Thermodynamic Study and Spectroscopic Analysis of a Charge-Transfer Complex between 3,5-Diamino-1,2,4-Triazole and 6-Methyl-1,3,5-Triazine-2,4-Diamine with Chloranilic Acid

Studying of charge-transfer (CT) and proton transfer interactions is essential due to their important role in many biological field and industrial applications. The current work will add more information’s about the nature of interaction between 3,5-diamino-1,2,4-triazole (DAT) and 6-methyl-1,3,5-triazine-2,4-diamine (MTDA) with 3,6-dichloro-2,5-dihydroxy-p-benzoquinone (chloranilic acid CLA) which was studied spectrophotometrically in Ethanol (EtOH) and Methanol (MeOH) solvents at different temperatures. The molecular composition of the formed complexes was studied by applying continuous variation and spectrophotometric titration methods and found to be 1:1 charge transfer complex for both Complex (DAT:CLA) and (MTDA:CLA) which are produced. Minimum-Maximum absorbance’s method has been applied to calcu-late the formation constant K CT and molecular extinction coefficient (ε); they recorded high values confirming high stability of the produced complexes. Oscillator strength (f), transition dipole moment (μ), ionization potential (I P ) and dissociation energy (W) of the formed CT-complexes were also determined and evaluated; they showed solvent dependency. It is concluded that the formation constant (K CT ) of the complexes is found to depend on the nature of both electron acceptor and donors and on the polarity of solvents.


Introduction
Charge transfer (CT) or proton transfer (PT) complexation is one of the important operations which have many applications in many fields like a biological system such as DNA binding [1], antimicrobial activity, antibacterial, antifungal [2] and drug receptor [3]. Also, it used in controlling the speed of enzymatic reactions [4], modern technology like organic solar cells [5], electrical conductivity and optical properties [6].
Triazine compounds are well known due to their broad biological activity towards several diseases such as analgesic, anti-inflammatory, anti-oxidant, analeptic [7]. A great deal of attention has been paid to triazine derivatives endowed with antitumor activity [8], besides widely used in natural or synthetic, with a great variety of pharmacological effects [9].
Due to the importance of triazines in many fields and in continuation of our studies on charge or proton transfer, in this paper we would like to add some information's to the chemistry of triazanes through synthesis and characterization of a novel charge transfer complex including proton transfer hydrogen bonding between 3,5-diamino-1,2,4-triazole (DAT) as an electron donor is compared on their other electron donor 6-Methyl-1,3,5-triazine-2,4-diamine (MTDA) with electron acceptor chloranilic acid (CLA). This work presents the spectroscopic characterization of the charge transfer.
This work aims to compare between DAT and MTDA with CLA. The molecular composition of the formed complex will be identified through job's method of continuous variations and spectrophotometric titration methods. The formation constant (KCT) and molecular extinction coefficient (ε) and spectroscopic physical parameters were estimated and evaluated including oscillator strength (f), transition dipole moment (m), ionization potential (ID), and resonance energy (R N ) which were also calculated and analyzed. The bonding nature will be discussed through calculating thermodynamic parameters using Van't Hoff equation.

Materials, Stock Solutions and Instrumentation
Powdered DAT (purity 98%) was obtained from Across Organics, Powdered MTDA (purity 98%) was supplied by Alfa Aesar (Germany).; chloranilic acid (purity 98%) was obtained from Sigma Aldrich, and spectroscopic grade, ethanol and methanol were used without further purification. Standard stock solutions of DAT (5 × 10 −3 mol·L −1 ), MTDA (5 × 10 −3 mol·L −1 ) and CLA (5 × 10 −3 mol·L −1 ) were immediately prepared before each series of measurements by dissolving appropriate amount in 50 mL of solvent. All solutions were stored in dark place for at least one week.

Molecular Composition of the Formed HBCT Complex
The molecular composition of the formed HBCT-complexes was determined by

Formation Constant of the Formed Complex
Based on the electronic spectra of the HBCTI-complex at various donor's concentrations ( Figure 4), K F and ε were calculated using the Minimum-Maximum absorbances method [11].

MTDA-CLA
where A max is the maximum absorbance of the complex, A min is the minimum absorbance of the complex, A complex is the complexes absorbance values between A max and A min , and C Donor is the concentration of the added donor in mol·L −1 . The set of equilibrium constants were averaged. Can conclude from Table 1, there is a small variation in the formation constant between ethanol and methanol. The stability constant (K CT ) of the formed complex DAT-CLA found equal to 3.9030 × 10 3 L·mol − 1 and 3.1975 × 10 3 L·mol − 1 in EtOH and MeOH respectively. On the other hand, the stability constant (K CT ) of the formed complex MTDA-CLA found equal to 5.444 × 10 3 L·mol − 1 and 4.846 × 10 3 L·mol − 1 in EtOH and MeOH respectively.
From the above we conclude the Formation constant of MTDA-CLA is higher amounts and the higher stability of the complex DAT-CLA.
Where it seems that the stability of the formed complex is connected by solvent polarity where the less polar solvent methanol exhibited the smaller value of the formation constant than ethanol. Consequently, the trend of increasing the formation constants follows the order EtOH > MeOH, as can be seen from Table 1.
The high values of the stability constant are attributed to the high donating power of donor, the high electron affinity of CLA and the high electric permittivity for both ethanol and methanol. On the other hand, the increase of electron density on the complex nitrogen facilitates the formation of proton transfer hydrogen bonding between the H-donor Complexes (OH) and the H-acceptor of donors (ring nitrogen). This hydrogen bonding adds extra stability to the formed complex.

Determination of the Spectroscopic Physical Data
The stability of the formed complex can be evident from calculating the spectroscopic physical parameters These parameters included oscillator strength (f), transition dipole moment (µ) [12], charge transfer energy (E CT ) [13], ionization potential (I P ) [14] and resonance energy (R N ) [15].
The experimental oscillator strength (f), which is a dimensionless quantity, used to express the transition probability of the CT-band [16] and transition dipole moment (μ) is a valuable tool that confirms the existence of proton transfer interaction in the formed complex, calculated through the following equations [17]: is the half bandwidth of absorbance, max ε and max ν are the molar extinction coefficient and wave number at the maximum absorption of the complex, respectively.
The charge transfer E CT energy is a measure of the ease of charge transfer from donor to the acceptor and it represents the transitions π-π * and n-π * . Open Journal of Physical Chemistry The charge transfer energy is calculated based on Equation (4) [13] 1243.667 The ionization potential I D is the energy required to remove an electron from the donor's molecular orbital participating in charge transfer interaction and can be calculated from Aloisi and Piganator equation.
The dissociation energy (W) of the formed CT complex was calculated from the corresponding CT energy (E CT ), the ionization potential of the donor (I D ) and electron affinity of the acceptor (E A ) using the following relationship [18]: The resonance energy (R N ) is a ground state property that contributes to the stability of the formed complex. These parameters can be calculated using Equations (2)-(6).
[ ] 4 7.7 10 3.5 The calculated values of the different spectroscopic physical parameters in both Complex (DAT:CLA) and (MTDA:CLA) are reported in Table 2. As can be seen from Table 2, the values of the oscillator strengths are increased gradually on moving from methanol to ethanol, confirming the high probability of charge transfer with high polar solvent ethanol in consisting with the stability constant values. On the other hand, the transition dipole moment follows the same trend as the oscillator strength of methanol was smaller than ethanol. Hence one concludes from Table 2, the probability of H-transfer in ethanol is higher compared with methanol. Furthermore, one can deduct from oscillator strength and transition dipole moment, that the stability of the formed complex is attributed to the presence of two interactions, the CT (charge transfer) and PT proton transfer).
In Complex (DAT:CLA), the ionization potential recorded the same and small values in both EtOH and MeOH, asserting the formation of stable complex and one can conclude that the same donor molecular orbital interacts with CLA to produce the charge transfer complex. The obtained ionization potential values recorded small values due to the high basicity of DAT (two nitrogen ring and two amino groups). While The ionization potential in Complex (MTDA:CLA) was recorded the same and little higher value than Complex (MTDA:CLA) in both EtOH and MeOH respectively, due to the high basicity of MTDA (three nitrogen ring, two amino groups and one methyl group). This behavior suggests that the electron responsible for the basic strength of donors (n-electrons) is the same involved in the CT interaction of donors with CLA in EtOH and MeOH. Hence, the investigated donors behave as n-donor towards π-acceptor (CLA). This means that the highest occupied molecular (HOMO) is the non-bonding molecular orbital and the H-bond complexing sites of donor is pyridinic like nitrogen by its lone electron pair. Therefore, the CT interaction is attributed to the promotion of non-bonding electrons of the donors the lowest unoccupied π-molecular orbital of the acceptor CLA (LUMO). Consequently, one can deduce that the formed charge transfer complex is mainly n-π type in both EtOH and MeOH. It seems that the high donating power of donor from the presence of two ring nitrogen's and two amino in DTA and three nitrogen ring, two amino groups and one methyl group in MTDA groups is presumably responsible for this situation. The charge transfer energy recorded the same values in both EtOH and MeOH.

Determination of Thermodynamic Parameters
The thermodynamic properties of the complexes were studied by estimating the enthalpy change, ΔH˚ (k·J·mol −1 ), the entropy change, ΔS˚ (J·k −1 ·mol −1 ) and as well as Gibbs free energy change, ΔG˚ (k·J·mol −1 ) of the charge transfer reaction.
where ΔH˚ and ΔS˚ are the enthalpy and entropy of the CT complex formation, respectively. R is the gas constant (8.314 J·mol −1 ·k −1 ) and T is the absolute temperature in Kelvin. Plotting the values of lnK CT versus 1000/T, a straight line was obtained ( Figure 4). The slope and intercept of the line were equal to (−ΔH˚/R) and (ΔS˚/R), respectively; thus, the values of ΔH˚ and ΔS˚ were determined. The results obtained are given in Table 3 all complexes the value of formation constant decreases with increasing temperature in methanol and ethanol indicating that the CT reaction is exothermic, where the enthalpy of formation (−ΔH˚) recorded 1.28 and 1.34 k·J·mol −1 in EtOH and MeOH, respectively in Complex (DAT:CLA) and in complex (MTDA:CLA) was recorded 4.65 and 3.15 k·J·mol −1 in EtOH and MeOH, respectively. Consequently, the electron density increases on donors, leading to high −ΔH˚.
One observes in Table 3, that the entropy ΔS˚ recorded small different between EtOH and MeOH it was 64.5 and 62.6 J·k·mol −1 in Complex (DAT:CLA) while the entropy ΔS˚ recorded small different between EtOH and MeOH it was 59.4 and 60.0 J·k·mol −1 . Open Journal of Physical Chemistry The standard Gibbs free energy change of the complexation process (ΔG˚) was estimated from the K CT value at room temperature using Equation (9). The negative value of ΔG˚ (Table 3) indicates the spontaneous reaction between donors and acceptor with strong interaction as can be understood from the high negative value of ΔG˚.

Application of the Studied HBCTI Reaction
Based on the formation of stable purple colored CT-complex between CLA and donors, we proposed in this section a simple, rapid, and accurate spectrophotometric method for determination of donors. Hence, under the optimum reaction conditions Beer's plot at various 1:1 molar ratio between donors and CLA was constructed ( Figure 5). The regression equations in both EtOH and MeOH were studied by the least square method. Open Journal of Physical Chemistry  Table 4). The limits of detection and quantification were calculated according to the IUPAC definition [19]. The calculated values were listed in Table 4. They recorded small values confirming the high accuracy of methods studied. It has been found also that the confidence intervals of intercept and slope recorded small values confirming excellent linearity between the absorbance and concentration ( Table 4). The accuracy and precision of the method was established by performing analysis of solutions containing five different amounts (within Beer's law limits) of all donors and measuring the absorbance of their HBCT complexes with CLA in EtOH and MeOH. The concentration of donors was determined from the regression equation and then calculated the recovery percentages, the standard deviation SD, and relative standard deviation RSD. The recovery percentages recorder values near 100% with RSD ranging from 0.7547 to 0.8323 in Complex (DAT:CLA), while the complex (MTDA:CLA) value from 1.49 to 1.94 confirming high accuracy and precision of the proposed method ( Table 5). Comparison of the difference between the mean and true value [20] with the largest difference that could be executed as a result of indeterminate error ts n ± has been carried out and the results were collected in Table 5. It has been found that ( X µ − ) were less than ts n ± indicating that no significant difference exists between the mean and true values.

Conclusion
A charge and proton transfers complexation reaction of between 3,5-diamino-1,2,4-triazole (DAT) and 6-Methyl-1,3,5-triazine-2,4-diamine (MTDA) with 3,6-dichloro-2,5-dihydroxy-p-benzoquinone (chloranilic acid CLA) was studied spectrophotometrically in Ethanol (EtOH) and Methanol (MeOH) solvents at different temperatures have been investigated experimentally by using the spectroscopic techniques UVvis. We got a new band at λ max 524.5 nm both in ethanol and methanol in Complex (DAT:CLA), while in complex (MTDA:CLA) there are 524.5 nm and 524 nm in ethanol and methanol. The molecular composition of the complex was found to be 1:1 charge transfer complex for both Complex (DAT:CLA) and (MTDA:CLA) by job and spectrophotometric methods. The stability constant was determined in the investigated solvents; they exhibited that MTDA-CLA was higher amounts and the higher stability of the complex (DAT-CLA). The thermodynamic parameters were determined and evaluated; they showed solvent dependency. It is concluded that the formation constant (KCT) of the complexes is found to depend upon the nature of both electron acceptor and donors and on the polarity of solvents.