Molecular Pincers Using a Combination of N-H and C-H Donors for Anion Binding

Abstract

A naphthalene imide (1) and a naphthalene (2) bearing two pyrrole units have been synthesized, respectively, as anion receptors. It was revealed by ¹H NMR spectral studies carried out in CD₃CN that receptors 1 and 2 bind various anions via hydrogen bonds using both C-H and N-H donors. Compared with receptor 2, receptor 1 shows higher affinity for the test anions because of the enhanced acidity of its pyrrole NH and naphthalene CH hydrogens by the electron-withdrawing imide substituent. Molecular mechanics computations demonstrate that the receptors contact the halide anions via only one of the two respective available N-H and C-H donors whereas they use all four donors for binding of the oxyanions such as dihydrogen phosphate and hydrogen pyrophosphate. Receptor 1, a push-pull conjugated system, displays a strong fluorescence centered at 625 nm, while receptor 2 exhibits an emission with a maximum peak at 408 nm. In contrast, upon exposure of receptors 1 and 2 to the anions in question, their fluorescence was noticeably quenched particularly with relatively basic anions including F^-, H₂PO₄^-, HP₂O₇^3-, and HCO₃^-.

Keywords: anion receptor; association constant; fluorescence; hydrogen bond; molecular pincers; titration.

Figure 1

Molecular structures of receptors 1 and 2.

Scheme 1

Synthesis of receptors 1 and 2.

Figure 2

The lowest energy structures of receptors 1 and 2.

Figure 3

Optimized geometries of the complexes of receptors 1 and 2 with fluoride, chloride, dihydrogen phosphate, and hydrogen pyrophosphate as computed in the gas phase and the corresponding calculated binding energies (ΔE) for the interactions of receptors 1 and 2 with the anions.

Figure 4

Partial ¹H NMR spectra of (a) 1 (3 mM) only, (b) 1 + excess TBAF, (c) 1 + excess TBACl, (d) 1 + excess TBABr, (e) 1 + excess TBAI, (f) 1 + excess TBAHSO₄, (g) 1 + excess TBAH₂PO₄, (h) 1 + excess (TBA)₃HP₂O₇, and (i) 1 + excess TEAHCO₃ in CD₃CN.

Figure 5

Partial ¹H NMR spectra of (a) 2 (3 mM) only, (b) 2 + excess TBAF, (c) 2 + excess TBACl, (d) 2 + excess TBABr, (e) 2 + excess TBAI, (f) 2 + excess TBAHSO₄, (g) 2 + excess TBAH₂PO₄, (h) 2 + excess (TBA)₃HP₂O₇, and (i) 2 + excess TEAHCO₃ in CD₃CN.

Figure 6

Top: Proposed binding mode of receptor 1 for the fluoride anion. Bottom: Partial ¹H NMR spectra recorded during the titration of receptor 1 (3 mM) with TBAF in CD₃CN.

Figure 7

Job’s plots for the interaction of receptor 1 with TBAF (left) and TBAH₂PO₄ (right), respectively, in CD₃CN.

Figure 8

Top: Putative binding modes of receptor 1 for the dihydrogen phosphate anion. Bottom: Partial ¹H NMR spectra measured during the titration of receptor 1 (3 mM) with tetrabutylammonium dihydrogen phosphate (TBAH₂PO₄) in CD₃CN.

Figure 9

UV/Vis absorption and fluorescence spectra of 1 and 2 recorded in CH₃CN.

Figure 10

Top: Fluorescence spectra of receptors 1 (a) and 2 (b) in the absence and the presence of the indicated anions (as their respective TBA⁺ salts for all anions but the bicarbonate anion which was used in its TEA⁺ salt form) in CH₃CN. Bottom: Photographs showing the corresponding fluorescence changes of the CH₃CN solutions of receptors 1 (a) and 2 (b). Receptors 1 and 2 were excited at 448 nm and 307 nm, respectively.

Figure 11

Fluorescence spectral changes of receptor 1 (100 μM) observed during the titrations with (a) TBAF, (b) TBAH₂PO₄, (c) (TBA)₃HP₂O₇, and (d) TEAHCO₃ in CH₃CN. The excitation wavelength (λ_ex) was 448 nm. The black dotted arrows indicate the fluorescence spectra of receptor 1 in the presence of the indicated anion equivalents. The red T arrows indicate fluorescence spectral changes over the courses of titrations.