Synthesis, characterization, and photoluminescence of isostructural Mn, Co, and Zn MOFs having a diamondoid structure with large tetrahedral cages and high thermal stability

Article abstract of DOI:10.1039/b502007g

Three novel Mn, Co, and Zn MOFs containing large tetrahedral cages have been prepared and are stable up to 400 °C; X-ray diffraction revealed the frameworks are isostructural having a diamondoid structure of "hourglass" subunits connected by triangular carboxylate ligands. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b502007g

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COMMUNICATION
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Synthesis, characterization, and photoluminescence of isostructural
Mn, Co, and Zn MOFs having a diamondoid structure with large
tetrahedral cages and high thermal stability{
Daofeng Sun, Shengqian Ma, Yanxiong Ke, Tracy M. Petersen and Hong-Cai Zhou*
Received (in Cambridge, UK) 10th February 2005, Accepted 5th April 2005
First published as an Advance Article on the web 15th April 2005
DOI: 10.1039/b502007g
quadrupole moment. TATB should strengthen the resulting
MOFs even more.
Three novel Mn, Co, and Zn MOFs containing large
tetrahedral cages have been prepared and are stable up to
400 uC; X-ray diffraction revealed the frameworks are
isostructural having a diamondoid structure of ‘‘hourglass’’
subunits connected by triangular carboxylate ligands.
In this communication, we report three novel, isostructural
MOFs having a diamondoid structure with large tetrahedral cages,
namely, Co6O(TATB)4?(H+)2?(H2O)2?(Py) (Py 5 pyridine) (1),
Mn6O(TATB)4?(H+)2?(H2O)8?(DMF)2 (DMF 5 dimethylforma-
Zn6O(TATB)4?(H+)2?(H2O)5?(DMSO)4
Metal–organic frameworks (MOFs) have become a burgeoning
field of research in recent years due to their potential applications
in heterogeneous catalysis, non-linear optics, magnetism, and gas-
storage.1,2 They have also been designed with photoluminescence
properties.3 Careful selection of organic ligands and secondary
building units (SBUs) allows the construction of MOFs with
desired properties and structural-types.4,5 In the last decade, one
of the most frequently used ligands in MOF studies was the anion
of benzenetricarboxylic acid (H3BTC, Fig. 1).6 The expansion of
BTC to benzenetribenzoate (BTB, Fig. 1) by Yaghi and coworkers
recently resulted in new structural types with significantly increased
porosity.7
mide)
(2),
and
(DMSO 5 dimethylsulfoxide) (3). The protons in the formulae
were added to balance the overall charge of the MOFs (vide infra).
The TATB ligand, which has never been used as a ligand before
our work, was prepared using a modified literature method (see
supporting information{).
The hydro- or solvo-thermal reactions between H3TATB and
Co(NO3)2?6H2O, Mn(NO3)2 and Zn(NO3)2?6H2O resulted in the
formation of complexes 1, 2, and 3 respectively.{
Single-crystal diffraction analysis§ revealed that 1, 2, and 3 are
isostructural. The following discussions on structural aspects will
mainly focus on 3.
One of the main concerns in MOF materials is their thermal
stability. By introducing a triazine ring into a TATB ligand (the
anion of 4,49,40-s-triazine-2,4,6-triyl-tribenzoic acid, H3TATB,
Fig. 1), we hope to improve the thermal stability of the resulting
MOFs because H3TATB was used as a monomer for the
preparation of high-temperature-resistant polymers.8 Repulsions
between the C–H bonds on the central ring and those on the
peripheral rings play a key role in controlling the conformation of
BTB; the TATB ligand with no C–H bonds on the central ring is
not subject to such repulsions. Therefore, TATB is expected to be
flatter than BTB, and thus should produce new structural types in
MOF studies. In addition, the triazine ring is better suited for p–p
stacking than the central ring in BTB due to the increased
IR spectroscopic studies showed that the carboxyl groups of
H3TATB are deprotonated during the reaction. Each carboxylate
adopts a bidentate bridging coordination mode connecting a four-
coordinate Zn (Td-Zn) and a six-coordinate Zn (Oh-Zn) in a syn–
anti configuration (Fig. 2a).
Every six carboxylate groups from six TATB ligands bridge two
Td-Zn atoms and one Oh-Zn to form an ‘‘hourglass’’ structural
subunit. This unique hourglass arrangement can be attributed to
Fig. 1 Triangular carboxylate ligands.
{ Electronic supplementary information (ESI) available: details of the
synthesis of the ligand, drawings of TGA and XRD. See http://
www.rsc.org/suppdata/cc/b5/b502007g/
Fig. 2 View of (a) an hourglass subunit (colour scheme: aqua, Zn; red,
O; and grey, C), (b) m4-O atom connecting four hourglass subunits in a
tetrahedral geometry, and (c) a diamondoid network.
*zhouh@muohio.edu
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the fact that the octahedral ligand field stabilization energies for
Mn2+ (high spin d5), Co2+ (high spin d7) and Zn2+ (d10) are very
small, so there is little energy difference between tetrahedral and
octahedral ligand fields. There are examples in the literature in
which Zn,9 Mn and Co10 are arranged in such an hourglass
arrangement.
There are no discrete SBUs in the structure. Every hourglass
subunit binds two m4-O atoms and every m4-O atom connects four
˚
hourglass subunits (Zn–O: 2.067(1) A, Fig. 2b), forming an infinite
diamondoid network throughout the lattice (Fig. 2c).
As expected, the TATB ligand is almost planar in the three
coordination networks. For example, the average dihedral angle
between the central and peripheral rings is only 4.7u in 3. By
comparison, the average dihedral angle is 37.1u in MOF-14 in
which the BTB ligand was used.7a The TATB ligands appear in
pairs by forming p–p stacking in a staggered arrangement (Fig. 3).
The two triazine rings are stacked in such a way that the negatively
charged nitrogen atoms are aligned with the positively charged
carbon atoms from the other ring to maximize p–p stacking,
similar to those in compounds based on tripyridyltriazine.11 In
fact, the p–p stacking found in 2 is probably the strongest that has
Fig. 4 View of (a) a pair of TATB ligands (gold) covers the window of
the adamantine and connects two tetrahedral cages, and (b) every face of a
tetrahedral cage capped by another tetrahedral cage.
no weight loss from 300 to 440 uC, and 3 decomposes at higher
temperatures. For complexes 1 and 3, X-ray powder diffraction
patterns were measured to validate stability. The samples were
heated to 100, 200, 300, and 405 uC, respectively. XRD patterns of
both samples fit well with the simulated data, although slight shifts
of some peaks are observed at 300 or 405 uC (see supporting
information{). The thermal stability of all three MOFs from this
work is comparable to that of high-thermal-stability (430 uC)
MOFs in the literature.14 By comparison, MOF-177, also a Zn
MOF but with the BTB ligand, has a reported decomposition
temperature of 350 uC, although its high porosity may partially
account for the lesser thermal stability.7b
ever been found, the distance between the centres of the two
12
triazine rings being only 3.14 A. By comparison, typical p–p
˚
stacking interactions between two phenyl rings were found at
13
distances around 3.5 A. The pair of ligands cover each
˚
adamantane window in the diamond net (Fig. 4a), and every
two adamantanes shares the pair. Each ligand in the pair acts as
the face of a tetrahedral cage, in which every 12 Zn ions are
connected by four TATB ligands and four m4-O atoms, while the
m4-O atom acts as the vertex. The cage has an edge length of
The three MOFs in this report have been formulated based on
crystallographic studies and elemental analyses. In the formula of
3, the addition of two protons is the only way to balance the
overall charge of the MOF since 3 contains only Zn2+, which has
no other stable oxidation state. The oxidation states of Co and Mn
in 1 and 2 have been temporarily assigned based on crystal-
lographically determined M–O distances (Table 1). For 1 the
distances are consistent with the +2 oxidation state for both
four and six coordinate Co atoms.10 However, for 2, although the
Td-Mn–O distances are consistent with the +2 oxidation state, the
Oh-Mn–O distances are slightly shorter than those found in other
molecular compounds.10 This may imply a partial oxidation of the
central Mn atoms. Nevertheless, 2 can still be formulated as
containing only Mn2+ ions. The hourglass arrangement in 2 also
favours the d5 electron configuration and the Mn2+ oxidation state
assignment (vide supra).
3
…
˚
˚
18.218 A (O O distance) and a volume of 715 A , in which free
solvent molecules reside. Every face of the tetrahedron is covered
by another tetrahedron (Fig. 4b) to form a ‘‘super-tetrahedral
cage.’’ This super-cage is different from the T2 super-tetrahedron
consisting of four tetrahedra.3 The centre of the super-tetrahedron
in 3 contains a fifth tetrahedron surrounded by four tetrahedra.
Thermal gravimetric analysis (TGA) showed that complexes 1, 2
and 3 can be stable up to 400 uC. For complex 1, a weight loss of
5.08% from 50 to 340 uC corresponds to the loss of pyridine and
water molecules in the lattice (calcd.: 5.23%). Complex 1 is stable
up to 420 uC, and decomposes at higher temperatures. For
complex 2, the gradual weight loss of 13.32% from 50 to 260 uC
is consistent with the loss of all guest molecules in the lattice
(cacld.: 12.41%). There is no weight loss between 260 and 395 uC,
and complex 2 decomposes above 395 uC. For complex 3, there is
a weight loss of 15.66% from 50 to 300 uC, corresponding to the
loss of all solvent molecules in the lattice (cacl.d: 15.77%). There is
Photoluminescence studies of 3 showed that the excitation of the
solid samples of H3TATB and 3 at 268 nm produces the same
two luminescence peaks at 422 nm and 484 nm. These intense
blue luminescence signals may result from ligand-based p A p*
and n A p* transitions.15
˚
Table 1 Selected bond distances (A) in complexes 1–3
Td-M
Oh-M
M–O
M–O
M–m4-O
1
2
3
1.958(8)
2.00(2)
1.954(4)
2.073(3)
2.124(6)
2.067(1)
2.067(7)
2.06(2)
2.068(4)
Fig. 3 p–p Stacking of two triazine rings: the negatively charged nitrogen
atoms (in blue) are aligned with the positively charged carbon atoms (in
grey) in the other ring.
2664 | Chem. Commun., 2005, 2663–2665
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Crystal data for 3: C96H54N12O27Zn6, M 5 2199.73, cubic, space group
In conclusion, we have prepared three isostructural Mn, Co and
Zn MOFs with diamondoid networks containing large tetrahedral
cages using the unexplored TATB ligand. The introduction of
triazine rings into MOFs enhances the thermal stability signifi-
cantly. One of the reasons for the improved stability is due to the
exceptionally strong p–p stacking between every two adjacent
triazine rings. The introduction of flat nitrogen-containing
triangular ligands can be used as a general strategy to design
new MOF structural types with increased thermal stability.
The syntheses of MOFs using TATB ligands and other
transition metals are currently underway.
3
23
¯
˚
˚
Fd3, a 5 25.6887 (3) A, V 5 16952.2(3) A , Z 5 8, Dc 5 1.724 g cm
,
m 5 1.765 mm21, F(000) 5 8880, 18078 reflections measured, 964 unique
(Rint 5 0.0508) which were used in all calculations. Final residuals (for 109
parameters) were R1 5 0.0399, wR2 5 0.1224. CCDC 262520. See http://
www.rsc.org/suppdata/cc/b5/b502007g/ for crystallographic data in CIF or
other electronic format.
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This work was supported by Miami University, the donors of
the American Chemical Society Petroleum Research Fund (PRF-
38794-G3), the National Science Foundation (CHE-0449634,
ECS-0403669), and the Research Corporation (RI 1188).
Daofeng Sun, Shengqian Ma, Yanxiong Ke, Tracy M. Petersen and
Hong-Cai Zhou*
Department of Chemistry and Biochemistry, Miami University, Oxford,
Ohio, 45056, USA. E-mail: zhouh@muohio.edu; Fax: 513-529-8091;
Tel: 513-529-8091
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{ Synthesis of 1: A hydrothermal reaction of H3TATB (0.01 g, 2.27 6
1025 mol), Co(NO3)2?6H2O (0.025 g, 8.59 6 1025 mol) and 0.01 mL
pyridine in 12 mL H2O at 180 uC for 4 days resulted in the formation of
deep-blue crystals of 1 (yield: 28%). Anal. calcd. for 1 (%): C 54.20, H 2.57,
N 8.14; found: C 54.40, H 2.51, N 8.03.
Synthesis of 2: A mixture of H3TATB (0.01 g, 2.27 6 1025 mol),
Mn(NO3)2 (0.05 mL) and 0.025 mL pyridine in 1.5 mL
N,N-dimethylformamide was sealed in a Pyrex tube and heated to 110 uC
for 3 days, then cooled to room temperature. The resultant light-yellow
octahedral crystals were washed with DMF to give 2 (yield: 35%). Anal.
calcd. for 2 (%): C 52.01, H 3.25, N 8.33; found: C 51.54, H 3.34, N 8.03.
Synthesis of 3: A solvothermal reaction of H3TATB (0.005 g, 1.13 6
1025 mol), Zn(NO3)2?6H2O (0.020 g, 6.72 6 1025 mol), 1.5 mL
dimethylsulfoxide (DMSO), and 5 drops H2O2 (30% aq. solution) was
sealed in a Pyrex tube and heated to 90 uC, held for 12 h, and then heated
to 120 uC for 3 days (cooled to rt at a rate of 0.1 uC min21). The resultant
pale-yellow octahedral crystals were washed with DMSO to give the pure
sample (yield: 50%). Anal. calcd. for 3 (%): C, 48.67, H, 3.30, N, 6.55;
found: C, 48.39, H, 3.24, N, 6.49.
10 W. Clegg, I. R. Little and B. P. Straughan, Inorg. Chem., 1988, 27, 11,
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§ Crystal data for 1: C96H53Co6N12O26.5, M 5 2152.08, cubic, space group
3
23
¯
˚
˚
Fd3, a 5 25.7202 (5) A, V 5 17014.6(6) A , Z 5 8, Dc 5 1.680 g cm
,
m 5 1.236 mm21, F(000) 5 8696, 15971 reflections measured, 759 unique
(Rint 5 0.0702) which were used in all calculations. Final residuals (for 106
parameters) were R1 5 0.0753, wR2 5 0.2336. CCDC 262518.
Crystal data for 2: C96H55Mn6N12O27.5, M 5 2146.16, cubic, space
3
23
,
¯
˚
˚
group Fd3, a 5 25.764 (6) A, V 5 17102(7) A , Z 5 8, Dc 5 1.667 g cm
m 5 0.954 mm21, F(000) 5 8680, 6561 reflections measured, 443 unique
(Rint 5 0.0936) which were used in all calculations. Final residuals (for 109
parameters) were R1 5 0.0937, wR2 5 0.2425. CCDC 262519.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 2663–2665 | 2665