B. Ay et al. / Polyhedron 88 (2015) 164–169
165
was 254 nm, the flow rate was 1.0 mL minÀ1 and the retention
time was 15 min. Before injection of a sample, the column was
equilibrated with the mobile phase at a flow-rate of 1.0 mL minÀ1
for at least 30 min or until a steady baseline was obtained. Calibra-
tion curves of T and TQ were used for determination of the conver-
sion and yield values.
O
OH
OH
O
O
Picolinic acid
Nicotinic acid
N
OH
OH
Hydrothermal
Conditions
O
N
2.2. Synthesis of the coordination polymer 1
N
2,3-pyridinedicarboxylic acid
All starting materials were used directly without further purifi-
cation. A mixture of NiCl2Á6H2O (0.192 g, 0.8 mmol), pyridine 2,3-
dicarboxylic acid (0.134 g, 0.8 mmol), NaOH (0.064 g, 1.6 mmol)
and H2O (5 mL, 278 mmol) in the mole ratio 1.00:1.00:2.00:348
was sealed in a 23 mL Teflon reactor and kept under autogenous
pressure at 160 °C for 96 h. The mixture was cooled to room tem-
perature at a rate of 20 °C hÀ1 and turquoise colored crystals were
obtained. The heterogeneous solution mixture was separated from
the solid phase and the crystals were washed with water and then
dried at room temperature. Yield: 92.8% based on Ni. Initial pH:
3.60, final pH: 2.00. The coordination polymer is insoluble in com-
mon solvents (ethanol, methanol, acetonitrile etc) and water. Anal.
Calc. for C24H22N4Ni2O11: C, 43.64; H, 3.33; N, 8.49. Found: C,
43.12; H, 3.32; N, 8.19%. IR data (KBr pellet, cmÀ1): 3436(w),
3103(m), 2952(m), 1605(s), 1584(s), 1491(m), 1364(s), 1006(m),
779(s), 763(s), 588(s), 516(m).
Scheme 1. The behavior of the 2,3-pydc2À ligand under hydrothermal conditions.
O
Ni Catalyst
CH3CN, H2O2 and tBuOOH
O
HO
Scheme 2. Oxidation of T to TQ using a Ni(II) catalyst.
and neuropharmacological activities [39]. TQ, representing 18.4–
24% of the essential oil of black seed, possesses antibacterial, anti-
histaminic and anti-inflammatory activities [40]. The oxidation
product TQ is a crucial compound for research, such as for treat-
ment of cancer and other diseases. Especially in the last three
years, it has been proven to be effective in the treatment of cancer
of the pancreas [41], breast [42,43], prostate [44] and colon [45].
Also, it has anti-microbial [46], anti-histamine [47], anti-inflamma-
tory [48] activities and oxidative stress [49] properties.
2.3. X-ray structure determination
The crystallographic data collection for 1 was performed using a
Bruker AXS SMART CCD diffractometer with Mo K
a radiation at
In this article, we chose 2,3-pyridinedicarboxylic acid as a
ligand because of its interesting behavior under hydrothermal con-
ditions, and have synthesized and structurally characterized a
100 K. The APEX2 Crystallographic Suite was used for determina-
tion of the unit-cell parameters. The structure was solved by direct
methods and refined by full-matrix least-squares on F2 using the
SHELXTL crystallographic software package. Crystal data and struc-
ture refinement for 1 are summarized in Table 1 and selected bond
lengths and angles are given in Table 2. Anisotropic displacement
parameters, hydrogen coordinates and other crystallographic data
are available in the Supporting Information (SI) (Tables S1–S4). A
BRUKER AXS D8 model diffractometer was used for the powder
X-ray diffraction of the catalyst.
dinuclear coordination polymer, {[Ni2(NA)4(
l
-H2O)]Á2H2O}n (1).
The complex was obtained with higher yield and under different
conditions, such as starting materials, reaction temperature and
reaction time, than complexes of similar empirical formulas [50].
The thermal property and catalytic activity of complex 1 were
studied. The polymer showed high thermal stability and high
selectivity in the catalytic oxidation reaction of T to TQ (Scheme 2).
2. Experimental
2.4. Oxidation of thymol (T)
2.1. Materials and methods
T oxidation was carried out at room temperature and at 60 °C in
a three-necked flask (250 mL) equipped with a magnetic stirrer, a
reflux condenser and a temperature controller in an oil bath. T
(3.0 g, 20 mmol) and 20 mL acetonitrile were added successively
into the flask. The appropriate amount of oxidant, 30 wt.% of aque-
ous H2O2 (7.0 mL, 69.80 mmol) and 80 wt.% aqueous of the tert-
butyl hydroperoxide (6.0 mL, 48 mmol), were then added to the
reaction mixture. After heating this mixture to 25 and 60 °C, the
metal complex (0.1, 0.2 and 0.3 g) was added to start the reaction
and stirred incessantly for 2, 5, 8 and 24 h. The mixture of the reac-
tions was collected at given time intervals and was analyzed by
HPLC.
NiCl2Á6H2O, 2,3-pyridinedicarboxylic acid, NaOH, T, TQ, acetoni-
trile, tert-butyl hydroperoxide (ꢀ80%) and hydrogen peroxide
(30%) were purchased from commercial sources and used as
received. Hydrothermal synthesis was carried out in 23 mL PTFE-
lined stainless steel containers under autogenous pressure. Infra-
red spectra were recorded with KBr pellets on a Perkin-Elmer
RX-1 FT-IR spectrometer in the range 4000–400 cmÀ1. Thermo-
gravimetric analyses (TGA) were conducted in a nitrogen atmo-
sphere with a Perkin Elmer Pyris Diamond TG/DTA equipment at
a heating rate of 10 °C minÀ1. HPLC and headspace GC/MS were
used for the characterization of TQ, determination of the percent-
age conversion and selectivity values. The GC/MS was measured
on a Thermo Brand chromatograph with TR5MS capillary columns
3. Results and discussion
(60 m  0.25 mm i.d.; 25
conditions were injector temperature: 240 °C, beginning tempera-
ture: 50 °C, final temperature: 250 °C and heating rate: 3 °C minÀ1
lm film thickness). The chromatographic
3.1. Crystal structure of {[Ni2(NA)4(l-H2O)]Á2H2O}n (1)
.
For the HPLC analysis a Shimadzu HPLC system equipped with a
The coordination polymer was synthesized hydrothermally. It is
interesting to note that when the temperature was 160 °C, decar-
boxylation occurred and the pyridine-2,3-dicarboxylic acid was
transformed into nicotinic acid in the process of the hydrothermal
reaction [51,52]. Single crystal X-ray analysis showed that the
reversed phase C8 column (250 cm  4.6 mm column dimensions,
5 lm
particle sizes, Ascentis)Ò was used. CH3OH:CH3CN:H2O
(50:20:30, v/v) was used for the separation as the mobile phase.
The column temperature was 35 °C, the detection wavelength