Iron(III) complexes with pentadentate Schiff-base ligands: Influence of crystal packing change and pseudohalido coligand variations on spin crossover

Two novel iron(III) complexes involving pentadentate Schiff-base ligands, [Fe(L-Br)(L-1)], show temperature induced incomplete spin crossover of a gradual nature. While for the L-1 = NCSe- coligand the transition temperature (326 K or 317 K) lies above room temperature, the N-3(-) coligand causes its drop down to 143 K or 140 K (two values were obtained by different models). This shift is associated with a significant decrease of the enthalpy of the transition from about Delta H = 6.1 kJ mol(-1) to ca. Delta H = 1.7 kJ mol(-1) while the entropy of the transition is about Delta S = 19 J K-1 mol(-1) and Delta S = 12 J K-1 mol(-1), respectively. Two analogous complexes with Cl- and NCS- coligands remain high-spin over the whole temperature range. (C) 2014 Elsevier Ltd. All rights reserved.