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Published: Kem. Ind. 51 (11) (2002) 459–464
Paper reference number: KUI-35/2001
Paper type: Original scientific paper

Complexation of Fe(III) Ions with Crown Ethers in Acetonitrile

S. Marić, R. Kubiček and J. Budimir


The complex reaction between crown ethers and metal cation has been studied by different methods. It has been found that the crown ethers has a capacity to form stable stoichiometric complexes with certain cations. Although a vast literature has been accumulated in the last forty years, relatively small number of papers is concerned with interaction of transition-metal complexes with oxygen-containing macrocycles. Crown ethers form complexes with a Fe(III) cations, and with assumption that mobility of free cations is different from their complexes, conductometry has been used to study complexation of cation by crown ethers. The stoichiometry of formed complex was investigated by the molar ratio metod. Addition of the DC18C6 and 18C6 to the Fe3+ ion in solution causes continuous decrease in molar conductance. The slop of the corresponding mole ratio plots changes sharply at the point where the ligand to cation mole ratio was 1. The results obtained show that the molar ratio of crown ether to metal ions in ion-pair complex was 1:1, and stability of the complex is governed by temperature. In case of DB18C6 addition, gradual decrease in molar conductance of Fe3+ ion is observed which does not exibit any considerable change in curvature at mole ratio 1, indicating that a weaker 1:1 complex is formed. In polarographic investigation, the significant quantity for studyng the stability of the metal ion complexes was the half-wave potential (E1/2) of the differential pulse peak potential (Ep) of the complex and the free metal ion. The difference in E1/2 between the complex and metal ion was used in calculation of the stability constants of the complex. The addition of the DC18C6 shifts the potential Ep of the metal ions towards a more negative value. This shift was insignificant with addition of DB18C6. It has been shown, that besides the relative size of the internal hole of macrocyclic polyether and metal ionic diametar, the complex formation is affected by substituent groups on ligand ring. When two benzo group are attached to a crown ether ring, crown ether is more hydrophobic. Electrophilic substituents, like benzo groups decrease the basicity of oxygen attached to the macrocyclic ring and reduce the complexig power of polyether.

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iron, crown ethers, complexation, DP polarography