SCIENCE CHINA
Chemistry
• COMMUNICATIONS •
· SPECIAL TOPIC · Nano and Functional Materials
April 2013 Vol.56 No.4: 418–422
doi: 10.1007/s11426-013-4851-7
Linker extension through hard-soft selective metal coordination
for the construction of a non-rigid metal-organic framework
WEI ZhangWen1, YUAN DaQiang2, ZHAO XiaoLiang3,
SUN DaoFeng3* & ZHOU Hong-Cai1*
1
Department of Chemistry, Texas A&M University, College Station, TX 77843, USA
Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
3
Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering,
Shandong University, Jinan 250100, China
2
Received December 1, 2012; accepted December 26, 2012; published online February 21, 2013
A metal-organic framework (MOF) has been obtained by using a linker extension strategy. Three di-anions of 4-(3,5-dimethyl1H-pyrazol-4-yl)-benzoic acid coordinate to three Cu(I) ions forming an extended trigonal planar ligand, which links three
di-copper paddlewheel units giving rise to a Pt3O4 net.
MOF, linker extension, copper (I)
Metal-organic frameworks (MOFs) are a new class of porous materials that have experienced rapid development in
the last two decades [1–7]. Compared to traditional porous
materials such as zeolites and activated carbons, MOFs are
known for their tuneable pore structures, extraordinarily
high surface areas, and adjustable surface properties [8, 9].
These properties give MOFs great application potential in
gas storage and separation, most notably, hydrogen and
methane storage as well as carbon dioxide capture [10–25].
Prior work of others and ours has demonstrated that
high-pressure gravimetric hydrogen-adsorption capacity of
a MOF is proportional to its surface area [26]. In fact, a
large specific surface area is one of the main prerequisites
for MOF applications, especially for gas storage. Strategies
to reach higher surface area thus become a sought-after goal.
Extension of the length of an organic linker through organic
synthesis has become the most direct method to reach this
target [27–30]. Another approach is to use metal coordination to combine several short organic linkers into a
nano-sized linker [31, 32]. This approach is limited by the
*Corresponding authors (email: zhouh@tamu.edu; dfsun@sdu.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2013
competition between framework assembly and metalloligand formation. One strategy to overcome such a competition is to differentiate the two procedures by using two distinct sets of donor groups and metal ions. For instance, a
bi-polar ligand containing both a “hard” and a “soft” end
can be used to coordinate selectively with a “hard” metal
and a “soft” metal, respectively [33, 34]. In this communication, we report a non-rigid MOF possessing Pt3O4-net
topology constructed from a metal-coordination-extended
linker using such a strategy.
In order to obtain metal-coordination-extended linkers,
the short organic linker should be a bi-polar ligand in which
each end has its preference to a different type of metal ion.
It is known that the nitrogen donor atoms of a pyrazolate or
its derivatives can preferentially bind the Cu(I) ions to form
a Cu(I) triangle (Figure 1(a)) [35–37]. Hence, we designed
and synthesized a new bi-polar linker, namely 4-(4-carboxylatophenyl)-3,5-dimethylpyrazolate (dmpba), which can
connect to different metal ions with each end to construct
MOFs (Figure 1(b)).
It has also been documented that Cu(II) ions can be reduced to Cu(I) ions by N-containing compound or solvent
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[38]. Thus, a solvothermal reaction of the acid form of
dmpba (H2dmpba) and Cu(NO3)·2.5H2O in N,N-dimethylacetamide (DMA) in the presence of pyridine (Py) gave rise
to large block crystals of a MOF designated PCN-91 (PCN
stands for porous coordination network). Single X-ray diffraction revealed that PCN-91 is a MOF with the formula of
CuI6CuII3(dmpba)6(Py)(H2O)2·20DMA·12H2O, which was
further confirmed by elemental analysis, thermal gravimetric analysis (TGA), and X-ray photoelectron spectroscopy
(XPS) (Figure S1, S3, ESI†).
In the crystal structure, there is one metal-coordination
extended linker, CuI-dmpba, which consists of three dmpba
short linkers and three Cu(I) ions (Figure 2(a)). Clearly,
Cu(II) ions were partially reduced to Cu(I) ions by an unidentified reducing agent. Every three of these Cu(I) ions
selectively coordinate six nitrogen donor atoms of three
dmpba ligands to form a Cu(I) triangle with an average
CuI–N bond length of 1.831 Å and an average CuI∙∙∙CuI dis-
Figure 1 (a) The tri-copper unit (simplified); (b) 4-(4-carboxylatophenyl)3,5-dimethylpyrazolate (dmpba).
Figure 2 (a) The copper-coordination-extended linker, (b) a paddlewheel
SBU; (c) the Pt3O4-net in PCN-91; (d) the 3D framework of PCN-91 along
c axis showing interpenetration.
April (2013) Vol.56 No.4
419
tance of 3.185 Å. These distances are comparable to those
of other Cu(I) triangles of the same type [35–37]. It should
be pointed out that the CuI-dmpba linker is non-planar,
which is very similar to previously reported BTB (4′,4′henzene-1,3,5-triyl-tribenzoate) and TCBPB (1,3,5-tris(4′carboxy[1,1′-biphenyl]-4-yl-)benzene) ligands, but quite
different from our previous reported planar TATB (4,4,4s-triazine-2,4,6-triyltri benzoate) [39–41]. The three pyrazolato rings and Cu(I) ions are almost in the same plane and
the average dihedral angle between a pyrazolato ring and a
peripheral ring is 43.98°, slightly larger than those found in
BTB and TCBPB.
Each carboxylate group of a CuI-dmpba linker selectively
coordinate to two Cu(II) ions to form a Cu2 paddlewheel
secondary building unit (SBU) (Figure 2(b)), leading to a
porous 3D framework. The large pores resulted destabilize
the framework, thus a two-fold interpenetration is necessary
to stabilize the MOF (Figure 2(d)); attempts to obtain a
non-interpenetrated MOF were unsuccessful. If the CuIdmpba metal-coordination-extended linker and the Cu2
paddlewheel SBU can be considered as a 3-connected node
and 4-connected node, respectively, the framework possesses Pt3O4-net topology (Figure 2(c)). The solvent accessible volume of PCN-91 is 71.0% (30457.7 Å3 per 48391.0
Å3) calculated using PLATON [42]. The interpenetrated
framework possesses 1D channels with dimensions of
13.524 × 13.524 Å. Thermal gravimetric analysis showed
that PCN-91 is stable up to 270 C (Figure S1, ESI†). N2
sorption at 77 K for PCN-91 was examined and a type-I
isotherm was observed with a BET surface area of 644 m2/g
and Langmuir surface area of 729 m2/g (Figure 3).
It is interesting to note that PCN-91 possesses the same
topology with that of MOF-14 reported by Yaghi and
co-workers, which was constructed from a tricarboxylate
ligand BTB [39]. Both BTB and CuI-dmpba are non-planar
(Figure 4), which bind paddlewheel SBUs forming
Pt3O4-nets.
To understand such a complicated assembly procedure
involving two types of metal ions is not an easy task. Herein
we propose a preliminary reaction mechanism, which is
shown in Scheme 1. The Cu(II) ions in the reaction mixture
Figure 3
The N2 isotherm of PCN-91 at 77 K.
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Figure 4
Scheme 1
Wei ZW, et al.
Sci China Chem
April (2013) Vol.56 No.4
Extension of the non-planar linker.
A proposed mechanism for the assembly of PCN-91.
were partially reduced to Cu(I) ions in the reaction mixture.
The resulted Cu(I) ions selectively coordinated to the soft
nitrogen donor atoms of dmpba ligands to form CuI-dmpba
linkers, which further connected the Cu2 paddlewheel SBUs
giving rise to PCN-91.
In order to support the proposed reaction mechanism, we
decided to synthesize the reaction intermediate, the CuIdmpba linker. A direct reaction between dmpba and a Cu(I)
salt resulted in an uncharacterized precipitate. By using the
ester derivative of dmpba (dmpbae), and Cu(CH3CN)4BF4, a
tri-nuclear Cu(I) cluster, CuI3(dmpbae)3 (compound 1),
which crystallized in triclinic space group P-1, has been
synthesized and characterized. As shown in Figure 5, every
Cu(I) ion was coordinated by two nitrogen atoms from two
different dmpba ligands with an average Cu–N bond length
of 1.836 Å, and Cu∙∙∙Cu distance of 3.201 Å, consistent with
those found in PCN-91. The average dihedral angle between
the pyrazolato rings and the peripheral rings is 44.8°, also
comparable to that found in PCN-91. This tri-nuclear Cu(I)
cluster may be applied as a starting material for MOF assembly reactions because the ester group can be hydrolyzed
to a carboxylate group under basic or acidic condition. We
are currently using 1 as a starting material to synthesize
other MOFs.
Interestingly, it was found that the interpenetrated
framework is elastic and can change its unit cell parameters
Figure 5
The molecule structure of compound 1.
reversibly under different temperatures. The length of c axis
of the unit cell can vary from 26.35 to 27.52 Å while those
of a and b axes remain almost unchanged (Table S2, ESI†).
Powder X-ray diffraction (PXRD) patterns were also simulated based on the crystal structures (Figure 6). The [2 2 2]
diffraction peak was chosen as the basis for comparison
because of its sensitivity toward unit cell change and the
absence of interference. This peak lies at 9.05° at 95 K and
shifts to 8.51° at 295 K. This low temperature non-rigidity
is quite different from the frequently reported breathing effect
of MOFs, and it is also observed for other CuI-pyrazolato
complexes [43–45]. Recently, Li and coworkers suggested
that this flexibility comes from the interactions between the
two CuI-pyrazolato rings [2, 46, 47]. However, this cannot
explain why only the c axis changes with the temperature
variation. In our case, the translation of one of the two interpenetrated frameworks in PCN-91 relative to the other
along the crystallographic c axis is most likely responsible
for this peculiar non-rigidity effect.
In conclusion, we have synthesized a MOF, PCN-91,
Wei ZW, et al.
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April (2013) Vol.56 No.4
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Figure 6 PXRD patterns of PCN-91, the arrows indicate the diffraction
peak from the [2 2 2] lattice plane.
containing metal ions of two different oxidation states by
using a simple bi-polar organic linker dmpba through the
hard-soft-selective-coordination strategy. The metal-coordination-extended linker, CuI-dmpba, consists of three
dmpba linkers and three Cu(I) ions, was generated in situ
during the reaction. PCN-91 possessing Pt3O4-net topology
is non-rigid, which most likely arises from the relative
translation of the two interpenetrated frameworks along the
crystallographic c axis. Although the surface area of
PCN-91 is moderate, we have demonstrated a useful strategy for the construction of porous MOFs containing two types
of metal ions by using a hard-soft-selective-coordination
synthetic strategy. Using a series of bi-polar ligands to synthesize MOFs with different metal ions is currently underway in our laboratory.
This work was supported by the U.S. Department of Energy (DOE Grants
DE-FC36-07GO17033), and the Welch Foundation (A-1725). Microcrystal
diffraction was carried out with the assistance of Yu-Sheng Chen at the
advanced Photon Source (APS) on Beamline 15ID-B at Chem-MatCARS
Sector 15, which is principally supported by the National Science Foundation/Department of Energy under grant number NSF/CHE-0822838. Use
of the Advanced Photon Source was supported by the U. S. Department of
Energy, Office of Science, Office of Basic Energy Sciences, under Contract
No. DE-AC02-06CH11357.
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