Synthesis and Characterisation of a New Series of Bistable Iron(II) Spin-Crossover 2D Metal-Organic Frameworks

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 [FeN6] 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(3Xpy)(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.