Broken Symmetry and Elastic Frustration in Isostructural Mn Spin Crossover Crystals.

Symmetry breaking spin state transitions in two of three isostructural salts of Mn spin crossover cations, [Mn(3-OMe-5-NO-sal323)], with heavy anions are reported. The ReO (1) 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, P2/c), and a low temperature phase (S=1, C2/c). The AsF (2) 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 ). The SbF (3) complex undergoes a gradual transition between a high temperature phase (S=2, P ) and a low temperature spin state ordered phase (S=1/S=2, P- ). Correlation of the volume of the anion and the T of the transitions in these complexes and three analogous complexes with similar anions, BF (4), ClO (5), PF (6), reveals an increase in T upon increasing the anion volume. We rationalise that the volume of the anions used modulates the elastic interactions between the Mn sites in the lattice, with increasing elastic frustration with the larger anions resulting in a two-stepped transition for 1 and the stabilisation of the mixed (HS:LS) state to low temperature for 2 and 3.