Thermal‐, Pressure‐ and Light‐Induced Spin‐Crossover Behaviour in the Two‐Dimensional Hofmann‐Like Coordination Polymer [Fe(3‐Clpy)₂Pd(CN)₄]

Abstract

Abstract The thermal spin‐crossover behaviour, photoexcitation and subsequent relaxation, as well as the pressure‐induced spin‐crossover behaviour at 298 K are discussed for the non‐porous two‐dimensional Hofmann‐like coordination polymer [Fe(3‐Clpy) 2 Pd(CN) 4 ] ( 1 ). The title compound undergoes a two‐step, cooperative thermal‐induced SCO with critical temperatures T c1 ↓ = 159.6 K and T c1 ↑ = 164.5 K for the first step and T c2 ↓ = 141.4 K and T c2 ↑ = 148.4 K for the second step. Irradiation of the low‐spin state with green light (514 nm) at 10 K induced the photoexcitation of around 60 % of the iron(II) centres to the high‐spin state (LIESST effect). The subsequent cooperative relaxation processes were recorded at several temperatures and analysed. The effect of cooperativity on the photoexcitation was investigated by the LITH (light‐induced thermal hysteresis) procedure. At 300 K, the coordination polymer 1 underwent a cooperative spin‐crossover transition at around 0.6 GPa. The thermodynamic analysis of the pressure‐induced SCO transition afforded the enthalpy and cooperativity parameters Δ H HL ( p ) = 22.01 kJ mol –1 and Γ ( p ) = 7.47 kJ mol –1 , consistent with previous results.