Two-step spin-switchable tetranuclear Fe(II) molecular solid : Ab initio theory and predictions

Using density functional theory supplemented with on-site Coulomb U interaction in combination with ab initio molecular dynamics simulations, we investigate the spin-crossover (SCO) properties of a Fe(II) based cyanide-bridged square molecular system, [Fe-4(II)(mu-CN)(4)(bpy)(4)(tpa)(2)](PF6)(4) (where bpy = 2,2'-bipyridine and tpa = tris(2-pyridylmethyl) amine], exhibiting a two-step SCO transition. The ab initio calculated SCO temperatures are found to show remarkably good agreement with experimentally measured spin conversion temperatures [M. Nihei et al., Angew. Chem., Int. Ed. 44, 6484 (2005)]. Our theoretical study predicts further chemo switching of the spin state by introduction of guest molecules such as CO2, CS2, and H2O into the porous topology of the system, which would add another dimensionality to this interesting material.