No abstract

Titanium-based metal−organic frameworks (Ti-MOFs) are one of the most active fields of MOFs due to their intriguing catalytic properties; however, the fast reaction between the widely used carboxylate-based ligand (hard base) and Ti 4+ ion (hard acid) makes the syntheses of Ti-MOFs extremely challenging. In this work, we propose a new strategy by adopting sulfonic acid (soft base) as organic ligands to synthesize Ti-MOFs based on hard and soft acid and base (HSAB) theory. As an example of this strategy, seven titanium sulfonate metal−organic frameworks have been synthesized from 1,5-naphthalenedisulfonic acid (H 2 nds) and titanium isopropoxide and their structures are determined by single-crystal X-ray diffraction. These compounds exhibit permanent porosity, solvent stability, and light adsorption ability with a narrow band gap (2.67 eV). This work demonstrates a powerful method for the single-crystal syntheses of new Ti-MOFs, which will greatly promote the development of Ti-MOFs.

Flexible metal–organic frameworks (MOFs) are one kind of stimuli-responsive materials that exhibit reversible structural transformations in response to external stimuli. Exploring and understanding the stimuli response behavior of flexible MOFs is challenging, as it involves weak host–guest interaction. We report here the unique flexibility of MOF Zn­(int)­(Ad) (TIF-A1, Hint = isonicotinic acid, Had = adenine) induced by acetylene adsorption. TIF-A1 is rigid toward most gas molecules, while only C2H2 can induce the flexibility of TIF-A1. C2H2-loaded TIF-A1 is characterized by single-crystal X-ray diffraction and molecular modeling. It is revealed that the flexibility of TIF-A1 originates from the strong interaction between acetylene and the framework, which pushes the rotation of the int ligand and the expansion of the framework simultaneously. This work is helpful in deeply understanding the flexibility of MOFs and guides exploring new flexible MOFs in the future.