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(3)(dpa)(4)(MeCN)(2)](2+). Use of the chiral salt (NBu(4))(2)[As(2)(tartrate)(2)], (Λ-1 or Δ-1), resulted in the selective crystallization of the EMAC enantiomers as [Δ-Co(3)(dpa)(4)(MeCN)(2)](NBu(4))(2)[Λ-As(2)(tartarte)(2)](2), (Δ-2) and [Λ-Co(3)(dpa)(4)(MeCN)(2)](NBu(4))(2)[Δ-As(2)(tartrate)(2)](2) (Λ-2), respectively, in the P42(1)2 space group, whereas a racemic mixture of 1 yielded [Co(3)(dpa)(4)(MeCN)(2)][As(2)(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 D(4) 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.