Unsymmetrically substituted side-bridged cyclam derivatives and their Cu(ii) and Zn(ii) complexes

Abstract

A series of new compounds built up on the skeleton of 1,5,8,12-tetraazabicyclo[10.2.2]hexadecane (1,4-ethylene-bridged cyclam, 1,4-en-cyclam) were synthesized and characterized by means of NMR and MS spectroscopy and a single-crystal X-ray structure determination. The attempts to substitute the secondary amino group of the p-nitrobenzyl-1,4-en-cyclam 3, using acetic acid derivatives as the alkylating agents, led to unexpected substitution of one of the piperazine ring nitrogen atoms, yielding the monoquaternary derivatives 4^BrBr and 5^BrBr, respectively. In order to explain this reaction behaviour, the solution structure of the starting compound 3 was established using 2D-NMR techniques. The acid–base behaviour of the ligands and thermodynamic stability constants of their copper(II) and zinc(II) complexes were determined using potentiometric titrations. Stability constants of the investigated metal complexes are significantly lower than those of the cyclam complexes and comparable to those of the Me₄cyclam complexes. A copper(II) complex of amine 3 was prepared and characterized. In the solid state, this complex has the central copper atom surrounded by five donor atoms forming a coordination sphere with geometry between trigonal bipyramid and square pyramid.