Enantiomeric resolution and X-ray optical activity of a tricobalt extended metal atom chain.

A simple procedure based on anion exchange was employed for the enantiomeric resolution of the extended metal atom chain (EMAC) [Co(dpa)(MeCN)]. Use of the chiral salt (NBu)[As(tartrate)], (Λ- or Δ-), resulted in the selective crystallization of the EMAC enantiomers as Δ-Co(dpa)(MeCN)[Λ-As(tartarte)], (Δ-) and Λ-Co(dpa)(MeCN)[Δ-As(tartrate)] (Λ-), respectively, in the 422 space group, whereas a racemic mixture of yielded [Co(dpa)(MeCN)][As(tartrate)]·2MeCN (-), which crystallized in the 2/ space group. The local electronic and magnetic structure of the EMAC enantiomers was studied, exploiting a variety of dichroisms in single crystals. A strong linear dichroism at the Co K-edge was observed in the orthoaxial configuration, whereas it vanished in the axial orientation, thus spectroscopically confirming the crystal symmetry. Compounds Δ- and Λ- are shown to be enantiopure materials as evidenced by mirror-image natural circular dichroism spectra in the UV/vis in solution and in the X-ray range at the Co K-edge in single crystals. The surprising absence of detectable X-ray magnetic circular dichroism or X-ray magnetochiral dichroism signals at the Co K-edge, even at low temperature (3 K) and a high magnetic field (17 T), is ascribed to a strongly delocalized spin density on the tricobalt core.