Crystal Structures and Properties of Large Protonated Water Clusters Encapsulated by Metal−Organic Frameworks

A large ionic water cluster H(H2O)28+, consisting of a water shell (H2O)26 and an encaged species H(H2O)2+ as a center core, was trapped in the well-modulated cavity of a porous metal−organic framework (MOF) {[Co4(dpdo)12(PMo12O40)3]−}∞ and structurally characterized. Degeneration of the protonated water cluster H(H2O)28+ into a smaller cluster H(H2O)21+ and recovery of H(H2O)28+ from the resulting H(H2O)21+ cluster in a reversible way demonstrated the unusual stability of the protonated water clusters H(H2O)28+ and H(H2O)21+ in the robust crystal host. Proton transport and proton/potassium ion exchange through the channels of the crystal host have been investigated by a well-established fluorometry method. X-ray fluorescence experiments and X-ray structural analyses of the exchanged crystals confirmed the occurrence of the proton/potassium ion-exchange reaction and the transformation of the protonated water cluster H(H2O)28+ to an ionic cluster K(H2O)27+. Comparison of the H+/K+ exchange of H(H2O)28+ with that of its neighboring protonated water cluster H(H2O)27+ suggested that the abundance of hydrogen bonds associated with the hydronium/water cluster in the H(H2O)28+ cluster was essential for proton transport through the Grotthuss mechanism. On the basis of the results, our porous network could be described as a synthetic non-peptide ion channel, in terms of not only structural features but also the functions addressed. Direct observation of the structures of various large ionic water clusters trapped by porous MOFs, coupled with the proton/ion-exchange processes and the reversible dehydration/rehydration, provided valuable insights into the aqueous proton transfer and its mobility pertaining to the large protonated water clusters in the condensed phase.

一项关于大型离子水簇H(H2O)28+的研究，该簇由水壳(H2O)26和一个被囚禁的物种H(H2O)2+构成的中心核组成，被捕获于经过精细调控的多孔金属-有机框架(MOF) {[Co4(dpdo)12(PMo12O40)3]−}∞的腔隙中，并对其结构进行了表征。通过可逆的方式，将质子化水簇H(H2O)28+分解为较小的簇H(H2O)21+，并从产生的H(H2O)21+簇中恢复H(H2O)28+，展示了质子化水簇H(H2O)28+和H(H2O)21+在坚固的晶体宿主中的异常稳定性。采用成熟的荧光法对晶体宿主中的质子传输和质子/钾离子交换进行了研究。通过X射线荧光实验和X射线结构分析确认了离子交换反应的发生，以及质子化水簇H(H2O)28+向离子簇K(H2O)27+的转变。将H(H2O)28+的H+/K+交换与其相邻的质子化水簇H(H2O)27+的交换进行比较，表明与H(H2O)28+簇中的水合氢离子/水簇相关的氢键的丰富性对于通过Grotthuss机制进行质子传输至关重要。基于这些结果，我们的多孔网络可以被描述为一种合成非肽离子通道，不仅从结构特征上，也从所涉及的功能上进行了阐述。通过直接观察被多孔MOF捕获的各种大型离子水簇的结构，结合质子/离子交换过程和可逆的脱水/再水化，为水溶液中的质子转移及其在凝聚相中的迁移性提供了宝贵的见解。