Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Embonic Acid Functionalized Niobium Complexes with Selective Dye Sorption Properties Lv-Bing Yuan, Yan-Ping He, Lei Zhang,* and Jian Zhang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China S Supporting Information *
studied. In addition, on the basis of the water stability and large cavities of the three-dimensional (3D) packing framework of Nb12, we investigated its selective sorption ability toward dye molecules with different molecular sizes, such as methylene blue (MB) and rhodamine B (RhB). The reaction of NbCl5 and H4L in a solution of iPrOH/DMF (3:1, v/v) at 100 °C yielded a binuclear niobium(V) complex Nb2. Single-crystal X-ray diffraction reveals that compound Nb2 crystallizes in the monoclinic P2/c space group. In Nb2, the asymmetric unit contains a H2[Nb2(μ2-O)(L)2(OiPr)2] moiety besides some solvent molecules squeezed by the PLATON program because of disorder. Each NbV center is six-coordinated by O atoms including one from a terminal OiPr solvent molecule, giving rise to a distorted octahedral coordination environment. In Nb2, each half of the L ligand binds to one NbV center with the naphthylic −OH and one O atom of the carboxylic group. Two NbV centers are further connected by a μ2-O atom to result in the final binuclear species (Figure 1a). The 3D packing super-
ABSTRACT: Presented here are binuclear Nb2, tetranuclear Ti2Nb2, and dodecanuclear Nb12 complexes decorated by embonic acid ligands. These structures feature a unique Ti−O−Nb or Nb−O−Nb subunit. Besides, the unprecedented dodecanuclear Nb12 cluster further displays a porous three-dimensional packing framework and stability in water, affording an interesting selective uptake for methylene blue molecules over rhodamine B.
P
olyoxometalates (POMs) are an important class of molecular materials because of their unique structural variety and wide applications in various fields such as magnetism, catalysis, medicine, and photophysics.1−5 Polyoxoniobates (PONs) represent an interesting branch of POM chemistry, which have been studied for more than 60 years. Up to now, a great number of PON clusters have been reported,6−12 such as {Nb6},6 {Nb20},7 {Nb24},8 {Nb27},9 {Nb31},9 {Nb32},10 and several Keggin-type PONs {TNb12} (T = Si, Ge).11 Meanwhile, some transition-metal-containing PONs13−21 have also been prepared and characterized, including {V 8 Nb 4 8 }, 1 3 {Co 14 Nb36},14 {Cu24Nb 56 },15 {Ni 10 Nb 32 },16 {Ti 12Nb 6},17 {K12Nb96},18 etc. More recently, Zheng and co-workers set a new record for the family of PONs and report a 114-nuclearity {Li8Nb114},22 which is the largest Nb−O cluster reported to date. On the other hand, functionalization of POMs by organic ligands is also of great significance, and many successful examples of hybrid POMs have been presented.26 However, compared to the pure inorganic PONs, niobium complexes decorated by functional organic species are less investigated.23−25 Earlier, Whitmire’s group synthesized several low-nuclearity heterometallic bismuth−niobium salicylate complexes.24,25 Recently, Amini and co-workers reported a tetranuclear niobium complex with a 8-hydroxy-2-methylquinoline or 5-chloro-8-hydroxyquinoline ligand.23 The introduction of ligands to niobium oxo clusters is still a challenging project and will greatly benefit their structural diversity and also physical properties. Herein, we report the synthesis and structural characterization of binuclear, tetranuclear, and dodecanuclear niobium oxo materials decorated by embonic acid (H4L) ligands, namely, [H2Nb2(μ2-O)(L)2(OiPr)2]·1.5iPrOH·1.5DMF (Nb2; OiPr = isopropyl alcohol; DMF = N,N-dimethylformamide), [H4Ti2Nb2(μ2-O)2(L)4(OnPr)2]·7nPrOH (Ti2Nb2; OnPr = npropanol), and [H 10 Nb 12 (μ 2 -O) 12 (μ 3 -O) 8 (L) 6 (OiPr) 6 ]· 7iPrOH·9DMF (Nb12), respectively. Their thermal stability and ultraviolet−visible (UV−vis) absorption spectra were also © XXXX American Chemical Society
Figure 1. (a) Structure of Nb2 and (b) its 3D packing supermolecular structure. (c) Structure of Ti2Nb2 and (d) its 3D packing supermolecular structure.
structure of these Nb2 complexes shows small channels along the c axis (Figure 1b). There is no typical hydrogen bond, and the binding interaction may be due to π−π interaction (3.5−4.8 Å) between the phenyl rings (Figure S1). It is worth noting that a similar dimeric titanium(IV) complex based on H4L has been reported by Baghel and Rao.27 However, Received: December 28, 2017
A
DOI: 10.1021/acs.inorgchem.7b03265 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
molecular structure, there are abundant weak hydrogen bonds (C−H···O ∼ 3.6−3.8 Å) and π−π interaction (6.1 Å) between two adjacent clusters (Figure S3). To the best of our knowledge, metal oxo clusters with layered core structures such as Nb12 are rarely synthesized. One famous example is the {Ti18O27} cluster in which the titanium oxide core can be viewed as a decked trimer of three near-planar-pentagonal {Ti(Ti5)} building units connected with μ2-O atoms.28 The successful construction of Nb12 indicates that Nb−O can also form such beautiful layer structures. To investigate the thermal stability and phase purity of the obtained complexes, thermogravimetric analyses (TGA) and powder X-ray diffraction (PXRD) patterns were measured. The results show that all of the samples have multistep weight loss before 320 °C, which is related to the removal of guest and coordinated solvent molecules (Figures S5, S7, and S9). They are thermally stable up to 400 °C before their structures collapse. The recorded PXRD patterns are quite similar to those simulated from single-crystal data, confirming the phase purity of the prepared samples (Figures S6, S8, and S10). More interestingly, Nb12 also presents good water stability. When the fresh crystals of Nb12 were immersed in water for 4 h, they maintained their original appearance and cell parameters, although the diffraction intensity became weaker. The PXRD pattern of Nb12 samples immersed in water showed that most of the peak positions were well consistent with the as-synthesized one, although some main peaks were widened and merged (Figure S9). The stability of Nb12 should be attributed to the abundance of weak interaction forces between clusters. We also studied the UV−vis absorption of these materials by using diffuse-reflectance spectroscopy (Figure S14). According to the Kubelka−Munk function, the calculated optical band gaps of Nb2, Ti2Nb2, and Nb12 are ∼1.97, ∼1.44, and ∼1.81 eV, respectively, which are obviously lower than those of Nb2O5 nanocrystals (∼3.40 eV) and other reported Nb−O materials.29 The relatively smaller band gaps of these materials might be attributed to functionalization of the H4L ligands in their structures. Although many metal−organic framework materials with dyeloading functions have been reported,30 the application of cluster materials in this field is still rare. Considering the high water stability and large channels of the 3D packing structure of Nb12, we decided to investigate its sorption ability toward dye molecules with different molecular sizes (Figure S15). We first explored its affinity to the cationic dye MB. Typically 30 mg of freshly prepared Nb12 crystals was immersed in 3 mL of an aqueous solution containing MB (10−2 M). The concentration of the dye was monitored by its maximum sorption wavelength using an UV−vis spectrophotometer. Obviously, the concentration of MB decreased with time, and the original blue solution of MB gradually faded to become colorless (Figure 3a), while for larger-sized RhB, its sorption by Nb12 was significantly poor and the solution still kept the rose color after 4 h (Figure 3b). In addition, the PXRD results confirm that the structure of Nb12 is still stable after dye uptake (Figure S17). The selective uptake for MB by Nb12 in the presence of RhB was also validated. A total of 30 mg of Nb12 crystals was soaked in an aqueous solution containing an equal concentration of MB and RhB. The obtained results clearly indicated that Nb12 could selectively absorb MB from the MB/RhB mixture, with the solution color changing from dark blue to rose (Figure 3c). Such sorption selectivity of Nb12 for MB could be attributed to the size effect. The crystals of Nb12 are unstable and soluble in the DMF
when a mixture of NbCl5 and Ti(OiPr)4 was used, their solvothermal assembly with H4L in the presence of ethylenediamine and nPrOH gave rise to tetranuclear structure Ti2Nb2. Structural analysis revealed that Ti2Nb2 crystallizes in a monoclinic crystal system with the C2/c space group and is featured by two Ti−O−Nb subunits. As shown in Figure 1c, both the TiIV and NbV centers are six-coordinated and present coordinated modes similar to that of NbV atoms in Nb2. However, only the NbV center is bonded by one terminal OnPr solvent molecule. Interestingly, two Ti−O−Nb subunits are held together by two bridging carboxyl groups, forming the final tetranuclear structure. Such dispersed Ti2Nb2 clusters further pack into a 3D superstructure, as seen in Figure 1d. There are weak hydrogen bonds [C−H···O (∼3.5 Å) and O−H···O (∼3.3 Å)] between two adjacent clusters (Figure S2). Interestingly, when 3 drops of 1,2-diaminocyclohexane was added to the synthetic reaction system of Nb2, a dodecanuclear cluster structure Nb12 with six Nb−O−Nb subunits was obtained (Figure 2b). The niobium oxide core in Nb12 can also be viewed
Figure 2. (a) Molecular structure of Nb12. (b) Double-layer cluster core of Nb12. (c) 3D packing supermolecular structure.
as a decked dimer of two near-planar-pyramidal {Nb6} building units connected by six μ2-O atoms with a layer-to-layer distance of ca. 3.7 Å. As shown in Figure 2a, the innermost three NbV centers of each {Nb6} building unit are seven-coordinated and connected through one μ3-O atom to form a {Nb3} subunit. Such a {Nb3} unit is further linked to three other six-coordinated NbV centers by three μ3-O atoms, generating the pyramidal {Nb6}. In particular, the apical NbV atoms are bonded by one OiPr solvent molecule, and the H4L ligands in Nb12 present coordinated modes similar to those in Nb2. Because of the high molecular symmetry of Nb12, which crystallizes in the trigonal crystal system with space group P63/m, the supramolecular packing of these clusters forms a beautiful 3D honeycomb-like superstructure with channels along the c axis. The diameter of the aperture is approximately 15 Å (Figure 2c). In this supraB
DOI: 10.1021/acs.inorgchem.7b03265 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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Communication
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03265. Additional figures, TGA, PXRD patterns, and UV−vis (PDF) Accession Codes
CCDC 1586762−1586764 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Lei Zhang: 0000-0001-7720-4667 Jian Zhang: 0000-0003-3373-9621 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the NSFC (Grants 21473202 and 21673238), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB20000000), and the Natural Science Foundation of Fujian Province (Grant 2017J06009).
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REFERENCES
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Figure 3. Time-dependent UV−vis absorption spectra of MB (a), RhB (b), and MB/RhB (c) in the presence of Nb12. The inserted photographs show the color change before and after dye uptake.
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DOI: 10.1021/acs.inorgchem.7b03265 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.7b03265 Inorg. Chem. XXXX, XXX, XXX−XXX
