Hysteresis in the ground and excited spin state up to 10 T of a [MnIII6MnIII]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](BPh₄)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 R. This entails a crystallographic S₆ 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.