Edge-directed [(M2)2L4] tetragonal metal-organic polyhedra decorated using a square paddle-wheel secondary building unit

Article abstract of DOI:10.1039/b925335a

A general strategy was developed for edge-directed self-assembly of tetragonal metal-organic polyhedra (MOPs) having a C₄ symmetry Cu^II₂(COO)₄ paddle-wheel as a secondary building unit, using C₂ symmetric dicarboxylic ligands as pincer-type primary building units. The Royal Society of Chemistry.

Full text of DOI:10.1039/b925335a

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Edge-directed [(M2)2L4] tetragonal metal–organic polyhedra decorated
using a square paddle-wheel secondary building unitw
M. Jaya Prakash, Minhak Oh, Xinfang Liu, Kwi Nam Han, Gi Hun Seong and
Myoung Soo Lah*
Received (in Cambridge, UK) 1st December 2009, Accepted 28th January 2010
First published as an Advance Article on the web 15th February 2010
DOI: 10.1039/b925335a
A general strategy was developed for edge-directed self-assembly
of tetragonal metal–organic polyhedra (MOPs) having a C4
symmetry CuII2(COO)4 paddle-wheel as a secondary building
unit, using C2 symmetric dicarboxylic ligands as pincer-type
primary building units.
separated manually under an optical microscope (Fig. S1w).
MOP-1a, [(Cu2)2L1 (DMF)4], is composed of individual
4
tetragonal-shaped cages, and each cage is composed of four
bidentate ligands and two Cu2(COO)4 paddle-wheel moieties
(Fig. 1b).z The two carboxylate groups in the ligand are on the
same side to serve as a pincer-type ligand, and the ligand serves
as an edge of the tetragonal cage. In MOP-1a, four ligands are
connected with two paddle-wheel moieties, with each paddle-
wheel moiety coordinated with two DMF solvent molecules.
The outer dimension of the cage between the phenyl residues
of the ligands on opposite sides is B24 A, and the dimension
of the cage between the metal ions of the paddle-wheel SBUs is
B17 A (Fig. S2w). The window of the cage formed by two
ligands is B7.1 A wide and B6.4 A high (Fig. S3w). The
dimensions of the inner cavity of the MOP are B11 A across
the phenyl groups and B5 A between the metal ions of the
paddle-wheel SBUs (Fig. S4w).
Two different types of inter-cage p–p stacking interactions
have been observed (Fig. S5 and Table S4w). Type-I inter-
action occurs bilaterally between the terminal phenyl groups
of the ligands; type-II interaction also occurs bilaterally, but
between the terminal phenyl group and the ethynyl group of
the ligands. The alternating interactions between the cages
lead to 1D chains (Fig. S6w), which in turn interact with
neighboring chains via van der Waals interactions only to
form a 2D sheet (Fig. S7w); the 2D sheets are further stacked
into a 3D packing structure (Fig. S8w). Even though the
tetragonal MOPs in the crystal are compactly arranged,
the rigidity of the ligands in the corresponding MOP and the
intercage p–p stacking interactions between the MOPs render
solvent space in the crystal packing structure. In the crystal
structure of MOP-1a, the solvent cavity volume of 1117 A3 per
unit cell represents B20.6% of the whole crystal volume.
Metal–organic polyhedra (MOPs) are a class of metal–organic
systems developed for various potential applications including
recognition and storage,1 catalysis,2 delivery, and synthetic
membranes for ionic channels.3 MOPs of various high
symmetries have been reported, such as O, T, or I symmetries.4
The MOPs are developed by two strategies, edge-directed
linear components connected at the corners5 and face-directed
facial components connected at the edges6 or at the corners.7
Some MOPs with tetragonal symmetry have been reported in
the literature, where C2 symmetric bidentate ligands with CuII,
PdII, CoII, PdII or MnII metal ions to form edge-directed8,9
or C4 symmetric tetradentate ligand with RhIV to form
2
face-directed tetragonal systems.10 Even though several highly
symmetric decorated MOPs, such as cuboctahedral11 or
octahedral12 cages, have been prepared using a secondary
building unit approach, there are only a few reports of low
symmetry decorated cage systems, such as an augmented
tetragonal MOP.9c,10
Here we report preparations of tetragonal MOPs decorated
with a Cu2(COO)4 paddle-wheel secondary building unit
(SBU), where primary building units, C2 symmetric pincer-type
ditopic ligands, self-assemble with CuII to generate a C4
symmetric Cu2(COO)4 paddle-wheel as a tetratopic SBU
(Fig. 1a). Each ligand occupying the edge of the tetragonal
cage connects to two square-planar Cu2(COO)4 paddle-wheels
to form an augmented tetragonal MOP.
A
solvothermal reaction of 3,30-[1,3-benzenediyldi-
(ethynyl)]dibenzoic acid (H2L1) with Cu(NO3)2ꢀ3H2O in
N,N0-dimethylformamide (DMF) resulted in the mixture of
crystals having two slightly different morphologies, block-shaped
greenish-blue crystals as a pseudo-polymorph (MOP-1a) as a
major form and rectangle-shaped blue crystals as another
pseudo-polymorph (MOP-1b) as a minor form, which were
Department of Chemistry and Applied Chemistry,
Hanyang University, Ansan, Kyunggi-do, 426-791, Korea.
E-mail: mslah@hanyang.ac.kr; Fax: +82 31 436 8100;
Tel: +82 31 400 5496
w Electronic supplementary information (ESI) available: Experimental
procedure, crystallographic details. CCDC 744143–744145. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b925335a
Fig. 1 (a) A reaction scheme for the self-assembly of augmented
tetragonal MOPs. (b) An ORTEP representation of the one cage unit
of MOP-1a with 20% of thermal ellipsoid probability displacement.
Hydrogen atoms were omitted for clarity.
ꢁc
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Similar to MOP-1a, the crystals of MOP-1b
([(Cu2)2L1 (DMF)4]) are also composed of isostructural
4
tetragonal-shaped cages in crystallographic inversion centers
(Fig. S9w). The cage itself in MOP-1b is chemically identical to
that of MOP-1a but has a higher symmetry imposed by the
crystallographic restraint.
The cages in MOP-1b are also involved in an inter-cage p–p
stacking interaction to form 1D chains, where only the type-I
interaction is utilized (Fig. S10 and Table S4w). The 1D chains
in MOP-1b interact with the neighboring chains to form 2D
sheets via van der Waals interactions (Fig. S11(a)w), and then
form a 3D packing structure (Fig. S11(b)w). A slightly different
packing in the MOP-1b leads to the slightly different solvent
cavity volume of 3179 A3 per unit cell, which represents
B26.6% of the whole crystal volume.
Fig. 2 Gas sorption isotherms of MOP-1a. Filled symbols represent
adsorption isotherms, and empty symbols represent desorption
isotherms.
from that of the as-synthesized MOP-1a, there is some
similarity between the PXRD pattern of the activated
MOP-1b and that of the activated MOP-1a (Fig. S15w), which
suggests that both the structures transform to the unknown
but similar polymorphic form in their packing structures when
they are activated, which might lead to the similar gas sorption
behaviors.
The powder X-ray diffraction (PXRD) pattern of the bulk
sample of MOP-1a resembles the simulated pattern obtained
from the single-crystal structure of MOP-1a (Fig. S12w). The
activated sample of MOP-1a, which was prepared by soaking
in DMF, methanol, and methylene chloride and then by
vacuum-drying overnight at 90 1C, showed a PXRD pattern
different from that of the as-synthesized material. The
substitution of the ligated solvent DMF to methanol and/or
the removal of the guest solvents from the solvent cavity
lead to a structural transformation of MOP-1a to another
polymorphic form. This transformation is irreversible: the
PXRD pattern of the activated sample resoaked in fresh
DMF solvent was completely different from those of both
the as-synthesized and activated samples.
Because of the limited solubility of both MOP-1a and
MOP-1b in aqueous and ordinary organic solvents, we have
introduced the methoxy group into the ligand to improve the
solubility of the MOPs. We prepared another similar kind of
tetragonal-cage MOP-2, ([(Cu2)2L2 (DMF)4]), using 3,30-[1,3-
4
benzenediyldi(ethynyl)]bis(4-methoxy)benzoic acid (H2L2) as
a methoxy-derivatized ligand. The solvothermal reaction of
H2L2 with Cu(NO3)2ꢀ3H2O in the DMF + CH3CN (1 : 1)
solvent mixture resulted in blue, block-shaped crystals. The
tetragonal cage in the MOP-2 is very similar to those described
in the MOP-1a and MOP-1b structures, except that there are
two additional methoxy groups per ligand (Fig. S16w). As in
MOP-1a and MOP-1b, a type-I inter-cage p–p stacking
interaction between the tetragonal cages in the MOP-2
structure leads to 1D chains (Fig. S17 and Table S4w). The
1D chains interact with the neighboring chains via a type-III
inter-cage p–p stacking interaction to form a 2D sheet
(Fig. S18w) based on the two different kinds of inter-cage
p–p stacking interactions, and 3D packing is accomplished via
weak van der Waals interaction between the 2D sheets
(Fig. S19w). The solvent cavity of the MOP-2 crystal takes
39.1% of the whole crystal volume (2884 A3 per unit cell),
which is a significantly larger proportion than in MOP-1a and
MOP-1b crystals. The phase purity of MOP-2 was confirmed
by comparing the PXRD pattern of the bulk sample with that
simulated from the single crystal structure (Fig. S20w). The
activation of MOP-2 causes a loss in crystallinity and leads to
the gas sorption behavior of the nonporous material.
The PXRD pattern of the bulk MOP-1b was different from
that of bulk MOP-1a, which confirmed the packing of the
MOPs in the two pseudo-polymorphs differed. The PXRD
pattern of the bulk MOP-1b matched the simulated PXRD
pattern from the single crystal structure of MOP-1b with [010]
preferred sample orientation rather than with random sample
orientation (Fig. S13w).
The sorption behaviors of the activated MOP-1a, which
had been vacuum-dried at 120 1C after soaking in DMF,
methanol, and methylene chloride, have been studied for N2
and H2 at 77 K and for CO2 and CH4 at 195 K (Fig. 2). The N2
sorption isotherm did not indicate any appreciable amount of
adsorption, which strongly suggested that the activated
MOP-1a did not have any accessible pore for N2 gas. The
H2 and CH4 sorption behaviors were also similar to N2, and
only a very small amount of gas sorption was observed.
However, the CO2 sorption isotherm was type-1 with
347 cm3 gꢂ1 capacity at 195 K and 1 bar. We speculate that
the activation of MOP-1a might lead to a structure having an
accessible pore with a pore window that only allows the CO2
gas molecule. We speculate that even though the window size
of the tetragonal cage unit is large enough to allow all the
gas molecules to be adsorbed, the activation of MOP-1a
might lead to a packing structure having the window of the
tetragonal cage partially blocked by adjacent cages and the
adjusted window size only suitable to CO2 gas molecule to
penetrate.
The solution integrity of MOP-2 could be investigated using
nuclear magnetic resonance (NMR) spectroscopy in DMF
solvent, because the solubility of the complex could be
improved by the introduction of methoxy groups into the
1
ligand. MOP-2 in d7-DMF gave broad peaks in the H-NMR
spectrum because of the electronic effect of the paramagnetic
CuII ion, which contrasts the sharp peaks in the 1H-NMR
spectrum of the free ligand (Fig. S21w). Two pairs of singlet
and multiplet peaks between 2.7 and 3 ppm and a peak at
The sorption behaviors of the activated MOP-1b (Fig. S14w)
are similar to those of the activated MOP-1a. Although the
PXRD pattern of the as-synthesized MOP-1b was different
ꢁc
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around 8.0 ppm were attributable to DMF molecules in two
different environments, which indicated that the ligated DMF
molecules were not replaced by the solvent DMF molecules in
the NMR time scale. Similar behavior has been observed for
other CuII complexes, such as [CuII2(Indo)4(DMF)2] (HIndo:
Indomethacin [1-(4-chlorobenzoyl)-5-methoxy-2-methyl-1H-
indole-3-acetic acid), containing a Cu2(COO)4 paddle-wheel
unit with ligated DMF molecules.13 Even though there are six
chemically different aromatic protons and methyl protons of
the methoxy group in the complex, only three broad peaks
were observed for the aromatic protons at 9.66, 8.30 and
7.33 ppm, and one for the methyl protons of the methoxy
group at 3.65 ppm. The reduction in the number of observed
peaks around the aromatic region might be attributed to the
extreme broadening of some proton peaks by the influence of
paramagnetic CuII centers.
l = 0.72999 A) = 0.847 mmꢂ1, 67 393 reflections were collected,
33 839 were unique [Rint = 0.0571]. R1(wR2) = 0.0830 (0.2427) for
27 707 reflections [I 4 2s(I)], R1(wR2) = 0.0911 (0.2534) for all
reflections. Crystal data for MOP-1b: [(Cu2)L12(DMF)2]
(C54H38N2O10Cu2), fw = 1001.94 g molꢂ1, monoclinic, space group
C2/c, a = 11.245(2) A, b = 37.235(7) A, c = 28.875(6) A,
b
= 98.01(3)1, V = Z = 8, m (synchrotron,
11972(4) A3,
l = 0.75000 A) = 0.766 mmꢂ1, 53 617 reflections were collected,
14 391 were unique [Rint = 0.0864]. R1(wR2) = 0.0815 (0.2357) for
12 173 reflections [I 4 2s(I)], R1(wR2) = 0.0900 (0.2452) for all
reflections. Crystal data for MOP-2: [(Cu2)L22(DMF)2]ꢀ3.25DMFꢀ
2.25MeCNꢀ0.5H2O (C72.25H76.50N7.50O17.75Cu2), fw = 1460.99 g molꢂ1
A,
,
monoclinic,
space
group
P21/n,
a
=
13.187(3)
b = 29.985(6) A, c = 18.713(4) A, b = 93.93(3)1, V = 7382(3) A3,
Z = 4, m (synchrotron, l = 0.75000 A) = 0.648 mmꢂ1, 61 282
reflections were collected, 19 718 were unique [Rint = 0.0464].
R1(wR2) = 0.0716 (0.2019) for 16 424 reflections [I 4 2s(I)],
R1(wR2) = 0.0829 (0.2153) for all reflections.
1 (a) S. Tashiro, M. Tominaga, M. Kawano, B. Therrien, T. Ozeki
and M. Fujita, J. Am. Chem. Soc., 2005, 127, 4546; (b) A. V. Davis
and K. N. Raymond, J. Am. Chem. Soc., 2005, 127, 7912;
(c) P. Mal, B. Breiner, K. Rissanen and J. R. Nitschke, Science,
2009, 324, 1697.
2 (a) M. D. Pluth, R. G. Bergman and K. N. Raymond, Science,
2007, 316, 85; (b) M. Yoshizawa, M. Tamura and M. Fujita,
Science, 2006, 312, 251.
Although the NMR spectrum of MOP-2 indicated that the
ligand and some solvent DMF molecules were engaged with
the Cu2(COO)4 paddle-wheel unit, the information regarding
the solution integrity of MOP-2 was limited, and we have
taken the atomic force microscopy (AFM) image of MOP-2 in
DMF. Discrete MOP molecules were observed as individual
particles on a mica sheet with an average height of B1.8 nm,
which is smaller than the approximate dimension of the MOP
from single crystal structure analysis, 2.4 nm in diameter
(Fig. S22w). Height contraction of a similar extent (B30%)
was observed for the other MOPs in the AFM images, in the
tapping mode of scanning the samples.7b,c
3 M. Jung, H. Kim, K. Baek and K. Kim, Angew. Chem., Int. Ed.,
2008, 47, 5755.
4 S. R. Seidel and P. J. Stang, Acc. Chem. Res., 2002, 35, 972.
5 (a) D. L. Caulder, R. E. Powers, T. N. Parac and K. N. Raymond,
Angew. Chem., Int. Ed., 1998, 37, 1840; (b) M. Tominaga,
K. Suzuki, M. Kawano, T. Kusukawa, T. Ozeki, S. Sakamoto,
K. Yamaguchi and M. Fujita, Angew. Chem., Int. Ed., 2004, 43,
5621; (c) Y. Liu, V. C. Kravtsov, D. A. Beauchamp, J. F. Eubank
and M. Eddaoudi, J. Am. Chem. Soc., 2005, 127, 7266.
6 (a) N. Takeda, K. Umemoto, K. Yamaguchi and M. Fujita,
Nature, 1999, 398, 794; (b) K. Umemoto, H. Tsukui,
T. Kusukawa, K. Biradha and M. Fujita, Angew. Chem., Int.
Ed., 2001, 40, 2620; (c) B. Olenyuk, J. A. Whiteford,
A. Fechtenkotter and P. J. Stang, Nature, 1999, 398, 796.
We have demonstrated the preparation of the augmented
tetragonal MOPs using pincer-type C2 symmetric ditopic
dicarboxylic ligands. The pincer-type ligands self-assembled
with CuII ions to form tetragonal MOPs having two square-
planar tetratopic Cu2(COO)4 paddle-wheels as a secondary
building unit and the rigid bent linkage moieties of the ligands
as edges of the tetragonal cage. Depending on the crystal
structures, the tetragonal MOPs are differently arranged by
use of various kinds and combinations of p–p stacking
interactions between the conjugated moieties of the ligands
in addition to van der Waals interactions. The different
packings lead to different proportions of solvent cavities in
the crystal structures. The guest removal from the cavities
caused irreversible structural transformations to an unknown
crystalline or amorphous phase. The microporosity of the
activated MOPs was demonstrated in MOP-1a and MOP-1b,
whereas MOP-2 did not show any gas sorption. The solution
integrity of MOP-2, which has improved the solubility
property compared with MOP-1a and MOP-1b with no
methoxy residues, was characterized using NMR spectroscopy
and further supported by AFM studies.
7 (a) C. Bruckner, R. E. Powers and K. N. Raymond, Angew. Chem.,
¨
Int. Ed., 1998, 37, 1837; (b) D. Moon, S. Kang, J. Park, K. Lee,
R. P. John, H. Won, G. H. Seong, Y. S. Kim, G. H. Kim, H. Rhee
and M. S. Lah, J. Am. Chem. Soc., 2006, 128, 3530; (c) J. Park,
S. Hong, D. Moon, M. Park, K. Lee, S. Kang, Y. Zou, R. P. John,
G. H. Kim and M. S. Lah, Inorg. Chem., 2007, 46, 10208;
(d) T. K. Ronson, J. Fisher, L. P. Harding and M. J. Hardie,
Angew. Chem., Int. Ed., 2007, 46, 9086; (e) Z. Lu, C. B. Knobler,
H. Furukawa, B. Wang, G. Liu and O. M. Yaghi, J. Am. Chem.
Soc., 2009, 131, 12532.
8 (a) L. J. Barbour, G. W. Orr and J. L. Atwood, Nature, 1998, 393,
671; (b) D. A. McMorran and P. J. Steel, Angew. Chem., Int. Ed.,
1998, 37, 3295; (c) N. L. S. Yue, D. J. Eisler, M. C. Jennings and
R. J. Puddephatt, Inorg. Chem., 2004, 43, 7671; (d) Z.-M. Liu,
Y. Liu, S.-R. Zheng, Z.-Q. Yu, M. Pan and C.-Y. Su, Inorg. Chem.,
2007, 46, 5814; (e) B. Wu, D. Yuan, B. Lou, L. Han, C. Liu,
C. Zhang and M. Hong, Inorg. Chem., 2005, 44, 9175.
9 (a) C.-Y. Su, Y.-P. Cai, C.-L. Chen, M. D. Smith, W. Kaim and
H.-C. Z. Loye, J. Am. Chem. Soc., 2003, 125, 8595; (b) C.-Y. Su,
Y.-P. Cai, C.-L. Chen, H.-X. Zhang and B.-S. Kang, J. Chem.
Soc., Dalton Trans., 2001, 359; (c) F. Dai, H. He, A. Xie, G. Chu,
D. Sun and Y. Ke, CrystEngComm, 2009, 11, 47.
This work was supported by KRF (Grant 2008-
313-C00424), NRF (Grants 2009-0084799 and 2009-0090894),
and CHBM. The authors also acknowledge PAL for beam line
use (2009-2063-03).
10 F. A. Cotton, P. Lei, C. Lin, C. A. Murillo, X. Wang, S.-Y. Yu and
Z.-X. Zhang, J. Am. Chem. Soc., 2004, 126, 1518.
11 (a) M. Eddaoudi, J. Kim, J. B. Wachter, H. K. Chae, M. O’Keeffe
and O. M. Yaghi, J. Am. Chem. Soc., 2001, 123, 4368;
(b) H. Furukawa, J. Kim, K. E. Plass and O. M. Yaghi, J. Am.
Chem. Soc., 2006, 128, 8398; (c) H. Furukawa, J. Kim, N. W. Ockwig,
M. O’Keeffe and O. M. Yaghi, J. Am. Chem. Soc., 2008, 130, 11650.
12 (a) Z. Ni, A. Yassar, T. Antoun and O. M. Yaghi, J. Am. Chem. Soc.,
2005, 127, 12752; (b) M. J. Prakash, Y. Zou, S. Hong, M. Park,
M.-P. N. Bui, G. H. Seong and M. S. Lah, Inorg. Chem., 2009, 48, 1281.
13 S. Ramadan, T. W. Hambley, B. J. Kennedy and P. A. Lay, Inorg.
Chem., 2004, 43, 2943.
Notes and references
z Crystal data for MOP-1a: [(Cu2)2L14(DMF)4] (C114H90N6O22Cu4),
fw = 2150.08 g molꢂ1, triclinic, space group P1, a = 17.475(4) A,
ꢀ
b = 18.723(4) A, c = 18.786(4) A, a = 82.05(3)1, b = 62.82(3)1,
g
= 87.67(3)1, V = Z = 2, m (synchrotron,
5414.3(19) A3,
ꢁc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 2049–2051 | 2051