Unveiling the Structure-Modulator Relationships in Thorium-Based Metal-Organic Framework Crystallization

Acid modulator has been the most widely employed ingredient for highly crystalline metal-organic framework (MOF) synthesis. However, the mechanistic understanding of thorium (Th)-based MOF crystallization remains a great challenge due to the intrinsic properties of fast olation and oxolation reactions of Th species in solution. Here, we constructed a series of Th-based MOFs by adding different modulators (formic acid, acetic acid, trifluoroacetic acid, and benzoic acid) to a synthetic solution along with tetratopic 1,3,5,7-adamantane tetracarboxylic acid (H₄ATC), a three-dimensional (3D) ligand with a rigid aliphatic backbone. This work presents an in-depth study of the structure-modulator relationship between the H₄ATC ligand and coordinating modulators in the Th-based MOF crystallization process. Crystal structures of these Th-based MOFs reveal that formic acid and acetic acid modulators can compete with the H₄ATC ligand to form NU-52 and NU-54; these MOFs possess Th nodes linked by the corresponding modulator. Alternatively, usage of trifluoroacetic acid and benzoic acid modulators results in NU-53 and NU-55; these MOFs possess Th nodes coordinated by only the H₄ATC ligand, regardless of the modulator amount. This work highlights that both the identity and amount of modulator play a crucial role in determining the resulting Th-based MOF structures when H₄ATC is selected as the coordinated ligand.