New 1,4,8,11-tetraazacyclotetradecane-5,7-dione ligands appended with additional 2-methylthiophene(s) pendant: crystal structure and properties of their copper(II) complexes

Two new macrocyclic dioxotetraamine ligands, 1-(2-methylthiophene)-1,4,8,11-tetraazacyclotetradecane-5,7-dione(H2L1) and 1,11-bis(2-methylthiophene)-1,4,8,11-tetraazacyclotetradecane-5,7-dione (H2L2), have been synthesized and characterized. The resulting dioxocyclams readily coordinate to Cu-II. The Cu-II complex of H2L2 has been isolated as a single crystal and the structure determined by X-ray diffraction analysis. The copper atom is in a square-planar environment with four basal nitrogen atoms. The solution behaviour of the Cu-II complexes, CuL1 and CuL2, has been further studied by e.s.r., u.v.-vis. and cyclic voltammetric techniques. A remarkable redshift has been observed for the maximum absorption band in the electronic spectra of CuL1 or CuL2 compared with that of the unsubstituted copper species (CuL). Electrochemical studies suggest that the introduction of thiophene pendant(s) to the macrocycle destabilizes the Cu-III ion compared with the unsubstituted dioxocyclam, and the reason for this is discussed.