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American Journal of Oil and Chemical Technologies
Journal Website: http://www.petrotex.us/2013/02/17/317/
SYNTHESIS AND CHARACTERIZATION OF TWO NEW CADMIUM
COMPLEXES OF PYRIDINE-2,5-DICARBOXYLIC ACID N-OXIDE
Latif, AL, Mirzaei, M*, Eshtiagh-Hosseini, H
Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, 917751436, Iran.
(E-mail:[email protected])
Abstract:
According to research conducted in our research group on the pyridinedicarboxylatefamily due to their various
applications, we decided to oxygenate nitrogen of pyridine ring of pyridine-2,5-dicarboxylic acid as N-oxide to
investigate synthesis, coordination modes, and structure types of these compounds in view of crystal engineering
concepts. In this work, we report the syntheses of two new coordination complexes based on pyridine-2,5dicarboxylic acid N-oxide (2,5-H2pydco) as a ligand with CdII metal ion in the presence of heterocyclic amines such
as 1,10-phenanthroline (phen) and 2,2'-bipyridine (2,2'-bipy). The synthesized ligand and complexes were
characterized by physico-chemical approaches including elemental analysis (CHN), infrared spectroscopy (IR),
mass spectrometry, and flame atomic absorption spectroscopy. Additionally a CSD search revealed that 397
complexes of pyridine-2,5-dicarboxylic acid (2,5-H2pydc) are reported, which only 4 of them are derived from
pyridine-2,5-dicarboxylic acid N-oxide.
Keywords:Pyridine-2,5-dicarboxylic acid N-oxide, cadmium, crystal engineering, coordination complexes
1. Introduction
In recent years, pyridinedicarboxylic acid derivatives have attracted much interest in designing and synthesis of
novel compounds with desirable properties. Also they are a class of organic compounds with variety roles in biology
and medicine and they are present in many natural products, such as alkaloids, vitamins, and co-enzymes [1]. The
carboxylate groups are widely used as building blocks too because they exhibit diverse coordination modes, such as
monodentate terminal and monodentate bridging, bidentate chelating, and bidentate bridging [2]. The different
coordination modes of carboxylate groups enhance the robustness of the resulting architectures. On the other hand,
the negative charge of carboxylate groups compensates the positive charge induced by the metal centre and can
mitigate the counter ion effect [3]. Furthermore, the flexibility of carboxylate groups is always efficient to form
fascinating topologies. Besides supramolecular contacts, hydrogen bonding or π···π stacking interactions further
make the whole framework more stable [4-6]. The above-mentioned advantages of carboxylate groups are
frequently employed in the design, syntheses, and crystallization of coordination frameworks. Also, the organic
aromatic polycarboxylate ligands, especially 1,4-benzenedicarboxylate, 1,3,5-benzenetricarboxylate, and 1,2,4,5benzenetetracarboxylate, have been extensively applied in the construction of a rich variety of infinite highdimensional structures because of their diverse coordination modes and high structural stability [7, 8].
Nevertheless, pyridinedicarboxylic acid N-oxide remains considered less so far [9-11]. Therefore, in this paper, we
chose pyridine-2,5-dicarboxylic acid N-oxide to build new coordination complexes because it can adopt more
complicated bridging modes and serve as multi-connected nodes, which will be helpful for constructing higher-
connected frameworks with novel topologies compared with pyridyl-dicarboxylic acid or pyridyl-monocarboxylic
acid N-oxide.In fact, N-oxide functionalization of pyridine could increase coordination capacities and flexibility of
the resulting ligands together with the enrichment of their coordination or bridging modes [12].
We synthesized pyridine-2,5-dicarboxylic acid N-oxide (pydco) hoping to change of coordination modes and
geometry of desired complexes of CdII metal ion in the presence of heterocyclic amines. The synthesized compounds
formulated as [Cd(pydco)(phen)2]·3H2O (1), [Cd(pydco)(2,2'-bipy)2]·H2O (2) are characterized by some physicochemical methods such as elemental analysis, infrared spectroscopy (IR), mass spectrometry, and atomic absorption
spectroscopy.
1. Materials and Methods
1.1. Materials
All reagents were purchased commercially and used without further purification. The infrared spectra were recorded
in the range of 4000–600 cm–1 on a Buck 500 scientific spectrometer using KBr discs. The C, H, and N elemental
analyses were performed on a Thermo Finnigan Flash model 1112 EA microanalyzer. Mass spectra were recorded
on Varian CH7A. Atomic absorption analysis was carried out on a flame atomic absorption spectroscopy, Shimadzu
AA 670, Japan. Melting points determined on a Barnstead Electrothermal 9300 apparatus.
1.2. Methods
1.1.2. Synthesis of pyridine-2,5-dicarboxylic acid N-oxide (2,5-H2pydco)
Pyridine-2,5-dicarboxylic acid (1 g, 6 mmol) and a solution of Na 2WO4·2H2O (0.06 g, 6 mmol) in 30% hydrogen
peroxide (9 ml) are heated at 90-100 °C with vigorous stirring for 50 min. Then an additional amount of 30%
hydrogen peroxide (22 ml) is added portionwise over a period of 2h until all insoluble substance has vanished. After
an additional 3 h of heating, the reaction mixture is allowed to stand for several hours. The crystalline solid is
filtered, and dried in the air; yield: (95%); m.p: 198 °C. Anal.Calcd.for C 7H5NO5; C, 45.91; H, 2.75; N, 7.65%.
Found: C, 48.00; H, 2.51; N, 7.87%. IR (KBr pellet, cm−1) ν: 3446(br), 2634(m), 1726(s), 1646(m), 1511(m),
1402(s), 1231(s), 1099(m), 765(s). m/z = 183 (M+, 43), 183(92), 138(95), 122(95), 94(27.5), 78(92.5), 29(92.5).
2.1.2. Synthesis of[Cd(pydco)(phen)2]·3H2O (1)
A solution of H2pydco (0.037 g, 0.2 mmol) in water (10 ml) was added dropwise to a solution of 1,10phenanthroline (0.039 g, 0.2 mmol) in water (5 ml) and the mixture was stirred at room temperature for 2h. Then, a
solution of CdCl2·2H2O (0.043 g, 0.2 mmol) in water (5 ml) is added to the reaction mixture and the stirring was
continued for a another 2 h, transferred to a 30 ml Teflon-lined stainless steel vessel and heated at 130 °C for 3 days
under autogenous pressure. After the reactant was cooled slowly to room temperature, white needle like crystals of 1
were obtained. Yield: (50% based on Cd). m.p: 262 °C. Anal.Calcd.for C31H26CdN5O8; C, 52.52; H, 3.70; N, 9.88%.
Found: C, 51.47; H, 3.48; N, 9.90%. IR (KBr pellet, cm−1) ν: 3456(br), 1716(w), 1672(m), 1436(s), 1206(s), 765(s).
3.1.2. Synthesis of[Cd(pydco)(2,2′-bipy)2]·H2O (2)
A solution of H2pydco (0.055 g, 0.3 mmol) in ethanol-water (1:1; 15 ml) was added with stirring to a solution of
2,2'- bipyridine (0.047 g, 0.055 mmol) in ethanol-water (1:1; 5ml) for 1 h. Then, a solution of CdCl2·2H2O (0.066 g,
0.3 mmol) in water (5 ml) was added dropwise to this solution and stirred at room temperature for 3h. White needle
like crystals of 2 were obtained by slow evaporation of solvent. Yield: (40% based on Cd). m.p: 220 °C. Anal.Calcd.
for C27H21CdN5O6; C, 54.98; H, 3.39; N, 11.23%. Found: C, 55.56; H, 3.33; N, 10.98%. IR (KBr pellet, cm−1) ν:
3566(br), 1691(s), 1600(m), 1551(m), 1400(s), 1229(m), 765(s).
2. Results and discussion
The complexes of 1 and 2 were obtained in the presence of 1,10-phenanthroline and 2,2'- bipyridine, respectively.
Infrared spectroscopy data show vibrations due to the water, carboxylic acid, and amine fragments of 1 and 2
(Figure 1). Strong bands at 1672 and 1436 cm−1 for 1 and also1691 and 1400 cm−1 for 2 are attributed to asymmetric
and symmetric stretching vibrations of the COO− group, respectively. The separation between ʋas(COO−) and
ʋs(COO−) is used to diagnose coordination modes. In our case the difference between ʋ as(COO−) and ʋs(COO−) is
more than 200 cm−1 indicating that the carboxylate group is coordinated unidentately to cadmium(II). Moreover, the
bands in the 1243–1206 cm−1 regions can be assigned to the N–O stretching vibrations of the pyridine N-oxide
group. There are also broad absorption bands at 3427–3158 cm−1 attributed to the O−H group of the uncoordinated
2
water molecules. In the range 3100-2500 cm−1 are bands derived from (=CH) vibrations of the aromatic rings.
Consequently the infrared spectroscopic data and elemental analyses of 1 and 2 are fully consistent with together.
Also, the presence of CdII ion in the synthetic complexes was confirmed by the analysis with flame atomic
absorption spectroscopy. The proportion of Cd II ion in complexes 1 and 2 were 0.9% and 0.6% w/w, respectively.
According to a survey of the Cambridge Structural Database (CSD, Version 5.36, update 2016) for complexation of
Cd metal ion by 2,5-H2pydc and 2,5-H2pydco, it is clear that few studies have been done on ligand 2,5-H2pydc and
any reports concerning complexation between 2,5-H2pydco and CdII have not been stated ( see Figure 2).
Figure1. IR spectra of (a) 2,5-H2pydco, (b) Complex 1, and (c) Complex 2
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Figure 2. Distribution of the complexation of cadmium metal ion with (a) 2,5-pydc and (b) 2,5-pydco ligands
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3. Conclusion
In conclusion, continuing with our previous works on synthesizing supramolecular compounds containing
pyridinedicarboxylic acid N-oxides [9-11], two new coordination complexes have been synthesized and
characterized. The red shift of bands ʋas(COO−), ʋs(COO−), and ʋas(COO−) and ʋ(NO) confirm formation of these
targeted complexes. In these complexes, 2,5-H2pydco ligand acted as a bidentate and could be coordinated to one
metal ion by only the N–oxide moiety and an oxygen atom of one carboxylate groups that resulted in the formation
of a 6-membered chelate ring with metal ion.
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