Singlet ground states in compounds with a [Mn III4O2]8+ core due to broken degeneration

Abstract

Two new tetranuclear compounds with a formula [Mn^III₄(μ-O)₂(μ-4-RC₆H₄COO)_7−m(L)_2m(phen)₂](ClO₄)_1+m, where R = MeO or ^tBu and m = 0 or 1, were synthesised and studied structurally and magnetically. The core of these compounds comprises a central Mn₂O₂ rhombus to which two terminal ions are attached – one to each oxo bridge. There are two types of bridges that alternately bind the central and terminal ions, those having a triple (μ-O)(μ-RCOO)₂ or a double (μ-O)(μ-RCOO) bridge. The fit of the magnetic data of analogous compounds has so far been performed considering two different magnetic interactions, that between central ions (J₁) and those between terminal and central ions (J_ct), leading to ground states with S_T = 2 or 3, or to five energetically degenerate ground states with S_T = 0–4, depending on the J₁/J_ct ratio. In contrast, the compounds presented herein show an isolated S_T = 0 ground state, and it was necessary to distinguish the two types of magnetic interactions between central and terminal ions (J₂ and J₃) to achieve a good fit of the experimental data. The differentiation of these interactions causes a spin state redistribution: the degeneration of S_T = 0–4 breaks and the states with S_T ≠ 0 become unstable as J₂ and J₃ become more different. Nevertheless, the assignment of these states to a particular spin configuration was unachievable because the composition of these states changes upon decreasing the J₃/J₂ ratio. The importance of considering the relative orientation of Jahn–Teller axes is also highlighted.