Mono- and trinuclear Cu^II complexes with tripodal ligands

Mononuclear [Cu(HL)] (1) and trinuclear [Cu₃(L)₂] (3) complexes were synthesized, where H₃L (1,1,1- tris[(salicylideneamino)methyl]ethane) is a tripodal ligand obtained by condensation of l,l,l-tris(aminorriethyl)ethane and salicylaldehyde in a 1:3 mole ratio. Another mononuclear complex [Cu(HL7′)] (2), where one of the three arms was disconnected probably due to thermal hydrolysis, was also obtained. The crystal structures, electronic spectra, and electrochemical and magnetic properties of the complexes were studied. In 1, one of the arms of H₃L is not coordinated to the metal atom, and the ligand serves as a tetradentate N₂O₂ ligand, the complex assuming a slightly distorted square-planar structure. In the trinuclear complex, an imine nitrogen atom and a phenolate oxygen atom of one arm of each terminal unit coordinate to the central Cu^II ion to form a linear trinuclear complex. The coordination geometry about each Cu is square planar. All of the complexes involve φ-φ stacking interactions between the molecules in the crystal structures, which are responsible for weak magnetic interactions. An electrochemical study of 3 in DMSO showed that the central Cu^II is more easily reduced to Cu^I than the terminal Cu ions. The mononuclear complexes, 1 and 2, show a quasi-reversible Cu^II/I couple almost at the same position as that of the terminal Cu ions of 3.