Hydrothermal synthesis of a novel μ-dihydroxo-bis(2,6- pyridinedicarboxylatoaquachromium(III)) complex and investigation of its catalytic activity

Article abstract of DOI:10.1016/j.ica.2011.12.043

A new chromium(III) compound [Cr(H₂O)(OH)(C₇H ₃NO₄)]₂, μ-dihydroxo-bis(2,6- pyridinedicarboxylatoaquachromium(III)), has been successfully synthesized by the hydrothermal reaction of 2,6-pyridinedicarboxylic acid (H₂pydc) and CrCl₃·6H₂O in the alkali medium. This metal complex was characterized by elemental analysis, IR spectroscopy, thermal analysis and single-crystal X-ray diffraction. The X-ray analysis showed that the structure is molecular and consists of units of two Cr(H₂O)(OH) (C₇H₃NO₄) groups linked through two OH groups. In the complex, each Cr centre is in a distorted octahedral environment, defined by two hydroxyl bridges, an aqua ligand and a tridentate pyridinedicarboxylate anion. The thermal stability and catalytic properties of the complex were also investigated. The complex affected the oxidation of thymol to thymoquinone with 100% selectivity and a conversion rate of 20.25%.

Full text of DOI:10.1016/j.ica.2011.12.043

Inorganica Chimica Acta 387 (2012) 15–19
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Hydrothermal synthesis of a novel
l-dihydroxo-bis(2,6-pyridinedicarboxylatoaqu-
achromium(III)) complex and investigation of its catalytic activity
a,
⇑
a
b
b
Burak Ay , Emel Yildiz , Stephanie Jones , Jon Zubieta
a
Department of Chemistry, Arts and Science Faculty, Cukurova University, Balcali 01330, Adana, Turkey
Department of Chemistry, Syracuse University, Syracuse, NY 13244, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 October 2011
Received in revised form 5 December 2011
Accepted 22 December 2011
Available online 29 December 2011
2
A new chromium(III) compound [Cr(H O)(OH)(C H NO )] , l-dihydroxo-bis(2,6-pyridinedicarboxylato
2
7
3
4
aquachromium(III)), has been successfully synthesized by the hydrothermal reaction of 2,6-pyridinedicar-
boxylic acid (H pydc) and CrCl O in the alkali medium. This metal complex was characterized by ele-
Á6H
mental analysis, IR spectroscopy, thermal analysis and single-crystal X-ray diffraction. The X-ray analysis
showed that the structure is molecular and consists of units of two Cr(H O)(OH)(C NO ) groups linked
2
3
2
2
7
H
3
4
through two OH groups. In the complex, each Cr centre is in a distorted octahedral environment, defined by
two hydroxyl bridges, an aqua ligand and a tridentate pyridinedicarboxylate anion. The thermal stability
and catalytic properties of the complex were also investigated. The complex affected the oxidation of thy-
mol to thymoquinone with 100% selectivity and a conversion rate of 20.25%.
Keywords:
Cr(III) complex
2
,6-Pydc
Hydrothermal synthesis
Heterogeneous catalysis
Crystal structure
Ó 2011 Elsevier B.V. All rights reserved.
Thymoquinone
1
. Introduction
food industries as active components, and the synthetic routes of-
ten involve epoxides or alcohols as intermediates [15,16]. Some
The synthesis of organic–inorganic hybrid compounds synthe-
sized under hydrothermal conditions at temperatures of 100–
00 °C is of great interest. This synthetic strategy not only affords
the opportunity to prepare complexes with novel structures and
properties but also provides an environmentally friendly approach
for the construction of coordination compounds through crystal
engineering [1,2]. For this purpose, the pyridinedicarboxylato
reactions of considerable commercial importance are: epoxidation
of limonene to limonene oxide [17,18]; and oxidation of carvacrol
or thymol to thymoquinone, a compound with antitumor and
hepatoprotective effects [19a,19b–21]. In this vein, thymol can be
oxidized to thymoquinone (Scheme 1), which has a commercial va-
lue considerably higher than that of the precursor. Since the natu-
ral sources of thymoquinone are limited to certain plants such as
Nigella sativa, Callitris articulate, there is a growing interest in its
production from other sources. It is well established that the chem-
ical transformation of abundant and cheap natural products can
make available other more valuable products.
2
(
pydc) ligands, such as 2,3-pydc, 2,4-pydc, 2,6-pydc, 3,4-pydc,
and 3,5-pydc, and d-block metal ions have been extensively used
3–8]. Coordination compounds incorporating pyridinedicarboxy-
[
lato ligands have been shown to form a wide range of interesting
network topologies, for example, 1D chains, 2D layers and 3D open
frameworks [9–11]. The most useful strategy for constructing such
structures is to employ appropriate bridging ligands. In general,
Herein, we report the hydrothermal synthesis and crystal
structure of new chromium compound [Cr(H
2 7 3 4 2
O)(OH)(C H NO )]
(C NO = 2,6-pyridinecarboxylate), which shows high catalytic
H
7 3
4
2
,6-pyridinedicarboxylic acid is an effective tridentate ligand,
activity in the oxidation of thymol, as well as high thermal stability.
To the best of our knowledge, no other researchers’ studies on the
oxidation of thymol in the presence of Cr(III) metal substituted
which can, for example, react with main group elements to give
stable heterocycles. Transition metal ions and pyridinedicarboxyl-
ate ligands prefer to form mono, multi-nuclear or 1D coordinating
structures due to the steric hindrance of the ligand [12–14].
One potential application of transition metal pyridinecarboxyl-
ate complexes is the oxidation of organic substrates. For example,
monoterpenes are widely used in pharmaceutical, cosmetic and
2
H pydc have been reported before in literature.
2
. Experimental
2.1. Materials and methods
⇑
Corresponding author. Tel.: +90 322 3386084/2442; fax: +90 322 3386070.
E-mail address: bay@cu.edu.tr (B. Ay).
CrCl
3
Á6H
2 2
O, H pydc, NaOH, thymol (2-isopropyl-5-methylphenol),
thymoquinone (2-isopropyl-5-methyl-pbenzoquinone), acetonitrile
0
020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.12.043

1
6
B. Ay et al. / Inorganica Chimica Acta 387 (2012) 15–19
2.3. Crystal structure determination
O
The X-ray single-crystal structure was performed on a Bruker
SMART system equipped with a CCD detector, using MoK radia-
[
Cr(OH)(H O)(C H NO )]
a
2
7
3
4
2
tion (k = 0.71073 Å) at 90 K. Data were corrected for Lorentz and
polarization effects, and absorption corrections were made using
SADABS [22]. The structure solution and refinement were carried
out using the SHELXL-96 [23] software package. The structure was
solved by direct methods and the nonhydrogen atoms refined
Solvent, H O2
2
HO
O
2
against F anisotropically. Crystal data are summarized in Table 1
and selected bond distances and bond angles are given in Table
Scheme 1. Oxidation of thymol.
2. The remained data of bond lengths and angles, atomic positional,
anisotropic displacement and hydrogen coordinates parameters
are available in the Supporting information (Tables S1–S5).
and hydrogen peroxide (30%) were purchased from Sigma–Aldrich.
Other reagents and solvents used were HPLC grade and obtained from
Sigma–Aldrich. The synthesis was carried out in Parr acid digestion
bombs with 23 mL poly(tetrafluoroethylene) lined stainless steel con-
tainers under autogenous pressure. The reactants were stirred briefly
and initial pH measured before heating. The initial and final pH values
of the reactions were measured using the Hanna 211 pH meter. Infra-
2
.4. Oxidation of thymol
Thymol oxidation was carried out at 60 °C in a three-necked flask
(
250 mL) equipped with a magnetic stirrer, a reflux condenser and a
temperature controller. Thymol (3.0 g, 20 mmol) and 20 mL solvent
acetonitrile and ethanol) were added successively into the flask.
Then, the appropriate amount of 30 wt.% aqueous H (6 mL)
(
red spectra was obtained on a Perkin–Elmer RX-1 (KBr disk; 4000–
2 2
O
À1
4
00 cm ) FTIR spectrometer. Thermogravimetric analysis (TGA)
was added to provide the required thymol-to-hydrogen peroxide
molar ratio. After heating this mixture to 60 °C, the catalyst (0.2 g)
was added to the mixture to initiate the reaction. The reaction was
monitored by taking samples at different times (1, 3, 5, 7 and 15 h).
was performed with a Perkin–Elmer Pyris Diamond TG/DTA N
2
(20–
800 °C range) at a heating rate of 10 °C/min. Characterization, % selec-
tivity and % conversion studies of the oxidation product thymoqui-
none were analyzed by high performance liquid chromatography
(
HPLC) and headspace GC/MS.
3
. Results and discussion
Headspace GC/MS was performed using a Thermo Scientific
Trace GC Ultra ISQ chromatograph and helium as the carrier gas.
3.1. Description of structure
TR5MS capillary columns (60 m  0.25 mm i.d.; 25
lm film thick-
ness) were used. The chromatographic conditions were as follows:
À1
The title complex was obtained by the hydrothermal method in
initial temperature: 50 °C; heating rate: 3 °C min ; final tempera-
alkali medium. The crystallographic analysis revealed that the
ture: 250 °C; injector temperature: 240 °C. The percentage of each
compound in the reaction mixtures was estimated directly from
the corresponding peak area. A Shimadzu HPLC system equipped
Table 1
with a reversed phase C18 column (250 Â 4.6 mm i.d., 5
l
m parti-
cle sizes, Phenomenex ) was used for HPLC analysis. Aliquots of
l were injected and separation was carried out with an iso-
Crystal data and structure refinement for the compound.
Ò
Formula
14 2 2 12
C H12Cr N O
20 l
Formula weight
T (K)
504.26
90(2)
cratic mobile phase consisting of methanol–acetonitrile–water
À1
Wavelength (Å)
Color
0.71073
Green
(
60:20:20, v/v/v) with a flow-rate of 1.0 mL min and at 35 °C col-
umn temperature. The detection wavelength was 254 nm for
detecting thymol and thymoquinone, which has a low absorptivity
at 254 nm. This wavelength was chosen since they gave the maxi-
mum absorbance. Prior to injection of a sample, the column was
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
triclinic
P1ꢀ
7.1299(12)
7.9558(14)
8.8443(15)
81.679(3)
69.763(3)
65.360(3)
427.85(13)
2
À1
equilibrated with the mobile phase at a flow-rate of 1.0 mL min
for at least 30 min or until a steady baseline was obtained. Percent-
age of thymol conversion and thymoquinone yield in the reaction
mixtures were estimated from the calibration curves.
a
(°)
b (°)
(°)
c
3
V (Å )
Z
D
2.2. Synthesis of catalyst
À3
calc. (mg/m
)
1.957
1.346
254
À1
Absorption coefficient (mm
F(000)
)
A mixture of CrCl
.5 mmol), NaOH (0.045 g, 1.1 mmol) and H
3
Á6H
2
O (0.133 g, 0.5 mmol), H
2
pydc (0.084 g,
0
2
O (5 mL, 278 mmol)
Crystal sizes (mm)
h range for data collection
Index ranges
0.18 Â 0.12 Â 0.06
2.45–28.08
in the mole ratio 1.00:1.00:2.20:278 was placed in a 23 mL Tef-
lon-lined Parr acid digestion bomb and heated for 5 days at
À9 6 h 6 9, À10 6 k 6 10,
À11 6 l 6 11
1
60 °C under autogenous pressure. Upon cooling to room temper-
Reflections collected
Independent reflections
Completeness to h
4246
ature, green crystals of compound suited for single crystal X-ray
diffraction analyses were obtained. The solution was filtered,
and the product was washed with water and acetone and dried
in the air. Yield: 61% based on Cr. Initial pH, 3.40; final pH,
2065 [Rint = 0.0225]
28.08%, 98.9%
multiscan
Absorption correction
Maximum and minimum transmission 0.9236–0.7937
Data/restraints/parameters
2065/0/160
1.067
1
.90. The crystals are insoluble in water and common solvents.
Goodness-of-fit (GOF) on F2
Anal. Calc. For C14
C, 33.12; H, 2.44; N, 5.19%. IR data (KBr pellet, cm ): 3467(w),
H
12Cr
2
N
2
O
12: C, 33.35; H, 2.40; N, 5.56. Found:
Final R indices [I > 2
R indices (all data)
r
(I)]
R
R
1
= 0.0402, wR
= 0.0473, wR
2
= 0.1009
= 0.1056
À1
1
2
Largest difference in peak and hole
0.793 and À0.478
3
6
075(m), 1634(s), 1433(w), 1367(s), 1087(s), 924(m), 745(m),
86(w), 560(s), 439(s).
À3
(
e Å
)

B. Ay et al. / Inorganica Chimica Acta 387 (2012) 15–19
17
Table 2
Fig. S2). Thermogravimetric analysis (TGA) was performed under
atmosphere at 1 atm with a heating rate of 10 °C/min on a Per-
Selected bond lengths (Å) and angles (°) for complex.
N
2
Cr(1)–O(6)
Cr(1)–O(6)
Cr(1)–N(1)
Cr(1)–O(5)
Cr(1)–O(3)
Cr(1)–O(1)
O(6)–Cr(1)
O(6)–Cr(1)–O(6)
O(6)–Cr(1)–N(1)
O(6)–Cr(1)–N(1)
O(6)–Cr(1)–O(5)
O(6)–Cr(1)–O(5)
N(1)–Cr(1)–O(5)
O(6)–Cr(1)–O(3)
O(6)–Cr(1)–O(3)
N(1)–Cr(1)–O(3)
O(5)–Cr(1)–O(3)
O(6)–Cr(1)–O(1)
1.947(2)
1.950(2)
1.978(2)
1.9885(19)
2.0120(18)
2.0290(17)
1.946(2)
78.48(10)
93.59(9)
172.06(8)
174.22(8)
95.74(8)
92.19(8)
93.92(8)
102.18(8)
78.63(8)
87.25(8)
93.09(8)
O(6)–Cr(1)–O(1)
N(1)–Cr(1)–O(1)
O(5)–Cr(1)–O(1)
O(3)–Cr(1)–O(1)
Cr(1)–O(6)–Cr(1)
Cr(1)–O(6)–H(4)
Cr(1)–O(6)–H(4)
Cr(1)–O(5)–H(6)
Cr(1)–O(5)–H(5)
C(6)–N(1)–Cr(1)
C(2)–N(1)–Cr(1)
C(7)–O(3)–Cr(1)
C(1)–O(1)–Cr(1)
101.29(8)
78.46(8)
88.03(8)
156.40(8)
101.52(10)
112(3)
132(3)
114(3)
121(3)
118.51(17)
118.60(17)
117.00(15)
116.44(15)
kin–Elmer Diamond TG/DTA. The TGA curve of the compound
exhibits thermal stability up to 278 °C. There are three weight loses
in the region from 278 to 800 °C. The first weight loss of 3.63% be-
tween 278 and 292 °C, corresponds to the loss of the coordinated
water molecule (calc. 3.57%). The second weight loss of 7.01% be-
tween 353 and 376 °C, corresponds to the loss of two hydroxyl
bridges (calc. 6.75%). The last weight loss of 57.60% between 418
and 472 °C, corresponds to the decomposition of molecular organic
2 3
ligands (calc. 56.55%). The final residue was probably Cr O
(remaining weight: found, 31.79%; calc. 32.74%). The observed
thermal behavior reflects the structural features and the TGA re-
sults basically agree with the formula of metal complex.
3.4. Catalytic activity
Percent selectivity and characterization studies of the thymo-
Symmetry transformations used to generate equivalent atoms: #1 Àx + 2, Ày + 2,
Àz + 1.
quinone were carried out using headspace GC/MS. The headspace
GC/MS chromatogram and spectrum of the oxidation product thy-
moquinone are shown in Fig. S3 (Supporting information). The oxi-
dation of thymol gave thymoquinone with 100% selectivity.
The catalytic activity of the title compound was evaluated for
the oxidation of thymol in the presence of acetonitrile and ethanol
with hydrogen peroxide using a standard HPLC analysis technique.
Table 3 summarizes the results, i.e., percentage of thymol conver-
sion. Conversion values were determined from thymol and thymo-
quinone calibration curves. Hydrogen peroxide is an inexpensive
ꢀ
complex crystallized in the triclinic system, space group P1. The
mononuclear subunit unit of the complex consists of a Cr(III),
two OHs, one deprotonated H
2 2
dpca ligand, and one H O molecule
(Fig. 1). The binuclear molecular assembly results from the linking
of mononuclear subunits through hydroxyl bridges. The Cr atom
has slightly distorted octahedral coordination geometry, defined
by one nitrogen and two carboxylate oxygen atoms from 2,6-pydc
and one oxygen atom from hydroxyl in the equatorial plane while
two oxygen atoms from coordinated water and a hydroxyl group
occupying the axial positions. The bond distance for Cr(1)–N(1)
of 1.978(2) Å is similar to those of related Cr(III) complexes,
2
and environmental friendly oxidant because H O is its reduction
product. Hydrogen peroxide alone was unable to oxidize thymol
to a significant extent. Oxidation of thymol cannot occur, unless
a catalyst promotes the reaction, an observation was in agreement
with the observations made by other researchers. The catalytic
activity of the Cr complex is comparable with other known cata-
lysts for the oxidation of phenols to quinones in terms of the for-
mation of by-product. It was not observed any secondary product
formation during the HPLC analysis (Supporting information
Fig. S4). Nevertheless, the formations of benzoquinones derivatives
and non-identified products have been reported in the similar oxi-
dation studies in literature [32–34]. It is also noteworthy that un-
1
.961(4)–2.054(4) Å [24,25]. The bond lengths for Cr(1)–O(1)
(
2.0290(17), Cr(1)–O(3) (2.0120(18), Cr(1)–O(5) (1.9885(19) and
Cr(1)–O(6) (1.1947(2) Å are similar to that for Cr–N (1.978(2) Å),
suggesting that carboxylic oxygen atoms have similar coordination
ability to nitrogen atoms in this complex, although carboxylic oxy-
gen atoms have stronger coordination ability than nitrogen atoms
in other complexes containing nitrogen and oxygen atoms [26].
Although dinuclear and mononuclear Cr complexes of 2,6-dicarb-
oxyl pyridine have similar Cr–O/Cr–N bond lengths and angles,
the solubility and thermal behavior properties of the dinuclear
complex are much better than the mononuclear complex [27].
der similar reaction conditions with CrCl
compound, no oxidation products were detected during the first
3
Á6H
2
O in place of the title
1
0 h of reaction.
To determine the performance of the catalyst, the percentage of
thymol conversion was plotted as a function of different reaction
times. The results of thymol oxidation reactions at 1, 3, 5, 7 and
3.2. IR spectra
1
5 h in the presence of acetonitrile and ethanol are shown in the
The infrared spectrum of the complex (Supporting information
Fig. 2. Using ethanol, it was only possible to convert 4.92% of the
thymoquinone after 15 h of reaction. In spite of the low thymol
conversion, the Cr catalyst gave approximately 100% selectivity
for the thymoquinone formation. Typical HPLC chromatograms of
oxidation of thymol recorded at 0, after 3 and 15 h of reaction
are available in the Supporting information (Fig. S4). When using
acetonitrile, catalyst performance was optimized giving the high-
est thymol conversion of 20.25% after 15 h with close to 100%
selectivity.
Fig. S1) exhibits strong and broad absorption bands in the range of
À1
3
m
075–3500 cm which are assigned as the characteristic peaks of
(OH) vibrations from the water molecules and bridge hydroxyl
À1
groups [28]. The strong vibration appearing at 1634 cm corre-
sponds to symmetric stretching vibrations of the carboxylate group
29], indicating the carboxylate oxygen atoms are coordinated to
[
the metal centre. The large frequency shifts from those of free li-
gand also indicate strong coordination of the nitrogen and carbox-
ylate oxygen of the ligand to the Cr(III) acceptor centre. The
À1
changes observed in the 700–200 cm region for the complex
3.5. Stability of the catalyst
are attributed to the coordination of the ligands to the metal.
À1
New bands of medium intensities at 560 and 438 cm can be as-
A salient advantage of heterogeneous catalysts is facile separa-
signed to the
m(Cr–N) [30] and m(Cr–O) [31] modes, respectively.
tion from the reaction products. The recovered heterogeneous cat-
alysts can thus be reused in subsequent reactions. In order to
assess the stability of the catalyst, the solid was filtrated at the
end of the each reaction and washed with acetonitrile, then dried
at 50 °C. The dried catalyst was used under the same reaction
3
.3. Thermal properties
Thermogravimetric analysis (TGA) was carried out to examine
the thermal stability of the complex (Supporting information
2 2
conditions (thymol/H O /acetonitrile, 0.2 g Cr(III) catalyst, 60 °C).

1
8
B. Ay et al. / Inorganica Chimica Acta 387 (2012) 15–19
Fig. 1. Symmetric unit of compound with atom numbering scheme (50% thermal ellipsoids).
At the end of third reaction, thymol conversion did not change sig-
Table 3
nificantly (Table 4). The recycling property is a very important
parameter for a heterogeneous catalyst. This study showed that
Cr(III) catalyst may be readily recycled three times.
Conversion in the oxidation of thymol catalyzed with
Cr(III) complex.
Reaction time (h)
Thymol conversion (%)
OH CH CN
C
2
H
5
3
4
. Conclusion
1
3
5
7
1.68
3.10
4.36
4.62
4.92
6.15
9.40
11.64
13.85
20.25
We have synthesized a novel Cr(III) complex from the reaction
mixture of H
mal methods. The structure consists of dinuclear molecules
Cr(H O)(OH)(C NO )] connected by hydrogen bonding. The
2
pydc and CrCl
3
Á6H
2
O in alkali medium by hydrother-
1
5
[
2
7
H
3
4
2
Cr ions are bridged by two OH groups, and the geometry of the
compound is significantly different from that of the previously re-
ported compound with the similar composition, [Co(C
7 3 4
H NO )(-
H
2
O) ] [3]. The most important findings are their solubilities and
2
thermal behaviors between mono and dinuclear Cr complexes. Be-
cause of the presence of the hydroxyl bridges, dinuclear Cr com-
plex is insoluble in hot/cold many common organic and inorganic
solvents (Supporting information Table S6) and more stable than
mononuclear Cr complex [24]. These features are very important
for the availability of heterogeneous catalysts. Variations in reac-
tion temperature, stoichiometry and pH in the hydrothermal do-
main often result in unusual structures.
Thymoquinone is a compound with a commercial value consid-
erably higher than its precursor thymol. Thymoquinone can be eas-
ily obtained by catalytic oxidation of this precursor using
environmentally cheap oxidizing agent, hydrogen peroxide and
the title compound as a heterogeneous catalyst. The oxidation of
thymol with 20.25% conversion gives thymoquinone with 100%
selectivity. This percent conversion is similar to those of related cat-
alytic studies [32,34]. In addition, the catalyst showed same activity
after repeated use and is stable to 278 °C. Our researches in this as-
pect is going on to prepare novel dinuclear complexes constructed
by dicarboxylic acid ligands such as 2,3-pydc, 2,4-pydc, 3,5-pydc,
Fig. 2. The percentage of the thymol conversion in the presence of acetonitrile and
ethanol.
3
,5-pyrazole and their catalytic properties will be examined.
Acknowledgment
Table 4
The stability of the catalyst.
This present work was supported by the Research Unit of
Cukurova University (Grant No. FEF 2009 YL51).
Number of
recycle
Thymol
conversion (%)
3
CH CN
Appendix A. Supplementary material
1
2
3
20.25
19.67
19.51
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.12.043.

B. Ay et al. / Inorganica Chimica Acta 387 (2012) 15–19
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