Trinuclear Cage-Like ZnII Macrocyclic Complexes: Enantiomeric Recognition and Gas Adsorption Properties

Three zinc(II) ions in combination with two units of enantiopure [3+3] triphenolic Schiff-base macrocycles 1, 2, 3, or 4 form cage-like chiral complexes. The formation of these complexes is accompanied by the enantioselective self-recognition of chiral macrocyclic units. The X-ray crystal structures of these trinuclear complexes show hollow metal-organic molecules. In some crystal forms, these barrel-shaped complexes are arranged in a window-to-window fashion, which results in the formation of 1D channels and a combination of both intrinsic and extrinsic porosity. The microporous nature of the [Zn(3)1(2)] complex is reflected in its N-2, Ar, H-2, and CO2 adsorption properties. The N-2 and Ar adsorption isotherms show pressure-gating behavior, which is without precedent for any noncovalent porous material. A comparison of the structures of the [Zn(3)1(2)] and [Zn(3)3(2)] complexes with that of the free macrocycle H(3)1 reveals a striking structural similarity. In H(3)1, two macrocyclic units are stitched together by hydrogen bonds to form a cage very similar to that formed by two macrocyclic units stitched together by Zn-II ions. This structural similarity is manifested also by the gas adsorption properties of the free H(3)1 macrocycle. Recrystallization of [Zn(3)1(2)] in the presence of racemic 2-butanol resulted in the enantioselective binding of (S)-2-butanol inside the cage through the coordination to one of the Zn-II ions.