Azide bridged dicopper and dinickel complexes: Structure and magnetism

The quest for coordination complexes with novel magnetic properties, which may have potentially useful applications in materials science, has focussed attention on superexchange bridges with the ability to propagate ferromagnetic coupling between paramagnetic transition metal centers. The flexi-dentate azide ligand can bridge two metal centers in two ways, in the mu(2)-1,1- or mu(2)-1,3- fashion. In the 1,3-mode superexchange is exclusively antiferromagnetic in nature, whereas in the 1,1-mode the magnetic role of the azide bridge depends in large measure on the angle it subtends at the two metal centers. For dicopper(II) systems with small angles (<108 degrees) ferromagnetic coupling dominates, while at larger angles the azide has been shown to propagate antiferromagnetic exchange. Control of the molecular dimensions, which in turn can influence the geometry at the azide, can lead to control of the magnetic properties. Examples of dicopper(II) and dinickel(II) complexes illustrating the ferromagnetic and antiferromagnetic realms of the azide bridge will be discussed, in addition to the design features of the dinucleating ligands required to bring the two metal centers into juxtaposition in a controlled fashion.