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

Abstract : 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 (NBu4)(2)[As-2(tartrate)(2)], (Lambda-1 or Delta-1), resulted in the selective crystallization of the EMAC enantiomers as [Delta-Co-3(dpa)(4)(MeCN)(2)](NBu4)(2)[Lambda-As-2(tartarte)(2)](2), (Delta-2) and [Lambda-Co-3(dpa)(4)(MeCN)(2)](NBu4)(2)[Delta-As-2(tartrate)(2)](2) (Lambda-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)]center dot 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 Delta-2 and Lambda-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.
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Anandi Srinivasan, Miguel Cortijo, Vladimir Bulicanu, Ahmad Naïm, Rodolphe Clérac, et al.. Enantiomeric resolution and X-ray optical activity of a tricobalt extended metal atom chain. Chemical Science , The Royal Society of Chemistry, 2018, 9 (5), pp.1136-1143. ⟨10.1039/c7sc04131d⟩. ⟨hal-01714521⟩

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