Coordination Chemistry of Nitronyl Nitroxide Radicals Has Memory

Nitronyl nitroxide free radicals, and particularly their coordination compounds, have played a major role in the development of molecular-based magnetic materials. Nevertheless, recent results show that the metal-nitroxide bond remains a source of very diverse and singular bistable magnetic systems. Furthermore, metal-nitroxide systems provide a rare opportunity for studying the role of the various interactions governing magnetic properties or electron transfer. From this point of view, they contribute to extending our fundamental knowledge, with prospects for switchable and multifunctional materials. Herein, we review three types of bistability in these systems from our own research which are illustrative of the wealth of these systems. These comprise molecular entities with Cu(II)-nitroxide bonds giving rise to a molecular-spin transition, and coordination polymers with Mn(II)-nitroxide bonds which may behave as magnets in some cases or are bistable by valence tautomerism in others.