Cobalt(III) Complexes of a Stereospecific Ligand Derived from (S)-Proline

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Abstract
The quadridentate ligand, (2S,2'S)-1,1'-(ethane-1,2-diyl)bis(pyrrolidine-2-carboxylic acid) dihydrochloride (H2pren,2HCl), has been derived from (S)-proline. The preparations of the cobalt(III) complexes Na [Co(pren)(CO3)],3H2O, [CO(pren)(H2O)2] ClO4,- 2H2O and [Co(pren)(H2O)Cl],1·5H2O are described. All exist as cis-α geometrical isomers. 13C N.M.R. and absorption spectral measurements confirm the cis-α arrangement. The complex cis-α- [Co(pren)(H2O)Cl],1·5H2O rapidly aquates with retention of configuration. The compounds having actual or near C2 symmetry exhibit a strong negative c.d. band under the 1A1g → 1T1g cubic absorption band which is due to the 1A1 →1E tetragonal component, consistent with the Δ absolute configuration imposed by the (S) configuration of the asymmetric carbon atom in the (S)-proline moiety. The o.r.d. spectrum of cis-α-Na [Co(pren)CO3],3H2O in aqueous solution has a negative Cotton effect at low energy as expected for the Δ absolute configuration.

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