Frustrated flexibility in metal-organic frameworks

Roman Pallach,Andrea Machalica,Philip A Chater,Rochus Schmid,Marvin Kloß,Julian Keupp,Michael T Wharmby,Kai Terlinden,Sebastian Henke,Christian Sternemann,Suresh K Vasa,Julia Kotschy,Louis Frentzel-Beyme,Rasmus Linser

doi:10.1038/s41467-021-24188-4

Abstract

Stimuli-responsive flexible metal-organic frameworks (MOFs) remain at the forefront of porous materials research due to their enormous potential for various technological applications. Here, we introduce the concept of frustrated flexibility in MOFs, which arises from an incompatibility of intra-framework dispersion forces with the geometrical constraints of the inorganic building units. Controlled by appropriate linker functionalization with dispersion energy donating alkoxy groups, this approach results in a series of MOFs exhibiting a new type of guest- and temperature-responsive structural flexibility characterized by reversible loss and recovery of crystalline order under full retention of framework connectivity and topology. The stimuli-dependent phase change of the frustrated MOFs involves non-correlated deformations of their inorganic building unit, as probed by a combination of global and local structure techniques together with computer simulations. Frustrated flexibility may be a common phenomenon in MOF structures, which are commonly regarded as rigid, and thus may be of crucial importance for the performance of these materials in various applications.

Highlights

Stimuli-responsive flexible metal-organic frameworks (MOFs) remain at the forefront of porous materials research due to their enormous potential for various technological applications
Metal-organic frameworks (MOFs) are a class of porous crystalline materials constructed from inorganic building units, which are interconnected by organic linkers[10,11]
Repeated solvent exchange with CH2Cl2 followed by careful drying under dynamic vacuum (~10–4 kPa) at 100 °C for 24 h gives the guest-free, dried materials dryMOF-5-CX

Summary

Introduction

Stimuli-responsive flexible metal-organic frameworks (MOFs) remain at the forefront of porous materials research due to their enormous potential for various technological applications. Several MOFs undergo reversible structural transformations in response to external stimuli, such as guest adsorption, changes in temperature, mechanical pressure, light irradiation or electric fields,[12] and form an important subclass, termed soft porous crystals or flexible MOFs13–15. Guest-removal leads to a transition of the flexible MOF from an expanded phase to a contracted phase, in which intra-framework dispersion interactions are maximized Based on these fundamental observations, we demonstrate here a new kind of stimuli-driven structural responsiveness of functionalized MOFs, which we term frustrated flexibility. We select an intrinsically rigid and nonresponsive MOF structure type, which does not allow for correlated (and crystalline-to-crystalline) structural changes involving a hinging or wine rack flexibility mechanism

Methods

Results

Conclusion