Study of complexation processes between kryptofix 222 with Cd 2+ and Y3+ metal cations in some pure and binary mixed non-aqueous solvents using conductometric method

Abstract

Crown ethers which first synthesized in 1967 by Pederson[1], are macrocyclic polyethers which are noted for their remarkable selectivity towards specific<br><br>cations [2] and formation of stable and selective complexes with them [3]. Cryptands are three dimensional analogues of crown ethers [4]. Most of them are<br><br>available as the commercial name ‘kryptofix’. In the present work, the complexation reactions between Cd2+ and Y3+metal cations with the macrocyclic<br><br>ligand, kryptofix222 (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8] hexacosane),were studied in acetonitrile-ethanol (AN-EtOH), acetonitrileethylacetate<br><br>(AN-EtOAc) binary mixed solvent solutions, and in pure acetonitrile (AN), pure ethanol (EtOH) and pure ethylacetate at different temperatures<br><br>using conductometric method. The conductance data show that in most solvent systems, kryptofix222 forms a 1:1 complex [M:L] with Cd2+ and Y3+cations,<br><br>but in some of the solvent systems,in addition of formation a 1:1 [M:L] complex, 1:2 [ ] and 1:3[ ] complexes are probably formed in solutions.<br><br>The stability order of (krytofix222.Cd) 2+ and (kryptofix222.Y) 3+ complexes in pure studied solvents at 25 was found to be: AN&gt; EtOH.<br><br>The stability constants of the 1:1 complexes were determined using a computer program, Genplot [5] . The results obtained in this study, show that the<br><br>(kryptofix222.Cd)2+ complex is more stable than (kryptofix222.Y) 3+ in most solvent systems at various temperatures and the stability constants of the 1:1<br><br>complexes are affected by the nature and also the composition of the binary solvent solutions. A non-linear behavior was observed for the changes of logKf of<br><br>the complexes versus the composition of the binary mixed solvents. The values of the standard thermodynamic parameters (ΔH°c , ΔS°c) were obtained from<br><br>temperature dependence of the equilibrium constants of (kryptofix222.Cd) 2+ and (kryptofix222.Y) 3+ complexes using the van‘t Hoff plots. The obtained<br><br>results show that the complexation processes between Cd2+ and Y3+ cations with C222 in all cases are entropy stabilized but from the enthalpy view point,<br><br>depending on the nature and composition of the mixed solvents they are exothermic and endothermic or athermic.