Palladium(II) coordination with polyazacycloalkanes

In view of the growing interest of palladium(II) for medical applications and the important role of polyamines in exploiting its properties, this review combines the published literature on the coordination of Pd(II) with small polyazacycloalkanes (three and four nitrogen atoms), such as 1,4,7-triazacyclononane, 1,5,9-triazacyclododecane, cyclen, cyclam, as well as their derivatives or close scaffolds such as pyclen. Finally, the rare examples of the use of aza-ligands with 103Pd and 109Pd radioisotopes for therapeutic purposes are also presented. The coordination of the Pd(II) ion is mainly square planar, often slightly distorted depending on the nature of the ligand. A few rare five-coordinated complexes with square pyramidal or trigonal bipyramidal geometries have been described in cases where the steric or topological requirements of the ligand outweigh the loss of stabilization due to geometric distortion. Considering the marked structural preference of Pd(II) for square planar coordination, this review allows us to propose hypotheses on the nature of new Pd(II) complexes whose properties could be exploited, especially in nuclear medicine.