Metal complexes with macrocyclic ligands. Part XXII. Synthesis two of bis‐tetraaza‐macrocycles and study of the structures, electrochemistry, VIS and EPR spectra of their binuclear Cu²⁺ and Ni²⁺ complexes

The two bis‐macrocycles 4 and 5 in which the tetraaza units are separated by a chain of different length, have been synthesized using 1,4,7‐tritosyl‐1,4,7,11‐tetraazacyclotetradecane as starting compound and bifunctional alkylating agents. The bis‐macrocycles give binuclear complexes with Ni 2+ and Cu 2+ , the properties of which have been studied to obtain information about the interaction of the two subunits as a function of the distance. The VIS spectra of the Ni 2+ and Cu 2+ complexes indicate that both metal ions are in a square‐planar geometry as expected from the results of the analogous complexes with 1,4,7,11‐tetraazacyclotetradecane 7 . Cyclic voltammetry and differential pulse polarography of the binuclear Ni 2+ complexes in CH 3 CN show a single two‐electron step for ligand 5 , whereas two distinct one‐electron redox processes can be observed for ligand 4 , indicating that the two metal ions interact with each other when the chain length is shorter. Similarly, the EPR studies of frozen solutions of the binuclear Cu 2+ complexes clearly show that a magnetic dipolar interaction between the two paramagnetic centers exists, and that the strength of it depends upon the length of the bridge. Finally, from the X‐ray structures of the binuclear Ni 2+ complexes with 4 and 5 , it is seen that the two rings are kept apart as far as possible, the distances between the two metal ions determined in the solid correlate well with the observations in solution.