Spontaneous Resolution Induced by Self-Organization of Chiral Self-Complementary Cobalt(III) Complexes with Achiral Tripod-Type Ligands Containing Three Imidazole Groups
2002-01-05T00:00:00Z (GMT) by
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(H<sub>3</sub>L<sup>6</sup>)](ClO<sub>4</sub>)<sub>3</sub>·H<sub>2</sub>O (<b>6</b>) and [Co(H<sub>3</sub>L<sup>7</sup>)](ClO<sub>4</sub>)<sub>3</sub>·0.5H<sub>2</sub>O (<b>7</b>) were synthesized, where H<sub>3</sub>L<sup>6</sup> = tris[2-(((imidazol-4-yl)methylidene)amino)ethyl]amine and H<sub>3</sub>L<sup>7</sup> = tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine. Each complex induces the chirality of clockwise (<i>C</i>) and anticlockwise (<i>A</i>) enantiomers due to the screw coordination arrangement of the achiral tripod-type ligand around the Co(III) ion. The fully protonated (<b>6</b>, <b>7</b>), the formally hemi-deprotonated (<b>6</b>‘, <b>7</b>‘), and the fully deprotonated (<b>6</b>‘ ‘, <b>7</b>‘ ‘) 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 (<b>6</b>, <b>7</b>). The fully deprotonated complexes (<b>6</b>‘ ‘, <b>7</b>‘ ‘) form a hydrogen-bonded network structure, in which the <i>C</i> and <i>A</i> enantiomers coexist and are connected through a water molecule. The formally hemi-deprotonated species [Co(H<sub>1.5</sub>L<sup>6 or 7</sup>)]<sup>1.5+</sup>, 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(H<sub>3</sub>L<sup>6 or 7</sup>)]<sup>3+</sup> and [Co(L<sup>6 or 7</sup>)]<sup>0</sup>, in which adjacent molecules with the same chirality are arrayed in an up-and-down fashion. In the crystal lattices of the perchlorate salts (<b>6</b>‘, <b>7</b>‘), the perchlorate ions are located in the cavity, and the homochiral layer consisting of <i>C</i> enantiomers and the adjacent layer consisting of <i>A</i> enantiomers are stacked alternately to give an achiral crystal. The chloride salt of the hemi-deprotonated complex [Co(H<sub>1.5</sub>L<sup>6</sup>)]Cl<sub>1.5</sub>·4H<sub>2</sub>O (<b>6a</b>‘) 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 <b>6a</b>‘ 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.