metal cation complexation

Journal of Molecular Liquids 142 (2008) 53–56

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Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m o l l i q

Electric conductance and semi-empirical studies on two thiophene derivatives/metal cation complexation F.I. El-Dossoki Chemistry Department, Faculty of Education, Suez-Canal University, Port-Said, Egypt

A R T I C L E

I N F O

Article history: Received 23 March 2008 Accepted 25 April 2008 Available online 6 May 2008 Keywords: Thiophene Conductance Semi-empirical calculation

A B S T R A C T The formation constants for 1:1 Stoichiometric complexes of 2,4-diamino-3,5-dicyano thiophene (DADCT) and 2amino cyclohexane thiophene-3-carbonitrile (ACTC) with transition metal cations (Mn+2 , Ni+ 2 , Cu+ 2 , Zn+ 2 , Cd+ 2 , UO+2 2 , La+ 3 and Zr+ 4 ) in 50% (V./V.) ethanol–water and methanol–water solvents have been determined conductometrically at different temperatures. A semi-empirical PM3 calculations were also used to predict the structure of the metal complex by calculating the enthalpy of formation, the geometrical parameters and Mulliken charges of the free ligands and the suggested structures of the formed complexes. The values of the different thermodynamic parameters (ΔG, ΔH and ΔS) have been obtained. The results show that the complexation reactions are all exothermic except in the case of La+ 3-DADCT, which is endothermic reaction. The formation constant for the transition metal cations-ACTC complexes were larger than that for the transition metal cations-DADCT complexes. Also, the formation constants for all studied complexes in ethanol–water solvent were higher than that in methanol– water solvent. Using the SPSS computer program, a second order relation was found between Log k and the ionic radius (r) of the cations under investigation. The semi-empirical PM3 calculations show that there are two suggested structures of the complexation of (DADCT) with the studied metal ions. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The preparation and reactions of (DADCT) [1,2] and (ACTC) have been described [3]. Recently the proton ionization process were studied thermodynamically for some heterocyclic amines [3–5]. Conductometric titration method is considered to be one of the important methods for studying the complexation process between different ligands and metal ions [6–8] not only in aqueous solutions but also in nonaqueous or mixed ones. Only one conductometric study has concerning with the complexation process of (DADCT) with (Cu+ 2, Ni+ 2, Zn+ 2, Co+ 2 and Mn+ 2) in 20% npropanol–water solvent [6]. Computational chemistry has allowed us to take a closer look at the complex at the molecular level which unable to go under the experimental measurements[9]. The empirical technique of using the mixed solvent in the determination of the metal ion/ligand complexation has been used extensively[7]. Hence, we aims in this paper to study in details the complexation reactions of (DADCT) and (ACTC) with some transition metal cations (Mn+ 2 , Ni+ 2 , Cu+ 2 , Zn+ 2 , Cd+ 2 , UO+2 2 , La+ 3 and Zr+ 4 ) in 50% (V./V.) (ethanol–water ) and (methanol–water) solvents at different temperatures using the electrical conductivity technique and a semi-empirical PM3 calculations.

NiCl2 , CuCl2 , ZnCl2 , CdCl2 , UO2(NO3)2 , LaCl3 , ZrOCl2 , ethanol and methanol were supplied by Adwic and BDH analytical grade. Bidistilled water was used to prepare the organic-aqueous mixtures (50% V./V.). The conductometric measurements were carried out using adijital conductivity meter of a type Jenway , 4310 , with asensitivity 0.01 μΩ− 1 cm− 1. The temperature was controlled in only ±0.1 °C using ultrathermostate of a type MLW,UB (8542). In a typical experiment, 10 ml. of 1 × 10− 4 M metal salt solution was placed in a double jacketed cell

2. Experimental DADCT and (ACTC) thiophene compounds (Skeleton I ) were prepared and purified using the method described earlier [3]. MnCl2 , E-mail address: [email protected]. 0167-7322/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2008.04.013

Fig. 1. The conductometric titration curves for DADCT with some cations.

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F.I. El-Dossoki / Journal of Molecular Liquids 142 (2008) 53–56

[L]t = total ligand concentration added to the solution and [L] = free ligand concentration. The complexation reaction between the studied metal cations and (DADCT) ,(ACTC) were modeled and the reaction scheme was suggested to have the following mechanism to give either tetrahedral or octahedral complex. L þ Mn þðH2 OÞx ↔L  MðH2 OÞn þ mðH2 OÞ

ð3Þ

where M (Mn+ 2, Ni+ 2, Cu+ 2 and Zn+ 2 ), n = 2–4, m = 0–4, x = 2–6. 3. Results and discussion The measured specific conductance (KS) of Mn+ 2 , Ni+ 2 , Cu+ 2 , Zn+ 2, UO+2 2 , La+ 3 and Zr+ 4 solution was monitored as a function of the DADCT and ACTC /metal ion mole ratio [L]/[M] at 20, 25, 30, 35 °C in 50% (V/V) ethanol–water and methanol–water solvent (Figs. 1 and 2 as example). The resulting plots show that, the addition of the ligand to the metal salt solution causes a gradual decrease in the specific conductance, which tends to level off at high ligand to metal ion mole ratios. The results then indicating that the complexed cation is less mobile than the solvated one [11]. From Figs. 1 and 2, it is seen that, for all metal cation–ligand system studied, the slope of the corresponding specific conductance–mole ratio plots change at the point where the ratio is one , indicating of a 1:1 stoichiometric complex between DADCT, ACTC and the transition metal cations used. The complex formation constant were evaluated using Eqs. (1) and (2) and listed in Table 1. The values of the complex formation constants show that the nature of the solvent (i.e. dielectric constant and the donor number of the solvent), the chemical structure of the thiophene derivatives used and the temperature degree have a very fundamental effect on the stability of the resulting complexes. The stability constant of all 1:1 stoichiometric complexes with DADCT and ACTC was found to be decreased in methanol–water than in ethanol–water solvents. This can be illustrated , since in the complexation process, the ligand must compete with solvent molecules for the cations, variation of the solvent is expected to change the apparent binding abilities of the complexes [6]. Actually, there is an inverse relationship between the stability of the complexes and the solvation power of the solvents, as expressed by the [12,13] donor numbers. Several works which show the same type of solvent effect on the stability of different metal ion–ligand complexes are reported earlier [14,15].

Fig. 2. The conductometric titration curves for ACTC with some cations.

connected to the ultra-thermostate at the desired temperature (20– 35 °C). Then known amount of 1 × 10− 3 M (DADCT) and (ACTC) solution were added by means of a micropipete. The conductance of the mixture was then measured after each addition and after stirring.

The formation constants (K) for 1:1 metal ion-DADCT and ACTC stoichiometric complexes , can be expressed [10] as in Eq. (1): K¼

Λ m −Λ obs: ðΛ obs: −Λ C Þ½L

ð1Þ

where ½L ¼ ½Lt −

½M t ðΛ m −Λ obs Þ ðΛ m −Λ C Þ

ð2Þ

Λm = molar conductance of metal ion before addition of the ligand (thiophene derivatives DADCT or ACTC), Λobs = observed molar conductance of the solution during the titration, ΛC = molar conductance of the complexed metal ion, [M]t = total metal salt concentration,

Table 1 The values of the stability constant of the transition metal cation–thiophen complexes Ligand

DADCT

T

293 298 303 308

ACTC

293 298 303 308

Solvent

EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O

Log K Mn+ 2

Ni+ 2

Cu+ 2

Zn+ 2

Cd+ 2

UO+2 2

La+ 3

Zr+ 4

4.8500 4.8200 4.8200 4.8000 (3.1920) 4.8100 4.7800 4.7910 4.7520 (3.188) 4.5500 4.4100 4.5010 4.3900 4.4800 4.3720 4.4610 4.3320

4.7360 4.7034 4.7119 4.6783 (3.732) 4.6740 4.6411 4.6360 4.6031 (3.725) 4.7918 4.6912 4.7788 4.6849 4.7510 4.6770 4.7225 4.6484

4.6481 4.6200 4.6291 4.6029 (4.2340) 4.6120 4.5860 4.5948 4.5685 (4.222) 5.4500 5.1717 5.3100 4.9854 5.2500 4.7922 5.1900 4.7871

4.5933 4.5811 4.5846 4.5790 (3.509) 4.5755 4.5690 4.5654 4.5596 (3.494) 4.7001 4.6911 4.6945 4.6872 4.6800 4.6730 4.6765 4.6688

4.6909 4.5160 4.6800 4.5112 4.6705 4.5071 4.6601 4.5022 4.7120 4.5521 4.7018 4.5421 4.6985 4.5390 4.6970 4.5371

4.6552 4.5501 4.6225 4.4009 4.5945 4.2500 4.5657 4.1511 5.8261 4.6901 5.8172 4.4809 5.8099 4.2785 5.8013 4.2750

4.5519 4.5503 4.8085 4.8011 5.0610 5.0531 5.3137 5.3060 5.5047 4.8812 5.4247 4.8122 5.3450 4.7421 5.2633 4.6706

5.5430 5.3613 5.1675 4.9404 4.7671 4.5260 4.3061 4.1921 5.8131 5.8008 5.6041 5.5218 4.9830 4.9002 4.3608 4.2779

Values in parthenes were in 20% n-propanol–H2O solvent [6].

F.I. El-Dossoki / Journal of Molecular Liquids 142 (2008) 53–56

55

Table 2 The free energy change of the formation of the transition metal cation– thiophene complexes −ΔG (k J mol− 1)

Ligand

T

Solvent

DADCT

293

EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O

298 303 308 ACTC

293 298 303 308

Mn+ 2

Ni+ 2

Cu+ 2

Zn+ 2

Cd+ 2

UO+2 2

La+ 3

Zr+ 4

27.182 27.014 27.475 27.360 27.877 27.703 28.226 27.996 25.500 24.716 25.656 25.023 25.965 25.339 26.281 25.521

26.543 26.360 26.858 26.667 27.089 26.899 27.313 27.119 26.855 26.292 27.239 26.704 27.536 27.107 27.822 27.386

26.050 25.893 26.386 26.237 26.730 26.579 27.069 26.915 30.544 28.985 30.267 28.417 30.427 27.774 30.576 28.203

25.743 25.675 26.133 26.101 26.518 26.481 26.896 26.862 26.342 26.291 26.759 26.718 27.124 27.084 27.551 27.506

26.290 25.309 26.676 25.714 27.069 26.122 27.455 26.524 26.408 25.512 26.801 25.891 27.231 26.307 27.672 26.730

26.089 25.501 26.349 25.086 26.629 24.632 26.898 24.456 32.652 26.285 33.159 25.542 33.673 24.797 34.178 25.186

25.511 25.502 27.409 27.367 29.332 29.287 31.305 31.259 30.851 27.356 30.921 27.430 30.978 27.484 31.008 27.516

31.065 30.047 29.455 28.161 27.629 26.236 25.369 24.697 32.579 32.510 31.944 31.475 28.880 28.400 25.691 25.203

Table 3 The enthalpy change of the formation of the transition cation-thiophene complexes Ligand

Solvent

DADCT

EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O

ACTC

−ΔH (k J mol− 1) Mn+ 2

Ni+ 2

Cu+ 2

Zn+ 2

Cd+ 2

UO+2 2

La+ 3

Zr+4

31.879 70.136 10.930 7.1730

12.752 8.8283 19.128 6.3759

9.5640 19.128 71.729 31.880

2.5500 6.6532 7.6511 7.9699

3.5611 12.752 9.9213 10.930

15.302 46.527 5.4651 38.256

95.640 89.264 26.566 25.849

148.77 133.89 219.97 208.67

In comparing the resulting data with that found in literature (Table 1) , it was shown that the formation constants of DADCT-M+n in 50%(V/V) ethanol–water and methanol–water solvents were higher than that in 20% n-propanol–water solvent. This can be related to the differences in the dielectric constants and the donor numbers of the solvents. The results also indicating that the stability constant of (DADCT-M+n) complexes in all used solvents were lower than that of (ACTC-M+n) complexes. This may be due to the different chemical structure of the two thiophene derivatives used (Skeleton I). Using the SPSS computer program [16] the following second order relations (Eqs. 4–7) was found between Log k and the ionic radius (r) of the cations under investigation (dissimilar electronic configurations): For DADCT in MeOH–H2O mixtures: Logk ¼ 2:5r2 −5:2r þ 7:1

ð4Þ 2

with square correlation coefficient (R ) = 0.93. For DADCT in EtOH–H2O mixtures: Logk ¼ 4:98−0:17r2 −0:17r

ð5Þ

with R2 = 0.87. For ACTC in MeOH–H2O mixtures: Logk ¼ 20:4r2 −36:1r þ 20:2

We can evaluate the different thermodynamic parameters (ΔG, ΔH and ΔS) depending on Van't Hoff's isochore. The Gibbs free energy changes ΔG can be calculated from Eq. (8) ΔG ¼ −RTlnK

The Log K values have plotted against (T ), which gives a straight line with slope equals −ΔH(2.303R)− 1, from which the enthalpy changes ΔH can be computed from Eq. (9) ΔS ¼ ðΔH−ΔGÞ=T:

Table 4 The entropy change of the formation of the transition metal cation–thiophene complexes

DADCT

T

293

2

with R = 0.97. For ACTC in EtOH–H2O mixtures: Logk ¼ 17:2r2 −0:31r þ 18

298 303

ð7Þ 308

with R2 = 0.84 From Table 1, it was also noted that the stability or the formation constant for all complexes decreases as the temperature increase except the La+ 3-DADCT complex in ethanol–water and methanol– water solvents which increase as the temperature increase. This indicating that the complexation process for all complexes in all solvents used is exothermic one except that of La+ 3-DADCT which is endothermic process.

ð9Þ

The values of the different thermodynamic parameters are recorded in Tables (2–4). The free energy change values of all the metal ion complexes behaves the same trend as Log k. The negative values of ΔG and ΔH, indicating that the complexation reactions were a spontaneous one. The enthalpy of formation of the metal ion/DADCT complexes were estimated using the semi-empirical PM3 calculations and the results

Ligand

ð6Þ

ð8Þ −1

ACTC

293 298 303 308

Solvent

EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O EtOH–H2O MeOH–H2O

−ΔS (k J mol− 1 K− 1) x 10− 2 Mn+ 2

Ni+ 2

Cu+ 2

Zn+ 2

Cd+ 2

UO+2 2

La+ 3

Zr+ 4

1.00 14.1 1.07 14.2 1.13 14.4 1.08 14.3 -6.29 -6.67 -6.32 6.68 -6.35 -6.69 -6.37 -6.70

-4.70 -5.98 -4.73 -5.98 -4.73 -5.96 -4.72 -5.94 -2.64 -6.79 -2.72 -6.82 -2.77 -6.84 -2.82 -6.82

-6.01 -2.69 -6.03 -2.77 -6.16 -2.84 -6.07 -2.91 +11.2 +0.99 10.9 1.16 10.9 1.36 10.8 1.19

-8.10 -6.66 -8.11 -6.72 -8.10 -6.73 -8.09 -6.75 -6.38 -6.25 -6.41 -6.29 -6.42 -6.31 -6.46 -6.34

-7.76 -4.28 -7.75 -4.35 -7.76 -4.41 -7.75 -4.47 -5.62 -4.98 -5.66 -5.02 -5.71 -5.07 -5.76 -5.13

-3.68 +7.17 -3.71 7.19 -3.74 7.22 -3.76 7.16 -9.28 +4.08 -9.29 4.26 -9.31 4.44 -9.32 4.24

-41.3 -39.2 41.29 39.14 41.24 39.12 41.21 39.13 -1.46 -0.51 -1.46 -0.53 -1.45 -0.54 -1.44 -0.54

40.2 35.4 40.0 35.5 39.9 35.5 40.0 35.4 63.9 60.1 63.1 59.4 63.0 59.5 63.1 59.5

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Table 5 The enthalpy of formation(in K J /mol) of (DADCT) suggested complexes as calculated applying the semi-empirical PM3 program at 298 K Ligand

Ratio (L:M)

Structure

Mn+ 2 complex

Cu+ 2 complex

Ni+ 2 complex

Zn+ 2 complex

(DADCT)

1:1 1:1 2:1 2:1

I II I II

−1023.246 −990.242 +1423.50 +1377.80

−907.478 −906.642 +1262.34 +1261.50

−856.900 −467.742 + 119077 +650.800

−163.438 −671.308 +826.660 +226.001

were recorded in Table (5). The results show that the most possible modes of structures that DADCT can coordinate the metal ion in a bidentate fashion are through cyano nitrogen and amino nitrogen (structure I) or through cyano nitrogen and thiophen sulfur(structure II). Also we can noticed that structure (I) is preferred than structure (II) where it has more enthalpy of formation except in case of Zn which prefer structure (II). As shown from Table (5), the calculated enthalpies of formation for the complexes of stoichiometric ratio 1:1(L:M) has a high negative values while that of 2:1(L:M) stoichiometric complexes has a high positive values. This indicate the highest probability of the formation of 1:1(L:M) stoichiometric complexes and the lowest probability to form 2:1(L:M) stoichiometric complexes. This results confirm the experimental results in this study that the complexes of stoichiometric ratio 1:1(L:M) has been formed between the ligand and the studied cations. The difference in the values of enthalpy of formation calculated applying the semi-empirical PM3 program and the experimental results is due to the effect of the medium where solvents were used in the experimental process while the semiempirical PM3 calculations were made in the gas phase.

Table 6 The geometrical parameters (in Å) and Mulliken charges (q) of the free (DADCT) ligand and its suggested complexes

Bond length Structure I (N6`C6) (C1–N1) (M–N1) (M–N6) Structure II (C4–S) (N5`C5) (M–S) (M–N5) Charges Structure I q N1 q N6 qM Structure II qS q N5 qM

Free (DADCT)

Mn+ 2 (DADCT)

Cu+ 2 (DADCT)

Ni+ 2 (DADCT)

Zn+ 2 (DADCT)

1.1647 1.3320 – –

1.9799 2.3976 1.9044 1.4759

1.3976 1.8648 1.807 1.8560

1.3901 1.7015 2.1112 1.9980

1.3801 1.5984 2.3668 2.5210

1.6507 1.1632 – –

2.5133 1.2924 2.3563 1.4791

2.4760 1.250 2.3850 1.8557

1.8012 1.2146 2.4021 2.0022

1.7883 1.1801 2.5410 2.7718

+0.494 −0.026 –

+0.947 +0.816 −1.225

+ 1.112 + 0.196 − 0.882

+0.501 +0.056 −0.555

+0.266 +0.036 −0.385

−0.136 −0.095 –

+0.485 +0.902 −1.146

+ 0.547 + 0.162 − 0.742

+0.525 −0.131 −0.555

+0.426 −0.094 −0.270

The Mulliken charges (q) on the different atoms and the bond lengths in the free ligand (DADCT) and their modeled complexes with (Mn+ 2, Ni+ 2, Cu+ 2 and Zn+ 2) were determined applying the semiempirical PM3 calculations and recorded in Table (6). From the Mulliken charges, it was observed that M(+n) received electrons in both structures and hence acquired more negative charge. On the other hand, there is an electron depletion on the atoms S, N5, N1 and N6 accompanied by a flow of electrons towards M(+n). Also there is a change in the electron density on the atoms C1, C2, C6, C3, C4 and C5 where there is a delocalization of Π-electrons along the (N5`C6–C2fC1–N1) in structure (I) and along (N5`C5–C4–S) in structure (II). This delocalization of electrons is indicative of complex formation. A significant change was also observed in the bond length as a result of the complexation processes, where the bonds (N6`C6) and (C1–N1) becomes taller than that in the free ligand (DADCT) before complexation as a result of a formation of (M–N1) and (M–N6) bonds structure (I). Also in structure (II), bonds (C4–S) and (N5`C5) becomes taller than that in the free ligand as a result of a formation of (M–S) and (M–N5) bonds in the complexes. In compairing the stabilities of the studied complexes, it was noticed that Mn+ 2 N Cu+ 2 N Ni+ 2 N Zn+ 2 as shown from the values of enthalpy of formation, geometrical parameters and the Mulliken charges. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

V.K. Gewald, H. Bottcher, B. Schlinke, Chem. Ber. 99 (1966) 94. M.S. Mawas, M. Sugluta, H.P.S. Chaula, J. Heterocycl. Chem. 15 (1978) 949. K. Gewald, Chem. Ber. 98 (1965) 3571. M.M. Emara, M.M. Bahr, Bull. Soc. Chim. Fr. 125 (1984) 1–2. A.O. Alyoubi, Y. Obaid, M.M. Emara, Orient. J. Chem. 7 (2) (1991) 78. A.A. Mohamed, Al-Azhar, Bull. Sci 12 (2) (2001) 65–72. G.H. Zimmerman, R.H. Wood, J. Phys. Chem. 31 (2002) 995–1017. A.V. Sharygin, R.H. Wood, G.H. Zimmerman, V.N. Baloshov, J. Phys. Chem. B 106 (2002) 7121–7134. N.Z. Atay, T. Varnali, Turk. J. Chem. 26 (2002) 303–309. Y. Takeda, Bull. Chem. Soc. Jpn. 56 (1983) 3600. M.K. Amini, M. Shamsipur, Inorg. Chim. Acta 183 (1991) 65–96. V. Gutmann, E. Wychera, Inorg. Nucl. Chem. Lett. 2 (1966) 257. Y. Marcus, J. Solution Chem. 12 (1983) 271. M. Shamsipur, S. Madaeni, S. Kashanian, Talanta 36 (1989) 773. S. Kashanian, M.B. Gholivand, S. Madaeni, A. Nikrahi, M. Shamsipur, Polyhedron 7 (1988) 1227. M.J. Norusis, The SPSS guide to data analysis for SPSS / PC + (Znded), SPSS Inc., Chicago, IL, U.S.A., 1991