Tuning Adsorption-Induced Responsiveness of a Flexible Metal–Organic Framework JUK‑8 by Linker Halogenation

Flexible stimuli-responsive
metal–organic frameworks have
become promising candidates for numerous applications in gas-related
technologies; however, the methods of fine tuning their responses
are still limited and sought after. In this work, we demonstrate control
over the adsorption properties of a flexible platform by incorporating
halogen substituents (X = F, Cl, Br, I) into an eightfold interpenetrated
isoreticular series [Zn­(oba)­(X-pip)]n (JUK-8X; X-pip = 4-pyridyl-functionalized
benzene-1,3-dicarbo-5-halogenohydrazide; oba2– =
4,4′-oxydibenzoic carboxylate). The introduced halogen atoms
allow for precise tuning of CO2 gate-opening pressures
from p/p0 = 0.08 for
the parental JUK-8 to 0.78 for the chlorine-functionalized JUK-8Cl. The presence of fluorine or chlorine
substituent in the X-pip linker practically does not influence the
maximum molar CO2 uptake as compared to JUK-8, whereas larger bromine or iodine atoms increase
this uptake by 59 and 48%, respectively. Utilizing in situ powder
X-ray diffraction (PXRD) during CO2 adsorption for a model JUK-8F, we propose a detailed mechanism of phase
transitions including positions of the adsorbed gas molecules for
the two loaded phases. Density functional theory calculations supported
by in situ PXRD measurements at a saturation pressure shed light on
the unusual CO2 adsorption properties of JUK-8Br and JUK-8I. Overall,
our report demonstrates the use of halogen interactions for the control
of a gas-responsive system and provides insightful guidance for the
further development of flexible, adaptable materials.