Cyanide iron complexes

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

Environmental. The toxicity of cyanide in the aquatic environment or natural waters is a result of free cyanide, ie, as HCN and CN . These forms, rather than complexed forms such as iron cyanides, determine the lethal toxicity to fish. Complexed cyanides may revert to free cyanide under uv radiation, but the rate is too slow to be a significant toxicity factor. Much work has been done to estabhsh stream and effluent limits for cyanide to avoid harmful effects on aquatic life. Fish are extremely sensitive to cyanide, and the many tests indicate that a free cyanide stream concentration of 0.05 mg/L is acceptable (46), but some species are sensitive to even lower concentrations. 

The complex cyanides of transition metals, especially the iron group, are very stable in aqueous solution. Their high co-ordination numbers mean the metal core of the complex is effectively shielded, and the metal-cyanide bonds, which share electrons with unfilled inner orbitals of the metal, may have a much more covalent character. Single electron transfer to the ferri-cyanide ion as a whole is easy (reducing it to ferrocyanide, with no alteration of co-ordination), but further reduction does not occur. 

The lithium enolate 2a (M = Li ) prepared from the iron propanoyl complex 1 reacts with symmetrical ketones to produce the diastercomers 3 and 4 with moderate selectivity for diastereomer 3. The yields of the aldol adducts are poor deprotonation of the substrate ketone is reported to be the dominant reaction pathway45. However, transmetalation of the lithium enolate 2a by treatment with one equivalent of copper cyanide at —40 C generates the copper enolate 2b (M = Cu ) which reacts with symmetrical ketones at — 78 °C to selectively produce diastereomer 3 in good yield. Diastereomeric ratios in excess of 92 8 are reported with efficient stereoselection requiring the addition of exactly one equivalent of copper cyanide at the transmetalation step45. Small amounts of triphcnylphosphane, a common trace impurity remaining from the preparation of these iron-acyl complexes, appear to suppress formation of the copper enolate. Thus, the starting iron complex must be carefully purified. 

Since ferrous iron usually colors minerals green, and ferric iron yellow or brown, it may seem rather remarkable that the presence of both together should give rise to a blue color, as in the case of vivianite. It may be pointed out, however, that this is by no means a unique instance of such an effect. Even apart from the artificial substances, Prussian and Turnbull s blues, which are complex cyanides containing both ferric and ferrous iron, there are several blue minerals in which the color seems explainable only on this basis. The most noteworthy of these are crocidolite and related amphiboles iolite and the blue tourmaline or indicolite. Other instances may perhaps be discovered, should this subject ever be investigated as it deserves to be . 

In higher plants, elevated cyanide concentrations inhibited respiration (through iron complex-ation in cytochrome oxidase) and ATP production and other processes dependent on ATP, such as... 

The therapeutic effects of sodium nitroprusside depend on release of nitric oxide which relaxes vascular muscle. Sodium nitroprusside is best formulated as a nitrosonium (NO+) complex. Its in vivo activation is probably achieved by reduction to [Fe(CN)5NO]3, which then releases cyanide to give [Fe(CN)4NO]2, which in turn releases nitric oxide and additional CN to yield aquated Fe(II) species and [Fe(CN)6]4 (502). There are problems associated with its use, namely reduced activity due to photolysis (501) and its oxidative breakdown due to the action of an activated immune system (503), both of which release cyanide from the low-spin d6 iron complex. 

Electronic and vibrational spectroscopy continues to be important in the characterization of iron complexes of all descriptions. Charge-transfer spectra, particularly of solvatochromic ternary diimine-cyanide complexes, can be useful indicators of solvation, while IR and Raman spectra of certain mixed valence complexes have contributed to the investigation of intramolecular electron transfer. Assignments of metal-ligand vibrations in the far IR for the complexes [Fe(8)3] " " were established by means of Fe/ Fe isotopic substitution. " A review of pressure effects on electronic spectra of coordination complexes includes much information about apparatus and methods and about theoretical aspects, though rather little about specific iron complexes. ... 

Solubility data (pA sp) for two dozen hexacyanoferrate(II) and hexacyanoferrate(III) salts, and Pourbaix (pe/pH) diagrams for iron-cyanide-water, iron-sulfide-cyanide-(hydr)oxide, iron-arsenate-cyanide-(hydr)oxide, and iron-copper-cyanide-sulfide-(hydr)oxide, are given in a review ostensibly dedicated to hydrometallurgical extraction of gold and silver. " The electrochemistry of Prussian Blue and related complexes, in the form of thin films on electrodes, has been reviewed. ... 

Cyanide-bridged complexes involving iron-diimine-cyanide complexes are discussed in Section 5.4.3.5.8. 

Iron in both the +2 and +3 valence states forms several stable hexacoordi-nated octahedral complexes with cyanide (CN ) ion, known as ferrocyanide or hexakiscyanoferrate(4—), [Fe(CN)6] and ferricyanide or hexakiscyanofer-rate(3-), [Fe(CN)6]3-, respectively. The simple iron(II) cyanide, Fe(CN)2 is unstable and all iron cyanide compounds known are coordination complexes. 

Iron Blue, Cl Pigment Blue 27, which has been known by various names over the years, perhaps the best known being Prassian Blue, is ferric ammonium ferricyanide, FeNH Fe(CN)g(xH20. The hrst step in its preparation involves the precipitation of complex iron(ii) cyanides, e.g. potassium hexacyanoferrates(ii) with iron(i) salts, e.g. the sulfate or chloride, in an aqueous solution in the presence of ammonium... 

The well-known alkylation of ferrocyanide ion to form isocyanide iron complexes (48) can be explained by an insertion mechanism if the metal is alkylated initially, and then metal alkyl adds across a cyanide group. This mechanism also explains how external radioactive cyanide ion can enter the isocyanide ligands (48). 

Although iron, cobalt, and nickel occur in the same triad in Group VIII., the three elements differ considerably in their ability to form addition compounds with ammonia. Iron forms few ammino-salts, most of which are unstable, and its tendency to complex-salt formation of the ammine type appears in the complex cyanides and not in the ammines themselves. 

Iron blue pigments are produced by the precipitation of complex iron(II) cyanides by iron(II) salts in aqueous solution. The product is a whitish precipitate of iron(II) hexacyanoferrate(II) M 2Fe1I[Fe1I(CN)6] or M11Fe11[Fe11(CN)6], (Berlin white), which is aged and then oxidized to the blue pigment [3.180]. 

Vanadium(II) reacted with SCN- and diphenylguanidine (L) to form a ternary complex with a V SCN L ratio of 1 2 2.176 Other ternary complexes like phenylguanidine iron(II) cyanide are being exploited in titrimetric determination of cyanide ions, for example. 

In order to prevent the reduction between iron(II) and formaldoxime occurring, another iron complexing agent (potassium cyanide) was used in the presence of a reductant (ascorbic acid) that reduces iron(III) to iron(II). Aluminium, titanium, uranium, molybdenum and chromium also form light-coloured complexes that normally do not interfere in the determination of manganese in water or plant material by this method. If the aluminium or titanium concentrations are higher than 40 ppm an additional masking flow of tartrate is recommended [31]. 

History. Braun and Tschemak [23] obtained phthalocyanine for the first time in 1907 as a byproduct of the preparation of o-cyanobenzamide from phthalimide and acetic anhydride. However, this discovery was of no special interest at the time. In 1927, de Diesbach and von der Weid prepared CuPc in 23 % yield by treating o-dibromobenzene with copper cyanide in pyridine [24], Instead of the colorless dinitriles, they obtained deep blue CuPc and observed the exceptional stability of their product to sulfuric acid, alkalis, and heat. The third observation of a phthalocyanine was made at Scottish Dyes, in 1929 [25], During the preparation of phthalimide from phthalic anhydride and ammonia in an enamel vessel, a greenish blue impurity appeared. Dunsworth and Drescher carried out a preliminary examination of the compound, which was analyzed as an iron complex. It was formed in a chipped region of the enamel with iron from the vessel. Further experiments yielded FePc, CuPc, and NiPc. It was soon realized that these products could be used as pigments or textile colorants. Linstead et al. at the University of London discovered the structure of phthalocyanines and developed improved synthetic methods for several metal phthalocyanines from 1929 to 1934 [1-11]. The important CuPc could not be protected by a patent, because it had been described earlier in the literature [23], Based on Linstead s work the structure of phthalocyanines was confirmed by several physicochemical measurements [26-32], Methods such as X-ray diffraction or electron microscopy verified the planarity of this macrocyclic system. Properties such as polymorphism, absorption spectra, magnetic and catalytic characteristics, oxidation and reduc-... 

Fig. 6 Photocurrent/potential characteristics for Ti02 ( ), and rhenium ( ) and iron (A) cyanide complexed Ti02 electrodes. The Ti02 was illuminated by UV (380 nm), the others by visible light.

Fig. 7 Photocurrent efficiency (charge transfer/photon flux) of rhenium (ig)and iron (A)cyanide complexed Ti02 photoelectrodes with that of untreated Ti02 ( )for comparison. The extension of the spectral response into the visible is evident.

Very recently Geus and co-workers [44, 45] have applied another method based on chemical complexes. This is the complex cyanide method to prepare both monocomponent (Fe or Co) and multicomponent Fischer-Tropsch catalysts. A large range of insoluble complex cyanides are known in which many metals can be combined, e.g. iron(n) hexacyanide and iron(m) hexacyanide can be combined with iron ions, but also with nickel, cobalt, copper, and zinc ions. Soluble complex ions of molybdenum(iv) which can produce insoluble complexes with metal cations are also known. Deposition precipitation (Section A.2.2.1.5) can be performed by injection of a solution of a soluble cyanide complex of one of the desired metals into a suspension of a suitable support in a solution of a simple salt of the other desired metal. By adjusting the cation composition of the simple salt solution, with a same cyanide, it is possible to adjust the composition of the precursor from a monometallic oxide (the case when the metallic cation is identical to that contained in the complex) to oxides containing one or several foreign elements. 

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