Dynamic spin-crossover in [Fe-II(TRIM)(2)]Br-2 and [Fe-II(TRIM)(2)](HCO2)(2) investigated by Mossbauer spectroscopy and magnetic measurements

The magnetic and Mossbauer data for the structurally related complexes [Fe- II(TRIM)(2)]Br-2 (1) and [Fe-II(TRIM)(2)] (HCO2)(2) (2), (TRIM = 4-(4-imida zolyl-methyl)-2-(2-imidazolylmethyl)imidazole) evidence a dynamic spin conv ersion between the (1)A(1) and T-5(2) spin states with T-1/2 = 340 (1) and 330 K (2), Delta H = 12.7 kJ mol(-1) (1) and Delta S = 37 J K-1 mol(-1) (1) . Three structurally related complexes including [Fe-II(TRIM)(2)](2+) and d ifferent uncoordinated anions have a Delta H similar to 14 kJ mol(-1) gap b etween the high- and low-spin states and very different T-1/2: 170 (Cl-2), 220 (PhCO2)(ClO4), 340 K (Br-2). The Delta S-vib decrease (Delta H/T-1/2 - Delta S-elec) parallels the electronegativity decrease of the uncoordinated anions and the hydrogen-bond lengths increase for these related complexes. (C) 1999 Elsevier Science B.V. All rights reserved.