Supramolecular fullerene-porphyrin chemistry. Fullerene complexation by metalated "jaws porphyrin" hosts

Article abstract of DOI:10.1021/ja017555u

Porphyrins and fullerenes are spontaneously attracted to each other. This new supramolecular recognition element is explored in discrete, soluble, coordinatively linked porphyrin and metalloporphyrin dimers. Jawlike clefts in these bis-porphyrins are effe

Full text of DOI:10.1021/ja017555u

Published on Web 05/14/2002
Supramolecular Fullerene-Porphyrin Chemistry. Fullerene
Complexation by Metalated “Jaws Porphyrin” Hosts
Dayong Sun,† Fook S. Tham,† Christopher A. Reed,*,† Leila Chaker,‡ and
Peter D. W. Boyd*,‡
Contribution from the Departments of Chemistry, UniVersity of California,
RiVerside, California 92521-0403, and The UniVersity of Auckland, PriVate Bag,
Auckland, New Zealand
Received November 15, 2001
Abstract: Porphyrins and fullerenes are spontaneously attracted to each other. This new supramolecular
recognition element is explored in discrete, soluble, coordinatively linked porphyrin and metalloporphyrin
dimers. Jawlike clefts in these bis-porphyrins are effective hosts for fullerene guests. X-ray structures of
the Cu complex with C60 and free-base complexes with C70 and a pyrrolidine-derivatized C60 have been
obtained. The electron-rich 6:6 ring-juncture bonds of C60 show unusually close approach to the porphyrin
or metalloporphyrin plane. Binding constants in toluene solution increase in the order Fe(II) < Pd(II) <
Zn(II) < Mn(II) < Co(II) < Cu(II) < 2H and span the range 490-5200 M-1. Unexpectedly, the free-base
porphyrin binds C60 more strongly than the metalated porphyrins. This is ascribed to electrostatic forces,
enhancing the largely van der Waals forces of the π-π interaction. The ordering with metals is ascribed to
a subtle interplay of solvation and weak interaction forces. Conflicting opinions on the relative importance
of van der Waals forces, charge transfer, electrostatic attraction, and coordinate bonding are addressed.
The supramolecular design principles arising from these studies have potential applications in the preparation
of photophysical devices, molecular magnets, molecular conductors, and porous metal-organic frameworks.
porphyrin light-harvesting devices,9-11 molecular conductors or
magnets,12-14 medicine,15 and porous metal-organic frame-
Introduction
As discrete molecules, the chemistry of fullerenes has been
explored in three identifiable phases. Early investigations
emphasized their synthesis and redox chemistry, and under-
standing the new type of bonding presented by their curved π
surfaces.1 Organic functionalization chemistry followed.2-5
Now, the supramolecular chemistry of fullerenes is attracting
attention.6,7
This paper concerns a newly recognized supramolecular
recognition element, the attraction of the curved π surface of a
fullerene to the center of the flat π surface of a porphyrin or
metalloporphyrin. In contrast to the traditional paradigm, it is
not necessary to match a concave host with a convex guest.
The interaction has attracted attention from a fundamental point
of view,8 and there are potential applications in fullerene-
works.16
The close association of a fullerene and a porphyrin was first
recognized in the solid-state porphyrin-fullerene assembly of a
covalent fullerene-porphyrin conjugate.17 In the crystal structure
of this species, the 2.75 Å approach of a fullerene carbon atom
to center of a porphyrin plane was notably shorter than
separations of familiar π-π interactions. Graphite and typical
arene/arene separations are in the range 3.3-3.5 Å. Interfacial
porphyrin/porphyrin separations are >3.2 Å, fullerene/arene
approaches lie in the range 3.0-3.5 Å, and fullerene/fullerene
separations are typically ca. 3.2 Å.17 Two conclusions were
drawn.
First, the close approach was proposed to reflect an attractive,
structure-defining π-π interaction. The frequency with which
it has appeared in subsequent research supports this notion.
* To whom correspondence should be addressed. E-mail: chris.reed@
ucr.edu and pdw.boyd@auckland.ac.nz.
† University of California, Riverside.
‡ The University of Auckland.
(9) Guldi, D. M. Chem. Commun. 2000, 321-327.
(10) Segura, J. L.; Martin, N. Chem. Soc. ReV. 2000, 29, 13-25.
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Chem.-Eur. J. 2000, 6, 1629.
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Stuttgart: New York, 1994.
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London, 1999.
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C. A. J. Org. Chem. 1997, 62, 3642-3649.
9
6604
J. AM. CHEM. SOC. 2002, 124, 6604-6612
10.1021/ja017555u CCC: $22.00 © 2002 American Chemical Society

Supramolecular Fullerene-Porphyrin Chemistry
A R T I C L E S
Reports soon appeared of cocrystallates of fullerenes with
octaethylporphyrin (H2OEP) and metallooctaethylporphyrins,18
and with tetraphenylporphyrins8 (H2TPP). More examples
followed,19-25 and fullerenes are attracted to other flat π
surfaces.13,26-28 The curved to flat π-π attraction has been
analyzed by molecular mechanics modeling and shown to be
largely the result of van der Waals dispersion forces.8 Further,
the interaction is not merely a feature of solid-state cocrystal-
lizations; it persists in solution.8 This has been shown elegantly
with covalently linked bis-porphyrin hosts that take up fullerene
guests having binding constants up to 7 × 105 M-1 in
toluene.29-31 These values can exceed those of traditional
concave hosts such as calixarenes,32 cyclotriveratralenes,33,34 and
resorcarenes,35 showing that the matching of convex and
concave surfaces is not a requirement for strong supramolecular
π-π complexation.
Second, it was proposed that the fullerene-porphyrin π-π
interaction involved some degree of electrostatic attraction or
charge transfer.17 In particular, it was proposed that the electron-
rich “double bond” at the 6:6 ring juncture of C60 or C70 was
attracted to the protic center of the free-base porphyrin. This
proposal was counter to the prevailing notion of fullerenes acting
as electron acceptors. Indeed, the electron-accepting ability of
fullerenes is probably their most characteristic chemical prop-
erty.36 This is illustrated in metalloporphyrin chemistry by the
reaction of CrII(TPP) with C60 in tetrahydrofuran to give the
Cr(III) fulleride salt [Cr(THF)2(TPP)][C60]37 or the reaction of
SnII(TPP) with C60 in the presence of N-methylimidazole to give
[SnIV(N-MeIm)2(TPP)][C60]2.37 However, because of the de-
localized nature of the C60 LUMO, fullerene electronegativity
is fundamentally a global property, whereas the proposed
fullerene/porphyrin interaction is a more local one. One approach
to address the charge transfer question is to study metalated
porphyrin hosts and determine how the fullerene responds as a
function of metal. The roles of hard, first row transition metals
in metalloporphyrins can be expected to be quite different from
soft metals in phosphine complexes such as Ni(C60)(PEt3)2
or IrCl(C60)(CO)(PPh3)239 because hard metals have little or no
opportunity to engage in π back-bonding.
38
We have reported that the complex of Fe(TPP)+ with C60 is
green rather than purple, the expected color of the combined
unperturbed chromophores.19 This implies charge transfer via
coordinate bonding. The Fe atom is slightly out-of-plane toward
the C60 indicating the presence of a weak axial coordinate bond
with at least some degree of covalence. The orbitals involved
were identified by density functional theory in closely related
complexes of Fe(TPP)+ with η2-bonded arenes.19 Because the
Fe(III) center is cationic and its dz2 orbital is only half occupied,
it would be difficult to argue that the direction of charge transfer
is not with the fullerene as the donor. On the other hand, many
fullerene-M(OEP) structures have been interpreted as indicating
no covalent interaction,18 and some structures apparently have
the less electron-rich 5:6 (rather than a 6:6) ring-juncture bond
closest to the metalloporphyrin.20,22 Another recent study
concludes that these molecular complexes have no charge
transfer in the ground state.23 Evidence that C60 has sufficient
ligand field strength to cause a high to low spin-state change in
Mn(TPP) has also been forwarded,23 but this contradicts field
strength deductions based on other Mn(TPP) chemistry40 and
isoelectronic Fe(III) chemistry.19 A recent communication
reports that binding constants for C60 increase as a function of
the metal in a bis-porphyrin host in the order Ag(I) < Ni(II) <
Cu(II) < Zn(II) < free base < Co(II) < Rh(III).41 With the
exception of rhodium(III), the differences are quite small, and
the ordering has not been interpreted. The ordering changes
slightly with C70.41 Synthesizing a coherent understanding of
these sometimes conflicting observations is the goal of this
paper. We address the problem by studying the complexation
of C60 and C70 guests in metalated “jaws porphyrin” hosts.
(18) 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-
7097.
(19) Evans, D. R.; Fackler, N. L. P.; Xie, Z.; Rickard, C. E. F.; Boyd, P. D. W.;
Reed, C. A. J. Am. Chem. Soc. 1999, 121, 8466-8474.
(20) Ishii, T.; Aizawa, N.; Yamashita, M.; Matsuzaka, H.; Kodama, T.; Kikuchi,
K.; Ikemoto, I.; Iwasa, Y. J. Chem. Soc., Dalton Trans. 2000, 4407-4412.
(21) Olmstead, M. M.; de Bettencourt-Dias, A.; Duchamp, J. C.; Stevenson, S.;
Dorn, H. C.; Balch, A. L. J. Am. Chem. Soc. 2000, 122, 12220-12226.
(22) Ishii, T.; Aizawa, N.; Kanehama, R.; Yamashita, M.; Matsuzaka, H.;
Kodama, T.; Kikuchi, K.; Ikemoto, I. Inorg. Chim. Acta 2001, 317, 81-
90.
(23) Konarev, D. V.; Neretin, I. S.; Slovokhotov, Y. L.; Yudanova, E. I.; Drichko,
N. V.; Shul’ga, Y. M.; Tarasov, B. P.; Gumanov, K. L.; Batsanov, A. S.;
Howard, J. A. K.; Lyubovskaya, R. N. Chem.-Eur. J. 2001, 7, 2605-2616.
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S.; Miyasaka, H.; Kodama, T.; Kikuchi, K.; Ikemoto, I.; Tanaka, H.;
Marumoto, K.; Kuroda, S.-I. J. Chem. Soc., Dalton Trans. 2001, 2975-
2980.
Results and Discussion
Design and Synthesis of Jaws Porphyrin. The design of
“jaws” porphyrins evolved from an interplay of molecular
modeling and experiment. The basic bis-porphyrin motif was
cut from the common zigzag structural arrangement seen in self-
assembling tetraphenylporphyrin/fullerene cocrystallates.8 Vari-
ous linkers such as those illustrated in Figure 1 were used to
construct bis-porphyrins as fullerene binding hosts maintaining
this motif. The design criteria, based on cocrystallate structures,
allowed for a porphyrin-porphyrin intercenter distance of 11.5-
12 Å with interplanar porphyrin angles of 40-60°. Host-guest
complexes with C60 were constructed and subjected to geometry
optimization with molecular mechanics calculations. In each
case, the gas-phase binding enthalpy was determined together
with the geometry of the free bis-porphyrin host. It became clear
(25) Ishi-i, T.; Iguchi, R.; Snip, E.; Ikeda, M.; Shinkai, S. Langmuir 2001, 17,
5825-5833.
(26) Andrews, P. C.; Atwood, J. L.; Barbour, L. J.; Nichols, P. J.; Raston, C. L.
Chem.-Eur. J. 1998, 4, 1384-1387.
(27) Balch, A. L.; Olmstead, M. M. Coord. Chem. ReV. 1999, 185-186, 601-
617.
(28) Hochmuth, D. H.; Michel, S. L. J.; White, A. J. P.; Williams, D. J.; Barrett,
A. G. M.; Hoffman, B. M. Eur. J. Inorg. Chem. 2000, 593-596.
(29) Tashiro, K.; Aida, T.; Zheng, J.-Y.; Kinbara, K.; Saigo, K.; Sakamoto, S.;
Yamaguchi, K. J. Am. Chem. Soc. 1999, 121, 9477-9478.
(30) Sun, D.; Tham, F. S.; Reed, C. A.; Chaker, L.; Burgess, M.; Boyd, P. D.
W. J. Am. Chem. Soc. 2000, 122, 10704-10705.
(31) Nishioka, T.; Tashiro, K.; Aida, T.; Zheng, J.-Y.; Kinbara, K.; Saigo, K.;
Sakamoto, S.; Yamaguchi, K. Macromolecules 2000, 33, 9182-9184.
(32) Atwood, J. L.; Koutsantonis, G. A.; Raston, C. L. Nature 1994, 368, 229-
232.
(33) Steed, J. W.; Junk, P. C.; Atwood, J. L.; Barnes, M. J.; Raston, C. L.;
Burkhalter, R. S. J. Am. Chem. Soc. 1994, 116, 10346-10347.
(34) Matsubara, H.; Hasegawa, A.; Shiwaku, K.; Asano, K.; Uno, M.; Takahashi,
S.; Yamamoto, K. Chem. Lett. 1998, 923-924.
(38) Fagan, P. J.; Calabrese, J. C.; Malone, B. Acc. Chem. Res. 1992, 25, 134-
142.
(39) Balch, A. L.; Olmstead, M. M. Chem. ReV. 1998, 98, 2123-2166.
(40) Kirner, J. F.; Reed, C. A.; Scheidt, W. R. J. Am. Chem. Soc. 1977, 99,
2557-2563.
(41) Zheng, J.-Y.; Tashiro, K.; Hirabayashi, Y.; Kinbara, K.; Saigo, K.; Aida,
T.; Sakamoto, S.; Yamaguchi, K. Angew. Chem., Int. Ed. 2001, 40, 1858-
1861.
(35) Fox, O. D.; Drew, M. G. B.; Wilkinson, E. J. S.; Beer, P. D. Chem.
Commun. 2000, 391-392 and references therein.
(36) Reed, C. A.; Bolskar, R. D. Chem. ReV. 2000, 100, 1075-1119.
(37) Stinchcombe, J.; Pe´nicaud, A.; Bhyrappa, P.; Boyd, P. D. W.; Reed, C. A.
J. Am. Chem. Soc. 1993, 115, 5212-5217.
9
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A R T I C L E S
Sun et al.
Figure 1. Representative “jaws porphyrin” hosts and the calculated structures of their C60 host-guest complexes. From left to right: H4JawsP′′, H4JawsP′,
and H4JawsP.
that some hosts were significantly strained in the host-guest
complexes. Of the three shown in Figure 1, only H4JawsP and
H4JawsP′ gave plausible binding energies and unstrained
structures.
spectrum showed more C84 complex than C60 complex (Figures
S1 and S2). This indicates that C84 binds to the porphyrin host
more strongly than does C60, an observation that is consistent
with the displacement of C60 by C70 from the host in solution
experiments.30 Greater van der Waals interactions are possible
with higher fullerenes. The result with a gadolinium endohedral
fullerene is even more interesting. A minor peak due to Gd@C82
in the spectrum of a mixture with C60, C70, C76, C78, and C84
becomes the major peak of 1:1 complexes when treated with
H4JawsP (Figures S3 and S4). This suggests that endohedral
fullerenes bind more strongly to porphyrin hosts than do empty
fullerenes, consistent with the high selectivity that porphyrin-
appended silica stationary phases show in fullerene and endo-
hedral fullerene chromatography.42 Stronger binding of endo-
hedral fullerenes relative to same-size empty fullerenes sug-
gests an increased electrostatic component to the porphyrin-
fullerene interaction. The exterior surface of endohedral fullerenes,
derived from electropositive metals such as gadolinium, must
have fulleride character. This would enhance attraction to the
positive center of the porphyrin and help to explain why
metalloporphyrins have been successful in crystallizing interest-
ing endohedral fullerenes.21
Soft ionization MALDI mass spectrometry turns out to be
an excellent rapid qualitative assay for fullerene binding to
porphyrin hosts. Using a dithronol matrix, 1:1 fullerene:bis-
porphyrin complexes give exact mass peaks corresponding to
both the supramolecular complexes and their constituent parts,
and the abundance ratios of complexes relative to constituent
parts roughly correlate with binding constants derived in
solution. By contrast, cocrystallized monomeric porphyrin/
fullerene complexes retain no complexation when gas-phase ions
are produced under comparable conditions. Of the three
examples illustrated in Figure 1, H4JawsP′′ showed little
evidence of binding to C60, H4JawsP′ showed some interaction,
and H4JawsP (right side of Figure 1) was the best.
These results are consistent with 1H NMR results in solution.
Upfield shifts of the central N-H proton at -2.3 ppm indicate
fullerene complexation because of the ring current effect of the
fullerene on the porphyrin. The larger the upfield shift is, the
greater the binding.8 When equimolar amounts of the bis-
porphyrins and C60 are mixed in toluene-d8/chloroform-d3, the
shifts for 10-4 M solutions were 0.01, 0.04, and 0.11 ppm for
H4JawsP′′ to H4JawsP′ to H4JawsP, respectively. The corre-
sponding gas-phase binding energies from molecular mechanics
The PdCl2 linkage to the meta-pyridyl functionality in
H4JawsP optimizes fullerene fit and preorganization. The meta-
tert-butyl substituents were added for good solubility in low
dielectric organic solvents. Divalent metals (Pd, Zn, Cu, Co,
Fe, and Mn) were readily inserted into H4JawsP via standard
methods, except that Ni(II) competition with Pd in the linkage
thwarted the synthesis of Ni2(JawsP). Host/guest complexation
were estimated as 32.0, 44.9, and 57.4 kcal mol-1
.
MALDI also appears to be a useful indicator of selectivity.
For example, when a mixture of C60 and smaller amounts of
higher fullerenes (including C70, C76, C78, and C84) was treated
with somewhat less than 1 equiv of Cu2JawsP, the MALDI
(42) Xiao, J.; Savina, M. R.; Martin, G. B.; Francis, A. H.; Meyerhoff, M. E. J.
Am. Chem. Soc. 1994, 116, 9341-9342.
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Supramolecular Fullerene-Porphyrin Chemistry
A R T I C L E S
Figure 3. Perspective drawing of the crystal structure of the complex of
N-methylpyrrolidine-functionalized C60 with free-base jaws porphyrin
showing the pairing common to all C60 structures.
atom protrudes out of the porphyrin plane in the opposite
direction (0.024 Å from the mean N4 plane) because of the
overall doming and asymmetric warping of the porphyrin. As
in the Pd analogue where the closest approach is 2.85 Å,30 this
indicates a relatively insignificant role for coordinate bonding.
For comparison, the closest Fe‚‚‚C distance in [Fe(C60)(TPP)]+
is 2.63 Å, and the Fe atom is slightly out-of-plane toward the
fullerene.19
Figure 2. Perspective drawing of the crystal structure of Cu2(JawsP)‚C60.
A pair of trans 3,5-di-tert-butylphenyl substituents on each porphyrin have
been omitted for clarity. Thermal ellipsoids are shown at 50% level.
with fullerenes was typically investigated in toluene solution.
Arene solvents are known to solvate fullerenes and porphyrins
well,43 so the choice of toluene sets up a real competition. The
binding constants are not artificially exaggerated by solvent/
solute incompatibility.
Third, inspection of the crystal packing reveals a fullerene-
fullerene van der Waals contact. The complexes occur in pairs
allowing van der Waals contact of the fullerenes. Interestingly,
the 6:6 ring-juncture bonds are in perfect alignment, as though
set up for a [2 + 2] cycloaddition reaction. The C‚‚‚C separation
is 3.29 Å. Even when the fullerene is derivatized with N-
methylpyrrolidine, the same alignment is observed in pairs of
complexes (see Figure 3). The C‚‚‚C separation is almost
identical at 3.30 Å. In fact, the pairing feature is found in all
three C60 jaws porphyrin structures. It suggested to us that a
2:1 bis-porphyrin:fullerene complex might form with a dimeric
fullerene such as C120 or C120O. However, using mass spec-
trometry as mentioned earlier, we found evidence only for the
1:1 complex with pure C120O, so crystallization studies were
not pursued. The carbon atom closest to the porphyrin center
in the N-methylpyrrolidine structure (Figure 3) is three atoms
removed from the functionality. Mulliken population analysis
in a semiempirical MO calculation45 shows this to be the most
negative atom that is sterically accessible to the porphyrin plane,
indicating an electrostatic component to the orientation of the
fullerene. This carbon atom (C15F) is 2.76 Å from the 4N mean
plane.
Although fullerene/fullerene C‚‚‚C contacts at ca. 3.3 Å
appear in several crystal structures8 and can be significant,46
they are clearly subservient to porphyrin-fullerene interactions.
All fullerene-porphyrin cocrystallates have fullerene-porphyrin
interactions, but not all have fullerene-fullerene contacts.
The H4(JawsP)‚2C70 structure differs from the other structures
in that an “extra” fullerene is accommodated in the lattice. As
shown in Figure 4, this fullerene acquires the familiar por-
phyrin-fullerene interaction by association with the “outside”
X-ray Structures of Fullerene Complexes. Crystals of jaws
porphyrin/fullerene complexes suitable for X-ray structure
determination were obtained for four derivatives: (a) C60 with
a mixture of Pd(II) and free-base porphyrin, Pd1.5H(JawsP)‚
C60, reported earlier,30 (b) C60 with the copper(II) metalated
porphyrin, Cu2(JawsP)‚C60, (c) a common functionalized fullerene,
N-methylpyrrolidino-C60, with free-base porphyrin, and (d) C70
with free-base porphyrin, H4(JawsP)‚2C70. As is common in
fullerene crystallography, disorder diminishes the detailed
information available from these structures, but the essential
structural features can be deduced. All show the now familiar
attraction of the fullerene to the center of the porphyrin or
metalloporphyrin. In all three C60 structures, it is a 6:6 rather
than the 6:5 ring-juncture bond that most closely approaches
the porphyrin or metalloporphyrin.
Three features of the Cu2(JawsP)‚C60 structure are worthy
of comment. First, the 24-atom porphyrin core is slightly domed
and ruffled. The doming may be to wrap the fullerene more
effectively (see Figure 2) and is presumably the result of
maximizing attractive van der Waals contact. Ruffling of
metalloporphyins is related to the size of the central metal, small
metals such as Cu(II) typically leading to S4 ruffling.44 A similar
warping of macrocycles is observed in fullerene complexes with
metalloporphyrazines28 and other metallo-tetraphenylporphy-
rins.23
Second, the Cu‚‚‚C distances are relatively long (2.83 and
3.06 Å to a 6:6 ring juncture), and the copper atom is not drawn
out of the porphyrin plane toward the fullerene. In fact, the Cu
(43) Noviandri, I.; Bolskar, R. D.; Lay, P. A.; Reed, C. A. J. Phys. Chem. B
1997, 101, 6350-6358.
(44) Scheidt, W. R.; Lee, Y. J. Struct. Bonding 1987, 64, 1-70.
(45) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209-220.
(46) Barbour, L. J.; Orr, G. W.; Atwood, J. L. Chem. Commun. 1998, 1901.
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A R T I C L E S
Sun et al.
Figure 5. Variable temperature 13C NMR spectrum of a 1:3 mixture of
Pd2(JawsP) and C60. The asterisk marks solvent peaks (toluene).
Table 1. C60 13C NMR Data on M2(JawsP)‚C60 in Toluene
δ at −90 °C
(ppm)
coalesence
temp (°C)
binding
constant (M )
-1
M
Pd(II)
Zn(II)
2H
Cu(II)
Mn(II)
Fe(II)
Co(II)
140.8
140.2
140.0
140.5 (br)
a
-75
-60
-50
-45
-15
-45
25
815 ( 120
1945 ( 750
5200 ( 120
4860 ( 250
2760 ( 120
490 ( 15
Figure 4. Perspective drawing of the crystal structure of H4(JawsP)‚2C70
showing C70 complexed within, and cocrystallized outside, the free-base
jaws porphyrin.
of the jaws porphyrin. In this way, it is reminiscent of earlier
cocrystallates.8 The C70 that is complexed within the jaws of
the porphyrin is bound “side-on” rather than “end-on”, confirm-
ing structural deductions based on 13C NMR.30 This presumably
maximizes van der Waals attraction since the flatter equatorial
regions of C70 have more contact with the porphyrin than the
more highly curved polar regions.8
190 (br)
192 (br)
2975 ( 120
a Extremely broad.
NMR Spectroscopy. Variable temperature NMR spectros-
copy is a powerful technique for assessing the thermodynamic
and kinetic aspects of fullerene complexation.30 In the present
work, we use NMR spectroscopy to determine the relative
binding constants of C60 to M2(JawsP) as a function of metal,
with the added complication that paramagnetic metals introduce
chemical shifts that are intrinsically temperature-dependent.
Toluene-d8 was used as solvent, and 13C-enriched C60 (10-
15%) was used to enhance signal quality.
Figure 5 shows the variable temperature 13C NMR spectrum
of a 1:3 mixture of the palladium(II) metalated porphyrin,
Pd2(JawsP), and C60. At -90 °C, two fullerene peaks are seen
at 141.0 and 142.6 ppm, assigned to complexed and uncom-
plexed C60, respectively. At this temperature, the complex must
be in the slow exchange regime. Upon warming, coalescence
of these two signals is seen at -75 °C. At -30 °C, the sharpness
of the single peak at 142.5 indicates rapid exchange between
complexed and uncomplexed fullerene. The chemical shift
reflects a weighted average, plus the minor effects of temper-
ature on the intrinsic chemical shifts. As summarized in Table
1, similar results are obtained with the zinc-metalated porphyrin
with respect to both chemical shift and coalescence temperature.
The observations are completely reversible and are similar to
UV-Vis Spectroscopy. Small red shifts of the Soret bands
of porphyrins and metalloporphyrins (up to 7 nm) have been
noted upon complexation of fullerenes.23,29 Shifts in the lower
energy R,â bands are smaller or nonexistent. Because of the
high dilution required to keep porphyrin absorptivities on-scale
in a spectrometer, it is difficult to gain the complete measure
of these small shifts in solution. However, when 10-5 M toluene
solutions of the jaws porphyrin derivatives are treated with high
concentrations of C60, small shifts are sometimes observed. For
the free base and the Zn complex, the Soret band is red-shifted
by 2-3 nm upon complexation. For Cu and Co, the change is
insignificant. For Pd, there is a 1 nm blue shift. Red shifting
may reflect the donation of electron density from the axial ligand
to the porphyrin ring, lowering the energy of the π to π*
transition,47,48 consistent with the fullerene acting as a weak
donor. However, the shifts are too small for definitive inter-
pretation.
(47) Gouterman, M.; Schwarz, F. P.; Smith, P. D.; Dolphin, D. J. Chem. Phys.
1973, 59, 676.
(48) Nappa, M.; Valentine, J. S. J. Am. Chem. Soc. 1978, 100, 5075-5080.
9
6608 J. AM. CHEM. SOC. VOL. 124, NO. 23, 2002

Supramolecular Fullerene-Porphyrin Chemistry
A R T I C L E S
those with free-base porphyrin whose chemical shift in the slow
exchange limit is 140.0 ppm.30 Both the Pd(II) and the Zn(II)
metalloporphyrins and the free-base porphyrin produce upfield
chemical shifts on C60, consistent with a ring current effect from
a diamagnetic π system of the porphyrin. This is confirmed in
the 13C CPMAS NMR spectra of cocrystallates such as H2TPP‚
C60‚3toluene which shows a 3.2 ppm upfield shift of the C60
signal relative to free C60 powder. In both cases, the room-
temperature peak is sharp, indicating rapid tumbling of C60 on
the NMR time scale.49
In the solution measurements, the coalescence temperature
of the free-base complex (-50 °C) is higher than that of the
zinc complex (-60 °C) and palladium (-75 °C), indicating that
C60 binds more strongly to H4JawsP than the Pd or Zn
porphyrins in the order Pd < Zn < 2H. This conclusion can be
drawn because the chemical shift differences between bound
and unbound C60 in each case are of similar magnitude.
Competition experiments confirm this conclusion. When C60 is
titrated into a 2:3 mixture of H4JawsP and Pd2(JawsP), peaks
1
in the H NMR spectrum of the free-base porphyrin respond
immediately, whereas those of the palladium-metalated por-
phyrin respond only after ca. 1 equiv of C60 has been added.
For example, in a mixture containing H4JawsP, Pd2(JawsP), and
C60 in mole ratio 2:3:4.2, significant shifts can be seen in the
peaks of H4JawsP but not those of Pd2(JawsP). These data are
shown in Figure 6.
Titrations of toluene solutions of the three diamagnetic
systems (Pd, Zn, and free base) with 13C-enriched C60 were
followed by 13C NMR spectroscopy. The variation of chemical
shift allowed calculation of the binding constants, and these are
listed in the first three entries of Table 1. The ordering, Pd <
Zn < 2H, is consistent with the order of increasing coalescence
temperature.
Figure 6. 1H NMR spectrum of (a) a 2:3 mixture of H4JawsP (O) and
Pd2(JawsP) (b) with no C60 present and (b) upon addition of an overall
stoichiometric deficit of C60 in ratio 2:3:4.2. Peaks due to H4JawsP, marked
with O, change their chemical shift significantly between (a) and (b).
2
When /3 of an equivalent of C60 is added to the copper(II)
porphyrin, there is a 2.4 ppm shift upfield in the room-
temperature spectrum (Figure S5), not unlike (but somewhat
less than) that seen in diamagnetic metalloporphyrins or the free-
base porphyrin. This can be understood in terms of the singly
cally similar variable temperature 13C NMR spectra. Extraction
of low-temperature limits and relative binding constant data is
made more difficult by the overlay of significant intrinsic
changes of the chemical shifts as a function of temperature
arising from paramagnetism. The spectra for Co2(JawsP)‚C60
are illustrative of the data. Figure 7a shows data with a
stoichiometric deficit of C60 to obtain the chemical shift of the
complex in the slow exchange regime. At -90 °C, this peak is
observed at 192 ppm. Figure 7b shows data with a 2:3
stoichiometric excess of C60 to cleanly observe the coalescence
phenomenon, which occurs at 25 °C. Data for the Mn and Fe
systems are available as Figures S6 and S7, and a summary is
provided in Table 1.
For each of the Mn, Fe, and Co systems, the 13C chemical
shifts relative to free C60 are downfield. This is an expected
result in paramagnetic systems with axial symmetry if the axial
magnetic susceptibility is less than the component perpendicular
to the principal axis. For Fe(II) and Co(II) porphyrins, this is
known to be the case.52,53
2
2
occupied dx -y orbital of Cu(II) which directs the unpaired spin
toward the porphinato N atoms rather than toward the fullerene.
The resulting anisotropy in the magnetic susceptibility, as
reflected in the EPR g values for Cu(II)TPP, is small.50 Estimates
of the pseudocontact or dipolar shift due to this anisotropy,51
based on X-ray structures of porphyrin fullerene conjugates,
suggest a maximum average downfield shift of ca. 0.9 ppm.
Combined with the upfield shift due to the porphyrin ring current
(ca. 3.2 ppm), the reduced total shift of ca. 2.4 ppm is readily
rationalized. The coalescence temperature for the copper system
(ca. -45 °C) is slightly higher than those of Pd, Zn, and free
base suggesting similar binding strength. This is borne out by
detailed fitting of the NMR data which leads to the fourth entry
in Table 1.
Although significantly broadened by the presence of unpaired
spin, each of the other paramagnetic porphyrin systems inves-
tigated [M ) Co(II), Mn(II), and Fe(II)] shows phenomenologi-
Affinity Considerations. The affinity of M2(JawsP) for C60
increases in the order Fe(II) < Pd(II) < Zn(II) < Mn(II) <
Co(II) < Cu(II) < 2H. Studies on a related cyclic bis-porphyrin
system41 show Ag(I) < Ni(II) < Cu(II) < Zn(II) < free base
< Co(II) , Rh(III) for C60. There are differences in the ordering
(49) Johnson, R. D.; Bethune, D. S.; Yannoni, C. S. Acc. Chem. Res. 1992, 25,
169-175.
(50) Manoharan, P. T.; Rogers, M. T. Electron Spin Resonance of Metal
Complexes; Plenum: New York, 1969; pp 143-173.
(51) Horrocks, W. D. In NMR of Paramagnetic Molecules: Principles and
Applications; La Mar, G. N., Horrocks, W. D., Holm, R. H., Eds.; Academic
Press: New York, 1973; Chapter 4.
(52) Goff, H.; La Mar, G. N.; Reed, C. A. J. Am. Chem. Soc. 1977, 99, 3641.
(53) La Mar, G. N.; Fulton, G. P. J. Am. Chem. Soc. 1976, 98, 2119-2123.
9
J. AM. CHEM. SOC. VOL. 124, NO. 23, 2002 6609

A R T I C L E S
Sun et al.
Figure 7. Variable temperature 13C NMR spectra of Co2(JawsP) and C60 at (a) 2:1 mole ratio and (b) 2:3 mole ratio.
between these two studies, but overall the variation of binding
constants as a function of metal is relatively small. Omitting
fullerenes, thereby altering the magnitude of the van der Waals
component to the binding energy.
Rh(III), they vary by less than 900 M-1
.
It has been suggested that the 2.7-3.0 Å approach of
fullerenes to metalloporphyrins is too distant for covalent
bonding.18,20 This is based primarily on the observation that
metal-fullerene distances are “too long for covalence” and the
metals are typically not drawn out of their porphyrin planes
toward the fullerenes. However, length is a rather subjective
criterion of bonding, and metal out-of-plane displacements with
weak axial ligands are manifest clearly only in discrete five-
coordinate complexes. The presence of face-to-face porphyrin
dimers in many X-ray structures effectively adds a competing
interaction in the sixth “vacant” coordination site.44 Under these
circumstances, correlations of axial ligand binding strength often
fail to correlate with out-of-plane displacements,19 and, for weak
ligands such as fullerenes and arenes, displacements may be
reduced to the level of structural insignificance. We believe it
is necessary to view coordinate bonding as a continuum out to
at least 3.0 Å.19 At these distances, the strength of the bonding
is obviously very weak, and its degree of covalence small, but
the displacement of Fe(III) from the porphyrin plane in
Fe(C60)(TPP)+ indicates that in favorable circumstances it can
be structurally observable. Small changes in UV-vis spectra,
the green color of Fe(C60)(TPP)+, and the measurable perturba-
tion of the metalloporphyrin EPR spectra in the presence of
fullerenes23 all suggest that some degree of charge transfer from
the fullerene to the porphyrin via coordinate bonding can occur
in favorable circumstances. The magnitude, however, is small
and, in many cases, is not a separable effect.
Theory suggests that the attraction of fullerenes to porphyrins
and metalloporphyrins is largely van der Waals in nature.8 In
addition to these dispersion forces, the attraction must also be
subject to the subtle interplay of small effects from differences
in solvation, electrostatics, charge transfer, and coordinate bond
formation.
Interestingly, the free-base porphyrins bind C60 somewhat
more strongly than most metalloporphyrins. We ascribe this to
an electrostatic attraction of the electron-rich 6:6 ring-juncture
bond of the fullerene to the electropositive N-H center of the
porphyrin. This highlights the importance of an electrostatic
component augmenting the π-π host-guest interaction. It is
consistent with the observation of stronger binding of endohedral
fullerenes and the orientation of the fulleropyrrolidine discussed
above.
In the absence of a coordinate interaction, the greater number
of electrons associated with metals relative to H atoms leads to
the expectation of stronger van der Waals interactions with
metalated porphyrins relative to free-base porphyrins. However,
the data largely confound this expectation. This means that
coordinate bonding and dispersion forces due to the metal are
rather weak, and less than the electrostatic component present
in the free-base interaction. One qualifying consideration is that
the van der Waals attraction should not be considered constant
throughout the series of metalloporphyrins. Differing degrees
of porphyrin ruffling will lead to varying contact areas with
9
6610 J. AM. CHEM. SOC. VOL. 124, NO. 23, 2002

Supramolecular Fullerene-Porphyrin Chemistry
A R T I C L E S
The report of somewhat stronger binding of C60 to Rh(III)41
(and possibly Ru(II) porphyrins)22,54 relative to the other metals
investigated to date is readily understood. A second row metal
utilizing a formal vacant coordination site, whose low-spin d6
configuration can profit from ligand field stabilization energy
when engaged in covalent bonding, has more opportunity to
make a significant axial ligand bond. Low-spin d6 rhodium(III)
may even engage in π back-bonding into the fullerene LUMO,
taking bonding elements from each of the otherwise fairly
distinct classes of metal-fullerene interactions represented by
magnets and molecular conductors, and in the creation of new
porous metal-organic frameworks.16
Experimental Section
General. All manipulations involving air-sensitive materials were
performed in a Vacuum Atmospheres glovebox (O2, H2O, 0.5 ppm).
Toluene and THF were dried by distillation from Na/benzophenone
outside the glovebox and again inside the box prior to use. Toluene-d8
was dried over molecular sieves and filtered through a 0.2 µ syringe
filter prior to use. “Jones reductor” Zn(Hg) was prepared by treating
Zn pellets with dilute HgCl2 solution, washing with dilute hydrochloric
acid then water, and drying under vacuum. 13C-enriched C60 (10-15%)
was purchased from MER Corp. Meso-3-pyridyl-triphenylporphyrin and
the PdCl2-linked dimer, H4JawsP, were prepared as previously de-
38
phosphine complexes such as Ni(C60)(PEt3)2 and the hard
metalloporphyrin complexes discussed herein.
The remaining structural question is why most metallo-
porphyrin interactions occur with the 6:6 ring-juncture bond of
C60, while some seem to prefer the 5:6 ring juncture. The 6:6
ring-juncture bond is the shorter, more electron-rich (olefinic)
bond, although both have double bond character due to π
delocalization and aromaticity. The 5:6 interaction is observed
in MII(OEP)‚C60 (M ) Pd, Cu, Ni) and RuII(CO)(OEP)‚C60,
and the presence of π antibonding interactions has been
suggested.20,22 If coordinate bonding is involved, we expect the
6:6 interaction to be preferred because it can better provide σ
donation to the metal. However, in the Pd, Cu, and Ni structures,
the M‚‚‚C distances are all very long (2.99-3.07 Å), correlating
with the low affinities. The 5:6 orientations may simply reflect
a lack of significant preference and control by crystal packing
interactions whose influences are difficult to assess. Particular
conformations of the ethyl substituents in these three compounds
have been noted.20,22 The 5:6 orientation with shorter metal-
carbon distances in the ruthenium(II) case (2.83-2.86 Å)22 is
more difficult to rationalize. However, packing effects from the
ethyl groups of the OEP may again be a factor. More data are
needed to learn whether there is generality to this observation.
The report that the order of binding constants changes slightly
between C60 and C7041 shows that subtle differences in solvation
energies are another factor affecting affinities.
scribed.30 C120O55,56 and N-methylpyrrolidine-derivatized C60 were
57
prepared by literature methods. Host-guest complexes were prepared
by additions of fullerenes and porphyrins in toluene. NMR spectra were
recorded in toluene-d8 on an Inova 500 MHz spectrometer and internally
referenced to residual protio peaks of the solvent. MALDI TOF mass
spectra were recorded on a PerSeptive Biosystems Voyager-DE STR
instrument operated in linear mode with laser intensity of ca. 2000
eV. Air-stable samples were mixed with dithronol matrix. Air-sensitive
samples were used directly, loading under anaerobic conditions.
Titrations were performed with 10-3 M solutions of porphyrins and
incremental additions of 10-3 M solutions of 13C-enriched C60 dissolved
in the porphyrin stock solution. Binding constants were determined
from the variation in 13C NMR shifts using the WinEQNMR program.58
Molecular Modeling. Host/guest complexes were constructed using
the program CERIUS2.59 Geometry optimization was carried out using
the universal force field.60 Gas-phase binding energies were estimated
by comparison of the energy of the porphyrin-fullerene complex with
the optimized energies of C60 and the bis-porphyrin. In this model, the
gas-phase interaction energy of a single porphyrin with C60 is between
28 and 30 kcal mol-1 depending on the porphyrin substituents. Bis-
porphyrins that give effective fullerene binding are found to have about
twice this energy, while those with weaker binding are less. The main
feature contributing to weaker binding is the strain imposed on some
hosts upon complexation. Semiempirical MO calculations using the
PM3 method45 were carried out using SPARTAN (Version 5.1, Wave
function Inc.). A Mulliken population analysis was performed on the
PM3 optimized structure to estimate the atomic charge distribution on
the fullerene.
H4JawsP′′ and H4JawsP′. These were prepared by slow addition
of 100 mg of 5-(3-aminophenyl)10,15,20-(p-methylphenyl)porphyrin61
dissolved in 5 mL of dichloromethane containing triethylamine to a 5
mL solution containing 14.2 mg of either 1,4- or 1,3-di(chlorocarbonyl)-
benzene. The mixture was stirred under a nitrogen atmosphere for 24
h at room temperature. The product was precipitated by addition of
hexane and purified by chromatography on a silica gel column using
dichloromethane and dichloromethane/ether (9.8:0.2) eluents. Both
products gave parent ions in FAB mass spectra using m-nitrobenzyl
alcohol as a matrix: H4Jaws′′ m/z 1473.6 [M+ 1473.6, C102H76N10O2];
H4Jaws′ m/z 1473.616 (accurate mass) [M+ 1473.618, C102H76N10O2].
Pd2(JawsP). Meso-3-pyridyl-triphenylporphyrin was heated with a
large excess of PdCl2(DMSO)2 in toluene for ca. 4 h or in CHCl3 for
Overall, it is clear that a number of small effects conspire to
affect binding constants in ways that do not always lend
themselves to easy deconvolution. Depending on the physical
property that is measured, the effect may or may not be
detectable. A continuum of weak bonding forces from coordinate
bonding to dispersion forces, that is, from weak ligation to
solvation,19 is preferable to arbitrary limits or categorical
statements about presence or absence.
Conclusions
The identification of an attractive fullerene-porphyrin in-
teraction represents a new supramolecular recognition element.
It can be fine-tuned with metals in the porphyrins through a
subtle interplay of coordinate bonding, charge transfer, elec-
trostatics, and solvation energy effects. It differs from traditional
π-π interactions by the closeness of the approach and the
surprising affinity of a curved molecular surface to a planar
surface. This offers new opportunities to assemble discrete
supramolecular complexes as well as extended solids. We
anticipate that the molecular design principles emanating from
these studies will be most useful in the manipulation of
photophysical properties, adjusting charge transfer in molecular
(55) Lebedkin, S.; Ballenweg, S.; Gross, J.; Taylor, R.; Kra¨tschmer, W.
Tetrahedron Lett. 1995, 36, 4971-4974.
(56) Taylor, R.; Barrow, M. P.; Drewello, T. J. Chem. Soc., Chem. Commun.
1998, 2497-2498.
(57) Drovetskaya, T.; Reed, C. A.; Boyd, P. D. W. Tetrahedron Lett. 1995, 36,
7971-7974.
(58) Hynes, M. J. J. Chem. Soc., Dalton Trans. 1993, 311-331.
(59) CERIUS2; Molecular Simulations: San Diego, CA, 1997; Version 3.5.
(60) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W.
M. J. Am. Chem. Soc. 1992, 114, 10024-10035.
(61) Liddell, P. A.; Sumida, J. P.; MacPherson, A. N.; Noss, L.; Seely, G. R.;
Clark, K. N.; Moore, A. L.; Moore, T. A.; Gust, D. Photochem. Photobiol.
1994, 60, 537.
(54) Maruyama, H.; Fujiwara, M.; Tanaka, K. Chem. Lett. 1998, 805-806.
9
J. AM. CHEM. SOC. VOL. 124, NO. 23, 2002 6611

A R T I C L E S
Sun et al.
ca. 2 days to accomplish Pd pyridyl linking and porphyrin metalation
in one step. Completion of reaction was checked by 1H NMR via loss
of the pyrrole N-H resonance at -2.14 ppm, and the product was
purified by flash chromatography on silica gel with chloroform as
eluent. m/z 2292 [M + H] plus monomer fragment at 1055. See Figure
(E2 - 1 ) 0.936) and systematic absent reflections indicated one
possible space group, Ccca. The space group Ccca was later determined
to be correct. Patterson method was used to locate the Pd and Cu atoms.
Subsequent isotropic least-squares refinements led to an electron density
map from which all of the non-hydrogen atoms were identified in the
asymmetry unit of the unit cell, including the four partially occupied
hexane solvent molecules. The site occupancy factors for the four
hexane molecules were 50, 47, 36, and 17%, which gave a total of 1.5
hexane molecules in the asymmetry unit. The C60, the four partially
occupied hexane molecules, and the nine disordered methyl groups were
modeled as rigid groups using the EQIV, SADI, DFIX, DELU, SIMU,
ISOR commands. The C60 and the Pd1 atom were located on a 2-fold
rotation axis (-x, y, 1.5 - z symmetry operator). Atomic coordinates
and isotropic and anisotropic displacement parameters of all the non-
hydrogen atoms were refined by means of a full matrix least-squares
procedure on F 2. All H-atoms were included in the refinement in
calculated positions riding on the atoms to which they were attached.
The refinement converged at R1 ) 0.0598, wR2 ) 0.1638, with I >
2σ(I). The largest peak/hole in the final difference map was 1.088/-
0.791 e/Å3.
1
S8 for simulation of isotopes. H NMR: 9.81 (1H, s, pyridyl), 9.16
(1H, d, pyridyl), 8.94 (6H, m, phenyl), 8.65 (3H, d, phenyl), 8.06 (6H,
m, pyrrole), 7.83 (2H, m, pyrrole), 7.69 (1H, t, pyridyl), 6.78 (1H, t,
pyridyl), 1.34 and 0.91 (54H, m, t-Bu) ppm. UV-vis (toluene): 422
(Soret), 526, 602 nm.
Cu2(JawsP). H4JawsP in CHCl3 was treated 2 equiv of Cu(OAc)2
dissolved in minimum methanol under reflux for 2 h. Purification was
achieved by column chromatography on silica gel with dichloromethane
as eluent. m/z 2206 [M + H]; free base was absent. 1H NMR (broad):
9.18, 7.82, 7.58, 6.43, 1.34 ppm. UV-vis (toluene): 421 (Soret), 541,
580 nm.
Co2 (JawsP). This was prepared in a manner similar to Cu2(JawsP)
using Co(OAc)2. m/z 2195 [M+]; free base was absent. 1H NMR
(broad): 11.70, 9.86, 9.09, 1.33, 0.92. UV-vis (toluene): 420 (Soret),
526, 606, 655 nm.
Zn2(JawsP). This was prepared in a manner similar to Cu2(JawsP)
using Zn(OAc)2. m/z 2211 [M + H]; free base was absent. H NMR:
H4(JawsP)‚(N-methylpyrrolidino-C60)‚0.5hexane. Black crystals
were grown by layered diffusion of hexanes into a toluene solution of
1:1 2-(N-methylpyrrolidino)-C60 and Cu2(JawsP). Details of the
1
64
10.18 (1H, pyridyl), 9.32 (1H, pyridyl), 9.17, 9.13 (6H, phenyl), 8.99,
8.81 (3H, phenyl), 8.34, 8.29, 7.94 (8H, pyrrole), 8.13 (1H, pyridyl),
6.78 (1H, pyridyl), 1.49 and 0.92 (54H, t-Bu) ppm. UV-vis (toluene):
426 (Soret), 552, 592 nm.
Mn2(JawsP). This was prepared under anaerobic conditions by
treatment of H4Jaws with an equal weight of MnBr2 in toluene/THF
under reflux in the presence of K2CO3 for 2 days. After filtration, the
product was purified by recrystallization from toluene/THF. UV-vis
(toluene): 429 (Soret), 452, 595, 638 nm. m/z (no matrix) 1004 [M+
for monomeric Mn porphyrin]; free base was absent.
structure solution and refinement are given in the Supporting Informa-
tion. Orthorhombic Ccca: a ) 27.585(3) Å, b ) 43.432(4) Å, c )
29.246(3) Å, R ) 90°, â ) 90°, γ ) 90°, V ) 35039(6) Å3, Z ) 8, Dc
) 1.115 g/cm3, 2θmax ) 46.52°, 83 188 measured reflections, 12 601
independent reflections, 7734 with I > 2σ(I) used for refinement to
give R1 ) 0.0624, wR2 ) 0.1617.
H4(JawsP)‚2C70‚3toluene‚1.5hexane. Black crystals were grown by
layered diffusion of hexanes into a 1:1 toluene solution of H4(JawsP)
and C70. Details of the structure solution and refinement are given in
Fe2(JawsP). This was prepared in a manner similar to Mn2(Jaws)
using FeBr2. Stirring with product solution over Zn(Hg) pellets removed
traces of Fe(III) porphyrin. UV-vis (toluene): 432 (Soret), 455, 545
nm. m/z (no matrix) 1006 [M+ for monomeric Fe porphyrin]; free base
was absent.
the Supporting Information. C134H150Cl2N10Pd)(C70)2‚(C7H8)3(C6H14)1.5
,
monoclinic C2/c, a ) 39.032(6) Å, b ) 30.454(5) Å, c ) 38.496(6)
Å, R ) 90°, â ) 106.912(3)°, γ ) 90°, V ) 43781(12) Å3, Z ) 8, Dc
) 1.264 g/cm3, 2θmax ) 34.46°, 47 939 measured reflections, 13 269
independent reflections, 6144 with I > 2σ(I) used for refinement to
give R1 ) 0.1196, wR2 ) 0.3376.
X-ray Structure Determinations. Cu2(JawsP)‚C60‚1.5hexane.
Black crystals were grown by layered diffusion of hexanes into a toluene
solution of 1:1 Cu2(JawsP) and C60. A fragment of a thin needle (0.27
× 0.21 × 0.04 mm3) was used. The crystal was mounted onto a glass
fiber and coated with epoxy resin to prevent the loss of solvent of
crystallization. X-ray intensity data were collected at 213(2) K with
the CCD detector placed at a distance of 4.8450 cm from the crystal.
On the basis of a orthorhombic crystal system, the integrated frames
yielded a total of 93 497 reflections at a maximum 2θ angle of 43.94°
(0.95 Å resolution), of which 10 882 were independent reflections (Rint
) 0.0996, Rsig ) 0.0722, redundancy ) 8.6, completeness ) 100%)
and 6566 (60.3%) reflections were greater than 2σ(I). The unit cell
parameters were a ) 27.903(2) Å, b ) 43.578(3) Å, c ) 29.242(2) Å,
R ) 90°, â ) 90°, γ ) 90°, V ) 35 556(5) Å3, Z ) 8, calculated
density Dc ) 1.190 g/cm3. Absorption corrections were applied
(absorption coefficient µ ) 0.424 mm-1) to the raw intensity data
using the SADABS program in the SAINTPLUS software.62 The
maximum/minimum transmission factors were 0.9833/0.8942. The
Bruker SHELXTL (Version 5.1) software package63 was used for phase
determination and structure refinement. The distribution of intensities
Acknowledgment. We thank Dr. Robert Bolskar of TDA for
samples of higher and endohedral fullerenes, Prof. Michael J.
Hynes for a copy of WinEQNMR, and Drs. Parimal Paul, Daniel
Stasko, Alex Clark, Steven Kennedy, Richard Kondrat,
Mehrnoosh Sadeghi, and Yi Meng for assistance. This work
was supported by the National Institutes of Health (GM 23851)
and the Marsden Fund of The Royal Society of New Zealand
(00-UOA-030).
Supporting Information Available: MALDI mass spectra
(Figures S1-S4 and S8), NMR spectra (Figures S5-S7), and
X-ray data (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
JA017555U
(63) SHELXTL Software Reference Manual; Bruker Analytical X-ray System,
Inc.: Madison, WI, 1997-1998; Version 5.1.
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