In situ generation of functionality in a reactive haloalkane-based ligand for the design of new porous coordination polymers

Article abstract of DOI:10.1021/ic401924d

Herein, we report new porous coordination polymers (PCPs) via a facile synthetic approach called "in situ generation of functionality in the ligand". Upon a synthetic process of PCPs, a neutral (-CH₂OH) or a cationic functionality (-CH₂-[4,4′-bipyridine]⁺) was generated on a isophthalate ligand from a reactive haloalkane (-CH ₂Br) moiety, affording two new PCPs. The PCPs have two-dimensional layered structures with large potential solvent-accessible voids for CO ₂ adsorption.

Full text of DOI:10.1021/ic401924d

Communication
pubs.acs.org/IC
In Situ Generation of Functionality in a Reactive Haloalkane-Based
Ligand for the Design of New Porous Coordination Polymers
Prakash Kanoo,† Ryotaro Matsuda,*,† Hiroshi Sato,† Liangchun Li,† Hyung Joon Jeon,†
and Susumu Kitagawa*,†,‡
†Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Nishikyo-ku, Katsura, Kyoto 615-8510, Japan
‡Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku,
Katsura, Kyoto 615-8510, Japan
S
* Supporting Information
employed a new dicarboxylate ligand, 5-(bromomethyl)-
ABSTRACT: Herein, we report new porous coordination
polymers (PCPs) via a facile synthetic approach called “in
situ generation of functionality in the ligand”. Upon a
synthetic process of PCPs, a neutral (−CH2OH) or a
cationic functionality (−CH2-[4,4′-bipyridine]+) was gen-
erated on a isophthalate ligand from a reactive haloalkane
(−CH2Br) moiety, affording two new PCPs. The PCPs
have two-dimensional layered structures with large
potential solvent-accessible voids for CO2 adsorption.
isophthalic acid (Br-iptH2), functionalized with a reactive
bromomethyl (−CH2Br) backbone. The −CH2Br group
transforms to a −CH2OH group during the synthesis of PCP1,
while it reacts with 4,4′-bipyridine to generate an exotic single
negatively charged isophalate ligand {5-(4,4′-bipyridinium
methyl)isophthalate} in PCP2 (Scheme 1). We have been able
Scheme 1. Synthesis of PCPs via in Situ Transformation of the
Br-iptH2 Ligand to OH-ipt and bpy-ipt at RT
he design and synthesis of new porous coordination
T
polymers (PCPs) or metal−organic frameworks have long
been fascinating to material scientists1 for their widespread
applications such as adsorption,2 separation,3 catalysis,4 etc. In
recent times, focus has been on the construction of PCPs with
functional pores for targeting specific applications, such as
selective adsorption,2c molecular recognition,5 specific sensing,6
etc. However, such ligand designs and their subsequent
integration of functional groups into the targeted framework
present synthetic challenges to material scientists. For applicable
use of PCPs, the development of a facile synthetic process is
necessary for the fabrication of new compounds with complex
ligand systems. In this context, in situ ligand transformation is
one of the promising methods to reduce synthetic steps and also
to fabricate new functional pores that are otherwise inaccessible
by the conventional stepwise synthesis.7 Moreover, an in situ
method is highly viable for the synthesis of multitopic organic
ligands with a varying scale of denticity.8 To date, many PCPs
have been isolated via in situ ligand formation and exhibit
interesting properties.9 Benzyl halide (except fluoride), a reactive
species, is known to undergo facile nucleophilic substitution
reactions with versatile nucleophiles. When substitution
reactions occur during PCP synthesis, there is large possibility
of introducing various functional groups into the benzyl part. In
particular, when a neutral nucleophile such as an amine group is
substituted with a halide group, it is possible to introduce a
positive charge into the ligand.10 Therefore, we adopted the use
of a precursor ligand with a benzyl bromide group and attempted
to synthesize new PCPs via in situ modification.
to achieve transformation of a −CH2Br group to a −CH2OH
group under mild reaction conditions at RT unlike at higher
temperatures (>50 °C), which are usually employed (Figure S1
in the SI).11
PCP1 was synthesized at RT by the slow diffusion of a N,N-
dimethylformamide (DMF) solution of Cu(NO3)2·3H2O into
an aqueous solution of Br-iptH2 in the presence of NaHCO3,
which kept the solution basic to accelerate the substitution
reaction. Blue hexagonal crystals were collected after 10 days,
washed with a DMF/H2O solution, and used for single-crystal X-
ray diffraction (XRD) studies. In a similar fashion, PCP2 was
synthesized by the slow diffusion of an ethanol solution of
Cu(NO3)2·3H2O into an aqueous solution of Br-iptH2 and bpy
in the presence of NaHCO3. Blue block-shaped crystals of PCP2
formed after 7 days. Single-crystal XRD analysis suggests that
PCP1 and PCP2 crystallize in space groups R3m and P2 /n,
̅
1
respectively. Both compounds have two-dimensional (2D)
Herein, we report two new PCPs, {[Cu(OH-ipt)(H2O)]·
guest}n (PCP1) and {[Cu2(bpy-ipt)2(bpy)0.5(H2O)3]·(2NO3)·
guest}n (PCP2), that have been assembled through in situ ligand
transformation at room temperature (RT). To realize this, we
Received: July 25, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic401924d | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
structures (Figures 1 and 2) with potentially large accessible
voids. Interestingly, PCP1 has a (6,3)-kagome topology with a
́
crystallographic requirements, the third layer, C, is placed at 2/3x,
2/3y, 2/3z (coordinates are chosen with reference to x, y, z of layer
A), thus leading to ABC-type packing in the crystal. This unusual
ABC packing creates a highly porous structure with two
distinguishable nanochannels along the crystallographic [211]
and c directions (Figure S4 in the SI). The approximate
dimensions of the nanochannels are 3 × 4 and 6 × 6 Å2. It is
important to mention that the pore surfaces of the nanochannels
are decorated with −CH2OH groups from the OH-ipt ligand
(Figure 1c). The surface also has copper(II) atoms coordinated
by water molecules, the removal of which would potentially
provide unsaturated copper(II) sites. The potential void space of
the crystal, as calculated by PLATON13 software, was found to be
5290 Å3, which is approximately 60.6% of the unit cell volume.
PCP2 also has a 2D layered structure (Figures 2 and S5 in the
SI) that assembles with an unusual single negatively charged
carboxylate ligand bpy-ipt formed in situ. The most plausible
mechanism for formation of the bpy-ipt ligand is the nucleophilic
attack of the pyridyl nitrogen atom of bpy on the −CH2Br group
of Br-iptH2. The multitopic bpy-ipt ligand features an unusual
coordination mode through two carboxylate groups and a pyridyl
group, which is never realized with a normal isophthlate ligand.
The structure features two crystallographically independent
copper centers with square-pyramidal geometry (Figure 2a). Cu1
is coordinated to four different bpy-ipt ligands in the equatorial
plane with coordinations furnished by two carboxylate and two
bpy moieties of the ligand. The fifth coordination at the axial
position is provided by a water molecule. The coordination
environment of Cu2 is different from that of Cu1, with two bpy-
ipt, one bpy, and one water molecule occupying the equatorial
plane. A water molecule at the axial position fulfills the square-
pyramidal geometry requirement. Cu1 and Cu2 are connected
via the isophthalate part of one bpy-ipt ligand to generate a 2D
layered structure. Interestingly, in the 2D layer itself, the bpy
ligand acts as a beam and connects two Cu2 centers, providing
additional stability in the layer. Strong π−π interaction of the
benzene ring of the isophthalate moiety (Figure S6 in the SI)
between the 2D layers leads to the formation of a three-
dimensional (3D) supramolecular structure. The framework is
cationic because of the monoanionic nature of the bpy-ipt ligand,
and the residual charge is balanced by NO3− counteranions in the
structure (Figure S7 in the SI). The 3D supramolecular structure
(Figure 2c) possesses large channels that are occupied with guest
water molecules. The guest molecules were highly disordered in
the channels and, hence, the SQUEEZE14 tool was applied to
refine the guest-free structure. The approximate dimensions of
the nanochannels, calculated considering the van der Waals
surface, are 11.5 × 11.5 Å2 (Figure S8 in the SI). The potential
solvent-accessible void in guest-free PCP2 turns out to be ∼6000
Å3, which is 62% of the unit cell volume.
Figure 1. Crystal structure of PCP1: (a) [Cu2(CO2)4] building unit; (b)
2D kagome layer composed of [Cu2(CO2)4] and OH-ipt (−OH is
́
disordered); (c) framework structure of PCP1 viewed along the
crystallographic c direction showing hexagonal channels. An enlarged
view (right-hand side) shows the location of pendant hydroxyl groups
(large balls) in the nanochannel. Color specification: copper, blue;
oxygen, red; carbon, gray (hydrogen atoms are not shown for clarity).
Figure 2. Crystal structure of PCP2: (a) coordination environment of
two different copper(II) centers; (b) 2D layered structure of PCP2; (c)
framework structure viewed along the crystallographic a direction
showing large channels. Color specification: copper, blue; oxygen, red;
carbon, gray; nitrogen, olive (hydrogen atoms are not shown for clarity).
Thermogravimetric analysis (TGA) suggests that PCP1 and
PCP2 is stable up to 280 and 230 °C, respectively. The purity of
the as-synthesized compounds was confirmed by powder XRD
(PXRD) studies (Figures S10 and S11 in the SI). The desolvated
PXRD of PCP1 suggests a structural change upon guest removal.
PCP2 shows the unusual behavior of crystal-to-amorphous
transformation15 upon guest removal (Figure S11 in the SI).
When heated at 120 °C for 12 h, PCP2 transforms to an
amorphous phase, as suggested by the PXRD pattern. The
amorphous phase, when soaked in a 1:1 H2O/EtOH solvent
mixture for 24 h, returns to its original crystalline phase. Hence, it
can be concluded that PCP2 is reversible with respect to a
crystal-to-amorphous phase change.
[Cu2(CO2)4] secondary building unit (Figure 1a,b). Usually 2D
compounds with kagome
́
topology pack in an AA fashion;9g,12
however, PCP1 packs in an unusual ABC fashion along the
crystallographic c direction (Figure S2 in the SI). Upon analyzing
the crystal structure in detail, we found profound influence of the
−CH2OH group for the observed packing. The basic building
unit of the framework is a [Cu2(CO2)4] core connected to four
OH-ipt ligands and two water molecules. The coordinated water
molecules of layer A interact with the −CH2OH group of OH-ipt
of the neighboring layer B through hydrogen bonding (Figure S3
in the SI). This enforces layer B to displace by 1/3x + 1/3y in the
ab plane. At the same time, layer B locates itself at 1/3z toward the
crystallographic c direction. As a consequence, to meet the
B
dx.doi.org/10.1021/ic401924d | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
The degassed PCPs, PCP1′ and PCP2′ were tested for their
gas sorption behavior at low temperatures. As can be seen from
Figure 3, the compounds show significant uptake of CO2 gas at
by the “ERATO Kitagawa Integrated Pores Project” from the
Japan Science and Technology Agency.
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functionality with a reactive haloalkane-based ligand under mild
reaction conditions to assemble two PCPs of copper(II). The
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́
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̈
ASSOCIATED CONTENT
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X-ray crystallographic file (CIF), experimental details, and NMR,
TGA, and PXRD data. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
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Corresponding Authors
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
P.K. acknowledges the Japan Society for the Promotion of
Science for a postdoctoral fellowship. This work was supported
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dx.doi.org/10.1021/ic401924d | Inorg. Chem. XXXX, XXX, XXX−XXX