Chiral recognition and conglomerate crystallization induced by self-organization of cobalt(III) complexes of a tripodal ligand containing three imidazole groups

The effect of a counteranion on chiral recognition inducing conglomerate crystallization of a cobalt(III) complex is reported. An achiral tripodal ligand involving three imidazole groups, tris{[2-{(imidazol-4-yl)methylidene}amino]- ethyl}amine (H₃L), was prepared by condensation of tris(2-aminoethyl)amine and 4-formylimidazole in a 1:3 mole ratio. The reaction of H₃L and trans-[Co^IIICl₂(py) ₄]⁺ afforded the chiral (Δ or Λ) [Co ^III(H₃L)]³⁺ complex. The formally hemideprotonated complexes [Co^III(H_1.5L)]X _1.5·nH₂O (where X = Cl, Br, I, BF₄, ClO₄, or PF₆) were synthesized by controlled deprotonation of the uncoordinated imidazole NH groups of [Co(H₃L)³⁺. In crystals of the hemideprotonated complex, two components, [Co(H ₃L)]³⁺ and [Co(L)], with the same absolute configuration are linked by imidazole-imidazolate hydrogen bonds to form an extended homochiral 2D sheet structure, which is composed of a hexanuclear unit with a trigonal void. There are two ways of stacking the sheets: One is via homochiral stacking, and the other is via heterochiral stacking. When the size of the counterion is small (i.e., X = Cl, Br, I, or BF₄), adjacent homochiral sheets with the same chirality are stacked to form a homochiral crystal (conglomerate). With large anions (i.e., ClO₄^- and PF₆^-), a homochiral sheet consisting of Δ enantiomers and a sheet consisting of Λ enantiomers are stacked alternately to give a heterochiral crystal (a racemic crystal). Optically active Λ-[Co(H_1.5L)](ClO₄)_1.5·H ₂O was synthesized from Λ-[Co(H₃L)]³⁺, and the crystal structure was compared to that of the racemic complex. There are two conflicting factors governing the crystal structure: the skeletal density; the size of the channels. The 2D sheets are more closely packed in the homochiral crystal than in the heterochiral crystal. However, the channels, where the counterions are accommodated, are smaller in the homochiral crystal, and the steric congestion between the anions increases with increasing anion size. The heterochiral crystal has a flexible, zigzag channel structure, and the size of the channels can increase to accommodate larger anions. Thus, complexes with large anions (i.e., ClO₄^- and PF₆ ^-) preferentially form heterochiral crystals rather than homochiral crystals.