Octaazacryptand Complexation of the Fluoride Ion

Abstract

The fundamental properties of proton, fluoride, and chloride binding of the octaazacryptand L have been investigated by potentiometry and NMR spectroscopy. Successive protonation constants of L are 11.18(16), 9.43(08), 7.59-(07), 5.78(02), and 4.39(33), with the last value corresponding to a two-proton step. 'H NMR spectra at 250 MHz indicate that proton exchange is slow on the NMR time scale in the pH region where the two-proton step occurs. It is proposed that this results from disruption of an intemal hydrogen bond network and heightened electrostatic repulsions present in tetraprotonated L. NMR data also suggest that protonation occurs predominantly at the secondary amines over the pH range studied. Triprotonated L binds fluoride significantly (log K = 3.6(4)) near neutral pH, and further protonation of L increases the fluoride complex stability (log K = 11.2(5) for the hexaprotonated fluoride complex). 'H NMR data show that the rate of fluoride exchange decreases with solution pH. The magnitude of the aqueous fluoride binding constant, the substantial NMR shifts, and the published solid state structure suggest fluoride is bound inside the cavity of protonated L. MM2 calculations predict the strain energy of L increases by 10.2 kcal/mol when the expected nitrogen-halide hydrogen bond length increases from that of fluoride to that of chloride. Size selectivity of L is proposed to explain the fact that fluoride is bound over lo7 times stronger than chloride.

Anion Cryptates: Synthesis, Crystal Structures, and Complexation Constants of Fluoride and Chloride Inclusion Complexes of Polyammonium Macrobicyclic Ligands

“…Similar results were obtained in an independent work [32]. Moreover, 2.6H' presents also a high F-/Cl-selectivity; the stability constant for F-is, nevertheless, somewhat smaller than for 1'6H+, since only one H-atom is now available on each N-atom for interacting with F-.…”

“…1 and 2), which are listed in Table 1. For ligand 1, a cumulated value has been obtained for the last two protonations, corresponding to a two-proton uptake (see also [32] [33]).…”

Halide Anion Recognition in Water by an Hexaprotonated Octaaza-Cryptand: A Molecular Dynamics Investigation

“…1.9 Â 10 À2 mol L À1 , about ten times more concentrated than in the complexation experiments reported by Lehn et al [18] or Smith et al [20] . After 4000 steps of energy minimization with conjugate gradients, 20 ps of MD were performed keeping the solute rigid (BELLY option of AMBER), [25] allowing for water relaxation.…”

“…[15,16] Halide anion complexation by a neutral calixarene in organic solution has recently been reported. [17] This paper deals with the halide anion complexation by an hexaprotonated bicyclic octaaza-cryptand (Figure 1), studied experimentally by Lehn et al [18,19] This ligand, referred to later as L 6 , forms in acidic aqueous solution, a complex with F À of high stability (logK 10.55, according to Lehn et al [18] and 11.2 acording to Smith et al [20] ) and displays a spectacular F À /Cl À selectivity (> 10 8 ). The inclusive nature of the F À complex, noted hereafter as LF 5 , is supported by NMR spectroscopy in solution, and by an X-ray structure, [18] where the six ammonium N sites form a quasi trigonal prismatic arrangement with F À´´´N distances ranging from 2.76 to Figure 1.…”

Aspects of Anion Coordination from Historical Perspectives

“…Unlike in O-BISTREN (11), fluoride accepts hydrogen bonds from all six secondary ammonium groups of the cage 15. Solution studies carried out by Hay, Smith et al in 1995 using potentiometric techniques led to a surprisingly large value of 11.2 logarithmic units for the interaction of the hexaprotonated macrocycle with fluoride anion measured in 0.1 M KNO 3 [124]. The reported F − /Cl − selectivity at pH = 5.9 expressed as log K s (F − complex)/log K s (Cl − complex) was also an exceptionally high 7.5.…”