Robust Multifunctional Yttrium-Based Metal Organic Frameworks with Breathing Effect


Phosphonate- and yttrium-based metal-organic frameworks (MOFs), formulated as [Y(H(5)btp)]center dot 5.5H(2)O (1), [Y(H(5)btp)]center dot 2.5H(2)O (2), (H3O)[Y-2(H(5)btp)(H4btp)]center dot H2O (3), and [Y(H(5)btp)]center dot H2O center dot 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) (H(8)btp) 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 Schafli symbol of {4(13).6(2)}{4(8).6(6).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 x 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.



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Firmino, ADG; Mendes, RF; Antunes, MM; Barbosa, PC; Vilela, SMF; Valente, AA; Figueiredo, FML; Tome, JPC; Paz, FAA

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We wish to thank Fundacao para a Ciencia e a Tecnologia (FCT, Portugal), the European Union, QREN, FEDER through Programa Operacional Factores de Competitividade (COMPETE), CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT ref. UID/CTM/50011/2013), QOPNA (FCT ref. UID/QUI/00062/2013), and CQE (FCT ref. UID/QUI/00100/2013), financed by national funds through the FCT/MEC and when appropriate cofinanced by FEDER under the PT2020 Partnership Agreement. We also thank FCT for funding the R&D project FCOMP-01-0124-FEDER-041282 (ref. FCT EXPL/CTM-NAN/0013/2013). FCT is also gratefully acknowledged for the Ph.D. grants SFRH/BD/84495/2012 and SFRH/BD/84231/2012 (to A.D.G.F. and R.F.M., respectively), the postdoctoral research grants SFRH/BPD/89068/2012, SFRH/BPD/94381/2013, and SFRH/BPD/96665/2013 (to M.M.A., to S.M.F.V., and to P.C.B., respectively), and the Development Grant IF/01174/2013 (to F.M.L.F.).

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