Insights into the crystal-packing effects on the spin crossover of [Fe^II(1-bpp)]²⁺-based materials

Abstract

Iron(II) complexes of the [Fe(II)(1-bpp2)](2+) type (1-bpp = 2,6-di(pyrazol-1-yl)pyridine) have been intensively investigated in the context of crystal engineering of switchable materials because their spin-crossover (SCO) properties dramatically depend on the counterions. Here, by means of DFT + U calculations at the molecular and solid state levels we provide a rationale for the different SCO behaviour of the BF4(-) and ClO4(-) salts of the parent complex; the former features Fe(II) complexes with a regular coordination geometry and undergoes a spin transition, whereas the Fe(II) complexes of the latter adopt a distorted structure and remain in the high-spin state at all temperatures. The different SCO behaviour of both salts can be explained on the basis of a combination of thermodynamic and kinetic effects. The shape of the SCO units at high temperature is thermodynamically controlled by the intermolecular interactions between the SCO units and counterions within the crystal. The spin trapping at low temperatures in the ClO4(-) salt, in turn, is traced back to a kinetic effect because our calculations have revealed the existence of a more stable polymorph having SCO units in their low-spin state that feature a regular structure. From the computational point of view, it is the first time that the U parameter is fine-tuned on the basis of CASPT2 calculations, thereby enabling an accurate description of the energetics of the spin transition at both molecular and solid-state levels.

Iron(II) Complexes of 2,4-Dipyrazolyl-1,3,5-triazine Derivatives—The Influence of Ligand Geometry on Metal Ion Spin State

“…Hence, the high-spin nature of the complexes in the solid state is not simply imposed by their coordination geometry, 43,51 but genuinely reflects the weak ligand field imposed by the bpt ligand donors. derivatives often undergo SCO in solution near room temperature.…”

“…Thus, a is computed to be 1.405 Å in [Fe(bpp) . 43,51 The additional distortion energy, centre, but do not cause it. ( Figure S25 and Table S11).…”

Understanding the Influence of the Electronic Structure on the Crystal Structure of a TTF-PTM Radical Dyad

“…It must be mentioned that DFT-functionals incorporating dispersion correction has been a common method for studying polymorphism in molecular crystals. 1,[18][19][20][21][22][23][24] The minimum energy structure of the different polymorphs has been obtained by performing successive variable-cell geometry relaxations, in which the lattice parameters as well as the atomic positions are optimized simultaneously until the atomic forces are smaller than 1.0 E −5 atomic units. We have chosen to compare optimized structures (instead of the reported crystals) for two reasons.…”

Disclosing the Ligand- and Solvent-Induced Changes on the Spin Transition and Optical Properties of Fe(II)-Indazolylpyridine Complexes

“…Herein, we continue our inspections on one of the most prominent families of SCO compounds, the one based on the [Fe II (1-bpp)] 2+ core (1-bpp = 2,6-di(pyrazol-1-yl)pyridine) [10], which has reported striking examples on how subtle modifications on the crystal packing may lead to notably-different SCO behaviors. For instance, the use of BF 4´a s the counterion results into a full SCO transition with a small hysteresis loop [23], whereas the use of ClO 4´r esults in the SCO molecules being kinetically trapped in its HS state [19,24]. Alternatively, modifications of the 1-bpp unit have been recently reported, obtaining (among others [25,26] (5) [27].…”

“…So far, the application of quantum chemistry to aid in this quest has been limited by the lack of an accurate and computationally-efficient methodology to work in the solid state of those compounds and, apart from few exceptions [6,[17][18][19][20], it has been essentially focused on the study of the individual molecules [21]. However, some advances have been made in the field and, especially, in the application of DFT + U to calculate the adiabatic energy difference between the LS and HS phases of SCO compounds in their unit cells, thus, accounting for all the crystal packing effects and weak interactions [22].…”