Thermodynamic Study of the Substitution Reaction between Cis-[ Pt ( p-MeC 6 H 4 ) 2 ( SMe 2 ) 2 ] Complex and pyridine-3-ol Ligand *

The equilibrium constants and other thermodynamic parameters for the adduct formation between cis-[Pt(p-Tol)2(SMe2)2] complex as an acceptor and 3-hydroxy pyridine (pyridine-3-ol) ligand as a donor in acetone, ethanol and benzene at various temperatures (13-20°C) have been spectrophotometrically measured. Our studies showed that ΔH(KJ.mol) of the reaction in ethanol, acetone and benzene are 6.442, 43.1 and 46.247 respectively. The related entropies (J.mol.K) have been obtained 89.82, 218.5 and 236 respectively, too. In addition, the amount of ΔG(kJ.mol) for the reaction occurred in solvents mentioned above are -20.324, -22.013 and 24.081.


Introduction
Platinum complexes are now among the most widely used drugs for the treatment of cancers [1,2].Among them, cis-platin is one of the effective antitumor agents used to treat various types of human cancers [3,4].In this view, ligand substitution reactions on monomeric square planar platinum(II) complexes have been extensively studied [5].During the past two decades, there has been a great interest in ligand substitution reactions of organoplatinum(II) complexes containing one or more Pt-C bonds [6][7][8][9][10].Commonly, ligand exchange has been recognized as the initial step of many fundamental processes involving square-planar organometallic complexes [11].So many related studies could be found in areas such as thermal decomposition of alkyl compounds [12][13][14], insertion of olefins into the M-H bond [15], oxidative addition and reductive elimination reactions [16], geometrical isomerization [17], electrophilic attack at the metal-carbon bond [18], exchange reactions [19] and the fluxionary motions of coordinated ligands [20].These mentioned fields of study might be considered from thermodynamic and kinetic aspects of chemical reactivity of platinum or other metal complexes [21][22].
In the present study, we describe the thermodynamics of cis-[Pt(p-Tol) 2 (3-hydroxy pyridine) 2 ] complex formation, starting by cis-[Pt(p-Tol) 2 (SMe 2 ) 2 ] precursor, and due to the donating characteristics of solvents, we then obtain the thermodynamic parameters for the ligand exchange and substitution process of this reaction in some solvents.

Experimental
The compounds 3-hydroxy pyridine, acetone, ethanol, benzene was of analytical grade.Cis-[Pt(p-Tol) 2 (SMe 2 ) 2 ] complex was prepared with a previously reported method [23].For this purpose, in a typical experiment a freshly prepared solution of p-tolyl lithium (4 ml) was added slowly to a stirred, ice-cooled solution of finely powdered [PtCl 2 (SMe 2 ) 2 ] (200 mg; 0.5 mmol) in dry ether (10 ml).The reaction mixture was stirred for 2 h at 0C and subsequently hydrolyzed with a few milliliters of saturated solution of NH  (20 ml) in order to convert the coexisted dimeric species into the monomer.The solvent was evaporated and the residue was washed with a few milliliters of acetone.The reaction yield was 74% with a melting point of 88C (d).The product was identified with its 1 H NMR spectrum and used as initial complex in this study.

Instrumental
Thermodynamic studies were carried out by a Shimadzu-2550 spectrophotometer with temperature control using an Index Instrument GD120 constant-temperature bath.

Method
3ml of initial complex (2×10 -4 mol L -1 in ethanol) containing a little SMe 2 due to prevent dimerization in solution, was transferred into the thermostatic cell compartment of the UV-Vis spectrophotometer, which was kept at a constant temperature by circulating water.Then, the ligand (0.05 mol L -1 in ethanol) was added stepwise as it is common in the regular thermodynamic studies.Finally, the K eq values were extracted using the fitting of obtained data.

Thermodynamic Studies
The complex was obtained from the reaction shown in equations 1 in which X, Y and Z are the initial complex, the ligand and the product complex respectively.
(1) The absorption measurements were carried out in the range of 330-360 nm after the equilibrium was achieved.In this range, the ligand shows no absorption.The equilibrium constants and molar absorption of the complex were determined by measuring the absorbance in various concentrations of 3-hydroxy pyridine and constant cis-[Pt(p-Tol) 2 (SMe 2 ) 2 ] concentration by non-linear curve fitting using KaleidaGraph program [24].
When 3-hydroxy pyridine as a donor (D) reacts with cis-[Pt(p-Tol) 2 (SMe 2 ) 2 ] ions as an acceptor (A) to form the new complex, then K eq for the equation 1 is: The observed absorbance of the solution is: (3) The donor shows no absorption in the range of 330 to 360 nm.Thus, the above equation will be summarized as below: Besides, the initial concentration of X is obtained by the Eq.5: (5) As a result, the product concentration in equilibrium time will be obtained by the following equations: (6) and: (7) and: Substituting [X] eq and [Z] eq into Eq.5, then we obtain the following result: ( where ϵ is the molar absorption of the species denoted. Figure 1 shows the increase in absorption bands in the range of 330-360 nm for the reaction between cis-[Pt(p-Tol) 2 (SMe 2 ) 2 ] and C 5 H 5 NO at various additions of ligand in C 2 H 5 OH at 13°C.As it is shown, increase in the absorption was carried out after the first addition of new ligand, and the absorption was moderately constant after a while.If the reaction is completed, it is no need to add more ligand.If not, more addition of ligand is necessary because there is an equilibrium state between reactants and product.While the new ligand is added in separate steps, the absorption increases and moderately gets constant after a while again, i.e. there is an equilibrium between reactants and product, and finally, the reaction is almost completed because there is no obvious change in absorption after the last addition of ligand.If we select just the final spectrum of each addition and collect the absorption in the selected wavelength, 330nm, we can draw the plot of absorptions vs. molarities of the ligand as shown in   The same is valid for other systems, as well.The equilibrium constants of the studied cis-[Pt(p-Tol) 2 (C 5 H 5 NO) 2 ] complex in three various solvents are shown in table 1.K eq = equilibrium constant.

Thermodynamic parameters
The thermodynamic parameters of the studied reaction of platinum(II) complex were calculated by the van't Hoff equation ,10: (10) in which K eq and R are the equilibrium and gas constants respectively, and T is the temperature in the Kelvin scale.The values of ΔHº and ΔSº were obtained as 6441.69J.mol-1, 89.82 J.mol-1.K-1 from the slope and the intercept of the related linear plot (lnK f vs. 1/T) respectively as shown in Figure 4. Furthermore, the ΔG o of the reaction is calculated as -20.32 kJ.mol -1 in ethanol.5 shows the curves in all three solvents.We can get the thermodynamic parameters using the Van The results in table 1 and 2 show that the equilibrium constants and Stability constants are dependent on the solvent and the trend is related to the donor number of them.The Gutmann donor number for benzene, acetone and ethanol are 1, 17, and 31.5 respectively.The equilibrium constants for ethanol are smaller than acetone and benzene.Therefore, the equilibrium constant decreases with increasing the donor number.In other words, the fourcoordinated complex was found to be more stable in a solvent with a higher donor number.Consequently, the trend for this influence on the reactivity of the studied complex toward a given N-donor is as below: Benzene > Acetone > Ethanol

Figure 2 .
The m1, m2, and m3 values show , , and respectively based on the curve fit definition in KaleidaGraph program.

Figure 2 .
Figure 2. The plot of Abs vs. molarities of the ligand for cis-[Pt(p-Tol) 2 (SMe 2 ) 2 ] reaction with C 5 H 5 NO at T=13 o C in C 2 H 5 OH.

Table 2 .
't Hoff equation.The results are shown in table 2. The plot of LnK eq vs. 1/T for cis-[Pt(p-Tol) 2 (SMe 2 ) 2 ] reaction with C 5 H 5 NO in different solvent.Thermodynamic Parameter Values for cis-[Pt(p-Tol) 2 (SMe 2 ) 2 ] reaction with C 5 H 5 NO at various temperatures in ethanol, acetone and benzene at 25°C.ΔH o , ΔS o and ΔG o are enthalpy, entropy and Gibb's energy of the reaction.