A quadruply stranded metallohelicate and its spontaneous dimerization into an interlocked metallohelicate

Article abstract of DOI:10.1002/anie.200703162

(Figure Presented) Making up a foursome: Self-assembly of 4,4′-(3-pyridinemethoxy)benzophenone (L) and Pd(NO₃) ₂ resulted in the quantitative formation of a quadruply stranded metallohelicate [Pd₂(L)₄], which undergoes spontaneous dimerization to an unprecedented chiral interlocked metallohelicate [Pd ₂(L)₄]₂ (see X-ray structure; C black, H white, N blue, O red).

Full text of DOI:10.1002/anie.200703162

Communications
DOI: 10.1002/anie.200703162
Supramolecular Chemistry
A Quadruply Stranded Metallohelicate and Its Spontaneous
Dimerization into an Interlocked Metallohelicate**
Morihiko Fukuda, Ryo Sekiya,* and Reiko Kuroda*
Coordination-driven self-
assembly of discrete supra-
molecular systems ([MxLy]
systems;
M = transition-
metal ion, L = ligand) has
become one of the most
intensely researched fields
of current supramolecular
chemistry. This is because
of its excellent features
such as the ease of prepara-
tion and high yield of target
[MxLy] supramolecules as
well as its potential applica-
tions in material sciences.[1]
Over the past two decades,
extensive studies have been
made on the fabrication of
elaborate [MxLy] systems
with various morpholo-
gies.[2] Among them, metal-
lohelicates[3] have received
Scheme 1. Schematic illustration of the dimerization of two quadruply stranded metallohelicates (1) to an
interlocked metallohelicate (2).
particular attention because of their potential use as struc-
tural components of functional materials as well as their
inherent chirality.[4] Although single-, double-, and triple
metallohelicates have been well-documented,[4,5] quadruply
stranded metallohelicates have been rarely reported.[6]
Herein we report the design and synthesis of a quadruply
stranded metallohelicate and its spontaneous dimerization
into an interlocked structure (Scheme 1). Previously, Fujita
et al. used rigid ligands and reported an interesting self-
assembly of an interlocked supramolecular structure from ten
components, as well as the reorganization of two different
preformed cages into an interlocked structure by partial
decomposition.[7] The study described herein shows that a
metallohelicate with flexible ligands dimerizes to form an
interlocked structure while ultimately keeping the integrity of
the monomer unit.
The novelty of our design for the ligand which would give
rise to the quadruply stranded metallohelicate lies in the use
of benzophenone as the origin of twist: Benzophenone
usually adopts a nonplanar C2 symmetry as a result of steric
repulsion between the hydrogen atoms of neighboring phenyl
rings and, in solution, the enantiomers are in equilibrium by
[*] Prof. R. Sekiya, Prof. R. Kuroda
Department of Life Sciences
Graduate School of Arts and Sciences
The University of Tokyo
3-8-1 Komaba, Meguro-ku, Tokyo 153-8902 (Japan)
Fax: (+81)3-5454-6601 (for R.S.)
Fax: (+81)3-5454-6600 (for R.K.)
E-mail: csekiya@mail.ecc.u-tokyo.ac.jp
ckuroda@mail.ecc.u-tokyo.ac.jp
M. Fukuda, Prof. R. Kuroda
Department of Biophysics and Biochemistry
Graduate School of Science
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Prof. R. Kuroda
Japan Science and Technology Agency
ERATO-SORST
Kuroda Chiromorphology Team
4-7-6 Komaba, Meguro-ku, Tokyo 153-0041 (Japan)
À
rapid rotation of the phenyl rings at the Ccarbonyl Cphenyl bonds.
We coupled the benzophenone framework with two flexible
3-pyridylmethoxy groups to create the bridging ligand 4,4’-
bis(3-pyridinemethoxy)benzophenone (L1; Scheme 2). Its
twisted structure was confirmed by PM6 calculations[8] (see
Figure S1 in the Supporting Information), which show a
dihedral angle of 59.68 between the two phenyl rings of the
benzophenone moiety. We expected that self-assembly of L1
and PdII ions, which prefer to adopt a square-planar geometry,
[**] We are grateful to the single-crystal X-ray structure analysis groupof
the RIGAKU corp. for measurement of the diffraction data of 2. This
work was partly supported by a Grant-in-Aid for Scientific Research
(17750122) from the Ministry of Education, Culture, Sports, and
Technology of Japan.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
706
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would give the target quadruply stranded metallohelicate
[Pd2(L1)4] (1).
The ligand L1 was synthesized in 88% yield from 3-
(chloromethyl)pyridine hydrochloride and 4,4’-dihydroxy-
benzophenone.[9] Colorless crystals of X-ray quality were
obtained by slow evaporation of a mixed solvent solution of
CH2Cl2 and hexane at room temperature. Crystallographic
analysis[10] showed L1 to have C2 symmetry with a dihedral
angle of 49.88 (Figure 1). The 1H NMR spectrum of L1
([D6]DMSO) showed a sharp singlet at d = 5.20 ppm corre-
sponding to the methylene protons (Figure 2a), thus indicat-
ing a rapid interconversion of the enantiomers on the NMR
time scale.
Figure 2. 1H NMR spectra (500 MHz, 293 K, [D6]DMSO) of a) 4,4’-
bis(3-pyridinemethoxy)benzophenone (L1), b) [Pd2(L1)4](NO3)4 (1-
(NO3)4), and c) [Pd2(L1)4]2(NO3)8 (2(NO3)8). The symmetrically inde-
pendent parts of L1 and 1 are colored red. Those of 2 are colored red
or blue.
Figure 1. ORTEP drawing of 4,4’-bis(3-pyridinemethoxy)benzophenone
(L1; 50% probability ellipsoid). Top: front view; bottom: side view.
Nitrogen and oxygen atoms are represented as shaded ellipsoids.
Scheme 2.
The reaction of L1 with Pd(NO3)2 (2:1 ratio) in
[D6]DMSO for one hour at room temperature yielded a
clear light brown solution (Scheme 2).[9] The ESI mass
spectrum (see Figure S2 in the Supporting Information)
showed a family of prominent signals at m/z 619.9, 960.9,
and 1984.9, which were assigned to [1(NO3)]3+, [1(NO3)2]2+,
and [1(NO3)3]+, respectively. These signals indicate the
of L1. An NOE signal was also observed between Ha and Hf,
but not between Hb and Hf. This finding indicates that Ha
points into the inner cage of 1 and is proximal to Hf of the
ligand as well as to Hf of the three neighboring bridging
ligands, whereas Hb points outward. These structural features
are consistent with those of energy-minimized structures of 1
calculated with the universal force-field method (Figure 3).[12]
The two lowest energy structures have D4 symmetry, with
azimuthal angles q relating the two {PdN4} units of 67.58 and
43.48. In fact, the simple 1H NMR spectrum suggests 1 has an
1
formation of the target metallohelicate. The H NMR spec-
trum of the solution showed the quantitative formation of 1:
The pyridyl protons, in particular Ha and Hd, showed large
downfield shifts, a characteristic of metal–ligand complex-
ation (Figure 2a,b, and Table S1 in the Supporting even higher symmetry in solution. However, a variable-
Information). The other pyridyl protons displayed much
smaller changes in their chemical shifts. Several attempts to
crystallize 1 were unsuccessful.[11] A 2D NOESY spectrum
(see Figure S3 in the Supporting Information) was thus
recorded to elucidate the structure. The NOE signals between
the methylene protons (He) and the a- and g-pyridyl protons
(Ha and Hb) indicate that the pyridyl rings and the methylene
groups are nearly parallel, as observed in the crystal structure
temperature NMR spectrum of 1 in [D6]DMSO/[D4]MeOD
[13]
(1:1)
showed a broadening of the signal corresponding to
He upon cooling (see Figure S4 in the Supporting
Information). Thus, one possible explanation of the higher
symmetry is rapid dynamic averaging on the NMR time scale
of various conformers, including the energy-minimized struc-
tures, in solution, although the possibility of C4v or D4h
symmetry cannot be excluded entirely.[14]
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Communications
Figure 3. The two lowest energy minimized structures of 1 calculated
with molecular mechanics calculation using universal force field
method (only (M)-twisted 1 is shown).[14] Color scheme: gray, carbon;
white, hydrogen; blue, nitrogen; red, oxygen; dark green, palladium.
Inset: the definition of the azimuthal angle q.
Interestingly, complex 1 was gradually converted into a
new complex (2) in solution at room temperature, and the
rate of the conversion was accelerated on increasing the
temperature. To identify the new complex, a freshly prepared
solution of 1 in [D6]DMSO was heated at 353 K for about
24 h.[9] The 1H NMR spectrum of the resultant solution
(Figure 2c) showed a family of new signals, but no signals
for 1. Further heating of the solution at 353 K resulted in no
further change in the NMR spectrum, thus indicating 1 is a
kinetic product and 2 is a thermodynamically stable one. It is
noteworthy that no signals of intermediates were detected by
NMR spectroscopy during the conversion. The ESI mass
spectrum (see Figure S5 in the Supporting Information)
showed a family of new prominent signals at m/z 620.0,
756.3, 961.6, 1301.9, and 1985.3, which were assigned as
[Pd4(L1)8(NO3)2]6+
,
[Pd4(L1)8(NO3)3]5+, [Pd4(L1)8(NO3)4]4+,
[Pd4(L1)8(NO3)5]3+, and [Pd4(L1)8(NO3)6]2+, respectively. This
finding suggests that 2 is a dimerized complex of 1.
X-ray quality single crystals of 2 were obtained by slow
diffusion of ethyl acetate vapor into a solution of 2 in DMF at
room temperature. Remarkably, crystallographic analysis
revealed that 2 is an interlocked metallohelicate composed
of two {Pd2(L1)4} units (Figure 4).[15] This structure is, to the
best of our knowledge, the first example of such a structure.
Complex 2 resides on a C4 symmetry axis, with the four PdII
atoms located on it. Interestingly, the two {Pd2(L)4} units in a
given molecule of 2 twist in the same direction. The azimuthal
angles q of the {Pd2(L)4} units are 21.8 and 8.98 (or their
negative values), which are smaller than those of the energy-
Figure 4. a) ORTEP drawing of the X-ray crystal structure of the
interlocked metallohelicate 2 (30% probability ellipsoid). Color
scheme: gray, carbon; blue, nitrogen; red, oxygen; green, palladium.
b) Side view and c) topview of the space-filling illustrations of the
X-ray crystal structure of 2, with one color for each dimer.
complex is not dictated by the crystal packing force (see
Figure S6 in the Supporting Information).
Complex 2 retains its interlocked structure in solution.
Figure 2b and c show that each signal of the bridging ligands
splits into two signals, which suggests that the left and right
minimized structures of 1. This arrangement is mainly due to halves of the bridging ligands experience magnetically differ-
the compact structure of 2: CH/p and p/p interactions
between the neighboring bridging ligands were indicated by
the interatomic distances (representative CH/p interactions
are shown in Figure S7 in the Supporting Information).
Complex 2 has three cages in which nitrate or solvent
molecules may be incorporated.[16] As the space group is
centrosymmetric P4/n, mirror-image molecules with opposite
twist are present in the crystal. No strong intermolecular
interactions were found between different complexes of 2,
and DMF molecules were present in the intermolecular space.
ent environments. This proposal is supported by the 13C NMR
spectrum, in which all 13C signals except for that of the
carbonyl carbon atom (singlet at d = 192.2 ppm) are split into
two signals (see Figure S8 in the Supporting Information).
The proton signals were fully assigned on the basis of H-H
COSY (see Figure S9 in the Supporting Information) and
NOESY (see Figure S10 in the Supporting Information)
spectra. The protons of the benzophenone framework (Heo,
Hfo, Hei, and Hfi) and a 3-pyridinemethoxy arm (Hbi, Hci, and
Hei), except the a-pyridyl protons (Hai and Hdi), experience
These observations suggest that the stereochemistry of the upfield shifts (see Table S3 in the Supporting Information).
708
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Among them, the upfield shift of Hei is the largest (Dppm
=
1H NMR spectra of the solutions, which were rapidly cooled
À1.35 ppm). These shifts are explained by the shielding effect
of the neighboring bridging ligands. The large upfield shift of
Hei was expected from the X-ray structure of 2, in which the
methylene protons lie just above the phenyl rings of the
benzophenone moieties (see Figure S7 in the Supporting
Information). The NOESY spectrum provided important
evidence for the interlocked structure (see Figure S10 in the
Supporting Information). Of particular interest are the
interactions between the b- or g-pyridyl protons (Hci or Hbi)
and the benzophenone protons. Normally no NOE signals are
observed because of the long distance between them, as found
in 1 (see Figure S3 in the Supporting Information). This
evidence shows that 2 retains its interlocked structure in
solution.
Despite the C4 symmetry of 2 in the solid state, its
1H NMR spectrum is simple, which suggests apparent higher
symmetry in solution. Variable-temperature NMR measure-
ments of 2 in [D6]DMSO/[D4]MeOD (1:1) showed that, upon
cooling the solution, the signals for the protons of the
benzophenone frameworks and the methoxy units, especially
Hei, Hfi, and Hgi, broadened quite significantly compared to
those of the pyridyl protons (Figure 5). This result indicates
to room temperature for measurement of the spectra, showed
families of signals of L1 and 1 as well as of 2 (see Figure S11a
in the Supporting Information). Very small amounts of a
black precipitate, most likely Pd metal, were formed in the
NMR sample tubes. This result means that 2 is highly
thermodynamically stable and decomposes slowly to 1 and/
or L1 only at high temperature (> 363 K). After seven days at
room temperature, the decomposed 1 was almost completely
converted back into 2 (see Figure S11b in the Supporting
Information). The remarkable stability of 2 may be ascribed
to its compact structure arising from CH/p, p/p, and van der
Waals contacts between the neighboring bridging ligands, as
found in the crystal structure.
In summary, we have designed and synthesized a new
quadruply stranded metallohelicate [Pd2(L1)4] (1), which is
formed quantitatively through self-assembly of Pd(NO3)2 and
the bridging ligand of 4,4’-bis(3-pyridinemethoxy)benzophe-
none (L1). Complex 1 undergoes spontaneous dimerization
into the unprecedented interlocked metallohelicate
[Pd2(L1)4]2 (2). We are currently investigating the possibility
of dynamic interconversion of chirality as well as the ability of
1 and 2 to encapsulate anions.
Received: July 14, 2007
Keywords: cage compounds · helical structures ·
.
interlocked structures · self-assembly · supramolecular chemistry
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that rotation of the phenyl rings becomes slow on the NMR
time scale. The signals for Hfi and Hgi (“inner” phenyl rings,
see Figure 2c) were much broader than those of Hfo and Hgo
(“outer” phenyl rings, see Figure 2c) at 223 K. This effect is
due to the steric crowding in the central region of 2. These
low-temperature NMR measurements indicate that in solu-
tion 2 is in rapid interconversion between various conformers,
including the enantiomeric pairs of C4 symmetry found in the
crystal structure.
We further investigated the thermal stability of 2. Single
crystals of 2 were dissolved in [D6]DMSO and then the
solutions were heated at various temperatures. Below 353 K,
2 was stable, without decomposition. After heating the
solutions at 363, 373, and 383 K for several hours, the
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Communications
[7] M. Fujita, N. Fujita, K. Ogura, K. Yamaguchi, Nature 1999, 400,
4015.4, 2qmax = 54.48 were a = 20.8580(8), b = 20.8580(8), c =
33.150(2) Š, and V= 14421.1(1) Š3. The intensity data were
collected on a Rigaku Saturn-724 CCD diffractometer (MoKa
radiation, l = 0.71073 Š) at 103 K. A total of 145124 reflections
were collected of which 16491 reflections were independent
(Rint = 0.0791). The crystal structure was solved by direct
methods using the SIR-2004 program[20] and refined by succes-
sive differential Fourier syntheses and full-matrix least-squares
procedures using the SHELXL-97 program.[19] Anisotropic
thermal factors were applied to all non-hydrogen atoms. All
hydrogen atoms were generated geometrically. The structure
was refined to final R1 = 0.124 for 11715 data [I > 2s(I)] with 592
52 – 55.
[8] J. J. P. Stewart, MOPAC 2007, http://openmopac.net/.
[9] See the Supporting Information for details.
[10] Crystal data for L1 (C25H20N2O3): Mr = 198.2, colorless crystal,
crystal dimensions 0.40 ” 0.40 ” 0.25 mm3, orthorhombic, space
group Pbcn (no. 60), Z = 8, 1calcd = 1.39 gcmÀ3, F(000) = 831.9,
2qmax = 55.88 were a = 7.1974(5), b = 6.4495(5), c = 40.935(3) Š,
and V= 1900.16(2) Š3. The intensity data were collected on a
Bruker SMARTAPEX CCD diffractometer (Mo
radiation,
Ka
l = 0.71073 Š) at 103 K. A total of 10678 reflections were
collected of which 2243 reflections were independent (Rint
=
0.0254). The crystal structure was solved by direct methods
using the SHELXS-97 program[18] and refined by successive
differential Fourier syntheses and full-matrix least-squares
procedures using the SHELXL-97 program.[19] Anisotropic
thermal factors were applied to all non-hydrogen atoms. All
hydrogen atoms were generated geometrically. The structure
was refined to final R1 = 0.047 for 2032 data [I > 2s(I)] with 137
parameters, wR2 = 0.113 for all data, GOF = 1.088, and residual
electron density max/min = 0.357/À0.264 eŠÀ3..
parameters, wR2 = 0.388 for all data, GOF = 1.391, and residual
À3
electron density max/min = 1.485/À1.636 eŠ
. The diffuse
electron density arising from the disordered and unidentified
moieties was treated with the SQUEEZE routine within the
PLATON software package.[17] The final 1calcd, F(000), and Mr
values reflect known contents only. Statistical prior to treatment
of data with SQUEEZE were R1 = 0.203 [I > 2s(I)], wR2 = 0.538
for all data, and GOF = 2.111. CCDC 652008 (L1) and 652007
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[11] Crystallization of 1 from DMF or DMSO solutions gave only
crystals of 2. We have reasoned that during crystallization, 1 was
gradually converted into the thermodynamically more stable 2.
See text for details.
[16] The highly disordered state of the incorporated molecules meant
that none of them could be located, and hence in the final
refinement, the electron density within the cages was treated
with the SQUEEZE routine in the PLATON program pack-
age.[17]
[17] A. L. Spek, PLATON, University of Glasgow, Glasgow (Scot-
land), 1998.
[18] G. M. Sheldrick, SHELXS-97: Program for X-ray Crystal
Structure Solution, University of Gꢀttingen, Gꢀttingen (Ger-
many), 1997.
[19] G. M. Sheldrick, SHELXL-97: Program for X-ray Crystal
Structure Refinement, University of Gꢀttingen, Gꢀttingen
(Germany), 1997.
[20] M. C. Burla, R. Caliandro, M. Camalli, B, Carrozzini, G. L.
Cascararo, L. De Caro, C. Giacovazzo, G. Polidori, R. Spagna,
SIR-2004: A program for automatic solution and refinement of
crystal structures.
[12] A. K. Rappe, K. S. Colwell, C. J. Casewit, Inorg. Chem. 1993, 32,
3438 – 3450.
[13] As [D6]DMSO freezes at 291 K, we used [D6]DMSO/[D4]MeOD
(1:1) mixed solutions for the measurement of the variable-
temperature NMR spectra of 1 and 2. Splitting of the He signal of
1 was not observed at the lowest possible temperature for the
solvent.
[14] The possibility of L1 having C2v symmetry with a planar
conformation in the complexes and a box shape for 1 was
pointed out by one of the referees. PM6 calculations suggested
that benzophenone with a planar conformation has a higher heat
of formation than the other conformations because of the steric
repulsion between hydrogen atoms of neighboring phenyl rings.
[15] Crystal data for 2 (Pd(C25H20N2O3)2(DMF)): Mr = 972.4, color-
less crystal, crystal dimensions 0.11 ” 0.10 ” 0.04 mm3, tetragonal,
space group P4/n (no. 85), Z = 8, 1calcd = 0.90 gcmÀ3, F(000) =
710
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