Highly stable superprotonic conductivity (>10 −2 S cm −1 ) has been achieved through the unprecedented solvent-freecoordinative urea insertion in MOF-74 [M 2 (dobdc), M = Ni 2+ , Mg 2+ ; dobdc = 2,5-dioxido-1,4-benzenedicarboxylate] without an acidic moiety. The urea is bound to open metal sites and alters the void volume and surface functionality, which triggers a significant change in proton conductivity and diffusion mechanism. Solid-state 2 H NMR revealed that the high conductivity was attributed to the strengthening of the hydrogen bonds between guest H 2 O induced by hydrogen bonds in the interface between H 2 O and the polarized coordinated urea.
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.
We present deuteron quadrupole coupling constants (DQCC) for hydroxyl‐functionalized ionic liquids (ILs) in the crystalline or glassy states characterizing two types of hydrogen bonding: The regular Coulomb‐enhanced hydrogen bonds between cation and anion (c–a), and the unusual hydrogen bonds between cation and cation (c–c), which are present despite repulsive Coulomb forces. We measure these sensitive probes of hydrogen bonding by means of solid‐state NMR spectroscopy. The DQCCs of (c–a) ion pairs and (c–c) H‐bonds are compared to those of salt bridges in supramolecular complexes and those present in molecular liquids. At low temperatures, the (c–c) species successfully compete with the (c–a) ion pairs and dominate the cluster populations. Equilibrium constants obtained from molecular‐dynamics (MD) simulations show van't Hoff behavior with small transition enthalpies between the differently H‐bonded species. We show that cationic‐cluster formation prevents these ILs from crystallizing. With cooling, the (c–c) hydrogen bonds persist, resulting in supercooling and glass formation.
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