A self-assembled multiporphyrin cage complex through three different zinc(II) center formation under well-balanced aqueous conditions

Article abstract of DOI:10.1002/anie.201208040

Construction of a self-assembled cage complex through three different Zn^II centers is achieved using a Zn porphyrin ligand with four 2,2′-bipyridin-5-yl (bpy) groups. The multiporphyrin cage encapsulates guest molecules unsymmetrically by π-π interactions. Well-balanced aqueous conditions, which allow the formation of both tris(bpy) and hydrated bis(bpy) Zn^II units, result in the unsymmetrical yet well-defined supramolecular structure. Copyright

Full text of DOI:10.1002/anie.201208040

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DOI: 10.1002/anie.201208040
Supramolecular Chemistry
A Self-Assembled Multiporphyrin Cage Complex through Three
Different Zinc(II) Center Formation under Well-Balanced Aqueous
Conditions**
Takashi Nakamura, Hitoshi Ube, Motoo Shiro, and Mitsuhiko Shionoya*
Coordination-driven self-assembly of organic ligands and
metal ions is a powerful method for construction of large and
well-defined molecular architectures.[1] A large number of
excellent examples for supramolecular coordination struc-
tures have been reported,[2] such as squares, tetrahedra,
octahedra, and cuboctahedra. In particular, supramolecular
hosts with an isolated inner space possess unique functions
such as molecular recognition,[3] catalytic reactions,[4] control
of guest motion,[5] and stabilization of unstable species,[6]
reflecting geometrical characteristics of each inner space.
It is well understood that both precise design of organic
ligands with multiple binding sites and deliberate choice of
Figure 1. Metal-mediated self-assembly leading to well-defined supra-
molecular complexes. a) Metal-mediated self-assembly providing uni-
form polyhedral complexes (a tetrahedron and an octahedron are
shown) comprised of chemically equivalent metal centers. b) ZnII-
mediated self-assembly forming a supramolecular cage complex
[Zn1116(H2O)18](OTf)22 (2) through the formation of three different
kinds of ZnII centers from a C4-symmetric Zn-porphyrin ligand 1. A
H2O ligand bound to the axial position of Zn-porphyrin 1 is omitted
from the chemical structure for clarity and for ambiguity of the axial
ligand in solution. The ZnII centers depicted in the same color (purple,
orange, and green) are equivalent with each other.
metal species are important in setting up an initial condition
for their spontaneous supramolecular assembly. For instance,
upon addition of a certain metal ion to organic ligands
possessing multiple metal binding sites of the same type,
resultant self-assembled structures become highly symmetric
with chemically equivalent metal centers under a given
condition (Figure 1a). For more intricate self-assembled
structures, now-common procedures are differentiation of
binding sites and/or use of more than one kind of metal ion.[7]
Moreover, several polynuclear complexes with different
metal centers constructed from one kind of metal ions and
bridging ligands possessing two chemically equivalent biden-
tate units have been reported.[8] These examples have proven
that combination of even simple ligands and metal ions can
lead to intricate structures. In most cases, however, the
binding stoichiometry of ligands to metal centers stays
constant in each complex. Thus, a method to generate
unsymmetrical structures with more than one kind of metal
center from a simple organic ligand would open a new
direction towards more elaborate supramolecular functions.
In this regard, we envisioned that simultaneous formation
of several different metal centers under well-balanced com-
plexation conditions with coordinating co-solvents would
have great potential as a strategy to construct intricate self-
assembled structures. Herein we report a supramolecular
cage complex, [Zn1116(H2O)18](OTf)22 (2) (OTf = CF3SO3),
formed from Zn(OTf)2 and a C4-symmetric Zn-porphyrin
ligand 1 in an aqueous–organic mixed solvent (Figure 1b).
Although the ligand 1 has four chemically equivalent (2,2’-
bipyridin)-5-yl (bpy) groups attached to a central Zn-porphy-
rin, the resulting cage complex 2 possesses three different
kinds of ZnII centers with bpy ligands. Notably, two different
kinds of hydrated bis(bpy) ZnII units were formed in addition
to a tris(bpy) unit under a well-balanced aqueous condition,
leading to complex 2 with an unsymmetrical yet well-defined
structure. Its triangular bipyramidal inner space surrounded
by six Zn-porphyrin rings allowed unsymmetrical inclusion of
up to two p-acceptor guest molecules, 2,7-dinitro-9-fluore-
none (3),[9] through p–p interactions with Zn-porphyrin rings
on the inner wall.
[*] T. Nakamura, Dr. H. Ube, Prof. Dr. M. Shionoya
Department of Chemistry, Graduate School of Science
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
E-mail: shionoya@chem.s.u-tokyo.ac.jp
Dr. M. Shiro
Rigaku Corporation
Metalloporphyrins have metal- and/or ligand-centered
photochemical and redox properties, and so far many studies
have been conducted to develop functional self-assembled
molecular systems.[10,11] Molecular design has often been done
making the most of C4-symmetry of porphyrin ring. For
instance, Nitschke et al. have recently reported a self-assem-
bled M8L6 cubic cage complex formed from C4-symmetric
3-9-12 Matsubaracho, Akishima, Tokyo 196-8666 (Japan)
[**] This research was supported by Global COE Program and KAKENHI
from the Japan Society for the Promotion of Science (JSPS) and
MEXT (Japan). T. Nakamura thanks JSPS Research Fellowship for
Young Scientists.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208040.
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tetrakis(pyridylimine) ligands and Fe(OTf)2.[12] A Zn-porphy-
rin ligand 1 used in our study possesses four bpy groups
directly attached to the meso positions of the central Zn-
porphyrin ring. X-ray crystallographic analysis of 1 revealed
its rigid structure (Supporting Information, Figure S9). Owing
to the steric repulsion between the b hydrogen of the
porphyrin ring and the hydrogen at the 4-position of bpy,
the bpy groups do not take coplanar conformation with the
central porphyrin ring. The dihedral angles between the bpy
groups and the porphyrin ring were in the range from 61 to
878 in the crystal structure.
ZnII was selected for coordination-driven self-assembled
structures, because ZnII is coordinatively labile and thus
adaptable in coordination behaviors. It is known that ZnII and
2,2’-bipyridine form mono(bpy), bis(bpy), and tris(bpy) ZnII
complexes, depending on synthesis conditions for counter-
anions and solvents.[13] In this work, well-balanced complex-
ation conditions were developed for the formation of a well-
defined anisotropic supramolecular cage structure with both
bis(bpy) and tris(bpy) ZnII units. We envisioned that such
synthesis conditions could be realized by controlling the
amount of coordinating solvent. Moreover, the rigidity of the
tetrakis bidentate ligand 1 was considered a key factor to
determine the relative positions of different kinds of metal
centers leading to an unsymmetrical yet finite structure.
Complexation of ligand 1 and Zn(OTf)2 was examined by
1H NMR spectroscopy in an aqueous–organic mixed solvent
CDCl3/CD3OD/D2O = 10:10:1 (Figure 2). Under these con-
ditions, 2,2’-bipyridine and Zn(OTf)2 were expected to form
either hydrated bis(bpy) complex, [Zn(bpy)2(D2O)2]2+, or
Figure 2. Characterization of ligand 1 and ZnII complex. a)–c) 1H NMR
spectra of 1 and 2 (500 MHz, 300 K). a) 1 in CDCl3/CD3OD=1:1 (v/
v%). b) 1 and Zn(OTf)2 (11:6 Zn(OTf)2/1) after heating at 608C for
3 h in CDCl3/CD3OD/D2O=10:10:1. c) Crystals of 2 dissolved in
CD3CN. d) X-ray crystal structure of [Zn1116(H2O)18]22+ in which one
Zn-porphyrin 1 is highlighted and color-coded in red, yellow, green,
and blue, to present its C1 symmetry in 2. See also Figure 3 for the
structure of 2. e) Chemical structure and labeling of hydrogen atoms
of 1. Colors are coded according to the structure in (d).
1
tris(bpy) complex, [Zn(bpy)3]2+. The H NMR spectrum of
the ligand 1 showed that 1 had a C4 symmetry in solution; that
is, four bpy groups were chemically equivalent on the NMR
timescale (Figure 2a). Upon addition of Zn(OTf)2 and
heating the reaction mixture at 608C for 3 h, a set of sharp
signals assignable to a supramolecular cage complex 2
1
appeared (Figure 2b). As many as 36 different aromatic H
signals were observed, indicating that desymmetrized 1 exhib-
ited a C1 symmetry in the complex 2. In a synthetic scale
experiment, ligand 1 and Zn(OTf)2 underwent complexation
in CHCl3/CH3OH/H2O = 10:10:1, and the complex 2 was
isolated in 76% yield after recrystallization from CH3CN/
H2O/Et2O. ESI-MS analysis of this product in a CH3CN
solution clearly found a set of signals assignable to complex 2
with the composition [Zn1116](OTf)22 (Supporting Informa-
tion, Figure S11). A UV/Vis absorption spectrum of 2 showed
Soret and Q bands characteristic to Zn-porphyrin, while they
were broadened and red-shifted compared with those of
ligand 1 (Supporting Information, Figure S39).
Crystals suitable for X-ray analysis were obtained by slow
vapor diffusion of CH2Cl2 into CH3OH solution of 2. The X-
ray analysis unambiguously revealed a cage framework of
[Zn1116(H2O)18]22+, featuring six Zn-porphyrin rings and
eleven bpy-ZnII units (Figure 3).[14] The shape of complex 2
can be perceived as an enneahedron with all of its faces being
quadrilaterals (Figure 1b). There exist three different kinds of
six-coordinate ZnII centers in 2 apart from Zn1 of porphyrin
1 (Figure 3c). Two Zn4 centers at the both ends of 2 (depicted
in purple in Figure 3) were bound by three bpy groups, which
Figure 3. The X-ray crystal structure of 2. TfOÀ anions, free solvents,
and hydrogen atoms of H2O coordinating to ZnII are omitted for
clarity. Three different kinds of ZnII centers bound by bpy groups are
colored in purple (Zn4), orange (Zn2), and green (Zn3). Zn1 centers
in the porphyrin rings are colored in gray; colors of the other atoms
are based on CPK coloring. a) and b) Views from a C2 symmetry axis
(a) and from a C3 symmetry axis (b). ZnII atoms and porphyrin rings
are shown as a space-filling model. The others are shown as a stick
model. c) Coordination geometries of three different kinds of bpy-ZnII
units. Hydrogen atoms are omitted for clarity.
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is denoted as cis-{Zn4(bpy)3}. Six Zn2 centers positioned in
the shoulder of 2 (orange) were coordinated by two bpy and
two H2O molecules, which is denoted as cis-{Zn2(bpy)2-
(H2O)2}. Three Zn3 ions in the middle (green) also formed
hydrated cis-{Zn3(bpy)2(H2O)2}, but relative positions of
pyridine rings at the edge of ligand 1 were in the trans
geometry around the Zn3 centers, while those of Zn2 units
were in the cis geometry. Each Zn1 of the porphyrin part of
1 was coordinated by one axial H2O ligand from outside of the
cage complex to form a square-pyramidal geometry in which
Zn1 was displaced by 0.14(2) ꢀ from the N4 plane. When
taken together, the stoichiometry of the cationic framework
of 2 is [Zn1116(H2O)18]22+. In terms of D-L configuration of the
metal centers in 2, cis-{Zn4(bpy)3} (purple) and cis-{Zn2-
(bpy)2(H2O)2} (orange) have the same optical isomerism,
while cis-{Zn3(bpy)2(H2O)2} (green) has the opposite isomer-
ism.
1
The H NMR spectrum of 2 was fully analyzed by 1H–1H
1
COSY and H–1H NOESY NMR spectroscopy, which indi-
cated that complex 2 is stable in solution and its basic
structure is the same as that obtained by X-ray analysis
(Figure 2c; Supporting Information, Figures S13–S16,
Table S1). In the NOESY measurement, several strong
diagnostic interligand NOE signals were observed for bis-
(bpy) ZnII units ((a2, a4), (g2, g4), (a3, g3)) and for several b
protons of porphyrin rings ((h2, g4), (h’2, h’1), (h’3, d3)).
Furthermore, proton signals with characteristic chemical
shifts (h2, f2, h’1, etc.) were well-explained by the shielding
and deshielding effects from aromatic rings of the neighbor-
ing ligands.
Figure 4. Unsymmetrical encapsulation of 2,7-dinitro-9-fluorenone (3)
inside triangular bipyramidal shaped cavity of the cage complex 2.
a) Visualization of the triangular bipyramidal shaped inner space of 2
surrounded by six Zn-porphyrin rings. The volume was calculated to
be 730 ꢀ3 (Connolly surface, probe radius: 4.0 ꢀ). b) Unsymmetrical
encapsulation of 2,7-dinitro-9-fluorenone (3) inside the cavity of 2. Up
to two p-acceptor guest molecules 3 were captured on the Zn-
1
porphyrin ring of the inner wall by p–p interactions. c) H NMR
H2O molecules play a vital role in constituting the
framework of [Zn1116(H2O)18]22+ in light of the fact that
24H2O molecules coordinate to 15ZnII centers in the crystal
state. Actually, a mixed solvent containing about 5% of H2O
facilitated the generation of complex 2, whereas the complex-
ation in non-aqueous CDCl3/CD3OD = 1:1 did not efficiently
proceed the formation of 2 (Supporting Information, Fig-
ure S37). This result suggests that H2O molecules act as
important stopper ligands in the ZnII-mediated self-assembly
process. The presence of H2O was also important for
stabilization of hydrated structures of complex 2 in the
solution. In hot CH3CN, complex 2 was converted into
a structural isomer in which D–L isomerism of three bis(bpy)
Zn3 units (depicted in green) in the middle of the cage were
reversed from that of 2, whereas in the presence of solvent
amount of H2O (CH3CN/H2O = 20:1 (v/v)), this conversion
was not observed and complex 2 remained stable (Supporting
Information, Figures S17–S29, Tables S2–S3). In summary,
the moderate coordination ability of H2O towards ZnII
centers allowed the formation of both tris(bpy) and hydrated
bis(bpy) ZnII units, leading to the unsymmetrical yet well-
defined self-assembled complex 2.
spectrum of guest-inclusion complexes [32ꢁ2] and [3ꢁ2] (500 MHz,
250 K, CD3CN). The ratio of [32ꢁ2]:[3ꢁ2] was 3.5:1. 1H NMR signals of
one side of 3 encapsulated in [32ꢁ2] at the edge of the cavity and
those of the other side are denoted with (i, j, k) and (i’, j’, k’),
respectively.
To confirm characteristic guest inclusion ability of cage 2,
encapsulation of 2,7-dinitro-9-fluorenone (3), a symmetrical
p-acceptor molecule exhibiting charge-transfer properties,[9a]
was examined. This guest molecule was captured unsym-
metrically within the triangular bipyramidal shaped cavity of
2, by p–p interactions with p-donor Zn-porphyrin on the
cavity wall (Figure 4b). Upon mixing guest 3 with cage 2 in
CD3CN, a host-guest complex [32ꢁ2] that encapsulates two
molecules of 3 was formed as a major species, as confirmed by
1D and 2D 1H NMR measurements at 250 K (Figure 4c;
1
Supporting Information, Figures S30–S36). H NMR signals
of encapsulated 3 of [32ꢁ2] appeared at d = 8.27, 6.28, 5.17,
À0.33, À0.66, and À1.36 ppm (2H value each), respectively.
The signals for one side of 3 ((i, j, k) = (À0.66, À1.36,
À0.33 ppm)) were greatly shifted upfield (Dd = À8.41 ꢀ
À9.90 ppm) from those in the absence of cage 2, which
indicates that only one side of 3 lay deep at the edge of the
cavity of 2 and was considerably affected by the shielding-
effect from Zn-porphyrin rings. In the NOESY measurement,
diagnostic NOE signals were observed between protons of
one side of encapsulated 3 and those of cage 2 at the edge of
inner cavity ((j, g1), (j, h’2), (k, h2)), which supports the
The [Zn1116(H2O)18]22+ framework of 2 (Figure 3a,b) has
a triangular bipyramidal shaped inner space as revealed by X-
ray analysis. Its cavity (ca. 9 ꢁ 18 ꢀ) is surrounded by six Zn-
porphyrin rings, and its volume was calculated to be about
730 ꢀ3 (Figure 4a). The cavity has three windows (d ꢀ 4 ꢀ) in
the middle of 2. With these features, this cavity could be used
for multiporphyrin-based functions.
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Information, Figure S34). These results also mean that the
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balanced aqueous condition. Such unsymmetrical encapsula-
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In conclusion, we have constructed a supramolecular
metal complex [Zn1116(H2O)18](OTf)22 (2) composed of
a combination of Zn(OTf)2 and a C4-symmetric, tetrakis(bi-
pyridyl) Zn-porphyrin ligand 1. Three different kinds of bpy-
ZnII units, one kind of tris(bpy), and two kinds of hydrated
bis(bpy) ZnII units, are formed stably to result in a discrete
cage complex with a well-defined shape and size. In this self-
assembly process, H2O molecules, which possess a moderate
coordination ability for ZnII, play vital roles in the formation
and stabilization of 2. Furthermore, the Zn-porphyrin hex-
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molecules 3 unsymmetrically within its triangular bipyramidal
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Received: October 5, 2012
Published online: November 19, 2012
Keywords: bipyridine · host–guest systems · porphyrins ·
.
self-assembly · supramolecular chemistry
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À0.40 eꢀÀ3
. See the Supporting Information for details.
CCDC 881253 (1) and 881254 (2) 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.
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