Ligand field effects on the transport of 3d metal ions across a bulk liquid membrane by lipophilic tetraamine carriers

The 2 lipophilic tetraamines 6-hexadecyl-1,4,8,11-tetraazaundecane (I) and 7-hexadecyl-1,5,9,13-tetraazatridecane (II) were synthesized and used as carriers for the transport of metal ions across liq. membranes. Extn. expts. from aq. solns. of MX2 (M = Ni, Cu, Zn) to a CH2Cl2 layer contg. I or II were carried out. The metal is extd. in the org. phase as the MLX2 species (L = I or II), and in particular Cu is extd. to a larger extent than Ni and Zn, and because of the competition of the H+ and M2+ ions for the tetraamine, the process is pH controlled and the Cu extn. yield is fairly dependent upon the nature of the inorg. anion X-, decreasing in the order Br- > Cl- > CO3- > NO3- > ClO4-, a parallel sequence to the ligating tendencies of the axially bound counterions. Transport expts., performed by interfacing 2 aq. layers by a bulk CH2Cl2 membrane, made 5 × 10-4 M in the tetramine carrier I or II, and driven by a metal concn. gradient, showed that Cu is transported selectively in the presence of equimolar amts. of Ni and Zn ions. Selectivity is strongly enhanced by the use of I rather than II as carrier.