Analysis of complexation between new bidentate bis-NHC ligand and some metal cations at different temperature

In this research, the determination and complexation process between 3,3’-(2,2’-(4-methyl-phenylenesulfonamido)bis(ethane-2,1-diyl))bis(1-benzyl-3H-benzo[d]imidazol-1-ium)dibromide with Ni 2+ , Zn 2+ , Pd 2+ , Ag + , and Hg 2+ cations in the binary mixture of methanol (MeOH) and water (H 2 O) at different temperatures (15, 25, 35 and 45ºC) were studied using a conductometric method. The results show that the stoichiometry of the complexes in all binary mixed solvents for Ni 2+ , Zn 2+ , and Pd 2+ were 1:1 (M:L), while in other cases 1:2 (M:L) and 2:1(M:L). The stability constants (log ) of complex formation have been determined by fitting molar conductivity curves using a computer program (GENPLOT). The obtained data shows that in the pure methanol solvent system, the stability order is Ni 2+ < Pd 2+ <Zn 2+ <Hg 2+ <Ag + and the complexation process seems more stable in pure methanol in most cases. The thermodynamic parameters ) , , ( ο ο ο C C C S H G ∆ ∆ ∆ were determined conductometrically. The complexes in all cases were found to be enthalpy destabilized but entropy stabilized. The experimental data was tested by using an artificial neural network (ANN) program and was in good agreement with the estimated data.


A B S T R A C T
In this research, the determination and complexation process between 3,3'-(2,2'-(4-methyl-phenylenesulfonamido)bis(ethane-2,1-diyl)) bis(1-benzyl-3H-benzo[d]imidazol-1-ium)dibromide with Ni 2+ , Zn 2+ , Pd 2+ , Ag + , and Hg 2+ cations in the binary mixture of methanol (MeOH) and water (H 2 O) at different temperatures (15, 25, 35 and 45ºC) were studied using a conductometric method. The results show that the stoichiometry of the complexes in all binary mixed solvents for Ni 2+ , Zn 2+ , and Pd 2+ were 1:1 (M:L), while in other cases 1:2 (M:L) and 2:1(M:L). The stability constants (log ) of complex formation have been determined by fitting molar conductivity curves using a computer program (GENPLOT). The obtained data shows that in the pure methanol solvent system, the stability order is Ni 2+ < Pd 2+ <Zn 2+ <Hg 2+ <Ag + and the complexation process seems more stable in pure methanol in most cases. The thermodynamic parameters ) , , ( were determined conductometrically. The complexes in all cases were found to be enthalpy destabilized but entropy stabilized. The experimental data was tested by using an artificial neural network (ANN) program and was in good agreement with the estimated data.

Introduction
In early 1968, Wanzlick and Schönherr pioneered scientists that convey N-Heterocyclic carbene (NHC) complexation to a gained general acceptance among the researchers [1]. This effort was followed by Ardueng who found the stable crystalline carbene in 1991 [2]. In the meantime, numerous studies have been reported with the various applications so far. NHCs ligands synthesized from benzimidazole and imidazole were founded as attractive ligands for complexation due to their structure variety and stability [3]. Generally, it is more stable than other types of carbenes, such as the Fisher and Schrock carbenes [4]. Furthermore, these types of ligands can bond to either hard and soft transition metal ions or atoms through strong chelation [5][6][7][8][9]. Several articles have described the interesting features of NHC ligands and their metal complexes in detail, especially on their C-C coupling catalysis activities [10][11][12][13]. Recently, its application as an anticancer activity was also reported [14][15][16][17][18]. Several bidentate bis-NHC ligands derived from benzimidazole and imidazole with a different bridging linker have been reported [19][20][21][22][23]. The types of bridging linkers play the main role in the coordination of metal complexes. Different types of bridging linkers will be offered for the different conformations of complexes. This is related to the flexibility, length, and size of linkers [5]. In addition, different cavity sizes of ligand, ionic radii of metal [24], and solvent systems are the important factors that can influence the stability of complexation formation [25]. To our certain knowledge, there have been few reports of the thermodynamic study of bidentate bis-NHC ligand complexes with different metal ions. This encouraged us to investigate the effects of pure and binary solvent mixtures on stability constants and thermodynamic parameters of complexes. In our study, bidentate bis-NHC ligand connected by sulfonamide moiety was designated and synthesized by a simple and efficient method [3,19], namely 3,3'-(2,2'-(4-methyl-phenylenesulfonamido) bis(ethane-2,1-diyl))bis(1-benzyl-3H-benzo[d] imidazol-1-ium)dibromide (NHCL) (Fig. 1). To determine the stability, selectivity and stoichiometry of NHCL-M n+ complexes with different metal cations, the conductometric technique was chosen [26]. This technique has several advantages such as great sensitivity [27], low cost [28] as well as simple experimental arrangement [29] compared with other techniques such as spectrophotometry, calorimetry [30], and NMR spectroscopy [31], and potentiometry [32][33][34][35]. The sensors based on electrochemical methods have been used to measure the analyte concentrations in water samples. Hence, electroanalytical techniques such as, potentiometry, voltammetry, and conductometry have been extensively reported. Other techniques such as inductively coupled plasma mass spectrometry [36], co-precipitation [37], flame atomic absorption spectrometry (F-AAS) [38], inductively coupled plasma optical emission spectrometry (ICP-OES) [39], and electrothermal atomic absorption spectrometry (ETAAS) [40] have also been used for the determination cations in water samples after complexation process. Thus, this study can contribute to a better understanding of ligand character and behavior in coordination chemistry, and the solvent effect in its complexation process.

Chemicals and Instruments
The chemicals and metal ions used, which are nickel acetate, zinc acetate, palladium acetate, silver acetate, and mercury nitrite were purchased from Sigma-Aldrich (USA) and MERCK (Germany). All chemicals were analytical grade and used without further purification. Deionized distilled water and methanol with HPLC grade available from MERCK (Germany) were used as a solvent. The conductometric measurements were carried out using a digital Thermo Scientific conductivity device in a JULABO F12 thermostat water bath with a constant temperature maintained within ±0.01ºC. A conductometric cell model Orion 013005MD with a cell constant of 0.99cm -1 was used throughout the studies.

Synthesis of the bidentate bis-NHC ligand, NHCL
Several steps synthesized NHCL (4) from the diol (1) are shown in Figure 2. The synthesis of compounds (2) and (3) has been reported previously [19]. NHCL (4) was prepared according to the modification method designated as follows [3]. Benzyl bromide (0.1710 g, 1.0 mmol) was stirred in 20 ml of 1,4-dioxane, and then compound (3) (0.2296 g, 0.5 mmol) was added to it. The reaction mixture was refluxed at 100 ºC for 12 hours and the pale yellow precipitated was obtained. The product was collected by filtration, washed with fresh 1,4-dioxane (2×5 mL) and diethyl ether (2×5 mL) and dried in vacuo to give a pale yellow powder (4).

Analysis procedure
These experimental designs were prepared according to the altered procedure [41-43] and were applied to all metal cations; Ni 2+ , Zn 2+ , Pd 2+ , Hg 2+ , and Ag + . The formation constant of the complexes will be obtained by using the procedure designated as follow. A solution of the metal ion with a concentration, of 5.0 × 10 -5 M was prepared and fixed in a titration cell. After that, the L with concentration 2.5 × 10 -3 M was added to the titration cell using a micropipette. During the reaction, the desired temperature was fixed and a magnetic stir has been used to form a homogenized condition in the titration cell. The conductivity values were measured before and after each titration of ligands solution.

Results and discussion
The general reaction for complex formation (1:1) can be stated by Equation 1 and the corresponding equilibrium constant ( f K ), is given by Equation 2.  , the values of other species involved by using the appraised amount of the formation constants at the current iteration step of the program. Refinement of the parameters is continued until the sum-of-squares of the residuals between calculated and observed values of the conductance for all experimental points is minimized. The output of the program GENPLOT comprises refined parameters, the sumsquares and the standard deviation of the data [45]. For determination of the stability constants of complex formation between the NHC ligand and various metal cations, the conductometric method has been selected as the best method in numerous studies [25,41,46]. To study the complexation reaction of L with the Ni 2+ , Zn 2+ , Pd 2+ , Hg 2+ , and Ag + cations, the changes in molar conductivity (Ʌ m ) of the solution were supervised as a function  Figure 3. As can be seen in Figure 3 ( Figure 3 (B) observed a slightly decreasing in Ʌ m with increasing the ligand concentration, which indicated that ligand mobility is less than free solvated metal cation. However, it shows the increment after breaking points. This is indicated that the mobility of complexes in MeOH-H 2 Table 1 show the increase of stability constants (log f K ) for NHCLM n+ complexes with an increase of temperature in most of the solvent systems. This is an indication for an endothermic complexation reaction between ligands and metal cations in the solution [36]. The obtained data shows that in the pure MeOH solvent system the stability constant is varying as Ni 2+ < Pd 2+ <Zn 2+ <Hg 2+ <Ag + and in the most cases complexation process seems more stable in pure MeOH. The results proved that the stability of the resulting complexes was influenced by the nature of the solvent system. In the reaction mixture solution, the ligand should be able to excess metal cations which are solvated by solvent molecules to form a complex. Hence, dissimilarities in the nature of the solvent system may influence in the binding properties of NHCL, and subsequently, the stability and selectivity metal complexes.  Figure 4. This pattern is probably due to solvent-solvent interaction that changed the structure of the solvent mixtures and consequently changed the solvation properties of the metal ions, ligand and the resulting complexes [25,41]. In other cases, the increments of the stability constant value by the reducing of mol% of MeOH have been observed.  .7), would be increased and cause to form the complexes. In order to better understand the complexation proceed discussed, it is useful to consider the enthalpy and entropy contribution to these reactions. The The data demonstrated that these complexations process are temperature dependent. The value of standard enthalpy (  [39]. The calculated results are summarized in Table 2.  The results reveal, in the most cases the changes in The variation of log f K for all complex cases in contrast with cationic radius for studied cations in MeOH-H 2 O binary mixture at 25ºC were presented in Figure 7. As it is seen in Figure 7, the stability constant of complexes increase as the size of a metal cation increases from Ni 2+ to Ag + . These results can be explained due to solvent system effect. Usually the small metals cations will be more solvated than the bigger metals cations in a same solvent system and decrease its mobility. Consequently, the competition ligand with solvent molecules was increase and resulted in the decreasing of stability constant [44].

Computational study
With the purpose to elucidate the obtained experimental results, a density functional theory (DFT) study was conducted. The DFT calculations were carried out with the GAUSSIAN 09 software package, with the B3LYP/LANL2DZ basis set. The binding energy ∆E in the complexation between NHCL and M n+ in MeOH pure solvent is defined by the following formula,  Figure 8(A, B, C), respectively. While the calculated results of binding energies for all complexes in MeOH pure solvent were listed in Table 3. It is clear from Table 3 that the binding energy

Mathematical Modeling
Artificial neural network (ANN) as a mathematical (or computational) model that is inspired by the structure and function of biological neural networks in the brain, is one of the most successful technologies in the last two decades [49][50][51][52]. In this research work, the ANN was applied for the simulation of property parameters correlation, and a good agreement in the experimental and predicted value is obtained. A 4-11-1 (input layer-hidden layer-output layer) network structure was used as shown in Figure 9.   The obtained constant formations of complex reactions between all metal studied cations and the proposed ligand using the various mole percentages of water in H 2 O-MeOH binary media system at different temperatures were visualized in Figure 11 by drawing surface and contour plots of the stability constant (log K f ) for the complex formation as a function of the mol% of H2O in the binary mixtures and different studied cationics radius. As seen in Figure 11, increasing of mol% of water and cationic radius causes a decrease and an increase in the constant stability of complexes (log K f ). The maximum constant formation of complexes was obtained in zero mol percentage of water (100% of MeOH) and the highest amount of cationic radii related to Ag + cation (1.15Å).This result indicated that this model is valid for the estimation of constant stability of complexes of Ni 2+ , Pd 2+ , Zn 2+ , Hg 2+ and Ag + in MeOH-Water binary mixtures at different temperatures. The estimated results based on the ANN program were in a good agreement with obtaining experimental results.

Conclusion
According to the obtained results, the complexation of Ni 2+ , Zn 2+ , Pd 2+ , Hg 2+ and, Ag + cations with NHC ligand can be explained in terms of the size-fit concept, where the NHC ligand forms a most stable complex with a cation having a size which fits best with its cavity size. The obtained data shows that in the pure MeOH solvent system the stability constant is varying as Ni 2+ < Pd 2+ <Zn 2+ <Hg 2+ <Ag + and the complexations process seems more stable in pure MeOH and pure H 2 O.The result also showed that in most cases, the NHCLM n+ complexes was enthalpy destabilizer but entropy stabilizer. However, some results have shown that increment in stability is constant with the decrease of mol% MeOH in the solvent system. The effects of mole% of water and cationic radii of studied cations on the complexation reactions were investigated and high correlation between experimental data and ANN kinetic model was obtained which is a proof of the high performance of conductometric method for the complex formation study.