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An atypical coordination in hexacyanometallates: Structure and properties of hexagonal zinc phases

In hexacyanometallates, the involved transition metals are usually found with octahedral coordination. The exception corresponds to the hexagonal zinc phases where this metal appears tetrahedrally coordinated to N ends from the CN ligands. Those zinc hexacyanometallates where such atypical coordination appears were identified and for four of them the crystal structure was refined from X-ray diffraction powder patterns using the Rietveld method. Zinc hexacyanoferrates (III), hexacyanocobaltate (III), hexacyanoiridate (III) and the mixed zinc-cesium hexacyanoferrate (II) were found to be dimorphic, cubic (Fm-3m) and hexagonal (R-3c), related to the zinc atom in octahedral or tetrahedral coordination, respectively. In the absence of an exchangeable cation, the hexagonal phases result anhydrous. This last feature was attributed to a low polar character for the pores surface. The M?ssbauer spectrum of hexagonal zinc hexacyanoferrate (III) is an unresolved quadrupole splitting doublet (Δ=0.18 mm/s). The iron nucleus is sensing a weak electric field gradient related to a relatively high symmetry for its ligands and charge environment. The IR spectrum appears to be an excellent sensor to identify the coordination for the zinc atom in a given sample. For the tetrahedral coordination, the CN stretching absorption was found at least 8 cm-1 above the frequency observed for this vibration in the octahedral one. For hydrated phases, the crystal water evolves on heating preserving the material porous framework. The temperature at which the material becomes anhydrous parallels the polarizing power of the charge balancing cation sited within the channels. Hexagonal Zn-Cs ferrocyanide becomes anhydrous at 100 °C, while for the Zn-Na analogue a heating close to 200 °C is required. The stability temperature range for the anhydrous phases depends on the nature of the engaged hexacyanometallate anion; the higher stability was observed for hexacyanoferrates (II). Zinc ferricyanide shows the weaker magnetic interaction for the hexagonal modification due to an unfavourable geometry for the overlapping path between the unpaired electrons on the iron(III) atoms. The open 3D porous network is formed by relatively large ellipsoidal cavities, three per cell, communicated through elliptical openings (windows), six per cavity. For dimorphic zinc hexacyanometallates (III), the most compact structure (higher density) corresponds to the hexagonal modification, however, it has the largest cavity windows and cavity (pore) size, and also the higher thermal stability.

Thermal-induced changes in molecular magnets based on Prussian blue analogues

The thermal-induced changes in molecular magnets based on Prussian blue analogues, M3[Fe(CN)6]2·xH2O (M = Mn, Co, Ni, Cu, Zn, and Cd), were studied from infrared, X-ray diffraction, thermo-gravimetric, Moessbauer, and magnetic data. Upon being heated, these materials loose the crystalline water that enhances the interaction between the metal centers, as has been detected from Moessbauer spectroscopy data. At higher temperatures, a progressive decomposition process takes place, liberating CN- groups, which reduces the iron atom from Fe(III) to Fe(II) to form hexacyanoferrates(II). The exception corresponds to the cobalt compound that undergoes an inner charge transfer to form Co(III) hexacyanoferrate(II). In the case of zinc ferricyanide, the thermal decomposition is preceded by a structural transformation, from cubic to hexagonal. For M = Co, Ni, Cu, and Zn the intermediate reaction product corresponds to a solid solution of M(II) ferricyanide and ferrocyanide. For M = Mn and Cd the formation of a solid solution on heating was not detected. The crystal frameworks of the initial M(II) ferricyanide and of the formed M(II) ferrocyanide are quite different. In annealed Mn(II) ferricyanide samples, an increasing anti-ferromagnetic contribution on heating, which dominates on the initial ferrimagnetic order, was observed. Such a contribution was attributed to neighboring Mn(II) ions linked by aquo bridges. In the anhydrous annealed sample such interaction disappears. This effect was also studied in pure Mn(II) ferrocyanide. The occurrence of linkage isomerism and also the formation of Ni(III), Cu(III), and Zn(III) hexacyanoferrates(II) were discarded from the obtained experimental evidence.