Deciphering crystal packing effects in the spin crossover of six [Fe ^II (2-pic) ₃ ]Cl ₂ solvatomorphs

In this work we investigate how the crystal packing affects the spin crossover (SCO) of [Fe ^II (2-pic) ₃ ]Cl ₂ . Four alcohol solvatomorphs of this compound have been reported to present a transition from a singlet low spin (LS, S = 0) to a quintet high spin (HS, S = 5) state at different temperatures, whereas two other solvatomorphs remain in the HS state along the entire range of temperatures explored. Given that all these solvatomorphs are isostructural, this behavior hints at crystal packing effects playing an important role in the spin transition. With the aim of deciphering their origin, we performed periodic DFT+U+D2 computations on the crystal lattices of the six solvatomorphs to quantify the importance of all possible molecular and intermolecular contributions to their spin-state energetics. We demonstrate that the spin crossover of the alcohol solvatomorphs of [Fe ^II (2-pic) ₃ ]Cl ₂ (1) is strongly influenced by the change of intermolecular interactions. We prove that isolated molecules of 1 would not undergo SCO, whereas intermolecular interactions are crucial to allow the spin transition in the solid state. The key interactions are those between (i) different SCO molecules (stabilizing the LS) and (ii) between SCO molecules and counterions (stabilizing the HS). Moreover, we show that these contributions are proportional to the crystal expansion associated with the phase change, in a way that larger volume expansions imply larger changes in these contributions, with the overall effect of stabilizing the LS state of the material. Finally, we discuss how our results challenge the common perspective that associates the cooperativity of a spin transition with the number- and strength- of intermolecular contacts in the crystal.