New insights into metal ion–crown ether complexes revealed by SEIRA spectroscopy

Abstract

We demonstrate a powerful spectroscopic technique, surface-enhanced infrared absorption (SEIRA) spectroscopy, not only for detecting host–guest complexes in solution but also for examining the relationship between the guest selectivity, complex structure, and solvent effect. We synthesize thiol derivatives of 15-crown-5 and 18-crown-6 [2-(6-mercaptohexyloxy)methyl-15-crown-5 (15C5-C₁OC₆-SH) and 2-(6-mercaptohexyloxy)methyl-18-crown-6 (18C6-C₁OC₆-SH)], which are adsorbed on gold surfaces through S–Au bonds. The IR difference spectra of the M⁺·15C5-C₁OC₆ (M = Li, Na, K, Rb, and Cs) complexes on gold are observed using aqueous solutions of MCl by SEIRA spectroscopy. The spectra show a noticeable change in the C–O stretching vibration at around 1100 cm⁻¹. The spectral patterns of M⁺·15C5-C₁OC₆ are similar for Li⁺ and Na⁺, and for K⁺, Rb⁺, and Cs⁺; the interaction between the metal ions and 15C5-C₁OC₆ changes drastically between Na⁺ and K⁺ in the series of alkali metal ions. On the other hand, the equilibrium constant for complex formation determined by the IR intensity shows clear preference for Na⁺ ions. We also observe the IR difference spectra of M⁺·18C6-C₁OC₆ in methanol and compare them with those in water. The spectral patterns in methanol are almost the same as those in water, but the equilibrium constant in methanol does not show preference for any ion, different from the K⁺ preference in water. From these findings we attribute the origin of the ion selectivity of 15C5 and 18C6 in solution to the interaction between the metal ions and the crown ethers in the complexes or the solvation energy of free ions. In the case of 15C5-C₁OC₆ in water, the preference of Na⁺ over K⁺, Rb⁺, and Cs⁺ can be attributed to the strength of the interaction or the size matching between metal ions and 15C5-C₁OC₆; the Na⁺ selectivity over Li⁺ ions is dominated by the solvation energy of free ions. For 18C6-C₁OC₆ in methanol, the equilibrium constant for complex formation becomes much bigger in methanol than that in water and loses the selectivity in methanol, because the solvation energy in methanol is fairly smaller than that in water, predominating the contribution from the strength of the interaction between metal ions and 18C6-C₁OC₆. The IR spectra measured by SEIRA spectroscopy are quite sensitive to properties of host–guest complexes such as the intermolecular interaction, the structure, and the orientation against the gold surface. However, the evidence for guest selectivity emerges primarily in the intensity of the spectra, rather than band positions or spectral patterns in the IR spectra.