Dication Containing Three Aromatic Ring Structure-Directs toward a Chiral Zeolite, Spans Three Cavities, and Effectively Traps Water

The chiral zeolite STW has been synthesized using
a large dication containing three aromatic rings (two imidazolium
moieties plus a benzene one). The integrity of the occluded organics
is proven by 1H and 13C liquid NMR of the zeolite
dissolved in HF/D2O. Molecular simulations strongly suggest
that each aromatic ring is located in a different cavity so that each
dication crosses two medium pore 10-membered-ring (MR) windows. This
is confirmed by Rietveld refinement against synchrotron powder X-ray
diffraction data, which suggests that there is also trapped water
in the cavities, explaining a significant excess of H found in this
material. The presence of water is very soundly confirmed by Fourier-transform
infrared experiments, which also show that this water cannot leave
the zeolite even after heating at 180 °C under vacuum. Molecular
simulations allow us to conclude that this is due to the blockage
of the 10 and 8MR windows by the occluded organics. The peculiar situation
of an organic structure-directing agent spanning three cavities has
important implications for the structure-direction concept that are
briefly discussed. Several chiral derivatives of this dication have
also been studied, and their possibility to enantioselectively direct
the crystallization of STW has been analyzed.