Microporous lanthanide metal-organic frameworks containing coordinatively linked interpenetration: Syntheses, gas adsorption studies, thermal stability analysis, and photoluminescence investigation

Article abstract of DOI:10.1021/ic801948z

Under solvothermal conditions, the reactions of trigonal-planar ligand, TATB (4,4′,4"-s-triazine-2,4,6-triyl-tribenzoate) with Dy(NO ₃)₃, Er(NO₃)₃, Y(NO ₃)₃, Yb(NO₃)₃,

Full text of DOI:10.1021/ic801948z

Inorg. Chem. 2009, 48, 2072-2077
Microporous Lanthanide Metal-Organic Frameworks Containing
Coordinatively Linked Interpenetration: Syntheses, Gas Adsorption
Studies, Thermal Stability Analysis, and Photoluminescence
Investigation
Shengqian Ma,* Daqiang Yuan, Xi-Sen Wang, and Hong-Cai Zhou*
Department of Chemistry, Texas A&M UniVersity, P.O. Box 30012, College Station, Texas 77842,
and Chemical Sciences & Engineering DiVision, Argonne National Laboratory,
Argonne, Illinois 60439
Received October 15, 2008
Under solvothermal conditions, the reactions of trigonal-planar ligand, TATB (4,4′,4′′-s-triazine-2,4,6-triyl-tribenzoate)
with Dy(NO3)3, Er(NO3)3, Y(NO3)3, Yb(NO3)3, gave rise to four microporous lanthanide metal-organic frameworks
(MOFs), designated as PCN-17 (Dy), PCN-17 (Er), PCN-17 (Y), and PCN-17 (Yb), respectively. The four porous
MOFs are isostructural, with their crystal unit parameters shrinking in the order of PCN-17 (Dy), PCN-17 (Y),
PCN-17 (Er), and PCN-17 (Yb), which also reflects the lanthanides’ contraction trend. All of them adopt the novel
square-planar Ln4(µ4-H2O) cluster as the secondary building unit and contain coordinatively linked doubly
interpenetrated (8,3)-connected nets. In addition to exhibiting interesting photoluminescence phenomena, the
coordinatively linked interpenetration restricts the pore sizes and affords them selective adsorption of H2 and O2
over N2 and CO, as well as renders them with high thermal stability of 500-550 °C as demonstrated from TGA
profiles.
Introduction
ences.7 Compared to first-row transition metals, lanthanides
have a larger coordination sphere and more flexible coordi-
nation geometry. These characteristics make it difficult to
control the final topology of a lanthanide MOF.8 So far,
research for reported lanthanide MOFs has been focused on
magnetic and photoluminescent properties;9 however, the
construction of porous lanthanide MOFs and their potential
application in gas adsorption have been much less developed.10
Recently, we have been interested in the use of a trigonal-
planar ligand, TATB (4,4′,4′′-s-triazine-2,4,6-triyl-triben-
Metal-organic frameworks (MOFs)1 have in the past
decade become an active research area and attracted great
attention from both academia and industry because of their
fascinating topology2 and tantalizing application potential
in fields such as catalysis,3 magnetism,4 separation,5 and gas
storage.6 There exists an escalating interest in the synthesis
of MOFs using lanthanides owing to their unique optical and
magnetic properties and characteristic coordination prefer-
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* To whom correspondence should be addressed. E-mail: sma@anl.gov
(S.M.), zhou@mail.chem.tamu.edu (H.C.Z.).
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2072 Inorganic Chemistry, Vol. 48, No. 5, 2009
10.1021/ic801948z CCC: $40.75  2009 American Chemical Society
Published on Web 01/27/2009

Lanthanide Metal-Organic Framework
zoate), to promote the formation of dinuclear and polynuclear
secondary building units (SBUs) in porous MOFs constructed
from first-row transition metals.11 Many interesting SBUs
such as dinuclear paddlewheel,11b,f trinuclear hourglass,11a,d,g
trinuclear µ3-oxo-centered basic-carboxylate,11e and tetra-
nuclear µ4-oxo-centered square-planar cluster11c have been
observed in porous MOFs which demonstrate high surface
areas and significant hydrogen uptake.11 The extraordinary
ability of TATB to promote polynuclear SBUs can mainly
be attributed to the planarity of the ligand and its tendency
to encourage π-π stacking in MOFs.
In continuation of this theme, we have extended the
application of the TATB ligand for the construction of
lanthanide MOFs with novel architectures and interesting
properties.12 Under solvothermal conditions, the reactions
of the TATB ligand with Dy(NO3)3, Er(NO3)3, Y(NO3)3, and
Yb(NO3)3 afforded four microporous lanthanide MOFs,
designated as PCN-17 (Dy), PCN-17 (Er), PCN-17 (Y)and
PCN-17 (Yb), respectively (PCN represents Porous Coor-
dination Network). The four MOFs are isostructual, and
contain coordinatively linked interpenetration which confines
their pore sizes for selective adsorption of H2 and O2 over
N2 and CO. In this contribution, we present the syntheses
and detailed structure description of the four isomorphous
MOFs along with the gas adsorption studies, thermal stability
analysis, and photoluminescence investigation.
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Experimental Section
Synthesis of PCN-17 (Yb). A mixture of H3TATB (0.01 g) and
Yb(NO3)3 (0.025 g) in 1.2 mL of DMSO (dimethylsulfoxide) with
five drops of H2O2 (30%, aq.) was sealed in a Pyrex tube, heated
to 145 °C (temperature increase rate, 2 °C/min), allowed to stay
for 72 h, and cooled to 35 °C (temperature decrease rate, 0.2 °C/
min). The brown crystals obtained were washed with DMSO twice
to give pure PCN-17 (Yb) with the following formula: Yb4(µ4-
H2O)(C24H12N3O6)8/3(SO4)2 ·3H2O·10C2H6SO. Elemental analysis
for PCN-17 (Yb), calculated: C 34.71%, H 3.47%, N 3.85%; found:
C 33.87%, H 3.41%, N 3.68%.
Synthesis of PCN-17 (Dy). The procedure is similar to the
synthesis of PCN-17 (Yb), but with 0.025 g of Dy(NO3)3 instead
of Yb(NO3)3 used. The resultant brown crystals were washed with
DMSO twice to give the pure compound, PCN-17 (Dy), with the
formula of Dy4(µ4-H2O)(C24H12N3O6)8/3(SO4)2 ·3H2O·10C2H6SO.
Elemental analysis for PCN-17 (Dy), calculated: C 35.22%, H
3.52%, N 3.91%; found: C 33.40%, H 3.47%, N 3.54%.
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Synthesis of PCN-17 (Er). The procedure is similar to the
synthesis of PCN-17 (Dy), but with 0.025 g of Er(NO3)3 instead of
Dy(NO3)3 utilized. The resultant pink crystals were washed with
DMSO twice to give the pure compound, PCN-17 (Er), with the
following formula: Er4(µ4-H2O)(C24H12N3O6)8/3(SO4)2 ·3H2O·10C2-
H6SO. Elemental analysis for PCN-17 (Er), calculated: C 34.99%,
H 3.49%, N 3.88%; found: C 33.61%, H 3.53%, N 3.52%.
Synthesis of PCN-17 (Y). The procedure is similar to the
synthesis of PCN-17 (Er), but with 0.025 g of Y(NO3)3 instead of
Er(NO3)3 utilized. The resultant yellow brown crystals were washed
with DMSO twice to give the pure compound, PCN-17 (Y), with
the following formula: Y4(µ4-H2O)(C24H12N3O6)8/3(SO4)2 · 3H2O ·
10C2H6SO. Elemental analysis for PCN-17 (Y), calculated: C
38.98%, H 3.97%, N 4.33%; found: C 37.56%, H 3.93%, N 4.18%.
(All the Ln(NO3)3 used above (Ln ) Dy, Er, Y, Yb) were prepared
by dissolving Ln2O3 in HNO3 aqua solutions followed by evapora-
tion to crystallize Ln(NO3)3 salts).
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Single-Crystal X-ray Crystallography. Single crystal X-ray
data were collected on a Bruker Smart Apex diffractometer
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Inorganic Chemistry, Vol. 48, No. 5, 2009 2073

Ma et al.
Table 1. Crystal Dataa and Structure Refinement of PCN-17 (Dy), PCN-17 (Er), PCN-17 (Y), and PCN-17 (Yb)
PCN-17 (Dy)
PCN-17 (Er)
PCN-17 (Y)
PCN-17 (Yb)
formula
FW
C72H62N8O31S6Dy4
2377.66
C72H62N8O31S6Er4
2396.70
C72H62N8O31S6Y4
2083.30
C72H62N8O31S6Yb4
2419.82
crystal system
space group
a, Å
b, Å
c, Å
cubic
Im3m
26.348(3)
26.348(3)
26.348(3)
90.00
90.00
90.00
18292(3)
6
1.295
cubic
Im3m
26.268(4)
26.268(4)
26.268(4)
90.00
90.00
90.00
18126(5)
6
1.317
cubic
Im3m
26.311(3)
26.311(3)
26.311(3)
90.00
90.00
90.00
18214(4)
6
1.09
cubic
Im3m
26.225 (1)
26.225(1)
26.225(3)
90.00
90.00
90.00
18037(2)
6
1.337
j
j
j
j
R, deg
ꢁ, deg
γ, deg
V, Å3
Z
dcacl, g/cm3 cm-1
GOF
1.033
0.0601, 0.1668
1.079
0.0947, 0.2529
1.107
0.0921, 0.2555
1.094
0.0969, 0.2685
2
b
R1, wR2
a Obtained with graphite-monochromated Mo KR (λ ) 0.71073 Å) radiation, R1) ∑|Fo| - |Fc|/∑|Fo| and wR2) {[∑w(Fo - Fc2)2]/[∑w(Fo )2]}1/2
.
b
2
equipped with an Oxford Cryostream low temperature device and
a fine-focus sealed-tube X-ray source (Mo KR radiation, λ )
0.71073 Å, graphite monochromated) operating at 45 kV and 35
mA. Frames were collected with 0.3° intervals in ꢀ and ω for 30 s
per frame such that a hemisphere of data was collected. Raw data
collection and refinement were done using SMART. Data reduction
was performed using SAINT+ and corrected for Lorentz and
polarization effects.13 The structure was solved by direct methods
and refined by full-matrix least-squares on F2 with anisotropic
displacement using SHELX-97.14 Non-hydrogen atoms were refined
with anisotropic displacement parameters during the final cycles.
Hydrogen atoms on carbon were calculated in ideal positions with
isotropic displacement parameters set to 1.2 × Ueq of the attached
atom. Absorption corrections were applied using SADABS after
the formula of the compound is determined approximately.13
Solvent molecules in the structure were highly disordered and were
impossible to refine using conventional discrete-atom models. To
resolve these issues, the contribution of solvent electron density
was removed by the SQUEEZE routine in PLATON.15 In all of
the four structures, the Ln (Ln ) Yb, Er, Dy, Y) atoms are
disordered and each Ln is refined as occupying two equally
populated positions. CCDC-617998 (PCN-17 (Yb)) and CCDC-
644651 (PCN-17 (Dy)) contain the supplementary crystallographic
data which can be obtained free of charge at www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic Data
Center, 12, Union Road, Cambridge CB21EZ, UK; fax: (+44)
1223-336-033; or deposit@ccdc.cam.ac.uk).
Gas Adsorption Measurements. Gas adsorption measurements
were performed with a Beckman Coulter SA 3100 surface area
and pore size analyzer. The fresh samples were evacuated under
dynamic vacuum (<10-3 torr) at 250 °C overnight to remove the
guest molecules. Before the measurement, each sample was
evacuated again by using the “outgas” function of the surface area
analyzer for 2 h at 250 °C. A sample of 100 mg was used for N2
(99.999%) adsorption measurement, and was maintained at 77 K
with liquid nitrogen. In the hydrogen adsorption measurement, high
purity hydrogen (99.9995%) was used. The regulator and pipe were
flushed with hydrogen before connecting to the analyzer. The
internal lines of the instrument were flushed three times by utilizing
the “flushing lines” function of the program to ensure the purity of
H2. The measurement was maintained at 77 K with liquid nitrogen.
Similar to the procedures used for H2 measurement at 77 K, highly
pure O2 (99.99%), CO (99.99%), and CO2 (99.99%) were used for
their respective gas adsorption measurements. All the gases used
for the measurements were purchased from Linde Gas LLC,
Cincinnati, Ohio, U.S.A. The temperatures at 195 K were main-
tained with an acetone-dry ice bath. To prevent condensation of
CO and O2 at 77 K, the pressure ranges were below 448 torr and
156 torr, respectively. For all adsorption isotherms, P0 represents
a relative saturation pressure given by the Beckman Coulter SA
3100 surface area and pore size analyzer during the measurements:
at 77 K, P0 was 757 torr for N2, 441 torr for CO, and 151 torr for
O2. For hydrogen 757 torr was used as a relative standard.
Results and Discussion
Description of the Structures. Single crystal X-ray
analysis revealed that the four MOFs are isostructural with
j
each other, crystallizing in space group Im3m (Table 1). The
trend of lanthanide contraction from Dy to Yb can be
reflected by the reduction in their unit cell parameters as
shown in Table 1. The unit parameters of PCN-17 (Y) lie in
between PCN-17 (Er) and PCN-17 (Dy) and can be ascribed
to the Y3+ ion radius of 1.02 Å which is between the radius
of Er3+ (1.00 Å) and that of Dy3+ (1.03 Å).16 The four MOFs
all adopt the square-planar Ln4(µ4-H2O) SBU (Ln ) Dy, Er,
Y, Yb), with the µ4-H2O residing at the center of the square
of four Ln atoms (Figure 1a). All of the four Ln atoms in
the SBU are in the same plane and coordinate seven oxygen
atoms (four from four carboxylate groups of four different
TATB ligands, two from the bridging sulfate ligand generated
in situ, and one from the µ4-H2O). The distances between
the Ln and carboxylate oxygen range from 2.152 Å to 2.187
Å, and those of Ln and sulfate oxygen fall in the range of
2.147 to 2.324. The Ln-µ4-H2O distances vary from 2.703
to 3.008 Å, indicating very weak LnsH2O bonding (Table
2). The µ4-H2O bridged square-planar structural motif has
been previously observed in some lanthanide complexes,17
but it is slightly different from that in PCN-9 where a µ4-O
atom resides at the center of a square of four cobalt atoms.11c
The rationale for the assignment of the µ4-center as H2O
molecule instead of O atom lies in the charge balance of the
whole framework.
(13) SAINT+, version 6.22; Bruker Analytical X-Ray Systems, Inc.:
Madison, WI, 2001.
(14) Sheldrick, G. M. SHELX-97; Bruker Analytical X-Ray Systems, Inc.:
Madison, WI, 1997.
(16) Lide, D. R. Handbook of Chemistry and Physics, 73rd ed.; CRC Press,
Int.: Boca Raton, FL, 1992-1993.
(17) Deacon, G. B.; Forsyth, C. M.; Harika, R.; Junk, P. C.; Ziller, J. W.;
Evans, W. J. J. Mater. Chem. 2004, 14, 3144–3149.
(15) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.
2074 Inorganic Chemistry, Vol. 48, No. 5, 2009

Lanthanide Metal-Organic Framework
Figure 1. Structures of PCN-17 (Dy, Er, Y, Yb): (a) Ln4(µ4-H2O) SBUs; (b) octahedral cage; (c) cuboctahedral cage; (d) an octahedral cage enclosed by
an cuboctahedral cage form from another network; (e) space-filling packing of unactivated sulfate-bridging doubly interpenetrated (8,3)-connected net (viewed
from (1 0 0) direction); (f) space-filling packing of activated sulfate-bridging doubly interpenetrated (8,3)-connected net with open tiny pores resulted from
the removal of the µ4-H2O of the SBUs (viewed from (1 0 0) direction). (Pink, Ln (Ln ) Dy, Er, Y, Yb); gray, carbon; red, oxygen; yellow, sulfur).
Table 2. Selective Bond Distances of PCN-17 (Dy), PCN-17 (Er), PCN-17 (Y), and PCN-17 (Yb)
PCN-17 (Dy)
PCN-17 (Er)
PCN-17 (Y)
PCN-17 (Yb)
Ln-O (carboxylate)/Å distances (Å)
Ln-O (sulfate)/Å distances (Å)
Ln-µ4-H2O/Å distances (Å)
Ln-Ln (opposite) distances of Ln4(µ4-H2O) SBUs/Å
π-π interaction of the TATB ligand pairs/Å
2.187
2.324
2.819
6.392
3.351
2.160
2.147
2.776
6.407
3.394
2.168
2.268
3.008
6.584
3.473
2.152
2.198
2.703
6.335
3.441
In all of the four structures, every TATB ligand links three
Ln4(µ4-H2O) SBUs, every Ln4(µ4-H2O) SBU connects eight
trigonal-planar TATB ligands and four sulfate ligands to form
an infinite framework (Supporting Information, Figure S1a).
Alternatively, they can be viewed comprising the infinite
SBU with each sulfate bridging two Ln4(µ4-H2O) clusters
and each cluster connecting four sulfates (Supporting Infor-
mation, Figure S1b). There exist two types of cages in their
structures: one is a truncated octaheral cage defined by six
Ln4(µ4-H2O) SBUs at the corners and eight TATB ligands
on the faces (Figure 1b), and the other one is a cuboctahedral
cage enclosed by eight truncated octahedral cages occupying
the vertices of the cube (Figure 1c). Every cuboctahedral
cage connects eight truncated octahedral cages via face-
sharing, and each truncated octahedral cage links six cub-
octahedral cages to form a (8,3)-net framework (Supporting
Information, Figure S2).
Two such (8,3)-nets are mutually interpenetrated because
of the π-π interactions of the TATB ligand pairs (Supporting
Information, Figure S3) as reflected in the short distance
between the centers of the two triazine rings of the TATB
ligand pairs (Table 2). The staggered TATB ligand pairs
resulting from strong π-π interactions lead to the truncated
octahedral cages of one set framework enclosed by the
cuboctahedral cages of the other set framework with the
triangular face-sharing (Figure 1d). This closes the windows
of the truncated octahedral cages and also reduces the size
of the opening of the cuboctahedral cages, which is further
restricted by the bridging sulfate ligands. Overall, their
structures can be described as sulfate-bridged doubly inter-
penetrated (8,3)-connected nets. When using the space-filling
model to pack their frameworks, no open pores can be found
along (1 0 0) direction (Figure 1e) or (1 1 0) direction
(Supporting Information, Figure S4a), although some tiny
open pores (∼ 2.0 Å) can be observed along (1 1 1) direction
(Supporting Information, Figure S4b). However, the removal
of the µ4-H2O in the Ln4(µ4-H2O) SBUs can result in some
openings of ∼3.5 Å which are the only passages to allow
gas molecules to pass through (Figure 1f).
Gas Adsorption Studies. To test their permanent porosi-
ties, gas adsorption studies were performed utilizing fully
activated samples. Unlike our previous reported porous
MOFs,11 soaking freshly prepared PCN-17 (Dy, Er, Y, Yb)
in methanol and dichloromethane could not remove the high-
boiling-point H2O and DMSO guest molecules, and the
solvent-exchanged samples took up neither N2 nor H2 even
after thermal activation at 100 °C. This can be attributed to
retention of the µ4-H2O in the Ln4(µ4-H2O) SBU, which
precludes the entrance of methanol or dichloromethane for
solvent exchange despite the existence of small openings of
∼2.0 Å along (1 1 1) direction (excluding van der Waals
radii18). Alternatively, the four porous MOFs were then
(18) Bondi, A. J. Phys. Chem. 1964, 68, 441–451.
Inorganic Chemistry, Vol. 48, No. 5, 2009 2075

Ma et al.
Figure 2. CO2 adsorption isotherms of PCN-17 (Dy, Er, Y, Yb) at 195 K.
Table 3. Gas Adsorption Summary of PCN-17 (Dy), PCN-17 (Er),
PCN-17 (Y), and PCN-17 (Yb)
PCN-17 PCN-17 PCN-17
(Dy)
(Er)
(Y)
PCN-17
BET surface area (m2/g)a
pore volume (cm3/g)b
H2 uptake (cm3/g, at 77K,
1 atm)
selectivity of O2 over N2
(at P/P0 ) 1)
738
0.31
110
606
0.25
90
814
0.33
123
820
0.34
105
Figure 3. O2, H2, N2, and CO adsorption isotherms of (a) PCN-17 (Yb);
(b) PCN-17 (Er); (c) PCN-17 (Dy); (d) PCN-17 (Y) at 77 K.
2.3
1.9
2.5
1.5
1.9
1.7
9.0
3.6
selectivity of H2 over CO
(at P/P0 ) 1)
pores freely (Table 3). However, the amounts of N2 and CO
adsorbed by PCN-17 (Yb) are very limited, and the selectiv-
ity of H2 over CO is 3.6 but the selectivity of O2 over N2 is
9.0 (Table 3). Nevertheless, PCN-17 (Dy, Er, Y) can uptake
moderate amounts of N2 and CO with type-I behaviors. The
volumes of adsorbed N2 and CO by PCN-17 (Dy, Er, Y)
are comparable or lower than the volumes of adsorbed H2
despite the fact that H2 is a supercritical gas at 77 K and are
much lower than those of adsorbed O2. This indicates that
the pore sizes of PCN-17 (Dy, Er, Y) are not open enough
to let N2 and CO gas molecules go through freely. The
selectivity of H2 over CO is 1.9, 1.5, and 1.7 for PCN-17
(Dy), PCN-17 (Er), and PCN-17 (Y), respectively, while the
selectivity of O2 over N2 is 2.3, 2.5, and 1.9 (Table 3). In
view of the kinetic diameters of 2.89 Å for H2, 3.46 Å for
O2, 3.64 Å for N2, and 3.76 Å for CO,21 it can be inferred
that the pore opening of PCN-17 (Yb) should be between
3.46 and 3.64 Å in diameter, and those of PCN-17 (Er), PCN-
17 (Dy), and PCN-17 (Y) should be around 3.7 Å. These
are also consistent with the crystallographically observed
aperture sizes (Ln-Ln distances in Table 2) of the four MOFs.
The different aperture sizes of the four porous MOFs may
presumably ascribe for the difference of their sorption
properties. The adsorption selectivity of H2 and O2 over N2
and CO shown by PCN-17 is not common,5a,c,h,j promising
its application in the separation of nitrogen and oxygen, as
well as the separation of hydrogen from carbon monoxide
in fuel cell applications.
a Calculated with CO2 adsorption data at 195 K using the Brunauer-
Emmett-Teller (BET) equation. b Calculated with CO2 adsorption data at
195 K using Dubinin-Radushkevich equation.
activated in a way similar to the activation of PCN-17 (Yb)12
by evacuating the freshly prepared samples at 250 °C under
a dynamic vacuum overnight. Considering their limited pore
size of ∼3.5 Å even after the removal of the µ4-H2O in the
Ln4(µ4-H2O) SBUs, CO2 (kinetic diameter: 3.33 Å19) adsorp-
tion isotherms measured at 195 K instead of N2 (kinetic
diameter: 3.64 Å19) adsorption isotherms measured at 77 K
were employed to estimate their surface areas. As shown in
Figure 2, CO2 adsorption isotherms of the four fully activated
MOFs reveals typical type-I behaviors as expected for
microporous materials. Fittings of the Brunauer-Emmett-
Teller (BET) equation20 to the adsorption isotherms of CO2
give the estimated surface areas of 738 m2/g, 606 m2/g, 814
m2/g, and 820 m2/g for PCN-17 (Dy), PCN-17 (Er), PCN-
17 (Y), and PCN-17 (Yb), respectively (Table 3). Using the
Dubinin-Radushkevich equation,21 the pore volumes of
PCN-17 (Dy), PCN-17 (Er), PCN-17 (Y), and PCN-17 (Yb)
are estimated to be 0.31 cm3/g, 0.25 cm3/g, 0.33 cm3/g, and
0.34 cm3/g respectively (Table 3).
To check their potential for selective gas adsorption
applications, N2, H2, O2, and CO adsorption studies were
carried out at 77 K. As shown in Figure 3, all of the four
porous MOFs can adsorb a large amount of O2 (258 cm3/g
for PCN-17 (Dy), 196 cm3/g for PCN-17 (Er), 170 cm3/g
for PCN-17 (Y), and 210 cm3/g for PCN-17 (Yb)) and a
moderate amount of H2 (110 cm3/g for PCN-17 (Dy), 90
cm3/g for PCN-17 (Er), 123 cm3/g for PCN-17 (Y), and 105
cm3/g for PCN-17 (Yb)) with typical type-I behaviors. This
indicates that O2 and H2 molecules can pass through their
Thermal Stability Analysis. To investigate their thermal
stabilities, thermal gravimetric analysis (TGA) was performed
on a Perkin-Elmer TGA 7 instrument. As shown in Figure
4, PCN-17 (Dy)/(Er)/(Yb) have similar thermal stability and
are stable up to 500 °C. The first weight loss from 20 to
430 °C corresponds to the loss of 10 DMSO, 3 H2O guest
solvent molecules, and 1 µ4-H2O molecule, which is followed
by a steady plateau up to 500 °C. Their frameworks start to
collapse with the loss of TATB ligands from 500 to 700 °C.
As for PCN-17 (Y), it is stable up to 550 °C followed by
(19) Beck, D. W. Zeolite Molecular SieVes; Wiley & Sons: New York, 1974.
(20) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60,
309–319.
(21) Dubinin, M. M.; Radushkevich, L. V. Dokl. Akad. Nauk USSR 1947,
55, 327–329.
2076 Inorganic Chemistry, Vol. 48, No. 5, 2009

Lanthanide Metal-Organic Framework
to the intraligand π to π* transitions.11a,d As shown in Figure
4, PCN-17 (Dy) exhibits characteristic Dy3+ emissions
4
6
resulted from the F9/2 f HJ (J ) 15/2, 13/2, and 11/2)
transitions: the emission band at ∼ 485 nm corresponds to
the 4F9/2 f6H15/2 transition; the band at ∼579 nm arises from
the 4F9/2 f6H13/2 transition; and the weak band at ∼667 nm
is attributed to the 4F9/2 f 6H11/2 transition.24 Only very weak
intraligand fluorescent emission from the TATB ligand has
been observed, and no charge transfer bands have been found
because the energy transfer between TATB and Dy3+ is
inefficient.11a,d,25
Figure 4. TGA profiles of PCN-17 (Dy, Er, Y, Yb).
Conclusions
In summary, four isostructural lanthanide microporous
MOFs, PCN-17 (Dy, Er, Y, Yb) based on novel square-
planar Ln4(µ4-H2O) SBUs have been constructed and struc-
turally described. They contain coordinatively linked inter-
penetration which not only yields high thermal stability up
to 550 °C as demonstrated from TGA profiles but also
confines their pore sizes affording interesting gas adsorption
properties. The selective adsorption of H2 and O2 over N2
and CO exhibited by PCN-17 (Yb) promises its potential
applications in the separation of nitrogen and oxygen, the
separation of hydrogen from carbon monoxide in fuel cell
applications, and hydrogen enrichment of the N2/H2 exhaust
in ammonia synthesis.
Figure 5. Room temperature solid-state photoluminescence spectra of
H3TATB and PCN-17 (Dy).
Acknowledgment. This work was supported by the U.S.
Department of Energy (DE-FC36-07GO17033) and the U.S.
National Science Foundation (CHE-0449634). H.-C. Z.
acknowledges the Research Corporation for a Cottrell Scholar
Award and Air Products for a Faculty Excellence Award.
S.M. acknowledges Dr. Yujuan Liu for the photoluminescent
measurements and the Director’s Postdoctoral Fellowship
from Argonne National Laboratory.
the decomposition of its framework with the loss of three
TATB ligands. Their thermal stabilities demonstrated from
TGA profiles are among the highest of reported porous
MOFs,22 albeit some dense structures can be stable up to
600 °C.23 Their unusual stabilities can be attributed to the
coordinatively linked interpenetration. Nevertheless, H2
adsorption isotherms of the samples heated at high temper-
atures revealed they could only maintain their framework
integrities at up to 480 °C (Supporting Information, Figure
S5).12
Photoluminescent Investigation. Lanthanide compounds
are known for their photoluminescent properties.9 Excited
at 325 nm at room temperature, H3TATB exhibits a broad
emission band at ∼ 420 nm (Figure 5), which can be ascribed
Supporting Information Available: Further details are given
in crystallographic information files (CIF) of PCN-17 (Er, Y), with
structure graphics, and in Figures S1-S5. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC801948Z
(24) (a) Chandler, B. D.; Cramb, D. T.; Shimizu, G. K. H. J. Am. Chem.
Soc. 2006, 128, 10403–10412. (b) Sun, Y.-Q.; Zhang, J.; Yang, G.-
Y. Chem. Commun. 2006, 1947, 1949. (c) Sun, Y.-Q.; Zhang, J.; Yang,
G.-Y. Chem. Commun. 2006, 4700, 4702.
(25) Thirumurugan, A.; Pati, S. K.; Green, M. A.; Natarajan, S. J. Mater.
Chem. 2003, 13, 2937–2941.
(22) Park, K. S.; Ni, Z.; Coˆte´, A. P.; Choi, J. Y.; Huang, R.; Uribe-Romo,
F. J.; Chae, H. K.; O’Keeffe, M.; Yaghi, O. M. Proc. Nat. Acad. Sci.
U.S.A. 2006, 103, 10186–10191.
(23) de Lill, D. T.; Cahill, C. L. Chem. Commun. 2006, 4946, 4948.
Inorganic Chemistry, Vol. 48, No. 5, 2009 2077