Broken symmetry and elastic frustration in isostructural Mn III spin crossover crystals

Symmetry‐breaking spin‐state transitions in two of three isostructural salts of MnIII spin‐crossover cations, [MnIII(3‐OMe‐5‐NO2‐sal2323)]+, with heavy anions are reported. The ReO4− salt undergoes two‐step spin crossover which is coupled with a re‐entrant symmetry‐breaking structural phase transition between a high temperature phase (S = 2, C2/c), an intermediate ordered phase (S = 1/S = 2, P21/c), and a low temperature phase (S = 1, C2/c). The AsF6− complex undergoes an abrupt transition between a high temperature phase (S = 2, C2/c) and a low temperature ordered phase (S = 1/S = 2, P‐1). The SbF6− complex undergoes a gradual transition between a high temperature phase (S = 2, P‐1) and a low temperature spin‐state ordered phase (S = 1/S = 2, P‐1). Correlation of the volume of the anion and T1/2 in these complexes and three analogous complexes with similar anions, BF4−, ClO4−, PF6−, reveals an increase in T1/2 upon increasing the anion volume. We rationalise that the volume of the anions modulates the elastic interactions between the MnIII sites, with increasing elastic frustration with the larger anions resulting in a two‐stepped transition for [MnIII(3‐OMe‐5‐NO2‐sal2323)]ReO4 and the stabilisation of the mixed (HS:LS) state to low temperature for [MnIII(3‐OMe‐5‐NO2‐sal2323)]AsF6 and [MnIII(3‐OMe‐5‐NO2‐sal2323)]SbF6.