Layered MetalâOrganic Frameworks Based on Octahedral

Communication pubs.acs.org/crystal

Layered Metal−Organic Frameworks Based on Octahedral Lanthanides and a Phosphonate Linker: Control of Crystal Size Filipe A. Almeida Paz,*,† Sérgio M. F. Vilela,†,‡ and Joaõ P. C. Tomé‡,§ †

Centre for Research in Ceramics and Composite Materials (CICECO) and ‡Organic Chemistry, Natural Products, and Food Products (QOPNA), Department of Chemistry, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal § Department of Organic Chemistry, Ghent University, B-9000 Gent, Belgium S Supporting Information *

ABSTRACT: The hydrothermal reaction between lanthanide salts and residues of (benzene-1,3,5-triyltris(methylene))triphosphonic acid (H6bmt) promotes the formation of a new series of isotypical layered lanthanide−organic frameworks (LnOFs), [Ln2(H3bmt)2]·H2O [where Ln3+ = Eu3+ (1), Gd3+ (2), Tb3+ (3), Dy3+ (4), Ho3+ (5), Er3+ (6), Tm3+ (7), and Yb3+ (8)]. The crystal structure was unveiled from powder X-ray diffraction data (both laboratory and synchrotron) in tandem with other characterization techniques (namely thermogravimetry, thermodiffractometry, vibrational spectroscopy, and elemental analysis). It is shown that the lanthanide contraction leads, on average, to a reduction of the average crystallite size, up to the nanometer scale in the case of compound 8. The unique structural features of the ∞2[Ln2(H3bmt)2] layers are discussed in detail, in particular, the six-coordination environment of the lanthanide cations and its topological relationship with the formation of the first 5,5L4 binodal network based on chelating phosphonate groups.

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Our research group has been focused on the development of new multitopic organic ligands11 based on phosphonate groups to prepare novel functional lanthanide−organic frameworks (LnOFs).12 More recently, we developed the synthesis in large scale of (benzene-1,3,5-triyltris(methylene))triphosphonic acid (H6bmt; Scheme 1) and prepared two families of materials: a

etal−Organic Frameworks (MOFs) nowadays remain one of the most studied families of compounds,1 either because of their interesting architectures 2 and design construction principles,2a,3 or because of their direct application in the construction of functional devices and in industrial processes.4 Indeed, the symbiotic collage of metallic centers (or their aggregates) and multitopic organic ligands can direct unique structural features with interesting functionalities such as catalytic activity;5 photoluminescence;6 magnetism;7 in medicine;8 separation, storage, or purification of gases or other molecules;9 and proton conductivity.10 The vast majority of the MOFs reported in the literature are based on transition metal centers coordinated to ligands based on carboxylate groups, with in recent years the use of imidazoles gaining also some terrain. In contrast with this, the use of phosphonate groups is significantly less common, even though they can bring some interesting structural features into the developed MOFs: (i) they mimic the tetrahedral connectivity iconic of the zeo-type materials; (ii) the existence of three oxygen atoms improve metal connectivity leading to more robust and even thermally stable networks; (iii) they permit chemical modification by design due to the presence of pendant carbon-based groups. Nevertheless, the use of phosphonates also brings some disadvantages, particularly when combined with rare-earth cations (usually characterized by larger coordination numbers), such as the formation of poorly crystalline materials, typically powder due to a fast rate of crystal nucleation and growth. © XXXX American Chemical Society

Scheme 1. (Benzene-1,3,5triyltris(methylene))triphosphonic Acid (H6bmt)

zeolitic 3D system, formulated as [Ln2(H3bmt)2(H2O)2]· H2O,13 and a 1D [La(H4bmt)(H5bmt)(H2O)2]·3H2O14 chain. Both systems exhibited interesting photoluminescent properties with the latter compound also being a remarkable heterogeneous catalyst, even surpassing commercially available MOFs for the same reaction. In this Communication, we report a new series of isotypical layered LnOFs formulated as [Ln2(H3bmt)2]·H2O [where Ln3+ = Eu3+ (1), Gd3+ (2), Tb3+ Received: June 16, 2014 Revised: July 29, 2014

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dx.doi.org/10.1021/cg500875m | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. Scanning electron microscopy images of the as-prepared bulk [Ln2(H3bmt)2]·H2O compounds [where Ln3+ = Eu3+ (a), Gd3+ (b), Tb3+ (c), Dy3+ (d), Ho3+ (e), Er3+ (f), Tm3+ (g), and Yb3+ (h)], emphasizing the average crystallite reduction promoted by the lanthanide contraction effect.

Figure 2. Final Rietveld plot (laboratory X-ray diffraction data) of [Eu2(H3bmt)2]·H2O (1). Observed data points are provided as the red line; the best fit profile (upper trace - black) and the difference pattern (lower trace - blue) are drawn as solid lines, respectively. Green vertical bars indicate the angular positions of the allowed Bragg reflections. Refinement details are given in Table S1 in the SI. Inset: (a) Distorted octahedral coordination environment for the crystallographically independent Eu3+ cation composing the crystal structure of 1. Symmetry transformations used to generate equivalent atoms: (i) 1.5 − x, −0.5 + y, 1.5 − z; (ii) 1 − x, −y, 2 − z; (iii) 1 − x, 1 − y, 2 − z; (iv) 0.5 + x, 0.5 − y, −0.5 + z. (b) Mixed ball-and-stick and polyhedral crystal packing representation of 1 viewed in perspective down the [010] crystallographic direction.

(3), Dy3+ (4), Ho3+ (5), Er3+ (6), Tm3+ (7), and Yb3+ (8)], in which the lanthanide metallic centers exhibit a rare octahedral coordination environment among phosphonate-based networks, which further induces a progressive reduction of the average particle size up to the nanometer scale (in the case of 8). The reaction between H6bmt and different lanthanide chloride salts based on progressively smaller cationic radius (i.e., metals from Eu3+ to Yb3+) under typical hydrothermal conditions (180 °C for 72 h) led to the isolation of a series of isotypical microcrystalline powders as clearly evidenced by

powder X-ray diffraction (Figure S4 in SI). The synthetic conditions are identical to those we used to prepare the zeolitic system [Ln2(H3bmt)2(H2O)2]·H2O (where Ln3+ = La3+, Ce3+, Pr3+, and Nd3+),13 but instead of isolating large single-crystals, microcrystalline powders were systematically obtained. As further emphasized in Figure 1, the use of lanthanides with higher atomic number (also corresponding to those with smaller ionic radius) leads to a progressive reduction of the average particle size (up to the nanometer scale when Yb3+ cations are used). This well-known lanthanide contraction effect has been reported to be at the genesis of distinct B



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Figure 3. Schematic representations of the two-dimensional (2D) neutral layer composing the crystal structure of compound [Eu2(H3bmt)2]·H2O (1), emphasizing (a) the intermetallic distances along the lanthanide phosphonate chain running parallel to the b-axis, and (b,c) the relationship between the hybrid layer and its topological representation as a bimodal 5,5L4 network (for a detailed and comparative description of the topological features, see the SI). Color scheme as in Figure 2 (please note: the center of gravity of the H3bmt3− organic ligand is represented as a light-blue ball).

As shown in Figure 2 (inset - left) the crystal structure of [Ln2(H3bmt)2]·H2O is formed by a single Ln3+ cation coordinated to six independent phosphonate groups describing a slightly distorted octahedral coordination environment, {EuO6} (see the SI for a detailed comparative analysis of the distortion degree of the metallic centers of 1 and 8). This structural feature contrasts with what is usually observed and reported in related materials, particularly those based on H6‑xbmt−x residues: in both the 1D [La(H4bmt)(H5bmt)(H2O)2]·3H2O14 and 3D [Ln2(H3bmt)2(H2O)2]·H2O13 compounds the analogous geometry for the metallic center is instead based on octacoordinated coordination environments, being for the latter reduced to seven upon dehydration of the framework. A search in the literature reveals that reports on hexacoordinated crystalline lanthanide phosphonate materials are scarce: besides the detailed structural study from the research groups of Ma and Zheng20 on the mixed-metal Ln3+Cu2+ 3D isotypical series based on 2-pyridylphosphonic acid, only two other reports of simple 1D chains based on La3+ and Nd3+ are known to date.21 Relaxing the coordination sphere of the cation to allow the substitution of one phosphonate group by other moiety, but still restricting to a hexacoordinated coordination environment, only one other family of mixedmetal materials has been described to date.22 In short, the occurrence of octahedral lanthanide coordination environments

structural modifications of LnOF architectures: variation of the framework dimensionality from 1D to 3D,15 modification of metallic coordination spheres16 and framework topologies,17 formation of specific coordination modes,18 and even at the tuning of specific functionalities (e.g., uptake of CO2 and CH4 gases).19 To the best of our knowledge the reduction in average crystal size effect observed for the [Ln2(H3bmt)2]·H2O compounds reported herein has not yet been reported to date for LnOF architectures. Noteworthy, the progressive broadening of the reflections of the powder patterns of heavier lanthanides (Figure S4 in SI) does not seem to arise from size effects, but instead they are attributed to crystal strain as modeled for the Yb3+-based material (see technical section in the SI). The structural features of the isotypical [Ln2(H3bmt)2]·H2O series could only be unveiled from ab initio structural solution using laboratory powder X-ray diffraction data (Figure 2), in combination with data derived from other (advanced) characterization techniques such high-resolution powder X-ray diffraction data (synchrotron), thermodiffractometry, thermogravimetry, vibrational spectroscopy, and elemental analysis (see SI for detailed information). Excellent agreements for the Rietveld plots were obtained for both the Eu3+ and the Yb3+based materials (SI Table S1, Figures S1−S3 and also the dedicated technical section). C



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In this context we are now designing new organic ligands that simultaneously permit the existence of this metallic environment while inducing the formation of porous 3D networks.

in the 2D (layered) [Ln2(H3bmt)2]·H2O arises as a truly unique, distinctive feature rarely encountered among this type of compounds. The self-assembly of the {LnO6} octahedral unit (Figure 2a) with the crystallographically independent multitopic H3bmt3− ligand promotes the formation of a hybrid 2D network as depicted in Figure 3. Chains of lanthanide phosphonate are disposed parallel to the b-axis of the unit cell, with the shortest Eu···Eu intermetallic distance being of 5.190(7) Å. Interchain distances along the layer range between 9.656(7) and 9.969(7) Å. Based on a pure topological abstraction, this 2 ∞ [Eu2(H3bmt)2] layer can be envisaged as a binodal 5,5connected network of the 5,5L4 topological type, being to the best of our knowledge the first of its kind based exclusively on chelating phosphonate groups bound to a relatively rigid organic core (see dedicated section in the SI for additional details). Noteworthy, this type of mathematical abstraction of the network can even unveil strong relationships with the previously reported 3D [Ln2(H3bmt)2(H2O)2]·H2O13 photoluminescent materials: removing one internodal connection from the 3D networks (hence mimicking the reduction of the coordination number of the lanthanide from seven to six), one obtains the same topological type herein described for layered 1 (see the SI for additional details and schematic representations). Individual ∞2[Eu2(H3bmt)2] layers stack along the [101] direction of the unit cell to yield the relatively densely packed crystal structure of 1 (inset in Figure 2). The connectivity of the organic H3bmt3− residues permits the existence of several protonated phosphonate groups decorating the surface of the layers (Figure 3c) that serve as supramolecular anchoring points of strong O−H···O hydrogen bonds involving solely the phosphonate groups: as depicted in Figure S5 (in the SI) adjacent layers are glued together by two distinct graph set motifs, R22(8) and R22(10),23 which ultimately cooperatively contribute to the structural robustness of the compound. Remarkably, the partially occupied water molecule of crystallization does not appear to be strongly engaged in additional supramolecular contacts. This structural feature agrees well with the registered FT-IR spectrum for 1 (Figure S11 in SI): two relatively sharp bands centered at ca. 3697 and 3397 cm−1 are attributed to the asymmetric and symmetric stretching vibrational modes of water, respectively; the high wavenumber at which these bands appear is a strong indication of their poor involvement with the remaining hybrid network. Indeed, as observed from the various thermal investigations performed, this water molecule is easily released from the material at around 200 °C without phase transition (Figures S9 and S10 in SI). In conclusion, we have shown that the hydrothermal reaction of lanthanides with progressively smaller ionic radii (i.e., from Eu3+ to Yb3+) with residues of H6bmt leads to the formation of a new series of isotypical layered compounds exhibiting, on average, smaller size for the crystalline particles for the heavier cations. Among the various structural unique features of the 2 ∞ [Ln2(H3bmt)2] layers described in this short Communication, the presence of six-coordinated octahedral environments is of great importance. Indeed, this new family of compounds has shown that with multitopic chelating phosphonate-based organic ligands it is possible to isolate networks having low coordination numbers for the cations and a fully water-free first coordination sphere. This is expected to have strong implications in the design of more photoluminescent materials.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic Information Files (CIFs) for 1 and 8. Experimental details on the powder X-ray diffraction studies and additional structural characterization: evaluation of the distortion degree of the octahedral, supramolecular contacts and comparative detailed topological studies. Detailed synthetic procedures, thermogravimetry, thermodiffractometry, and vibrational spectroscopy studies on the various members of the studied isotypical series. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +351 234 247126. E-mail: fi[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Fundaçaõ para a Ciência e a Tecnologia (FCT, Portugal), the European Union, QREN, FEDER through Programa Operacional Factores de Competitividade (COMPETE), Laboratório Associado Centro de Investigaçaõ em Materiais Cerâmicos e Compósitos, CICECO (FCOMP01-0124-FEDER-037271; ref. FCT PEst-C/CTM/LA0011/ 2013), and QOPNA (FCOMP-01-0124-FEDER-037296; ref. FCT PEst-C/QUI/UI0062/2013) for their general funding scheme. FCT is also gratefully acknowledged for funding the R&D FCOMP-01-0124-FEDER-041282 (ref. FCT EXPL/ CTM-NAN/0013/2013), and for the postdoctoral research scholarhip No. SFRH/BPD/94381/2013 (to SMFV). We further wish to thank the European Synchrotron Radiation Facility (ESRF, Grenoble, France) for granting access to the ID31 beamline through the R&D project CH-3692.

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