Rational design of single-molecule magnets: a supramolecular approach

Abstract

Since the discovery that Mn₁₂1212OAc acts as a single-molecule magnet (SMM), an increasing number of transition metal complexes have been demonstrated to behave as SMMs. The signature of a SMM is a slow relaxation of the magnetization at low temperatures accompanied by a magnetic hysteresis. 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 (S_t) and an easy-axis magnetic anisotropy. The extensive research on Mn₁₂1212OAc and other SMMs has established more prerequisites for a rational development of new SMMs besides the high-spin ground state 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 ligand 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 (talen^t-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 [M^ttt₆6M^ccc]ⁿ⁺nn+ by the assembly of three molecular building blocks. Attractive van der Waals interactions of the tert-butyl phenyl units of two triplesalen trinuclear building blocks increase the driving force. In this respect, we have been able to synthesize the isostructural series [Mn^IIIIIIIIIIII₆666Cr^IIIIIIIIIIII]³⁺3+3+3+, [Mn^IIIIII₆6Fe^IIIIII]³⁺3+, and [Mn^IIIIII₆6Co^IIIIII]³⁺3+ with [Mn^IIIIIIIIIIII₆666Cr^IIIIIIIIIIII]³⁺3+3+3+ being a SMM. A detailed analysis and comparison of the magnetic properties of the three heptanuclear complexes and the tetranuclear half-unit [Mn^IIIIII₃3Cr^IIIIII]³⁺3+ provides significant insight for further optimization of the SMM properties. 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.