Ab initio study of spin-crossover mechanism in Fe(II) complexes with thiazole-based chelating ligands using density...

Spin-crossover (SCO) phenomena in Fe(II) complexes, especially those with octahedral coordination, are of growing interest for their potential in molecular electronics, sensors, and memory devices. These materials exhibit reversible switching between high-spin and low-spin states in response to external stimuli such as temperature or pressure. In this study, we investigate three Fe(II) complexes [Fe(4bt) $_3^{}$ ](ClO $_4^{}$ ) $_2^{}$ , [Fe(2bt) $_3^{}$ ](ClO $_4^{}$ ) $_2^{}$ .MeOH, and[Fe(3tpH) $_3^{}$ ](ClO $_4^{}$ ) $_2^{}$ to understand their spin-state behavior in relation to both intramolecular and intermolecular interactions. Our computational results indicate that [Fe(2bt) $_3^{}$ ](ClO $_4^{}$ ) $_2^{}$ .MeOH and [Fe(3tpH) $_3^{}$ ](ClO $_4^{}$ ) $_2^{}$ undergo spin-crossover transitions with temperature, whereas [Fe(4bt) $_3^{}$ ](ClO $_4^{}$ ) $_2^{}$ stabilizes in the low spin state. Intermolecular interactions such as $\pi$ - $\pi$ stacking and O-H contacts significantly modulate the electronic structure and spin-state energetics. By comparing isolated molecular complexes with their crystalline counterparts, we highlight the critical influence of crystal packing on the SCO mechanism. These insights contribute to the rational design of Fe(II)-based materials with tunable magnetic properties.

Spin-polarized density functional theory (DFT) calculations were carried out using the Vienna Ab initio Simulation Package (VASP). The Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) was employed, along with Grimme's D2 dispersion correction to account for van der Waals interactions. The projector augmented wave (PAW) method was used to describe core-valence interactions. Strong correlation effects in Fe 3d orbitals were treated using the PBE+U method.