Hysteresis in the ground and excited spin state up to 10 T of a [MnIII6MnIII]3+ triplesalen single-molecule magnet

We have synthesized the triplesalen-based single-molecule magnet (SMM) [MnIII6MnIII]3+ as a variation of our SMM [MnIII6CrIII](BPh4)3. The use of the rod-shaped anion lactate (lac) was intended to enforce a rod packing and resulted in the crystallization of [MnIII6MnIII](lac)3 in the highly symmetric space group R. This entails a crystallographic S6 symmetry of the [MnIII6MnIII]3+ molecules, which in addition are all aligned with the crystallographic c axis. Moreover, the molecular environment of each [MnIII6MnIII]3+ molecule is highly symmetric. Single-crystals of [MnIII6MnIII](lac)3 exhibit a double hysteresis at 0.3 K with a hysteretic opening not only for the spin ground state up to 1.8 T, but also for an excited state becoming the ground state at ≈ 3.4 T with a hysteretic opening up to 10 T. Ab initio calculations including spin-orbit coupling establish a non-magnetic behavior of the central MnIII low-spin (l.s.) ion at low temperatures, demonstrating that predictions from ligand-field theory are corroborated in the case of MnIII l.s. by ab intio calculations. Simulations of the field- and temperature-dependent magnetization data indicate that [MnIII6MnIII]3+ is in the limit of weak exchange (J ≪ D) with antiferromagnetic interactions in the trinuclear MnIII3 triplesalen subunits resulting in intermediate S* = 2 spins. Slight ferromagnetic interactions between the two trinuclear MnIII3 subunits lead to a ground state in zero-field that is approximately described by a total spin quantum number S = 4. This ground state exhibits only a very small anisotropy barrier due to the misalignment of the local zero-field splitting tensors. At higher magnetic fields of ≈ 3.4 T, the spin configuration changes to an all-up orientation of the local MnIII spins, with the main part of the Zeeman energy needed for the spin-flip being required to overcome the local MnIII anisotropy barriers, while only minor contributions of the Zeeman energy are needed to overcome the antiferromagnetic interactions. These combined theoretical analyses provide a clear picture of the double-hysteretic behavior of the [MnIII6MnIII]3+ single-molecule magnet with hysteretic openings up to 10 T.