A Calix[6]arene Receptor Rigidified by a Self-assembled Triammonium Cap: X-ray and NMR Characterization of the Binding of Polar Neutral Guests

Abstract

A calix[6]arene has been rigidified by three primary ammonium arms self-assembled with the counter anions. This supramolecular edifice provides a well-defined conic hydrophobic cavity closed at the narrow rim by a tricationic site. X-ray and NMR analyses show that the resulting polarized host behaves as a remarkable endo-receptor for small molecules such as amides, alcohols and even nitriles. This study highlights the efficiency of a system that associates a cationic site with a hydrophobic cavity to host dipolar neutral molecules.

Keywords:

• Host-guest
• Cationic receptor
• Molecular recognition
• Calix[6]arene
• Self-assembly

Acknowledgements

Part of this work was supported by the Région Haute-Normandie. We thank Quentin Benard for microscopy analyses of the single crystals and the students of the MST CIC RSA1 2003–04 class for the preparation of the 1,3,5-tris-O-methylated calix[6]arene.

Notes

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Corresponding authors. Email: [email protected]; [email protected]

The ¹H NMR spectrum of 2 in DMSO-d ₆ showed an upfield shifted resonance for the methoxy groups at 2.04 ppm and the two ArH and tBu signals displayed large Δδ shifts (0.75 ppm and 0.60 ppm, respectively).

In DMSO-d ₆, the conformation adopted by 1 is the same as in CDCl₃ as shown by the upfield shifted methoxy resonances (2.20 ppm).

Considering a distance of ca. 4 Å between a tricationic center (q=+3e) and a fixed dipole with a dipolar moment μ=3.8 D (3.82 D for DMF or 3.92 D for MeCN) in chloroform (ϵ=4.8), the calculated electrostatic interaction is: W = - 1/(4 \pi \varepsilon _{0}) \times \vert q \vert \mu / \varepsilon d^{2} = - 2.89 \hspace{0.167em} \times \hspace{0.167em} 3 \times 3.8/(4.8 \hspace{0.167em} \times \hspace{0.167em} 0.4^{2}) \approx - 40 \hspace{0.167em} kJ \hspace{0.167em} mol^{ - 1}.

The corresponding guest resonances were: DMF (at 216 K): {\rm \delta _{CH_{3}} = - 0.15 \hspace{0.167em} ppm} and −0.53 ppm and {\rm \delta _{HCO} = 6.79 \hspace{0.167em} ppm;} AcNH₂ (at 223 K): −0.74; EtOH (at 223 K): {\rm \delta _{CH_{3}} = - 1.77 \hspace{0.167em} ppm} and δ_CH2O=undetected (signals were overlapped by those of the tBu groups); PrOH (at 213 K): {\rm \delta _{CH_{3}} = - 2.23 \hspace{0.167em} ppm,} {\rm \delta _{CH_{2}} = - 1.50 \hspace{0.167em} ppm} and δ_CH2O=undetected (signals were overlapped by those of the tBu groups); CH₃CH₂CONH₂ (at 263 K): {\rm \delta _{CH_{3}} = - 1.85 \hspace{0.167em} ppm} and {\rm \delta _{CH_{2}} = 0.22 \hspace{0.167em} ppm;} ClCH₂CONH₂ (at 263 K): {\rm \delta _{CH_{2}} = 2.00 \hspace{0.167em} ppm;} CH₃CN (at 223 K): {\rm \delta _{CH_{3}} = - 1.29 \hspace{0.167em} ppm.}

The resonances for the included (±)-propane-1,2-diol (at 220 K) were: {\rm \delta _{CH_{3}} = - 2.22 \hspace{0.167em} ppm,} {\rm \delta _{CH_{2}O} = 2.03 \hspace{0.5em} and \hspace{0.5em} 2.20 \hspace{0.167em} ppm,} and δ_CHOH=undetected (signals were overlapped by those of the tBu groups).

The addition of H₃PO₄ to 1 in CDCl₃ led to the precipitation of the salt.