Two Novel 3d-4f Heterometallic Frameworks Assembled from a

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Two Novel 3d-4f Heterometallic Frameworks Assembled from a Flexible Bifunctional Macrocyclic Ligand Xian-Dong Zhu,‡,† Zu-Jin Lin,† Tian-Fu Liu,† Bo Xu,† and Rong Cao*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China ‡ College of Biological and Chemical Engineering, Anhui Polytechnic University, Wuhu 241000, People’s Republic of China S Supporting Information *

ABSTRACT: Two novel 3d-4f heterometallic porous metal− organic frameworks with a flexible bifunctional macrocyclic ligand have been successfully synthesized and characterized. The mononuclear [M(TETA)]2− complexes exhibiting two kinds of connection models to lanthanide ions give rise to three-dimensional pillared-layer frameworks with highly solvent accessible volume. Interpenetration is avoided by using the nanosized secondary building units of transition metal macrocyclic complex.

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functionalized macrocyclic polyamine ligand, 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (H4TETA), as flexible bifunctional organic linker to construct porous MOFs. Previous studies of other groups shown that the combination of H4TETA with 3d metals usually produce mononuclear or binuclear complexes in mild room temperature.7 The metal ion is coordinated by four amino nitrogens of the macrocycle and two carboxylates on the pendant arms, whereas the other two carboxylates are protonated and not involved in coordination. On the other hand, our former study has demonstrated multidimensional lanthanide-based MOFs can be synthesized through hydrothermal conditions but with slim cavities, in which the flexible linkers coordinate to metal ions just using pendant carboxylate arms, whereas nitrogen atoms on the ring are free of coordination.8a On the basis of these facts, our strategy was to build porous MOFs by interconnecting transition metal macrocyclic complex [M(TETA)]2− possessing uncoordinated carboxylate oxygens with lanthanide cations having affinity for oxygen (Scheme 1). Moreover, bulkiness of the macrocyclic complex often prevents interpenetration of the network. As we know, interpenetration is a major impediment in the achievement of channels or cavities in MOFs even though it occasionally provides open structures. Fortunately, prismatic crystals of [Eu2Zn3(TETA)3(H2O)4]·12H2O (1) and [Gd2Cu3(TETA)3(H2O)2]·6H2O (2) have been successfully assembled via hydrothermal reaction, which represents the first two novel 3d-4f heterometallic porous MOFs with a flexible macrocyclic polyamine ligand through elaborate design.

ssembly of metal−organic frameworks (MOFs) with porous architecture is currently of great interest due to their potential applications in ion exchange, gas storage and separation, sensing probe, and catalysis.1 Compared to conventionally used microporous inorganic materials such as zeolites, these organic−inorganic hybrids have the advantage of rational design by adopting different metal cations and organic ligands. Up to now, most attention has been focused on the homometallic porous MOFs, especially transition metal carboxylate networks. Heterometallic MOFs with open structures, especially containing lanthanide ions, are still less developed.2,3 The challenge of forming permanently porous materials is apparent, which can be attributed to the higher coordination numbers and variable geometries of 4f metals frequently causing lattice interpenetration and, consequently, making a crystalline solid with no pores. Furthermore, the competitive reaction between 3d and 4f metals chelated to the same ligand tends to give homometallic complexes rather than 3d-4f heterometallic ones. Some general design and assembly strategies toward porous MOFs have been systematically investigated, such as secondary building units (SBUs), secondary building blocks (SBBs), pillared-layer approach, and reticular synthesis.4 The selection of appropriate organic linkers with given shape and function would be a crucial step of the assembly process. In recent years, conformational rigid polycarboxylates, such as 1,3,5-benzenetricarboxylates and its analogues, have been demonstrated to be efficient skeletons to construct porous MOFs.5 Compared with rigid ligands, flexible ligands with multiple conformations will endow the assembly system with structural diversity and more intriguing topologies as well as properties.6 It is therefore an important aspect that is worthwhile paying attention to. Keeping this in mind, we have recently selected a tetraaza© 2012 American Chemical Society

Received: June 23, 2012 Revised: August 23, 2012 Published: August 28, 2012 4708

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the axial direction than the other two (Table S2, Supporting Information). Interestingly, this is in contrast to the geometry found for the [Cu(TETA)]2− species, which is elongated along the O−Cu−O axis.6c The 14-membered macrocyclic ring adopts trans-III configuration according to the nomenclature of Bosnich, Poon & Tobe.10 As the two carboxylate oxygens and other two acetate groups not involved in coordination with Zn2+ are further connected to lanthanide ions, the mononuclear [Zn(TETA)]2− complex can be considered as multidentate SBUs. Notably, three crystallographically unique [Zn(TETA)]2− SBUs exhibit two kinds of connection models in the construction of the polymeric structure of complex 1. First, [Zn(TETA)]2− SBUs connect to four Eu3+ atoms with its four acetate groups adopting μ4κ1:κ2:κ1:κ2 or μ4-κ1:κ1:κ1:κ1 binding modes and further extended to form the two-dimensional (2D) heterometallic layers in the ab plane (Figure 2). Second, [Zn(TETA)]2− SBU chelates two

Scheme 1. Perspective Representation of the Transition Metal Macrocyclic Complex [M(TETA)]2− Considered As Multidentate SBUs

Single-crystal X-ray diffraction, elemental analysis, and vibrational spectroscopic studies performed on complexes 1 and 2 reveal that they are almost isostructural except for the different number of coordinated and lattice waters.9 Hence, the crystal structure of compound 1 is depicted here in detail. The asymmetric unit consists of one crystallographically unique Eu3+ motif and three Zn2+ atoms. As illustrated in Figure 1, Eu1

Figure 2. View of the 2D heterometallic layers of complex 1 in the ab plane.

Eu3+ atoms with its two acetate groups adopting μ2-κ2:κ2 binding mode, which link 2D heterometallic layers giving rise to a 3D pillared-layer framework with 1D hexagonal channels with dimensions of about 17.82 × 15.03 Å2 along the [100] direction (Figure 3). All coordinated water and unbonded carboxylate oxygens point toward the channels, forming hydrophilic cavities filled with a mass of guest water molecules. The void volume of the channels without the guest molecules, calculated by PLATON,11 is about 44.7%. In compound 1, interpenetration is avoided by using the nanosized [Zn(TETA)]2− SBUs. The highly solvent accessible volume therefore has been achieved (Figure 4). The analogous FT-IR spectra of complexes 1 and 2 are consistent with the structure analysis (Figure S1 in Supporting Information). The spectrum of complex 1 clearly exhibits the characteristic vibrational bands for carboxylate groups (COO−···Ln3+) at 1584 cm−1 for the asymmetric stretching and at 1461 cm−1 for symmetric stretching.12 Meanwhile, the absence of the band ranging from 1690 to 1730 cm−1 (υCO for COOH) demonstrates the complete deprotonation of the carboxylate groups. The strong and broad absorption centered

Figure 1. ORTEP representation of the macrocyclic ligand and coordination environment of metal ions with thermal ellipsoids at 30% probability level in complex 1.

atom is nine-coordinated by seven carboxylate oxygens from five TETA4− ligands and two water molecules, which displays distorted tricapped triprismatic geometry. The Eu−O bond lengths vary from 2.338(5) to 2.600(5) Å. The Eu−O−Eu bond angles are in the range of 51.20(18)−151.1(2)°. The coordination polyhedrons around the three Zn2+ centers are both slightly distorted trans-octahedra, the equatorial planes of which comprise two amino nitrogens of the macrocyclic ring and two carboxylate oxygens on the pendant arms; the other two nitrogens occupy the remaining apical coordinate sites. This is clearly indicated by the longer Zn−N bond distances in 4709

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Complex 1 exhibits an intense, characteristic red europium emission upon excitation with a wavelength of 393 nm. As shown in Figure S3 in Supporting Information, transitions from the excited 5D0 state to the different J-levels of the lower 7F state were observed in the emission spectrum (J = 0−2, 4), i.e., 5 D0 → 7F0 at 579 nm, 5D0 → 7F1 at 591 nm, 5D0 → 7F2 at 614 nm, and 5D0 → 7F4 at 693 nm. It is well-known that the transition 5D0 → 7F0 is strictly forbidden in a field of high symmetry and, therefore, the Eu(III) ions in complex 1 should occupy sites with low symmetry and no inversion center should be present for these sites, in agreement with the crystal structural analysis. The emission intensity of the so-called hypersensitive 5D0 → 7F2 transition is almost nine times compared to the 5D0 → 7F1 transition, which also demonstrates a lower symmetric coordination environment of the Eu(III) ions in the structure.13 To summarize, the first two novel 3d-4f heterometallic porous MOFs with a flexible bifunctional macrocyclic ligand have been successfully synthesized through elaborate design. The mononuclear [M(TETA)]2− complexes exhibiting two kinds of connection models to lanthanide ions give rise to 3D pillared-layer frameworks with highly solvent accessible volume. This work clearly illustrates that interpenetration can be avoided by using the nanosized SBUs of transition metal macrocyclic complex. We believe that this strategy may have some general application in the search for building porous hybrid materials. Further investigation focusing on the stability of the pores, properties of these compounds, and construction of analogues MOFs is now in process.

Figure 3. Perspective view of 3D pillared-layer framework of complex 1 along the [100] direction; the guest water molecules are omitted for clarity.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic file for compounds 1 and 2 in CIF format; selected bond lengths and angles for 1 and 2 (Tables S1−S2); FT-IR spectra for 1 and 2 (Figure S1); TGA curve for 1 and 2 (Figure S2); fluorescence spectrum for 1 (Figure S3). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-591-83796710. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported from 973 Program (2011CB932504, 2012CB821705), 863 Program (2011AA03A407), NSFC (91022007, 21171003), NSF of Fujian Province (E0520003), Fujian Key Laboratory of Nanomaterials (2006L2005), and NSF of Anhui Province (1208085QB44).

Figure 4. Space-filling model of complex 1 along the a axis showing the large 1D hexagonal channels; the guest water molecules are omitted for clarity.

at 3412 cm−1 is attributed to the presence of water molecules in the compound. The two compounds show similar thermal stability, as measured by thermogravimetric analysis (TGA). For compound 1, it reveals that a continuous weight loss of 13.95% occurs from 25 to 195 °C, which can be attributed to the release of crystallization guest water molecules subsequently the coordinated water molecules (calc. 13.90%). No further weight loss was observed until it reached 270 °C, at which the decomposition of the complex occurred (Figure S2 in Supporting Information). TGA data show the enhanced stability of coordination framework, which is a very significant value given that only flexible macrocyclic ligand are involved.

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REFERENCES

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