Extraction-ability and -selectivity of tetra-aza-crown ethers for transition metal cations

Abstract

In order to investigate the relative effects of the differences between the structures and lipophilicities of 1 10-dioxa-4, 7, 13, 16-tetra-azacyclo- octadecane (TA-18-crown-6) and the tetrabenzyl derivative of 1,10-dioxa-4, 7, 13, 16-tetra-azacyclo-octadecane (TBTA-18-crown-6) on their extraction-abilities and -selectivities for transition metal cations, constants of the overall extraction (logK ex ) of 1:1 (M:L) complexes, the distribution (K D ) for two diluents (CH 2 Cl 2 and CHCl 3 ) with different dielectric constants have been determined at 25 ± 0.1°C. The magnitude of logK ex is largely determined by that of K D . The equilibrium constants of TA-18-crown-6 have been compared with those of TBTA-18-crown-6. It is found that: (i) logK ex sequences of TA-18-crown-6 and TBTA-18-crown-6 for transition metals in CH 2 Cl 2 lie in order: Fe 3+ > Cu 2+ > Mn 2+ > Co 2+ > Cd 2+ > Ni 2+ > Zn 2+ and Fe 3+ > Cu 2+ > Co 2+ > Mn 2+ > Ni 2+ > Cd 2+ > Zn 2+ respectively; (ii) the stability sequences of two types of tetra-aza-crown ethers with the transition metal cations in CHCl 3 are the same as follows: Fe 3+ > Ni 2+ > Cu 2+ > Co 2+ > Zn 2+ > Cd 2+ > Mn 2+ , and (iii) unusual selectivities are observed for transition metal-tetra-aza-crowns, e.g. the high Fe 3+ /M n+ selectivity factors (S f ) of TA-18-crown-6, except for the competitive- extractions for the special case in CHCl 3 of TBTA-18-crown-6, it was found that the Mn 2+ /M n+ values were relatively higher according to the other transition metal cations. A systematic sequence in these two types of solvents is not found for a given transition metal cation in terms of the variation of selectivity with the tetra-aza-crown ethers. The results provide alternatives for the rational design of other specific ligands on the transition metal cations.