Copper(II) and gallium(III) complexes of trans-bis(2-hydroxybenzyl) cyclen derivatives: absence of a cross-bridge proves surprisingly more favorable.

Abstract

Two cyclen (1,4,7,10-tetraazacyclododecane) derivatives bearing trans-bis(2-hydroxybenzyl) arms, the 1,7-(2-hydroxybenzyl)-1,4,7,10-tetraazacyclododecane (H2do2ph) and its cross-bridged counterpart (H2cb-do2ph), have been synthesized, aiming toward the possible use of their copper(II) and gallium(III) complexes in nuclear medicine. The protonation of both compounds was studied in aqueous solution as well as their complexes with Cu(2+) and Ga(3+) cations. The complexes of both ligands with Ca(2+) and Zn(2+) metal ions were also studied due to the abundance of these cations in biological media. In mild conditions the complexes of Ca(2+) and Ga(3+) with H2cb-do2ph did not form. The behavior of the two ligands and their complexes was compared by the values of the equilibrium constants, the data of varied spectroscopic techniques, the values of redox potentials of their copper(II) complexes, and the resistance of the complexes to acid dissociation. It was expected that, as found for related pairs of cyclen and cyclam (1,4,8,11-tetraazacyclotetradecane) derivatives, the cross-bridged macrocyclic derivative could be an excellent ligand for the complexation of copper(II). Additionally, the N-2-hydroxybenzyl groups were chosen due to their known ability to coordinate the gallium(III) cation. Due to the small size of the latter cation and its particular propensity to form hexacoordinate complexes, it was also expected that there would be a good ability of both ligands for the uptake of Ga(3+). Surprisingly, the results revealed that the cyclen derivative H2do2ph is the best ligand for the coordination of Cu(2+) and Ga(3+) cations, not only from their thermodynamic stability as expected but also from their kinetic inertness, when compared with its cross-bridged counterpart.