Design of flexible lewis acidic sites in porous coordination polymers by using the viologen moiety

Article abstract of DOI:10.1002/anie.201203834

Charged up: A novel porous coordination polymer with a charged organic surface (COS) comprising a zwitterionic organic linker, viologen, has been synthesized. The COS shows strong Lewis acidity accompanied with flexibility, and the isosteric heat of H₂ adsorption is 9.5 kJ mol^-1, which is comparable to that of open metal sites (OMSs). Copyright

Full text of DOI:10.1002/anie.201203834

Angewandte
Chemie
DOI: 10.1002/anie.201203834
Structure Elucidation
Design of Flexible Lewis Acidic Sites in Porous Coordination Polymers
by using the Viologen Moiety**
Masakazu Higuchi, Kohei Nakamura, Satoshi Horike, Yuh Hijikata, Nobuhiro Yanai,
Tomohiro Fukushima, Jungeun Kim, Kenichi Kato, Masaki Takata, Daisuke Watanabe,
Shinji Oshima, and Susumu Kitagawa*
Considerable effort has been devoted to the design of metal–
organic architectures and a variety of frameworks have
emerged through self-assembly processes involving metal
ions and organic linkers.[1] The synthesis of coordination
polymers with a channel structure, polymers which have been
called porous coordination polymers (PCPs) or metal–
organic frameworks (MOFs),[2,3] are of great interest because
of their unique functions, such as gas storage,[4] separation,[5]
and catalysis.[6,7] Among the PCPs, frameworks having Lewis
acidic sites have been highlighted because of their gas-
capturing properties[8] or catalytic activities.[7] The main
strategy for preparing the Lewis acidic sites is to introduce
open metal sites (OMSs). In contrast, we have achieved the
fabrication of charged organic surfaces (COSs) using a pyr-
idinium moiety in the porous framework.[9] Because the guest-
accessible interior of the pore is mainly organized by the
organic moiety, these COSs interact effectively with guest
molecules. Another noteworthy point of the introduction of
COSs is flexibility, which is based on the flexible nature of the
PCP framework.[3,10] PCPs often show a flexible contraction/
Scheme 1. Synthesis of 1ꢀ0.5DMF·3.5H2O.
expansion of the framework through guest accommodation,
and if we could incorporate COSs onto the flexible network,
the obtained framework would show induced-fit capture of
guests at the COSs, a process which is difficult to achieve with
the use of OMSs. For the purpose of the construction of
flexible COSs, we employed a viologen derivative as an
organic linker because of its intrinsic Lewis acidity[11] and the
dynamic motion of the aromatic rings.[12] Herein we report the
synthesis of a PCP bearing a viologen motif, the strength of
the Lewis acidity, and the adsorption properties.
[*] Dr. M. Higuchi, Prof. Dr. S. Kitagawa
Institute for Integrated Cell-Material Sciences, Kyoto University
69 Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501 (Japan)
E-mail: kitagawa@icems.kyoto-u.ac.jp
The reaction of Zn(NO3)2·6H2O with 1,4-naphthalenedi-
carboxylic acid (1,4-H2ndc) and 1,1-bis(4-carboxybenzyl)-
4,4’-bipyridinium bis(hexafluorophosphate) (H2bcbpy·2PF6)
in N,N’-dimethylformamide (DMF) affords the PCP {[Zn(1,4-
K. Nakamura, Dr. S. Horike, Dr. Y. Hijikata, Dr. N. Yanai,
T. Fukushima, Prof. Dr. S. Kitagawa
Department of Synthetic Chemistry & Biological Chemistry
Graduate School of Engineering, Kyoto University
Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
ndc)(bcbpy)]·(0.5DMF)(3.5H2O)}n
(1ꢀ0.5DMF·3.5H2O)
(Scheme 1). The bcbpy, which is a zwitterionic ligand, acts
as a neutral organic linker and 1ꢀ0.5DMF·3.5H2O does not
include any counter anions. The crystal structure of
1ꢀ0.5DMF·3.5H2O was determined by single-crystal X-ray
crystallography at 223 K. The Zn2+ ion is tetrahedrally
coordinated by two 1,4-ndc ligands and two bcbpy ligands
(Figure 1a) to give two-dimensional (2D) interpenetrated
layers along the ac plane (Figure 1b). The 2D layers are of the
44-sql topology (see Figure S1 in the Supporting Information).
The interpenetrated 2D layers are stacked along the b axis to
form a 3D structure because of the p–p interaction between
the 1,4-ndc and the viologen moiety of the bcbpy (Figure 1c).
The 1ꢀ0.5DMF·3.5H2O possesses 1D channels along the
c axis with a cross-section of 4.7 ꢀ 4.1 ꢁ2 (Figure 1d). The
0.5DMF and 3H2O sit in the cavity and the 0.5H2O is
between the 2D sheets. The pore surface is formed by bcbpy
and 1,4-ndc ligands (Figure 1e). The carbonyl oxygen atom of
the DMF is located 2.48 and 2.64 ꢁ from the two a-hydrogen
Dr. J. Kim, Prof. Dr. M. Takata
Japan Synchrotron Radiation Research Institute, RIKEN
1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198 (Japan)
Dr. K. Kato, Prof. Dr. M. Takata
Spring-8 Center, RIKEN
1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148 (Japan)
D. Watanabe, S. Oshima
Hydrogen & New Energy Research Laboratory, Research &
Development Division, JX Nippon Oil & Energy Corporation
8, Chidoricho, Naka-ku, Yokohama 231-0815 (Japan)
[**] The synchrotron radiation experiments were performed at the
BL44B2 in the SPring-8 with the approval of the RIKEN (Proposal
No. 20090066). This work was supported by the JX Nippon Oil &
Energy Corporation, ERATO “Kitagawa Integrated Pores Project” of
the Japan Science and Technology Agency (JST), and the New
Energy and Industrial Technology Development Organization
(NEDO).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203834.
À
atoms of the viologen moiety (Figure 2a), and it forms a C
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Table 1: The chemical shift of acetone carbon atom 13C NMR resonance
=
and infrared n(C O) band position of acetone absorbed in different PCPs
together with those of acetone.
Interactive
site
Acetone as probe
À1
=
d [ppm]
n(C O) [cm
]
pure acetone (gas)
HKUST-1ꢀacetone
PCP(Na+)ꢀacetone[a]
MOF-74ꢀacetone
1ꢀacetone
–
206
–
212.1
213.6
214.2
217.1
1731
1708
1699
–
1695
1690
[c]
OMS (Cu2+
)
OMS (Na+)
OMS (Zn2+
)
)
)
[d]
COS (Vio2+
COS (Vio2+
[b]
[b]
1ꢀ0.5acetone·3.5H2O
[a] PCP(Na+):[CdNa(2-stp)(dabco)0.5]. 2-stp=2-sulfonylterephthalate,
dabco=1,4-diazabicyclo[2,2,2]octane. [b] Charged organic sites by di-
cationic viologen. [c] Not detected because of paramagnetic CuII in
HKUST-1. [d] Not observed. Vio=viologen.
carbonyl carbon atom 13C NMR resonance of 1ꢀ0.5aceto-
ne·3.5H2O appeared at d = 217.1 ppm, which is downfield
from its normal resonance position at d = 206 ppm (Table 1).
The observed downfield shift is larger than that of MOF-
74(Zn2+)16 (d = 213.6 ppm) and PCP(Na+) (d = 212.1 ppm),[15]
both of which have OMSs on the pore surface. In addition, in
=
the infrared spectra of 1ꢀ0.5acetone·3.5H2O, the n(C O)
band of acetone was observed at n˜ = 1690 cmÀ1. This band
position has a shift that is Dn˜ = 41 cmÀ1 lower than its value of
n˜ = 1731 cmÀ1 when in pure acetone gas.[17] This shift also
=
indicates that the C O bond of acetone is weakened by the
=
hydrogen bonding between bcbpy and acetone. This n(C O)
band at n˜ = 1690 cmÀ1 is lower than that of HKUST-1(Cu2+
)
[18]
(n˜ = 1708 cmÀ1) and PCP(Na+) (n˜ = 1699 cmÀ1). These results
demonstrate that 1 has sufficient Lewis acidic sites, probably
Figure 1. a) Crystallographic environment around the Zn2+ ion.
b) Interpenetrated 2D sheet. c) 2D sheets stacking along the b axis.
d) 3D assembled structure of 1ꢀ0.5DMF·3.5H2O. e) Pore surface
around the DMF guest. Hydrogen atoms and guests in the pore are
omitted.
À
because of the involvement of the oxygen atom with two C H
groups in the viologen ligand which are comparable to those
based on OMSs. To obtain information on the charged state of
the viologen compounds, Raman spectra were measured for
1ꢀ0.5DMF·3.5H2O and 1ꢀ0.5acetone·3.5H2O. Together
with the n˜ = 1299 cmÀ1 obtained for H2bcbpy·2PF6, the
Raman shifts at n˜ = 1303 cmÀ1 for 1ꢀ0.5DMF·3.5H2O and
1ꢀ0.5acetone·3.5H2O show the dication form of the viologen
moiety (see Figure S4 and Table S1 in the Supporting
Information).[19]
To observe directly the flexibility of the Lewis acidic site
in the framework, the crystal structure of 1ꢀ0.5aceto-
ne·3.5H2O was determined by single-crystal X-ray crystallog-
raphy at 278 K. The carbonyl oxygen atom of acetone is
located 2.43 ꢁ from the two a-hydrogen atoms of the
Figure 2. Interaction between the viologen part of bcbpy and DMF in
1ꢀ0.5DMF·3.5H2O (a) and acetone in 1ꢀ0.5acetone·3.5H2O (b).
H···O hydrogen bond.[13] Thermogravimetric analysis (TGA)
performed on 1ꢀ0.5DMF·3.5H2O showed a weight loss of
12.4% from 298 to 500 K (see Figure S2). This loss corre-
sponds to the loss of 0.5DMF and 3.5H2O per formula unit
(calc. 12.4%).
To investigate the Lewis acidity of the pore surface, we
employed acetone as a probe guest. The 13C isotropic
chemical shift of carbonyl groups in acetone is sensitive to
the interaction of the carbonyl oxygen with Brønsted and
Lewis acids.[14,15] When the crystal of 1ꢀ0.5DMF·3.5H2O was
immersed in acetone, acetone was exchanged with DMF to
form 1ꢀ0.5acetone·3.5H2O. The chemical shift of the acetone
viologen moiety, and results from C H···O hydrogen bonding,
À
as in the case of 1ꢀ0.5DMF·3.5H2O (Figure 2). The exchange
of guests affords two notable changes in the structure of
1ꢀguests. First, the distance between the two a-hydrogen
atoms is 4.29 ꢁ in 1ꢀ0.5acetone·3.5H2O, a distance which is
larger than that for 1ꢀ0.5DMF·3.5H2O (3.73 ꢁ). This change
corresponds to the elongation of the interpenetrated 2D sheet
along the a axis. Second, the dihedral angle of the two pyridyl
rings of the viologen moiety in 1ꢀ0.5acetone·3.5H2O (61.18)
is larger than that in 1ꢀ0.5DMF·3.5H2O (45.78), and thus
promotes shortening of the distance between the stacked 2D
sheets and the pore diameter of the 1D channels along the
2
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b axis. These structural reorientations suggest that the two
bcbpy ligands relocate to capture the acetone molecule
suitably with an accompanying rotation of the viologen
moiety. Similar behavior in the framework was observed at
298 K by an analysis of the synchrotron X-ray powder
diffraction (see Figure S6 and Table S3 in the Supporting
Information).[20]
We also investigated the gas adsorption property of guest-
free 1 (see Figure S5 in the Supporting Information). After
a degassing treatment of 1ꢀ0.5acetone·3.5H2O at 463 K for
12 hours under vacuum, the N2 sorption isotherm at 77 K for
1 was examined. The adsorption isotherm of nitrogen exhibits
Type-I behavior, thus indicating that 1 maintains permanent
microporosity. This result corresponds to the cell volume of
1 revealed by the analysis of X-ray powder diffraction. 1 also
adsorbs gas-phase acetone from the low pressure region at
298 K, thus suggesting a strong interaction between the
acetone and the viologen framework. For acetone absorbed in
1 (1ꢀacetone), a 13C chemical shift for the carbonyl carbon
À
Figure 4. Difference IR spectra in the H H stretching region for
hydrogen adsorbed on 1 at 99 K in the pressure range as shown in the
figure. The IR spectrum at 0.5 kPa of H2 pressure is used as the blank
because of a drastic change of spectra from 0 to 0.5 kPa (see Figure S7
in the Supporting Information).
finally tending toward 7.5 kJmolÀ1 at 0.4 wt% (Figure 3b).
These relatively high values of 9.5–8.5 kJmolÀ1 at low cover-
age of 0–0.1 wt% are comparable to those of OMSs[21] despite
the organic surface of 1. To obtain information on H2
adsorbed on 1, we carried out variable-temperature infrared
(VTIR) measurements below 99 K and at H2 pressures
ranging from 0 to 500 kPa (Figure S8). The VTIR spectra of
H2 adsorbed on microporous materials can be very informa-
tive in explaining and differentiating the active sites on the
interior surface. The difference VTIR spectra mainly show
three peaks at n˜ = 4112, 4095, 4088 cmÀ1 (Figure 4) and no
other peaks were observed in the region of n˜ = 4040–
4000 cmÀ1. This data indicates that there is no chemisorptive
site in 1.[22] The sharp doublet at n˜ = 4095 and 4088 cmÀ1 is
ascribed to para- and ortho-H2, respectively, based on
previous reports.[22,23] These values are compared with those
for PCPs with OMSs such as Mg-MOF-74 (n˜ = 4091,
4085 cmÀ1) and for alkali-cation-exchange zeolites such as
Li-FER (n˜ = 4090 cmÀ1). Furthermore they are a lower fre-
quency shift than that for PCPs with OMSs such as HKUST-
=
atom resonance of d = 214.2 ppm and n(C O) band at n˜ =
1695 cmÀ1 were obtained. These data lead us to the conclusion
that the degassed form of 1 maintains strong Lewis acidic sites
originating from the viologen sites. The lack of change in the
Raman shift associated with the viologen ligand of 1ꢀacetone
shows that the viologen moiety of the bcbpy ligand remains in
the dication form.
Furthermore to assess the heat of H2 adsorption for 1,
which bears the flexible Lewis acidic site, H2 adsorption
isotherms were also investigated at 77 K, 82 K, and 87 K
(Figure 3). These data were fitted using a virial-type equation
(see Figure S7 in the Supporting Information). The heat of
adsorption was found to be 9.5 kJmolÀ1 at zero coverage,
1 (n˜ = 4097, 4090 cmÀ1) and Cr3(btc)2 (n˜ = 4116, 4110 cmÀ1
;
see Table S4 in the Supporting Information). This compar-
ison, the lack of an exposed Zn2+ in 1, and the result of the
NMR and IR (Table 1) spectra allow the assignment of this
À
doublet to para- and ortho-H2 adsorbed at two of the C H
groups in the viologen ligand. The other peaks from ranging
from n˜ = 4130 to 4100 cmÀ1 (peak top: 4112 cmÀ1 at 500 kPa)
apparently show a lower shift than that for MOF-5 (peak top:
4130 cmÀ1) and HKUST-1 (from 4150 to 4120 cmÀ1; Table S4),
and is assigned to peaks adsorbed on the organic moiety in
those frameworks. This result indicates that the organic
À
moiety, except for two C H groups in the viologen ligand, can
also perturb H2 more than neutral organic parts such as
terephthalate in MOF-5 and trimesate in HKUST-1. There-
fore we can conclude that flexible COSs created by a viologen
moiety provide Lewis acidic sites and wide surfaces, which can
be effective in strongly adsorbing and perturbing H2, and is
thus comparable to OMSs. The intensities of bands at n˜ = 4112
and 4088 cmÀ1 have been measured as a function of temper-
ature (T), and standard enthalpy (DH0) and entropy (DS0)
were calculated to be À5.2 kJmolÀ1 and À87 JmolÀ1 KÀ1,
~
*
Figure 3. a) H2 adsorption isotherms for 1 at 77 K( ), 82 K( ),
&
87 K( ). b) Isosteric heat of H2 adsorption (Qst) for 1.
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Published online: && &&, &&&&
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Keywords: hydrogen · Lewis acids · metal–organic frameworks ·
polymers · structure elucidation
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4
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Communications
Structure Elucidation
M. Higuchi, K. Nakamura, S. Horike,
Y. Hijikata, N. Yanai, T. Fukushima, J. Kim,
K. Kato, M. Takata, D. Watanabe,
S. Oshima, S. Kitagawa*
&&&& — &&&&
Design of Flexible Lewis Acidic Sites in
Porous Coordination Polymers by using
the Viologen Moiety
Charged up: A novel porous coordination
polymer with a charged organic surface
(COS) comprising a zwitterionic organic
linker, viologen, has been synthesized.
The COS shows strong Lewis acidity
accompanied with flexibility, and the
isosteric heat of H2 adsorption is
9.5 kJmolÀ1, which is comparable to that
of open metal sites (OMSs).
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!