Framework Cationization by Preemptive Coordination of Open Metal Sites for Anion‐Exchange Encapsulation of Nucleotides and Coenzymes

Abstract

Cationic frameworks can selectively trap anions through ion exchange,a nd have applications in ion chromatography and drug delivery.However,cationic frameworks are muchr arer than anionic or neutral ones.H erein, we propose ac oncept, preemptive coordination (PC), for targeting positively charged metal-organic frameworks (P-MOFs). PC refers to proactive blocking of metal coordination sites to preclude their occupation by neutralizing ligands such as OH À . We use 20 MOFs to showt hat this PC concept is an effective approach for developing P-MOFs whose high stability, porosity,and anion-exchange capability allowimmobilization of anionic nucleotides and coenzymes,i na ddition to chargeand size-selective capture or separation of organic dyes.T he CO 2 and C 2 H 2 uptake capacity of 117.9 cm 3 g À1 and 148.5 cm 3 g À1 ,respectively,at273 Kand 1atm, is exceptionally high among cationic framework materials.Crystalline porous materials (CPMs) with exchangeable ions are useful for many ion-exchange applications,s uch as pollutant extraction. [1] In nature,C PMs (for example,z eolites) usually have an anionic framework. Cationic framework materials with anion exchange capability are desired for av ariety of applications,s uch as removal of toxic ClO 4 À and AsO 4 3À ,s eparation of nucleotides and amino acids,o r immobilization of anionic pharmaceuticals. [2] Recent advances in MOFs have opened opportunities for developing P-MOFs. [3] In fact, cationic frameworks were among the earliest MOFs to be studied. [4] Examples include those formed by Cu + (or Ag + )and neutral linkers,such as 4,4'bipyridine.H owever,t hese P-MOFs have limited structural variety and low stability.O ne promising method for creating stable P-MOFs is to use clusters. [5] But not all cluster types are suitable,because of the extra attached negative ligands owing to the increased node connectivity,a swell as the trapping of simple anions,such as O 2À ,can make the overall charge of the cluster neutral or negative.Itisthe ratio between neutral and negative ligand groups that plays apivotal role in the overall charge of acluster.Synthetic strategies capable of controlling this ratio are thus desirable.An analysis shows that both tetrameric [Zn 4 O(RCOO) 6 ] and dimeric M 2 (RCOO) 4 (solvent) 2 clusters usually form neutral MOFs. [6] In comparison, trigonal planar trimers could give ap ositive cluster,s uch as [M 3 O(RCOO) 6 L 3 ] + , when all of the Mi ons are trivalent and all of the Ld onor groups are neutral. [7] There are,h owever, two pitfalls that could neutralize trimers:t he inclusion of M 2+ or the occupation of one Ls ite by an anionic ligand (such as OH À ). [8,9] To overcome the neutralizing effect of M 2+ ,t he simplest solution is to employ metal ions (such as In 3+ )t hat are unlikely to exist as M 2+ .H owever,i ti sa dvantageous to use transition metal ions because of their desirable properties, such as low cost, low toxicity,a nd high stability for applications in aqueous environments.P revious studies on trimers with ions such as Co...

A Modulator‐Induced Defect‐Formation Strategy to Hierarchically Porous Metal–Organic Frameworks with High Stability

“…[13] In stark contrast, the pristine UiO-66 (window size: ca. For the encapsulation of functional guest molecules, such as dyes, polyoxometalates, and metalloporphyrins, which are usually larger in sizes than the micropores of MOFs such as UiO-66, HP-UiO-66 unambiguously displays its advantages.…”

“…5.5 nm) in HP-UiO-66, the defect generation might cause the framework to be positively charged, facilitating the trapping of anionic dyes such as R250. [13] In stark contrast, the pristine UiO-66 (window size: ca. 0.6 nm) cannot uptake such a large molecule and its color remains unchanged after even longer soaking time (Figure 3 a).…”

Pore‐Environment Engineering in Multifunctional Metal‐Organic Frameworks

“…Zhao et al . introduced neutral pyridyl into the system to modify the exposed metal sites in M 3 O(COO) 6 cluster by preemptive coordination . By changing the type of ligands and the incorporation ratio of pyridyl group and carboxylate groups, three different structures were obtained (Figure ).…”

“…Coordination environment around trimers with preemptive coordination strategy and three specific possibilities to construct cationic frameworks through direct synthesis. Reproduced with permission . Copyright © 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.…”

Design Strategy for the Controlled Generation of Cationic Frameworks and Ensuing Anion-Exchange Capabilities

“…58 The initial strategy for obtaining a positively charged framework consisted of employing neutral polytopic organic ligands such as polypyridine-or imidazole-based linkers; 59−61 however, these structures generally elicited poor stability and limited tunability. Subsequent attempts to control ionicity were elegantly demonstrated by Bu and co-workers 62 in secondary building units consisting of planar trimers, where control over the oxidation state of the metal ions (M II versus M III ) as well as the preemptive coordination of neutral ligands to the open metal sites of the trimer can indeed yield cationic frameworks. Similarly, the direct stripping of terminally coordinated F − anions from the metal clusters of MOFs has also been shown to generate a cationic scaffolding.…”