Helicoidal self-assembly of (1R,2S)-ephedrine within nanoporous aluminophosphates: transfer of chirality to spatial distribution of dopants

Abstract

In an attempt to produce chiral nanoporous catalytic materials, (1R,2S)-ephedrine (EPH) is used as structure-directing agent (SDA) for the synthesis of crystalline nanoporous aluminophosphates. Experiments showed that this molecule tends to drive the crystallization of the AFI aluminophosphate framework in the presence of dopants (Mg, Co, Zn or Mn). The selection of EPH as SDA was based on i) its self-assembly behaviour, as revealed by UV-Vis fluorescence spectroscopy, which drives EPH to form dimers through stacking of the aromatic rings, ii) the presence of two asymmetric atoms, which promote a helicoidal arrangement of the dimers within the one-dimensional pores of the AFI framework, where consecutive dimers are always rotated by an angle of +30 deg, as evidenced by molecular-mechanics calculations, and iii) the presence of NH2 + groups, where the positively-charged H atoms will tend to direct the spatial distribution of the negative charge associated to the isomorphic substitution of divalent (2+) dopants by Al3+ through the formation of strong H-bonds. A multilevel experimental and computational study shows that the helicoidallyarranged H-bonded-to-N atoms of EPH (white balls in Fig. 1) locate close to the framework walls and thus a strong electrostatic interaction between the NH2 + groups and the negative charge associated to the dopant incorporation is established (dashed lines), promoting a transfer of the chirality from the helicoidal assembly of EPH dimers to a double-helix distribution of the dopants (green and blue balls in Fig. 1).