Rational Design of Single-Molecule Magnets

Abstract

AbstractSingle-molecule magnets possess a superior property in comparison to other polynuclear but only paramagnetic transition metal complexes: single-molecule magnets can be magnetized and they remain magnetized, even in the absence of an external magnetic field. They exhibit a hysteresis in the magnetization in analogy to the well-established solid-state magnets. Due to these promising properties, single-molecule magnets have attracted a great deal of research. However, the key property, the blocking temperature, has not been increased since the discovery of the first single-molecule magnet Mn12. A reason for this failure may be found in the prevalence of serendipitous approaches to new single-molecule magnets: metal ions and small ligands are reacted in the hope to obtain a new single-molecule magnet. The massive characterization of Mn12 has shed light on necessary requirements for a polynuclear transition metal complex to behave as a single-molecule magnet. Besides the usually accepted two requirements (i.e. a high spin ground state and a magnetic anisotropy), a control of the molecular topology seems to be highly demanded. In order to reduce the tunneling through the anisotropy barrier, the rhombicity of the spin ground state should be close to zero. This requires at least a molecular C3 symmetry. Additionally, the overall metal ion arrangement should be lower than a cubic. Otherwise, the local magnetic anisotropies cancel each other by projecting onto the spin ground state. We have performed a ligand design in accordance to the above given requirements, which will be presented here. The triplesalen ligand combines the phloroglucinol bridging unit for high spin ground state and a salen like coordination environment for local magnetic anisotropies. In addition, this ligand is C3 symmetric and imposes a C3 symmetry on its complexes. The first example of a rationally designed single-molecule magnet, Mn6Cr3+, will be described in some detail.KeywordsMagnetic AnisotropyPrussian BlueSpin Ground StateSalen LigandMolecular Building BlockThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.