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

Phosphonate- and yttrium-based metal–organic frameworks (MOFs), formulated as [Y­(H₅btp)]·5.5H₂O (<b>1</b>), [Y­(H₅btp)]·2.5H₂O (<b>2</b>), (H₃O)­[Y₂(H₅btp)­(H₄btp)]·H₂O (<b>3</b>), and [Y­(H₅btp)]·H₂O·0.5­(MeOH) (<b>4</b>), 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 <b>2</b> and <b>3</b> were further studied to ascertain their performance as heterogeneous catalysts and proton conductors, respectively, with outstanding results being registered for both materials. Compound <b>2</b> 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 <b>3</b> 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