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

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 are identified with infrared and 1H nuclear magnetic resonance spectroscopies and characterized by their reactivities with polar compounds such as alcohols. Terminal OH, paired mu 2-OH, and aqua groups on nodes are catalytic sites in numerous reactions. Relatively unreactive hydroxyl groups (such as isolated mu 2-OH groups) may replace reactive groups and inhibit catalysis; some node hydroxyl groups (e.g., mu 3-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. It is posited that understanding and controlling this chemistry will lead to tailored MOFs and improved adsorbents and catalysts. Many metal-organic frameworks consist of nodes such as Zr6O8 connected by organic linkers. Hydroxyl groups are present in various forms on the nodes, characterized by infrared and NMR spectroscopies. They have contrasting reactivities, and many play central roles in catalysis, with terminal hydroxyl groups being among the most reactive.image