Water Soluble and Ultra-Stable Ti4L6 Tetrahedron with Coordination

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Water Soluble and Ultra-Stable Ti4L6 Tetrahedron with Coordination Assembly Function Yan-Ping He, Lv-Bing Yuan, Guang-Hui Chen, Qi-Pu Lin, Fei Wang, Lei Zhang, and Jian Zhang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09463 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017

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Journal of the American Chemical Society

Water Soluble and Ultra-Stable Ti4L6 Tetrahedron with Coordination Assembly Function Yan-Ping He, Lv-Bing Yuan, Guang-Hui Chen, Qi-Pu Lin, Fei Wang, 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. ABSTRACT: We have successfully constructed a tetrahedral Ti4L6 cage with calixarene-like coordination active vertices. It is further featured by high solubility and stability in H2O and DMF/H2O solution, affording interesting stepwise assembly function with other metal ions. Through trapping different amounts of Co or Ln ions, the Ti4L6 tetrahedra can be organized into various dimensional architectures, including Ti4L6-Co3 cage, Ti4L6-Ln2 cage, Ti4L6-Ln2 chain and three-dimensional (3D) Ti4L6-Ln framework. Unusual mixed-valence phenomenon was observed in the Ti4L6 cage, whose Ti3+/4+ compositions were adequately identified by ESR and XPS analyses. More remarkably, the calixarene-like oxygen vertices of the Ti4L6 cage can also be used for the recognition of C3-symmetric dye molecules through N-H⋅⋅⋅O hydrogen bonding. Accordingly, driven by visible light, selective and efficient homogeneous photodecomposition of acid blue 93 (AB-93) and alkali blue 4B (AB-4B) were successfully achieved. Therefore, this work not only represents a milestone in constructing symmetric Ti-based cages with interesting coordination assembly function, also brings a new method for preparing technologically important soluble photo-active cages.

INTRODUCTION Polyhedral coordination cages have attracted considerable attention in the past two decades due to their aesthetically appealing structures1-2 and wide applications in various fields.3-7 Currently, most of the research interests in this field have been devoted to the construction of discrete cages themselves.8 The further assembly of cages into advanced supramolecular materials is relatively less developed. Only a few cages have been organized into networks through labile coordination sites,9-10 interlocked molecules by mechanical bonds,11 and metallohydrogels using weak interactions.12 It still remains a challenge to find efficient methodology for the cage-based further assembly. Reasonably, if a cage could possess coordination function itself, we would easily assemble it with metal ions to form supramolecular architectures. The realization of such proposal depends on the construction of coordination active cages, which are still very rare to date. On the other hand, the preparation of cages mostly involves the metal-directed self-assembly between metal ions and organic ligands. Thus the geometric and electronic characteristics of the selected metal nodes greatly determine the structures and properties of the targeted supramolecular complexes. Compared to the intensively applied Pd,13-15 Pt,16-17 Fe,18-20 Zn,21-22 Co23-24 and lanthanides25-26, metal-organic cages based on the earth abundant Ti are still relatively less. 27-29 As for tetrahedral Ti4L4 and Ti4L6 cages, to the best of our knowledge, only several examples were reported by Raymond.30-31 Considering the excellent photocatalytic performances of titanium containing materials,32-33 the Ti-based cages would also have good potential for photo active applications. 34-35

Based on our former success in titanium-oxo clusters, we decided to investigate the assembly of symmetric polynuclear-

Ti cages (PTCs) and selected the cheap embonic acid (H4L) as a two-fold-symmetric organic linker (Scheme 1). Its central sp3 carbon atom can rotate freely to adopt desired bend angle for the potential Ti4L6 tetrahedral cage construction. Meantime, this ligand contains abundant carboxylate and phenol groups to endow it with strong coordination affinity towards Ti ions. Moreover, as shown in Scheme 1, if the Ti ion is chelated by adjacent phenol and carboxylate oxygen, the η0η1-type carboxylate groups from three edge-L may form a calixarenelike vertex. Then, the four coordination active vertices can further encapsulate other metal ions to acquire additional functionality or assemble the cages into advanced structures.

Scheme 1. The assembly of tetrahedral M4L6 cage with calixarenelike coordination active vertices for potential metal ion trapping.

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Following the above structural design, Ti(OiPr)4 or TiCl4 was solvolthermally assembled with embonic acid (H4L) in the presence of ethylenediamine (en), n-propanol and N,Ndimethylformamide (DMF), giving rise to anionic tetrahedral Ti4L6 cages in orthorhombic or cubic supramolecular arrangements (PTC-101 or PTC-102) (Table 1, Figures 1 and S4). Expected coordination active vertices are found in this Ti4L6 tetrahedron. Moreover, electrospray ionization mass spectrometry (ESI-MS) analysis indicates that it is soluble and stable in H2O and DMF/H2O solution. Such unique characters allow the Ti4L6 cage to further trap Co or Ln ions to achieve advanced assemblies from 0 to 3-dimensionality (PTC-103 to 106). Similar Zr4L6 (PTC-101(Zr) and PTC-102(Zr)) and Hf4L6 (PTC-101(Hf) and PTC-102(Hf)) tetrahedra were also constructed, proving the universality of our method. Interestingly, electron spin resonance (ESR) spectrum and X-ray photoelectron spectroscopy (XPS) studies confirm the existence of Ti3+ ions in the Ti4L6 cages of PTC-101 and PTC-102, making them rare mixed-valence Ti3+/4+ complexes. And the aqueous soluble Ti4L6 cages can also act as visible light active homogenous photocatalysts for dye decomposition. Moreover, such photodecomposition process is not only efficient, but also quite selective which should be attributed to the molecular recognition ability of Ti4L6 cage through its calixarene-like oxygen vertices.

tron spin resonance (ESR) spectra were recorded on a Bruker ER-420 spectrometer with a 100 kHz magnetic field in the X band at room temperature.

Table 1. Summary of the cage compositions and characteristics of the crystallographicly studied cage-based compounds.

Synthesis of PTC-102(Zr) and PTC-102(Hf): Substituting TiCl4 with ZrCl4 and HfCl4 in the above synthetic procedure for PTC-102.

Complex

Cage Composition

Space Group

Structure Dimensions

PTC-101

[Ti4L6]

Fddd

0D

PTC-102

[Ti4L6]

I23

0D

PTC-102(Zr)

[Zr4L6]

I23

0D

PTC-102(Hf)

[Hf4L6]

I23

0D

PTC-103

[Co3(Ti4L6)(H2O)15]

P-3c1

0D

PTC-104

[Eu2(Ti4L6)(H2O)8(DMF)6]

C2/c

0D

PTC-105

[Tb2(Ti4L6)(H2O)13]

C2/c

1D

PTC-106

[Eu(Ti4L6)(H2O)3]

P213

3D

Synthesis of (Me2NH2)8[(Ti4L6)]·4(n-PrOH)·2(en)·6(DMF) PTC-101: Ti(OiPr)4 (160 μL, 0.5 mmol), H4L (155 mg, 0.4 mmol) and 2 drops of ethylenediamine (en) were added to 6 mL of n-propanol/DMF (3:1, v/v) and mixed at room temperature. The resultant solution was heated at 100 °C for three days. After cooled to room temperature, red crystals of PTC101 were obtained. (yield: 78% based on H4L). Elemental analysis for C188H226N18O46Ti4, Calcd (%): C, 61.60; H, 6.21; N, 6.87. Found: C, 61.56; H, 6.27; N, 6.79. Synthesis of PTC-101(Zr) and PTC-101(Hf): Substituting Ti(OiPr)4 with Zr(OnPr)4 or HfCl4 (en replaced by NaOH (10 mg, 0.25 mmol)) in the above synthetic procedure for PTC101. Synthesis of (Me2NH2)10[(Ti4L6)]Cl·7(n-PrOH)·4(DMF) PTC102: TiCl4 (90 μL, 0.4 mmol), H4L (120 mg, 0.3 mmol) and 2 drops of en were added to 6 mL of n-propanol/DMF (3:1, v/v) and mixed at room temperature. The resultant solution was heated at 100 °C for two days. After cooled to room temperature, red crystals of PTC-102 were obtained. (yield: 65% based on H4L). Elemental analysis for C191H235N14O47ClTi4, Calcd (%): C, 61.90; H, 5.92; N, 5.29. Found: C, 61.95; H, 5.89; N, 5.30.

Synthesis of (Me2NH2)2[Co3(Ti4L6)(H2O)15]·30(H2O)·14(DMF) PTC-103: PTC-101 (or PTC-102) (146 mg (or 148 mg), 0.04 mmol) and CoCl2·6H2O (48 mg, 0.2 mmol) were added to 6 mL of DMF/H2O (2:1, v/v) and mixed at room temperature. The resultant solution was heated at 80 °C for three days. After cooled to room temperature, red crystals of PTC-103 were obtained. (yield: 88% (or 56%) based on PTC-101 (or PTC102)). Elemental analysis for C184H276N16O95Co3Ti4, Calcd (%): C, 48.03; H, 6.05; N, 4.87. Found: C, 48.11; H, 6.01; N, 4.92. The experimental Co:Ti molar ratio of 1:1.34 was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Synthesis of PTC-103(Mn): Substituting CoCl2·6H2O with MnCl2·4H2O in the above synthetic procedure for PTC-103.

EXPERIMENTAL SECTION All reagents were purchased commercially and used without further purification. All syntheses were carried out in a 20 ml vial under autogenous pressure. Elemental analysis (C, H, and N) was carried out on a Vario Micro E III analyzer. All powder X-ray diffraction analyses were recorded on a Rigaku Dmax2500 diffractometer with Cu Kα radiation (λ = 1.54056 Å) with a step size of 0.05°. Thermal stability studies were carried out on a NETSCHZ STA-449C thermoanalyzer with a heating rate of 10 °C/min under a nitrogen atmosphere. ESIMS was carried out on Impact II UHR-TOF (Bruker). UV-Vis absorption spectra were measured on a Perkin-Elmer Lambda 950 UV-Vis spectrophotometer. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Physical Electronics Quantum 2000 instrument, equipped with a monochromatic Al Kα source (Kα = 1486.6 eV) at 300 W under UHV. Elec-

Synthesis of (Me2NH2)2[Eu2(Ti4L6)(H2O)8(DMF)6]·24(H2O)·9 (DMF) PTC-104: PTC-101 (or PTC-102) (146 mg (or 148 mg), 0.04 mmol) and Eu(NO3)3·6H2O (120 mg, 0.27 mmol) were added to 6 mL of DMF/H2O (1:1, v/v) and mixed at room temperature. The resultant solution was heated at 60 °C for two days. After cooled to room temperature, red crystals of PTC-104 were obtained. (yield: 43% (or 26%) based on PTC101 (or PTC-102)). Elemental analysis for C187H257N17O86Eu2Ti4, Calcd (%): C, 48.67; H, 5.61; N, 5.16. Found: C, 48.71; H, 5.67; N, 5.22. Synthesis of PTC-104(Y, Pr, Nd, Sm, Gd): Substituting Eu(NO3)3·6H2O with relative Ln salts in the above synthetic procedure for PTC-104. Synthesis of (Me2NH2)2[Tb2(Ti4L6)(H2O)13]·22(H2O)·8(DMF) PTC-105: PTC-101 (or PTC-102) (146 mg (or 148 mg), 0.04 mmol) and Tb(NO3)3·6H2O (67 mg, 0.15 mmol) were added


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to 6 mL of DMF/H2O (2:1, v/v) and mixed at room temperature. The resultant solution was heated at 60 °C for one day. After cooled to room temperature, red crystals of PTC-105 were obtained. (yield: 72% (or 43%) based on PTC-101 (or PTC-102)). Elemental analysis for C166H214N16O79Tb2Ti4, Calcd (%): C, 47.39; H, 5.32; N, 5.12. Found: C, 47.45; H, 5.41; N, 5.09.

RESULTS AND DISCUSSION

Synthesis of PTC-105(Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu): Substituting Tb(NO3)3·6H2O with relative Ln salts in the above synthetic procedure for PTC-105. Synthesis of (Me2NH2)5[Eu(Ti4L6)(H2O)3]·16(H2O)·7(DMF) PTC-106: PTC-101 (or PTC-102) (146 mg (or 148 mg), 0.04 mmol) and Eu(NO3)3·6H2O (34 mg, 0.075 mmol) were added to 6 mL of DMF/H2O (2:1, v/v) and mixed at room temperature. The resultant solution was heated at 80 °C for six hours. After cooled to room temperature, red crystals of PTC-106 were obtained. (yield: 95% (or 87%) based on PTC-101 (or PTC-102)). Elemental analysis for C169H199N12O62EuTi4, Calcd (%): C, 54.36; H, 5.37; N, 4.51. Found: C, 54.42; H, 5.34; N, 4.55. Synthesis of PTC-106(Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu): Substituting Eu(NO3)3·6H2O with relative Ln salts in the above synthetic procedure for PTC-106. X-ray Crystallography. Crystallographic data of PTC-101 to PTC-106 were collected on a Supernova single crystal diffractometer equipped with graphite-monochromatic Cu Kα or Mo Kα radiation (λ = 1.54056 Å or 0.71073 Å) at 273 K. Absorption correction was applied using SADABS.36 Structure was solved by direct method and refined by full-matrix leastsquares on F2 using SHELXTL.37 In these structures, some cations and free solvent molecules were highly disordered and could not be located. The diffused electron densities resulting from these residual cations and solvent molecules were removed from the data set using the SQUEEZE routine of PLATON38 and refined further using the data generated. In addition, for structures PTC-102 to PTC-105, their crystals under investigation showed no significant intensity above 1.05 Å resolutions. Because of the low diffraction intensity, some benzene rings of the L ligands could not be located in PTC-105. Crystal data and details of data collection and refinement of PTC-101 to PTC-106 including PTC-102(Zr) and PTC-102(Hf) were summarized in Table S1. CCDC 1552882-1552889 contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Centre.

Figure 1. Structures of the ∆∆∆∆-[Ti4L6] and ΛΛΛΛ-[Ti4L6] isomers in PTC-101. Photos of its crystals and solution in DMF-H2O are also illustrated in the middle. Atom color code: green Ti; red O; gray C.

Characterizations of the Ti4L6 Cages. Single-crystal X-ray diffraction analysis reveals that the Ti4L6 cage in PTC-101 and PTC-102 displays a tetrahedral geometry with the four Ti atoms situating at the vertices and the six L ligands defining the edges (Figure 1). Each Ti center is chelated by three carboxylphenol groups from three different ligands, giving rise to distorted octahedral coordination geometry. Another uncoordinated carboxyl oxygen atom is exposed outside. Thus the three carboxyl groups from three different ligands around the same Ti atom form a calixarene-like arrangement filled with coordination active oxygen sites. The ligands in the Ti4L6 cage are in the twisted forms, with an average intersection angle of 87.4o between two naphthalene rings in PTC-101 (Figures S3). The Ti···Ti distances are in the range of 8.79 and 8.92 Å. The free spaces inside and between Ti4L6 cages are occupied by the disordered amine cations and solvent molecules. Interestingly, this Ti4L6 cage is a chiral one (Figure S2). The guest molecules significantly influence the cage resolution, with PTC-101 containing the ∆∆∆∆-[Ti4L6] and ΛΛΛΛ-[Ti4L6] isomers in its achiral crystal structure. However, the crystallization of PTC102 occurred spontaneous resolution, leading to a chiral structure with simple ∆∆∆∆-[Ti4L6] isomers.

Photocatalysis. Photodecomposition of dyes (AB-93, AB-4B, MB, MO, RhB) by PTC-101 (PTC-106) was investigated under visible light irradiation (LED-T8). Typically 20 mg photocatalyst of PTC-101 (PTC-106) was added to 2.5 mL H2O/DMF (4:1, v/v), and then 50 µL of dye aqueous solution (1.25 × 10-2 M) was added to the mixture in the dark for 5 min to achieve an adsorption−desorption equilibrium, followed by visible light irradiation. The concentration of dye was determined at its maximum absorption wavelength using an UV−vis spectrophotometer. Figure 2. Negative-mode ESI-MS spectrum of PTC-101 in DMF/H2O solution.



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As mentioned above, in the structure of the Ti4L6 cage, there are four calixarene-like vertices filled by oxygen atoms, which should be active for coordination to metal ions. And in the view of charge balancing, the anionic nature of the Ti4L6 cage also makes it very easily to combine cationic metal ions. Therefore, this Ti4L6 should be an ideal candidate for the investigation on the stepwise assembly of metal-organic cages. Before carrying out this proposal, the solution behaviors of the Ti4L6 cage were studied. It is very interesting to find that the crystals of PTC-101 and PTC-102 can be readily dissolved in water (saturated at 2.5 g·L˗1 at 80 °C), making this Ti4L6 tetrahedron a rare example of water soluble cages (Figure S5). When DMF was further introduced as co-solvent, the solubility of Ti4L6 cages significantly increased (Figure 1). Then, ESI-MS spectra of the H2O and DMF/H2O solution of PTC-101 were recorded (Figures S5 and 2), confirming the high stability of the Ti4L6 cage after dissolution.

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Coordination Assembly of the Ti4L6 Cages with Metal Ions. Based on the coordination activity and solubility/stability of the Ti4L6 tetrahedron, its stepwise assembly with other metal ions was investigated. At the beginning, Co2+ ions were added to the DMF/H2O solution of PTC-101 which was heated at 80°C for three days, forming red hexagonal-prism crystals of PTC-103. Structural analysis reveals that PTC-103 crystallizes in trigonal crystal system with space group P-3c1. As shown in Figure 3a, each Ti4L6 cage catches three Co atoms by three formerly uncoordinated carboxyl oxygen atoms at one vertex of the tetrahedron. Each Co center is six-coordinated to form a distorted octahedral geometry. The 3D packing superstructure of these Ti4L6-Co3 cages shows honeycomb-like channels with an effective window size of 15.5 Å along the c-axis (Figure 3b). Similar reactions using DMF/H2O solution of PTC-102 as cage source also produced PTC-103. In addition, an isostructural compound PTC-103(Mn) could also be obtained by substituting Co2+ with Mn2+ in the synthetic procedure, confirmed by crystallographic unit cell index and powder XRD pattern comparison (Figure S22). Encouraged by this result, more cage assembly procedures were studied using trivalent Ln3+ ions with higher coordination number instead of the above Co2+ ion. Interestingly, a series of Ti4L6-Ln compounds with different structural dimensionalities were readily synthesized by tuning the concentration of Ln3+ ions. The cage:Ln molar ratio of 1:6.8 gave the formation of PTC-104, which crystallizes in the monoclinic system with a space group of C2/c. As shown in Figure 3c, each Ti4L6 cage captures two Eu atoms by two uncoordinated carboxyl oxygen atoms at two vertices of the tetrahedron, and each Eu atom is eight-coordinated. Such dispersed 0D Ti4L6-Eu2 cages further pack into a 3D superstructure with small channels along the caxis (Figure 3d). When increasing the cage:Ln molar ratio to 1:3.8, red crystals of PTC-105 were obtained, in which each Ti4L6 cage catches three Tb atoms by three uncoordinated carboxyl oxygen atoms at three vertices of the tetrahedron (Figures 3e and S8). There are two crystallographically independent Tb atoms (Tb1 and Tb2) in the asymmetric unit. Ii is worth noting that Tb1 atom links two adjacent cages to form a Z-shaped infinite chain, which further packs into a 3D dense superstructure (Figure S9). Finally, by further increasing the cage:Ln molar ratio to 1:1.9, a 3D Ti4L6-Ln framework structure of PTC-106 was successfully built. It crystallizes in a symmetric cubic crystal system with space group of P213. Although each Ti4L6 cage in PTC-106 also traps three lanthanide atoms, here each Eu atom connects three Ti4L6 cages to generate a 3D framework structure (Figures 3f and 3g). From the viewpoint of topology, the framework of PTC-106 can be described as a 3connected network by reducing both Ti4L6 cages and Eu atom as nodes.

Figure 3. Molecular and packing structures of PTC-103 (a, b) and PTC-104 (c, d). Z-shaped chain structure of PTC-105 (e). Molecular structure (f) and 3D framework topology (g) of PTC-106.

Interestingly, such assembly of Ti4L6 cage with Ln ions can be efficiently applied to almost the whole range of rare-earth elements. Through replacing the used Eu or Tb ions during the synthesis of PTC-104, PTC-105 and PTC-106 with other lanthanides, three series of Ti4L6-Ln complexes can be readily prepared. And comparison PXRD analyses clearly confirm that they are isostructural as PTC-104, PTC-105 and PTC-106 (Figures 4 and S24). Moreover, both the DMF/H2O solutions of PTC-101 and PTC-102 can be applied as the sources of Ti4L6 cages (Figure S28).


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Figure 5. (a) Crystals of PTC-102, PTC-102(Zr) and PTC-102(Hf). (b) ESR spectra of PTC-102, PTC-102(Zr) and PTC-102(Hf). (c) XPS Ti2p band of PTC-102, indicating the deconvoluted signals.

Figure 4. PXRD patterns of the two series of Ti4L6-Ln compounds that are isostructural as PTC-105 (a) and PTC-106 (b), respectively.

Photo and Electronic Characteristics of the Ti4L6 Cages. To further verify the applicability and universality of our construction methodology for tetrahedral cage, other tetravalent metal ions like Zr4+ and Hf4+ were introduced instead of Ti4+ into to the assembly system with embonic acid. As a result, isostructural Zr4L6 and Hf4L6 cages were obtained in the structures of PTC-101(Zr), PTC-101(Hf) and PTC-102(Zr), PTC-102(Hf), respectively. Although presenting structural similarity, Ti, Zr, Hf-based tetrahedra show different photophysical properties. All the Ti-complexes display red colors, while the Zr, Hfcomplexes are yellow (Figures 5a and S11). UV−vis diffuse reflection spectra studies (Figure S12) confirm that Ticomplexes have relatively lower bandgaps around 2.0 eV, which should be mainly due to the penetration of the highest occupied phenols levels into the bandgap of the Ti/O core.39 The calculated bandgaps of the yellow Zr(Hf)-based compounds are ∼2.60 eV, which are obviously higher than the Tianalogues.

Electron spin resonance (ESR) analysis was applied to study the electronic characteristics of the metal ions in the prepared cage compounds. It is interesting to find that the ESR spectrum of PTC-102 exhibits strong signal at g = 1.985 that could be assigned to Ti3+ (Figure 5b).40 The other signal at g = 2.038 should be attributed to O2− which is generated by reducing atmospheric O2 with the surface Ti3+ ions.41 The presence of Ti3+ in PTC-102 was further confirmed by X-ray photoelectron spectroscopy (XPS) studies. As shown in Figure 5c, the broad peaks indicate the multiple oxidation states of Ti ions. After deconvolution, the signals at 459.01 and 464.86 eV can be attributed to Ti4+, whilst the signals at 457.82 and 463.53 eV belong to Ti3+.42 The relative content of Ti3+ in PTC-102 was obtained by comparing the XPS peak areas and roughly estimated to be 25%,43 corresponding to one Ti3+ per Ti4L6 cage. However, the exact positions of Ti3+ ions can not be determined by crystallographic analysis, possibly due to the statistics distribution of Ti3+/4+ ions. Similar but much weaker ESR and XPS signals are also found in the spectra of PTC-101 (Figures S13 and S14). The production of these Ti3+ ions might be attributed to the applied organic amines which have already proven to have reductive activities under solvolthermal conditions.44-45 It is well known that Ti3+-doped TiO2 shows significantly enhanced visible-light driven photocatalytic performances.43, 46 However, this kind of molecular mixed-valence Ti3+/4+ complexes is still very rare. Thus, here we supply a new approach towards the important Ti3+/4+-O materials. In comparison, no signals can be detected in the ESR spectra of PTC102(Zr) and PTC-102(Hf). XPS studies also confirm the complete Zr4+ and Hf4+ compositions (Figures S15 and S16). Therefore, although Zr and Hf ions show similar cage assembly behaviors as Ti, their redox characters are quite different.



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lecular recognition and results in poor photodecomposition activities (Figure S36).

Figure 6. (a) Photos of solutions of AB-93, PTC-101 and photodecomposition process of AB-93. (b)Absorption peak of AB-93 at different time intervals. Initial conc. of AB-93: 2.45 × 10-4 M; catalyst dose: 20 mg PTC-101 (2.14 × 10-3 M).

Molecular Recognition based Photoactivities of the Ti4L6 Cages. Considering the excellent photocatalytic performances of Ticomplexes, the photoactivities of the T4L6 cages were evaluated using dye photodecomposition as models. Based on its high aqueous solubility, the solution of PTC-101 was used as homogenous photocatalyst. And due to its lower bandgap, visible light (LED lamp for the normal room lighting) has been selected as the irradiation source. It is very interesting to find that the obtained T4L6 cages are active for the decomposition of acid blue 93 (AB-93), which can almost be completely decomposed within 10 min (Figure 6). Moreover, the capacity of this homogenous photocatalyst is also very high, which can still retain its efficiency after continuously adding four doses of AB-93 (Figure S30). However, its photoactivities towards other dye molecules are very poor, such as methylene blue (MB), methyl orange (MO) and rhodamine B (RhB) (Scheme 2, Figure S37). The intensities of the typical absorption peaks of these three dyes do not show obvious decreases even after 1h. We think the above photocatalytic selectivity of the T4L6 cage might be attributed its different supramolecular recognition ability towards these dye molecules. As shown in Scheme 2, from the geometric point of view, the calixarene-like oxygen vertices of the T4L6 cage match very well with the three –NHgroups in AB-93. Thus the molecules of AB-93 can be drawn close to the T4L6 cages by three N-H⋅⋅⋅O hydrogen bonds in the solution, which can advance the energy or electron transfer between them and further promote the decomposition of AB93. ESI-MS analysis of the products after AB-93 photodecomposition indicates the bond-break at the secondary amine position, which further confirms the important effect of the above N-H⋅⋅⋅O hydrogen bonding (Figure S38). As for MB, MO and RhB, lack of such geometry matching could not produce mo-

Scheme 2. (a) Structures of the tested dye molecules. (b) Illustration of the proposed recognition of AB-93 by Ti4L6 cage through three N-H⋅⋅⋅⋅O hydrogen bonds.

In contrast, another dye molecule, alkali blue 4B (AB-4B) which has similar but less symmetric structural characteristics than AB-93 could also be decomposed by the T4L6 cage, but with lower efficiency (completed in 30 min) (Figures S33 and S35). Furthermore, the heterogeneous state of the T4L6 cages (PTC-106) shows significantly lower activities than the homogenous cage (Figures S32 and S34). This might be due to the reasons that the dye molecules can only be adsorbed into the surface of the solids. And in the solid state, it would also be difficult for the T4L6 cages to move and rotate to recognize AB93 and AB-4B through hydrogen bonding. CONCLUSIONS In summary, a water soluble and stable tetrahedral Ti4L6 cage with universal coordination assembly abilities and selective dye photodecomposition activities has been reported here. Attributed to its four calixarene-like active vertices, this Ti4L6 cage shows interesting stepwise assembly function with other metal ions. The Ti4L6-Co3 cage and different Ti4L6-Ln structures from simple cage to 1D chain and 3D framework were successfully constructed. Moreover, the calixarene-like oxygen vertices of the Ti4L6 cage have also been used for the recognition of dye molecules through N-H⋅⋅⋅O hydrogen bonding. As a result, the homogenous Ti4L6 cage presents efficient and selective photodecomposition activities towards AB-93 and AB-4B. Therefore, our work not only provides an efficient methodology to build coordination active cages for advanced supramolecular materials assembly, but also opens a new way


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to produce photoactive cage materials with good activity and selectivity.

ASSOCIATED CONTENT Supporting Information The additional experimental details and figures. This material is available free of charge via the Internet at http://pubs.acs.org. The X-ray crystallographic coordinates for structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition numbers 15528821552889 (PTC-101 to PTC-106). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

AUTHOR INFORMATION Corresponding Author [email protected]; [email protected].

ACKNOWLEDGMENT This work is supported by NSFC (21425102, 21473202, 21403235, 21521061 and 21673238), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000) and FJKJT (2017J06009).

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SYNOPSIS TOC


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Water Soluble and Ultra-Stable Ti4L6 Tetrahedron with Coordination

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