Robust Multifunctional Yttrium-Based Metal–Organic Frameworks with Breathing Effect

Phosphonate- and
yttrium-based metal–organic frameworks
(MOFs), formulated as [Y­(H5btp)]·5.5H2O
(1), [Y­(H5btp)]·2.5H2O (2), (H3O)­[Y2(H5btp)­(H4btp)]·H2O (3), and [Y­(H5btp)]·H2O·0.5­(MeOH) (4), were prepared
using a “green” microwave-assisted synthesis methodology
which promoted the self-assembly of the tetraphosphonic organic linker
[1,1′-biphenyl]-3,3′,5,5′-tetrayltetrakis­(phosphonic
acid) (H8btp) with Y3+ cations. This new family
of functional materials, isolated in bulk quantities, exhibits a remarkable
breathing effect. Structural flexibility was thoroughly studied by
means of X-ray crystallography, thermogravimetry, variable-temperature
X-ray diffraction, and dehydration and rehydration processes, ultimately
evidencing a remarkable reversible single-crystal to single-crystal
(SC–SC) transformation solely through the loss and gain of
crystallization solvent molecules. Topologically, frameworks remained
unaltered throughout this interconversion mechanism, with all compounds
being binodal 6,6-connected network with a Schäfli symbol of
{413.62}­{48.66.8}. Results
show that this is one of the most stable and thermally robust families
of tetraphosphonate-based MOFs synthesized reported to date. Porous
materials 2 and 3 were further studied to
ascertain their performance as heterogeneous catalysts and proton
conductors, respectively, with outstanding results being registered
for both materials. Compound 2 showed a 94% conversion
of benzaldehyde into (dimethoxymethyl)­benzene after just 1 h of reaction,
among the best results registered to date for MOF materials. On the
other hand, the protonic conductivity of compound 3 at
98% of relative humidity (2.58 × 10–2 S cm–1) was among the highest registered among MOFs, with
the great advantage of the material to be prepared using a simpler
and sustainable synthesis methodology, as well as exhibiting a good
stability at ambient conditions (temperature and humidity) over time
when compared to others.