Theoretical study on high-spin to low-spin transition of {Fe(pyrazine)[Pt(CN)4]}: Guest-induced entropy decrease

A porous coordination polymer (PCP), {FeII(pyrazine)[PtII(CN)4]}, adsorbs CS2 molecules to induce spin transition from high-spin (HS) to low-spin (LS) state. To elucidate this mechanism, we investigated flexibility of the PCP framework, namely rotation of pyrazine ligands, with DFT method and evaluated the rotational entropy difference ( Δ S rot HS – LS ) between the HS and LS states with Fourier grid Hamiltonian method. The Δ S rot HS – LS value is considerably large in the absence of CS2. The CS2 adsorption occurs between two pyrazine ligands to suppress the pyrazine rotation in both states, which decreases Δ S rot HS – LS to induce the HS → LS transition at room temperature.