thumbnail ~ketchef Prussian Blue, an Inorganic Evergreen Andreas Ludl lnstitut fur Anorganische Chemie Universitat Bern CH-3000 Bern 9. Switzerland
The first published report about a synthetic coordination compound dates back to the early 18th century. It describes a novel blue dye prepared from such exotic ingredients as dried oxblood. Since the synthesis of this pigment was accomplished in Berlin, in the Kingdom of Prussia, it subsequently was called Berlin Blue or, more commonly, Prussian Blue. Other labels as well, like prussiate or cyanide, derived from the Greek expression for blue, are traced hack to this chemical event. Later chemical analysis identified Prussian Blue as a complex iron cyanide. Considering its long history and its perpetual attraction for chemists the metaphorical label "evergreen" may be used for this compound. To this day Prussian Blue continues to attract the scientific curiosity of chemists, an attraction which undoubtedly arises from its startling blue color. Neither Feaq3+nor Fe(CN)fi4-, the two' standard starting materials, absorb light in this spectral region. The blue color thus clearly originates in a cooperative interaction of the iron ions in Prussian Blue. The formation of this pigment may now he written as
-
4FeW3++ 3Fe(CN)e4- Prussian Blue, Fer[Fe(CN)sls.xHzO (X
= 1k16)
The very low solubility presents evidence for the polymeric nature of the solid product. Diffraction experiments using X-rays and neutrons reveal a cubic lattice. The cyanide ions a d a s bridges and link the two structurally distinct iron ions. The resulting three-dimensional framework also provides positions for the water molecules. Because of the 4:3 ratio of Fe(II1) and Fe(II), 25% of its Fe(I1) sites are vacant. Applying the standard rules to determine oxidation states from a given stoichiometry we notice that i t is not possible to derive an integer number for the oxidation state of iron in Prussian Blue, in contrast to common coordination compounds. Rather, we observe the coexistence of four Fe(II1) and three Fe(I1). The occurrence of more than one oxidation state of the same element in a particular compound is, by definition, the characteristic property of a mixed valence compound. In the systematic investigation of mixed valence compounds which are known for some 40 elements, Prussian Blue plays a prominent role as a prototype. For a brief discussion of some properties of Prussian Blue related to the coexistence of two valences of iron it suffices to focus our attention on the sequence Fe(II1)-N-C-Fe(I1)which represents the essential structural element of the three-dimensional polymeric framework. The assignment of the oxidation states, Fe(I1) in a carbon octahedron, Fe(II1) in a nitrogen-water environment (ferric ferrucyanide) has been unambiguously established by a variety of physical techniques such as infrared, photoelectron, and Mossbauer spectroscopy. The electronic interaction between the two oxidation states of iron, the mixed valence interaction, is responsible nut just
edited by O.S.U College of New Rochelle New Rochelle. NY 10801
MARY VIRGINIAORNA,
for the intense blue color alone. Owing to the corresponding partial delocalization of the valence electrons Prussian Blue is a semiconductor. Moreover, the same Fe(I1)-Fe(II1)interaction is reflected in the magnetic properties. Below 5.5 K Prussian Blue becomes a ferrumagnet, a rather unique behavior among coordination compounds.l T o prepare Prussian Blue nowadays dried oxblood is not an essential chemical. A crvstalline samole is obtained bv the following procedure: 90 mmoles of Fed13 and 67 m moles of H4Fe(CN)fi(obtained from KdFe(CN)s by ion exchange) are dissolved in 1.5L of 10 M HCI. Slow diffusion of water vapor into this vellow solution produces tiny, almost black crystals of ~ r u s s i a nBlue within approximately 10 weeks. A much quicker and very easy preparative route consists of simply mixing solutions of FeCI3 and K4Fe(CN)~ leading to a very voluminous blue precipitate or colloidal suspension. ?wing to the extremely small particle size such quickly precipitated samples may incor~orateor absorb significant and variable amounts of potas&nn, hence the oversimplified formula KFeFe(CN)e for the so-called "soluble" Prussian Blue. Interestingly, the same blue pigment is obtained when the oxidation states of the starting compounds are interchanged, i.e., by mixing solutions of F e S 0 ~ 7 H z and 0 K Z F ~ ( C NThe ) ~ . resulting solid has been called Turnbull's Blue, ferrous ferritechniaues. cvanide. A varietv"of soectrosconic . . . however. unambiguously proved it to be identical with Prussian Blue; in other words. the formation of the solid precipitate is accom. . panied hy an elcctron tmt~sferreactiun or valence inrerrhange. Rwardless uf the rhoice d starting materials the blue mixed val&e compound invariably is ferric ferrocyanide possessing the characteristic structural element Fe(II1)-N-C-Fe(I1). In addition to its role in basic research in inorganic chemistry we have to mention the application of Prussian Blue in various fields. I t is emoloved in aualitative tests for the . presence of nitrogen in organic compounds as well as in inoreanic analvtical chemistrv. Prussian Blue is encountered even in the wine making proEess when exc&siron in the wine is removed bv controlled addition of KdFe(CN)n. Itsmost important application, however, is as a low-priced blue pigment. The industrial production is based on the reaction of Na4Fe(CN)Gand FeSOaIHzO in the presence of (NH4)2S04 initially producing the insoluble iron(I1) ferrocyanide, the socalled Berlin White. Ammonium-containing Prussian Blue is generated by subsequent oxidation with bichromate or chlorate. This nrocedure leads to oroducts with narticle sizes oi a few h u n d r h A as required fc;r the technicai application as a pigment. During the 1960's the United Stares produced approximately 10 million pounds per year ot'l'rusiian Blue, the industrial product being alsu known as Iron Blue, Pigment Blue. .Milmi l3lue. French Rlue, err. The use as pigment for ~olserhslene and the manutacture of printing inks and carbon . . . paper are representative examples iilustrahg the practical application of Prussian Blue. l For recent studies see D. B. Brown (Editor). "Mixed Valence Compounds." D. Reidel Publishing Company. 1980; F. Herren. P. Fischer. A. Ludi, and W. Hilg, Inorg. Chem., 19, 956 (1980).
Volume 58 Number 12 December 1981
1013
? _v
9.
