Synthesis, characterization and thermal properties of sodium pyruvate thiosemicarbazone and some of its metal complexes

1 Introduction

Pyruvic acid is an organic acid. It is also a ketone, as well as being the simplest alpha-keto acid. The carboxylate ion (anion) of pyruvic acid (CH₃COCOO⁻) is known as pyruvate and is a key intersection in several metabolic pathways (Cody et al.,2000). The coordination behaviour of aliphatic carboxylic acids with aldehyde or ketone groups is interesting due to the presence of multi-coordination sites. The anions of these acids can act as monodentate, bridging bidentate or chelating ligands showing ambidentate character (Raju and Sivasankar, 2009). Various transition metal complexes with bi, tri and tetradentate Schiff bases containing nitrogen, sulfur and oxygen donor atoms play an important role in biological systems and represent interesting models for metalloenzymes, which efficiently catalyze the reduction of dinitrogen and dioxygen (Frausto and Williams, 1991; Kaim and Schwederski, 1996). Furthermore, metal complexes of thiosemicarbazone derivatives have attracted special attention due to their biological activity (Scovill et al., 1982) and medicinal properties (Petering et al., 1964; El Asmy et al., 1987). In this work we reported the synthesis and characterization of sodium pyruvate thiosemicarbazone (SPTSC). This molecule coordinates to a variety of divalent metal ions M²⁺ (M = Fe, Co, Ni, Cu or Cd) and one type of complexes with octahedral geometry are obtained. Thermal analyses (TG and DTG) of (SPTSC) and the complexes have been carried out to study their thermal stability.

2 Experimental

3 Results and discussions

Sodium pyruvate thiosemicarbazone (SPTSC), Scheme 1 was prepared by the aqueous reaction between sodium pyruvate and thiosemicarbazide hydrochloride in mole ratio of (1:1). The Schiff base ligand was characterized by elemental analyses, FTIR, UV–Vis and H NMR spectroscopy. The reactions of SPTSC with a variety of divalent metal ions (M = Fe, Co, Ni, Cu or Cd), in a 2:1 stoichiometric ratio, respectively, gave rapid deposition of microcrystalline solids, and despite numerous attempts, we have been unable to produce crystals suitable for X-ray structural analysis. However, the color, melting points, elemental analysis and molar conductivity values, Table 1, of these materials indicate that they all are neutral complexes with the formulation [M(SPTSC)₂], Scheme 2. Thermal analyses (TG and DTG) have been carried out for the free ligand (SPTSC) and all its metal complexes in order to study the thermal properties and stability of all of them.

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Scheme 1

General structural formulae of (SPTSC).

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Scheme 2

Proposed structures of the complexes.

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The molar conductivity data of all complexes in DMSO, Table 1 correspond to their nonelectrolytic nature (Geary, 1971). The room temperature magnetic moment data of Fe(II), Co(II), Ni(II) and Cu(II) complexes are 5.1, 4.6, 2.9 and 1.86, respectively. These data indicate the high spin nature of these complexes. Due to the completely filled (d) orbitals the cadmium complexes are diamagnetic.

The ¹H NMR spectrum of (SPTSC) in DMSO-d₆ Fig. 1A, showed three characteristic signals, singlet at 2.05 ppm which could be assigned to protons of methyl group (CH₃), doublet at 8.6 ppm, this signal may be attributed to the protons of NH₂ group. The third signal was observed at 10.63 ppm as a singlet which can be assigned to the proton of NH group. The ¹H NMR spectrum of [Cd(SPTSC)₂] in DMSO-d₆ Fig. 1B showed no change in the number of signals compared with the spectrum of free ligand (slightly different in the signals positions). This indicates that the coordination of SPTSC to the metal ions has occurred through sulfur atom as thione (i.e. there is no deprotonation of hydrazide nitrogen site and thiolization of C⚌S group) (Al-Karawi et al., 2009).

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Figure 1

¹H NMR spectra A₁ and A₂ for SPTSC, B for [Cd(SPTSC)₂].

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In order to study the binding mode of the Schiff base ligand (SPTSC) to the metal ion in complexes, the IR spectrum of the free ligand was compared with the spectra of the complexes. The IR spectrum of (SPTSC) Fig. 2A shows band in the regions 3406 cm⁻¹ and 1734 cm⁻¹ which are assigned to the υ(NH) and υ(C⚌O) vibrations, respectively. The spectrum also shows band with double peaks in the range of (3292–3282) cm⁻¹, which could be attributed to the asymmetrical and symmetrical stretching vibrations of NH₂ group. The u_asym and u_sym stretching of carboxylate ion (COONa) were observed at 1626 cm⁻¹ and 1340 cm⁻¹, respectively (Raju and Sivasankar, 2009). The bands due to u(C⚌N) and u(C⚌S) vibrations are located in the regions 1516 and 1271 cm⁻¹, respectively. The (C⚌S) group is less than (C⚌O) group and has a considerably weaker band; in consequence the band is not intense. Identification is therefore difficult and uncertain. Spectra of compounds in which the (C⚌S) group is attracted to a nitrogen atom show an absorption band in the general (C⚌S) stretching region. In addition several other bands in the broad region of 1515–700 cm⁻¹ can be attributed to vibrations involving interaction between (C⚌S) stretching and (C–N) stretching (Silverstien et al., 2005). All the IR spectra of complexes show bands around (3427–3379 cm⁻¹) which are due to stretching vibrations of NH group. The u_asym and u_sym stretching of NH₂ group were observed in all spectra of complexes, Table 2. Also the spectra show bands in the range (1228–1207 cm⁻¹) which could be assigned to stretching vibration of C⚌S group. The stretching vibration of (C⚌N) group is found to shift to higher wave numbers in the spectra of all complexes in comparison with free ligand, suggesting the coordination of the azomethane nitrogen atom to the central metal ions, in agreement with previous studies (Rejani et al., 1999; Al-Karawi, 2009). The u_asym and u_sym stretching of carboxylate group in all the complexes spectra are comparable with that of (SPTSC), which suggests the ionic nature of pyruvate ion in the complexes.

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Figure 2

FTIR Spectra of (A) SPTSC, (B) [Co(SPTSC)₂], (C) [Cd(SPTSC)₂].

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UV–Vis spectrum of Schiff base ligand (SPTSC) Fig. 3, exhibited an intense absorption peak at (33,003 cm⁻¹) related to the (π–π∗) transition. The (n–π∗) transition may be obscured by this absorption band. The UV–Vis spectra of all prepared complexes showed hypsochromic shift of ligand band except Cd(II) complex which caused bathochromic shift, Table 3. The appearance of new medium intensity bands at lower wave numbers were observed either merged or slightly separated from the intra ligand (π–π∗) band. These bands were mainly attributed to charge transfer transition. Further bands with very low extinction coefficients appeared in the visible and near IR region. These bands were attributed to ligand field (d–d) transitions (Lever, 1968; Figgis, 1966; Sutton, 1969). The electronic spectrum of Fe(II) complex showed the (π–π∗) transition plus two additional bands which could be attributed to CT and spin allowed transition in octahedral geometry (Lever, 1968; Figgis, 1966; Sutton, 1969; Lokesh and Sulekh, 2006). The electronic spectrum of the Co(II) complex recorded in DMSO solution displayed three peaks at (27,700 cm⁻¹), (19,723 cm⁻¹) and (14,749 cm⁻¹). These bands may be assigned to the CT, (⁴T₁g^(F) → ⁴A₂g^(F)) and (⁴T₁g^(F) → ⁴T₂g^(F)) transitions, respectively (Lever, 1968; Figgis, 1966; Sutton, 1969; El Asmy et al., 2005). The position of these bands suggests an octahedral environment around the cobalt (II) atom. The spectrum of Ni(II) complex displayed absorption bands at (26,525 cm⁻¹) and (15,380 cm⁻¹). These bands could be attributed to the spin allowed transitions (³A₂g → ³T₁g^(P)) and (³A₂g → ³T₁g^(F)), respectively, (Lever, 1968; Figgis, 1966; Sutton, 1969; El Asmy et al., 2005). The position of these bands confirms an octahedral geometry of nickel (II) complex. The electronic spectrum of Cu(II) complex shows band at (31,847 cm⁻¹) which should be assigned to the CT. The spectrum also displayed a broad band in the range (14,285–13,947 cm⁻¹). This band corresponded to the transition (²B₁g → ²A₁g). The position of these bands confirm the distorted octahedral Cu(II) complex (Lever, 1968; Figgis, 1966; Sutton, 1969). The electronic spectrum of Cd(II) complex exhibited bands at (32,362 cm⁻¹) and (24,390 cm⁻¹), which could be attributed to the ligand band (π–π∗) and CT transitions, respectively. No ligand field transitions are observed because of filled d–orbital. Since the d¹⁰ configuration affords no crystal field stabilization, the stereochemistry depends on size and polarizing power of the M^II cation and the steric requirement of the ligand (Figgis, 1966).

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Figure 3

UV–Vis spectra of (1) SPTSC, (2) [Co(SPTSC)₂], (3) [Cu(SPTSC)₂].

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3.1

3.1 Thermal analyses

The content of a component in a complex changes with its composition and structure. Thus, the content of such component can be determined based on the mass losses of these components in the thermogravimetric plots of the complex (Hatakeyama and Liu, 1998; Al Shihri, 2004). Therefore, the thermogravimetric analysis (TGA and DTGA) (50–900 °C) for prepared complexes was recorded to distinguish between the coordinate and hydrate solvents and to give an insight into the thermal stability of the studied complexes. In order to study the thermal stability of the prepared complexes, the TG plot of the free ligand was compared with the TG plots of prepared complexes. The TG plot of (SPTSC) Fig. 4 showed that it was decomposed in three successive steps. These decomposition steps occurred in the temperature range 150–350 °C. The first decomposition peak occurred at 150 °C with a percentage weight loss of 3.7%. This decomposition step was due to a loss of H₂O molecule. The other decomposition peaks may be attributed to degradation of thiosemicarbazide and sodium pyruvate moieties, Table 4.

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Figure 4

Thermogravimetric analysis TG and DTG of (A) SPTSC, (B) [Co(SPTSC)₂], (C) [Ni(SPTSC)₂], (D) [Cd(SPTSC)₂], (E) [Cu(SPTSC)₂].

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Table 4 Thermal degradation data.

Compound	Stage	TG – temperature range (°C)	TG mass loss (%)		Residue
			Found	(Calcd.)
(SPTSC)	I	150–180	3.7	(4.7)	C4H6N3O2NS
	II	210–230	35.9	(34.9)	C3H6N3S
	III	250–350	37.3	(39.0)	C2H3N
[Co (SPTSC)2]	I	330–400	65.2	(64.5)	Co N2S2
	II	430–520	5.6	(6.6)	Co S2
	III	575–680	5.6	(7.5)	Co S
[Ni(SPTSC)2]	I	320–400	68.5	(67.8)	NiNS2
	II	520–600	3.8	(3.3)	NiS2
	III	680–760	5.6	(7.5)	NiS
[Cu (SPTSC)2]	I	230–280	45.2	(45.6)	C6H6N2O4Cu
	II	310–380	20.3	(20.0)	C2N2O2Cu
	III	400–440	8.2	(8.8)	CuNO2
[Cd (SPTSC)2]	I	320–360	48.7	(50.6)	Cd N2O2S2
	II	450–650	14.4	(12.6)	Cd S2

The curves obtained for [Co(SPTSC)₂], [Ni(SPTSC)₂] and [Cd(SPTSC)₂] Fig. 4 showed no decomposition peaks below 300 °C. This highly confirms that the complexes have been formed by the coordination of metal ion to the (SPTSC) donor atoms only (i.e. there is no coordinate and hydrate solvents). Also the curves showed thermal stability up to 330 °C for [Co(SPTSC)₂], 320 °C for [Ni(SPTSC)₂] and [Cd(SPTSC)₂] Fig. 4. This leads to say that the prepared complexes have more thermal stability than free ligand (SPTSC) and the thermal stability of (SPTSC) has been enhanced by the coordination to metal ions. The final decomposition steps of the complexes are attributed to complete decomposition of the complexes leaving MS residue (M = Co, Ni) and CdS₂ for Cd(II) complex. The details of degradation data of SPTSC and all prepared complexes are illustrated in Table 4.