Isolation and crystallographic characterization of ErSc2N@C80: An endohedral fullerene which crystallizes with remarkable internal order

Article abstract of DOI:10.1021/ja001984v

The Er_nSc₃-_nN@C₈₀ (n = 0-3) family of four endohedral fullerenes has been prepared by vaporization of graphite rods packed with 2% Sc₂O₃/3% Er₂O₃/95% graphite powder in a Kraetschmer-Huffman fullerene generator under dynamic flow of helium and dinitrogen. ErSc₂N@C₈₀ has been isolated in pure form via three stages of high-pressure liquid chromatography and characterized by mass spectrometry. The first structure of a mixed metal endohedral, ErSc₂N@C₈₀, has been determined by single-crystal X-ray diffraction at 90 K on ErSc₂N@C₈₀·Co^II(OEP)·1.5C ₆H₆·0.3CHCl₃, which was obtained by diffusion of a solution of ErSc₂N@C₈₀ in benzene into a solution of Co^II(OEP) (OEP is the dianion of octaethylporphyrin) in chloroform. The structure of ErSc₂N@C₈₀ consists of a planar ErSc₂N unit surrounded by an icosahedral C₈₀ cage. The nominal Er-N distance is 2.089(9) A and the Sc-N distance is, as expected, shorter, 1.968(6) A. Despite its location within the C₈₀ cage, the ErSc₂N unit displays a remarkable degree of order within the solid-state structure. The metal ions make close contact with individual carbon atoms of the cage with shortest Sc-C distances, in the range of 2.03-2.12 A, and shortest Er-C distances of 2.20 and 2.22 A. Two different, but equally populated, orientations of the I_h C₈₀ cage were required to describe the fullerene portion of the structure. Although these C₈₀ cages are located on a crystallographic mirror plane, that plane does not coincide with a mirror plane of the cages themselves. Consequently, the cage is disordered over four superimposed sites.

Full text of DOI:10.1021/ja001984v

12220
J. Am. Chem. Soc. 2000, 122, 12220-12226
Isolation and Crystallographic Characterization of ErSc2N@C80: an
Endohedral Fullerene Which Crystallizes with Remarkable Internal
Order
Marilyn M. Olmstead,† Ana de Bettencourt-Dias,† J. C. Duchamp,‡,§ S. Stevenson,‡,
Harry C. Dorn,*,‡ and Alan L. Balch*,†
Contribution from the Department of Chemistry, UniVersity of California, DaVis, California 95616 and the
Department of Chemistry, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061
ReceiVed June 5, 2000
Abstract: The ErnSc3-nN@C80 (n ) 0-3) family of four endohedral fullerenes has been prepared by
vaporization of graphite rods packed with 2% Sc2O3/3% Er2O3/95% graphite powder in a Kra¨tschmer-Huffman
fullerene generator under dynamic flow of helium and dinitrogen. ErSc2N@C80 has been isolated in pure form
via three stages of high-pressure liquid chromatography and characterized by mass spectrometry. The first
structure of a mixed metal endohedral, ErSc2N@C80, has been determined by single-crystal X-ray diffraction
at 90 K on ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚0.3CHCl3, which was obtained by diffusion of a solution of
ErSc2N@C80 in benzene into a solution of CoII(OEP) (OEP is the dianion of octaethylporphyrin) in chloroform.
The structure of ErSc2N@C80 consists of a planar ErSc2N unit surrounded by an icosahedral C80 cage. The
nominal Er-N distance is 2.089(9) Å and the Sc-N distance is, as expected, shorter, 1.968(6) Å. Despite its
location within the C80 cage, the ErSc2N unit displays a remarkable degree of order within the solid-state
structure. The metal ions make close contact with individual carbon atoms of the cage with shortest Sc-C
distances, in the range of 2.03-2.12 Å, and shortest Er-C distances of 2.20 and 2.22 Å. Two different, but
equally populated, orientations of the Ih C80 cage were required to describe the fullerene portion of the structure.
Although these C80 cages are located on a crystallographic mirror plane, that plane does not coincide with a
mirror plane of the cages themselves. Consequently, the cage is disordered over four superimposed sites.
Introduction
Recently the preparation of the novel endohedral, Sc3N@C80,
which contains the planar tetra-atomic Sc3N unit within the
fullerene cage has been reported.7,8 This novel endohedral is
formed by conducting the normal Kra¨tschmer-Huffman arc
fullerene preparation with scandium oxide-doped graphite rods
in a dynamic atmosphere that contains dinitrogen in addition
to helium. This process, the trimetallic nitride template (TNT)
method, produces Sc3N@C80 in an abundance which exceeds
that of C84, the most prevalent of the fullerenes with masses
greater than C70, and makes macroscopic quantities of Sc3N@C80
available for chemical and physical characterization. Sc3N@C80
has been characterized by 13C , 14N , and 45Sc NMR spectros-
copy, UV/vis spectroscopy, and by a single-crystal X-ray
diffraction study of the cocrystallized solid, Sc3N@C80‚CoII-
(OEP)‚0.5C6H6‚1.5CHCl3 (OEP is the dianion of octaethylpor-
phyrin).9,10,11 Although the crystallographic work revealed the
The original recognition1 of the unique stability of C60 was
rapidly followed by news that metal ions could be incorporated
into the central portion of fullerene cages to produce a family
of endohedral fullerenes such as La@Cn.2 However, progress
in exploring the chemical and physical properties of the
endohedral fullerenes has been slowed by three factors: the low
yields in which most endohedrals are produced, the frequent
low solubility of the endohedrals, and the air sensitivity of some
of these species. Nevertheless, many endohedrals have been
detected with one, two, or three atoms, generally electropositive
metal atoms, incorporated within the carbon cages, which range
in size from U@C28 to La3@C106 and beyond.4 Two recent
3
reviews of the field are available.5,6
† University of California, Davis.
‡ Virginia Polytechnic Institute and State University.
§ Permanent address: Emory and Henry College, Emory, Virginia, 24327-
0943.
(6) Nagase, S.; Kobayashi, K.; Akasaka, T.; Wakahara, T. In Fullerenes:
Chemistry, Physics, and Technology; Kadish, K. M., Ruoff, R. S., Eds.
John Wiley and Sons: New York, 2000, p 395.
(7) Stevenson, S.; Rice, G.; Glass, T.; Harich, K.; Cromer, F.; Jordan,
M. R.; Craft, J.; Hadju, E.; Bible, R.; Olmstead, M. M.; Maitra, K.; Fisher,
A. J.; Balch, A. L.; Dorn, H. C. Nature 1999, 401, 55.
Permanent address: Luna Nanomaterials, 2851 Commerce Street,
Blacksburg, VA 24060.
(1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R.
E. Nature 1985, 318, 162.
(2) Heath, J. R.; O’Brien, S. C.; Zhang, Q.; Liu, Y.; Curl, R. F.; Kroto,
H. W.; Tittel, F. K.; Smalley, R. E. J. Am. Chem. Soc. 1985, 107, 7779.
(3) Guo, T.; Diener, M. D.; Chai, Y.; Alford, M. J.; Haufler, R. E.;
McClure, S. M.; Ohno, T.; Weaver, J. H.; Scuseria, G. E.; Smalley, R. E.
Science 1992, 257, 1661.
(4) Alvarez, M. M.; Gillan, E. G.; Holczer, K.; Kaner, R. B.; Min, K.
S.; Whetten, R. L. J. Phys. Chem. 1991, 95, 10561.
(5) Shinohora, H. In Fullerenes: Chemistry, Physics, and Technology;
Kadish, K. M., Ruoff, R. S., Eds. John Wiley and Sons: New York, 2000,
p 357.
(8) Dorn, H. C.; Stevenson, S.; Craft, J.; Cromer, F.; Duchamp, J.; Rice,
G.; Glass, T.; Harich, K.; Fowler, P. W.; Heine, T.; Hajdu, E.; Bible, R.;
Olmstead, M. M.; Maitra, K.; Fisher, A. J.; Balch, A. L. Proceedings of
the IWEPNM2000 Conference; Kirchberg/Tyrol, Austria, March 4-10,
2000; American Institute of Physics; in press.
(9) Crystallization of fullerenes is frequently accompanied by orientational
disorder, which impedes detailed structural study. However, cocrystallization
of fullerenes with CoII(OEP) and other porphyrins produces crystals in which
the fullerene frequently is ordered or sufficiently ordered to allow analysis
of its structure.10,11
10.1021/ja001984v CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/18/2000

Characterization of ErSc2N@C80
J. Am. Chem. Soc., Vol. 122, No. 49, 2000 12221
location of the Sc3N unit within the fullerene cage, residual
electron density in the vicinity of the cage indicated that one or
more alternative cage orientations were present, but this aspect
of disorder was unresolved.
The empty-cage fullerenes, C80 along with C74 and C72, have
been considered as “missing fullerenes” because of their low
abundances in raw fullerenes soot and/or in raw soot extract.12
Seven isomeric structures of the C80 cage [with symmetries D2,
D5d, C2V, C2V′ (two distinct isomers), D3, D5h, and Ih] fulfill the
isolated pentagon rule.13 Theoretical calculations indicate that
these idealized structures have open shell structures and are
subject to Jahn-Teller effects, which lower their symmetries.14
The empty cage C80 isomers D2 and D5d (D5) are nearly equal
in energy and are the most stable isomers. The stability of the
entire range of C80 isomers decreases in the following order:
D2 and D5d (D5) > C2v > C2V′ (Cs) > D3 (C3) > D5h (Cs) > Ih
(D2), where the lowered symmetry of the affected cages is given
in parentheses. Two forms of the empty cage C80 have been
isolated and identified as the D2 and D5d isomers on the basis
of their 13C NMR spectra.15,16
Figure 1. Mass spectrum of the raw CS2 extract that was utilized to
obtain ErSc2N@C80. The intense mass clusters due to C60 and C70 are
not shown.
troscopic and crystallographic data on Sc3N@C80 are consistent
with the presence of a carbon cage with Ih symmetry for which
the (Sc3+)3(N3-)@(C806-) electronic distribution is probable.7
Here we report the preparation of the ErnSc3-nN@C80 (n )
0-3) family of endohedrals, the isolation of ErSc2N@C80, and
its detailed structural characterization by single-crystal X-ray
diffraction.
Despite the low intrinsic abundance of C80 in fullerene
extracts, the formation of endohedral fullerenes by the standard
Kra¨tschmer-Huffman technique has provided access to several
other C80-based endohedrals including La2@C80, Pr2@C80,17
Ce2@C80,18 and M@C80 (M ) Ca, Sr, Ba).19 Sufficient
quantities of La2@C80 have been purified and isolated so that
a number of spectroscopic,20 electrochemical,21 and chemical
properties22 of this endohedral fullerene have been explored.
Electronic structure calculations have shown that the stability
of the Ih of isomer of C80 increases markedly when six electrons
are added to the cage to provide a closed-shell electronic
structure.23,24 As a result, the Ih isomer becomes the most stable
Results and Discussion
Synthesis of the ErnSc3-nN@C80 (n ) 0-3) Family and
Isolation and Characterization of ErSc2N@C80. Vaporization
of graphite rods packed with 2% Sc2O3/3% Er2O3/95% graphite
powder (with cobaltII oxide as a catalyst) in a Kra¨tschmer-
Huffman fullerene generator under dynamic flow of helium and
dinitrogen produces a black soot. Extraction of this raw soot
with cold carbon disulfide produces a reddish-orange solution
of soluble empty cage and metal encapsulated fullerenes. A
portion of the mass spectrum obtained from the material in the
carbon disulfide extract is shown in Figure 1. Mass clusters
arising from the presence of the ErnSc3-nN@C80 family are
present in the following order of decreasing abundance: Er-
Sc2N@C80 > Er2ScN@C80 > Sc3N@C80 > Er3N@C80. In
addition, the customary peaks due to C76, C78, and C84 are seen
along with those of Er2@C82. The intense features coming from
the more abundant C60 and C70 molecules are not shown.
ErSc2N@C80 was isolated from the raw soot and purified via
three stages of high-pressure liquid chromatography (HPLC).
The first stage utilized a pentabromobenzyl column with carbon
disulfide as eluant and an automated HPLC procedure that has
been outlined previously.26 The C84-C88 fraction from the
pentabromobenzyl column was then collected and further
separated using a Buckyclutcher column with a toluene mobile
phase. To remove coeluting impurities in this fraction, a third
round of HPLC utilized a Buckyprep column with toluene as
eluant. The final HPLC trace is shown in Figure 2, along with
the mass spectrum obtained from this sample. ErSc2N@C80
forms reddish-brown solutions in carbon disulfide and benzene
which are stable to air. Although the overall yield of ErSc2N@C80
represents 3-5% of the total soluble extract (see Figure 1), it
is very difficult to remove the TNT endohedral metallofullerenes
(Er3N@C80 and ScEr2@C80) impurities because of chromato-
graphic coelution with very similar elution times on the
Buckyprep column. Nevertheless, from 10 packed graphite rods,
150 mg of soluble extract was obtained and 1-2 mg of purified
ErSc2N@C80 was recovered.
structure for (C80)-6 25
For La2@C80, calculations reveal that
.
the Ih structure is favored with a formal (La3+)2(C80)6- electron
distribution within the molecule. The 13C and 139La NMR spectra
of solutions of La2@C80 have been interpreted in terms of an Ih
structure for the C80, with the metal atoms undergoing rapid
circular motion within the carbon cage.20 Similarly, the spec-
(10) Olmstead, M. M.; Costa, D. A.; Maitra, K.; Noll, B. C.; Phillips, S.
L.; Van Calcar, P. M.; Balch, A. L. J. Am. Chem. Soc. 1999, 121, 7090.
(11) Boyd, P. D. W.; Hodgson, M. C.; Rickard, C. E. F.; Oliver, A. G.;
Chaker, L.; Brothers, P. J.; Bolskar, R. D.; Tham, F. S.; Reed, C. A. J. Am.
Chem. Soc. 1999, 121, 10487.
(12) Wan, T. S. M.; Zhang, W.; Nakane, T.; Xu, M.; Inakume, H.;
Shinohara, H.; Kobayashi, K.; Nagase, S. J. Am. Chem. Soc. 1998, 120,
6806.
(13) Fowler, P. W.; Manoloupoulos D. E. An Atlas of Fullerenes. Oxford
University Press: Oxford, 1995; p 254.
(14) Kobayashi, K.; Nagase, S.; Akasaka, T. Chem. Phys. Lett.. 1995,
245, 230.
(15) Hennrich, F. H.; Michel, R. H.; Fischer, A.; Richard-Schneider, S.;
Gilb, S.; Kappes, M. M.; Fuchs, D.; Bu¨rk, M.; Kobayashi, K.; Nagase, S.
Angew. Chem., Int. Ed. Engl. 1996, 35, 1732.
(16) Wang, C.-R.; Sugai, T.; Kai, T.; Tomiyama, T.; Shinohara, H. Chem.
Commun. 2000, 557.
(17) Ding, J.; Yang, S. J. Am. Chem. Soc. 1996, 118, 11254.
(18) Ding, J.; Yang, S. Angew. Chem., Int. Ed. Engl. 1996, 35, 2234.
(19) Dennis, T. J. S.; Shinohara, H. Chem. Commun. 1998, 883.
(20) Akasaka, T.; Nagase, S.; Kobayashi, K.; Wa¨lchli, M.; Yamamoto,
K.; Funasaka, H.; Kako, M.; Hoshino, T.; Erata, T. Angew. Chem., Int. Ed.
Engl. 1997, 36, 1643.
(21) Suzuki, T.; Maruyama, Y.; Kato, T.; Kikuchi, K.; Nakao, Y.; Achiba,
Y.; Kobayashi, K.; Nagase, S. Angew. Chem., Int. Ed. Engl. 1995, 34, 4,
1094.
(22) Akasaka, T.; Nagase, S.; Kobayashi, K.; Suzuki, T.; Kato, T.;
Kikuchi, K.; Achiba, Y.; Yamamoto, K.; Funasaka, H.; Takahashi, T. Angew.
Chem., Int. Ed. Engl. 1995, 34, 2139.
(23) Fowler, P. W. Chem. Phys. Lett. 1986, 131, 444.
(24) Gillan, E. G.; Yeretzian, C.; Min, K. S.; Alvarez, M. M.; Whetten,
R. L.; Kaner, R. B. J. Phys. Chem. 1992, 96, 6869.
(25) Fowler, P. W.; Zerbetto, F. Chem. Phys. Lett. 1995, 243, 36.
(26) Stevenson, S.; Dorn, H. C.; Burbank, P.; Harich, K.; Anal. Chem.
1994, 66, 2675.

12222 J. Am. Chem. Soc., Vol. 122, No. 49, 2000
Olmstead et al.
superimposed upon one another. Because of this disorder in the
cage, it was necessary to refine it as a rigid group using idealized
coordinates. In this model, the C-C bond distances at 6:6 ring
junctions are held at 1.4263 Å, while the C-C bond distances
at 6:5 ring junctions are 1.4276 Å. This model produces a C80
cage with an 8.19-Å diameter between carbon atoms at the
junction of two six-membered rings and one five-membered ring
and an 8.133-Å diameter between carbon atoms that join three
six-membered rings. Because the C80 cage was disordered and
treated as a rigid group, it was not possible to determine whether
the incorporation and localization of the ErSc2N unit within the
cage produced any distortion of the cage.
Figure 6 presents information about the ErSc2N group and
its relationship to the cage around it. Part a shows the ErSc2N
group alone. The N and Er atoms lie on a crystallographic mirror
plane. The nominal Er-N distance is 2.089(9) Å and the Sc-N
distance is, as expected, shorter, 1.968(6) Å. The Er-N-Sc
angle is 119.1(3)° and the Sc-N-Sc angle is 121.3(6)°. The
ErSc2N unit is planar within experimental error, with the sum
of the two Er-N-Sc angles and the Sc-N-Sc angle equal to
359.6°. The Er and Sc thermal ellipsoids are elongated in
directions which are indicative of motion of the ErSc2N group
along the walls of the C80 cage. RMS analysis of the thermal
motion compensates for this distortion and indicates that the
range for the Er-N bond length is 2.09-2.20 Å, and the range
for the Sc-N bond length is 1.97-2.06 Å.29 The nominal
nonbonded Sc‚‚‚Sc and Sc‚‚‚Er distances are 3.50 and 3.43 Å,
respectively.
Figure 2. The HPLC trace for the purified sample of ErSc2N@C80
utilized in this work. The insert shows the mass spectrum of this sample.
Parts b and c of Figure 6 show the relationships between the
ErSc2N group and the C80 cage surrounding it. Because there
are two orientations of the C80 cage within the crystal, there
are also two slightly different molecules of ErSc2N@C80
produced by coupling these cage orientations with the ErSc2N
portion. These molecules differ in regard to the interactions of
the metal ions with the cage. Figures 7 and 8 show projections
of the metal ions onto the inner surface of the C80 cages of the
two distinct molecules. As it is seen in these drawings, each
metal ion resides close to a single carbon atom of the cage,
with Sc-C distances in the range 2.03-2.12 Å and Er-C
distances of 2.20 and 2.22 Å. In molecule 1, shown in Figure
7, all of the metal atoms reside near carbon atoms that are
located at the intersection of two hexagons and one pentagon.
In molecule 2, as shown in Figure 8, the Er atom and one of
the Sc atoms also lie over carbon atoms that are located at the
intersection of two hexagons and one pentagon, but the other
Sc atom lies over a carbon atom that resides at the intersection
of three hexagons. Thus, the arrangement of scandium ions over
carbon atoms is statistical. Within the icosahedral C80 cage, there
are 60 carbon atoms that reside at the intersection of two
hexagons and one pentagon, but there are only 20 carbon atoms
that reside at the intersection of three hexagons.
In addition to the major site for the ErSc2N group discussed
above, examination of difference electron density maps during
refinement indicated that there were other, less populated sites
for the ErSc2N group within the C80 cage. The ErSc2N group
shown in Figure 6 has a fractional occupancy of 0.80. In addition
there are three sites of electron density, one on the crystal-
lographic mirror plane, the others in general positions, which
have been assigned to erbium atoms. Three other sites of lesser
electron density in general positions within the cage have been
assigned as scandium atoms. All of these sites are roughly 1.9-
2.1 Å from the central nitrogen atom and are close to the
Figure 3. Perspective view of the independent molecules and their
relative orientations within crystalline ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚
0.3CHCl3 with 50% thermal contours for all non-hydrogen atoms. Only
one orientation of the C80 cage is shown. The site on the left is partially
occupied by chloroform and benzene, which are shown superimposed.
Crystallization and Structural Analysis of ErSc2N@C80‚
CoII(OEP)‚1.5C6H6‚0.3CHCl3. Because the sample of ErSc2-
N@C80 itself did not appear to form X-ray diffraction quality
crystals, cocrystallization with CoII(OEP)9,10 was attempted and
found to yield a suitable material: ErSc2N@C80‚CoII(OEP)‚
1.5C6H6‚0.3CHCl3. Figure 3 shows the individual molecular
components of the solid and their relative orientations. The solid
consists of five independent molecules that occupy four sites.
One site, which is shown in the upper left of Figure 3, is
fractionally occupied by a molecule of benzene and a molecule
of chloroform. Figure 4 shows a stereo diagram that illustrates
the molecular packing within the solid.
ErSc2N@C80. The ErSc2N@C80 molecule sits at a site of
crystallographic mirror symmetry. However, the C80 cage itself
is orientationally disordered, and none of the 15 mirror planes
of any one of the icosahedral cages coincides with the
crystallographic mirror plane.27,28 Figure 5 presents a stereo
drawing which shows the two orientations of the cages
(27) Other cases in which the fullerene does not utilize the crystal-
lographic site symmetry are known. For example, in C60‚5Ag(NO3) the C60
cage resides at a site of mm symmetry but is disordered over four
orientations.
(28) Olmstead, M. M.; Maitra, K.; Balch, A. L. Angew. Chem., Int. Ed.
Engl. 1999, 38, 231.
(29) Johnson, C. K. Crystallographic Computing; Munksgaard: Copen-
hagen, 1970; p. 220.

Characterization of ErSc2N@C80
J. Am. Chem. Soc., Vol. 122, No. 49, 2000 12223
Figure 4. View of unit cell of ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚0.3CHCl3 which shows crystallographic packing of the components. Only one
orientation of the C80 cage is shown, and the disordered chloroform is not shown.
Figure 5. Drawing which shows a superposition of the two orientations of the C80 cage in ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚0.3CHCl3. One cage
is shown with a solid line connecting the carbon atoms (small circles); the other cage orientation has open lines connecting the carbon atoms.
fullerene cage. All can be assembled into planar ErSc2N units
that resemble the ErSc2N unit shown in Figure 6, but each of
these has a different orientation within the cage (see supporting
information).
distance, 2.63 Å) in [Fe(TTP)(C60)]+ has been described as
having a covalent Fe-C interaction that is somewhat different
from symmetrical η2-bonding.36
In addition to these fullerene-porphyrin interactions, there
are significant porphyrin-porphyrin contacts with pairwise,
face-to-face contact. As is the case with C60‚2CoII(OEP)‚CHCl3
and with C70‚CoII(OEP)‚C6H6‚CHCl3, the face-to-face porphy-
rin-porphyrin contact is greater in the fullerene cocrystals than
it is in pristine CoII(OEP) itself. Thus, the lateral shift (LS, 1.534
Å), mean plane separation (MPS, 2.994 Å), and the Co‚‚‚Co
separation (3.364 Å) in ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚
0.3CHCl3 are shorter than the corresponding distances in CoII-
(OEP) (LS, 3.38 Å; MPS, 3.33 Å, Co‚‚‚Co separation, 4.742
Å) but are similar to those in C70‚CoII(OEP)‚C6H6‚CHCl3 (with
LS, 1.67 Å; MPS, 3.19 Å, Co‚‚‚Co separation, 3.392 Å). This
close face-to-face arrangement in ErSc2N@C80‚CoII(OEP)‚
1.5C6H6‚0.3CHCl3 is facilitated by the positioning of all of the
ethyl groups on the opposite side of the porphyrin plane from
the adjacent CoII(OEP) molecule. In contrast, CoII(OEP) alone
has four ethyl groups on one side of the porphyrin plane and
four on the opposite side.30
CoII(OEP) and Its Interaction with ErSc2N@C80 and
Itself. The CoII(OEP) molecule resides on a crystallographic
mirror plane which coincides with a molecular mirror plane that
bisects the cobalt atom, N(1), and N(3). The geometry of the
CoII(OEP) molecule is entirely normal. The Co-N distances
[Co-N(1), 1.969(8); Co-N(2), 1.985(6); and Co-N(3), 1.976-
(7) Å] in ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚0.3CHCl3 are similar
to those in CoII(OEP) [Co-N, 1.967(3), 1.975(2) Å],30 in C60‚
2CoII(OEP)‚CHCl3 [Co-N, 1.954(5)-1.985(6) Å], and in C70‚
CoII(OEP)‚C6H6‚CHCl3 [Co-N, 1.964(5)-1.967(5) Å].10
The CoII(OEP) molecule is positioned so that the ethyl groups
of the porphyrin form an octapoidal embrace about the fullerene
and the porphyrin plane is adjacent to the C80 cage. The fullerene
is too far from the cobalt atoms for any normal covalent bonding
between them. The closest approaches of the cobalt atom to
the C80 cage involves C(9B), which is 2.706 Å from the cobalt
atom, and C(78A), which is 2.746 Å from the cobalt atom.
Although these distances are too long to represent true η2-
coordination,31 they are shorter than the normal van der Waals
contact seen between graphite layers (3.4 Å),32 between adjacent
porphyrins (3.2 Å and larger),33 and between neighboring
fullerenes (>3.2 Å).34,35 The shorter Fe-C60 contact (Fe-C
Discussion
The TNT approach has been successfully utilized to prepare
the set of four endohedrals, ErnSc3-nN@C80 (n ) 0-3). In the
process described here, the raw soot that was obtained is
particularly rich in the mixed metal species, ErSc2N@C80, which
has been separated and isolated in pure form.
(30) Scheidt, W. R.; Turowska-Tyrk, I. Inorg. Chem. 1994, 33, 1314.
(31) Balch, A. L.; Olmstead, M. M. Chem. ReV. 1998, 98, 2123.
(32) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell
University Press: Ithaca, NY, 1960; p 260.
(35) Balch, A. L.; Lee, J. W.; Noll, B. C.; Olmstead, M. M. J. Chem.
Soc., Chem. Commun. 1993, 56.
(33) Scheidt, W. R.; Lee, Y. J. Struct. Bonding (Berlin) 1987, 64, 1.
(34) Bu¨rgi, H. B.; Restori, R.; Schwarzenbach, D.; Balch, A. L.; Lee, J.
W.; Noll, B. C.; Olmstead, M. M. Chem. Mater. 1994, 6, 1325.
(36) Evans, D. R.; Fackler, N. L. P.; Xie, Z.; Rickard, C. E. F.; Oliver,
A. G.; Chaker, L.; Brothers, P. J.; Bolskar, R. D.; Boyd, P. D. W.; Reed,
C. A. J. Am. Chem. Soc. 1999, 121, 8466.

12224 J. Am. Chem. Soc., Vol. 122, No. 49, 2000
Olmstead et al.
Figure 6. Drawings which show (a) the ErSc2N unit alone with thermal
elipsoids, and b and c, the two molecules of ErSc2N@C80 (without
thermal elipsoids) which arise due to the cage disorder. In b and c, the
dashed lines connect the metal ions to the nearest carbon atoms.
The crystallographic data indicate that the ErSc2N unit is
tightly packed within the C80 cage, because the Sc-N and Er-N
distances, along with the shortest Sc-C and Er-C distances,
are all shorter than the shortest comparable bond lengths in other
compounds that these metal ions form. Moreover, these distances
are (>0.25 Å) shorter than comparable mean bond lengths given
in the Cambridge Structural Data Base (CSD).37 The nominal
Sc-N distance in ErSc2N@C80 is 1.968(6) Å, but a search of
the CSD reveals that the mean Sc-N distance is 2.251 Å in 82
observed examples and that the previous shortest Sc-N distance
was 2.039 Å (for dichloro-bis(tetrahydrofuran)-bis(trimethyl-
silyl)amido-scandium).38 Other relevant comparisons include
tris(bis(dimethylsilyl)amido)(tetrahydrofuran)-scandium with
Sc-N distances of 2.063(2), 2.064(2), and 2.079(2) Å39 and
(η5-cyclopentadienyl)(octaethylporphyrin)scandium with an av-
erage Sc-N distance of 2.190(2) Å.40 Similarly, the nominal
Er-N distance in ErSc2N@C80 is 2.089(9) Å, but the mean
Er-N distance in the CSD is 2.411 Å in 128 observed examples,
and the previous shortest Er-N distance was 2.151 Å (for bis-
(µ2-triphenylphosphineiminato)-tris(η5-cyclopentadienyl)-(tri-
Figure 7. Projections which show the positions of the metal ions with
respect to the adjacent walls of the C80 cage for one orientation of the
cage (molecule 1).
phenylphosphineiminato)-di-erbium).41 In a particularly relevant
comparison, the average Er-N distance in [tetrakis(tetrahydro-
furan)lithium]-[tetrakis(diphenylamido)erbium] is 2.26 Å.42
Similar considerations pertain to the shortest Sc-C and Er-C
distances inside the cage. Thus, the shortest Sc-C distances in
ErSc2N@C80 range from 2.029 to 2.121 Å, but the mean Sc-C
distance is 2.430 Å in 73 examples in the CSD, and the shortest
Sc-C distance in the CSD is 2.204 Å for the σ-Sc-C bond in
bis(trimethylsilylmethyl-)-N,N-bis(di-isopropylphosphinomethyl-
dimethylsilylamido)-scandium.43 In other relevant examples, the
average Sc-C distance in (η5-cyclopentadienyl)(octaethylpor-
phyrin)scandium is 2.494(4) Å,39 and the average Sc-C distance
in di-(µ-chloro)-bis(di-(η5-cyclopentadienyl)scandium) is 2.46
Å.44 Similarly, the shortest Er-C distances in ErSc2N@C80 are
(37) Allen, F. H.; Davies, J. E.; Johnson, O. J.; Kennard, O.; Macrae, C.
F.; Mitchell, E. M.; Mitchell, G. F.; Smith, J. M.; Watson, D. J. Chem. Inf.
Comput. Sci. 1991, 31, 187.
(38) Karl, K.; Sybert, G.; Massa, W.; Dehnicke, K. Z. Anorg. Allg. Chem.
1999, 625, 375.
(39) Anwander, R.; Runte, O.; Eppinger, J.; Gerstberger, G.; Herdtweck,
E.; Spiegler, M. J. Chem. Soc., Dalton Trans. 1998, 847.
(40) Arnold, J.; Hoffman, C. G. J. Am. Chem. Soc. 1990, 112, 8620.
(41) Anfang, S.; Harms, K.; Weller, F.; Borgmeier, O.; Lueken, H.;
Schilder, H.; Dehnicke, K. Z. Anorg. Allg. Chem. 1998, 624, 159.
(42) Wong, W.-K.; Zhang, L.; Xue, F.; Mak, T. C. W. Polyhedron, 1997,
16, 2013.
(43) Fryzuk, M. D.; Giesbrecht, G.; Rettig, S. J. Organometallics, 1996,
15, 3329.
(44) Atwood, J. L.; Smith, K. D. J. Chem. Soc., Dalton Trans. 1973,
2487.

Characterization of ErSc2N@C80
J. Am. Chem. Soc., Vol. 122, No. 49, 2000 12225
complexes. Ab initio molecular orbital and density functional
calculations have placed the Sc atoms 2.358 Å from the nearest
carbon atoms in Sc2@C84 47 and 2.322 Å from the closest carbon
atoms in Sc2@C80.48 These are longer distances than those
observed in ErSc2N@C80. However, fewer atoms are present
in the interior of Sc2@C84 and Sc2@C80 than in ErSc2N@C80.
Room temperature X-ray powder diffraction data have been
analyzed for Sc3@C82 by a combination of Rietveld and
maximum entropy method (MEM) methods.49 The results
produced a model in which an equilateral triangle of scandium
atoms with a Sc‚‚‚Sc distance of 2.3(3) Å resides within a C82
cage having C3V symmetry and with the Sc atom 2.52(2) Å from
the closest carbon atom of the C82 cage. A similar analysis of
X-ray powder diffraction data for Sc2@C84 indicated that the
Sc‚‚‚Sc distance is 3.9(1) Å and that the shortest Sc‚‚‚C contact
is 2.4(2) Å.50 Again the Sc‚‚‚C contacts observed in ErSc2N@C80
are shorter.
Bonding within ErSc2N@C80 can be considered to consist
of four components. First, there is the covalent C-C bonding
that forms the cage itself. This cage then mechanically entraps
the ErSc2N unit. In addition to the mechanochemical encapsula-
tion, there is a strong ionic component to the bonding in the
endohedral. The ErSc2N@C80 molecule can be thought in formal
terms to consist of three concentric rings of charge starting with
the core nitride (N3-), which is surrounded by three M3+ ions,
which are then encapsulated by the (C80)6- cage. This formal
charge distribution utilizes the characteristic M3+ oxidation state
for both scandium and erbium and places a 6- charge on the
C80 cage. These added electrons on the fullerene uniquely
stabilize the Ih symmetry cage structure relative to the six other
isomeric C80 cage geometries, as noted earlier.23,24 Given the
close contacts inside the cage, there is no doubt significant
orbital overlap that leads to covalent interactions among the
components and to sharing of electron density so that the
effective charges on the individual components are reduced
below those given by the formal ionic model.
Although the ErSc2N unit is firmly ensconced within the
fullerene cage, it is also likely to be able to ratchet around inside
the cage. The nature of the thermal ellipsoids shown in part A
of Figure 7 is suggestive of such motion. The observation of
additional, less populated sites of electron density within the
cage also indicates that there are alternative locations for the
group and again suggests that this group is able to move within
the fullerene cage. The mobility of atoms within fullerenes cages
has been suggested previously from NMR studies on La2@C80,20
Sc3N@C80,7 Sc2@C84,51 and from EPR studies on Sc3@C82.52,53,54
Crystals of ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚0.3CHCl3 and
Sc3N@C80‚CoII(OEP)‚0.5C6H6‚1.5CHCl3 are isomorphic. Al-
though they differ in the content of cocrystallized solvent
molecules, they have very similar arrangements of the fullerene
and cobalt porphyrin components. It is remarkable that the
Figure 8. Projections which show the positions of the metal ions with
respect to the adjacent walls of the C80 cage for the other orientation
of the cage (molecule 2).
2.195 and 2.215 Å, but the mean Er-C distance in the CSD is
2.769 Å, and the shortest Er-C distance is 2.412 Å (in triphenyl-
tris(tetrahydrofuran)-erbium),45 and for comparison, the average
Er-C distance in di-µ-chloro-bis(di-(η5-cyclopentadienyl)er-
bium) is 2.59 Å.46 In considering the comparison of these M-C
distances, it is important to realize that the orientation of the
M-C unit in ErSc2N@C80 differs significantly from that in
conventional π-bonded ligands, such as η5-cyclopentadienyl
groups. In ErSc2N@C80, the metals reside near single carbon
atoms, whereas in (η5-C5H5) complexes, which are particularly
numerous in the CSD, the metal is centered over the cyclopen-
tadienide ring, and consequently for such compounds the M-C
distances are naturally longer than they would be if the metal
were located directly over a single carbon atom.
(47) Kobayashi, K.; Nagase, S.; Akasaka, T. Chem. Phys. Lett. 1996,
261, 502.
(48) Kobayashi, K.; Nagase, S. Chem. Phys. Lett. 1996, 262, 227.
(49) Takata, M.; Nishibori, E.; Sakata, M.; Inakuma, M.; Yamamoto,
E.; Shinohara, H. Phys. ReV. Lett. 1999, 83, 2214.
(50) Nishibori, E.; Takata, M.; Sakata, M.; Shinohara, H. J. Synchrotron
Radiat. 1998, 5, 977.
The positions of the scandium atoms within the fullerene cage
in ErSc2N@C80 can also be compared to structural information
obtained from theoretical calculations and from the analysis of
X-ray powder diffraction data on related scandium endohedral
(51) Miyake, Y.; Suzuki, S.; Kojima, K.; Kikuchi, K.; Kobayashi, K.;
Nagase, S.; Kainosho, M.; Achiba, Y.; Maniwa, Y.; Fisher, K. J. Phys.
Chem. 1996, 100, 9579.
(52) van Loosdrecht, P. H. M.; Johnson, R. D.; de Vries, M. S.; Kiang,
C.-H.; Bethune, D. S.; Dorn, H. C.; Burbank, P.; Stevenson, S. Phys. ReV.
Lett. 1994, 73, 3415.
(45) Bochkarov, L. N.; Stepantseva, T. A.; Zakharov, L. N.; Fukin, G.
K.; Yanovsky, A. I.; Struchkov, Yu. T. Organometallics 1995, 14, 2127.
(46) Lamberts, W.; Lueken, H.; Hessner, B. Inorg. Chem. Acta 1987,
134, 155.
(53) Shinohara, H.; Inakuma, M.; Hayashi, N.; Sato, H.; Saito, Y.; Kato,
T.; Bandow, S.; J. Phys. Chem. 1994, 98, 8598.
(54) Kato, T.; Bandow, S.; Inakuma, M.; Shinohara, H. J. Phys. Chem.
1995, 99, 856.

12226 J. Am. Chem. Soc., Vol. 122, No. 49, 2000
Olmstead et al.
ErSc2N portion within ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚0.3CHCl3
shows the high degree of order that it does with the erbium
atom localized largely on one site, a site on the crystallographic
mirror plane and furthest from the cobalt atom of the porphyrin.
A random distribution of the erbium ion over all three prominent
metal ion sites within the C80 cage seemed probable at the outset
of this project, but a much higher degree of localization is
observed. The most likely factor that contributes to ordering
the ErSc2N unit within the fullerene with respect to the solid-
state environment outside the fullerene is the dipole moment
that is created by the asymmetric ErSc2N unit itself. The
differences in electronegativity of scandium and erbium, the
differences in their sizes and in the Sc-N and Er-N distances
all contribute to creating this dipole.
column was then collected and further separated with a Buckyclutcher
column (25 cm × 10 mm, Regis Chemical, Morton Grove, IL) using
toluene as a mobile phase. Upon re-injection of the ErSc2N@C80
fraction, a single homogeneous peak was obtained. Due to coeluting
impurities of this ErSc2N@C80 fraction, a third column (Buckyprep,
25 cm × 10 mm, Phenomenex Co., Torrence, CA) had to be employed.
By using this Buckyprep column with toluene as the eluant, a final
sample of purified ErSc2N@C80 was obtained. Its negative-ion mass
spectrum and final HPLC trace are shown in Figure 2.
Crystal Growth for ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚0.3CHCl3:
Crystals of ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚0.3CHCl3 were obtained
by layering an orange-red solution of ca. 0.5 mg of ErSc2N@C80 in
0.5 mL of benzene over a red solution of 2.5 mg of CoII(OEP) in 1.5
mL of chloroform. After we allowed the two solutions to diffuse
together over a five day period, black crystals formed.
X-ray Data Collection for ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚
0.3CHCl3: The crystals were removed from the glass tube, together
with a small amount of mother liquor and immediately coated with a
hydrocarbon oil on the microscope slide. A suitable crystal was mounted
on a glass fiber with silicone grease and placed in the cold stream of
a Bruker SMART CCD with graphite monochromated Mo KR radiation
at 90(2) K. No decay was observed in 50 duplicate frames at the end
of the data collection. Crystal data for ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚
0.3CHCl3, fw ) 1976.34, black parallelepiped, 0.11 × 0.03 × 0.02
mm, monoclinic, space group C2/m, a ) 25.180(2), b ) 15.0633(13),
c ) 19.650(2) Å, â ) 94.791(2)°, V ) 7427.1(12) Å3, λ ) 0.71073 Å,
Z ) 4, Dc ) 1.767 Mg m-3; µ(Mo KR) ) 1.616 mm-1; 2Θmax ) 25.00°;
T ) 90(2) K; 34988 refl. collected; 6819 independent (Rint ) 0.143)
included in the refinement; no absorption correction performed;
programs used for solution and refinement, SHELXS-97, Sheldrick,
1990; full-matrix least-squares based on F2, SHELXL-97; Sheldrick,
1998; 455 parameters, 250 restraints, R1) 0.1515, wR2)0.263 for all
data; R1 ) 0.087 computed for 3721 observed data (>2σ(I)).
The structure was solved by Patterson and difference Fourier
methods. Hydrogen atoms were added geometrically and refined using
a riding model. The major ErSc2N unit, the non-hydrogen atoms of
the CoII(OEP) unit, and the carbon atoms of the benzene molecule with
0.50 occupancy were refined using anisotropic thermal parameters. The
carbon atoms of the C80 cages were refined as a rigid group utilizing
ideal coordinates with Ih symmetry and free isotropic thermal param-
eters.
Resolution of the disorder in the location of the C80 cage in
ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚0.3CHCl3 is particularly sig-
nificant. The earlier structural work on Sc3N@C80‚CoII(OEP)‚
0.5C6H6‚1.5CHCl3 noted that there was an unresolved issue of
residual electron density in the region of the C80 cage.7 At the
time, this electron density suggested disorder in the orientations
of the C80 cage, but the disorder could not be effectively
modeled. However, when the disordered model used in the
refinement of the ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚0.3CHCl3
structure is utilized in the refinement of the Sc3N@C80‚CoII-
(OEP)‚0.5C6H6‚1.5CHCl3 structure, the R factor of the latter
structure drops by ca. 2%, which suggests that a similar form
of disorder is present in the location of the Sc3N@C80 molecule
in that crystalline environment.
The Ih fullerene cage of C80 differs significantly from that of
other fullerenes that have been subject to chemical and structural
characterization, because the icosahedral isomer of C80 lacks
any pyracylene region (Stone-Wales patch)55 in which a 6:6
ring junction is abutted by two pentagons. In icosahedral C80,
the 6:6 ring junctions are abutted by a hexagon and a pentagon.
As a consequence, the Ih isomer of C80 lacks the sites that are
most chemically reactive in other fullerenes, and it is likely to
display distinctive chemical behavior. Similarly, the placement
of the metal atoms within the Ih cage of C80, as seen in Figures
7 and 8, may not be found in other endohedral fullerenes where
pyracylene patches are present.
Note Added in Proof. The structure of Sc3N@C78 shows the Sc
atoms located at the midpoints of the two carbon atoms at the center
of pyracylene patches. Olmstead, M. M.; de Bettencourt-Dias, A.;
Duchamp, J. C.; Stevenson, S.; Marciu, D.; Dorn, H. C.; Balch, A. L.
Angew. Chem. Int. Ed., Engl., in press.
Experimental Section
Production of ErSc2N@C80. Graphite rods (0.25 diameter, 6 in.
length) were core-drilled and subsequently packed with 180 mg cobalt
oxide in a mixture of 1.0 g of graphite powder, 0.49 g of Sc2O3, 2.0 g
of Er2O3 per 3.2 g of hollowed graphite rod. These rods were then
vaporized in a Kra¨tschmer-Huffman-type fullerene generator under
dynamic flow of He (1250 mL/min) and N2 (22 mL/min) to obtain
samples containing ErSc2N@C80. The resulting soot from this TNT
approach7,8 was then cold-extracted using carbon disulfide to obtain
the initial endohedral extract. An NI-DCI mass spectrum of a typical
ErSc stock solution is shown in Figure 1.
Acknowledgment. This article is dedicated to Prof. Fred
Wudl on the occasion of this 60th birthday. We thank the
National Science Foundation (Grants CHE 9610507 and CHE
0070291 to A.L.B.) and LUNA Innovations (to H.C.D.) for
support, the Gulbenkian Foundation for a postdoctoral fellow-
ship for A.dB.-D., and Professor R. L. DeKock and Dr. A.
Tamulis for useful information. The Bruker SMART 1000
diffractometer was funded in part by NSF Instrumentation grant
CHE-9808259.
Separation of ErSc2N@C80. The ErSc stock solution was separated
using a 3-stage HPLC approach. First, this initial extract was separated
on the pentabromobenzyl, PBB, column (25 cm × 10 mm, Phenomenex
Co., Torrance, CA) with CS2 as the mobile phase. For this stage, an
automated approach was utilized.26 The C84-C88 fraction from this PBB
Supporting Information Available: X-ray crystallographic
files in CIF format for ErSc2N@C80‚CoII(OEP)‚1.5C6H6‚
0.3CHCl3.This material is available free of charge via the
Internet at http://pubs.acs.org.
(55) Stone, A. J.; Wales, D. J. Chem. Phys. Lett. 1986, 128, 501.
JA001984V