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 (log
K
ex) of 1:1 (M:L) complexes, the distribution (
K
D) for two diluents (CH
2Cl
2 and CHCl
3) with different dielectric constants have been determined at 25 ± 0.1 °C. The magnitude of log
K
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) log
K
ex sequences of TA-18-crown-6 and TBTA-18-crown-6 for transition metals in CH
2Cl
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.
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