Theoretical modeling of spin crossover in metal-organic frameworks: [Fe(pz)2Pt(CN)4] as a case study

Metal-organic frameworks (MOFs) with spin-crossover behavior are promising materials for applications in memory storage and sensing devices. A key parameter that characterizes these materials is the transition temperature T1/2, defined as the temperature with equal populations of low-spin and high-spin species. In this study, we describe the development, implementation, and application of a novel hybrid Monte Carlo/molecular dynamics method that builds upon the Ligand Field Molecular Mechanics approach and enables the modeling of spin-crossover properties in bulk materials. The new methodology is applied to the study of a spin-crossover MOF with molecular formula [Fe(pz)2Pt(CN)4] (pz = pyrazine). The total magnetic moment of the material is determined as a function of the temperature from direct calculations of the relative equilibrium populations of both low-spin and high-spin states of each Fe(II) center of the framework. The T1/2 value, calculated from the temperature dependence of the magnetization curve, is in good agreement with the available experimental data. A comparison between the spin-crossover behavior of the isolated secondary building block of the framework and the bulk material is presented, which reveals the origin of the different spin-crossover properties of the isolated molecular system and corresponding MOF structure.