Symmetry breakings in a metal organic framework with a confined guest

The MOF $[{\text{Fe(tvp)}}_{2}{(\text{NCS})}_{2}]\ifmmode\cdot\else\textperiodcentered\fi{}2\text{BzCHO}$ is demonstrated to undergo a complex sequence of phase transitions and spin-crossover behavior of its constitutive ${\text{Fe}}^{\text{II}}$ ions upon adsorption of benzaldehyde guest molecules. Our study, combining Raman and synchrotron x-ray diffraction measurements on a single crystal, reveals that the conversion from the pure high-spin to the pure low-spin phases implies a rich sequence of intermediate phases, with symmetry breaking forming at least three different space groups. These different symmetries involve spin-state ordering, ligand ordering, and guest ordering, interpreted by combining magnetic susceptibility, Raman measurements, density functional theory, and force field molecular dynamics calculations.