Characterization and Photochemical Behaviour of Some Thiosemicarbazones and their Pb(II) Complexes

Introduction

Thiosemicarbazones and their complexes were the subject of many studies in recent years owing to their potential therapeutic uses, such as antitumoral, fungicidal, bactericidal  or antiviral and antiparasitic activities ^1-5.Their complexes  proved to be  good catalysts for a number of organic reactions ⁶ .They were also used for a range of metal analysis ⁷, for  applications in optical and information instruments and devices ⁸.

Recently, the photochemical behaviour   of some substituted aldehyde thiosemicarbazones was studied. As a result of these studies,it was concluded that the aldehyde residue greatly affects the photoheterocyclization reactions ⁹. Furthermore, many  oxidizing agents were adopted for efficient cyclization of semi-and thiosemicarbazones. In this respect,  the regiochemistry of the cyclization reaction to a great extent depends on the conditions of the reaction as well as the structure of  the starting compounds ¹⁰.

Advanced oxidation process(AOP) that uses hydrogen peroxide with UV- light was found to be an effective method enabling transformation refractory non-biodegradable and/or for toxic organic compounds into only carbon dioxide and water ^11-13

Therefore and in order to gain more information about the photochemical behaviour of thiosemicarbazones, the present investigation deals with the photolysis of some thiosemicarbazones of structures ( Fig.1.) and their lead(II) complexes.

Figure 1: Suggested structure of [Pb(PTSC)2](NO3)2. H2O.

Experimaental

Materials

All chemicals used were of analytical grades. The ligands used in this study, 2-acetylpyridine- thiosemicarbazone (PTSC), 4-aminoacetophenonethiosemicarbazone(ATSC) and 2-furaldehydethiosemicarbazone (FTSC), were prepared  according to published procedures¹⁴.

Preparation of Pb(II) complexes

The Pb(II) thiosemicarbazone complexes were prepared by adding an ethanolic solution (20ml) of the thiosemicarbazone ligand (PTSC, ATSC or FTSC) (2 mmol) to an ethanolic solution (10 ml) of the Pb(NO₃)₂ (1 mmol).  The mixture was refluxed on a water bath for 30 min to afford coloured solutions. These solutions were concentrated to half of their volume on a water bath. After cooling to room temperature, the isolated crystalline solids were  washed with ethanol and finally dried over CaCl₂.

Analysis and Physical Measurements

Carbon, hydrogen, nitrogen and sulfur contents of the complexes were determined by an Elementar-Analysensystem GmbH Vario El analyzer.  The i.r. spectra were recorded as KBr pellets on a 470 Shimadzu infrared spectrophotometer. The electronic  spectra were measured in DMF solutions on a Perkin Elmer, Lambda 35 UV/VIS spectrophotometer. Electrical conductivity of  10^{-4 M solution of the complexes were measured at 25 o}C on a Jenway 4330 conductivity meter. The thermogravimetric (TG) and differential thermal analysis (DTA) of the solid complexes were preformed using a DTG-60H thermogravimetric analyser  (Shimadzu) from ambient temperature to 750 ^oC with a 10 ^oC min^-1 heating rate in dynamic air atmosphere. The photolyses were achieved using an Osram HBO 200 w/2 Lamp as a light source. Cut-one filter of 299 nm was used. Irradiations were carried out in dimethyl formamide (DMF)  solutions in 1-cm spectrophotometer cells at room temperature. Progress of the photolysis was monitored by the above mentioned spectrophotometer.

Results And Discussion

The reaction of lead(II) nitrate with the thiosemicarbazones(L) is illustrated in equation(1).

Pb(NO₃)₂       +     2L   +   nH₂O (from solvent) →   Pb(L)₂.nH₂O         (1)

n = 0, 1 or 4

The resulting complexes are sparingly soluble in common organic solvents but fairly soluble in DMSO and DMF. Table I includes the physical and elemental analysis of these complexes. It should be mentioned that the origin of water included in the formulae of the complexes is the (not absolute) ethanol used for preparation of the complexes.

Conductivity

Conductance measurements of 10^{-4 M solutions of the complexes in DMF are in the range 132-152  S cm2} mol^-1 indicating their 1: 2 electrolytic nature ¹⁵.

IR Spectra

Table II includes the IR spectral bands of the complexes. The band around 1600 cm^{-1 in the spectra of the free ligands (1600 cm-1)  is ascribed to to} n(C=N) of the azomethine moiety¹⁶.  Sulphur coordination is manifested by a shift of n(C=S) vibration which appears around 900 cm^{-1 in the spectra of free ligands17} to lower wave numbers in all the complexes(820-850 cm^-1). On the other  hand, the bands in the region 3220–3440 cm^-1 tentatively ascribed to symmetrical and asymmetrical stretching mode n(NH₂) in the spectra of the ligands, undergo a noticeable change in the spectra of the complexes due to coordination of sulfur from the C=S(NH₂) group as reported earlier ¹⁸. This coordination is confirmed by the presence of a new band at 430–455 cm^{-1 19, 20}  that can be assigned to n (M–N) for all the complexes. The presence of ionic nitrate is indicated by the appearance of a band in the range 1740-1780 cm^{-1 21}.

Electronic spectra

The electronic spectral data of Pb(II) thiosemicarbazone complexes in DMF  are recorded in Table III. The highest energy band, observed in the range 37,736-39,524 cm^-1, is attributed to a p-p^* transition. The band appearing in the spectra of the complexes as a shoulder  in the range 27,714-31,446 cm^-1  is most probably related to an intraligand charge transfer transition ²².

Thermal studies

The thermal behaviour of the Pb(II) complexes under investigation was studied in dynamic air using thermogravimetric analysis (TGA), derivatives thermogravimety  (DTG) and differential thermal analysis (DTA). The thermal decomposition of the complexes was recorded from  ambient  temperature to 750 ⁰C (Table IV). The results show that the complexes generally decompose in several thermal events, i.e., three or five decomposition steps. The first step for all hydrated complexes represents a dehydration step which correlates well with the theoretical values corresponding to the elimination of water molecules present.  The end products  of the decomposition  were identified from the TG traces to be PbCO₃, PbSO₄ or PbS.

Based on the above results of elemental analysis, i.r. and electronic spectra, conductivity and thermal analysis, the structure of [Pb(PTSC)₂](NO₃)₂. H₂O  as an example is proposed and given in Fig. 2.

Figure 2

Photochemical  behaviour

Photolyses  of the free ligands

The studied thiosemicarbazones exhibit different behaviour upon irradiating their DMF solutions at λ_irr ≥ 299 nm and recording their spectral changes at various time intervals. For PTSC, a new band at  444   nm appeared, while the absorbance at 318 nm decreased. The photoreaction is rather clean as indicated by the presence of a sharp isosbestic point at 388 nm (Fig. 4). It is to be noted that the photolysis imparts a yellow color to  the solution. For ATSC and FTSC,  the spectral changes upon irradiation are manifested by an absorbance decrease at 336 and 360 nm respectively. Such a different photochemical behaviour can be correlated with the variable thiosemicarbazone structures  (Fig. 1). The main structure differences are the presence of electron-withdrawing group (pyridyl nucleus)  in the PTSC linked to the –C=N-N  hydrazone moiety. Based on the photochemical behaviour of some related thioesmicarbazones, a photocyclization reaction could by suggested to account for the development of the new band upon irradiation in case of PTSC. The first step of the photoreaction of PTSC could be the occurrence of a cyclization reaction leading to the formation of  3-methyl-3-pyridyl-1,2,4-triazolidine-5-thione  1, the second step is regarded as a photooxidation of 1 to give   3-methyl-3-pyridyl-∆ 2-1,2,4-triazoline-5-thione 2.In fact the first step takes place only  in the case of PTSC and does not occur in case of ATSC and FTSC. It is to be noted that the pyridyl moiety, which acts as electron-withdrawn group may play a crucial role in enhancing the photocyclization reaction. This is not surprising since it is plausible that the electron-withdrawn group attached to C=N-N- in case of PTSC induces a partial positive charge to the carbon atom. At the same time the NH₂– group acts as an internal nucleophile, thus driving the cyclization reaction given by equation (2). This result is also confirmed by the observation that there is no cyclization product formed in the acidic medium, since the protonation of the amine group retarded its action greatly as a nucleophilic group so that  no cyclization reaction was possible.

Photolyses of Pb(II) complexes

Upon irradiation of the Pb(II) complexes of the thiosemicarbazones under investigation in DMF at λ_irr ≥ 299 nm, the spectral changes recorded during irradiation show that these complexes behave in a similar mannar to their corresponding free thiosemicarbazones. For the [Pb(PTSC)₂](NO₃)₂. H₂O  a new  band is observed upon irradiation. It is thus proposed that an unstable intermediate  3 forms, which in turn transforms to 2 according to the equation 3:

The spectral changes during irradiation (λ_irr ≥ 299 nm) of DMF solution of both [Pb(ATSC)₂](NO₃)₂ and [Pb(FTSC)₂](NO₃)_2. 4H₂O show that the absorbance at 365 nm, assigned to an intraligand transition, decreases. This behaviour indicates the photodecomposition reaction of the complexes. It seems that the active excited state reached by light absorption is the intraligand excited state type. The electronic structure of this excited state formed in the primary photochemical step of course differs from that of the ground state and leads to a change in donor the properties of the ligand ²³, which certainly weakens the metal –ligand bond. A reaction scheme given  by equations 4-6 is suggested to account for this result.

S N represents a thiosemicarbazone ligand. The IL-excited state generated by light absorption (equation 4) transfers an electron to the S- atom (equation 5). The mechanism of simultaneous decomplexation of two chelate arms (equation 5) is already suggested for the photochemical reactions of some Schiff base complexes ²⁴. The resulting compleces seem to be unstable and decompose to unidentified products in a secondary thermal reaction  (equation 6).

It is noteworthy that the similarity of the photochemical behaviour between the free thiosemicarbazones studied and their Pb(II) complexes indicates that the complexation blocked some centers in the ligands which do not participate in the photochemical process namely S and N. These results supported not only the suggested photocyclization mechanism but also the proposed structure of the Pb(II) complexes (Fig. 3).

Photolyses  in presence of H₂O₂

The UV/H₂O₂ oxidation process is of greatimportance ^{25, 26}. The main feature of such processis the generation of a very powerful oxidizing species, namely hydroxyl radicals ^{27, 28}. The pathway of  this process is the reaction of these radicals with saturated aliphatic products that involves abstraction of an H atom in the rate-determing step ^{29, 30}. On the other hand, when the organic molecule contains a double bond, the abstraction of  H atom is competed by an addition of ^.OH radicals on this unsaturated bond ³¹.

In comparison with other advanced oxidation processes (AOPS), such as fenton, ozone, UV /O₃, UV/TiO₂, etc. photolysis of hydrogen peroxide shows some advantages such as the complete miscibility of H₂O₂ with water. Photolysis of hydrogen peroxide is a very simple reaction running with high efficiency:

The hydroxyl radicals generated by H₂O₂ photolysis in the presence of organic compounds may lead to a variety of  reactions, such as:

The radical species formed through the above reactions may cause  further reactions leading finally to complete mineralization.

Photolyses in the presence of halocarbon solvents

 

The radical species formed through the above reactions may cause  further reactions leading finally to complete mineralization.

Photolyses in the presence of halocarbon solvents

It is known that halocarbon solvents such as CCl₄, CHCl₃ and CH₂Cl₂ act as electron acceptors. Therefor, the effect of the presence of 50% of these solvents on the photolysis of thiosemicarbazones and their Pb(II) complexes  has been studied(λ_irr ≥ 299 nm). The results indicated that the photoreaction in presence of such halocarbon solvents are faster than that in pure DMF. To rationalize this behaviourIt it is assumed that an active excited state of the charge transfer to solvent (CTTS) type could be reached by light absorption in addition to the above mentioned intraligand charge transfer excited state. Consequently, the following mechanism is postulated to interpret the formation of photocyclization product in case of PTSC with a higher rate in presence of halocarbon solvents (CHCl₃ is taken as an example).It is known that halocarbon solvents such as CCl₄, CHCl₃ and CH₂Cl₂ act as electron acceptors. Therefor, the effect of the presence of 50% of these solvents on the photolysis of thiosemicarbazones and their Pb(II) complexes  has been studied(λ_irr ≥ 299 nm). The results indicated that the photoreaction in presence of such halocarbon solvents are faster than that in pure DMF. To rationalize this behaviourIt it is assumed that an active excited state of the charge transfer to solvent (CTTS) type could be reached by light absorption in addition to the above mentioned intraligand charge transfer excited state. Consequently, the following mechanism is postulated to interpret the formation of photocyclization product in case of PTSC with a higher rate in presence of halocarbon solvents (CHCl₃ is taken as an example).

Also, The photodissociation  increases in the presence of halocarbon solvents. The photodecomposition depends greatly on the nature of  the halocarbon solvents and increases in the following order:

CCl₄  >   CHCl₃   >   CH₂Cl₂

This order is in agreement with the redox potential of this halocarbon solvents (CCl₄ = – 0.78 V, CHCl₃= -1.67 V and CH₂Cl₂ = – 2.33 V).

Table 1: Physical properties and elemental analysis of the  Pb(II) complexes.

Found /(calc.)				S cm-2 mol– l	m.p/ 0C (Decomp)	Colour	M . F (M.Wt)	Complex
S %	N %	H %	C %
8.39 (8.65)	19.14 (18.98)	2.97 3.01))	26.14 (26.05)	152	205	light brown	C16H22N10O7PbS2 (737.80)	[Pb(PTSC)2](NO3)2. H2O
8.68 (8.57)	18.64 (18.73)	3.37 (3.23)	28.23 (28.90)	132	216	Brown	C18H24N10O6PbS2 (747.76)	[Pb(ATSC)2](NO3)2
7.91 (8.65)	14.77 (15.11)	3.14 (2.99)	19.39 (19.43)	143	222	Brown	C12H22N8O12PbS2 (741.68)	[Pb(FTSC)2](NO3)2.           .4H2O

 

Table 2

Table 3: Electronic spectral data of the Pb(II) complexes (cm^-1)

Assignment	nmax (cm1)	Complex
IL-charge transfer p- p * transition	31,446 37,736	[Pb(PTSC)2](NO3)2. H2O
IL-charge transfer p- p * transition	29,718 39,524	[Pb(ATSC)2](NO3)2
IL-charge transfer p – p * transition	27,714 38,162	[Pb(FTSC)2](NO3)2.4H2O

 

Table 4

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