Tris Amino-acidato Complex

In 1954, Krebs and Rasche reported on the partial resolution of mer-[Co(gly)j] on a starch colunm by elution with water. Later, Douglas and Yamada reported on the partial resolution of /uc-[Co(gly)3] the fac isomer was dissolved in water containing 10 % KCl in order to increase the solubility of the isomer, and the solution was eluted from a potato starch coliunn (2.2x70 cm) with 10% KCl solution. 

Gillard and Payne also used alumina chromatography for the preparations of 

Chromatography with D(-l-)-lactose hydrate columns was used in the partial resolution of tris(acetylacetonato)cobalt II) complex partial resolution of [Co(acac)3], and mer- and c-[Co(bzac)3] (bzac = benzoylacetonate) is also effected by column chromatography on (H-)-lactos . The [Co(bzac)3] complex prepared from Na3[Co(C03)3] 3 H2O was separated into its mer and fac geometrical isomers by column chromatography on Florisil (Florodin Co., suj rior to alumina), prior to the resolution. For the resolution, (-l-)-lactose dried at 110 °C and then sieved to 100 mesh, and a chromatographic column (3.9 x 230 cm), which narrowed to 3.0 cm at 

Other complexes such as [Co(acac) (tfac)3 n] (tfac = trifluoroacetylacetonate n = 1, 2, 3) [Co(acac)2(L-am)] (L-am = L-ala, L-val, iV-methyl-L-ala or A-methyl-L-val) and [Co(acac)(L-val)2] were separated into geometrical isomers or diastereoisomers by column chromatography on alumina. 

Thin-layer chromatography has found widespread use in investigations of metal complexes. Dutta et al. reviewed a number of studies of 1968 1977 with emphasis on TLC and paper chromatography. Celap et al. reported on the effect of the composition and structure of cobalt(III) complexes on their Rp values, in which the chromatographic separation, on silica gel, of 75 complexes using several solvent system are investigated. 

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For the tris(amino acidato)metal(III) complexes four geometrical and chiral isomers are possible, as shown in Scheme 2, where NO is the chelated amino acid anion, mer and fac refer to the meridional and facial geometrical isomers and A and A refer to the configuration at the metal centre. For the glycine complexes A and A are an enantiomeric pair, while for the optically pure forms of the other amino acids they form a diastereomeric pair and hence are easier to separate. For most of the simple bidentate amino acids of the kinetically inert metal ions Cr" , Co " and Rh ", the four isomers have been obtained. The isomers are distinguishable by their UV/visible, CD and NMR spectra. Not all the isomers can be found in certain cases, for example, with L-proline the k-fac isomer could not be prepared, a fact which was predicted on steric grounds." ... 

Several years later, the preparative work on the geometrical isomers was renewed by other workers, who applied the three methods to the preparation of geometrical-optical isomers, or diastereoisomers, of tris-complexes from optically active amino acids. In general, four diastereoisomers are possible for a tris(L- or o-amino-acidato) complex fac ->r), fac —), wer(-t-) and mer —). 

Figure 4.1 shows the spatial distribution of the alkyl substituents, R, in the four isomers of a tris-(L-a-amino-acidato) complex [Co(l-NH2CHRCOO)3]. The substituents are arranged either in a pseudo-axial or in a pseudo-equatorial orientation to the threefold or pseudo-threefold axis of the isomers. The A isomers of both fac and mer have pseudo-equatorial substituents, while the A isomers have pseudo-axial ones. In the mer-ts. isomer, two substituents are axially disposed at one end of the pseudo-threefold axis and the third at the opposite end. In the mer-h isomer, there is an exceptionally small distance between two substituents. 

The absorption and CD spectral data for several tri L-amino-acidato) complexes are cited in Table 4.3. 

Such a steric distribution of the alkyl substituents raises the question whether stereoselectivity would be found in the tris complex with an L-amino acid forming chelate rings much less puckered than the 1,2-diamine chelate rings. The stereoselectivity in tris(L- or D-amino-acidato)cobalt(III) has been mainly studied for the elucidation of the steric interactions between the substituents on the a-carbon atom of the ligand. 

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