Mobility
of the organic linkers in metal–organic frameworks
(MOFs) is an established phenomenon. Knowledge of the details of linker
motion in MOFs could provide a great deal of information about the
linker structure and the way the guest molecules interact with the
organic framework. However, the mobility of the organic linkers is
poorly characterized. The extent of the influence of the metal cation
or guest molecules on linker motion is still unknown for MOFs with
identical topologies. In this work, we have analyzed the rotational
dynamics of the phenylene ring fragments of terephthalate (1,4-benzenedicarboxylate,
bdc) linkers in the series of MOFs [M2(bdc)2(dabco)]·G (M = Co2+, Ni2+, Cu2+, Zn2+; dabco =1,4-diazabicyclo[2.2.2]octane; G = none
or dimethylformamide, DMF). We have established that the reorientational
motion of the phenylene rings is performed by π-flipping of
the plane of the ring about its C
2 axis.
The dynamics of the phenylene rings is insensitive to the variation
of the metal cation, whereas the loading of the guest DMF molecules
provides both a significant decrease of the rate of π-flips
and an increase of the activation energy for the motion of the phenylene
rings.
The complex [Zn2(tdc)2dabco] (H2tdc = thiophene-2,5-dicarboxylic
acid; dabco = 1,4-diazabicyclooctane) shows a remarkable increase
in carbon dioxide (CO2) uptake and CO2/dinitrogen
(N2) selectivity compared to the nonthiophene analogue
[Zn2(bdc)2dabco] (H2bdc = benzene-1,4-dicarboxylic
acid; terephthalic acid). CO2 adsorption at 1 bar for [Zn2(tdc)2dabco] is 67.4 cm3·g–1 (13.2 wt %) at 298 K and 153 cm3·g–1 (30.0 wt %) at 273 K. For [Zn2(bdc)2dabco], the equivalent values are 46 cm3·g–1 (9.0 wt %) and 122 cm3·g–1 (23.9 wt %), respectively. The isosteric heat of adsorption for
CO2 in [Zn2(tdc)2dabco] at zero coverage
is low (23.65 kJ·mol–1), ensuring facile regeneration
of the porous material. Enhancement by the thiophene group on the
separation of CO2/N2 gas mixtures has been confirmed
by both ideal adsorbate solution theory calculations and dynamic breakthrough
experiments. The preferred binding sites of adsorbed CO2 in [Zn2(tdc)2dabco] have been unambiguously
determined by in situ single-crystal diffraction studies on CO2-loaded [Zn2(tdc)2dabco], coupled with
quantum-chemical calculations. These studies unveil the role of the
thiophene moieties in the specific CO2 binding via an induced
dipole interaction between CO2 and the sulfur center, confirming
that an enhanced CO2 capacity in [Zn2(tdc)2dabco] is achieved without the presence of open metal sites.
The experimental data and theoretical insight suggest a viable strategy
for improvement of the adsorption properties of already known materials
through the incorporation of sulfur-based heterocycles within their
porous structures.
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