Complexes Containing Optically Active Tartaric Acid

Haines et al. investigated the stereoselective formation of bisfethyienediamine)-cobaltflll) complexes containing optically active tartaric acid from a reaction of [Co(C03)(en)2]Cl and S(—)-tartaric acid in water at steam-bath temperature, two optically active complexes, A-[Co(S-C4H40g)(en)2] Cl and A-[Co(S-C4H306)(en ], were isolated by TLC with silica gel. In the former complex, the tartrate ion acts as a bidentate chelate agent with one free carboxylic acid (IV), and in the latter the tartrate is trinegative. 

Similar results were reported on bis(phenanthroline) complexes containing tartaric and malic acids From a reaction of [Co(C03)(phen)2]Cl 5 H2O and R(+)-tartaric acid in the dark at room temperature, A-[Co(R-tart)(ph i)2]C104 3 H2O was isolated by cation-exchange chromatography with Dowex 50W-X8 r in. The corresponding R(+)-maUc complex, A-[Co(R-malato)(phen)2]C104 2 H2O was isolated in a similar way. 

Mellor, D. P. (ed.) Chelating Agents and Metal Chelates. New York and London Academic Press 1964, p. 199 

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The bases generally employed in such resolutions have been natural alkaloids, such as strychnine, brucine, and ephedrine. These alkaloids are more complex than the general case shown in the figure, in that they contain several chiral centres (ephedrine is shown in Section 3.4.4). Tartaric acid (see Section 3.4.5) has been used as an optically active acid to separate racemic bases. Of course, not all materials contain acidic or basic groups that would lend themselves to this type of resolution. There are ways of introducing such groups, however, and a rather neat one is shown here. 

Pasteur s crystals of tartaric acid are more complex because the molecules contain two chiral carbon atoms. These can cancel out internally in the molecule so that three molecular forms actually exist, the two optically active mirror image structures that cannot be superimposed on each other, as the laevorota-tory (/-) and dextrorotatory (d-) forms, and the... 

The existence of a carbon atom bound to four different groups is not the strict condition for optical activity. The essential point is that the molecule should be asymmetric. Inorganic octahedral complexes, for example, can show optical isomerism. It is also possible for a molecule to contain asymmetric carbon atoms and still have a plane of symmetry. One structure of tartaric acid has two parts of the molecule that are mirror images, thus having a plane of symmetry. This (called the meso-form) is not optically active. See also resolution). 

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