contacts, as was true for the C60 in 1. The C-F contacts to the C70 resemble those in the C60 structure, as shown in Figure 5. As an example of the wide variety of directionality in this interaction, in the region of the porphyrin plane there are 10 C-F‚‚‚C(fullerene) contacts ranging from 2.660 to 3.207 Å. One seems to be pointed toward the center of a six-membered ring, four sets of two are close to 5:6 junctions and one set of two is close to a 6:6 junction. In addition, there are 11 contacts to neighboring porphyrins that are in the 2.827-3.202 Å range. Of these, one set of two is close to a 5:6 junction, two sets of three are near both 5:6 and 6:6 junctions, one set of two is near a 6:6 junction, and a single contact is closest to a single C of the ball. The C-F‚‚‚C(fullerene) angles range from 111.7 to 171.7°. C60‚NiF20TPP‚8benzene, 3. Crystals of 3 grew as purple needles in the tetragonal space group I41/a. Structure 3 resembles 2 except that the porphyrin is a Ni2+ complex instead of a free base and the fullerene is C60 instead of C70. The two structures have similar packing of the molecular constituents. In 3, the disorder in the fullerene portion differs in that the 4h site symmetry is taken up by pyracylene disorder instead of by tipping and rotation of the whole C70 fullerene as occurs in 2. To minimize rotational motion, data were collected at 10(3) K. However, even at this temperature, some residual orientational disorder exists in the C60. The average Uiso for the porphyrin
Figure 6. Saddling of the porphyrin plane is seen in this drawing of C60‚NiF20TPP, 3.
portion is 0.014 Å2 and for the C60 portion is 0.022 Å2. There are some other differences in the structures of 2 and 3. A view of the porphyrin with two fullerenes is shown in Figure 6. The metalloporphyrin is saddled, with a mean deviation from the N4C20 plane of 0.117 Å. The C-F‚‚‚C(fullerene) contacts are longer than in 2, ranging from 3.167 to 3.310 Å within the host porphyrin and 3.226-3.240 Å to the nearest neighbor porphyrin. This may be due to a combination of the saddling and the smaller size of the fullerene. However, electronic effects due to the presence of the Ni2+ cannot be ruled out. This structure also shows stacking of benzene and pentafluorophenyl rings with a centroid-to-centroid distance of 3.839 Å. C70‚ClFe(F20TPP), 4. Black plates of 4 are orthorhombic, crystallizing in the space group Cmc21. The data for this structure was collected at 18(3) K yet again the fullerene suffers some residual disorder. The average equivalent isotropic thermal parameter for the C70 is 0.033 vs 0.017 Å2 for the porphyrin moeity. Here, the model used for the C70 is freely refining with only one orientation. The C70 and the porphyrin share a crystallographic mirror plane. This structure differs from the 1, 2 and 3 in that there is no incorporated benzene. The Fe3+ atom is coordinated to an axial chloride that blocks access by a fullerene to the flip side of the porphyrin. There are 13 contacts in the asymmetric unit between the F’s and carbons of the fullerene guest ranging from 3.110 to 3.359 Å. The shortest C-F‚‚‚C(fullerene) distance is 2.821 Å to a fullerene at another
112 Crystal Growth & Design, Vol. 6, No. 1, 2006
Olmstead
Figure 9. A projection down the bc plane of 4 illustrating the repeat of the structure, the 55° angle between the planes of the porphyrins, and the close C70-C70 contact of 3.447 Å. Table 2. Mean Deviations, In-Plane and Out-of-Plane Distortions from the Porphyrin Plane for 1-4, and Their Fullerene Free Counterparts compound
Figure 7. The compatible size of C70 and the porphyrin platform of ClFe(F20TPP).
symmetry position. As with the previous structure, the C70 molecule fits very neatly on the porphyrin platform. The good match in overall size is depicted in Figure 7. In the N4 core, the longer N‚‚‚N distance of 4.065 Å coincides with the long C70 axis, whereas the shorter N‚‚‚N distance of 3.998 Å matches the short C70 axis. By way of comparison, the structure ClFe(F20TPP)‚toluene also shows unequal core distances of 4.010, 4.038 Å25a (3.996, 4.032 Å in a second structure determination25b). Although there are no benzene-pentafluorophenyl stacks in this structure, there are two other kinds of intermolecular interactions that predominate in this structure and which are not important in the other three. These are shown in Figure 8. Probably the strongest is a bifurcated hydrogen bond between the C-H pyrrole and the axial chloride of the adjacent porphyrin. The C-H (pyrrole) distance is 0.95(5) Å, the H‚‚‚Cl distance is 2.83(5) Å, and the C-H‚‚‚Cl angle is 107(3)°. (This interaction also occurs in the ClFe(F20TPP)‚toluene structure although it is not bifurcated: the H‚‚‚Cl distance is 2.89 Å and the C-H‚‚‚Cl angle is 168.7°.) Metal halide M-Cl‚‚‚H-C interactions of this geometry have been identified as intermediate strength hydrogen bonds.26,27 The second kind of interaction is that between F atoms, two in the asymmetric unit of distances 2.863 and 2.933 Å. The former contacts direct two porphyrin planes at 55° to one another and lead to a ball-on-a-ledge packing arrangment, depicted in Figure 9. In addition, two short C70‚‚‚C70 contacts
out-of-plane distortions
1
0.031
2 H2F20TPP‚ dioxanea H2F20TPPb 3 NiF20TPPe
0.032 0.009
4
0.037
ClFeF20TPP‚ toluenec ClFeF20TPP‚ toluened
0.057
negligible sad ruf:sad, ∼78:22 sad:dom, ∼69:31 sad, dom, ruf
0.062
sad, dom, ruf
a
Figure 8. A view of the bifurcated hydrogen-bonding interactions between pyrrole hydrogens and chlorine of the ClFe(F20TPP) in 4.
ave ∆, (Å)
0.015 0. 117 0.273
waV(x):waV(y), ∼61:39 sad negligible
in-plane distortions bre and δ N-str bre bre bre, δ negatiVe N-str negligible negative bre bre, negatiVe N-str δ bre, δ negatiVe N-str δ bre, δ negatiVe N-str
Ref 10. b Ref 24. c Ref 25a. d Ref 25b. e Ref 29.
in the “belt” region of 3.447 Å are generated by a crystallographic glide, in contrast to 1, 2, and 3 which contain wellseparated fullerenes. Another difference in this structure is the displacement of the Fe from the least-squares N4 plane 0.45 Å toward the axial Cl atom. Consequently, the closest approach of the C70 to Fe is a distant 3.42 Å. Deformations of the Porphyrin Planes. The four cocrystals were examined for their mean porphyrin plane deviations, as well as in-plane and out-of-plane deformations using normalcoordinate structural decomposition (NSD).28 As shown in Table 2, there are no dramatic differences between structures 1, 2, and 4 and the two reported structures of H20F20TPP10,24 and one structure (two determinations) of ClFeF20TPP. For compound 3, the cocrystal with C60 shows significant flattening of the macrocycle and a shift from a ruffled to a saddled porphyrin conformation in comparison to the free porphyrin NiF20TPP.29 The flattening can be seen as an attempt to accommodate the fullerene on both sides of the porphyrin plane and a concomitant opening up of the cavity provided by the flanking C6F5 groups. In view of the existence of multiple polymorphs with different porphyrin conformations for many of the Ni porphyrins, this flexibility is not surprising. Further details of the calculations are provided in the Supporting Information. Conclusions The common structural feature among compounds 1-3 is proximity of the fullerene to both sides of the porphyrin plane. The closest approach of a fullerene carbon to the center of the porphyrin is ca. 2.7 Å, shorter than normal van der Waals distance. Solvate benzene molecules are found in both edgeto-face and face-to-face packing arrangements. Compound 4 is a different structure due to the axial coordination of the Fe by chloride, C-H‚‚‚Cl hydrogen bonding and lack of solvate benzene. In the structure of 4, fullerene-fullerene contacts occur in the absence of as many C-F‚‚‚C(fullerene) contacts as in 1-3. However, in all four of the structures, the approach of C60 or C70 to a porphyrin plane is supported by numerous C-F contacts. The exact nature of this interaction is difficult to
Fluorinated Tetraphenylporphyrins
quantify, but since C-F‚‚‚C(fullerene) contacts exist in preference to fullerene-fullerene contacts in 1, 2, and 3, they are likely to be on a par to those between fullerenes. We have observed that well-formed crystals are readily grown with the fluorinated tetraphenylporphyrins. Because porphyrins are easily modified, this suggests that the use of other cocrystallizing molecules such as H2F20TPP and their metallo derivatives may be of significant utility in the growth of fullerene crystals. Supporting Information Available: X-ray crystallographic information files (CIF) for compounds 1-4; thermal ellipsoid drawings of compounds 1-4; and summary of the NSD calculations. These materials are available free of charge via the Internet at http:// pubs.acs.org.
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CG050225R
