Lattice-Solvent Effects in the Spin-Crossover of an Fe(II)-Based Material. The Key Role of Intermolecular Interactions between Solvent Molecules

The spin transition of Fe(II) complexes is the subject of intensive synthetic and computational efforts. In this manuscript, we analyze the spin crossover (SCO) of [Fe( E -dpsp) 2 ] 2+ ( 1 ), which features a spin transition depending on the cocrystallizing solvent molecules. Whereas the use of acetone results in a hysteretic spin transition at ∼170 K, the use of propylene carbonate (PC) results in a permanent diamagnetic signal up to 300 K. By means of DFT+U+D2 calculations in the solid state of the material, we unravel the reasons for such different behavior. Our results allow us to ascribe the relatively low transition temperature of 1 (BF 4 ) 2 ·acetone to the distorted arrangement of the SCO molecules in the low-spin state of the material. In turn, intermolecular interactions play the primary role in the case of 1 (BF 4 ) 2 ·2PC. In particular, we found that solvent–solvent interactions actively promote the stability of the low-spin state due to the formation of PC dimers. These dimers would appear at larger distances in the high-spin phase, with the subsequent loss of phase stability. This is yet another proof of how subtle is the spin transition phenomenon in Fe(II)-based architectures.