Ferromagnetic spin coupling between endohedral metallofullerene La@C 82 and a cyclodimeric copper porphyrin upon inclusion

Article abstract of DOI:10.1021/ja203491s

The cyclic host cyclo-[P_Cu]₂ carrying two covalently connected Cu(II) porphyrin units can accommodate La@C₈₂, a paramagnetic endohedral metallofullerene, in its cavity to form the inclusion complex cyclo-[P_Cu]₂?La@C₈₂, which can be transformed into the caged complex cage-[P_Cu]₂? La@C₈₂ by ring-closing olefin metathesis of its side-chain olefinic termini. On the basis of electron spin resonance (ESR) and electron spin transient nutation (ESTN) studies, cyclo-[P_Cu]₂? La@C₈₂ is the first ferromagnetically coupled inclusion complex featuring La@C₈₂, whereas cage-[P_Cu]₂? La@C₈₂ is ferrimagnetic.

Full text of DOI:10.1021/ja203491s

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Ferromagnetic Spin Coupling between Endohedral Metallofullerene
La@C82 and a Cyclodimeric Copper Porphyrin upon Inclusion
Fatin Hajjaj,†,‡ Kentaro Tashiro,*,§ Hidefumi Nikawa,|| Naomi Mizorogi,|| Takeshi Akasaka,||
Shigeru Nagase,^ Ko Furukawa,^ Tatsuhisa Kato,# and Takuzo Aida*,†,‡
†School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
‡Responsive Matter Chemistry & Engineering Research Group, Advanced Science Institute, RIKEN, 2-1 Hirosawa, Wako, Saitama
351-0198, Japan
§National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
||Center for Tsukuba Advanced Research Alliance, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan
^Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan
#Institute for the Promotion of Excellence in Higher Education, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
S Supporting Information
b
Scheme 1. Schematic Illustration of the Structures of cy-
ABSTRACT: The cyclic host cyclo-[PCu]2 carrying two
clo-[PM]2⊃Fullerene and cage-[PM]2⊃Fullerene [R = C12H25;
covalently connected Cu(II) porphyrin units can accom-
modate La@C82, a paramagnetic endohedral metallofuller-
ene, in its cavity to form the inclusion complex cyclo-
[PCu]2⊃La@C82, which can be transformed into the caged
complex cage-[PCu]2⊃La@C82 by ring-closing olefin
metathesis of its side-chain olefinic termini. On the basis
of electron spin resonance (ESR) and electron spin transient
nutation (ESTN) studies, cyclo-[PCu]2⊃La@C82 is the first
ferromagnetically coupled inclusion complex featuring
La@C82, whereas cage-[PCu]2⊃La@C82 is ferrimagnetic.
M = 2H, Cu, Zn; Fullerene (blue-colored sphere) = La@C82,
C82] and Their Transformation by Ring-Closing Metathesis
s a first step toward the realization of ultrahigh-density
A
memory devices,1 ordered clustering of spin-active mol-
ecules by means of well-defined host/guest chemistry might give
a clue to the yet-unaddressed issue of how one can rationally
design ferromagnetic spin coupling.2 However, successful exam-
ples of supramolecular approaches are still very rare,3 and new
molecular motifs have been awaited. Although endohedral
metallofullerenes with paramagnetic character4 are quite attrac-
tive motifs, only internal spin coupling within their carbon cages
has been studied to date.4c In the present work, we took notice of
La@C82, which is considered to be a potential motif for supra-
molecular spin coupling because its paramagnetic character
originates from a carbon radical delocalized over the spherical
network of C82.5 Here we report that the cyclodimeric copper(II)
porphyrin cyclo-[PCu]2 (Scheme 1) is the first host molecule that
can be ferromagnetically coupled with La@C82. The research
strategy made use of our well-established inclusion chemistry of
fullerenes with cyclic dimers of metalloporphyrins.6À9 cyclo-[PCu]2,
which was newly designed to host La@C82, is paramagnetic.
Furthermore, it bears alkyl side chains with olefinic termini and
thus can be transformed into its caged analogue cage-[PCu]2 by
intramolecular ring-closing olefin metathesis (Scheme 1). This
“convertible” molecular design was inspired by our curiosity
about how the mode of host/guest spin coupling is affected by
geometry. From our previous experiences with higher fullerenes9a
and short-spaced cyclodimeric metalloporphyrin hosts,9b we
assumed that upon inclusion of La@C82, cyclo-[PCu]2 would
not allow such a large fullerene to be located in the geometrical
center of its cavity. In contrast, cage-[PCu]2 might force La@C82
to stay within its tetrapodal cage. As highlighted in the present
communication, these two inclusion complexes displayed spin-
coupling profiles that were quite different from one another.
For the synthesis of cyclo-[PCu]2, we first prepared the zinc
complex cyclo-[PZn]2 (Scheme 1) by fluoride-mediated coupling
of the corresponding zinc porphyrin precursors bearing bro-
moalkyl side chains and silylated phenolic groups, and this
complex was unambiguously characterized by MALDIÀTOF
mass spectrometry and 1H NMR spectroscopy (Figures S2 and
S7 in the Supporting Information).10 Next, cyclo-[PZn]2 was
transformed quantitatively into cyclo-[PCu]2 by demetalation
under acidic conditions followed by metalation with Cu(OAc)2.
Upon titration with La@C82 in toluene at 25 ꢀC, the Soret
absorption band of cyclo-[PCu]2 displayed a bathochromic red
shift from 418 to 425 nm (Figure S13),10 just like analogous
inclusion complexes reported to date.6À9 From this spectral change
profile, the association constant (Kassoc) of cyclo-[PCu]2⊃La@C82
Received: April 15, 2011
Published: May 22, 2011
r
2011 American Chemical Society
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Figure 1. (a) MALDIÀTOF mass and (b) absorption (toluene, 25 ꢀC)
spectral profiles of (i) cyclo-[PCu]2⊃La@C82 and (ii) cage-[PCu]2⊃
La@C82.
was evaluated as 1.5 Â 106 MÀ1 (Figure S13).10 Next, cyclo-
[PCu]2⊃La@C82 was subjected to ring-closing olefin metathesis
(Scheme 1). However, because of concomitant liberation of the
guest La@C82, cage-[PCu]2⊃La@C82 was obtained in only 20%
yield. We later found that the free-base form of cyclo-[PCu]2
(cyclo-[P2H]2) in the presence of La@C82 can be metathesized to
give a better yield (40%) of its cage product, cage-[P2H]2⊃La@
C82. Accordingly, the Kassoc value (1.5 Â 107 MÀ1) of cyclo-
[P2H]2⊃La@C82 (Figure S12)8,10 was 1 order of magnitude
greater than that of cyclo-[PCu]2⊃La@C82. The resul-
tant complex cage-[P2H]2⊃La@C82 was quantitatively con-
verted into cage-[PCu]2⊃La@C82 by metalation with Cu-
(OAc)2. MALDIÀTOF mass spectrometry of the product
showed only a single peak due to the host/guest complex
[Figure 1a(ii)]. In sharp contrast, like other fullerene-in-
cluded noncaged cyclodimeric metalloporphyrins reported
to date,6À9 cyclo-[PCu]2⊃La@C82 underwent complete dis-
sociation during MALDIÀTOF mass spectrometry, affording
the molecular-ion peaks of cyclo-[PCu]2 and La@C82 [Figure 1a(i)].
Hence, La@C82 is indeed caged by cage-[PCu]2 and not allowed
to escape from its tetrapodal cavity. Because of the presence of
the guest La@C82, both inclusion complexes showed a red-
shifted NIR absorption band at ∼1000 nm (Figure 1b inset).
However, the red shift for the caged version (ii) was more explicit
than that for the noncaged one (i).
Figure 2. (a) ESR and (b) ESTN spectra at À150 and À268 ꢀC,
respectively, of (i) cyclo-[PCu]2, (ii) cyclo-[PCu]2⊃La@C82, (iii) cage-
[PCu]2, (iv) cage-[PCu]2⊃La@C82, and (v) La@C82 (inset: magnified)
in frozen chlorobenzene.
above; Figure 2a(i,iii)] as well as that of La@C82 [Figure 2a(v)].
Of particular interest, ESTN spectroscopy clearly showed that
the spin ground state of cyclo-[PCu]2⊃La@C82 is a quartet [S = 3/2;
Figure 2b(ii)]. For reference, when diamagnetic C82 (the C2-
symmetric isomer)12 was incorporated into the cavity of cy-
clo-[PCu]2 in place of La@C82 (Kassoc = 2.1 Â 107 MÀ1; Figure
S14),10 neither the ESR nor the ESTN profile changed (Figure
S15).10 Therefore, it is clear that La@C82 in cyclo-[PCu]2⊃
La@C82 is ferromagnetically coupled with the copper(II)
porphyrin units of the host component (Figure 3a). In sharp
contrast, cage-[PCu]2⊃La@C82, as determined by ESTN spec-
In order to explore the possibility of ferromagnetic spin
coupling, we measured electron spin resonance (ESR) and elec-
tron spin transient nutation (ESTN)11 spectra of the above
inclusion complexes and their components in frozen chloroben-
zene (Figure 2). ESTN spectroscopy is a pulsed ESR method that
allows the investigation of the spin states of diluted analytes in
frozen solvents. As shown in Figure 2a(i), cyclo-[PCu]2 at À150 ꢀC
displayed just an ordinary ESR spectral feature analogous to that
of its monomeric copper(II) porphyrin precursor (Figure S15).10
Furthermore, the spin ground state of cyclo-[PCu]2, as determined
1
troscopy, was a doublet in its spin ground state [S = /2;
Figure 2b(iv)]. However, in view of the fact that its ESR
spectrum [Figure 2a(iv)] was substantially different from those
of the precursors cage-[PCu]2 [Figure 2a(iii)] and La@C82
[Figure 2a(v)], the host and guest spins indeed interact with
one another. Hence, from the doublet character of cage-[PCu]2⊃
La@C82 in its spin ground state, the host/guest spin coupling is
most likely ferrimagnetic (Figure 3b).
A density functional theory (DFT) study10 of a simplified
model of cyclo-[PCu]2⊃La@C82 suggested that its two Cu(II)
porphyrin units are not oriented parallel to one another and that
the geometrical center of the included La@C82 does not lie on
the CuÀCu axis of the host component (Figure 3a).13 Such a
noncentrosymmetric geometry is reasonable considering the
bulkiness of La@C82.9b In contrast, in the optimized structure
of cage-[PCu]2⊃La@C82, La@C82 is in the geometrical center and
the two copper(II) porphyrin units are aligned parallel to one another
(Figure 3b).13 When the energy profile of cyclo-[PCu]2⊃La@C82 as
1
by ESTN spectroscopy at À268 ꢀC, was a doublet [S = /2;
Figure 2b(i)]. Therefore, in cyclo-[PCu]2, the two copper-
centered spins hardly interact with one another. This is also
the case for its caged analogue (cage-[PCu]2), whose ESR and
ESTN spectral features [Figure 2a(iii),b(iii)] are similar to those
of cyclo-[PCu]2. Meanwhile, when cyclo-[PCu]2 was allowed to
include La@C82, a featureless ESR signal centered at 320 mT
resulted [Figure 2a(ii)]. This spectral profile is essentially
different from those of guest-free cyclo- and cage-[PCu]2 [see
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a function of spin state was analyzed by DFT calculations using
its optimized geometry (Figure 3a) as the initial state, the quartet
spin state was slightly but definitely more stable than the doublet
spin state (Table S1),10 in conformity with our experimental
result [Figure 2b(ii)]. In contrast, for the optimized structure of
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changed by altering the cluster geometry.
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have demonstrated that the former compound is the first host
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transformed into its caged analogue (cage-[PCu]2⊃La@C82). As
shown by DFT calculations, this transformation is likely accompa-
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host molecule with the ability to change its conformaton in response
to physical stimuli might lead to the development of stimuli-
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’ ASSOCIATED CONTENT
(13) The optimized model of cyclo-[PCu]2⊃La@C82 adopted a
CuÀLaÀCu angle and CuÀLa distances of 144.5ꢀ and 5.691 and
9.415 Å, respectively, while the corresponding angle and distances in
the model of cage-[PCu]2⊃La@C82 after optimization were 175.1ꢀ and
5.368 and 8.977 Å, respectively.
S
Supporting Information. Synthesis and characterization
b
of cyclo- and cage-[PM]2 and their complexes with La@C82 and
C82. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
TASHIRO.Kentaro@nims.go.jp; aida@macro.t.u-tokyo.ac.jp
’ ACKNOWLEDGMENT
This work was partially supported by a Grant-in-Aid for
Scientific Research on Innovative Areas (20108001, “pi-Space”)
from MEXT, Japan. F.H. thanks the JSPS for a Postdoctoral
Fellowship for Foreign Researchers.
’ REFERENCES
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