Synthesis and complexation behaviour of substituted polyaza-macrocycles

Abstract

Macrocyclic ligands which bind to transition metal ions are known to form stable and kinetically inert metal complexes. They might be potentially used as soil micronutrients which have properties over and above their more commonly used NTA, EDTA and DTPA acyclic counterparts. Work has been directed to the synthesis of functionalised macrocyclic ligands to bind to copper and nickel and other transition metals. Macrocyclic ligands have been selected to bind to the respective transition metals, under mild conditions, to form kinetically stable complexes. The stereoselective synthesis 1,8-disubstituted derivatives of 1,4,8,11- tetra-azacyclotetradecane (cyclam) has been achieved and copper and nickel complexes prepared. For the 1,8-dibutyl derivative, the square-planar nickel(II) complex exists as two diastereoisomers, as revealed by crystallographic analysis. The structure of the copper(II) complex of this ligand confirmed that a strong in-plane ligand field was conserved in square-planar complexes not withstanding the dialkylation at nitrogen. The structure of the copper (ID complexes of two 1,8-dicarboxymethyl derivatives of cyclam also reveal primary N(_4) co-ordination with carbonyl oxygens occupying the elongated axial sites. The tricyclic ligand 1,5,8,12-tetraazatricyclo[l0,2,2,2(^5,8)]octadecane is readily prepared from one of these diacids and the copper complex is kinetically stable in acidic solution. The synthesis of [12]N(_3) di- and tri-substituted triazacyclododecane ligands has also been effected through the intermediacy of monotosylamide derivatives. Copper(II) and nickel(II) complexes have been isolated as crystalline solids and have been characterised by spectrophotometric and FAB mass spectrometric methods.