Spontaneous resolution induced by self-organization of chiral self-complementary cobalt(III) complexes with achiral tripod-type ligands containing three imidazole groups

The progression from synthetically achiral ligand and metal ion, to isolated chiral metal complex, to homochiral two-dimensional (2D) assembly layer, and finally to conglomerate is presented. The cobalt-(III) complexes of achiral tripod-type ligands involving three imidazole groups with the chemical formulas [Co(H₃L⁶)](CIO₄) ₃·H₂O (6) and [CO(H₃L⁷)] (CIO₄)₃·0.5H₂O (7) were synthesized, where H₃L⁶ = tris[2-(((imidazol-4-yl)methylidene)amino)ethyl]amine and H₃L⁷ = tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine. Each complex induces the chirality of clockwise (C) and anticlockwise (A) enantiomers due to the screw coordination arrangement of the achiral tripod-type ligand around the Co(III) ion. The fully protonated (6, 7), the formally hemi-deprotonated (6′, 7′), and the fully deprotonated (6″, 7″) complexes were obtained as good quality crystals by adjusting the pH of the solutions. The crystal structures were determined by single-crystal X-ray analyses. There is no intermolecular network structure in the fully protonated complexes (6, 7). The fully deprotonated complexes (6″, 7″) form a hydrogen-bonded network structure, in which the C and A enantiomers coexist and are connected through a water molecule. The formally hemi-deprotonated species [Co(H_1.5L^6or7)]^1.5+, which functions as a self-complementary chiral building block, generates equal numbers of protonated and deprotonated molecules by an acid-base reaction to form an extended 2D homochiral layer structure consisting of a hexanuclear structure with a trigonal void as a unit. The 2D structure arises from the intermolecular imidazole-imidazolate hydrogen bonds between [Co(H₃L^{6 or 7})]³⁺ and [Co(L^{6 or 7})]⁰, in which adjacent molecules with the same chirality are arrayed in an up-and-down fashion. In the crystal lattices of the perchlorate salts (6′, 7′), the perchlorate ions are located in the cavity, and the homochiral layer consisting of C enantiomers and the adjacent layer consisting of A enantiomers are stacked alternately to give an achiral crystal. The chloride salt of the hemi-deprotonated complex [Co(H_1.5L⁶)]Cl_1.5·4H₂O (6a′) is found to be a conglomerate, in which the chloride ions are positioned in the intermediate region of the double layer, and layers with the same chirality are well stacked by adopting the up-and-down layer's shape to generate channels, and so form a chiral crystal. The circular dichroism (CD) spectrum of 6a′ showed a positive peak and a negative peak at 480 and 350 nm, respectively, and the spectrum of another crystal showed an enantiomeric CD pattern, providing further evidence of spontaneous resolution on crystallization.