Structure and ion-complexing properties of an aza-15-crown-5 ether dye: synthesis, crystallography, NMR spectroscopy, spectrophotometry and potentiometry

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Abstract
The perchlorate of 2-{2-[4-(13-aza-1,4,7,10-tetroxa-13-cyclopentadecyl)phenyl]ethenyl}-3-ethylbenzothiazolium (3) has been synthesised and its structure determined in the crystalline state by X-ray diffraction and in solution in acetonitrile by 1H NMR methods. Complexation with barium and silver cations in solution has been studied in comparison with other model azacrown molecules by spectrophotometric and potentiometric methods. With the exception of the puckered azacrown moiety, the structure of 3 is remarkably planar, consistent with extensive π-conjugation throughout the remainder of the molecule in both solid and solution states. Compound 3 forms a stable 1∶1 complex with Ba2+ ions in acetonitrile, with a stability constant of K1 = 80 ± 10 dm3 mol–1; the complexation results in major changes in the electronic absorption spectrum of 3, consistent with binding to the azacrown moiety. A model compound, phenylaza-15-crown-5 (2a), was found to bind Ba2+ to form both a 1∶1 ligand–metal complex (LM), with a stability constant of K1 = (2.0 ± 0.2) × 104 dm3 mol–1, and a 2∶1 (L2M) complex, with a stability constant of K2 = 220 ± 20 dm3 mol–1 for binding of the second ligand. The electronic absorption and NMR spectra both indicate binding of the ‘hard’ Ba2+ cation to the azacrown oxygen atoms. However, by contrast, the evidence from NMR data shows that the ‘soft’ Ag+ cation complexes with 2a mainly through a strong interaction with the azacrown nitrogen atom, giving a 1∶1 complex with a stability constant of K1 = 6 ± 1 dm3 mol–1, determined by potentiometry. No complexation of 3 with Ag+ is observed. Studies of the simple aza-15-crown-5 ether reveal strong complexation with both Ba2+ (K1 × K2 > 1012 dm6 mol–2) and Ag+ [K1 = (1.15 ± 0.10) × 104 dm3 mol–1 and K2 = 450 ± 20 dm3 mol–1].

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