Chelates of chromium

If the metallisable dye is insoluble in water, a miscible solvent such as ethanol or ethylene glycol may be added. Polar solvents such as formamide or molten urea have sometimes been preferred. It is likely that such solvents will preferentially displace water molecules and coordinate with the chromium (III) ion as the first step in the reaction. If colourless organic chelates of chromium, such as those derived from oxalic or tartaric acid, are used instead of or in addition to hydrated chromium (III) salts, the difficulty of replacing the strongly coordinated water molecules in the first stage of the reaction is eliminated. In this way the initial reaction can be carried out at high pH without contamination by the precipitation of chromium hydroxide. Use of the complex ammonium chromisalicylate (5.12) in this connection should also be noted (section 5.4-1). 

Although the halogenated chelates of chromium, cobalt, and rhodium would be difficult to prepare from the sensitive 3-halo-2,4-pentanediones, the copper (II) bromochelate was synthesized both from the bromodiketone and by direct bromination of copper acetylacetonate. The relatively labile copper chelates form much more rapidly than the kinetically stable chelates of chromium, cobalt, and rhodium. 

Chelation of chromium with amino acids or other compounds which maintain its solubility in the gastrointestinal tract increases absorption (Mertz and Roginski 1969). Chromium picolinate, a popular dietary supplement, is a chelated compound that is better absorbed than CrClj but is rapidly excreted (Stoecker 2001). 

The acid hydrolysis of [Cr(LL)3] [LL = NH2CH2CH2OH, H0CH2CH(NH2)CH3, and NH2CH2CH(OH)CH3] was discussed previously (Reference 12). A study of the tris(ethanolamine) complex in aqueous/ acetone mixed solvents has also appeared. The acid hydrolysis of the low-spin tris(chelates) of chromium(II) with 2,2 -bipyridine and... 

Determination of chromium by many of the methods cited above is problematic. Variable and nonquantitative recovery with chelation-solvent extraction... 

Diphenylcarbazone and diphenylcarbazide have been widely used for the spectrophotometric determination of chromium [ 190]. Crm reacts with diphenylcarbazone whereas CrVI reacts (probably via a redox reaction combined with complexation) with diphenylcarbazide [ 191 ]. Although speciation would seem a likely prospect with such reactions, commercial diphenylcarbazone is a complex mixture of several components, including diphenylcarbazide, diphenylcarbazone, phenylsemicarbazide, and diphenylcarbadiazone, with no stoichiometric relationship between the diphenylcarbazone and diphenylcarbazide [192]. As a consequence, use of diphenylcarbazone to chelate Crm selectively also results in the sequestration of some CrVI. Total chromium can be determined with diphenylcarbazone following reduction of all chromium to Crm. 

The use of /r-hydroxo or ju-alkoxo bridged polynuclear complexes of chromium, molybdenum, tungsten, or rhenium in this route leads to the formation of monomeric bis(NHC) complexes, to the elimination of hydrogen, and to the partial oxidation of the metal [Eq.(ll)]. Chelating and nonchelating imidazolium salts as well as benzimidazolium and tetrazolium salts can be used. 

Nondestructive reactions of trisacetylacetonates of chromium(lll), cobalt(lll), and rhodium(lll) are reviewed. Halogenation, nitration, thiocyanation, acylation, formylation, chloromethylation, and aminomethylation take place at the central carbon of the chelate rings. Trisubstituted chelates were obtained in all cases except acylation and formylation. Unsymmetrically and partially substituted chelates have been prepared. Substitutions on partially resolved acetylacetonates yielded optically active products. NMR spectra of unsymmetrically substituted, diamagnetic chelates were interpreted as evidence for aromatic ring currents. Several groups were displaced from the chelate rings under electrophilic conditions. The synthesis of the chromium(lll) chelate of mal-onaldehyde is outlined. 

A search of the literature revealed two instances in which metal acetylaceto-nate rings had been substituted without degradation of the chelate rings. Treatment of chromium (III) acetylacetonate with bromine in chloroform had been reported to yield two products—a tribromo- and a hexabromochelate (III and IV) (31). These structures were assigned on the basis of halogen analyses. Later work simultaneous with our own revealed that IV was actually a chloroform solvate of III (24). 

Certain other 1,3-dicarbonyl chelates were brominated with difficulty or not at all. For example, the trifluoro- and hexafluoroacetylacetonates (VI, R = CF3, R = CH3, and R = R = CF3) were not brominated under a variety of vigorous conditions. However, in the case of the chromium chelates of 1-phenyl-1,3-butanedione and dibenzoylmethane (VI, R = C( Hr), R = CH5, and R = R = C(iHr)), reaction with N-bromosuccinimide (NBS) was successful. That the electron density at the central carbon of the chelate ring is an important factor in the success or failure of these electrophilic substitutions is evident from the fact that the bis-(ethylenediamine)-2,4-pentanedionocobalt(III) cation cannot be brominated even under vigorous conditions. 

Treatment of chromium (III) acetylacetonate with acetic anhydride and boron trifluoride etherate yielded a complex mixture of acetylated chelates but very little starting material. Fractional crystallization and chromatographic purification of this mixture afforded the triacetylated chromium chelate (XVI), which was also prepared from pure triacetylmethane by a nonaqueous chelation reaction (8, 11). The enolic triacetylmethane was prepared by treating acetylacetone with ketene. The sharp contrast between the chemical properties of the coordinated and uncoordinated ligand is illustrated by the fact that chromium acetylacetonate does not react with ketene. 

The acetylacetonates of chromium, cobalt, and rhodium were found to react with dimethylformamide in the presence of phosphorus oxychloride to yield formyl-substituted chelates (10). This is a well known technique for the introduction of an aldehyde group into reactive aromatic systems. The formylation of the chelate rings is a slow reaction, and by controlling the conditions it is possible to... 

The Mannich reaction is a particularly good method of introducing a reactive functional group into a sensitive aromatic nucleus. The reaction has been very useful in ferrocene chemistry. Treatment of chromium acetylacetonate under Mannich conditions yielded a tris-V,N-dimethylaminomethyl chelate (XXXIII). This remarkable substance was very difficult to purify because of its extreme solubility in all solvents ranging from n-heptane to water. The trisamino chelate (XXXIII) is a deep purple, hydroscopic oil and behaves like a typical organic amine. Reaction of this amine with methyl iodide afforded a trisquater-nary ammonium salt (XXXIV), soluble in water but insoluble in organic solvents. When this salt (XXXIV) was treated with cyanide ion, trimethylamine was lost and the cyanomethyl chelate (XXXV) was formed. 

Therefore in an attempt to distinguish among mechanisms A, B, and C the acetylacetonates of chromium(III), cobalt(III), and rhodium(III) were partially resolved and the optically active chelates were then subjected to several electrophilic substitution reactions. 

Chromium(II) chelates of 3-(N-2-furfuralideneimino)propionic acid (HFP), o-(JV-ar-furfurali-dene)benzoic acid (HFB), and o-(N-ar-furfuralideneimino)ethanesulfonic add (HFE)284-286 have been found to have magnetic moments of approximately 4.8 BM at room temperature. A band near 14500 cm-1 (CHC13 solution) has been assigned to the % 5T transition and formation constants have been measured. However, there is no indication of difficulties arising from the air sensitivity of chromium(II) solutions. 

The synthesis and characterization of materials showing biological activity similar to that of GTF isolated from yeast is a logical objective. As already mentioned, Mertz found aqua and similar complexes of chromium to be more active than chelates with strong ligands. An exception to this was an unspecified cysteine complex,1193 prepared by C. L. Rollinson, which showed marked, but erratic, behaviour in GTF tests. Further investigation of this observation would be interesting, particularly as the crystal structure of a cysteine complex is now known.1185... 

Ding et al. [88] used anion-exchange chromatog-raphy-ICP-MS to determine different forms of chromium in chromium picolinate products which are used as dietary supplements and appear to assist in weight loss. A Dionex AS7 anion-exchange column was used to separate Cr(III)-EDTA chelate and Cr(VI) in the supplements. Only 1% total chromium recoveries were obtained and this was attributed to retention of the chromium species on-column. The use of RP-HPLC proved to be more effective and complete chromium recoveries were obtained, based on the amounts stated on the manufacturer s product labels. 

The replacement of methyl groups on acetylacetone by /-butyl groups causes a massive increase in steric hindrance to electrophilic substitution. The chromium(III) chelate of dipivaloylmethane can be chlorinated and nitrated only very slowly by means of forcing conditions (equation 68).281... 

The most important feature of the chromium compounds, as well as of the other derivatives of early transition 3difference between the derivatives of different oxidation states (T able 12.18). The alkoxides of chromium (II and HI) are in the majority the insoluble and non-volatile polymers. The most important exclusion from this rule appears to be the volatile chelate Cr(OCMe2CH2OMe)3 complex, and in the future, possibly, the number of the representatives of this family will increase. The derivatives of chromium (IV and VI) are monomers highly soluble in organic solvents. The lack of volatility for Cr(VI) compounds in contrast to those of Cr(IV) is caused apparently by the high electronegativity of the central atom, leading to thermal destruction and not to evaporation. 

Kakitani, T., Hata, T., Katsumata, N. (2007) Chelating extraction for removal of chromium, copper, and arsenic from treated wood with bioxalate. Environmental Engineering Science, 24(8), 1026-37. 

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