Modulation of the Dynamics of a Two-Dimensional Interweaving Metal–Organic Framework through Induced Hydrogen Bonding

TL;DR

Researchers synthesized two similar metal-organic frameworks and found that a small chemical substitution affects their structural flexibility and response to CO2.

Contribution

A new strategy for modulating MOF dynamics through hydrogen bonding is introduced, enabling control over framework rigidity.

Findings

TMU-27 undergoes a structural phase transition upon CO2 sorption.

TMU-27-NH2 remains rigid due to hydrogen bonding between layers.

Crystal size in TMU-27 correlates with transformation pressure during CO2 adsorption/desorption.

Abstract

Inducing, understanding, and controlling the flexibility
in metal–organic
frameworks (MOFs) are of utmost interest due to the potential applications
of dynamic materials in gas-related technologies. Herein, we report
the synthesis of two isostructural two-dimensional (2D) interweaving
zinc(II) MOFs, TMU-27 [Zn(bpipa)(bdc)] and TMU-27-NH2 [Zn(bpipa)(NH2-bdc)], based on N,N′-bis-4-pyridyl-isophthalamide
(bpipa) and 1,4-benzenedicarboxylate (bdc) or 2-amino-1,4-benzenedicarboxylate
(NH2-bdc), respectively. These frameworks differ only by
the substitution at the meta-position of their respective bdc groups:
an H atom in TMU-27 vs an NH2 group in TMU-27-NH2. This difference strongly influences their respective responses
to external stimuli, since we observed that the structure of TMU-27
changed due to desolvation and adsorption, whereas TMU-27-NH2 remained rigid. Using single-crystal X-ray diffraction and CO2-sorption measurements, we discovered that upon CO2 sorption, TMU-27 undergoes a transition from a closed-pore phase
to an open-pore phase. In contrast, we attributed the rigidification
in TMU-27-NH2 to intermolecular hydrogen bonding between
interweaving layers, namely, between the H atoms from the bdc-amino
groups and the O atoms from the bpipa-amide groups within these layers.
Additionally, by using scanning electron microscopy to monitor the
CO2 adsorption and desorption in TMU-27, we were able to
establish a correlation between the crystal size of this MOF and its
transformation pressure.

Flexibility
in metal−organic frameworks (MOFs) may
be key for further advances in their industrial applications. Therefore,
exploring what might trigger it is crucial, since even a minimal change
within their structure, such as the substitution of an H for an NH2 group in the position of one of its constituent ligands,
can influence the material’s behavior toward external stimuli
as a whole.

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