Size‐Selective Sorption of Small Organic Molecules in One‐Dimensional Channels of an Ionic Crystalline Organic–Inorganic Hybrid Compound Stabilized by π–π Interactions

Abstract

The design and syntheses of porous materials such as zeolites and metal-organic frameworks (MOFs) are areas of intense research because of their unique properties in gas storage, separation, and heterogeneous catalysis. [1] Crystalline microporous zeolites show shape-selective adsorption properties because the sizes of the channel apertures formed by the covalently bonded [TO 4 ] (T= Si, Al, P, Ti, etc.) and/or [MO 6 ] (M = V, Mn, Mo, etc.) units can be controlled. [1e,l-n] In contrast, the pore sizes of MOFs, which are constructed from coordinatively bonded metal ions and organic ligands, can be controlled by the lengths and functional groups of organic ligands. [1j,k] In particular, the use of aromatic units induces p-p interactions [2] and enables the rational control of the complexation of the building units. [3] In addition, the presence of the aromatic moieties means that the resulting framework would show unique structural and guest-sorption properties. For example, the guest sorption in a Cu II dihydroxybenzoate/4,4'-bipyridine MOF [3c] causes the gliding of p-p-stacked building units, which leads to the change in the framework structure, and a Co II benzenetricarboxylate MOF [3e] separates the aromatic units from the aliphatic hydrocarbons.Trinuclear metal carboxylates have often been used as building units for the construction of MOFs, [1i, 4] which mostly contain dicarboxylates as bridging ligands to connect the trinuclear metal units, and do not show p-p interactions among the building units. [1i, 4a-c] The use of aromatic units as terminal ligands of trinuclear metal carboxylates would induce p-p interactions, which lead to the unique structural and guest sorption properties. Based on these considerations, we have used pyridine as a terminal ligand of the trinuclear metal carboxylate macrocation [Cr 3 O(OOCH) 6 (pyridine) 3 ] + in this work. Ionic crystals normally possess densely packed crystal lattices because of the strong and isotropic coulombic interaction, and thus are nonporous.

Frontiers and applications of polyoxometalates-based porous ionic crystals

“…PPICs 最重要的性质是其多孔性。通常来说，PPICs 孔结构要远大于普通的多酸晶体 [15] 。比 如，(NH 4 ) 3 [PW 12 O 40 ]多酸晶格中的 NH 4 + 和[PW 12 O 40 ] 3− 紧密堆积，没有形成孔结构。也有一些研 究表明，在 pH 较低的还原性溶液中或者紫外照射下，几十甚至上百个钼原子能够自组装，形成 具有孔结构的大尺寸环形或球形多酸团簇，用于手性客体识别 [25~27] 。然而，这类尺寸较大的团 簇制备困难、组装过程难以控制 [28] 。PPICs 制备工艺简便，在室温下，POMs 阴离子和配合物阳 [29] 、多 酸阴离子 [30,31] 以及抗衡阳离子 [29] 的变化对于离子晶体孔道尺寸的影响。通过系统地研究发现， [20] 。 不同种类的抗衡阳离子对 POMs 溶解度和稳定性影响极大。 对于 PPICs 而言， 阳离子组成既有抗衡阳离子， 也有配位化合物大阳离子簇， 因此具有更为丰富的可调节性。 首先，PPICs 的配合物阳离子部分可引入特定的官能团和桥接/末端配体，对其孔结构和理化性质 等进行系统地调节。比如，引入芳香基团 [29,32~34] 、烯烃 [35] 或氰基 [31] 作 为 大 阳 离 子 的 π-π 堆叠而成的稳定一维通道 [29,33,34] 。此外，Mizuno 等人 [35] 首次将烯烃引入 PPICs 中，利用了 [31] 还将氰基引入 PPICs 中， 利用极性氰基稳定结晶水，防止阴阳离子紧密堆积，形成了一维中孔结构，如图 2 所示。其次， PPICs 的抗衡阳离子种类也很丰富。Mizuno 课题组 [36] 通过引入不同的一价阳离子：Na + (半径， ) 的晶体结构图 [36] ; (b) A=K + , Rb + , NH 4 + , Cs + ; (c) A=TMA + [36] ; (d) [31] ; (e), (f) PPICs 的疏水、亲水通道 [36] • nH 2 O (A= Na + ) [36] ; (b) A=K + , Rb + , NH [31] ; (e), (f) hydrophobic and hydrophilic channels of PPICs [36] 除阳离子部分外, POMs 阴离子也是 PPICs 的重要组成部分, 通过调变 POMs 阴离子类型, 所制备的离子晶体也会展现出不同的结构和性能。目前所报道的 PPICs 大多由廉价易得、化学稳 定性良好的 Keggin 型 POM 组成, 也有一少部分是基于 Dawson [30,37,38] 和 Anderson [39] 型多酸阴离 A c c e p t e d https://engine.scichina.com/doi/10.1360/TB-2021-1035 [40] 发现, 3− (M = Mo, W)团簇在充放电过程中分子和电子结构的变化 [41] 。此外, Chen 课 题组 [42,43] 发现, 较大的 Keplerate 型多酸团簇{Mo 72 V 30 }和{Mo 72 Fe 30 }也可以通过多次可逆的氧化…”

Inorganic Hybrid Materials with Encapsulated Polyoxometalates

“…Recently a new field of research has emerged based on the direct assembly of POMs and organic cations leading to the formation of a variety of supramolecular structures which have been used in various domains [ 48 , 49 , 50 , 51 , 52 , 53 , 54 ]. Various polyoxometalates have been used although Keggin structures were preferred for their higher stability and fully symmetric structure.…”

Synthesis, Crystal Structure, and Properties of Two 2D Lamella Coordination Polymers Generated from Unsymmetrically Carboxylate Bridging Ligand

“…[1][2][3][4][5] Though the effective and facile approach for the synthesis of such materials is still a great challenge due to the lack of the understanding in formation mechanisms, nevertheless many examples with interesting compositions and structures have been synthesized through judicious combination of a metal "node" and a ligand "spacer," as well as act synergistically to provide access to a vast diversity of structural topologies. [6][7][8] Multicarboxylate ligands are good candidates for the construction of MOFs solids organized by coordination bonds, hydrogen bonds, and π -π stacking interaction, due to their diverse coordination ability.…”