Constructing an 8-Connected Hydrogen-Bonded Organic Framework with Hafnium-Oxo Clusters

Hydrogen-bonded organic frameworks (HOFs) exhibit intriguing structural features with potential applications in various fields. However, constructing HOFs based on high-connectivity nodes is a challenging task. In this study, we report the synthesis of three new metal-hydrogen-bonded organic frameworks (M-HOFs), Hf₄-P4/n, Hf₄-Fddd-1, and Hf₄-Fddd-2, based on a charge-neutral tetranuclear hafnium-oxo cluster [Hf₄(μ₂-OH)₈L₈] (L = 1H-benzimidazole-2-carboxylate) as the building unit. The rigid benzimidazole-carboxylate ligand maintains the cluster's symmetry while providing hydrogen-bonding sites. By modulating crystallization conditions, three distinct supramolecular packing modes are achieved: Hf₄-P4/n forms an sql net via four pairs of hydrogen bonds and four groups of DMF-mediated van der Waals interactions between [Hf₄(μ₂-OH)₈L₈] units; Hf₄-Fddd-1 adopts a rarely seen 8-connected tsi net through eight pairs of intercluster hydrogen bonds; Hf₄-Fddd-2 exhibits a dia net with only four pairs of hydrogen bonds per cluster. Framework stability correlates with interaction strength and number. Notably, Hf₄-P4/n and Hf₄-Fddd-1 show superior solvent stability compared to Hf₄-Fddd-2. Porosity analysis reveals that Hf₄-Fddd-1 possesses interconnected channels, with a void fraction of 31.1% and a surface area of 277.7 m²/g. This work highlights metal-oxo clusters as potential building units for synthesizing complex HOFs with high-connectivity nodes.