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) [Co3(dpa)4(MeCN)2]2+. Use of the chiral salt (NBu4)2[As2(tartrate)2], (Λ-1 or Δ-1), resulted in the selective crystallization of the EMAC enantiomers as [Δ-Co3(dpa)4(MeCN)2](NBu4)2[Λ-As2(tartarte)2]2, (Δ-2) and [Λ-Co3(dpa)4(MeCN)2](NBu4)2[Δ-As2(tartrate)2]2 (Λ-2), respectively, in the P4212 space group, whereas a racemic mixture of 1 yielded [Co3(dpa)4(MeCN)2][As2(tartrate)2]·2MeCN (rac-3), which crystallized in the C2/c 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 D4 crystal symmetry. Compounds Δ-2 and Λ-2 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.