Rational Design of Single-Molecule Magnets

MSc Jan-Philipp Broschinski, MSc Jan Oldengott

A milestone in the field of molecule-based magnetic materials was the observation that [Mn12O12(OAc)16(H2O)4], Mn12, exhibits a hysteresis in the magnetization of pure molecular origin and has been the first member of a new class of molecule-based magnets called single-molecule magnets (SMMs). The origin of SMM behaviour is the existence of an appreciable thermal barrier U for spin-reversal called magnetic anisotropy barrier which is related to the combination of a large total spin ground state (St) and an easy-axis magnetic anisotropy. This energy-barrier U for spin reversal causes a slow relaxation of the magnetization at low temperatures. The magnetic bistability of SMMs promises access to devices for ultimate high-density memory storage and quantum computing. Since the discovery that Mn12 acts as a single-molecule magnet, a still increasing number of SMMs has been identified in the effort to enhance the magnetic anisotropy barrier. However, although great success in increasing the anisotropy barrier U has been reached, the blocking temperature TB, below that the hysteretic behaviour can be observed, did not increase accordingly due to quantum tunneling through the anisotropy barrier. The extensive research on Mn12 and other SMMs has established more prerequisites for a rational development of new SMMs besides the high-spin ground state St and the magnetic anisotropy: the symmetry should be at least C3 to minimize the quantum tunneling of the magnetization through the anisotropy barrier but lower than cubic to avoid the cancellation of the local anisotropies upon projection onto the spin ground state.

Based on these prerequisites, we have designed the new C3 symmetric ligand system triplesalen which combines the phloroglucinol bridging unit for high spin ground states by the spin-polarization mechanism with a salen-like ligand environment for single-site magnetic anisotropies by a strong tetragonal ligand field.

The C3 symmetric, trinuclear complexes of the triplesalen ligand (talent-Bu2)6- exhibit a strong ligand folding resulting in an overall bowl-shaped molecular structure. This ligand folding preorganizes the axial coordination sites of the metal salen subunits for the complementary binding of three facial nitrogen atoms of a hexacyanometallate unit. This leads to a high driving force for the formation of heptanuclear complexes [Mt6Mc]n+ by the assembly of three molecular building blocks.

In this respect, we have been able to synthesize an isostructural series [Mt6Mc]n+ with varying terminal ions Mt and central ions Mc , which acts as SMMs.

The modular assembly of the heptanuclear complexes from three molecular building blocks allows the fine-tuning of the molecular and steric properties of each building block without losing the driving force for the formation of the heptanuclear complexes. This possibility of rational improvements of our isostructural series is the main advantage of our supramolecular approach to realize SMM behaviour.

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