Anion and pH induced spontaneous resolution of Δ- and Λ-[M(H2Biim)3]SO4 (M = Ru2+, Co2+, Ni2+, Mn2+, Fe2+, and Zn2+) enantiomers

Abstract

The homochiral self-assembly of [M(H₂biim)₃]SO₄ (M²⁺ = Ru, 1; Co, 2; Ni, 3; Mn, 4; Fe, 5; and Zn, 6; H₂biim = 2,2′-biimidazole) complexes have been systematically observed. For complexes 1 and 2, the spontaneous resolution processes not only depend on the counter anion but are also impacted by the pH value of the reaction solution. The enantiomers Δ-1 and Λ-1, and Δ-2 and Λ-2 were isolated in acidic conditions, while rac-1 and rac-2·5H₂O were obtained in neutral conditions, respectively. For complexes 3–6, however, the spontaneous resolution processes are independent on the pH value of the reaction solution. Single crystal X-ray diffraction analysis reveals the enantiomers Δ and Λ are homochiral (10,3)-b three-dimensional networks constructed by the hydrogen bonds between the sulfate and H₂biim ligand, whereas rac-1 assembles into (6,3) two-dimensional layers (see CrystEngComm, 2009, 11, 1114–1121) and rac-2 into one-dimensional chains. The data observed here seem to suggest that the hydrogen bonding modes between the sulfate and H₂biim ligand play a crucial role in spontaneous resolution of the enantiomers. Furthermore, the solid state CD spectra and powder XRD patterns were also used to diagnose the obtained complexes.