Supramolecular chemistry controlled by packing interactions during structure-direction of nanoporous materials: Effect of the addition of methyl groups on ephedrine derivatives

Abstract

A combined experimental and computational study has been applied in order to understand the effect of the addition of subsequent methyl groups to chiral (1R,2S)-(-)-ephedrine on its structure-directing behavior during crystallization of the nanoporous 3 AFI aluminophosphate. Results show that bare ephedrine and the double-methylated N,N-dimethyl-ephedrinium display the most efficient structure-directing abilities, while the mono-methylated derivative shows a poor structure-directing capacity towards this framework. Fluorescence spectroscopy indicates that confinement of the mono-and dimethylated derivatives within the nanopores of the AFI channels involves a preferential incorporation of the SDA molecules as monomers. In contrast, occlusion of the bare ephedrine can lead to a strong supramolecular aggregation, depending on the synthesis conditions. Molecular simulations suggest that the stronger trend of ephedrine to form supramolecular aggregates when confined within the AFI channels is due to the formation of two intermolecular H-bonds between consecutive molecules in the dimers interface, which balances the strong electrostatic repulsion between closely-located positive charges of the N atoms under this supramolecular arrangement. The addition of subsequent methyl groups reduces the possibility of establishing these interdimer Hbond interactions that stabilize dimers while provoking steric repulsions, thus reducing the supramolecular aggregation.

Conformational Space of (1R,2S)-Dimethyl-Ephedrinium and (1S,2S)-Dimethyl-Pseudoephedrinium in the Synthesis of Nanoporous Aluminophosphates

“…If we take a close inspection to the resonance corresponding to C1of DMEP confined within the AFI framework (as labelled inFigure 1), we observe two clearly distinguishable bands at 7.7 and 14.7 ppm, one of them close to the resonance of this C atom in solution (at 6.0 ppm). As we previously reported,35 the two bands show up in all the MgAPO-5 materials prepared under very different synthesis conditions (Figure S2), with a similar intensity. When the crystallization temperature was increased, some degradation products start to show up, giving resonances at 45-49 ppm and 58 ppm(Figure 1), which are not present at 140…”

“…Synthesis of the quaternary cation (1R,2S)-dimethyl-ephedrinium hydroxide (DMEP) has been described in our previous work. 35 Synthesis of (1S,2S)-dimethyl-pseudoephedrinium (DMPS) was carried out similarly; details are given in the Supporting Information. were used as sources of Al, P, Mg, Zn, Co and Si, respectively.…”

ICP-2: A New Hybrid Organo-Inorganic Ferrierite Precursor with Expanded Layers Stabilized by π–π Stacking Interactions

“…following the same procedure as reported in our previous work. 30 The iodide salt was further transformed into the corresponding hydroxide (DMEPOH) through an anionic resin (Amberlite IRN-78, Aldrich), and concentrated up to 24.0 wt %. A similar procedure was followed for the preparation of the diastereoisomer (…”

“…26 We also prepared the methyl-and dimethyl-derivatives of (1R,2S)-ephedrine, and they led to the crystallization of the AFI framework, but with a notably poorer π-π-driven supramolecular chemistry. 30 Due to the interest in the structure-direction of zeolite materials with these chiral molecules, we have now extended the study of the structure-directing behavior of (1R,2S)-dimethylephedrinium and its diastereoisomer (1S,2S)-dimethylpseudoephedrinium cations to the synthesis of aluminosilicate-based zeolites (see Scheme 1). Due to the limited stability of this cation, the synthesis has been performed in fluoride medium in order to avoid a basic pH that would enhance the degradation of the cation during hydrothermal crystallization.…”

“…Fluorescence spectroscopy was used to analyze the supramolecular aggregation state of DMEP cations in ICP-2 (Figure 3). Phenyl ring gives a fluorescence band with a vibronic structure centered at 282 nm when the molecules are isolated (monomers).Supramolecular aggregation of phenyl-containing species through π-π stacking of their aromatic rings provokes a reorganization of the electronic levels which results in a strong red-shift of the fluorescence band 30,36. The fluorescence spectrum of ICP-2 showed the main fluorescence band at 360 nm, evidencing that the DMEP cations are involved in supramolecular aggregates through π-π stacking.…”