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

A porous coordination polymer (PCP), {Fe^II(pyrazine)[Pt ^II(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 (ΔSrotHS-LS) between the HS and LS states with Fourier grid Hamiltonian method. The ΔSrotHS-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 ΔSrotHS-LS to induce the HS → LS transition at room temperature.