New insights into metal ion-crown ether complexes revealed by SEIRA spectroscopy
New insights into metal ion-crown ether complexes revealed by SEIRA spectroscopy
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DOI：
10.1039/c5nj01787d
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发表时间：
2015-01-01
影响因子：
3.3
通讯作者：
Furutani, Yuji
中科院分区：
文献类型：
--
作者：
Inokuchi, Yoshiya;Ebata, Takayuki;Furutani, Yuji
我们展示了一个强大的光谱技术，表面增强红外吸收（SEIRA）光谱，不仅用于检测主客体复合物在溶液中，但也用于研究客体的选择性，复杂的结构，和溶剂效应之间的关系。我们合成了15-冠-5和18-冠-6的硫醇衍生物[2-（6-巯基己氧基）甲基-15-冠-5（15 C5-C1 OC 6-SH）和2-（6-巯基己氧基）甲基-18-冠-6（18 C6-C1 OC 6-SH）]，它们通过S-Au键吸附在金表面。本文用SEIRA光谱法，在MCl水溶液中，观察了金表面M+中心点15 C5-C1 OC 6（M = Li，Na，K，Rb，Cs）配合物的红外差谱。光谱显示在1100 cm（-1）附近的C-O伸缩振动有明显的变化。M+中心点15 C5-C1 OC 6的光谱模式对于Li+和Na+以及对于K+、Rb+和Cs+是相似的;金属离子与15 C5-C1 OC 6之间的相互作用在碱金属离子系列中的Na+和K+之间急剧变化。另一方面，由IR强度确定的复合物形成平衡常数显示出对Na+离子的明显偏好。我们还观察了M+中心点18 C6-C1 OC 6在甲醇中的红外差谱，并与在水中的红外差谱进行了比较。在甲醇中的光谱模式几乎与在水中的那些相同，但在甲醇中的平衡常数不显示出任何离子的偏好，不同于在水中的K+的偏好。从这些发现中，我们归因于起源的离子选择性的15 C5和18 C6在溶液中的金属离子和冠醚之间的相互作用的配合物或自由离子的溶剂化能。在水中的15 C5-C1 OC 6的情况下，Na+优于K+、Rb+和Cs+可以归因于金属离子与15 C5-C1 OC 6之间的相互作用或尺寸匹配的强度; Na+优于Li+离子的选择性由自由离子的溶剂化能支配。对于18 C6-C1 OC 6在甲醇中的络合物形成的平衡常数变得比在水中的大得多，并且失去了在甲醇中的选择性，这是因为在甲醇中的溶剂化能比在水中的溶剂化能小得多，主要来自金属离子与18 C6-C1 OC 6之间的相互作用强度的贡献。SEIRA光谱测量的红外光谱是相当敏感的主客体配合物的性质，如分子间的相互作用，结构和对金表面的取向。然而，客体选择性的证据主要出现在光谱的强度中，而不是红外光谱中的带位置或光谱图案。
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+center dot 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+center dot 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+center dot 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.