Ethylenediamine, complexes with resolution

Fanali, S., Ossicini, L., Foret, F., and Bocek, R, Resolution of optical isomers by capillary zone electrophoresis study of enantiomeric and distereoisomeric cobalt (III) complexes with ethylenediamine and amino acid ligands, /. Microcol. Sep., 1, 190, 1989. 

Bridged hydroxy complexes (208) of the bisoxalate type have been known for many years. Both a trihydrate and an anhydrous form are known. The trihydrate is isomorphous with the corresponding cobalt(III) complex (Durrant s salt) and the hydrous complex is slowly converted to the hydrate on exposure to moisture. Only a preliminary crystallographic characterization has appeared " it crystallizes as a meso rather than racemic complex. IR and magnetic studies are in accord with the hydroxy-bridged structure. Despite the crystallographic meso structure, the resolution of the complexes has been reported either photochemically or with cis- — )bis(ethylenediamine)dinitrocobalt(III) and leads to optically active complexes with spectra similar to those of the corresponding tris oxalate. 

The first resolution of an octahedral complex into its enantiomers was achieved in 1911 by A. Werner, who got the Nobel Prize in 1913, with the complex [Co(ethylenediamine)(Cl)(NH3)] [10]. Obviously, resolution is to be considered only in the case of kinetically inert complexes whose enantiomers do not racemize quickly after separation. This is a very important remark since, as noted above, the interesting complexes are those containing exchangeable sites required for catalytic activity and thus more sensitive to racemization. We will not discuss here the very rare cases of spontaneous resolution during which a racemic mixture of complexes forms a conglomerate (the A and A enantiomers crystallize in separate crystals) [11,12]. 

Note that these compounds are not enantiomers, but true diastereomers with different properties, and they may be separated by fractional crystallization. The asymmetric carbon atom has an 5 configuration in both diastereomers, but the chirality about the molybdenum atom is different. Thus the asymmetric carbon aids in the resolution of the molybdenum center, but its presence is not necessary for the complex to be chiral. It is merely necessary for the Schiff base to be unsymmetric, i.e., have one pyridine nitrogen and one imino nitrogen. If the bkJentate ligand had been ethylenediamine, bipyridine, or the oxalate ion, there would have been a mirror plane and no duality at the molybdenum. 

Tris (ethylenediamine) cobalt (III) chloride was first prepared by Werner.1 Resolution was effected through the chloride d-tartrate which was obtained by allowing the chloride (1 mol) to react with silver d-tartrate (1 mol). The correct ratio of chloride ion to tartrate ion is important and this has meant that it was necessary to isolate the pure solid chloride, the synthesis of which has been described by Work.2 In the present method the less soluble diastereo-isomer is isolated directly and the expensive and unstable silver d-tartrate is replaced by barium d-tartrate. The addition of activated carbon ensures rapid oxidation of the initial cobalt (II) complex and eliminates small amounts of by-products of the reaction. 

The tris(ethylenediamine) chromium (III) ion was first resolved by Werner6 by means of sodium 3-nitro-(+)-camphor. What has been said concerning the resolution of the corresponding rhodium compound holds true of the chromium compound, except that for the chromium compound the solubility difference of the diastereoisomeric chloride (+)-tartrates is so small that a resolution via these diastereoisomers has not been achieved.5,6 The method reported here is essentially the same as the one described for the rhodium complex but with minor alterations... 

The first successful resolution of coordination compounds was reported by Werner in 1911 and involved two series of complexes, the cw-chloroammine-bis(ethylenediamine)cobalt(III) series (with V. L. King) and the m-bromo-amminebis(ethylenediamine)cobalt(III) series (with E. Scholze). Of the two, the bromoammine series is easier to resolve because of the greater difference in solubility between the diastereoisomers that are formed with the resolving agent, silver ( + )-octahedral configuration first postulated by Werner for cobalt(III) in 1893. 

Perhaps Wemer s tour de force in the area of polynuclear cobalt(III) complexes was his successful resolution of the hexoP ion (VIII). He prepared this ion and its ethylenediamine analog by heating a basic solution of m-diaquotetramminecobalt(III) sulfate and deduced its structure by observing that the acid cleavage gave back the cis-diaquo complex and cobaltous ion (37). He later resolved the ion into its optical antipodes with d-camphor sulfonic acid and d-bromocamphor sulfonic acid and obtained the D- and l- forms as the bromides (35). His successful resolution... 

---