Singlet ground states in compounds with a [MnIII4O2]8+ core due to broken degeneration

Two new tetranuclear compounds with a formula [MnIII4(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 (Jct), leading to ground states with ST = 2 or 3, or to five energetically degenerate ground states with ST = 0–4, depending on the J1/Jct ratio. In contrast, the compounds presented herein show an isolated ST = 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 ST = 0–4 breaks and the states with ST ≠ 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.