Soft Mode Metal-Linker Dynamics in Carboxylate MOFs Evidenced by Variable-Temperature Infrared Spectroscopy

Area of Science:

• Materials Science
• Chemistry
• Solid-State Physics

Background:

• Metal-organic frameworks (MOFs) are typically perceived as having static metal-linker bonds.
• Structural dynamics in MOFs usually refers to pore "breathing" or guest molecule interactions.
• Dynamic bonding in MOFs could explain key phenomena like catalysis, negative thermal expansion, and crystal growth.

Purpose of the Study:

• To challenge the static bond perception in carboxylate-based MOFs.
• To demonstrate reversible metal-linker bonding in carboxylate MOFs, analogous to zeolitic imidazolate frameworks (ZIFs).
• To provide a new perspective for designing next-generation metal-organic materials.

Main Methods:

• Comprehensive analysis of carboxylate-based MOFs.
• Variable-temperature diffuse reflectance infrared Fourier transform spectroscopy (VT-DRIFTS).
• Ab initio plane wave density functional theory calculations.

Main Results:

• Evidence for reversible metal-linker bonding in carboxylate MOFs was observed.
• This dynamic bonding is driven by specific vibrational modes, similar to ZIF melting.
• Red-shifts of carboxylate stretches coupled to anharmonic oscillators indicate dynamic bonding.

Conclusions:

• Metal-linker bonds in carboxylate MOFs are not static but exhibit dynamic behavior.
• These dynamics are driven by vibrations, akin to soft modes in other materials.
• Understanding these dynamics opens new avenues for designing advanced MOFs with tailored properties.