Stereoselective interaction of chiral metal complexes in solution as studied by chromatography. I. Modes of chiral discrimination and optical resolution of anion complexes.

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Chromatographic resolution of some anion complexes, [Co(ox)2(en)]−, C1- and C2-cis(N)-[Co(ox)(gly)2]−, [Co(ox)2(gly)]2−, and [Co(ox)2(acac)]2− (ox=oxalate ion, en=ethylenediamine, gly=glycinate ion, and acac=acetylacetonate ion), was attempted on an ion-exchange column using as chiral selectors several optically active cation complexes such as [Co(en)3]3+, [Co(sep)]3+, [Co(pn)3]3+, [Co(chxn)3]3+, [Co(acac)(en)2]2+, and [Co(gly)(en)2]2+ (sep=1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]icosane, pn=propylenediamine, and chxn=trans-1,2-cyclohexanediamine). The elution orders and degrees of optical resolution attained were interpreted on the basis that chiral discrimination is effected along either the C3 (or pseudo C3) axis or the C2 (or pseudo C2) axis of the respective complexes; the cation complex faces the anion complex with its N–H protons hydrogen-bonded to the coordinated oxygen atoms of the anion complex along the (pseudo) C3 or C2 axis. Plausible association models were proposed, which indicate that homochiral (i.e., Δ-Δ or Λ-Λ) and heterochiral (i.e., Δ-Λ or Λ-Δ) combinations are favored when the cation complex directs its C3 and C2 axes to the anion complex, respectively, regardless of which axis (C3 or C2) of the anion complex is directed to the cation complex. Most of the experimental data are consistently explained within the framework of the association models proposed.
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