Hysteresis in the ground and excited spin state up to 10 T of a [(Mn6MnIII)-Mn-III](3+) triplesalen single-molecule magnet

Abstract

We have synthesized the triplesalen-based single-molecule magnet (SMM) [Mn III6Mn III] 3+ as a variation of our SMM [Mn III6Cr III](BPh 4) 3. The use of the rod-shaped anion lactate (lac) was intended to enforce a rod packing and resulted in the crystallization of [Mn III6Mn III](lac) 3 in the highly symmetric space group R3. This entails a crystallographic S 6 symmetry of the [Mn III6Mn III] 3+ molecules, which in addition are all aligned with the crystallographic c axis. Moreover, the molecular environment of each [Mn III6Mn III] 3+ molecule is highly symmetric. Single-crystals of [Mn III6Mn III](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 Mn III low-spin (l.s.) ion at low temperatures, demonstrating that predictions from ligand-field theory are corroborated in the case of Mn III l.s. by ab intio calculations. Simulations of the field- and temperature-dependent magnetization data indicate that [Mn III6Mn III] 3+ is in the limit of weak exchange (J ≪ D) with antiferromagnetic interactions in the trinuclear Mn III3 triplesalen subunits resulting in intermediate S* = 2 spins. Slight ferromagnetic interactions between the two trinuclear Mn III3 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 Mn III spins, with the main part of the Zeeman energy needed for the spin-flip being required to overcome the local Mn III 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 [Mn III6Mn III] 3+ single-molecule magnet with hysteretic openings up to 10 T. © 2012 The Royal Society of Chemistry.