None-linker-bonded node sites in metal organic framework are catalytic sites for many reactions. Works on understanding the bonding of ligands on these node sites and elucidating their key roles in MOF catalysis are emerging. Questions remain unsolved for the bonding modes of formate on some complex MOF nodes, including those on M₆O₈ node of UiO-66. In this work, we chose hcp UiO-66—incorporating paired Hf₆O₈ node bridged by 6 μ₂-OH (labeled as Hf₂₂O₂₂)—to provide insight for the identities of various formate species on MOF node sites and to show their different reactivity with water.
hcp Hf-UiO-66 was synthesized by the reaction of HfCl₄ and benzene-1,4-dicarboxic acid (H₂BDC) from DMF in the present of both formic acid (modulator) and H₂O at 150 °C. PXRD data show the MOF is stable up to 250 °C. The number of formate on MOF node determined by dissolving in NaOH/D₂O and digesting with ¹H NMR spectroscopy was 4.1 per node (2.2 on defect sites and 1.9 on μ₂-OH sites).
IR spectra in ν_CH region show formate ligands can bond to both defect sites and μ₂-OH sites on the Hf₁₂O₂₂ node, and they both can reversibly transit between bidentate and hydrogen bonded monodenate (with H₂O) in the present or absent of water. We found formate ligands on μ₂-OH sites are more reactivity with water (reactive at 80-120 °C) than those on defect sites (reactive at 150-200 °C) of the same nodes. The results also demonstrate formate ligands on defect site become more reactive with water on paired μ₂-OH and defect sites of hcp UiO-66 than those isolated defect site of UiO-66. Our data provide evidence for elucidating the molecular structure of ligands on metal-oxide-cluster MOF node sites, which are not achievable on the surface of bulk metal oxides.

Yue Xiao

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