A proton-displacive phase transition encompassing spin crossover in hydrogen-bonded iron(II) complexes
The iron(II) complex [Fe(bpp)2](isonicNO)2·HisocNO·5H2O (bpp = 2,6-bis(pyrazol-3-yl)pyridine; isonicNO = isonicotinate N-oxide anion) crystallizes in a triclinic phase with 100% of Fe2+ cations in their high-spin state state. It undergoes a partial spin crossover with symmetry breaking at T1 = 167 K to a mixed-spin phase (50% high-spin (HS), 50% low-spin(LS)) that is metastable below T2 = 116 K. Annealing the compound at lower temperatures results in a 100% LS phase that differs from the initial HS phase in the formation of a hydrogen bond between two water molecules of crystallisation and the transfer of a proton between two isonicNO anions. Both phenomena can also be detected by X-ray diffraction of the mixed-spin phase and could be explained in terms of H-bond cooperativity. The aim of this proposal is to characterize these two proton-displacive phase transitions that unusually take place along with spin crossover by locating more precisely the position of H atoms. Three different temperature points (240 K, 120 K and 95 K) will be collected. Considering the small crystal size and the number of experiments, we estimate that 16 days will be needed to complete the characterization.
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ROMERO Francisco; Cañadillas-Delgado, L. and Verónica Jornet-Mollá. (2019). A proton-displacive phase transition encompassing spin crossover in hydrogen-bonded iron(II) complexes. Institut Laue-Langevin (ILL) doi:10.5291/ILL-DATA.5-12-346
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