Switchable intermolecular communication in a four-fold rotaxane

Article abstract of DOI:10.1002/anie.201107104

Firmly tied: A four-fold rotaxane was prepared from a porphyrin unit with four alkylammonium chains and a phthalocyanine unit with four peripheral crown ethers. In a dinuclear Cu²⁺ complex of the four-fold rotaxane, the Cu²⁺-porphyri

Full text of DOI:10.1002/anie.201107104

Angewandte
Chemie
DOI: 10.1002/anie.201107104
Rotaxanes
Switchable Intermolecular Communication in a Four-Fold Rotaxane**
Yasuyuki Yamada, Mitsuhiro Okamoto, Ko Furukawa, Tatsuhisa Kato, and Kentaro Tanaka*
In biological systems, precise intermolecular electronic com-
munication occurs in, and is regulated by, proteins and their
complexes. For example, light-harvesting complexes have
metal ions or metal complexes, such as manganese clusters,
and organic cofactors, such as chlorophylls, carotenoids, or
ubiquinones, in the “right” spatial arrangement so that
electrons and energy can be efficiently transferred.[1,2] Elec-
tronic communication between molecules is sensitive to their
relative orientation. Large molecular scaffolds of proteins
provide precise frameworks for molecular arrays as well as
the ability of dynamic switching intermolecular communica-
tion, accomplished by conformational changes induced by
external stimuli. These natural molecular frameworks are
programmed as sequences of amino acids, but we could
achieve flexible intermolecular communication in a simpler
molecular architecture by using mechanically interlocked
supramolecular motives, such as catenanes and rotaxanes, in
which two or more molecular components are inseparable but
their interactions are flexibly convertible.[3,4] Syntheses of
rotaxanes have been well-established, and a large variety of
fascinating examples of rotaxane-based molecular systems
has been reported.[3–5] As a specific example of switchable
rotaxanes, Stoddart, Credi and co-workers reported elevator-
like motion of a molecule with crown ethers along the strings
on another molecule in a three-fold rotaxane, in which the
relative position between the molecules was switchable by
redox or acid-base reactions.[6] Sauvage and co-workers made
a porphyrin dimer in a cyclic [4]rotaxane and showed its
ability to act as switchable receptor.[7] Porphyrin, phthalocya-
nine, and their metal complexes have been applied to a broad
range of functionalized molecular systems. Intermolecular
electronic communication in their programmed arrays partic-
ularly has attracted a lot of interests in a wide range of fields,
from photomaterials[8] to pigments,[9] molecular devices,[10]
and catalysts.[11]
Herein, we report a mechanically linked cofacially
stacked dimer of a metalloporphyrin and metallophthalocya-
nine units by formation of a four-fold rotaxane and its
switchable spin–spin communication induced by external
stimuli. A rotaxane consisting of a secondary ammonium ion
and a crown ether has been recognized as a versatile building
+
block for supramolecular systems. R2NH2 ions are com-
[*] Dr. Y. Yamada, Prof. Dr. K. Tanaka
Department of Chemistry, Graduate School of Science
Nagoya University
plexed by dibenzo[24]crown-8 in a threaded manner as a
result of electrostatic stabilization and hydrogen bonds
between the negatively charged interior of the crown ether
and the cationic ammonium moiety.[12] We designed a
porphyrin unit with four alkylammonium chains [1·4H]4+
and phthalocyanine unit with peripheral crown ethers 2
which are expected to form a facially stacked dimer through
formation of a four-fold rotaxane (Figure 1).
Furo-cho, Chikusa-ku, Nagoya 464-8602 (Japan)
E-mail: kentaro@chem.nagoya-u.ac.jp
Dr. Y. Yamada
Reseach Center for Materials Science, Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8602 (Japan)
M. Okamoto, Prof. Dr. T. Kato
The four-fold rotaxane [3·8H]4+·4Clꢀ was obtained in
41% yield through formation of a pseudo-rotaxane between
the porphyrin [1·5H]5+·5BARFꢀ (BARFꢀ = tetrakis[(3,5-bis-
trifluoromethyl)phenyl]borate[13]) and the phthalocyanine 2
in chloroform and acetone at a ratio of 4:1, followed by
locking through Staudinger-phosphite reaction, which con-
verts the azide groups to larger phosphoramidate units.[14] The
structure of the four-fold [2]rotaxane [3·8H]4+·4Clꢀ was given
unambiguously by ESI-TOF mass spectrometry and NMR
spectroscopy (Figure 2). The ESI-MS data clearly showed m/z
values of 1365.0 (z = 3), 1377.0 (z = 3), and 1024.0 (z = 4),
which confirmed the interlocked structure between [1·4H]4+
and 2 (calculated m/z values for [3 + 7H]3+, [3 + 8H + Cl]3+,
and [3 + 8H]4+ are 1365.0, 1377.0, and 1024.0, respectively;
Figure 2b). The highly symmetric structure of [3·8H]4+·4Clꢀ
Department of Interdisciplinary Environment
Graduate School of Human and Environmental Studies
Kyoto University
Yoshidanihonmatsu-cho, Sakyo-ku, Kyoto 606-8501 (Japan)
Dr. K. Furukawa
Institute for Molecular Science
Myodaiji, Okazaki 444-8585 (Japan)
Prof. Dr. T. Kato
Institute for the Promotion of Excellence in Higher Education
Kyoto University
Yoshidanihonmatsu-cho, Sakyo-ku, Kyoto 606-8501 (Japan)
Prof. Dr. K. Tanaka
CREST (Japan) Science and Technology Agency
Honcho 4-1-8, Kawaguchi-shi, Saitama 332-0012 (Japan)
[**] We thank Dr. Miyo Yoshida of the chemical instrumentation facility,
research center for materials science, Nagoya University for
elemental analyses. This work was financially supported by the
Grant-in-Aids for Scientific Research on the innovative area
“Coordination Programming” (area 2107, grant number 21108012)
and the global COE program for “Elucidation and Design of
Materials and Molecular Functions” to K.T. from the Ministry of
Education, Culture, Sports, Science, and Technology (Japan).
1
was shown in the H NMR spectrum (Figure 2a). All signals
in the spectrum were assignable as one-quarter part of the
four-fold rotaxane. Especially diagnostic are the high-field-
shifted signals of the pyrrolic NH protons of both porphyrin
and phthalocyanine units (Ha and Ha in Figure 2a, respec-
tively), indicating a p–p stacking interaction and a close
spatial arrangement between the crown ethers and the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107104.
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Figure 1. Self-assembly of the four-fold rotaxane and synthesis of its dinuclear Cu2+ complex. Formation of a pseudo-rotaxane between porphyrin 1
with alkylammonium chains and phthalocyanine 2 with peripheral crown ethers, followed by locking through a Staudinger-phosphite reaction led
to the formation of a four-fold rotaxane [3·8H]4+·4Clꢀ. Reaction of [3·8H]4+·4Clꢀ with Cu(OAc)2 (2 equiv) yielded the dinuclear Cu2+ complex
[4·8H]4+·4Clꢀ.
symmetric four-fold [2]rotaxane structure, in which all
alkylammonium chains were mechanically locked by the
phosphoramidate moiety in each crown ring. To the best of
our knowledge, this is the first example of a facially stacked
heterodimer of porphyrin and phthalocyanine units.
Both metallized porphyrin and phthalocyanine units were
obtained by using Cu(OAc)2 to yield a dinuclear Cu2+
complex [4·8H]4+·4Clꢀ in 93% yield as blackish-green crystals
(Figure 1). The rotaxane [4·8H]4+·4Clꢀ has two types of acidic
protons, which are located at the four ammonium moieties
(-NH2+-) and the four phosphoramidate moieties (-NHP(O)-).
To neutralize the protons, initially we chose 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU) as a base, but the absorption
spectrum of [4·8H]4+·4Clꢀ remained unchanged upon addi-
tion of DBU. However, addition of the stronger phosphazene
base (P1-tBu) caused significant changes in the UV/Vis
absorption spectra of the dinuclear complex in CH2Cl2
(Figure 3). The addition of up to four equivalents of
phosphazene base changed the spectrum only slightly (Fig-
ure 3b), but with further addition, above four equivalents,
both Soret and Q bands gradually decreased with a well-
defined isosbestic point at 698 nm (Figure 3c). The titration
profile (Figure 3d) clearly showed a two-stage process. The
two stages were identified in control experiments using model
compounds. The first stage shows deprotonation of the
phosphoramidate moiety (-NHP(O)- to -NꢀP(O)-), whereas
Figure 2. 1H NMR (600 MHz) of the four-fold rotaxane [3·8H]4+·4Clꢀ
the second stage shows deprotonation of the ammonium
in CDCl3 at 293 K. The signals labeled with a–t (shown in pink) are
assigned to the resonance of porphyrin 1 with alkylammonium chains,
and the signals labeled with a–e (shown in orange) are assigned to
the resonances of the phthalocyanine 2 with peripheral crown ethers.
b) ESI-TOF MS spectra of [3·8H]4+·4Clꢀ (M for
+
moiety (-NH2 - to -NH-; see the Supporting Information).
The fully deprotonated spectrum reverted to that of [4·8H]4+
upon addition of trifluoroacetic acid. The change in absorb-
ance at 684 nm upon successive addition of base and acid
shows the reversibility of the protonation and deprotonation
processes without decomposition of the complex (Figure 3e).
Continuous-wave electron paramagnetic resonance (CW-
EPR) spectra of the dinuclear Cu2+ complex in a four-fold
rotaxane structure, [4·8H]4+, and its deprotonated species,
[4]4ꢀ, in a frozen dichloromethane solution were recorded at
10 K by a conventional X-band spectrometer (Figure 4a and
c, respectively). The spectrum of the protonated [4·8H]4+ was
well-simulated as a sum of two isolated doublet (S = 1/2) spins
[3·8H]4+ =C220H280N20O48P4). c) ESI-TOF MS spectra of [4·8H]4+·4Clꢀ
(M for [4·8H]4+ =C220H276N20O48P4Cu2). In the insets of (b) and (c) the
measured MS signals are compared with the calculated isotope
distribution patterns of the corresponding charge of the molecular
ions.
alkylammonium chains, which was observed in a ROESY
spectrum (see the Supporting Information). Consequently,
the porphyrin and the phthalocyanine units stacked in a C4-
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Figure 4. EPR spectra of the protonated and deprotonated forms of
Figure 3. Photometric titration of the dinuclear copper complex
[4·8H]4+·4Clꢀ with the phosphazene base (P1-tBu). The dinuclear
copper complex [4·8H]4+·4Clꢀ was dissolved in CH2Cl2 at a concen-
tration of 2.0ꢀ10ꢀ6 m at 293 K. a) The spectra change during addition
of 0–24 equivalents of P1-tBu. b) The spectral change during addition
of 0–4 equivalents of P1-tBu. c) The spectral change during the
addition of 4–24 equivalents of P1-tBu (the isosbestic point is located
at 698 nm). d) Plots of the absorbance at 684 nm. e) Reversible
changes in the absorbance at 684 nm after successive addition of
the dinuclear Cu2+ complex (B=magnetic field). a) Experimental (red
curve) and simulated (blue curve) EPR spectra of the dinuclear copper
complex [4·8H]4+·4Clꢀ in the absence of the phosphazene base (P1-
tBu) in CH2Cl2 at a concentration of 500 mm at 10 K. b) Plots of cT
versus T for [4·8H]4+·4Clꢀ in the absence of P1-tBu from T=4 to 50 K.
The blue curve shows the fit of cT versus T for [4·8H]4+·4Clꢀ using the
Curie–Weiss model (q=ꢀ1.0 K). c) Experimental (red curve) and
simulated (blue curve) EPR spectra of [4·8H]4+·4Clꢀ in the presence of
8 equivalents of P1-tBu in CH2Cl2 at a concentration of 500 mm at 10 K.
The inset shows experimental (red) and simulated (blue) EPR spectra
at the Dms =2 transition in the half-field region. d) Plots of cT versus
T for [4·8H]4+·4Clꢀ in the presence of 8 equivalents of P1-tBu from
T=10 to 60 K. The blue curve shows the fit of cT versus T for
[4·8H]4+·4Clꢀ in the presence of 8 equivalents of the P1-tBu using a
singlet–triplet model (J/kB =ꢀ7.0 K).
*
40 equivalents of P1-tBu ( !D) and 40 equivalents of trifluoroacetic
*
acid (D! ).
of Cu2+ centers in square-planar ligand fields of the porphyrin
and phthalocyanine units without exchange interaction (Fig-
ure 4a). The signal intensities slightly increased with increas-
ing temperature (Figure 4b). Since the profile was well-
simulated by a Curie–Weiss model with q = ꢀ1.0 K, there is a
very small antiferromagnetic mean-field interaction between
the molecules. Spectra of the deprotonated [4]4ꢀ showed a
pattern quite different from that of [4·8H]4+ (Figure 4c). The
spectrum of [4]4ꢀ showed a transition for Dms = 1 around
3000 Gauss with a fine structure arising from large zero-field
splitting and a seven-lines hyperfine structure corresponding
to two equivalents of copper atoms (I = 3/2) with spin
exchange between the two Cu2+ centers. The forbidden
transition at Dms = 2 was also observed around 1600 Gauss,
accompanied by the seven lines of hyperfine structure
because of the two equivalent copper atoms (I = 3/2). These
features were well-reproduced in simulation. From the
temperature dependence of the signal intensity (Figure 4d),
the energy gap between the ground-state singlet and the first
excited-state triplet was estimated to be only 7 K, and the
Cu2+–Cu2+ distance was estimated to be 4.06 ꢀ by a point–
charge approximation from the zero-field splitting parameter
(D = 41.8 mT). Thus, antiferromagnetic coupling between
Cu2+–porphyrin and Cu2+–phthalocyanine moieties was
detected in the deprotonated form of the four-fold rotaxane.
Although we obtained single crystals of the protonated
dinuclear complex, we have not yet been able to perform
crystallographic analysis because they are fragile. Neverthe-
less, the cofacial distance between porphyrin and phthalo-
cyanine units in the protonated form is presumable longer
than that in the deprotonated form, because the alkyl chain
length between the porphyrin and the ammonium moiety was
calculated to be about 9 ꢀ, which is longer than the Cu2+
–
Cu2+ distance in the deprotonated form and probably involves
the inherent charge repulsion between the negatively charged
interiors of the crown ethers and the lone pairs of the amine
and phosphoramidate ions. In fact, a comparison of 1H NMR
spectra of the free-base four-fold rotaxanes, protonated
[3·8H]4+, and deprotonated [3]4ꢀ, showed significantly stron-
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ꢀ1.97 (br, 2H), ꢀ4.19 ppm (br, 2H). MS (ESI-TOF, positive) m/z
1024.0: calcd for C220H280N20O48P4 ([3·8H]4+), found: 1024.0. Anal.
Calcd. for C228H304Cl10N20O52P4 ([3·8H]4+·4Clꢀ + 2Et2O + 2CHCl3 +
2H2O): C, 59.26; H, 6.62; N, 6.01. Found. C, 59.44; H, 6.73; N, 5.65.
(0.36% Error).
ger shielding effects in the signals of pyrrolic NH protons of
the porphyrin and phthalocyanine units in [3]4ꢀ. Conse-
quently, the switchable spin–spin communication (isolated
spins in the protonated form and antiferromagnetic coupling
in the deprotonated form) is attributed to the variable
distance of the two spin centers in the flexible four-fold
rotaxane structure.
Synthesis of dinuclear Cu2+ complex [4·8H]4+·4Clꢀ: The four-fold
rotaxane [3·8H]4+·4Clꢀ (12.3 mg, 2.9 mmol) and Cu(OAc)2 (1.6 mg,
8.8 mmol) were dissolved in a 1:1 mixture of CHCl3 and MeOH
(3.0 mL) and the resulting solution was heated at 508C for 23 h. After
the mixture was diluted with CHCl3 (30 mL), it was washed with H2O
(50 mL ꢁ 3) and brine (50 mL), dried over anhydrous Na2SO4, and
evaporated. The crude product was purified by silica gel column
chromatography (2 cmfꢁ 10 cm, CHCl3:MeOH = 4:1–1:1 and
CHCl3:MeOH:H2O:brine = 10:10:2:1). The fraction containing the
desired product was washed with H2O (3 ꢁ 50 mL) and brine (2 ꢁ
50 mL), dried over anhydrous Na2SO4, and concentrated under
reduced pressure to give blackish-green solid, which was dissolved in
MeOH (20 mL) and treated with ion exchange resin (IRA 400 J CL
(Clꢀ form), 10 mL). Recrystallized from CHCl3:MeOH = 9:1/toluene/
Et2O yielded [4·8H]4+·4Clꢀ as blackish-green prism crystals (11.8 mg,
93%). MS (ESI-TOF, positive): m/z 1053.9: Calcd for
C220H276Cu2N20O48P4 ([M]4+), Found: 1053.9. Anal. Calcd. for
In summary, we have clearly shown switchable spin–spin
communication between mechanically interlocked metal
complexes induced by external stimuli. The spin–spin inter-
actions are sensitively influenced by distance and relative
spatial configuration between the Cu2+ centers. In comparison
with biological templates such as proteins, the four-fold
rotaxane has a symmetric and simpler design in which Cu2+–
porphyrin and Cu2+–phthalocyanine moieties are, predictably,
cofacially stacked and are able to change their spatial
arrangements in response to external stimuli. The switching
ability of the intermolecular communication could possibly
extend from, for example the spin–spin interaction, reported
in this work, to electron and energy transfer as well as
catalytic reactions on metal centers. The number of ammo-
nium moieties on each peripheral alkyl chain of the template
porphyrin could be used to regulate the number of assembled
phthalocyanines, yielding one-dimensional stacked phthalo-
cyanine arrays. This concept would thus allow for preparation
of well-defined molecular architectures with switchable
functions related to nanomagnetism, conductivity, photonic
properties, or catalysis. The development of programmable
arrays of homogeneous and heterogeneous metal complexes
in the rotaxane framework is now underway in our laboratory.
C230H306Cl10Cu2N20O54P4
([4·8H]4+·4Clꢀ + 2Et2O + 2CHCl3 +
4H2O): C, 57.31; H, 6.40; N, 5.81. Found: C, 57.38; H, 6.56; N, 5.44.
(0.37% Error).
EPR measurements and simulations: CW EPR measurements
were performed at various temperatures from 4 to 60 K using a
Bruker model ELEXES E380 X-band spectrometer equipped with an
Oxford 900 cryostat. All samples were prepared in a concentration
range of 0.1–1.0 mm and deaerated by freeze-and-thaw cycles before
the EPR measurements. The EPR spectral simulation was performed
with the MATLAB subroutine package EasySpin,[15] distributed from
http://easyspin.org/. The EPR simulation parameters were for the
protonated [4·8H]4+: the porphyrin site, spin quantum number: S = 1/
2, g tensor: g = (2.047, 2.047, 2.186), hyperfine coupling (hfc) tensor of
Cu nuclear spin: ACu = (3.0, 3.0, 21.0)/mT, hfc tensor of N nuclear spin
: AN = (2.0, 2.0, 2.0)/mT. The phthalocyanine site, spin quantum
number: S = 1/2, g tensor: g = (2.059, 2.059, 2.157), hfc tensor of Cu
nuclear spin: ACu = (1.7, 1.7, 20.0)/mT, hfc tensor of N nuclear spin:
AN = (1.86, 1.86, 1.86)/mT. For the deprotonated [4]4ꢀ: spin quantum
number: S = 1, g tensor: g = (2.034, 2.034, 2.098), hfc tensor of Cu
nuclear spin: ACu = (1.25, 1.25, 10.8)/mT, spin–spin dipole interaction
parameters: D = 41.8 mT, E = 0.0 mT.
Experimental Section
Syntheses of [1·5H]5+·5BARFꢀ and 2 are provided in the Supporting
Information.
Synthesis of the four-fold rotaxane [3·8H]4+·4Clꢀ: To an acetone
solution (3.6 mL) of porphyrin [1·4H]4+·4BARFꢀ (107 mg, 18 mmol) a
CHCl3 solution (14.4 mL) of phthalocyanine 2 (38 mg, 18 mmol) was
added. The resulting mixture was stirred at ambient temperature for
1 h, and then P(OEt)3 (0.90 mL, 5.3 mmol) was added. After 73 h, the
resulting solution was poured into a 1:1 mixture of hexane and Et2O
(100 mL). The blackish-green precipitate was collected by centrifu-
gation and dissolved in MeOH (20 mL). Ion-exchange resin (IRA
400CJ (Clꢀ form), 20 mL) was added to the solution to remove
BARFꢀ. After the resin was removed, the crude product was purified
Received: October 7, 2011
Published online: December 1, 2011
Keywords: copper · phthalocyanines · porphyrinoids · rotaxanes
.
by
silica
gel
column
chromatography
(2 cmfꢁ 21 cm,
[1] R. E. Blankenship, Molecular Mechanisms of Photosynthesis,
Blackwell Science, Oxford, UK, 2002.
CHCl3:MeOH = 4:1–1:1—CHCl3:MeOH:H2O:brine = 20:20:2:1).
The fraction containing the desired product was washed with H2O
(50 mL ꢁ 3) and brine (50 mL ꢁ 2), dried over anhydrous Na2SO4, and
concentrated under reduced pressure to give a blackish-green solid,
which was dissolved in MeOH (10 mL) and treated with ion exchange
resin (IRA 400 J CL (Clꢀ form), 10 mL). Finally, recrystallization
from CHCl3:MeOH = 4:1/toluene/Et2O yielded [3·8H]4+·4Clꢀ as
blackish-green needles (31 mg, 41%). 1H NMR (600 MHz, CDCl3/
TMS): d = 8.26 (br, 8H), 8.10 (br, 8H), 7.62 (br, 4H), 7.51 (br, 4H),
7.37 (br, 8H), 7.21 (d, J = 7.7 Hz, 8H), 7.05 (d, J = 7.3 Hz, 4H), 6.98
(br, 4H), 6.94–6.93 (m, 8H), 6.91 (d, J = 7.7 Hz, 8H), 6.82–6.81 (m,
8H), 4.73 (br, 8H), 4.60 (br, 8H), 4.57 (br, 8H), 4.42–4.41 (m, 8H),
4.32–4.31 (m, 8H), 4.24 (s, 8H), 4.16 (br, 24H), 4.07–4.04 (m, 8H),
3.99–3.96 (m, 8H), 3.90–3.89 (m, 16H), 3.81–3.79 (m, 8H), 3.79 (br,
8H), 3.74–3.62 (m, 16H), 3.55 (t, J = 8.2 Hz, 8H), 3.07–3.03 (m, 4H),
2.00 (br, 8H), 1.91 (br, 8H), 1.68 (br, 16H), 0.90 (t, J = 6.9 Hz, 24H),
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