Enantiopure Chiral Coordination Polymers Based on Polynuclear Paddlewheel Helices and Arsenyl Tartrate.

Herein, we report the preparation of chiral, one-dimensional coordination polymers based on trinuclear paddlewheel helices [M₃(dpa)₄]2+ (M = Co(II) and Ni(II); dpa = the anion of 2,2'-dipyridylamine). Enantiomeric resolution of a racemic mixture of [M₃(dpa)₄]2+ complexes was achieved by chiral recognition of the respective enantiomer by [Δ-As₂(tartrate)₂]2- or [Λ-As₂(tartrate)₂]2- in N,N-dimethylformamide (DMF), affording crystalline coordination polymers formed from [(Δ-Co₃(dpa)₄)(Λ-As₂(tartrate)₂)]·3DMF (Δ-1), [(Λ-Co₃(dpa)₄)(Δ-As₂(tartrate)₂)]·3DMF (Λ-1), [(Δ-Ni₃(dpa)₄)(Λ-As₂(tartrate)₂)]·(4 - n)DMF∙nEt₂O (Δ-2) or [(Λ-Ni₃(dpa)₄)(Δ-As₂(tartrate)₂)]·(4 - n)DMF∙nEt₂O (Λ-2) repeating units. UV-visible circular dichroism spectra of the complexes in DMF solutions demonstrate the efficient isolation of optically active species. The helicoidal [M₃(dpa)₄]2+ units that were obtained display high stability towards racemization as shown by the absence of an evolution of the dichroic signals after several days at room temperature and only a small decrease of the signal after 3 h at 80 °C.