Characterization, Structure, and Reactivity of Hydroxyl Groups on Metal‐Oxide Cluster Nodes of Metal–Organic Frameworks: Structural Diversity and Keys to Reactivity and Catalysis

Abstract

Among the most stable metal–organic frameworks (MOFs) are those incorporating nodes that are metal oxide clusters with frames such as Zr6O8. This review is a summary of the structure, bonding, and reactivity of MOF node hydroxyl groups, emphasizing those bonded to nodes containing aluminum and zirconium ions. Hydroxyl groups are often present on these nodes, sometimes balancing the charges of the metal ions. They arise during MOF syntheses in aqueous media or in post‐synthesis treatments. They have been identified with infrared and 1H nuclear magnetic resonance spectroscopies and characterized by their reactivities with polar compounds such as alcohols. Terminal OH, paired μ2‐OH, and aqua groups on nodes are catalytic sites in numerous reactions. Relatively unreactive hydroxyl groups (such as isolated μ2‐OH groups) may replace reactive groups and inhibit catalysis; some node hydroxyl groups (e.g., μ3‐OH) are mere spectators in catalysis. There are similarities between MOF node hydroxyl groups and those on the surfaces of bulk metal oxides, zeolites, and enzymes, but the comparisons are mostly inexact, and much remains to be understood about MOF node hydroxyl group chemistry. We posit that understanding and controlling this chemistry will lead to tailored MOFs and improved adsorbents and catalysts.This article is protected by copyright. All rights reserved

Defect-enabling zirconium-based metal–organic frameworks for energy and environmental remediation applications

“…, open metal sites or coordinatively unsaturated sites (CUSs) on the hexa-zirconium nodes, which are largely saturated with hydroxyl/aqua (–OH/H 2 O) ligands or other occupying ligands, depending on the synthetic condition and activation process. 63,74,209–213 Such defects in Zr-MOFs can provide the following benefits for environmental applications: (a) additional or larger pore apertures and higher porosity, 33,34,43,76,214 allowing the additional accessibility and faster diffusion of the targeted molecule to internal active sites; (b) more Zr( iv ) CUSs as Lewis-acid sites and/or more active Zr-OH/Zr-H 2 O binding sites (note that at pH = 7, terminal aqua ligands are generally changed to hydroxo ligands); 215–217 (c) the increased hydrophilicity in the pore to facilitate the penetration of aqueous media. 207,208,218 Furthermore, the particle size and surface charge of MOFs are other important factors for the adsorption and removal of contaminates, which can also be tuned by the modulator-based synthesis.…”

Discovery of two predictable (3,18)-connected topologies based on Wells–Dawson type cages for the design of porous metal phosphonocarboxylate frameworks

“…(3,18)-connected frameworks have been rarely observed in MOFs and only 4 compounds have been reported to date. 28,60,61 The uniqueness of these structures lies in their novel connectivity and represents the first instances of (3,18)-connected nets in metal phosphonocarboxylate frameworks.…”

CO2-Based Stable Porous Metal–Organic Frameworks for CO2 Utilization

“…The TGA thermograms revealed a weight loss under 300 °C, which was largely attributed to the loss of guest molecules and the dehydration of the Zr 6 O 4 (OH) 4 nodes to Zr 6 O 6 . The ligand elimination might also happen during 200–300 °C . Then, the MOFs lost weight rapidly after 400 °C due to framework decomposition.…”