New polynuclear 4,4 '-bis-1,2,4-triazole Fe(II) spin crossover compounds

Two novel polynuclear Fe(II) spin crossover materials of formula [Fe(btr)(3 )] [Fe(btr)(2)(H2O)(2)](anion)(4), where btr = 4,4'-bis-1,2,4-triazole and anion = BF4-, PF6-, have been prepared and their spin transition characteri stics studied over the the temperature range 5-300 K. They both reveal inco mplete spin crossover behaviour. Two different Fe(II) lattice sites of the FeN6 and FeN4O2 type are distinguished by Fe-57 Mossbauer spectroscopy. The first site is responsible for the SC behaviour whereas the second one rema ins high-spin throughout the whole temperature range. This explains why it is not possible to switch all the Fe(II) ions to the low-spin state by appl ication of hydrostatic pressure for the BF4- derivative. The temperature de pendence of the population of these sites has been carefully analysed by Mo ssbauer spectroscopy. (C) 2001 Academie des sciences / Editions scientifiqu es et medicales Elsevier SAS.