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Structural basis for the design of selective complexing agents for transition metals

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Drummond, Laurel A. (1989) Structural basis for the design of selective complexing agents for transition metals. Doctoral thesis, Polytechnic of North London.

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Abstract

X-ray crystallographic analyses of a group of salicylaldoxime ligands, and their copper(II) complexes were carried out. The three of the copper(II) complexes studied, bis-(2-hydroxy-5-methylbenzaldehydeoximato)copper(II), [(L1)2Cu]n, bis-(2-hydroxy-5-methylbenzophenoneoximato)copper(II), [(L5 )2Cu] and bis-(2-hydroxy-3-t-butyl-methylbenzaldehydeoximato)copper(II), [(L7 )2Cu], existed as centrosymmetric molecules, with the copper atom at crystallographic inversion centre. Two bidentate ligands produce a planar arrangement of donor atoms around the copper. The molecules reassemble macrocyclic systems, due to effective intramolecular hydrogen bonding linking the two ligands.

To correlate chelating properties with intramolecular H-bond strengths an X-ray structural analysis was carried out on one ligand supplied by ICI, and thought to be 3,5-dimethyl-2-hydroxy acetophenone methoxyimine. Unexpectedly the molecule was found to be 3,5-dimethyl-4-hydroxy acetophenone methoxyimine. This was successful in explaining the anomolously low efficiency of extraction for copper(ll), shown by the related n-nonyl oxime ligand.

It is thought that the presence of syn and anti isomers in 2-hydroxybenzophenone oximes accounts for the low efficiency of copper extraction shown by reagents which contain this chelating unit. The syn and anti forms, of 2-hydroxy-5-methylbenzophenone oxime (L5H) were prepared separately. Significantly both forms appear to yield [L5]2CH] when treated with Cu(CH3CO2)2.H20 in a homogeneous solution, although there are indications that formation of the complex from the syn-form is slower.

Selectivity of complexation of metal ions by macrocyclic ligands were investigated. The stability constant data of zinc(II) and cadmium(II) complexes of a series of cyclic quadridentate ligands were compared. For the 15-membered N4 macrocyle L15, the propyl (L15p) and butyl (L15b) C-alkylated derivatives were synthesised for the first time, and the cadmium(II) complex of (L15b) was prepared. X-ray analysis were performed on the free ligand L15b, and the three metal complexes [Zn(L15)(NO3)(H2O)]NO3, [Cd(15)(NO3)NO3, and [Cd(L15b)(NO3)NO3. These results were then correlated with the results of stability constant measurements in discussing the probable behaviour of the ligands in the solution environment and hence to provide some insight into the effect of alkylation on the discrimination for zinc(II) over cadmium(II). The discrimation for zinc over cadmium is decreased on alkylation and the reason for this is discussed.

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