title: Synthesis and characterisation of a new series of bistable iron(II) spin-crossover 2D metal-organic frameworks.
abstract: Twelve coordination polymers with formula {Fe(3-Xpy)(2)[M(II)(CN)(4)]} (M(II): Ni, Pd, Pt; X: F, Cl, Br, I; py: pyridine) have been synthesised, and their crystal structures have been determined by single-crystal or powder X-ray analysis. All of the fluoro and iodo compounds, as well as the chloro derivative in which M(II) is Pt, crystallise in the monoclinic C2/m space group, whereas the rest of the chloro and all of the bromo derivatives crystallise in the orthorhombic Pnc2 space group. In all cases, the iron(II) atom resides in a pseudo-octahedral [FeN(6)] coordination core, with similar bond lengths and angles in the various derivatives. The major difference between the two kinds of structure arises from the stacking of consecutive two-dimensional {Fe(3-Xpy)(2)[M(II)(CN)(4)]}(infinity) layers, which allows different dispositions of the X atoms. The fluoro and chloro derivatives undergo cooperative spin crossover (SCO) with significant hysteretic behaviour, whereas the rest are paramagnetic. The thermal hysteresis, if X is F, shifts toward room temperature without changing the cooperativity as the pressure increases in the interval 10(5) Pa-0.5 GPa. At ambient pressure, the SCO phenomenon has been structurally characterised at different significant temperatures, and the corresponding thermodynamic parameters were obtained from DSC calorimetric measurements. Compound {Fe(3-Clpy)(2)[Pd(CN)(4)]} represents a new example of a "re-entrant" two-step spin transition by showing the Pnma space group in the intermediate phase (IP) and the Pnc2 space group in the low-spin (LS) and high-spin (HS) phases.
authors.affiliation: Institut de Ciència Molecular (ICMol)/Departament de Química Inorgánica, Universitat de València, Edifici d'Instituts de Paterna, Apartat de Correus 22085, 46071 València, Spain.
authors.forename: Víctor
authors.initials: V
authors.lastname: Martínez
authors.forename: Ana Belén
authors.initials: AB
authors.lastname: Gaspar
authors.forename: M Carmen
authors.initials: MC
authors.lastname: Muñoz
authors.forename: Gennadiy V
authors.initials: GV
authors.lastname: Bukin
authors.forename: Georgii
authors.initials: G
authors.lastname: Levchenko
authors.forename: José Antonio
authors.initials: JA
authors.lastname: Real