Dynamic spin–crossover in [FeII(TRIM)2]Br2 and [FeII(TRIM)2](HCO2)2 investigated by Mössbauer spectroscopy and magnetic measurements

The magnetic and Mössbauer data for the structurally related complexes [FeII(TRIM)2]Br2 (1) and [FeII(TRIM)2] (HCO2)2 (2), (TRIM=4-(4-imidazolyl-methyl)-2-(2-imidazolylmethyl)imidazole) evidence a dynamic spin conversion between the 1A1 and 5T2 spin states with T1/2=340 (1) and 330 K (2), ΔH=12.7 kJ mol−1 (1) and ΔS=37 J K−1 mol−1 (1). Three structurally related complexes including [FeII(TRIM)2]2+ and different uncoordinated anions have a ΔH∼14 kJ mol−1 gap between the high- and low-spin states and very different T1/2: 170 (Cl2), 220 (PhCO2)(ClO4), 340 K (Br2). The ΔSvib decrease (ΔH/T1/2−ΔSelec) parallels the electronegativity decrease of the uncoordinated anions and the hydrogen-bond lengths increase for these related complexes.