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.

In the search for new functional materials, sensory and memory are important functions that require switchable components. Iron(II) spin-crossover (SCO) complexes have been demonstrated to be particularly suitable to this end. [1][2][3] They reversibly switch between a diamagnetic (S = 0) low-spin state (LS) and a paramagnetic (S = 2) high-spin state (HS) under an external stimulus like temperature, pressure, or light. A sharp variation of the structure, magnetism, color, or dielectric constant of the system in response to these stimuli may occur in the solid state. Furthermore, hysteresis accompanies the firstorder spin transition (ST) when the structural changes are transmitted cooperatively through the whole solid. Due to their switching properties the SCO materials are potentially useful for rewritable optical, thermal, or pressure memories at the nanometer scale. To be useful in practice, a demanding set of material requirements must be met, such as room-temperature operation, non-destructive writing, and readout of the information. Nowadays, prototypes of thermal displays based on the one-dimensional SCO polymeric systems ) have been described. [4,5] This family of compounds has been the focus of much attention because they present steep STs with large, stable hysteresis loops and color changes (white and violet for the HS and LS states, respectively) around room temperature for a given x value. The effect of pressure on the ST in some of these complexes has also been studied. ) exhibiting similar cooperative SCO behavior accompanied by a drastic color change from yellow-orange (HS) to deep red (LS) has been synthesized. [7,8] A Raman spectroscopic study of the pressure effects on the SCO coordination polymer {Fe(pz)[Ni(CN) 4 ]}·2H 2 O demonstrated the occurrence of a piezohysteresis of 0.07 GPa for this material at room temperature. [9] Unfortunately, this ST was found to be rather incomplete; indeed, only 50 % of the SCO iron(II) ions were involved in the ST and the hysteresis loop was strongly distorted. However, these results have created solid expectancies for the creation of pressure displays at room temperature. As a continuation of our research devoted to the study of new cyanide-bridged bimetallic polymers with strong cooperative SCO properties, [7][8][9][10][11][12][13][14] we decided to investigate the influence of pressure on the magnetic and optical properties of the 3D coordination polymer {Fe(pmd)(H 2 O)[Ag(CN) 2 ] 2 }·H 2 O (pmd = pyrimidine) (1). Herein, we report an unprecedented observation of pressure-tunable thermal bistability and piezochromic bistability at room temperature in 1.The crystal structure and magnetic behavior at ambient pressure of 1 were reported previously.[ (Fig. 1). At atmospheric pressure, the compound undergoes a thermally induced first-order ST with hys- COMMUNICATIONSAdv. Mater.

The thermal spin‐crossover behaviour, photoexcitation and subsequent relaxation, as well as the pressure‐induced spin‐crossover behaviour at 298 K are discussed for the non‐porous two‐dimensional Hofmann‐like coordination polymer [Fe(3‐Clpy)2Pd(CN)4] (1). The title compound undergoes a two‐step, cooperative thermal‐induced SCO with critical temperatures Tc1↓ = 159.6 K and Tc1↑ = 164.5 K for the first step and Tc2↓ = 141.4 K and Tc2↑ = 148.4 K for the second step. Irradiation of the low‐spin state with green light (514 nm) at 10 K induced the photoexcitation of around 60 % of the iron(II) centres to the high‐spin state (LIESST effect). The subsequent cooperative relaxation processes were recorded at several temperatures and analysed. The effect of cooperativity on the photoexcitation was investigated by the LITH (light‐induced thermal hysteresis) procedure. At 300 K, the coordination polymer 1 underwent a cooperative spin‐crossover transition at around 0.6 GPa. The thermodynamic analysis of the pressure‐induced SCO transition afforded the enthalpy and cooperativity parameters ΔHHL(p) = 22.01 kJ mol–1 and Γ(p) = 7.47 kJ mol–1, consistent with previous results.

For the 2D coordination polymers [Fe(3-Fpy)(2)M(II)(CN)(4)] (M(II) = Ni, Pd, Pt), the pressure-induced spin crossover behavior has been investigated at 298 K by monitoring the distinct optical properties associated with each spin state. Cooperative first-order spin transition characterized by a piezohysteresis loop ca. 0.1 GPa wide was observed for the three derivatives. Application of the mean field regular solution theory has enabled estimation of the cooperative parameter, Γ(p), and the enthalpy, ΔH(HL)(p), associated with the spin transition for each derivative. These values, found in the intervals 6.8-7.9 and 18.6-20.8 kJ mol(-1), respectively, are consistent with those previously reported for thermally induced spin transition at constant pressure for the title compounds (Chem.-Eur. J.2009, 15, 10960). Relevance of the elastic energy, Δ(elast), as a corrective parameter accounting for the pressure dependence of the critical temperature of thermally induced spin transitions (Clausius-Clapeiron equation) is also demonstrated and discussed.