Synthesis, Spectroscopic, and Thermal Investigations of Metal Complexes with Mefenamic Acid

The novel metal complexes with empirical formulae M(mef)2⋅nH2O (where M =Mn(II), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II); mef is the mefenamic ligand) were synthesized and characterized by elemental analysis, molar conductance, FTIR-spectroscopy, and thermal decomposition techniques. All IR spectra revealed absorption bands related to the asymmetric (]as) and symmetric (]s) vibrations of carboxylate group. The Nakamoto criteria clearly indicate that this group is bonded in a bidentate chelate mode. The thermal behavior of complexes was studied by TGAmethods under non-isothermal condition in air. Upon heating, all compounds decompose progressively to metal oxides, which are the final products of pyrolysis. Cu(II), Zn(II), and Cd(II) complexes were also characterized by the coupled TG-FTIR technique, which finally proved the path and gaseous products of thermal decomposition. Additionally, the coupled TG-MS system was used to determine the principal volatile products of thermolysis and fragmentation processes of Mn(mef)2⋅3H2O and Co(mef)2⋅2H2O.


Introduction
Fenemates (N-arylated derivatives of anthranilic acid) are pharmaceutical compounds with distinct anti-inflammatory and analgesic, antipyretic activity. Their mode of biological action is based on inhibiting prostaglandin synthetase [1,2]. In particular, 2-(2,3-dimethyl-phenyl) aminobenzoic acid (mefenamic acid: (CH 3 ) 2 C 5 H 3 NHC 5 H 4 COOH (Figure 1)) is an effective nonsteroidal agent widely used for the treatment of mild to moderate pains, inflammation, ache, and fever [3][4][5]. Moreover mefenamic acid, as other anti-inflammatory drugs, is emerging as novel chemopreventive agents against cancer [6,7]. It is an active ingredient of numerous drugs which are present on the pharmaceutical market [8,9]. However, NSAID-induced side effects, particularly in the gastrointestinal tract and kidney, often limit their applications. For this reason, considerable efforts have been made to increase their activity while minimizing side effects [10].
It is well known that several transition metal complexes with nonsteroidal drugs are more effective and show significantly lower toxicity than that of their parent drugs [11,12]. In particular, divalent metal complexes with several NSAID are better anti-inflammatory candidates then NSAIDs, because they have unique structures that could interact with the target enzymes more specifically. In addition, metal ions introduce extra antioxidant activity [13] and antiproliferative activity against cancer [14][15][16]. Furthermore gastrointestinal toxicities associated with the administration of complexes are much lower than those of the parent drug and improved safety of these drugs [17].
Our studies were stimulated by the fact that majority of anti-inflammatory drugs are carboxylic acids with their carboxylate group prone to metal binding [11,[18][19][20]. Some of mefenamate complexes have been described in the literature. Brzyska and Ożga characterized complexes of rare earth metals with mefenamic ligand [21]. Tapacli and Ide described mefenamate compounds with Ca and Na ions [22]. Kovala-Demertzi et al. investigated the compounds with the formulae: SnPh 3 (mef) and SnBu 2 (mef) 2 [23].
The aim of the present work was to obtain the mefenamate complexes of Mn(II), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II), determine their chemical properties, and study their thermal decomposition patterns.

Synthesis.
All complexes were obtained according to similar procedures. The first step of synthesis was preparation sodium salt of ligand by dissolution of mefenamic (1 mmol) acid in 50 mL fresh precipitated aqueous-ethanol solution (1 : 1) of NaOH (0,02 mol⋅L −1 ). The mixture was heated up to 60 ∘ C and added to aqueous solution of metal chlorides (0,5 mol in 25 mL). The reaction mixture was kept in 60 ∘ C for 2 hours. After several days the solid precipitates were isolated by filtration, washed with hot water, and dried on air. Complex

Measurements.
The chemical compositions of all complexes were defined by the elemental analysis followed by the atomic absorption spectrometry. Hydrogen, carbon, and nitrogen contents were measured with the Vario EL III Elemental Analyzer. The metal content was determined in samples mineralized using the Anton Paar Multiwave 3000 closed system instrument. The mixture of concentrated HNO 3 (6 mL) and HCl (2 mL) was applied. Metal concentrations were measured by the FAAS with the GBC Scientific Equipment 932 plus spectrometer.
IR spectra were recorded on FTIR-8501 Shimadzu spectrophotometer over 4000-400 cm −1 range using KBr pellets. The thermal stabilities of complexes were studied by means of TGA techniques. The measurements were made with the Netzsch, TG 209 apparatus, and Q-1500 Derivatograph. Samples (1⋅10 −2 g) were heated (in ceramic crucibles) up to 1000 ∘ C, at a heating rate 10 ∘ C min −1 in air atmosphere. The analysis of solid decomposition products was performed using TG and DTG curves and supported by the X-ray diffractograms (Siemens D-5000 diffractometer, graphite monochromatized CuK radiation) of sinters, obtained by heating the complex samples up to temperatures defined from TG curves. A coupled TG-MS system was applied for analysis of volatile products of thermal decomposition and fragmentation processes. Data were processed using online connected computer system with commercial software (Derivatograph TG/DTA-SETSYS-16/18, coupled to a Mass Spectrometer QMS-422 model ThermoStart from Balzers); platinum crucible, mass sample: 4-6 mg. Dynamic measurements were carried out in argon atmosphere (at a flow rate 20 mL⋅min −1 ) with a heating rate 10 ∘ C⋅min −1 and an ion source temperature of ca. 150 ∘ C using 70 eV electron impact ionization. The TG-FTIR measurements were carried out in ceramic crucibles at flowing argon atmosphere (20 mL⋅min −1 ) using the Netzsch TG 209 apparatus coupled with Bruker FTIR spectrophotometer. The samples were heated up to 1000 ∘ C at a heating rate 10 ∘ C⋅min −1 . Molar conductivity (Λ M ) of all synthesized compounds was measured in 1⋅10 −3 mol⋅L −1 solutions of MeOH, DMSO, and DMF, according to procedure as described in [24].

Results and Discussion
The empirical formulae of complexes showing number of water molecules and molar conductivities are presented in Table 1. All synthesized solid complexes are stable in air.
They are practically insoluble in water, but on the contrary quite soluble in polar organic solvents (e.g., EtOH, MeOH).
Analysis of X-ray powder diffraction data reveals high level of crystallinity and proves that neither pair of investigated compounds is isostructural.  [25] are intermediates between those of nonelectrolytes and 1 : 1 electrolytes.

FTIR Spectra.
The IR spectra of all complexes exhibit a broad absorption band in the water stretching region (3300-3600 cm −1 ). Additionally, bands related to the water bending vibrations are observed (1615-1620 cm −1 ). In all spectra the valence vibrations of monodissociated carboxylic group are not observed. On the contrary, asymmetric (1575-1578 cm −1 ) and symmetric (1386-1394 cm −1 ) vibration of dissociated OCO − group are clearly observed (Table 2). These bands are affected by the coordination of mefenamic ligand to metal ions. The separation Δ] = ] as (OCO) − ] s (OCO) and the direction of the shifts of these bands in comparison to those values of sodium salt characterized the nature of the metal-carboxylate bonds. The bathochromic shifts of asymmetric (] as ) and hypsochromic shifts of symmetric (] s ) frequencies are also observed. The magnitude of separation and the bands direction show that carboxylate group of mefenamic ligand coordinated in bidentate chelate mode [26]. Apart from carboxylate group the mefenamic anion has amine group, available for coordination. The NH deformation vibrations in the IR spectra of mefenamic acid are very close to those observed in the IR spectra of its complexes. That indicates that the NH group does not participate directly in coordination.

Thermal
Analysis. The data obtained from TG, DTG, and DTA curves supported by chemical and X-ray diffraction    Table 3. The thermal decomposition curves of Cd(mef) 2 ⋅2H 2 O are shown on Figure 2. The thermal decomposition of the complexes is a multistages process. All compounds started to decompose by dehydration accompanied by the endothermic effect. Majority of complexes lose their water molecules in one step. Only for Co(II) and Zn(II) complexes water elimination is two-stage transformation. Thermolyses of these complexes are quite similar. The two-stage dehydration (60-110 ∘ C, 130-240 ∘ C and 100-160 ∘ C, 160-190 ∘ C for Co(II) and Zn(II) compound, respectively) is followed by organic ligand thermodestruction and metal carbonate formation. Further heating leads to metal oxides: ZnO and CoO. The latter is obtained via Co 3 O 4 intermediate. In the terminal step of pyrolysis the DTA curve exhibits exoeffects. Formation of oxides was confirmed by the powder X-ray diffraction technique ( Figure 3).
Mn(mef) 2 ⋅3H 2 O and Cu(mef) 2 ⋅2H 2 O decompose in a similar way. One-step dehydration takes place at 60-240 ∘ C and 100-180 ∘ C for Mn(II) and Cu(II) complex, respectively. Further increase of temperature results in high mass loss (ca. 80%) caused by mefenamic ligand decomposition and Mn 3 O 4 (via Mn 2 O 3 ) and CuO formation. The lowest thermal stability was determined for Ni(II) complex. Its decomposition starts at 50 ∘ C with 6,5% mass loss related to water molecules elimination. In the following step, the anhydrous compound decomposes (190-460 ∘ C) to NiCO 3 . This step is represented as two distinct exothermic peaks at 340 ∘ C and 430 ∘ C on the DTA curve. Further heating leads to formation of intermediate equimolar mixture of NiO and Ni, which   subsequently is fully oxided to nickel oxide (mass increase on the TG curve). Presence of NiO was confirmed by powder Xray diffraction of sinters prepared by heating compound to 750 ∘ C. The highest thermal stability is shown by Cd(II) complex. Its pyrolysis pattern closely resembles that of Ni(II) complex. Dehydration is single-step process and started at 110 ∘ C. The next mass loss (210-380 ∘ C) as observed on TG curve corresponds to destruction of organic ligand and leads to formation of CdCO 3 . The latter is associated with endoand exothermic effects as represented by two distinct DTA peaks at 300 and 340 ∘ C, respectively. Further weight loss is observed at 470-760 ∘ C and results from the CdO formation (maximum on DTA curves at 550 ∘ C).  of ion current intensities were observed for temperature ranges 120-280 ∘ C, 270-450 ∘ C, and 600-900 ∘ C.

TG-FTIR Measurement.
The coupled TG-FTIR technique was applied to Cd(mef) 2 ⋅2H 2 O, Cu(mef) 2 ⋅2H 2 O, and Zn(mef) 2 ⋅2H 2 O complexes. All experiments were performed in argon atmosphere. The IR spectra were recorded using the Gramm-Schmidt curves. The respective stacked plot as registered for Cd(mef) 2 ⋅2H 2 O is given in Figure 5. Analysis of IR spectra of gases evolved during the thermal decomposition indicates that pyrolysis schemes of all investigated complexes are similar and closely related to those recorded in air atmosphere. All spectra clearly confirm that the thermal decomposition begins from the dehydration process. On the TG curve in range 100-160 ∘ C for Cd(mef) 2 ⋅2H 2 O and Cu(mef) 2 ⋅2H 2 O and 100-190 ∘ C for Zn(mef) 2 ⋅2H 2 O ca. 6% mass losses connected with water losing were observed. The IR spectra which recorded up to 120 ∘ C for Cd(II) and Cu(II) complexes and up to 150 ∘ C for Zn(II) complex show bands in the wavenumbers 3750-3500 and 1750-1400 cm −1 corresponding to stretching and deformation vibrations of liberating water molecules. Anhydrous complexes are stable up to about 170 ∘ C which is confirmed by the lack of IR spectra of gaseous products. Further heating leads to organic ligand degradation indicated by a significant mass loss observed on the TG curve. The spectra recorded at 260 ∘ C contain bands in frequencies ranging 3000-2600 cm −1 (assigned to the stretching vibrations of the CH bond from the CH 3 and CH 2 groups); 1450-1350 cm −1 (corresponding to bending vibrations of the CH in the CH 3 and CH 2 groups); 1225-1000 cm −1 and 900-720 cm −1 (from bending vibration of CH in aromatic ring). Additionally, this spectrum shows the trace of water molecules in split of organic ligand destruction (tied in with NH vibration). Spectra which recorded up to 290 ∘ C contain the same patterns as described above augmented by additional bands (in the range 2500-2250 cm −1 and 750-650 cm −1 ) corresponding to CO 2 vibrations and trace CO oscillations (2250-2050 cm −1 ). Heating the sample above 450 ∘ C resulted in rising intensity of bands related to carbon oxides while in the same time decreasing those of water and complete absence of aromatic ring vibrations.

Conclusions
All investigated complexes have been obtained as crystalline hydrates. Majority of synthesized compounds in MeOH, DMSO, and DMF solutions do not dissociate or dissociate only in a very limited degree. IR spectra firmly confirmed that mefenamic ligands are directly coordinated to metal ions only through carboxylate group, in a bidentate chelate mode. According to the chemical, spectroscopic, and thermal data, we have proposed the formula of obtained complexes (Figure 6). All compounds decompose progressively starting with dehydration at the temperature range 60-150 ∘ C. Anhydrous compounds are stable up to almost 170 ∘ C. The most temperature persistence is Cd(II) complex, while the least one is the Ni(II) compound. Results of the TG-MS and TG-FTIR investigations correlate closely with those obtained by the TG-DTG system. The differences are related to final products and paths of dehydration.

Conflicts of Interest
The authors declare that they have no conflicts of interest.