The proton-ligand dissociation constant of 4-(4-amino-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-ylideneamino)-phenol (
Schiff-base coordination complexes have attracted great attention over the past decades due not only to their facile syntheses, their wide application, and the accessibility of diverse structural modifications, but also to their biological modeling applications, catalysis, design of molecular ferromagnets, and materials chemistry [
4-(4-Amino-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-ylideneamino)-phenol (
A ligand solution (0.001 mol·dm−3) was prepared by dissolving an accurately weighed amount of the solid in ethanol (Analar). Metal ion solutions (0.0001 mol·dm−3) were prepared from Analar metal chlorides in bidistilled water and standardized with EDTA [
The apparatus, general conditions, and methods of calculation were the same as in previous work [ 5 cm3 0.001 mol·dm−3 HCl + 5 cm3 1 mol·dm−3 KCl + 5 cm3 ethanol. 5 cm3 0.001 mol·dm−3 HCl + 5 cm3 1 mol·dm−3 KCl + 5 cm3 0.00l mol·dm−3 ligand. 5 cm3 0.001 mol·dm−3 HCl + 5 cm3 l mol·dm−3 KCl + 5 cm3 0.001 mol·dm−3 ligand + 10 cm3 0.0001 mol·dm−3 metal chloride.
For each mixture, the volume was made up to 50 cm3 with bidistilled water before the titration. These titrations were repeated for temperatures of 308 K and 318 K. The temperature was controlled to within ± 0.05 K by circulating thermostated water (Neslab 2 RTE 220) through the outer jacket of the vessel. The pH measurements were performed with a Metrohm 836 Titrando (KF & Potentiometric Titrator) equipped with a combined porolyte electrode. The pH-meter readings in the nonaqueous medium were corrected [
The average number of the protons associated with ligands (L1 and L2) at different pH values,
Thermodynamic functions for the dissociation of ligand (L1) in 10% (by volume) ethanol-water mixture and 0.1 mol · dm−3 KCl at different temperatures.
Dissociation | Gibbs energy | Enthalpy | Entropy | |
---|---|---|---|---|
|
constant | kJ |
kJ |
J |
pKH |
|
|
|
|
298 | 9.25 (0.10) | 52.77 | 70.56 | |
308 | 9.07 (0.09) | 53.48 | 31.75 | 70.57 |
318 | 8.90 (0.09) | 54.19 | 70.56 |
Standard deviations are given in parentheses.
Thermodynamic functions for the dissociation of ligand (L2) in 10% (by volume) ethanol-water mixture and 0.1 mol · dm−3 KCl at different temperatures.
|
Dissociation |
Gibbs energy |
Enthalpy |
Entropy |
---|---|---|---|---|
pKH |
|
|
− |
|
298 | 4.26 (0.09) | 24.31 | 24.92 | |
308 | 4.09 (0.09) | 24.12 | 31.73 | 24.72 |
318 | 3.91 (0.08) | 23.81 | 24.92 |
Standard deviations are given in parentheses.
The formation curves for the metal complexes were obtained by plotting the average number of ligands attached per metal ion ( The maximum value of The metal ion solution used in the present study was very dilute ( The metal titration curves were displaced to the right-hand side of the ligand titration curves along the volume axis, indicating proton release upon complex formation of the metal ion with the ligand. The large decrease in pH for the metal titration curves relative to ligand titration curves points to the formation of strong metal complexes [ For the same ligand at constant temperature, the stability of the chelates increases in the order Mn2+, Co2+, Ni2+, and Cu2+ [
Stepwise stability constants for ML and ML2 complexes of ligand (L1) in 10% (by volume) ethanol-water mixtures and 0.1 mol
|
298 K | 308 K | 318 K | |||
---|---|---|---|---|---|---|
log |
log |
log |
log |
log |
log |
|
Mn2+ | 6.84 (0.08) | 6.04 (0.11) | 7.02 (0.09) | 6.22 (0.10) | 7.19 (0.10) | 6.40 (0.11) |
Co2+ | 6.95 (0.09) | 6.13 (0.10) | 7.13 (0.09) | 6.31 (0.10) | 7.31 (0.11) | 6.48 (0.11) |
Ni2+ | 7.02 (0.09) | 6.19 (0.09) | 7.20 (0.10) | 6.37 (0.09) | 7.38 (0.11) | 6.54 (0.10) |
Cu2+ | 7.15 (0.09) | 6.34 (0.09) | 7.34 (0.09) | 6.52 (0.10) | 7.53 (0.11) | 6.70 (0.10) |
Standard deviations are given in parentheses.
Stepwise stability constants for ML and ML2 complexes of ligand (L2) in 10% (by volume) ethanol-water mixtures and 0.1 mol
|
298 K | 308 K | 318 K | |||
---|---|---|---|---|---|---|
log |
log |
log |
log |
log |
log |
|
Mn2+ | 7.33 (0.08) | 6.74 (0.09) | 7.50 (0.09) | 6.90 (0.10) | 7.67 (0.10) | 7.07 (0.10) |
Co2+ | 7.54 (0.09) | 6.85 (0.10) | 7.72 (0.10) | 7.02 (0.10) | 7.90 (0.10) | 7.20 (0.11) |
Ni2+ | 7.63 (0.09) | 6.92 (0.10) | 7.80 (0.11) | 7.10 (0.09) | 7.98 (0.11) | 7.27 (0.10) |
Cu2+ | 7.78 (0.09) | 7.07 (0.10) | 7.97 (0.10) | 7.26 (0.10) | 8.16 (0.11) | 7.44 (0.10) |
Standard deviations are given in parentheses.
The dissociation constant ( The A positive value of A large positive value of A negative value of
Thermodynamic functions for ML and ML2 complexes of ligand (L1) in 10% (by volume) ethanol-water mixture and 0.1 mol
|
|
Gibbs energy/kJ |
Enthalpy/kJ |
Entropy/J |
|||
---|---|---|---|---|---|---|---|
|
|
|
|
|
|
||
298 | 39.03 | 34.46 | 237.52 | 225.21 | |||
Mn2+ | 308 | 41.39 | 36.68 | 31.75 | 32.65 | 237.50 | 225.10 |
318 | 43.78 | 38.97 | 237.52 | 225.21 | |||
| |||||||
298 | 39.66 | 34.98 | 242.64 | 223.92 | |||
Co2+ | 308 | 42.05 | 37.21 | 32.64 | 31.75 | 242.52 | 223.91 |
318 | 44.51 | 39.46 | 242.64 | 223.92 | |||
| |||||||
298 | 40.06 | 35.32 | 243.98 | 225.07 | |||
Ni2+ | 308 | 42.46 | 37.57 | 32.65 | 31.75 | 243.87 | 225.06 |
318 | 44.94 | 39.82 | 243.98 | 225.07 | |||
| |||||||
298 | 40.79 | 36.18 | 252.55 | 230.96 | |||
Cu2+ | 308 | 43.29 | 38.45 | 34.46 | 32.65 | 252.43 | 230.84 |
318 | 45.85 | 40.79 | 252.55 | 230.96 |
Thermodynamic functions for ML and ML2 complexes of ligand (L2) in 10% (by volume) ethanol-water mixture and 0.1 mol
|
|
Gibbs energy/kJ |
Enthalpy/kJ |
Entropy/J |
|||
---|---|---|---|---|---|---|---|
|
|
|
|
|
|
||
298 | 41.82 | 38.46 | 243.82 | 229.45 | |||
Mn2+ | 308 | 44.23 | 40.69 | 30.83 | 29.92 | 243.72 | 229.25 |
318 | 46.70 | 43.05 | 243.82 | 229.45 | |||
| |||||||
298 | 43.02 | 39.09 | 253.93 | 237.64 | |||
Co2+ | 308 | 45.53 | 41.39 | 32.65 | 31.73 | 253.82 | 237.44 |
318 | 48.01 | 43.83 | 253.93 | 237.65 | |||
| |||||||
298 | 43.54 | 39.48 | 252.58 | 239.05 | |||
Ni2+ | 308 | 45.99 | 41.87 | 31.73 | 31.75 | 252.38 | 239.04 |
318 | 48.59 | 44.27 | 252.58 | 239.05 | |||
| |||||||
298 | 44.39 | 40.34 | 264.61 | 248.00 | |||
Cu2+ | 308 | 47.48 | 42.81 | 34.46 | 33.56 | 264.50 | 247.99 |
318 | 49.68 |
45.30 | 264.62 | 248.05 |
All the thermodynamic parameters of the stepwise stability constants of complexes are recorded in Tables The stability constants (log The negative value of The The