Trans-Tris chromium

CrOTSeCgHg, Chromium(O), (ii -benzene)-dicarbonyl(selenocarbonyl)-, 21 1, 2 Cr04P2C34H32, Chromium, tetracarbonyl-(tributylphosphine)(triphenylphos-phine)-, trans-, 23 38 CrOsSeCs, Chromium(O), pentacarbonyl-(selenocarbonyl)-, 21 1, 4 Cr04C33Hs7, Chromium(III), tris(2,2,6,6-tet-ramethyl-3,5-heptane-dionato)-,... 

The cis-isomers of the tris(a-amino acid) complexes of cobalt(III) and chromium(III) have larger extinction coefficients for band maxima of d-d transitions than the corresponding trans-isomers (19-24). Although the difference in the coordination environment between the cis- and trans-tris(hydroxamato)chromi-um(III) complexes is less than that in the amino acid complexes, differences in the absorption spectra of the cis- and trans-isomers of Cr(hydroxamate)3 complexes have been found, especially for the extinction coefficients of the high energy transition... 

Figure 3. A perspective drawing of trans-tris(benzohydroxamato) chromium(III) looking down the molecular (pseudo) threefold axis...

Figure 4. A comparison of the octahedral bonding geometries of trans-tris(ben-zohydroxamato)chromium(III) and cis-tris(benzohydroxamato)iron(IH)...

C21H1sOgSc, Tris(tropolonato)scandium(III), 40B, 944 C2iHi8CrN30g 2 C3H8O, trans-Tris(benzohydroxamato)chromium(III) propanol solvate, 45B, 1220... 

Simple 1,3-dienes also undergo a thermal monocyclopropanation reaction with methoxy(alkyl)- and methoxy(aryl)carbene complexes of molybdenum and chromium [27]. The most complete study was carried out by Harvey and Lund and they showed that this process occurs with high levels of both regio-and diastereoselectivity. The chemical yield is significantly higher with molybdenum complexes [27a] (Scheme 7). Tri- and tetrasubstituted 1,3-dienes and 3-methylenecyclohexene (diene locked in an s-trans conformation) fail to react [28]. The monocyclopropanation of electronically neutral 1,3-dienes with non-heteroatom-stabilised carbene complexes has also been described [29]. 

Thermal deamination of tris(ethylenediamine)chromium(III) complexes is a standard preparative method for cis- and trans-diacidobis(ethylenediamine) complexes421,422 and the thermal behaviour of the starting materials has been related to their crystal structures.423 The cyano complex cis-[Cr(CN)2(en)2]C104 in DMSO undergoes stepwise reduction III— 11 — I at the DME. The standard redox potential for the Cr /Cr11 couple is -1.586 V (versus SCE). 

A rate and equilibrium study of the cis/trans isomerization of tris(l,l,l-trifluoro-2,4-pentanedionato)chromium(III) in the vapour phase has been reported.761 The equilibrium constant 3.56 was independent of temperature in the range 118-144.8 °C, the same value being measured by both static and kinetic methods. The activation energies for isomerization were both less than 115 kJmol-1 the Cr—O bond energy is 210 kJ mol-1. On these grounds, a bond-rupture mechanism was rejected. A similar mechanism may operate for the isomerization in solution. 

Chromium(III) complexes of a number of polyhydroxamic acids, microbial iron sequestering and transport agents (siderochromes) have been reported.797,798 The kinetic inertness of the chromium(III) complexes allows the facile separation of isomers for the model complex tris(iV-methyl-( - )-methoxyacetylhydroxamato)chromium(III), D-cis, L-cis and the l/d-trans isomers have been separated.798 The chromium complexes of desferrioxamine B (191) have been investigated the possible isomers are illustrated below (192-196). The cis isomer was isolated in relatively pure form.799 Thiohydroxamate800 and dihydroxamate (rhodotorulic acid) complexes have also been studied.801... 

Malonic acid CH2(C02H)2 (H2mal) (209) has a coordination chemistry with chrommm(III) closely resembling that of oxalate. Malonic acid is a slightly weaker acid than oxalic acid and slightly more labile complexes are formed. The tris complex is the most extensively studied, prepared by the reduction of chromate solutions or the reaction of chromium(III) hydroxide with malonate.917,918 919 The cis and trans diaqua complexes may be prepared by the reduction of chromate with malonate the isomers are separated by fractional crystallization. The electronic spectrum of the tris complex is similar to that of the tris oxalate and a detailed analysis of these spectra has appeared.889... 

The more cationic halogen containing compounds produced other products. Cobalt bis-allyliodide produced cis-polybutadiene and the even more cationic chromium, produced cyclododecatriene. Only with the more cationic system which introduced trans-structures, was cyclization and reduction of the metal able to intercept the polymerization reaction. Cyclization was not possible in the less cationic cobalt which produces all cis-polybutadiene nor was the hydride transfer possible with the less anionic chromium tris-allyl compound. 

The carbonyls are disposed trans to each other, which is deduced from the single carbonyl in the infrared spectrum. A similar cation may be formed by mild oxidation of the chromium dicarbonyl phosphine, Cr(CO)2QP [QP = tris(o-diphcnylphosphinophenyl)phosphine]. 

N-methyl derivative resulted in oxidation of the ligand with concomitant reduction of Co (III) to Co (II). The preparation of tris (benzohydroxa-mato) chromium (III), Cr(benz)3, was successful and resulted in the separation and characterization of its two geometric isomers (2). The half-lives for isomerization of these complexes near physiological conditions is on the order of hours. To facilitate the separation of all four optical isomers of a simple model tris (hydroxamate) chromium (III) complex, we prepared (using Z-menthol as a substituent) the optically active hydroxamic acid, N-methyl-Z-menthoxyacethydroxamic acid (men). This resulted in the separation of the two cis diastereoisomers of tris(N-methyl-Z-menthoxyacethydroxamato) chromium (III) from the trans diastereoisomers and their characterization by electronic absorption and circular dichroism spectra. 

Thin layer chromatography of the tris (benzohydroxamato) chromium (III) complex resulted in two green bands, corresponding to the cis and trans isomers, whose elution Rst values bracketed that of the one broad reddish-brown band of the Fe(III) complex. As just described, the geometric isomers of the Fe(III) complex are in rapid equilibrium in solution, and as a result, the mixture of these isomers elutes as one band with an Rst value that is a weighted average of the two individual isomers. 

The tris (N-methyl-Z-menthoxyacethydroxamato) chromium (III) and -iron (II) complexes, Cr(men)3 and Fe(men)3, were also purified by thin layer chromatography. The iron complex gives one broad reddish-brown band whose elution Rst value is bracketed by the bluish-green bands of the cis and trans isomers of the Cr(III) complex (2). As with the tris(benzohydroxamate) complexes, this behavior is caused by the rapid equilibration of the kinetically labile ferric complex. 

Due to the rather complicated stereochemistry of siderophores, simple hydroxamate and phenolate chromium(III) complexes, for which only four isomers are possible, have been used as models for the correlation between absolute configuration and UV-VIS and CD spectra. Tris(benzhydroxamate)chromium(III) has been separated into the cis and trans geometrical isomer as confirmed by X-ray structural analysis183). 

The cis- and trans-isomers of several simple tris(hydrox-amato)chromium(III) complexes have been separated by thin layer and column chromatographic techniques (11, 12). Both of the corresponding iron(III) and cobalt(III) complexes showed only one band with both techniques. The assignment of geometries for the... 

The pure solid cis- and trans-isomers of tris(benzohydrox-amato)chromium(III) have been separated by simple extraction of the cis-isomer into chloroform by adding water to the chloroform/ acetone solution of both isomers. The trans-isomer, which is insoluble in chloroform, precipitates out of the solution (25). 

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