Dynamic breathing effect in metal-organic frameworks: Reversible 2D-3D-2D-3D single-crystal to single-crystal transformation


The flexible organic linker nitrilotris(methyl phosphonic acid) (H(6)nmp) was used to isolate a new family of MOFs by the self-assembly with La3+ cations using a fast and simple microwave synthetic approach. Sulfuric acid was included in the reaction media with two major functions: (i) to slow down the deprotonation of the organic linker, thus favouring crystal growth over nucleation; (ii) to block coordination sites usually occupied by phosphonic acids groups, with the aim to design more porous materials. The 2D layered material obtained, [La-2(H(4)nmp)(2)(H2O)(3)(SO4)]center dot 8H(2)O (1), undergoes spontaneous single-crystal to single-crystal (SC-SC) transformations at ambient temperature, which were followed by single-crystal and powder X-ray diffraction studies. The highly disordered water molecules in the channels are gradually removed originating in a consecutive fashion [La-2(H(4)nmp)(2)(H2O)(3)(SO4)]center dot 6H(2)O (2), [La-2(H(4)nmp)(2)(H2O)(3)(SO4)]center dot 2H(2)O (3) and [La-2(H(4)nmp)(2)(H2O)(2)(SO4)]center dot H2O (4). Materials are structurally very similar, being composed of "2D" layers formed by metallic centers in a zigzag conformation connected by the organic linker. While 1 and 3 are, in a topological perspective, identical (the sole difference being on the number of crystallization water molecules), being a 4-connected uninodal 2D networks with point symbol of {4(4).6(2)}, 2 and 4 are in turn a 4,5-connected binodal (point symbol of {4(4).6(2)}{4(4).6(4)}) and 5-connected uninodal (point symbol of {4(6).6(4)}) 3D networks, respectively. The main difference in these networks is the group that connects the "2D" layers: while in 2 the connection is achieved by the sulfate ion, in 4 the phosphonic acid residue is the main responsible by the formation of the 3D network, with the sulfate ion being coordinated to only one Ln(3+) center. Because of the continuous transformation in ambient conditions of 1-3, the full characterization description by elemental analysis, FTIR spectroscopy, thermogravimetry and electron microscopy (SEM and EDS) is presented only for the final and stable structure, 4. (C) 2016 Elsevier B.V. All rights reserved.



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Mendes, RF; Paz, FAA

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Funding agencies and projects: 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), and QOPNA (FCT UID/QUI/00062/2013), financed by national funds through the FCT/MEC and when appropriate co-financed 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).; Individual grants and scholarships: FCT is also gratefully acknowledged for the Ph.D. grant No. SFRH/BD/84231/2012 (to RFM).

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