Prussian Blue, analogs

It is possible to describe the site preference of metals for nitrogen or carbon coordination in a Prussian blue analog in terms of ligand field stabilization energies Shriver, Shriver and Anderson, 1965). In application, even this simple treatment requires a number of estimates nevertheless, it affords reasonable agreement with experimentally determined structures and provides a framework for the systemization of Prussian blue type structures. One expectation from this approach is that if one of the two metal ions has more than six d electrons and the other has six or less, the latter will be carbon coordinated and the former nitrogen coordinated. 

There are many other Prussian blue analogs for which the visible spectrum is complex and not understood. Heavy metal ferricyanides and hexacyano complexes of other transition metals with less than d configurations are in this class due to the presence of ligand-metal charge transfer processes. 

Mahmoud MA, El-Sayed MA (2007) Reaction of platinum nanocatalyst with the ferricyanide reactant to produce Prussian blue analog complexes. J Phys Chem C 111 17180... 

Prussian blue analogs are here defined as polynuclear transition metal cyanides of the composition M [M (CN)6]i xHzO a retallizing with a cubic unit cell. They are easily obtained as sparsely soluble precipitates by mixing solutions of a cyano complex M (CN)e with an appropriate salt of The compounds prepared by using the hexacyanometalate in the form of the most common potassium salt invariably contain different amounts of potassium, which in some cases can be exchanged by cesium... 

Very often the Prussian blue analogs have been formulated with a definite amount of potassium, e.g. KFeFe(CN)6, KFeCr CN)e, K2CuFe (CN)e. The sparse pubhshed analytical data (6,32,36), however, indicate that potassium has to be considered as an impurity of these often colloidal precipitates. Thus, the polynuclear cyanides containing potassium or other alkali ions are non-stoichiometric compounds rather than compounds showing a definite formula as far as the alkah ions are concerned. 

The precipitates of Prussian blue analog cyanides always contain variable amounts of water, which can be removed without significant effects on the X-ray diffraction pattern. Some of these water molecules can be replaced by other molecules such as ammonia or alcohol 37). It was therefore assumed that the water is present partly as zeolitic, partly as surface water 3). Recent infrared spectroscopic studies, however, reveal that, in addition, coordinated water molecules are also present 33, 38). 

The first structural investigations of Prussian blue analogs date back to 1936 when Keggin and Miles (2) studied the X-ray powder patterns of iron cyanides. They found a luiit cell of the face-centered type and deduced a structural model from these geometrical data. This description, which will be briefly outlined below, has until recently been accepted by many authors (3, 39—44) for the discussion of the structural properties of cubic polynuclear cyanides Mfc[M (CN)e]i cHgO. In many cases the physically equivalent description has been used 3, 42,... 

For the four most frequently realized stoichiometries of Prussian blue analogs M [M (CN)6]i rHaO the model of K gin and Miles postulates the structural properties summarized in Table 1. 

Fig. 1. The cubic unit cell of Prussian blue analogs M [MB(CN)6]j M- - (position 4a), Q M (position 4b), (S) position 8c (cf. text)...

It cannot be expected that a structural model derived purely from X-ray powder data would provide a complete and reliable description of the actual structure. One, and probably the most important feature of the model of Keggin and Miles, however, seems to be beyond any doubt namely the linear arrangement M- N—C—M —C—N—M- - along the edge of the unit cell. The unit cell constants of a wide variety of Prussian blue analogs have been determined. All the lattice constants measured so far are between 9.9 and 10.9 A. Since the C—N distance is known to be close to 1.14 A (16), the differences in the cell constants directly reflect the differences in the distances N and M —C. 

Table 2. Structural properties of Prussian blue analogs xH f)...

The validity of the modified model has been tested by several complete crystal-structure determinations. Up to the present, almost all the single-crystal studies have been carried out with compoimds of the stoichiometry Ms [MB(CN)6]2(M- = Mn2+, Cd +j M = Co +, Cr3+, Ir +). The compound CsaLiCo(CN)6 studied by Wolberg 50) is not considered as a pol5muclear cyanide in this context, being neither a transition metal nor a metal of class B 34). Despite great structural similarities with the Prussian blue analogs, it is rather an alkaline salt of the mononuclear cyanocobaltate III). This distinction is, of course, arbitrary. 

Table 4. Interatomic distances of Prussian blue analogs obtained from single-crystal studies...

In his compilation of structural data, Wyckoff 53) relates the structures of Prussian blue analogs to the K2PtCl6 type. Whereas this comparison is stoichiometrically obvious for the compounds M2M (CN)e. the ambident coordination behavior of the cyanide ligand is not considered as a structural element. The pol5mieric cyanide is here assumed... 

While the general principles of the crystal structures of Prussian blue analogs have been conclusively elucidated, there still remain problems to be solved. It would be of interest to improve the resolution of the structure analysis to obtain finer details of the bond distances, and especially to study the influence of different metal ions on the C—N distance. The most desirable goal, of course, is still to grow single crystals of the archetype of these compounds, Prussian blue. 

Whereas within the family of the cubic Prussian blue analogs a large number of lattice constants have been determined, little attention has been devoted so far to polymeric cyanides not belonging to the cubic system. It must be emphasized, however, that polynuclear cyanides having unit cell symmetries other than cubic are by no means rare exceptions. Hexacyanometalates(III) of Zn2+ and Cd2+ are obtained not only in a cubic modification but also as samples with complicated and not yet resolved X-ray patterns of definitely lower symmetry than cubic (55). The exact conditions for obtaining either modification are not yet known in detail. The hexacyanoferrates(II), -ruthenates(II), and -osmates(II) of Mn2+ and several modifications of the corresponding Co 2+ salts show very complicated X-ray powder patterns which cannot be indexed in the cubic system (55). Preliminary spectroscopic studies show the presence of nearly octahedral M C6-units in these compounds, too. 

A complete crystal structure analysis has been carried out for Mn2Ru(CN)a 8 HgO, the corresponding hexacyanoferrate(II) and hexacyanoosmate(II) showing very similar lattice constants (55). This structure also consists of a three-dimensional framework with the characteristic sequence, Ru—C—N—Mn, deviating slightly from linearity. Contrary to the Prussian blue analogs, the coordination sphere of ruthenium as well as that of manganese has a definite unique composition. Moreover, the structure is ordered, and there are no fractional... 

The structural relationship of this monoclinic structure to the cubic one of the Prussian blue analogs is not very obvious. A comparison can be made if the dinuclear MnN6(H20)4 group is replaced by a hypothetical MNe octahedron and the monoclinic unit cell deformed to a cubic one. The resulting hypothetical structure is cubic face-centered with positions 4 a and 4 b alternatively occupied by Ru and M (56). 

Figure 7 Typical structures of cubic Prussian-blue analogs (a) A(III)[B(III)(CN)6], AjBi (b) Cs(I)A(II) [B(III)(CN)6], CsiAiBi (c) A(II) 3[B(III)(CN)6]2 H20, A3B2. [B(C1 6] are the dark solid octahedra surrounded by CN (very small spheres), A are the light small spheres, C are the gray medium size spheres in (b) in (c) H2O are shown by the small light-gray spheres.

It is worth mentioning that a europium(III)-doped prussian blue analog (Eu-PB) film was modified chemically on the surface of a microdisk platinum working electrode to avoid the possible electrode fouling as well as to improve the ECL efficiency and detection sensitivity. After optimizing the conditions, the ECL intensity was in proportion to analyte concentration in the range from 0.01 to... 

Liu, H., Matsuda, K., Gu, Z., Takahashi, K., Cui, A., Nakajima, R., et al. (2003). Reversible valence tautomerism induced by a single-shot laser pulse in a cobalt-iron Prussian blue analog. Physical Review Letters, 90, 167403. 

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