Spontaneous Resolution Induced by Self-Organization of Chiral Self-Complementary Cobalt(III) Complexes with Achiral Tripod-Type Ligands Containing Three Imidazole Groups

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Abstract

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)](ClO4)3*H2O (6) and [Co(H3L7)](ClO4)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(H1.5L6 or 7)]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(H3L6 or 7)]3+ and [Co(L6 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(H1.5L6)]Cl1.5*H2O (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.

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