Pressure-Induced Cooperative Spin Transition in Iron(II) 2D Coordination Polymers: Room-Temperature Visible Spectroscopic Study

For the 2D coordination polymers [Fe(3-Fpy)₂M^II(CN)₄] (M^II = Ni, Pd, Pt), the pressure-induced spin crossover behavior has been investigated at 298 K by monitoring the distinct optical properties associated with each spin state. Cooperative first-order spin transition characterized by a piezohysteresis loop ca. 0.1 GPa wide was observed for the three derivatives. Application of the mean field regular solution theory has enabled estimation of the cooperative parameter, Γ(p), and the enthalpy, ΔH_HL(p), associated with the spin transition for each derivative. These values, found in the intervals 6.8–7.9 and 18.6–20.8 kJ mol^–1, respectively, are consistent with those previously reported for thermally induced spin transition at constant pressure for the title compounds (Chem.Eur. J. 2009, 15, 10960). Relevance of the elastic energy, Δ_elast, as a corrective parameter accounting for the pressure dependence of the critical temperature of thermally induced spin transitions (Clausius–Clapeiron equation) is also demonstrated and discussed.