Metal complexes of macrocyclic ligands. Part XXXVII. Synthesis of Heteroditopic Bis-macrocycles and Their Potential for Preparing Heterobinuclear Metal Complexes

A new and generally applicable synthetic path for the preparation of heteroditopic bis-macrocycles using tri-N-protected tetraazacycloalkanes as building blocks and bromoacetyl bromide as bridging reagent is described. In the first step, bromoacetyl bromide is used as acylating agent for one of the macrocycles, whereas in the second step it is used as alkylating agent for the second macrocycle, thus giving protected bis-macrocyclic amides (e.g. 6 ). After reduction of the amide moiety and deprotection, bis-azamacrocycles with an ethylene bridge are obtained (e.g. 8 ). The corresponding homoditopic bis-macrocycles 16 and 17 are also prepared for comparison purpose. Spectrophotometric studies indicate that bis-macrocycle 8 , which consists of a 12- and a 14-membered ring, binds two metal ions with equal affinity, whereas compound 13 , in which an unsubstituted (cyclam) and a trimethyl-substituted tetraazacyclotetradecane unit (Me₃cyclam) are bridged, shows selective metal-ion binding. The first metal ion is always incorporated into the cyclam unit, whereas the second one binds to the Me₃cyclam macrocycle. Thus, by sequential addition of two different metal ions, heterobinuclear complexes can easily be prepared. The electrochemistry of the binuclear Ni²⁺ complexes, studied by CV and DPV, as well as the EPR spectra of the binuclear Cu²⁺ complexes clearly indicate metal-metal interactions.