Nanocrystals of the three‐dimensional, spin‐crossover, porous coordination framework [Fe(pz)Pt(CN)4]⋅n H2O (pz=pyrazine; n≤2.5) have been synthesized from water‐in‐oil microemulsions. The surfactant‐free nanocrystals readily desorb water and the resulting anhydrous compounds exhibit thermally induced electronic bistability accompanied by a pronounced color change (see picture; HS=high spin, LS=low spin) close to room temperature.

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.

The two-dimensional spin crossover (SCO) polymers 2 M(CN) 4 ] (M II = Ni(1), Pd(2), Pt(3)) were nanostructured in the form of nanocrystals and nanoparticles, and their chemical, structural, and physical characterization was carried out using different experimental methods (powder X-ray diffraction, magnetic susceptibility measurements, M€ ossbauer and infrared spectroscopy, transmission and scanning electronic microscopy, etc.). Surfactant-free nanocrystals of average dimensions 400 Â 400 Â 30 nm (1a, 1a*, 2a, 3a) were synthesized from water in oil microemulsions (w/o) while nanoparticles of average size 200 Â 100, 100 Â 60, and 70 Â 30 nm were obtained in poly(vinylpyrrolidone) (PVP) coating polymer (1b-1d, 2b, 2b 0 , and 3b). The spin crossover process is drastically influenced by the dimensions of the crystal/particle. Nanocrystals of 1-3 exhibit a quasi complete first-order spin transition centered within the interval 200-225 K, while PVP-coated nanoparticles undergo continuous second order spin transition at much lower temperatures (ca. 160 K).

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Assembly of Fe II , 3-cyanopyridine and [Au(CN)2] 2 affords, in one-pot reaction, three coordination polymers that represent a genuine example of supramolecular isomerism with strong influence in the spin crossover regime of the Fe II ions.

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.

In Wasser‐in‐Öl‐Mikroemulsionen wurden Nanokristalle des porösen dreidimensionalen Koordinationsnetzwerks [Fe(pz)Pt(CN)4]⋅n H2O (pz=Pyrazin; n≤2.5) synthetisiert. Die tensidfreien Nanokristalle geben bereitwillig Wasser ab. So werden wasserfreie Verbindungen erhalten, deren elektronische Bistabilität sich in einem Farbwechsel (siehe Bild) nahe Raumtemperatur äußert.

Compound 2 ](CF 3 SO 3 ) 2 Á1H 2 O, where 4-terpyridone is 2,6-bis(2-pyridyl)-4(1H)pyridone, forms two polymorphs. Polymorph 1 displays a continuous spin conversion in the temperature region 300-120 K while polymorph 2 shows, on cooling, the onset of a continuous high-spin (HS) to low-spin (LS) conversion interrupted by an abrupt ''reverse'' spin transition in the temperature region 217-203 K. The formed unstable HS intermediate phase (IP) undergoes a strong cooperative ''normal'' spin transition characterised by a hysteresis loop 33 K wide. The structural data give support for a crystallographic phase transition, which takes place concomitantly with the ''reverse'' spin state transition.