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

摘要

Two novel polynuclear Fe(II) spin crossover materials of formula [Fe(btr)_3][Fe(btr)_2(H_2O_2)](anion)_4, where btr - 4,4'-bis-1,2,4-triazole and anion = BF_4~-, PF_6~-, have been prepared and their spin transition characteristics studied over the temperature range 5-300 K. They both reveal incomplete spin crossover behaviour. Two different Fe(II) lattice sites of the FeN_6 and FeN_4O_2 type are distinguished by ~(57)Fe Mossbauer spectroscopy. The first site is responsible for the SC behaviour whereas the second one remains 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 application of hydrostatic pressure for the BF_4~- derivative. The temperature dependence of the population of these sites has been carefully analysed by Mossbauer spectroscopy.