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. 2015 Jan 15;119(2):383-94.
doi: 10.1021/jp511040p. Epub 2015 Jan 6.

Experimental and theoretical studies on halide binding with a p-xylyl-based azamacrocycle

Lucky Ahmed  1 , Md Mhahabubur Rhaman, John S Mendy, Jing Wang, Frank R Fronczek, Douglas R Powell, Jerzy Leszczynski, Md Alamgir Hossain
Affiliations

Experimental and theoretical studies on halide binding with a p-xylyl-based azamacrocycle

Lucky Ahmed et al. J Phys Chem A. .

Abstract

A p-xylyl-based macrocycle L has been synthesized and its binding properties with halides have been investigated by (1)H NMR titrations, single crystal X-ray diffraction analysis, and density functional theory (DFT) calculations. As investigated by (1)H NMR titrations, the ligand preferentially binds a halide in a 1:2 binding mode, with the association constants (in log K2) of 2.82, 2.70, 2.28, and 2.20 for fluoride, chloride, bromide, and iodide, respectively. The overall binding trend was found to be in the order of fluoride > chloride > bromide > iodide, reflecting that the binding strength correlates with the relative basicity and size of the respective halide. Crystallographic studies indicate that the ligand forms 1:2 complexes with chloride, bromide and iodide. In the chloride complex, the ligand is hexaprotonated and each chloride is held via three NH···Cl(-) bonds. The ligand is tetraprotonated for the other complexes, where each halide is H-bonded to two secondary ammonium NH(+) groups via NH···X(-) bonds. The results of DFT calculations performed on [H6L](6+) at M062x/6-311G (d,p) level in both gas and solvent phases, suggest that the ligand binds halides with the binding energy in the order of F(-) > Cl(-) > Br(-) > I(-), supporting the experimental data obtained from (1)H NMR studies. Results from DFT calculations further indicate that a 1:2 binding is energetically more favorable than a 1:1 binding of the ligand.

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Figures

Scheme 1
Scheme 1. Chemical Structures of L and [H6L]6+
Figure 1
Figure 1
Partial 1H NMR spectra of H6L(TsO)6 in the presence of 5 equiv of various halides in D2O at pH = 2.1.
Figure 2
Figure 2
1H NMR titrations of H6L(Ts)6 (2 mM) with the increasing amount of NaF (R = [NaF]0/[ligand]0) in D2O at pH = 2.1.
Figure 3
Figure 3
Change in the chemical shifts of NCH3 (H1) and CH3NCH2 (H2) against the increasing ratio of NaF in D2O at pH = 2.1.
Figure 4
Figure 4
Crystal structure of the chloride complex, 1: (A) perspective side view of [H6L(Cl2)]Cl4 showing atom labeling on N and Cl; (B) perspective view down the two central amines and (C) space filling model of [H6L(Cl2)]4+ (water molecules are omitted for clarity).
Figure 5
Figure 5
Crystal structure of the bromide complex, 2: (A) perspective side view of [H4L(Br2)]Br2 showing atom labeling on N and Br; (B) perspective view showing three coordinate bromide between two macrocycles; (C) space filling model showing two bromide between two macrocycles.
Figure 6
Figure 6
Crystal structure of the iodide complex, 3: (A) perspective side view of [H4L(I2)(CH3OH)2]I2 showing atom labeling on N, I and O; (B) perspective view down the two aromatic units and (C) space filling model of [H4L(I2)(CH3OH)2]I2.
Figure 7
Figure 7
(A) Structure of the azamacrocycle, [H6L]6+; (B) electrostatic potential map for [H6L]6+ calculated at the M06-2X/6-311G(d,p) level of theory (red = less positive potential, blue = more positive potential).
Figure 8
Figure 8
Optimized structures of 1:1 complexes showing hydrogen bonds in [H6L(F)]5+ (A), in [H6L(Cl)]5+ (B), in [H6L(Br)]5+ (C), and [H6L(I)]5+ (D) in the gas phase at the M06-2X/6-311G(d,p) level.
Figure 9
Figure 9
Optimized structures of 1:2 complexes showing hydrogen bonds in [H6L(F)2]4+ (A), in [H6L(Cl)2]4+ (B), in [H6L(Br)2]4+ (C), and [H6L(I)2]4+ (D) in the gas phase at M06-2X/6-311G(d,p) level.
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