Electrical Conductance study of interactions of Cu2+ , Co2+ and Ni2+ metal cations with kryptofix-21 in acetonitrile - ethanol binary solvent solution

Abstract

The main characteristic of a crown ether is the complexation of the ether oxygens with various ionic species. The factors which influence the stability of the<br><br>metal cation -crown ether complexes include: the relative size of the cation and the cavity of the crown ether, the number of oxygen atoms in the ring, the<br><br>coplanarity of the oxygen atoms ,the symmetrical placement of the oxygen atoms, the basicity of the oxygen atoms, steric hindrance in the polyether ring ,the<br><br>nature of the cation and electrical charge on the cation [2,3] .The ability of the solvent molecules to compete with the donor atoms of the ligand towards the<br><br>coordination sites of the cation and also the solvation of the ligand and the resulting complex are another factors that can thermodynamically influence the<br><br>complexation process between such macrocyclic ligands and the metal cations [4,5].The complexation reactions between Co 2+, Cu2+, Ni2+ metal cations with<br><br>the macrocyclic ligand , kryptofix -21,in pure acetonitrile , ethanol and also in acetonitrile- ethanol (AN-EtOH) binary mixtures have been studied at<br><br>different temperatures using conductometric method. the conductance data show that the stoichiometry of the complexes in most cases is 1:1 [ML]. But in the<br><br>case of Cu2+ cation a 1:2 [ML2] complex is formed in pure EtOH , which shows that stoichiometry of the complexes may be changed by the nature of the<br><br>medium. The stability constants of the 1:1[M: L] complexes were determined using a computer program, GENPLOT. The stability order of complexes<br><br>between kryptofix -21 and Co2+, Ni2+, Cu2+ cations at 250C in the binary solvent solution of AN- EtOH (mol % AN 0) is : ( kryptofix-21. Ni ) 2+ &gt; (kryptofix-<br><br>21. Cu) 2+ &gt; ( kryptofix-21. Co ) 2+, but in case of another AN- EtOH binary solvent solution (mol % AN 25) it was found to be : (kryptofix-21. Cu ) 2+&gt;<br><br>(kryptofix-21. Co) 2+&gt; ( kryptofix-21. Ni) 2+ and in case of AN- EtOH binary mixed systems (mol % AN 50 and 75) the stability order changes as : ( kryptofix<br><br>- 21. Co) 2+&gt; ( kryptofix-21. Cu) 2+&gt; ( kryptofix-21. Ni) 2+ The order of the stability of the complexes in pure AN is (kryptofix - 21. Co) 2+&gt; (kryptofix-21. Ni)<br><br>2+.&gt; (kryptofix-21. Cu) 2+ .The values of stability constants of complexes ,which were obtained from conductometric data , show that the stability of<br><br>complexes is affected by the nature and composition of the binary mixed solvents and a non-linear behavior was observed for variation logKf of the<br><br>complexes versus the composition of the solvent system .The values of thermodynamic parameters (ΔHc<br><br>0, ΔSc<br><br>0 ) for formation of complexes show that in<br><br>most solution systems , the complexes are enthalpy destabilize but entropy stabilized.