Browning compounds

Aminoazobenzene is a very weak base, and consequently it will not form salts with weak organic acids, such as acetic acid, although it will do so with the strong mineral acids, such as hydrochloric acid. Aminoazobenzene is a yellowish-brown compound, whilst the hydrochloride is steel blue. The colour of the latter is presumably due to the addition of the proton to the phenyl-N-atom, the cation thus having benzenoid and quinonoid forms ... 

Two complexes of FeCl, with NH OH were received in crystalline state-one-brown by precipitation from the solution (HI) and pink-brown compound was received by lyophilic drying of the solution (IV). 

The synthesis of p-tolylsulfonylmethyl isocyanide is described in this Volume. The light-brown compound, m.p. 111-114°, was used without further piu ification. 

A number of nitrogen-fixing bacteria contain vanadium and it has been shown that in one of these, Azotobacter, there are three distinct nitrogenase systems based in turn on Mo, V and Fe, each of which has an underlying functional and structural similarity.This discovery has prompted a search for models and the brown compound [Na(thf)]+[V(N2)2(dppe)2] (dppe = Pli2PCH2CH2PPh2) has recently been prepared by reduction of VCI3 by sodium naphthalenide... 

Yellow to brown compounds are obtained when one or more nitro groups are conjugated with electron-donor substituents such as hydroxy or, of more importance, amino groups. Such dyes are relatively cheap. Picric acid, 2,4,6-trinitrophenol, can be regarded as the oldest synthetic dye it was produced in 1771 by Woulfe by treating indigo with nitric acid. 

The observation of radical anions has been confirmed by ESR measurements as illustrated by [Ir4(CO)12], (g = 2.002) 208). Similarly, a toluene solution of Co4(CO)i2 reacts with cobaltocene precipitating a brown compound which is extremely reactive and contains a cobaltocenium cation for each four cobalt atoms of the anion55. With excess cobaltocene (or alkali metals) in THF the reaction proceeds further as shown in Eq. (20),... 

VOSO4 reacts with morpholine-4-carbodithioate [mdtc (4)] (1 2) in aqueous solution, pH 4, to form a dull-brown compound which is unstable and slowly... 

Iridiuin(iv).—Group VII Donors. All reported studies of Ir concerned halogen complexes. Pure IrF4 has been prepared for the first time via reduction of IrFg on a hot filament of Ir or W. X-Ray powder pattern studies of this volatile, red-brown compound showed it to be different from IrFj or IrFs. A simple melt synthesis of K2lrFg has been developed involving the reaction ... 

The synthesis and some reactions of meso-ionic 1,2-dithioM-ones (388) have been recently reported. The brown compound 388, R = R = Ph, has been prepared by several methods (i) the reaction betw i l,l,3,3-tetrabromo-l,3-diphenylacetone (PhCBrjCOCBrjPh) and potassium ethyl xanthate, (ii) the reaction between 1,3-diphenyl-propanetrione hydrate and tetraphosphorus decasulfide, (iii) 1,3-diphenylpropanetrione with hydrogen sulfide-hydrogen chloride in ethanol-chloroform yields the salt 389, R = R = Ph, X = Cl, which gives the meso-ionic I,2-dithiol-4-one with triethylamine, pyridine, or aqueous sodium bicarbonate. ... 

Pyrrole A nitrogen-containing, heterocyclic compound found at low concentrations in crude. Pyrrole can readily polymerize in the presence of light to form dark-brown compounds. Pyrrole derivatives, such as indole, have been identified as sources which can lead to darkening of fuel color and eventual deposit formation. 

Tetrammino-cupric Iodide Monohydrate, [Cu(NH3)4]I2.H20, crystallises in dark blue tetrahedra and decomposes rapidly in air, with loss of ammonia and formation of a dark brown compound. If the solution of the salt in ammonia be warmed it changes in colour, and, on cooling, crystals of decammino-tricupric iodide, [Cu3(NII3 separate. This substance on heating is transformed into diammino-eupric iodide. 

Dipyridino-manganous Chloride, [Mn(C5H5X)2]Cl3, appears to exist in two forms, one rose-coloured, the other brown. The rose-coloured variety is obtained from the manganic derivative described above by pressing it between filter-paper for several days, or it may be precipitated from a solution of manganous chloride in absolute alcohol by excess of pyridine. It may also be produced by recrystallising the brown compound from absoluet alcohol. The conductivity is the same for both forms. The brown modification is prepared from a concentrated solution of the chloride mixed with excess of pyridine. The solution turns brown and a solid is precipitated. In the dry state... 

A truly remarkable bismuth-sulfur compound was recently reported by Muller et al. (equation 63).205 The dark brown compound contains an anion which is essentially two S7 rings joined by a spiro bismuth atom, and two of these bismuth groups are joined by an S6 chain (73). This bismuth coordination polyhedron is a highly distorted square pyramid, the apex of which is the sulfur of the S6 chain, d(Bi—SapiCai) 2.683 A (av) and d(Bi—Seq) 2.817 A. 

Dicyclopentadienyl derivatives in a formal oxidation state of 3.5 appear to be relatively common. A purple, paramagnetic compound with one impaired electron (1.32 BM) for two Ta atoms was isolated and formulated as a dimer (65 equation 93). 729 A similar reaction did not yield the niobium analog, but violet or red-brown compounds were observed as intermediates in the reduction of [MX2(Cp)2] to [MX(Cp)2] (X = C1, Br). They could be isolated in good yields ( 60%, Scheme 11).730... 

Whereas Ni(acac)2(H20)2 has a mononuclear (raws octahedral structure,1536 the complex Ni(acac)2 has been found to possess a trinuclear structure (195) in which Ni06 octahedra share faces via bridging acac groups.1534 A reddish brown compound having the same stoichiometry has been extracted from a solution of Ni(acac)2 in CS2. It is assumed that the former compound contains both square planar and octahedral species in a 1 3 ratio.1550... 

H. Rose observed that phosphine forms a brownish-yellow precipitate with a soln. of mercuric bromide and P. Lemoult observed that by the action of phosphine on a soln. of mercuric bromide in one of potassium bromide, a brown compound is formed, phosphorus bromomercuriate, P(HgBr)3.PHg2Br. 

Z. Asfari, C. Bressot, J. Vicens, C. Hill, J.-F. Dozol, H. Rouquette, S. Eymard, V. Lamare and B. Tournois, Cesium Removal from Nuclear Waste Water by Supported Liquid Membranes Containing Calix-bis-brown Compounds, in ACS Symposium Series 642 (Anaheim CA, April 2-6, 1995), eds. R. A. Bartsch and J. D. Way, American Chemical Society, Washington, DC, Editon edn., 1996, pp. 376-390. 

Fast precipation generally gives a light brown, fine-grained powder, while slow precipitation gives a dark brown, glasslike, brittle product. Some authors have found two products at the same time, for example Emons and coworkers found both a yellow and a brown compound in different parts of the apparatus used. 

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