Radical ligand-containing single-molecule magnets

Single-molecule magnets represent the ultimate size limit for spin-based information storage and processing; however, such applications require large spin relaxation barriers and blocking temperatures. Ongoing efforts to synthesize single-molecule magnets with higher barriers must take into consideration key physical parameters, such as spin ground state, S, and axial zero-field splitting parameter, D, which are both correlated to the barrier height. A third critical parameter that has received less attention is the exchange coupling constant, J. This constant determines the degree of separation between spin ground state and excited states, which must be sufficiently large in a single-molecule magnet to maintain slow magnetization dynamics at elevated temperatures, and also serves to shut down fast quantum relaxation pathways. Toward this end, one synthetic strategy to engender strong magnetic exchange is the incorporation of radical ligands into metal complexes. Within these complexes, the presence of direct exchange between paramagnetic ligand and metal units can result in exceptionally strong magnetic coupling, much stronger in fact than more common superexchange interactions. This review article provides a survey of radical ligand-containing single-molecule magnets, with a brief overview of other classes of metal-ligand radical complexes that could be exploited in the design of new single-molecule magnets. Furthermore, ligand-field and electronic structure considerations in dictating exchange strength and slow magnetic relaxation are highlighted, with the aim of helping to guide the synthesis of future radical ligand-containing single-molecule magnets with even stronger exchange coupling.