Structure of Prussian Blue

The crystalline structure of Prussian blue was first discussed by Keggin and Miles on the basis of powder diffraction patterns [5] and then has been determined more precisely... 

The Prussian blue/Prussian white redox activity with potassium as the countercation is observed in cyclic voltammograms as a set of sharp peaks with a separation of 15-30 mV. These peaks, in particular the cathodic one, are similar to the peaks of the anodic demetallization. Such a set of sharp peaks in cyclic voltammograms correspond to the regular structure of Prussian blue with homogeneous distribution of charge and ion transfer rates throughout the film. This obvious conclusion from electrochemical investigations was confirmed by means of spectroelectrochemistry [10]. 

The sharpness of Prussian blue/Prussian white redox peaks in cyclic voltammograms can be used as an indicator of the quality of Prussian blue layers. To achieve a regular structure of Prussian blue, two main factors have to be considered the deposition potentials and the pH of initial growing solution. As mentioned, the potential of the working electrode should not be lower then 0.2 V, where ferricyanide ions are intensively reduced. The solution pH is a critical point, because ferric ions are known to be hydrolyzed easily, and the hydroxyl ions (OH-) cannot be substituted in their... 

Fig. 12.47 Portion of Ihe crystal structure of Prussian blue showing the bridging by ambidentate cyanide ions. Circles represent iron(II) (O). iron(UI) (O), and oxygen in water ( ). The remaining interstitial or "zeoiitic" water in the cubic sites has been omitted for clarity, as hove most of the cyanide ious. In addition, some of the cyanide ions are replaced by water molecules coordinated to iron(III), and there are also vacancies in the structure. [Modified from Buser, H. J. Schwarzenbach, D. Petter, W. Ludi, A. Inorg. Chem. 1977, 16. 2704-2710. Reproduced with permission.]...

Buser, E.J., Schwarzenbach, D., Petter W. and Ludi A. 1977. The crystal structure of Prussian blue. Inorg. Chem., 16, 2704-2710. 

While bifunctionality is known for the halogens and many pseudohalogens, it is most pronounced for cyanide and influences the structures, properties and chemistry of many of its derivatives. Cyanide bridges were present in the first recorded synthetic inorganic complex, Prussian blue (ca. 1700), and cyanide linkage isomers were often proposed in the old literature but reasonable evidence for the existence of linkage isomers and the structure of Prussian blue is very recent. 

Figure 4 The structure of Prussian blue and related compounds. If none of the cube centre sites are occupied, the structure is that of ferric ferricyanide (both black and white Fe positions occupied by Fe111) if every second cube centre site (marked with a dotted circle) is occupied by K+, the structure is that of soluble Prussian blue (black = Fe11, white = Fe111) if all the centre sites are occupied by K+ (crosses as well as dotted circles) the structure is that of dipotassium ferrous ferrocyanide...

The structure of Prussian blue, which has been used as a pigment, and other similar materials such as Cu [Fen(CN)6](aq) or Mn3[Com(CN)6]2(aq) are based on a three-dimensional cubic framework with MA and MB atoms at the corners of a cube and with MA—N—C—MB links. There can be empty metal and CN sites depending on the stoichiometry, that is, on the valence of MA and MB. Water molecules can also be bound to Fera in Prussian blue as well as being interstitial as in zeolites. Reduction of Prussian blue gives Everitt s salt K2[FenFen(CN)6],... 

Figure 5.39. Molecular structures of other molecular magnetic materials. Illustrated are (a) tris(oxalato) metalates [M M (ox)3], where M I=Mn, Fe, Ni, Co, Cu, Zn M =Cr, Fe, Ru, and (b) the simplified crystal structure of Prussian Blue, with an example of the analogue structure [(tacn)8Co8(CN)i2], where the tacn ligand is 1,4,7-triazacyclononane. Reproduced with permission from (a) Min, K. S. Rhine-gold, A. L. Miller, J. S. Inorg. Chem., 2005, 44, 8433, and (b) Belttan, L. M. C. Long, J. R. Acc. Chem. Res., 2005, 38, 325. Copyright 2005 American Chemical Society.

Metal hexacyanoruthenates possess a lower symmetry. Several compounds have highly disordered structures, especially when no alkali cations are present for charge compensation. Such a complex defect structure has been found for a completely potassium free Prussian blue precipitated very slowly from a solution in concentrated hydrochloric acid [25, 26]. Here, the structure still remains cubic face-centered however, one-third of the [M - (CN)(, is vacant, randomly distributed and that space is filled with water molecules. The coordination sphere of the remaining m1 -1 ions is maintained unchanged however, the mean coordination sphere of the M ions is decreased (mW(nC)4.5(H2O)i.5). No iron ions occupy interstitial positions, that is, only two types of iron environments exist. Since that special kind of Prussian blue has been the first and hitherto only Prussian blue that could be obtained as sufficiently large crystals to perform a single crystal structure analysis, practically all textbooks, and later publications present that defect structure as the real structure of Prussian blue, completely forgetting that this defect structure is an extreme that forms... 

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. 

Figure 6.16 The structure of Prussian Blue. The charge balancing counter-ions are located at the centers of the cubes within the structure. Movement of ions in and out of the structure is limited by the size of the pores in the faces of the cube.

Figure 3 Three-dimensional cubic structure of Prussian blue.

Earliest examples of structurally investigated coordination polymers dates back to 1936, where Keggin and Miles reported the structure of Prussian blue and related compounds, and Griffth in 1943, who reported the crystal structure of silver oxalate. ... 

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