Potentiometric , spectrophotometric , conductimetric and thermodynamic studies on some transition metal complexes derived from 3-methyl-1-phenyl-and 1 , 3-diphenyl-4-arylazo-5-pyrazolones

A new 3-methyl-1-phenyl-4-arylazo-5-pyrazolone and 1, 3-diphenyl-4-arylazo-5-pyrazolone have been synthesized and characterized by elemental analysis, IR, mass and H NMR spectra. The acid dissociation constants (pKa values) of the investigated ligands were determined potentiometrically and spectrophotometrically. The stability constants of the transition metal ions (VO, Cr, Mn, Co, Ni, Cu and Zn) with the investigated ligands were determined potentiometrically at different ionic strengths (0.167, 0.1, 0.05 and 0.025 M) NaCl at 25oC and different temperature (25, 30, 35, and 45oC). The values of stability constants were found to decrease with increasing ionic strengths and temperature. The stoichiometries were studied using spectrophotometric and conductimetric methods, the results indicate the existence of 1:1 and 1:2 (M:L) metal:ligand species. The relationships between the stability constants of the complexes, ionization constants of the ligands have been discussed and correlated. The thermodynamic parameters (∆G, ∆H and ∆S) and the thermodynamic stability constants for all of the investigated complexes were determined potentiometrically.


INTRODUCTION
Pyrazolone and azopyrazolone compounds are widely used as analytical reagents, they are capable of form-ing chelates with a number of metal cations [1,2], the formation of which is accompanied by change in color, pH, conductivity, and absorption spectra [3].Azo derivatives have attracted much attention by virtue of their applicability as potential ligands for a large number of metal ions [1].The 4-position of pyrazoline-5-one system is highly reactive and undergoes coupling reaction with diazonium salts to give 4-arylazo derivatives [4].The azo-derivatives of 5-pyrazolones as well as their metal complexes have wide applications in dye industry as well as analytical reagents for determination of trace metals and it is predicted to have some medical and biological applications [5,6].Different methods were reported for the syntheses of azopyrazolone derivatives [7][8][9][10][11][12].
The present paper deals with the determination of the stability constants of the binary complexes of VO 2+ , Cr 3+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ metal ions with the investigated ligands (L1-L6) in the presence of 0.167, 0.1, 0.05 and 0.025 M at 25ºC.The thermodynamic parameters (∆G, ∆H and ∆S) and the thermodynamic stability constants of the investigated complexes were evaluated in the presence of 0.1 M NaCl in the temperature range 25-45ºC.The Irving and Rossotti pH-metric titration using Sarin and Munchi technique [13] was used to determine the acid dissociation constants as well as the formation constants for the various complexes at 25ºC.The acid dissociation constants were also done in 40% ethanolic buffer solutions of varying pH values spectrophotometrically [14].The molar ratio of the metal ions to ligands [M]/[L] were studied spectrophotometrically using molar-ratio and continuous variation methods and were also determined using conductimetric titrations in aqueous ethanolic solutions (40% v/v).

Materials and Methods
All chemicals used in this investigation were chemically pure grade derived from BDH.They include chlorides of Cr 3+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ and VOSO 4 • 3H 2 O, sodium hydroxide (NaOH), sodium nitrite (NaNO 2 ), hydrochloric acid (HCl), acetic acid (CH 3 COOH), sodium chloride (NaCl), ethylacetoacetate, ethylbenzoylacetate, phenyl hydrazine, ethanol, 2-hydroxyaniline (o-aminophenol), o-aminobenzoic acid, p-aminobenzoic acid and diethyl ether; purchased from BDH. Water used was bidistilled water; distillation process was carried out using both of condensation process and ion exchange technique.10 -3 M of azopyrazolone solutions were prepared by dissolving a known mass of the azo compound in a proper volume of ethanol.10 -3 M solution of the metal ion in 0.1 M HCl was prepared by dissolving the appropriate weight of the corresponding metal chlorides of Cr 3+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ and VOSO 4 •3H 2 O in a proper volume of 0.1 M HCl to prevent the hydrolysis of the metal salt solutions.Pure aqueous solutions of metal ions were also prepared for conductimetric studies.0.1 M HCl was prepared and standardized against standard sodium carbonate solution, the exact concentration of the HCl solution was determined and used in calculations (0.1185 M). 0.2 M (CO 2 -free) NaOH solution was prepared and standardized against standard HCl solution.NaCl solutions with different concentrations (1, 0.6, 0.3 and 0.15 M) were also prepared.A series of buffer solutions covering the range (1.5-12) of pH values were prepared according to Britton method [14] with the modification involving titration of 100 ml of the mixture (0.1 M solution of equal amounts of boric, acetic and phosphoric acids) with 0.5 M NaOH to the desired pH and then making with water up to 250 ml so as to keep the ionic strength constant at all pH values [15].
The pH measurements were carried out using a Jenway, pH-meter 3310 with a glass combined electrode.The water thermostat was a Thermo Haake WKL 26, Karlsruhe, Germany accurate to ±0.1ºC.The solutions were equilibrated in the thermostat for about 15 min before titration.The equations used for the various calculations [17] were programmed into IBM computer.

Determination of the Thermodynamic Parameters
The following thermodynamic parameters: ∆G °, ∆H ° and ∆S ° were determined for each chelate depending on their stability constants [18].The free energy of formation (∆G °) of a complex is related to its stability constant by the relation [18]: -∆G ° = 2.303 RT logβ where, R = universal gas constant, T = absolute temperature and logβ = stability constant of the complex.Enthalpy of formation (∆H °) and entropy (∆S °) were calculated by plotting logβ versus 1/T.We can specify the quantitative dependence of the stability constant on temperature from the relation [18]: By rearranging, we get This is a linear equation of the form y = mx + b, where y = logβ, m = -∆H °/(2.303R) = slope, x = 1/T, and b = ∆S °/(2.303R) = intercept.This means that if the values of K for a given reaction are determined at various temperature, a plot of logβ versus 1/T will be linear, with slope ∆H °/(2.303R) and intercept ∆S °/(2.303R).This result assumes that both ∆H ° and ∆S ° are independent of temperature over the temperature range considered.This assumption is a good approximation over a relatively small temperature range [18].

Conductimetric Measurements
The conductivities of VO 2+ , Cr 3+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ and Ni 2+ metal ions with the investigated ligands (L1-L6) were measured using conductivity meter (Philips pw 9526, digital conductivity meter).The experimental procedure involves a conductimetric titration of 40 ml alcohol-water mixture (50% v/v) solution containing 4 × 10 -5 M of a given azopyrazolone derivative against a standard solution of 10 -3 M aqueous metal ion solution using microburette with continuous magnetic stirring.The instrument reading was recorded after each addition.All of the conductivity values were corrected for the effect of dilution during titration.Corrections were made by multiplying conductivity value (instrument reading) by the ratio (V + v)/V, where V is the original volume of the titrand (40 ml and v is the added volume of titrant [19]).The corrected conductivity values were plotted versus the molar ratio (L:M).The resulting curves are composed of straight lines with inflection points indicating the number of ligands around each central metal ion.

Spectrophotometric Method Applied for Determination of PK a Values
The absorption spectra of azopyrazolone compounds under investigation (L1-L6) were scanned over a range of wavelengths in universal buffer solutions of different pH values.For this purpose a known volume of (10 -3 M) solution of the azopyrazolone derivative was added to the buffer solution in a 10 ml volumetric flask dropwise with continuous shaking.The mixture was then made up to the mark with the buffer solution of appropriate pH.The spectra were obtained at room temperature using spectrophotometer (Jenway, 6305 UV-VIS spectrophotometer).
The method applied for the determination of pK a values of the different azopyrazolone derivatives is the half height method [20].This method depends on the fact that the limiting absorbance (A 1 ) represents complete conversion of the compound from one form to other.Since pKa is equal to the pH value at which the two forms exist in equivalent amounts (pK a = pH (at A ½ ), the pH corresponding to half the height of the absorbance-pH curve (A ½ ) is equal to pK a .The (A ½ ) value is given by the relation: where A 1 = maximum absorbance, A min = minimum absorption.
The absorption spectra of 3 × 10 -5 M solutions of the azopyrazolones under investigation (ligands L1-L6) were dissolved in 40% v/v ethanolic buffer solutions of varying pH values.

Determination of the Molar Ratio of the Metal Lons to the Ligands Spectrophotometrically
The mole ratio of the metal ions to the ligands was studied spectrophotometrically using molar ratio and continuous variation methods.The spectrophotometric method was used to confirm the data obtained by conductimetric and pH-metric methods.UV absorption can be used to determine stoichiometry of the complexes and this method appears to be valuable for studying complexes with low stabilities.

Molar Ratio Method
In this investigation, the concentration of the metal ions were maintained constant and the ligands concentrations were varied [21], so a series of metal-ligand aqueous ethanolic solutions were prepared with different [L]/[M] ratios.The absorptions of these solutions were measured using UV spectrophotometer at λ max of the expected complex ML 2 .Absorbance versus [L]/[M] curves were drawn for all complexes.

Continuous Variation Method
This method was used to confirm the data obtained using molar ratio and conductivity methods.In this method, the mole fraction was varied by changing the concentrations of the two components, maintaining the total number of moles constant [21].A series of metal-ligand aqueous ethanolic solutions were prepared with different metal mole fractions.The mole fraction of the metal was plotted against the absorbance of the expected complex at λ max of the complex.The measured absorbance increase as the molar ratio [M]/([L] + [M]) increase until the actual molar ratio of the complex is reached, after this point the absorbance becomes lower because the metal has no absorption at λ max of the complex.

Determination of Stability Constants
Titration curves were obtained for the titrations of VO 2+ , Cr 3+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ metal ions in the presence of different molarities of NaCl and different temperatures.The stability constants of the formed complexes were investigated in the pH range of 4-6.
The mean values of   (the average number of protons associated with the ligand) for the investigated ligands (L1-L6) at different pH values were calculated from the titration curves of solutions 1 and 2 by employing the relationship derived by Irving and Rossotti [17].On plotting log  /(1 -  ) vs. pH, a straight lines having an intercept equal to pK a on the pH axis are obtained.From the titration curves of solutions 1-3, the metal + ligand formation number values  � (the average number of ligand molecules coordinated to the metal ion) of the metal complexes were obtained at various pH values.The  � values were found to be less or equal to 2, indicating the formation of 1:1 and 1:2 complexes.The  � values were calculated using the Irving and Rossotti formulation [17].The free ligand exponent pL was calculated from the so obtained values of  � by the equation: where, C M is the concentration of ions M n+ used, C L is the concentration of the ligand, y is the number of dissociable protons ( y = 1 for the investigated ligands), and V o is the original volume (50 ml), V c is the volume of alkali (NaOH) consumed to reach the same pH values in curve c corresponding to the titration of solution 3,    is the formation constant values of the investigated ligands, and B is the pH value.The mean pK a values obtained from the corresponding different experimental formation curves using the average value and straight line methods.The results obtained for proton-ligand systems (pK a values) were 3.98, 4.04, 7.68, 7.93, 3.45 and 3.66 for the investigated ligands L1-L6, respectively.Referring to these data, the pK a values of ligands L5 and L6 have the lowest values from all of the six ligands used in this work because the ionizble group (-COOH) locates in the para position where there is no attraction with the carbonyl group of the pyrazolone ring, so it is easy to lose H + ion, whereas the ligands L1 and L2 have pK a values higher than L5 and L6 because the ionizable group (-COOH) locates in ortho position, so there is a hydrogen bond between the carbonyl group of the pyrazolone ring and -COOH, as a result; it is more difficult to lose H + , so these two ligands have pK a values higher than L5 and L6 [22].The pK a values of ligands L3 and L4 have the highest values from all because the ionizable group of these two ligands (-OH) locates in ortho position with lower ability to librate H + ion because hydroxyl group has lower acidity than carboxyl group and because of the presence of hydrogen with the carbonyl group of the pyrazolone ring [23].Ligand L1 has pK a value < L2, and L3 < L4, and L5 < L6, that is because of the presence of additional phenyl group (electron withdrawing group) in ligands L1, L3 and L5.The formation curves for the complexes were obtained by plotting the relation between average number of ligands attached per metal ion ( �) and free ligand exponent (pL),  � and pL were calculated as previously mentioned.To compute successive stability constants (logβ 1 and logβ 2 ) the method of interpolation is used [17], where logβ 1 and logβ 2 are equal to the values of (pL) when ( �) = 0.5 and 1.5, respectively.logβ 1 and logβ 2 for all complexes are given in Table 1.

Relations between the Properties of Central Metal Lons and the Stability Constants of the Complexes
The transition metal ions form predominantly ionic and coordinate bonds.If the bonds are ionic, the born rela- tion (E = Z 2 /2r [1 -1/D]) [24] can be hold for the energy change on complexation of an ion of charge (Z) and radius (r) [25][26][27] in a medium of dielectric constant (D).Since the stability constant is related directly to this energy, logβ values should increase linearly with Z 2 /r.The stability constants of some transition metal complexes show different behaviors which suggest the probable existence of linearity and nonlinearity of logβ with Z 2 /r [28].The plots of logβ 1 and logβ 2 versus Z 2 /r of the transition metal ions (VO 2+ , Cr 3+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ ) complexes with the investigated ligands (L1-L6), does not exhibit linear increase of logβ with increase of Z 2 /r.Interpretation in term of the assumption about ionic character of metal-ligand on which the linearity based is not valid.The other probable case is steric effects.It is also interest to study the logβ 1 and logβ 2 values of the transition metal complexes with the investigated ligands (L1-L6) as a function of atomic number 1/r (the inverse of ionic radius) as well as EN (electronegativity) [28] of metal ions.In this investigation, it was found that the stability constants of the first raw transition metal complexes with the investigated ligands (L1-L6) (logβ values) are generally increase with decreasing ionic radius r (or increasing 1/r) and increase with increasing atomic number [29], also logβ increases with increasing electonegativity, this is because upon increasing electronegativity of the metal ions, the electronegativity difference between the metal atom and the donor atom of the ligand would expectedly associate with increase of covalent character of the metal-ligand bond.These relations are not linear and there are some exceptions from the mentioned observations because of the presence of steric effects and ionic bonds.The relations between the ionization constants (pK a ) of the ligands and the stability constants (logβ 1 , logβ 2 ) of their complexes with the transition metal ions (VO 2+ , Cr 3+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ ) can be studied.It is observed that as pK a of the ligand increases, the stability constant logβ of the complex increase.The semi linear correlation observed in this study demonstrates that factors which increase or decrease pK a of the ligand also affect the logK ML values for the metal ions in a parallel fashion.This means that substituent groups that tend to increase electron density on donor atom and hence tend to increase the coordination ability of the ligand increase logK ML , and also increase the value of pK a of the ionizable hydrogen [29].The assumption here is that ligand basicity is directly related to ligand stability i.e., more stable complexes being formed from basic ligands [29].Such correlation cannot be generalized because of the presence of some exceptions from linearity and because this result was observed form the ligands and the metal ions used in this study only.

Effect of Lonic Strength on Stability Constants
The stability constants of the metal ions (VO 2+ , Cr 3+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ ) complexes with the investigated ligands (L1-L6) were found to decrease with increasing ionic strength of the medium as shown in Table 2.For each complex the relationship between the stability constant and the square root of the ionic strength is plotted in Figures 1, 2 and was in agreement with the Debye-Hűckel equation [30].The thermodynamic stability constants (logβ °) were obtained by extrapolating the straight line plots of logβ versus √I to zero ionic strength.From the plots of logβ versus √I which illustrated in Figures 1 and 2, the logβ ° (thermodynamic stability constants) values were obtained and listed in Table 3.Also values of pK a were found to decrease with increasing ionic strength of the medium which are in agreement with Debye-Hűckel equation [30,31].The equation ∆G = -2.303RT logβ gives the relationship between the thermodynamic stability con-    stant and the free energy change according to the complex formation.

Effect of Temperature on Stability Constants
From the pK a and logβ values and their temperature dependence, the values of the thermodynamic functions ∆G, ∆H and ∆S were calculated [32].The values of stability constants in Table 4 reveal that the stability con-stants decrease with increasing temperature, along with the pK a value (Figures 3, 4).

Conductimetric Measurements
The calculated molar ratio [L]/[M] were plotted against the corrected molar conductance values.The results indicated that the conductance increases with the addition of the metal ion solutions due to the release of the highly conducting hydrogen ions as a result of chelation.In-   spection of the titration curves shows the presence of two distinctive breaks at metal to ligand molar rations of 1:1 and 1:2, respectively.

Spectrophotometric Determination of the pK a Values of the Investigated Ligands
The absorption spectra were recorded to investigate the spectral properties of the species liable to exist in such media and to determine the ionization constant (pK a ) values of the acidic groups present.Britton-Robinson universal buffers [14] were used to control the pH over the range 1.5-12.0.The maximum absorption of the ligand increases as pH of the buffer increase.The spectra in acidic solutions of pH 1.5-6.0 are characterized by a strong band absorbing maximally within the wavelength range 370-400 nm.These bands are due to absorption of the nonionized form liable to exist in such solutions and may be assigned to π-π* electronic transition within the ligand molecule influenced by intramolecular charge transfer.The spectra in alkaline solutions are characterized by the presence of a strong band absorbing maximally at the same range, which may be assigned to the absorption of the ionized form liable to exist at high pH values as a result of acid base equilibrium.It is noted in this investigation that the absorption bands assigned to the ionized form increases gradually by increasing of pH, attaining the maximum value at pH 10-12.0.The absorbance-pH curves show that the absorbance attains a limiting value at the extreme pH values in highly acidic or alkaline solutions indicating the existence of only one ionization step which is the ionization of -OH or -COOH groups.The variation of absorbance with pH is used for the calculation of ionization constants (pK a values) of the investigated ligands using the half height method [20].The results obtained are of the same order compared to those obtained potentiometrically and are 3.98, 4.08, 7.67, 7.98, 3.40 and 3.64 for the investigated ligands (L1-L6), respectively.The ionization of strong acidic carboxylic group is ionized at lower pH value, due to the high stability of the corresponding anion by resonance.They do not impart any spectral changes as the ionizable proton is not conjugated with the π-system of the molecule.

Determination of the Stoichiometry of the Complexes Spectrophotometrically
The mole ratio of the metal ions to the ligands was studied spectrophotometrically using molar ratio and continuous variation methods.The spectrophotometric method was used to confirm the data obtained by conductimetric method.UV absorption spectra can be used to determine stoichiometry of the complexes, and this method appears to be valuable for studying complexes with low stabilities.

Molar Ratio Method
It was observed that the absorption increases linearly as the ligand concentration increase, because of the formation of the complex until the solution reaches the actual molar ratio of the investigated complex.At this point all of the added materials were completely reacted and the absorption observed is the absorption of the investigated complex alone.After this point, the excess amount of the added ligand causes an inflection in the straight line that is because the ligand has an absorption value differ from that of the complex at λ max of the complex [21].
[L]/[M] ratio corresponding to the inflection point in (Abs-[L]/[M] curve) indicates to the actual [L]/[M] ratio of the investigated complex.It was found that all of the complexes in this investigation are able to be stable in the form ML 2 .

Continuous Variation Method
This method was used to confirm the data obtained using molar ratio and conductivity methods.In this method, The measured absorbance increases as the molar ratio [M]/([L] + [M] ) increase until the actual molar ratio of the complex is reached, after this point the absorbance becomes lower because the metal has no absorption at λ max of the complex.It was found that all of the curves have inflection points at mole fraction around (0.33); this means that all of the complexes in this investigation have the form ML 2 and this form is the most stable form.

CONCLUSIONS
The results obtained from the potentiometric measure-ments for proton-ligand systems (pK a values) were 3.98, 4.04, 7.68, 7.93, 3.45 and 3.66 for the investigated ligands L1-L6, respectively.It is observed that as pK a of the ligand increases, the stability constant logβ of the complex increase.The stability constants of the metal ions (VO 2+ , Cr 3+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ ) complexes with the investigated ligands (L1-L6) were found to decrease with increasing ionic strength of the medium which was in agreement with the Debye-Hűckel equation.The thermodynamic stability constants (logβ °) were obtained by extrapolating the straight line plots of logβ versus √I to zero ionic strength.Also the values of pK a were found to decrease with increasing ionic strength of the medium.The values of the thermodynamic functions ∆G, ∆H and ∆S were calculated.The values of stability constants reveal that the stability constants decrease with increasing temperature, along with the pK a value.Conductimetric measurements show the presence of two distinctive breaks at metal to ligand molar rations of 1:1 and 1:2, respectively.The results obtained from the spectrophotometric measurements are of the same order compared to those obtained potentiometrically and are 3.98, 4.08, 7.67, 7.98, 3.40 and 3.64 for the investigated ligands (L1-L6), respectively.The ionization of strong acidic carboxylic group is ionized at lower pH value due to the high stability of the corresponding anion by resonance.They do not impart any spectral changes as the ionizable proton is not conjugated with the π-system.The mole ratio of the metal ions to the ligands was studied spectrophotometrically using molar ratio and continuous variation methods.All the investigated complexes are found to be stable in the form ML2, which is also the most stable one.

Figure 1 .
Figure 1.Plots of logβ versus √I for L1 with the investigated metal ion complexes.

Figure 2 .
Figure 2. Plots of logβ versus I for L2 with the investigated metal ion complexes.

Figure 3 .
Figure 3. Plots of logβ versus 1/T for L1 with the investigated metal ion complexes.

Table 1 .
Collective data of stability constants (logβ 1 and logβ 2 ) for all of the investigated complexes.