Chiral aggregation and spontaneous resolution of thiosemicarbazone metal complexes

Chiral aggregation and spontaneous resolution of thiosemicarbazone metal complexes MnL2 (1 and 1') (HL = acetylpyrazine thiosemicarbazone) were achieved through cooperation of hydrogen bonding and pi-pi stacking interactions. Compound 1 crystallized in a chiral space group P4(1)2(1)2 and the molecules exhibited lambda-configuration. Head-to-tail pyrazine-amino hydrogen bonding linked the molecules together presenting a two-dimensional homochiral sheet. The interlayer pi-pi stacking interactions linked the layers in a homochiral array and extended the 2D homochirality to a three-dimensional network. Compound 1 crystallized in space group P4(3)2(1)2, the opposite handedness of P4(1)2(1)2. The molecules, the two-dimensional hydrogen-bonded sheets and the three-dimensional network all exhibited the opposite chiralities to those of 1. Solid state CD spectra measurements confirmed the occurrence of spontaneous resolution of MnL2 and demonstrated that one of the enantiomers of the compound was always in excess. The zinc complexes (2 and 3) and nickel complex 4 crystallized in the chiral space group P4(3)2(1)2. Similar two-dimensional pyrazine-amine hydrogen bonded chiral sheets and three-dimensional networks were found in the crystal packing patterns. The cadmium complex 5 crystallized in a chiral space group P2(1)2(1)2(1), however, it exhibited a similar chiral packing pattern. The only difference was that enlarged Cd-S and Cd-N bond lengths induced the potential 4(1) symmetry. All results indicated that the supramolecular synthon, the cooperation of the pyrazine-amino hydrogen bonding and the pi-pi stacking interactions, were robust enough so that the orientation of the molecules in the solid state could be predicted to a reasonable degree of accuracy.