Guest molecule release triggers changes in the catalytic and magnetic properties of a FeII-based 3D metal-organic framework

Article abstract of DOI:10.1021/ic500004s

A Fe^II-based metal-organic framework (MOF), {[Fe ₂(pbt)₂(H₂O)₂]·2H ₂O}_n, undergoes an irreversible dehydration, which triggers changes in the catalytic and magnetic properties of the MOF. These property changes are attributed to the high-spin to low-spin transition of 7.1% center Fe^II, which is demonstrated by ⁵⁷Fe Moessbauer, X-ray photoelectron spectroscopy, and UV/vis absorption spectra.

Full text of DOI:10.1021/ic500004s

Communication
pubs.acs.org/IC
Guest Molecule Release Triggers Changes in the Catalytic and
Magnetic Properties of a FeII-Based 3D Metal−Organic Framework
Zhouqing Xu,†,‡ Wei Meng,† Huijun Li,†,‡ Hongwei Hou,*,† and Yaoting Fan†
†Department of Chemistry, Zhengzhou University, Henan 450001, People’s Republic of China
‡Department of Physics and Chemistry, Henan Polytechnic University, Henan 454000, People’s Republic of China
S
* Supporting Information
irreversible release of one free water molecule to yield
ABSTRACT: A FeII-based metal−organic framework
{[Fe2(pbt)2(H2O)2]·H2O}n (2). Moreover, dehydration triggers
(MOF), {[Fe2(pbt)2(H2O)2]·2H2O}n, undergoes an irre-
versible dehydration, which triggers changes in the
catalytic and magnetic properties of the MOF. These
property changes are attributed to the high-spin to low-
spin transition of 7.1% center FeII, which is demonstrated
noticeable changes in the catalytic and magnetic properties.
Single-crystal X-ray analysis reveals that complex 1 crystallizes
in the orthorhombic space group Pbcn. The Fe1 and Fe2 centers
display distorted octahedral coordination geometry and are all
coordinated by five N atoms from three pbt2− ligands and one
aqua O atom. As shown in Scheme S1 in the SI, there are two
crystallographically independent pbt2− ligands exhibiting two
different conformations due to the Ctriazole−Ctriazole rotation
flexibility, but all act as tridentate ligands, binding to three FeII
ions through two chelate bonds and one monodentate linkage. In
such a manner, a pair of symmetry-related Fe1 centers are linked
by a terminal triazole moiety of pbt2− to give a [Fe1(trz)]2
dimeric unit (Figure 1a), and adjacent Fe2 centers are joined by
by 57Fe Mossbauer, X-ray photoelectron spectroscopy, and
̈
UV/vis absorption spectra.
etal−organic frameworks (MOFs) are extended struc-
M
tures composed of metal-based nodes, organic linkers,
and sometimes guest molecules.1 They represent an exception-
ally rich field of functional materials because of their fascinating
structural features and interesting properties.2 This versatility
stems from their unique structures formed through ion-covalent
association of center metal ions and organic ligands and/or weak
interactions.3 Undoubtedly, center metal ions play a crucial role
in the determination of the properties of MOFs. Several groups
have demonstrated that changing the metal node in MOFs
through a single-crystal-to-single-crystal transmetalation can
make previously inert MOFs catalytically active or improve the
gas-sorption properties of the framework.4 In addition, the
properties of MOFs can also be changed by modifying the
organic ligands. Hupp et al. and Yaghi et al. have reported that
isomorphous MOFs synthesized by the same center metal ions
and ditopic carboxylate decorated with variously sized organic
functional groups have different capacities.5
The crucial role of the metal ions or clusters and organic
ligands in the determination of the structures and properties of
MOFs has been extensively studied.6 However, research on the
influence of guest molecules on the properties of MOFs is still
rare.7 Actually, it is of great current interest to illustrate the subtle
relationship between the guest molecules and properties of the
frameworks because these insights may open new ways to study
catalysis, magnetism, etc.8 To explore the influence of the guest
molecules on the properties of MOFs, it is necessary to obtain
chemically and thermally stable crystals that could maintain the
framework when guest molecules are exchanged or removed. It
has been demonstrated that metal azolate frameworks
constructed by triazolate or pyrazolate derivatives and metal
ions can meet the above requirements.9 Therefore, we chose a
triazolate derivative, 5′-(pyridin-2-yl)-2H,4′H-3,3′-bis(1,2,4-tri-
azole) (H2pbt), to construct a FeII-based MOF,
{[Fe2(pbt)2(H2O)2]·2H2O}n (1). Interestingly, crystals of 1
were allowed to stand at 160 °C for 1 day, undergoing an
Figure 1. (a) [Fe2(trz)]2 unit. (b) 1D [Fe1Lb]n chain. (c) View of the
3D structure along the c axis.
pbt2− ligands to form 1D [Fe2(pbt)]n chains along the b axis
(Figure 1b), respectively. Interestingly, each [Fe1(trz)]2 unit
serves as a pillar to connect four adjacent chains (Figure S2 in the
SI). In this way, these 1D chains are extended into a 3D
coordination framework (Figure 1c). After removal of all guest
water molecules, 1 shows approximate rectangular channels, with
10.45 × 4.58 Å windows from the c axis encompassing a
considerable solvent-accessible volume about 547.8 Å3 per unit
cell calculated by PLATON (Figure 1c).
Received: January 2, 2014
Published: March 24, 2014
© 2014 American Chemical Society
3260
dx.doi.org/10.1021/ic500004s | Inorg. Chem. 2014, 53, 3260−3262

Inorganic Chemistry
Communication
In addition, there exist strong hydrogen-bonding interactions
between the water molecules and triazole N atoms in 1 (Table S2
in the SI). It is worth noting that the O3 water molecule was
fastened to the framework by the strong hydrogen bonds O3−
N10 [2.792(6) Å] and O3−O1 [2.925(6) Å], while the unstable
O4 lattice water molecule was sited in the cavities by weak
hydrogen bonds O4−O2 [3.030(6) Å] and O4−N3 [3.090(8)
Å; Figures S3 and S4 in the SI). These facts may give rise to
dehydration of these two lattice water molecules at different
temperatures. Thermogravimetric analysis (TGA) of 1 (Figure
S5 in the SI) confirms that the first weight loss of 2.85% in the
range of 58−108 °C corresponds to one lattice water molecule
per formula unit (calcd 2.97%), then an approximate plateau
region in the range of 108−165 °C is observed, and the second
weight loss of 8.62% in the range of 165−195 °C corresponds to
one lattice and two coordinated water molecules per formula unit
(calcd 8.91%).
Because FeII ions exhibit dynamic Jahn−Teller distortion and
can provide open Lewis acid sites, Fe MOFs should be highly
active catalysts for some reactions requiring Lewis acidity, such as
aldol condensation, ring opening of epoxides, etc. Aiming to
evaluate the catalytic activity of 1, we have selected a
representative aldol condensation reaction requiring Lewis
acidity. The reaction of benzaldehyde, pyridylaldehyde, and
furaldehyde with methanol to form the corresponding dimethyl
acetal was investigated at 40 °C for 20 h using a small quantity of
complex 1 as the catalyst.10 Gas chromatographic analysis shows
that conversion values of these reactions are 65%, 45%, and 62%,
respectively.
Figure 2. (a) χMT versus T plots for 1 (black) and 2 (red). (b) 57Fe
Mossbauer spectra of complexes 1 and 2.
̈
value descends more rapidly, reaching a value of 2.13 cm3 K
mol−1 at 2 K, which is likely due to zero-field splitting. In contrast
to complex 1, the χMT value of complex 2 decreases more quickly
than that of 1 to a value of ca. 3.70 cm3 K mol−1 at 50 K and then
descends more sharply, reaching a value of 0.429 cm3 K mol−1 at
2 K. The quick decrease of the magnetic susceptibilities for 2 may
be attributed to a reduction of the Fe−N bonds, which shortens
the Fe−Fe distance and strengthens the antiferromagnetic
coupling effect. It is worth noting that at 300 K the χMT value of 2
(6.367 cm3 K mol−1) is lower than that of 1 (6.90 cm3 K mol−1)
by about 0.533 cm3 K mol−1, which amounts to 7% of the χMT
value of 1.
Why can a subtle difference in the guest molecule of the MOFs
stir up considerable changes in the catalytic and magnetic
properties? The difference in the magnetic and catalytic behavior
was traced to intrinsic differences in the molecular structures of 1
and 2.
First, the single-crystal X-ray structure determination reveals
that through the release of one guest molecule the structure of 1
display three changes: (1) the trapped water (O4) was lost; (2)
all of the Fe−O and Fe−N bonds were shortened, and especially
it should be pointed out that the Fe2−O2 bond length was
shortened significantly from 2.179 to 2.062 Å (Tables S4 and S5
in the SI and Figure 3); (3) the unit cell a parameter decreases
from 23.507(5) to 22.696(5) Å, and the unit cell volume
decreases from 4817.8(17) to 4646.7(16) Å3. These changes may
bear some of the responsibility for the differences in the catalytic
and magnetic properties of 1 and 2. On the one hand, 2 has one
less lattice water than 1, which is helpful for the substrates to pass
through the surfaces and approach the active centers of 2. Then
the coordinated water molecules are replaced to generate the
required reaction intermediates. Thus, the catalytic activity of 2 is
superior to that of 1. On the other hand, the release of the O4
water molecule and the reduction of the Fe−N and Fe−O bonds
disrupt the preexisting hydrogen-bond interactions and shorten
the Fe−Fe distance, which strengthens the antiferromagnetic
coupling effect of the FeII centers. In this way, the magnetic
properties of the complex are modified.
Inspired by the lattice water molecules in the cavities, we
attempt to explore the possibility of guest species dehydration.
Elimination of the guest molecules disrupts the preexisting
noncovalent interactions and may modify the properties of the
complex.11 After 1 stood at 160 °C for 1 day, the color of its
crystals changed from yellow to dark red. TGA of the dark-red
crystals shows that three water molecules are lost before 195 °C
(Figure S5 in the SI). Therefore, the dark-red crystals have only
one water less than 1, and the formula should be
{[Fe2(pbt)2(H2O)2]·H2O}n (2). We tried to remove the other
lattice water molecule by heating. Disappointingly, the complex
maintained at 180 °C for 1 day lost single crystallinity. TGA of
complex 1 also shows that the O3 lattice water molecule is
removed together with two coordinated water molecules in the
range of 165−195 °C. The subtle component difference of these
two MOFs motivates us to evaluate the catalytic activity of
complexes 1 and 2. Following the same method, the catalytic
activity of complex 2 is investigated. To our satisfaction,
conversion of benzaldehyde, pyridylaldehyde, and furaldehyde
is improved to 87%, 56%, and 78%, respectively. The results
demonstrate that the catalytic activity of 2 is higher than that of 1.
In other words, the catalytic activity of the FeII-based MOF is
significantly improved by release of a free water molecule.
Because elimination of the guest molecules may lead to
changes in the magnetic properties of the complexes,8a we
investigate the magnetic behavior of these two complexes. In
Figure 2a is shown the χMT versus T plots of complexes 1 and 2
measured in an applied field of 1000 Oe over the temperature
range 300−2 K. For 1, the χMT value is 6.90 cm3 K mol−1 at 300
K, which corresponds to two quintet spin-state FeII centers. This
value is higher than the usual one for paramagnetic FeII
complexes in the high-spin (HS) state, possibly because of a
contribution of the orbital angular moment. The χMT value
decreases slowly in the 300−50 K range. Below 50 K, the χMT
Second, the color change from yellow of 1 to dark red of 2 and
the reduction of the Fe−N and Fe−O bonds are attributed to the
3261
dx.doi.org/10.1021/ic500004s | Inorg. Chem. 2014, 53, 3260−3262

Inorganic Chemistry
Communication
AUTHOR INFORMATION
Corresponding Author
*E-mail: houhongw@zzu.edu.cn. Fax: +86-371-67761744.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was funded by the National Natural Science
Foundation (Grants 21371155 and 91022013) and the Research
Foundation for the Doctoral Program of Higher Education of
China (Grant 20124101110002).
REFERENCES
■
(1) (a) Bradshaw, D.; Garai, A.; Huo, J. Chem. Soc. Rev. 2012, 41,
2344−2381. (b) Li, J.-R.; Sculley, J.; Zhou, H.-C. Chem. Rev. 2012, 112,
869−932. (c) He, Y.; Zhou, W.; Krishna, R.; Chen, B. Chem. Commun.
2012, 48, 11813−11831. (d) Lang, J.-P.; Xu, Q.-F.; Yuan, R.-X.;
Abrahams, B. F. Angew. Chem., Int. Ed. 2004, 43, 4741−4745.
(2) (a) Liu, D.; Lang, J.; Abrahams, B. F. J. Am. Chem. Soc. 2011, 133,
11042−11045. (b) Lu, Z.; Zhang, R.; Li, Y.; Guo, Z.; Zheng, H. J. Am.
Chem. Soc. 2011, 133, 4172−4174. (c) Xu, Z.; Wang, Q.; Li, H.; Meng,
W.; Han, Y.; Hou, H.; Fan, Y. Chem. Commun. 2012, 48, 5736−5738.
(d) Liu, D.; Ren, Z.-G.; Li, H.-X.; Lang, J.-P.; Li, N.-Y.; Abrahams, B. F.
Angew. Chem., Int. Ed. 2010, 49, 4767−4770.
Figure 3. View of the difference of the asymmetric unit of complexes 1
(left) and 2 (right) and the color change from 1 to 2. Symmetry codes:
A, −0.5 + x, 0.5 + y, 0.5 − z; B, 1 − x, 1 − y, 1 − z.
spin-state transition of the FeII centers from HS to low spin (LS).
We consider that the HS → LS transition may play a crucial role
in the changes of the catalytic and magnetic properties from 1 to
2. For one thing, the HS → LS transition empties the eg orbitals.
This implies that the presence of unoccupied antibonding eg
orbitals of the FeII centers can favor the electron-pair acceptance
from the substrates and, therefore, formation of the required
reaction intermediates.12 For another, associated with the HS →
LS transition, there is a decrease in the relative number of
unpaired electrons, and hence the χMT value of 2 is lower than
that of 1 at room temperature. Also, the reduction of the Fe−N
bonds shortens the Fe−Fe distance and strengthens the
antiferromagnetic coupling effect of the FeII centers.
(3) (a) Duriska, M. B.; Neville, S. M.; Batten, S. B. Chem. Commun.
2009, 5579−5581. (b) Cook, T. R.; Zheng, Y.-R.; Stang, P. J. Chem. Rev.
2013, 113, 734−777.
(4) (a) Dinca, M.; Long, J. R. J. Am. Chem. Soc. 2007, 129, 11172−
11176. (b) Zhao, J.; Mi, L.; Hu, J.; Hou, H.; Fan, Y. J. Am. Chem. Soc.
2008, 130, 15222−15223. (c) Zhang, Z.; Zhang, L.; Wojtas, L.; Nugent,
P.; Eddaoudi, M.; Zaworotko, M. J. J. Am. Chem. Soc. 2012, 134, 924−
927.
(5) (a) Deng, H.; Grunder, S.; Cordova, K. E.; Valente, C.; Furukawa,
́
H.; Hmadeh, M.; Gandara, F.; Whalley, A. C.; Liu, Z.; Asahina, S.;
Kazumori, H.; O’Keeffe, M.; Terasaki, O.; Stoddart, J. F.; Yaghi, O. M.
Science 2012, 336, 1018−1023. (b) Gadzikwa, T.; Farha, O. K.; Mulfort,
K. L.; Hupp, J. T.; Nguyen, S. T. Chem. Commun. 2009, 3720−3722.
(6) (a) Xuan, W.; Zhu, C.; Liu, Y.; Cui, Y. Chem. Soc. Rev. 2012, 41,
1677−1695. (b) Das, M. C.; Xiang, S.; Zhang, Z.; Chen, B. Angew.
Chem., Int. Ed. 2011, 50, 10510−10520.
(7) (a) Wang, X. Y.; Wang, L.; Wang, Z. M.; Gao, S. J. Am. Chem. Soc.
2006, 128, 674−675. (b) Halder, G. J.; Kepert, C. J.; Moubaraki, B.;
Murray, K. S.; Cashion, J. D. Science 2002, 298, 1762−1765.
(8) (a) Li, B.; Wei, R. J.; Tao, J.; Huang, R.-B.; Zheng, L. S.; Zheng, Z. J.
Am. Chem. Soc. 2010, 132, 1558−1566. (b) Lu, Z. Z.; Zhang, R.; Li, Y. Z.;
Guo, Z. J.; Zheng, H. G. J. Am. Chem. Soc. 2011, 133, 4172−4174. (c) Li,
C.-P.; Du, M. Chem. Commun. 2011, 47, 5958−5972.
(9) (a) Zhang, J. P.; Zhang, Y. B.; Lin, J. B.; Chen, X.-M. Chem. Rev.
2012, 112, 1001−1033. (b) Aromía, G.; Barriosa, L. A.; Roubeaub, O.;
Gameza, P. Coord. Chem. Rev. 2011, 255, 485−546.
(10) Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H. Chem. Commun.
2012, 48, 11275−11288.
(11) (a) Ohmori, O.; Kawano, M.; Fujita, M. J. Am. Chem. Soc. 2004,
126, 16292−16293. (b) Lv, G.-C.; Wang, P.; Liu, Q.; Fan, J.; Chen, K.;
Sun, W.-Y. Chem. Commun. 2012, 48, 10249−10251.
57Fe Mossbauer spectroscopy and XPS show that all Fe atoms
̈
of the two complexes are in the oxidation state of II (Figures 2b
and S10 in the SI). As shown in Figure 2b, at 300 K, the spectrum
of complex 1 displays a quadrupole doublet, with the isomer shift
(δ) and quadrupole splitting (ΔEQ) characteristic of HS FeII
(Table S1 in the SI). Because all FeII centers in complex 1 are
equivalent with a [FeN5O] octahedral geometry, there are only
two doublets at 300 K. As for complex 2, apart from the main
doublet (blue and green lines) characterized by the quadrupole
splitting with ΔEQ = 2.17 and 2.82 mm s−1 and isomer shift δ =
0.98 and 1.04 mm s−1 corresponding to the HS state of FeII (S =
2), a poorly resolved doublet (turquoise line) of relative area
7.1% appears with ΔEQ = 0.21 mm s−1 and an isomer shift value
of 0.55 mm s−1 and is assigned to LS FeII (S = 0). The existence of
LS is also confirmed by the UV/vis absorption spectra. As shown
in Figure S6 in the SI, the spin-allowed 1A1 → 1T1 d−d transition
of the LS FeII appears as a 620 nm shoulder of the intense singlet
metal-to-ligand charge-transfer band at 480 nm.13 This result is
consistent with the 7% reduction of the χMT value of 1.
In conclusion, a FeII-based MOF, 1, loses a free water molecule
and converts to 2. Dehydration bears the responsibility for the
reduction of the Fe−O and Fe−N bonds and for the HS → LS
transition of 7.1% of the center ions. These results give rise to
changes in the catalytic and magnetic properties of the FeII-based
MOF.
(12) (a) Niu, S.; Hall, M. B. Chem. Rev. 2000, 100, 353−406.
(b) Platero-Prats, A. E.; Snejko, N.; Iglesias, M.; Monge, A.; Gutirrez-
Puebla, E. Chem.Eur. J. 2013, 19, 15572−15582.
(13) (a) Decurtins, S.; Gtlich, P.; Hasselbach, K. M.; Hauser, A.;
Spiering, H. Inorg. Chem. 1985, 24, 2174−2178. (b) Lever, A. B. P.
Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier: Amsterdam, The
Netherlands, 1984; Vol. 33.
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data in CIF format, experimental details,
and spectroscopic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
S
3262
dx.doi.org/10.1021/ic500004s | Inorg. Chem. 2014, 53, 3260−3262