Two novel isostructural and heteroleptic Nd(III) and Dy(III)-organic frameworks constructed by 2,5-pyridinedicarboxylic acid and in situ generated 2-pyridinecarboxylic acid: Hydrothermal synthesis, characterization, photoluminescence properties and heterogeneous catalytic activities

Article abstract of DOI:10.1016/j.poly.2017.04.011

The crystal structures, heterogeneous catalytic activities and photoluminescent properties of two novel three dimensional (3D) metal organic frameworks (MOFs), [Nd(pyc)(pydc)(H₂O)]_n (1) and [Dy(pyc)(pydc)(H₂O)]_n (2) (H₂pydc?=?pyridine-2,5-dicarboxylic acid, Hpyc?=?2-pyridinecarboxylic acid) are presented. The synthesized complexes under hydrothermal conditions were characterized by the elemental analysis, inductively coupled plasma (ICP), fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), single crystal X-ray diffraction and powder X-ray diffraction (PXRD) analysis. For morphological analysis, field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) were used. The compounds are isostructural and each Ln³⁺ (Nd(III) and Dy(III)) centers are eight-coordinated. MOFs have the distinct 3D open-framework architectures due to the presence of Hpyc ligands, which are converted from H₂pydc ligands in-situ decarboxylation under the hydrothermal conditions. Photoluminescence properties and thermal stabilities of the compounds have been investigated. Their heterogeneous catalytic activities have also been examined on the oxidation of the thymol (T) to thymoquinone (TQ). The structures of the MOFs were stable after three catalytic cycles. The maximum T conversion values, 40.37% and 36.45% were recorded with nearly 100% selectivities by using 1 and 2, respectively.

Full text of DOI:10.1016/j.poly.2017.04.011

Accepted Manuscript
Two novel isostructural and heteroleptic Nd(III) and Dy(III)-organic frame-
works constructed by 2,5-pyridinedicarboxylic acid and in situ generated 2-
pyridinecarboxylic acid: Hydrothermal synthesis, characterization, photolumi-
nescence properties and heterogeneous catalytic activities
Burak Ay, Emel Yildiz, İbrahim Kani
PII:
S0277-5387(17)30280-2
DOI:
http://dx.doi.org/10.1016/j.poly.2017.04.011
POLY 12589
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
15 February 2017
11 April 2017
Please cite this article as: B. Ay, E. Yildiz, I. Kani, Two novel isostructural and heteroleptic Nd(III) and Dy(III)-
organic frameworks constructed by 2,5-pyridinedicarboxylic acid and in situ generated 2-pyridinecarboxylic acid:
Hydrothermal synthesis, characterization, photoluminescence properties and heterogeneous catalytic activities,
Polyhedron (2017), doi: http://dx.doi.org/10.1016/j.poly.2017.04.011
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Two novel isostructural and heteroleptic Nd(III) and Dy(III)-organic frameworks
constructed by 2,5-pyridinedicarboxylic acid and in situ generated 2-pyridinecarboxylic
acid: Hydrothermal synthesis, characterization, photoluminescence properties and
heterogeneous catalytic activities
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Burak Ay a,*, Emel Yildiz a, İbrahim Kani b
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a Çukurova University, Department of Chemistry, Arts and Science Faculty, 01330, Adana, Turkey
b Department of Chemistry, Anadolu University,26470, Eskişehir, Turkey
Phone: +90 322 338 60 84/2442-20, Fax: +90 322 338 60 70
bay@cu.edu.tr
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Abstract
The crystal structures, heterogeneous catalytic activities and photoluminescent properties
of two novel three dimensional (3D) metal organic frameworks (MOFs),
[Nd(pyc)(pydc)(H2O)]n (1) and [Dy(pyc)(pydc)(H2O)]n (2)
(H2pydc = pyridine-2,5-
dicarboxylic acid, Hpyc = 2-pyridinecarboxylic acid) are presented. The synthesized
complexes under hydrothermal conditions were characterized by the elemental analysis,
inductively coupled plasma (ICP), fourier transform infrared (FT-IR) spectroscopy,
thermogravimetric analysis (TGA), single crystal X-ray diffraction and powder X-ray
diffraction (PXRD) analysis. For morphological analysis, field emission scanning electron
microscopy (FESEM) and atomic force microscopy (AFM) were used. The compounds are
isostructural and each Ln3+ (Nd(III) and Dy(III)) centers are eight-coordinated. MOFs have
the distinct 3D open-framework architectures due to the presence of Hpyc ligands, which are
converted from H2pydc ligands in-situ decarboxylation under the hydrothermal conditions.
Photoluminescence properties and thermal stabilities of the compounds have been
investigated. Their heterogeneous catalytic activities have also been examined on the
oxidation of the thymol (T) to thymoquinone (TQ). The structures of the MOFs were stable
after three catalytic cycles. The maximum T conversion values, 40.37% and 36.45% were
recorded with nearly 100% selectivities by using 1 and 2, respectively.
Keywords: Hydrothermal synthesis, Nd(III) and Dy(III) Coordination polymers, MOFs,
Decarboxylation, Heterogeneous Catalyst, Thymoquinone
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1. Introduction
Metal organic frameworks (MOFs) have been received great attention because of their
diversiform architectures and potential applications in the areas of molecular recognition, ion
exchange, catalysis, luminescent, gas adsorption and separation processes [1-5]. MOFs
containing lanthanide ions (Ln3+) are the most popular class of coordination polymers. They
can generate different topological structures, porous materials, unpredictable and amazing
coordination polymers. Ln3+ ions own larger radius and higher affinity for hard donor centers
and ligands with oxygen or hybrid oxygen-nitrogen atoms. Additionally, these ions with
special luminescent resulting from 4f electrons, which illustrate the coordination polymers are
intriguing and remarkably suitable for the development of optical devices [6]. Subcritical
Water Hydrothermal Synthesis (SWHS) has become a powerful method in the synthesis of
MOFs which exhibits an ideal performance on increasing the crystal growth under high
temperature and pressure conditions. However, unexpected in situ reactions have been
observed under SWHS conditions, such as formation of a sulfide-sulfide bond, the hydrolysis
of carboxylate esters, cyclo addition of organic nitriles with azide and ammonia,
dehydrogenative carbon-carbon coupling and decarboxylation of aromatic carboxylates [7-
10]. These reactions are significantly important in synthetic chemistry. Hydro- and solvo-
thermal in situ ligand synthesis is of great interest in coordination chemistry for the
preparation of unusual coordination complexes as well as for the discovery of new organic
reactions [11, 12]. This method is also provide unique medium to the synthesis of high purity
catalysts used in obtain of natural drugs.
Nigella Sativa L. (black seed or black cumin) has been known as natural drug for hundred
years. Nigella Sativa L. contains important pharmacologically active substances such as
thymol (T), carvacrol, thymoquinone (TQ), dithymoquinone and thymohydroquinone [13].
TQ is the main component of the black seed, which can obtain oxidation of T by using
effective homogeneous or heterogeneous catalysts. It has higher commercial value than the
precursor. TQ, representing 18.4-24% of the essential oil of black seed, has many different
activities. TQ has been known for its antioxidant [14], antihistamine [15], antinociceptive
[16], neuropharmacological activity [17], oxidative stress [18], antimicrobial [19] and
anticancer [20, 21] properties. Especially, in the past five years, it has been proven to be
effective in the treatment of cancer diseases such as colon [22], pancreas [23], prostate [24]
and breast [25, 26]. Some publications have been reported for the oxidation of T to TQ with
heterogeneous and homogeneous catalysts [27-31]. Heterogeneous catalysts have several
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advantages comparing to homogeneous catalysts, such as good dispersion of active sites,
simple product isolation, easier separation from the reaction mixture, they are lesser
consumption, environment-free and reusability [32-34]. Thus, the heterogeneous solid base
catalysts have been recognized as potential alternatives to homogeneous catalysts.
In this study, we described the synthesis and characterization of two novel 3D open
lanthanide-organic frameworks [Nd(pyc)(pydc)(H2O)]n (1) and [Dy(pyc)(pydc)(H2O)]n (2)
containing Hpyc ligand, which was formed in situ from H2pydc and lanthanum(III) salts. We
observed that in situ hydrothermal decarboxylation reactions can occur at H2pydc when it
reacts with nitrate salts of Ln3+. Interestingly, the decarboxylation of H2pydc was observed
during hydrothermal synthesis of MOFs. The results indicated that the decarboxylation
occured on the 5-carboxylate group rather than the 2-carboxylate group of H2pydc. Therefore,
Hpyc takes part in the construction of both complexes as the second ligand (Scheme 2).
Amazingly, the carboxylate group of Hpyc involved coordination while the pyridyl nitrogen
atom could not. These novel heterogeneous catalysts with mixed ligands have been
synthesized using ‘one-pot’ methodology by SWHS. Their crystal structures, heterogeneous
catalytic activities, and photoluminescent properties of the MOFs have been examined in
details.
2. Experimental
2.1. Materials and methods
Nd(NO3)3.6H2O, Dy(NO3)3.6H2O, H2pydc, TQ, T, piperazine, hydrogen peroxide (30%,
H2O2) and tert-Butyl hydroperoxide (~80%, TBHP) were purchased from commercial sources
and used without further purification. All hydrothermal syntheses were carried out in 23 mL
PTFE-lined stainless steel containers under autogenous pressure. IR spectra were recorded
with KBr pellets on a Perkin-Elmer RX-1 FT-IR spectrometer in the range of 4000-400 cm-1.
Elemental analysis was performed on Thermo Flash 2000 CHNS. Perkin-Elmer Optima
2100DV ICP instrument was used for the quantitative lanthanide analyses. Perkin Elmer Pyris
Diamond TG/DTA equipment was used for the TG analyses. Rigaku Miniflex system with
CuKα radiation (λ= 1.54059 Å) was used for the PXRD studies. Solid state phosphorescence
excitation and emission spectra were recorded on a Perkin-Elmer LS 55 Luminescence
Spectrometer. The FESEM and AFM images were recorded using Carl Zeiss, SUPRA-55 and
Nanosurf FlexAFM, respectively. A Thermo Brand chromatograph with TR5MS capillary
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columns was used for the GC/MS analysis. HPLC analyses were performed on Shimadzu
HPLC system equipped with a reversed phase C8 column.
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I-H2pydc, II-Piperazine, III- Hpyc, IV-Pyrazine, V-Pyridine, VI-3-pyridinecarboxylic acid,
VII-N-nitrosopiperazine, VIII-N-N’-dinitrosopiperazine
Scheme 1. Schematic representation of in situ decarboxylation and the forming new ligands
in solution phase under hydrothermal conditions (i), and the coordination modes of the
ligands with using lanthanides (ii).
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2.2. Synthesis of [Nd(pyc)(pydc)(H2O)]n (1)
A solution of Nd(NO3)3.6H2O (0.132 g, 0.30 mmol), 2,5-pyridinedicarboxylic acid (0.150
g, 0.90 mmol), piperazine (0.026 g, 0.30 mmol), and H2O (5 mL, 278 mmol) with the mole
ratio of 1:3:1:927 was stirred before heating at 180 °C for 72 hours. The initial and final pH
values were 2.20 and 3.40, respectively. The heterogeneous solution mixture was separated
from the solid phase, and the crystals washed by the water, and dried at room temperature.
Dark yellow crystals (Fig. 1) suitable for X-ray diffraction were isolated in 48% yield (based
on Nd). Anal. Calcd. for C13H9N2O7Nd: C, 34.71; H, 2.00; N, 6.23. Found: C, 34.10; H, 2.14;
N, 6.34%. The ICP analysis (%) showed that 1 contained Nd: 31.76; Calcd. For Nd: 32.10. IR
data (cm-1): 3487(m), 3071(m), 1661(s), 1591(s), 1481(m), 1396(s), 1363(s), 1033(m),
947(w), 820(m), 771(m), 755(m), 699(m), 590(m), 506(s), 430(w).
2.3. Synthesis of [Dy(pyc)(pydc)(H2O)]n ( 2)
The preparation of MOF 2 was similar to that of MOF 1 except that Dy(NO3)3.6H2O
(0.132 g, 0.30 mmol) was used instead of Nd(NO3)3.6H2O. The initial and final pH values
were 3.25 and 3.75, respectively. Colorless crystals (Fig. 1) suitable for X-ray diffraction
were isolated in 72% yield (based on Dy). Anal. Calcd. for C13H9N2O7Dy: C, 33.35; H, 1.92;
N, 5.98. Found: C, 33.77; H, 1.88; N, 6.13%. The ICP analysis (%) showed that 2 contained
Dy: 33.69; Calcd. For Dy: 34.74. IR data (cm-1): 3668(m), 3136(m), 1661(s), 1631(s),
1592(s), 1560(s), 1488(m), 1407(s), 1369(s), 1287(w), 1160(w), 945(m), 832(m), 761(m),
693(m), 573(w), 524(m), 437(w).
2.4. X-ray structure determination
The crystallographic data collection for 1 and 2 were collected with a Bruker AXS APEX
CCD diffractometer equipped with a rotation anode at 296(2) K using graphite monocromated
Mo Kα radiation (λ = 0.71073 Å). Diffraction data were collected over the full sphere and
were corrected for absorption. A Bruker SMART [35] program package was used for the data
reduction. Crystal data and structure refinement for the MOFs are summarized in Table 1.
Structure solution was found with the SHELXS-97 [36] package using the direct methods,
and refined SHELXL-97 [37] against F2 using first isotropic and later anisotropic thermal
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parameters for all nonhydrogen atoms. Hydrogen atoms were added to the structure model on
calculated positions. Geometric calculations were performed with Platon [38].
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Figure 1. High definiton microscope views of the synthesized MOFs.
3. Results and discussion
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3.1. Descriptions of crystal structures of the catalysts
Single-crystal X-ray diffraction studies revealed that 1 and 2 are isostructural, whose
crystals fall in the same centrosymmetric space group (P21/c). The symmetric and asymmetric
X-ray crystal structures of the Nd(III) and Dy(III) complexes are shown in Fig. 2. The crystal
data and structure refinement for the MOFs are given in Table 1. The selected bond lengths
(Å) and bond angles (°) of 1 and 2 are given in Table S1 and S2. For the convenience of
depiction, the structure of 1 is described as a representative example in detail here. MOFs are
crystallizes in the monoclinic system, space group P21/c. Each asymmetric unit of Ln3+
complexes contains one metal atom, one H2pydc, one Hpyc and one water molecule (Fig. 3).
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Figure 2. Molecular structure of 1 (a), and 2 (b). All hydrogen atoms are omitted for the
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clarity.
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Figure 3. Asymmetric unit of 1 and 2.
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Figure 5(b) shows that two Hpyc and two H2pydc carboxylate groups function as a bidentate
bridge and connect two Nd(III) ions to form a dimeric molecule with a Nd1···Nd1 Å
separation of 4.532 Å. Within the binuclear unit the pydc ligands chelates exclusively to a
single Ln site through the nitrogen donor, and one carboxylate terminus while the second
carboxylate group of same pydc2- ligand bridges two Ln centers of dimer. Metal to metal
distance between two building binuclear units, [Ln2(H2pydc)4(Hpyc)2(H2O)]n linked through
two pydc2- ligands, is 6.843 Å. Moreover, pydc2- ligands act as a bridge to link different
Ln(III) ions of adjacent chains, resulting in three-dimensional network structure (Fig. 5a). As
shown in Fig. 5b, in 3D structure of 1, the distances among five adjacent Nd(III) ions are
4.532, 6.260, 7.952, 9.353, 10.833 and 11.663 Å . Each Ln(III) ion is further bonded to one
chelated pydc through N and O atoms, one carboxylato oxygen of neighboring pydc, and one
aqua molecule.
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Table 1
Crystal data and structure refinement for 1 and 2.
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Code
1
2
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Crystal size
C13H9N2O7Nd
450.47
296(2) K
C13H9N2O7Dy
467.72
296(2) K
0.71073 Å
Monoclinic
0.31 x 0.18 x 0.15 mm
0.71073 Å
Monoclinic
0.21 x 0.38 x 0.19 mm
Space group
P21/c
P21/c
Unit cell dimensions a = 9.353(3)Å α = 90°
b = 14.316(5) Å β = 95.413(13)°
a = 9.1977(11) Å α = 90°
b = 14.1014(17) Å β = 95.983(4)°
c = 10.4690(13) Å γ = 90°
1350.5(3) Å3
c = 10.556(5)Å γ = 90°
Volume
Z
1407.2(8) Å3
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Density (calculated) 2.126 Mg/cm3
2.300 Mg/cm3
Absorption
3.730 mm-1
coefficient
5.575 mm-1
F(000)
872
892
Theta range for data
collection
2.19 to 28.68°
2.23 to 28.31°
-11<=h<=12, -19<=k<=19,
-13<=l<=13
-12<=h<=11, -18<=k<=18,
-13<=l<=13
Index ranges
Reflections collected 59287
48051
Unique reflections
3441 [R(int) = 0.0340]
3317 [R(int) = 0.0267]
Max. and min.
transmission
0.6046 and 0.3910
0.6046 and 0.3910
Data / restraints /
parameters
3441 / 0 / 216
3317 / 1 / 216
Goodness-of-fit on F2 1.014
1.024
Final R indices
[I>2σ(I)]
R1 = 0.0226, wR2 = 0.0698
R1 = 0.0160, wR2 = 0.0406
R1 = 0.0166, wR2 = 0.0408
0.785 and -0.807 e.Å-3
R indices (all data)
Largest diff. peak and
hole
R1 = 0.0238, wR2 = 0.0708
0.889 and -1.071 e.Å-3
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The Ln3+ ion in the complexes is eight-coordinated and consists of six O-atoms and one N-
atom from pyridine carboxylate ligands, and one O-atom from water. The coordination
geometry of the Ln3+ ions can be described as a distorted square-antiprism (Fig. 4), with
atoms O3, O4, O6 and O7 (for 2) / O1, O2, O3 and O6 (for 1) forming the upper and O1, O2,
O5 and N1 (for 2) / O4, O5, O7 and N1 (for 1) forming the lower plane. The mean plane
ο
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angle between upper and lower squares is 5.579 and 6.790 for 1 and 2 respectively.
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Figure 4. Square antiprism coordination geometry of 1 and 2.
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The 2,5-pydc groups are coordinated to the Ln3+ ion and can be classified into three different
coordination modes (Fig. 6): Pyridine and carboxylate group N–C–O are in a chelating mode,
in which nitrogen of pyridine ring and one of the oxygen of terminus carboxylato group
coordinate to the same Ln3+ ion, the second oxygen atom of carboxylato group coordinate to
neighboring Ln3+, the second carboxylato group (O–C–O) of same ligand is in a bidentate
mode, in which two O atoms coordinate to two different Ln3+ ions to form a dimer. The 2-
pydc group is coordinated to the lanthanide ions with only one type of coordination mode in
which two oxygens of carboxylato group coordinate to two different center in dimeric unit
through bidentate bridging mode.
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a)
b)
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c)
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Figure 5. a) 3D structure of 1. b) Nd-Nd distances in the structure of 1. c) A schematic view
of the uninodal 6-connected 3D framework of complex 1 and 2.
The average bond lengths of Nd–O(carboxyl) and Dy–O(carboxyl) bonds of the present complexes
are 2.35(3) and 2.326 (2) Å, and the O–Nd–O and O–Dy–O bond angles lie in the range
62.15(8)–149.6(9) ° and 63.91-148.3 (7) °, respectively (Table S1-S2). A comparative study
shows that the average Dy–O and Nd–O bond lengths are consistent with reported Dy(III) and
Nd(III) complexes [39-42]. The bond length for the oxygen atom of the water molecule to Nd
and Dy ions are 2.457 (3) Å and 2.369 (2) Å, respectively. In the crystal structure, some
important strong intra- and inter-molecular hydrogen bondings were observed among the
water molecule, pydc and pyc ligands. The hydrogen bond details are presented in Table S3.
Topological analysis showed that complex 1 and 2 have the same uninodal 6-connected 3D
framework and display pcu topology with the point symbol of (412.63) (Fig. 5c) [42-a].
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Figure 6. The coordination modes of pydc and pyc for 1.
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3.2. In situ hydrothermal decarboxylation and formation
GC-MS analysis was performed for the solution phase to understand new ligands
formation, and the possible reaction mechanism of in situ decarboxylation under
hydrothermal conditions. Interestingly, five new ligands, namely 3-pyridinecarboxylic acid,
N-nitrosopiperazine, N,N’-dinitrosopiperazine, pyrazine and pyridine, formation were
observed in the solution phase (Scheme 1-i). This result supported to decarboxylation
reaction, and formation of new ligands (Fig. S1). Nitroso groups came from the nitrate salts of
the lanthanide metals, and linked to only piperazine ligands in two different ways under
hydrothermal conditions. The decarboxylation was observed at 5-carboxylate group and
formed Hpyc in the presence of Nd(NO3)3.6H2O and Dy(NO3)3.6H2O at 180 °C. As shown in
Scheme 1-ii, H2pydc and Hpyc ligands coordinated to the lanthanide atoms in different
coordination modes. Zhao et al. reported the in situ decarboxylation mechanism at high
temperature and pressure for the deprotonating of carboxylic acids. According to the
Arrhenius equation, dlnk/dT = E/RT2, the reaction rate accelerates with the increase of
temperature under confined compression [43]. Lanthanide salt species have an impact on in
situ decarboxylation reactions as well as temperature and pressure values. For instance, the
H2pydc do not lose its carboxyl group in the presence of chloride salts of lanthanide at low
reaction temperatures. In addition, if the same experiment is performed without metal salt in
the equal conditions, neither decarboxylation nor new ligand formations observe in the
solution phase by GC-MS analysis. Consequently, the nitrate salts of the lanthanides have a
catalytic role in decarboxylation and formation of new ligands at high temperature under
subcritical water conditions.
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3.3. FT-IR spectra
The FT-IR spectra of the free H2pydc and the synthesized MOFs are shown in Figs. S2-
S4. Their absorption bands are assigned to the υ(O-H) stretching vibrations of coordinated
water molecules in unit cell in the ranges of 3680-3300 cm-1 [44, 45]. The carboxyl band of
the ligand is found at 1730 cm-1. The absence of this band indicates that H2pydc are
completely deprotonated in the form of pydc2- anions upon reaction with the metal ions. For 1,
the two strong bands are at 1661 and 1396 cm-1 corresponding to the stretching vibrations of
asymmetric and symmetric carboxyl groups. For 2, these bands are observed at 1631 and
1407 cm-1, respectively [46, 47]. Other important supporting evidence is metal
nitrogen/oxygen absorption band to ensure the coordination. The Ln-N and Ln-O stretching
vibrations were observed between 400 and 550 cm-1. The medium bands at 506 and 430 cm-1
may be ascribed to υ(Nd-N) [48] and υ(Nd-O) [49]. The new bands, having medium
intensities at 524 and 437 cm-1, can be assigned to the υ(Dy-N) [50] and υ(Dy-O) [51]
stretching vibrations. These absorption values are absent in the infrared spectra of the ligand.
The obtained results are in agreement with the data from the single crystal X-ray analysis.
3.4. Thermal analysis and PXRD patterns
To determine thermal stability of the complexes, MOFs crystals were selected under an
optical microscope. The similar thermal curves were obtained due to the isostructural nature
of the compounds (Fig. 7). Their thermogravimetric (TG) analysis was performed in the
temperature range from 50 to 800 °C under N2 atmosphere with a heating rate of 10 ºC.min-1.
TG/DTG analysis supported the single crystal X-ray results of the MOFs (Figs. S5-S6). The
compounds are thermally stable up to ∼200 °C. Above this temperature, the two TG curves of
compounds exhibited two main steps of weight loss. The TG curve of 1 showed the first
weight loss of 4.20% in the temperature range 201 to 317 °C, corresponding to the loss of one
coordinated water molecule per formula unit (Calcd. 4.00%). The second significant weight
loss of 61.79% corresponds to the decomposition of H2pydc and Hpyc ligands in the
framework in the temperature range of 340 to 563 °C, (Calcd. 63.90%). The residual weight
of 33.98% is attributed to the final product of Nd2O3 (Calcd. 32.09%). The TG plot of 2 also
showed the weight loss of 3.36% in the first stage between 221 and 333 °C, attributable to the
loss of a coordinating water molecule in formula unit (Calcd. 3.85%). The next weight loss of
59.04% corresponds to the decomposition of H2pydc and Hpyc ligands in the framework in
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the temperature range of 409-646 °C, (Calcd. 61.40%). The remaining weight of 33.98%
corresponds to Dy2O3, which is slightly greater than the calculated value of 34.74% because of
uncompleted decomposition.
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Figure 7. Thermogravimetric curves of 1 (solid line) and 2 (dotted line).
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313
314
315
316
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To determine the phase purities of the synthesized MOFs, the PXRD analysis were
performed at room temperature. Simulated patterns were obtained by using the single crystal
data and diffraction-crystal module of the Mercury (Hg) program. The experimental and
simulated XRD patterns of the MOFs from single-crystal X-ray data were shown in Fig. S7.
These patterns were compared with each other in terms of peak positions. The PXRD patterns
of the compounds are in good agreement with each other on the basis of the single crystal
structures. The compounds have been successfully obtained as pure crystalline phases, and
have high purities. Their high resolution microscope images support these results (Fig. 1).
3.5. Morphologies of the MOFs
The surface morphologies and microstructures of the obtained MOFs were performed by
using FESEM and AFM analyses. These techniques were significantly helpful to determine
the particle sizes and identify the distinct morphologies of the MOFs. Although the MOFs
were synthesized with the H2pydc and Hpyc ligands, their morphologies were found in
different shapes and sizes vary from 1 to 100 µm due to the using Nd(III) and Dy(III) metals.
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The images of 1 and 2 show smart look with thick shaped structure or stratified, which
indicate crystalline nature. The stratified shape structures are due to the aggregation of a great
number of monomers leading to the polymerization.
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(a)
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(b)
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331
332
333
334
335
336
Figure 8. AFM images of 1 (a) and 2 (b).
The FESEM images supported that the synthesized MOFs were in polymeric form. The
3D topographic images of the MOFs were characterized by using AFM, as shown in Figs. 8-
9. The topographies determined in 1.2 × 1.2 µm2 for Nd(III) and 3.0 × 3.0 µm2 for Dy(III)
reveal bumpy areas. In addition, 1 (roughness 22.21 nm) showed a higher level of roughness
compared to 2 (roughness 15.68 nm). These values supported that the surfaces are uniform
and compact [52]. The image of 2 is found to be more relatively smooth and homogeneous
compared to 1. The image of 2 showed much less smooth with sharp and rough structure.
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360
Figure 9. FESEM images of single crystals of 1 (a-d) and 2 (e-h).
3.6. Photoluminescence properties
The photoluminescence properties of free H2pydc, and the synthesized MOFs were
examined in the solid state at room temperature, as shown in Fig. 10. The synthesized MOFs
exhibited similar wide bands ranging from 500 to 600 nm. It is worth mentioning that we
could not see any characteristic emissions of Nd3+ and Dy3+ ions in the emission spectra of the
complexes. To understand the nature of the photoluminescence properties of the MOFs, the
emission spectrum of free ligand was measured under the same conditions. It was obtained
three primary emission bands in the regions of 200-300 nm (λex: 190 nm) [53]. Compared to
the H2pydc, the emission broad band of 1 mainly ranges from 560 to 600 nm with the
maximum wavelengths at 580 nm under excitation of 302 nm. For 2, the emission wide band
ranges from 590 to 640 nm with the maximum wavelengths at 608 nm under excitation of 302
nm. The photoluminescence emission bands are red shifted in the MOFs. The red shift is
presumably related to the intraligand luminescence emission [54]. Considering the strong
emission peaks at 580 nm for 1, and 608 nm for 2, which were different from the emission
band of the free ligand greatly. The lower energy band would be assigned to the ligand-to-
metal charge transfer (LMCT) [55, 56], and observed emission values of the complexes are
supported to the coordinated ligands to Nd3+ and Dy3+ ions.
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Figure 10. Solid state photoluminescence spectra of H2pydc and MOFs at room temperature.
3.7. Catalytic activity studies
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Although homogeneous catalysts have higher selectivity, they cannot be easily recovered
from the reaction medium by filtration like heterogeneous catalysts. Catalyst recovery is a
very important property for the industrial processes. Heterogeneous catalysts are superior to
homogeneous catalysts due to this characteristic feature. Cheap and environmentally friendly
oxidant agents namely H2O2 and TBHP have mostly been used as oxidant agents in the
examination of the catalytic activities of MOFs towards oxidation reactions in a
heterogeneous medium. H2O2 and TBHP reagents were used as oxidant agents for the
catalytic reactions in acetonitrile. All reactions were performed in the temperature of 60 °C.
The catalytic activity studies were performed according to the reference [57]. The obtained T
conversion values (%) in different conditions are given in Table 2. The characterization of the
TQ was performed by GC/MS (Fig. S8). The catalytic performance of the catalysts was
determined by HPLC analysis. The retention times of the T and oxidation product, TQ were
detected at 254 nm by HPLC before the catalytic studies (Figs. S9-S10). The clear thing is T
cannot be oxidized to TQ without catalyst but only trace amount of product (Fig. S11).
Whereas, Using 1 and 2, T significantly oxidized to TQ (Figs. S12-S13). TQ is obtained in
trace amounts in the absence of catalyst (see Scheme 2). The conversion values were plotted
as a function of temperature in 60 °C for the determination of their catalytic activities.
According to HPLC analysis, maximum T conversion values were 40.37% with H2O2, and
32.71% with TBHP by using 1. On the other hand, 36.45% with H2O2 and 22.05% with TBHP
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401
were obtained in the medium of 2. It was not observed any by products in all the HPLC
chromatograms, so their selectivities were approximately 100% (Figs. S14-S17). Other
variables such as amount of the catalysts, type of the oxidant agents and reaction times are
shown in Table 4. The activities of the MOFs were optimized with 0.15 g of complex using
H2O2 and TBHP as oxidants at 60 °C. In both oxidant agents, T conversion values were
obtained maximum rate when the reaction time and MOFs amounts increased. To determine
the whether catalytic effect of the ligand and metal salts or not, the same catalytic reaction
medium were prepared. It was not observed any product in the HPLC chromatogram by using
only ligand. Trace amounts of TQ were obtained using both solely Nd(III) and Dy(III) salts
(Figs. S18-S20) for the same aim. These results showed that ligand and lanthanide salts were
not showed any significant catalytic activities in the oxidation of T to TQ. At the end of the
catalytic studies, it was not observed any by-product except main oxidation product by using
1 and 2 as catalysts, and oxidant agents. The maximum T conversion values, 40.37% and
36.45% were recorded with nearly 100% selectivities by using 1 and 2, respectively. As it
seen from the PXRD results and the high definition microscope views, the MOFs have high
purities and crystallinity properties.
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Scheme 2. Oxidation of T (a) to TQ (b) using catalyst and without catalyst.
3.8. Separation and reusability of the MOFs
Reusability is one of the important properties for the heterogeneous catalysts. They can
be recycled easily from the reaction medium using convenient solvents. To examine the
reusability of the MOFs, four sequential reactions were performed for 0.15 g MOFs using
hydrogen peroxide as an oxidant agent for 24 hours at 60 °C. After every reuse, the catalysts
were separated from the reaction mixture, filtered off, and washed with methanol (10 mL),
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420
acetone (10 mL), and then dried at room temperature. The conversion values remained
comparable worth after three tests. These results support the reusability of the catalysts
without any significant loss of activity (Table S4), but these values decreased in the fourth
tests. The obtained results showed that the MOFs have long usability for the oxidation of T to
TQ (Fig. 11). The MOFs contents of the filtrates were determined by ICP analysis after the
separation of the catalysts by filtration. At the end of the four runs, only 1.8% of 1 and 1.5%
of 2 were lost into the solution during the reusability reactions.
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Figure 11. The effects of oxidants on the oxidation reaction for the MOFs catalytic system.
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Table 2
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% T conversion values under different reaction conditions.
_________________________________________________________________________
H2O2
TBHP
____________________________________
___________________________________
Catalyst Reaction
Amount
(g)
T
Catalyst Reaction
Amount
(g)
T
Catalyst
Time
(hour)
Conversion
(%)
Catalyst
Time
(hour)
Conversion
(%)
___________________________________________________________________________
3
6
9
12
24
3
6
9
12
24
3
2.63
5.70
9.07
10.34
12.17
3.22
3
6
9
12
24
3
6
9
12
24
3
3.67
5.91
10.37
15.56
21.13
4.60
0.05
0.10
0.15
0.05
0.10
0.15
6.17
7.67
13.07
18.46
24.50
9.24
18.66
25.50
29.31
40.37
12.27
17.35
22.90
5.07
1
1
6
9
12
24
6
9
12
24
8.33
16.11
24.08
32.71
___________________________________________________________________________
3
6
1.64
4.16
3
6
1.66
2.70
0.05
0.10
0.15
9
5.29
7.58
9.14
3.22
0.05
0.10
0.15
9
3.55
6.06
7.89
3.33
5.70
8.64
11.28
14.44
5.17
12
24
3
6
9
12
24
3
6
9
12
24
3
6
9
12
24
3
6
9
7.19
10.58
14.17
18.23
6.07
11.80
19.91
28.67
36.45
2
2
7.27
11.37
16.57
22.05
12
24
12
24
441
___________________________________________________________________________
442
443
(Temperature: 60 °C; Catalyst: [Nd(pyc)(pydc)(H2O)]n (1) ; [Dy(pyc)(pydc)(H2O)]n (2) ; Solvent: Acetonitrile).
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459
In order to evaluate the efficiency of the present catalysts, we have compared the
obtained results of the oxidation of the T with those of other catalysts and shown in the Table
3. Although Mn(TDCPP)Cl and FePor catalysts show high selectivity and good T conversion,
they are homogeneous catalysts and can only be used once. Others, [MnIII(H2O)BW11O39]6-
shows 35-40% conversion but four major oxidation products, 2-isopropyl-5-methyl-1,4-
benzoquinone, 2-hydroxy-3-isopropyl-6-methyl-1,4-benzoquinone, 2-hydroxy-6-isopropyl-3-
methyl-1,4-benzoquinone and 4-hydroxy-3-isopropyl-6-methyl-1,2-benzoquinone, are found.
FePcTS shows 99% conversion, whereas three major products TQ and two different polymers
occur. As a heterogeneous catalyst, MnTBzPyP shows 100% selectivity and less than 18%
conversion. The synthesized heterogeneous catalysts show similar selectivity, which are
higher conversion values than the MnTBzPyP catalyst. Clearly, the present catalysts also give
higher yield than the previous heterogeneous catalytic studies. Their most important
advantage over other catalysts is that they can be used three times for the oxidation of T to
TQ.
Table 3
Comparison of the results obtained for the oxidation of the T with other catalysts.
Catalyst
Catalyst Type Oxidant T (°C)
Selectivity (%)
% T
Ref.
Mn(TDCPP)Cl
MnTBzPyP
[MnIII(H2O)BW11O39]6-
FePcTS
FePor
MOF 1
MOF 1
MOF 2
MOF 2
Homogeneous H2O2
Heterogeneous H2O2
Homogeneous H2O2
Homogeneous KHSO5
Homogeneous KHSO5
Heterogeneous H2O2
Heterogeneous TBHP
Heterogeneous H2O2
Heterogeneous TBHP
RT
RT
BP
RT
RT
60
60
60
60
98.9
100
-
18-31
-
100
100
100
100
-
[27]
[28]
[29]
[30]
[31]
TW
TW
TW
TW
<18
35-40
99
47.6
40.37
32.71
36.45
22.05
RT: Room temperature, BP: Boiling point, TW: This work
460
461
462
463
464
465
4. Conclusions
In conclusion, we have successfully synthesized two novel heterogeneous MOFs based
on pyridine-2,5-dicarboxylic acid, and in situ formed 2-pyridinecarboxylic acid under
hydrothermal conditions. The resulting decarboxylated ligand, 2-pyridinecarboxylic acid, is
20

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observed as the second organic linker in the frameworks. Isostructural Nd(III) and Dy(III)
complexes are eight-coordinated with the oxygen atoms from pyc-, oxygen and nitrogen
atoms from pydc2-, and water molecule. When the pyc- acts as a mono dentate ligand from the
O-donor atom, the pydc2- is coordinated via two oxygen and one nitrogen atoms, acting as a
tridentate ligand.
The heterogeneous catalysts showed high selectivity (100%) in the oxidation of T to TQ.
It was not observed any by-product in all HPLC chromatograms. The catalysts can be used for
three cycles and they are convenient heterogeneous catalysts for the oxidation of T to TQ.
Moreover, their photoluminescent spectra show that 1 and 2 have potential luminescent
properties at the maximum emissions at 580 and 608 nm, respectively.
Acknowledgements
The authors gratefully acknowledge financial support from the Research Unit of
Çukurova University (Grant No. FEF2013D5), and the Medicinal Plants and Medicine
Research Centre of Anadolu University, Eskişehir, Turkey, for the use of X-ray
Diffractometer.
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Appendix A. Supplementary material
CCDC 1456508 and 1456507 contains the supplementary crystallographic data for
complexes 1 and 2, respectively. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
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Graphical Abstract (Synopsis)
Two novel 3D lanthanide-organic frameworks have been synthesized under hydrothermal
conditions. The catalytic performances of MOFs were investigated and their selectivities were
measured as 100% for the oxidation of thymol to thymoquinone. The photoluminescence
properties of the MOFs have also been examined.
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Graphical Abstract (Pictogram)
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24