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Mar 6, 2018 - ABSTRACT: A series of dicarboxylates bridged titanium-oxo clusters with {Ti3(μ3-O)} building units have been synthesized through facile...
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Dicarboxylate Ligands Oriented Assembly of {Ti3(μ3‑O)} Units: From Dimer to Coordination Triangles and Rectangles Mei-Yan Gao, Xi Fan, 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 *

ABSTRACT: A series of dicarboxylates bridged titanium-oxo clusters with {Ti3(μ3-O)} building units have been synthesized through facile one-step solvothermal reactions. It is interesting to find that the geometric characteristics of the obtained supramolecular structures highly depend on the configuration of the applied dicarboxylate ligands. A linear dimeric [Ti3O]2L2 complex can be constructed using flexible cyclohexanedicarboxylic acid, while the introduction of rigid 2-nitro-1,4-benzenedicarboxylic acid gives rise to a triangular [Ti3O]3L3 structure. Moreover, unusual [Ti3O]4L4 rectangles have been achieved with more symmetric 5nitro-1,3-benzenedicarboxylic acid or terephthalic acid. Furthermore, a photochromic effect is observed on the obtained complexes upon UV−vis light irradiation in the presence of alcohol.

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reported over the past few decades,8,22−30 including the highest nuclearity carboxylate-chelated Ti52 cluster.31 However, the PTCs coordinated by dicarboxylate ligands with novel photochromic behavior are still rarely reported.32−34 In addition, to our knowledge, no study has been performed on how the configuration of the ligands will affect the geometrical structures of the obtained PTCs. As a follow-up of our previous research on PTCs,8−11,22,24,28,31,35 we report herein the synthesis, structural analysis, and the photochromic behavior of four crystalline PTCs with triangular {Ti3(μ3-O)} units stabilized by a series of flexible and/or rigid dicarboxylate ligands.

INTRODUCTION Polyoxotitanate nanoparticles are one family of the mostly investigated materials with wide applications in solar energy conversion, degradation of environmental pollutants, and photovoltaic cells.1−4 However, due to the lack of the precise structural information at the atomic level, the illustration of their electronic properties and theoretical working mechanism remains extremely challenging.1,4 Serving as the structure and reactivity model of polyoxotitanate nanoparticles, crystalline polyoxo-titanium clusters (PTCs) have been attracting tremendous attention in recent decades.5−13 The molecular structure and atomic connectivity of PTCs could be accurately determined by single-crystal X-ray diffraction analysis.6 More importantly, the geometrical and electronic structures of PTCs are highly tunable by different coordinating ligands.8,10,11 Therefore, studies on crystalline PTCs will provide some insights in exploring the optimal reaction condition, analyzing the detailed mechanism, and understanding the structure− property relationship at the atomic level.14,15 As a special class of cluster-based materials between molecular complexes and bulk metal oxides, the geometrical and electronic structures of PTCs are highly tunable by simply varying the experimental conditions of these solution-based synthetic approaches.8,16 The formation mechanism of PTCs is usually regarded as a self-assembly process,16 sharing the similar idea of coordination-driven self-assembly within supramolecular chemistry and materials science. It has been a successful strategy for the construction of various complicated or delicate supramolecular structures during the past few decades.17−21 To date, an increasing number of PTCs containing {Ti3(μ3-O)} units bridged with alkoxide and carboxylate ligands have been © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Materials and Instrumentation. All the reagents and solvents were purchased commercially from Energy Chemical and were used as received without further purification, except that Ti(OiPr)4 was acquired from Adamas-beta and isopropyl alcohol was purchased from Sinopharm Chemical Reagent Beijing. Fourier transform infrared spectroscopy (FTIR) data were collected on a PerkinElmer Spectrum 100 FT-IR Spectrometer. UV−vis absorption spectra were measured on a PerkinElmer Lambda 950 UV−vis spectrophotometer. A 300 W Xe lamp was used as the UV−vis light source to prepare samples for Electron Paramagnetic Resonance (EPR). EPR spectra were recorded on a Bruker ER-420 spectrometer with a 100 kHz magnetic field at 110 K. Powder X-ray diffraction (PXRD) analysis was performed on a MiniFlex2 X-ray diffractometer using Cu−Kα radiation (λ = 0.1542 nm) in the 2θ range of 5−50° with a scanning rate of 5° min−1. The thermogravimetric analyses (TGA) were performed on a Mettler Received: March 6, 2018

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DOI: 10.1021/acs.inorgchem.8b00586 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Data and Structure Refinement for PTC-71−PTC-74 CCDC No. formula Mr T [K] crystal system space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] V [Å3] Z ρc [gcm−3] μ [mm−1] reflns coll. unique reflns GOF R1 [I > 2σ(I)]a wR2 [I > 2σ(I)]b a

PTC-71

PTC-72

PTC-73

PTC-74

1810862 C64H132O26Ti6 1605.09 293(2) triclinic P1̅ 11.5900(8) 12.1679(10) 18.1311(13) 71.857(7) 87.365(6) 64.271(8) 2177.0(3) 1 1.224 5.013 15004 8588 1.026 0.0719 0.236

1810863 C96N3O45Ti9H177 2524.5 293(2) trigonal P3̅ 26.1873(12) 26.1873(12) 13.4302(6) 90 90 120 7976.2(8) 2 1.051 0.488 107570 10599 1.135 0.1417 0.4202

1810864 C128N4O60Ti12H236 3365.99 293(2) triclinic P1̅ 13.9946(6) 16.8218(7) 20.3445(8) 98.239(3) 103.734(4) 95.451(4) 4562.8(3) 1 1.225 0.569 34944 16151 1.158 0.1282 0.4032

1810865 C128H240O52Ti12 3185.99 293(2) monoclinic C2/c 27.5474(12) 29.505(2) 23.4898(17) 90 97.443(5) 90 18932(2) 4 1.118 4.612 27908 14094 1.106 0.1277 0.4269

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2. Photochromic Experiment. Typically, 100 mg of sample was added into 1 mL of ethanol in an EPR quartz tube. The mixture was illuminated under UV−vis light for around 6 h until the color of the mixture turned to purple-blue. Then, the liquid part was removed using a pipet; the remaining solid sample was frozen in liquid nitrogen and used for EPR measurements at 110 K.

Toledo TGA/SDTA 851e analyzer in a N2 atmosphere with a heating rate of 10 °C/min from 20 to 800 °C. Synthesis of PTC-71. Ti(OiPr)4 (0.92 mL, 3.0 mmol) and 1,4cyclohexanedicarboxylic acid (0.156 g, 0.91 mmol) were added to isopropyl alcohol (5.5 mL) and mixed at room temperature. The resultant solution was heated at 80 °C for 1 day. After cooled to room temperature, colorless crystals of PTC-71 were obtained. Yield: ∼300 mg (∼38% based on Ti). Synthesis of PTC-72. Ti(OiPr)4 (0.92 mL, 3.0 mmol) and 2-nitro1,4-benzenedicarboxylic acid (0.037 g, 0.18 mmol) were added to isopropyl alcohol (5.5 mL) and mixed at room temperature. The resultant solution was heated at 80 °C for 3 days. After cooled to room temperature, colorless crystals of PTC-72 were obtained. Yield: ∼400 mg (∼48% based on Ti). Synthesis of PTC-73. Ti(OiPr)4 (0.92 mL, 3.0 mmol) and 5-nitro1,3-benzenedicarboxylic acid (0.0400 g, 0.19 mmol) were added to isopropyl alcohol (5.5 mL) and mixed at room temperature. The resultant solution was heated at 80 °C for 3 days. After cooled to room temperature, colorless crystals of PTC-73 were obtained. Yield: ∼350 mg (∼42% based on Ti). Synthesis of PTC-74. Ti(OiPr)4 (0.92 mL, 3.0 mmol) and terephthalic acid (0.0926 g, 0.56 mmol) were added to isopropyl alcohol (5.5 mL) and mixed at room temperature. The resultant solution was heated at 80 °C for 3 days. After cooled to room temperature, colorless crystals of PTC-74 were obtained. Yield: ∼450 mg (∼56% based on Ti). X-ray Crystallography. Crystallographic data of PTC-71 and PTC-74 were collected on a Supernova single crystal diffractometer equipped with graphite-monochromatic Cu Kα radiation (λ = 1.54178 Å) at 273 K. Crystallographic data of PTC-72 and PTC-73 were collected on an Oxford Xcalibur Eos diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. Structure was solved by direct method and refined by full-matrix least-squares on F2 using the SHELXTL-2014.36,37 Nonhydrogen atoms were refined anisotropically, and all hydrogen atoms were generated geometrically. The SQUEEZE option of PLATON was used to eliminate the contribution of disordered guest molecules to the reflection intensities.38 The crystallographic data and structure refinement of four complexes are summarized in Table 1. CCDC 1810862−1810865 contain the supplementary crystallographic data for this paper.

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RESULTS AND DISCUSSION By adjusting the configuration of dicarboxylic ligands, four different types of PTCs are successfully synthesized via a similar solvothermal approach in isopropyl alcohol at 80 °C with high yields: Ti6(μ3-O)2(CAA)2(OiPr)16 (PTC-71, CAA = 1,4cyclohexanedicarboxylic acid), Ti9(μ3-O)3(2-NBA)3(OiPr)24 (PTC-72, 2-NBA = 2-nitro-1,4-benzenedicarboxylic acid), Ti12(μ3-O)4(5-NBA)4(OiPr)32 (PTC-73, 5-NBA = 5-nitro1,3-benzenedicarboxylic acid), and Ti12(μ3-O)4(NA)4(OiPr)32 (PTC-74, TPA = terephthalic acid) (see experimental details). The geometrical structure and nuclearity of the obtained PTCs largely depend on the configuration of corresponding dicarboxylate ligands (Scheme 1). The geometrical structure, composition, purity, and physical properties of these complexes are systematically investigated by various physical and spectroscopic techniques. The FTIR spectra of all four PTC complexes show the characteristic bands of the Ti-oxo core in the range of 1000−500 cm−1 as well as the IR modes of organic coordinating ligands ranging from 3500 to 1000 cm−1 (Figures S1−S4). The thermal behaviors of compounds PTC-71 and PTC-74 are similar (Figures S5 and S8), whose thermogravimetric analyses (TGA) curves display that the structures can be retained up to 400 °C. Complexes PTC-72 and PTC-73 reveal relatively lower thermal stability (Figures S6 and S7). Single-crystal X-ray diffraction analysis reveals that PTC-71 is crystallized in the triclinic space group P1̅ (Table 1), and all Ti atoms in this compound are six-coordinate with distorted octahedral coordination geometry (Figure 1a,b and Figure S9). The phase purity of complex PTC-71 was confirmed by powder X-ray diffraction (PXRD) measurement (Figure S10), B

DOI: 10.1021/acs.inorgchem.8b00586 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Compared to the flexible ligands like CAA, structurally rigid dicarboxylate ligands lead to the formation of porous PTCs with multiple {Ti3(μ3-O)} building blocks. The use of rigid 2NBA ligand with low symmetry yields a triangular [Ti3O]3L3 structure (PTC-72), while more symmetric rigid 5-NBA or TPA ligands result in an unusual rectangular [Ti3O]4L4 architecture (PTC-73 and PTC-74). Complex PTC-72 is crystallized in the trigonal P3̅ space group (Table 1). And the measured PXRD pattern of PTC-72 matches well with the simulated pattern, indicating its phase purity (Figure S10). Three {Ti3(μ3-O)} units are connected by three low symmetric 2-NBA ligands forming a triangular [Ti3O]3L3 structure. In each {Ti3(μ3-O)} unit, the Ti1 atom is five-coordinate with a hexahedral coordination geometry, while Ti2 and Ti3 atoms are six-coordinate with a distorted octahedral coordination environment (Figure 1c,d, and Figure S11). Each Ti2 atom is coordinated by two oxygen atoms from the carboxylate groups of two 2-NBA ligands; in contrast, the Ti1 or Ti3 atom is coordinated by only one oxygen atom from the 2-NBA ligand. All five-coordinate Ti atoms are located in the same side of the triangular [Ti3O]3L3 structure, revealing a C3 symmetry of the whole structure. The outer surface of complex PTC-72 is surrounded by 8*3 isopropyl molecules, including 6 isopropyl bridging ligands to link 2 adjacent Ti atoms and 18 terminal ligands. The length of the triangle is 1.15 nm (Figure 1c), with three nitro groups in 2-NBA ligands on the outer side of the triangle. The packing structure reveals the shortest distance of the adjacent clusters is 1.59 nm, which is longer than that in complex PTC-71 (Figure 2). By switching the coordinating ligands from geometrically low symmetric 2-NBA to more symmetric ones (e.g., 5-NBA and TPA), two unusual rectangular [Ti3O]4L4 complexes (PTC-73 and PTC-74) are obtained, in which four {Ti3(μ3-O)} units are linked by four 5-NBA or TPA ligands with an ideal square shape (Figure 3). The difference of the geometrical structure between complexes PTC-71 and PTC-73/PTC-74 is possibly attributed to the steric hindrance and Coulomb repulsion introduced by various ligands used in the synthesis. Singlecrystal X-ray diffraction analyses demonstrate that complexes PTC-73 and PTC-74 are crystallized in the space groups P1̅ and C2/c, respectively (Table 1). The phase purity of both complexes was confirmed by PXRD measurements, which largely match well with the simulated patterns (Figure S10). It has to be mentioned that the crystals of PTC-73 and PTC-74 are very fragile and unstable once taking out from the mother liquor. Complexes PTC-73 and PTC-74 exhibit a similar geometrical structure except for the different surface coordinating ligands. Very similar to that of complex PTC-72, the Ti1 atom in each {Ti3(μ3-O)} unit of PTC-73/PTC-74 is also fivecoordinate with a hexahedral environment, while Ti2 and Ti3 atoms are six-coordinate with a distorted octahedral environment (Figure 3). In addition, the coordination environment of Ti atoms by carboxylate groups of the ligands in complexes PTC-73 and PTC-74 can also be categorized to two types as that in complex PTC-72, in which each Ti2 atom is chelated by two carboxylate oxygen atoms from two coordinating ligands and the Ti1 or Ti3 atom is coordinated by one carboxylate oxygen atom from one coordinating ligand. In each complex, every {Ti3} core is surrounded by 8 isopropyl molecules except for the dicarboxylate ligands, including 2 bridging isopropyl ligands to connect 2 adjacent Ti atoms and 6 terminal ligands. Looking closely at the arrangement of coordinating ligands in each complex, it is found that two pairs of nitro groups in PTC-

Scheme 1. Illustration of the Assembly of Different PTCs

Figure 1. Ball-and-stick representations of molecular structures and {Ti3(μ3-O)} units of complexes PTC-71 (a, b) and PTC-72 (c, d).

which matches well with the simulated one based on the data of single-crystal analysis. The {Ti6} core is composed of two triangular {Ti3(μ3-O)} units with idealized C2 symmetry, which are linked together by two flexible CAA ligands. Due to the large flexibility of the CAA ligand, the {Ti6} core can be ideally regarded as a linear dimeric arrangement of two triangular {Ti3(μ3-O)} units. The outer face of complex PTC-71 is functionalized by 8*2 isopropyl molecules. Four of these 16 isopropyl ligands act as bridging ligands between the two adjacent Ti atoms, and the rest of them work as terminal ligands. The packing structure demonstrates the shortest distance of the adjacent clusters in PTC-71 (Figure 2) is 1.16 nm. It is interesting to note that some literatures have reported the Ti-based supramolecular coordination complexes (SCCs) using catechol type ligands.39−42 However, the ones we present here with multinuclear Ti-O building units are still relatively rare. To investigate how the configuration of the coordinating ligands could affect the formation of PTCs, we have replaced the flexible CAA ligand with some structurally rigid dicarboxylate ligands, such as 2-NBA, 5-NBA, and TPA. C

DOI: 10.1021/acs.inorgchem.8b00586 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Crystal packing structures of (a) PTC-71, (b) PTC-72, (c) PTC-73, and (d) PTC-74.

73 and PTC-74 is relatively longer with the values of 1.68 and 1.69 nm, respectively (Figure 2), which is largely due to the rigidity and steric hindrance of the coordinating ligands. Diffuse reflectance spectroscopic measurements were performed on four complexes to better understand their optical absorption properties. According to the Kubelka−Munk function,43 the optical band gaps of PTC-71, PTC-72, PTC73, and PTC-74 are calculated as 3.35, 2.95, 3.06, and 3.25 eV, respectively (Figures S12−S15). It is interesting to note that minor modification on the coordinating ligands (2-NBA for PTC-72, 5-NBA for PTC-73) influences both the geometrical structures (triangular shape for PTC-72, rectangular shape for PTC-73) and optical band gaps (2.95 eV for PTC-72, 3.06 eV for PTC-73) of the obtained PTCs. In order to investigate the photochemical properties of obtained PTCs, complexes PTC71-PTC-74 were illuminated under UV−vis light in the presence of alcohol. Upon irradiation, a photochromic behavior is clearly observed for all four complexes as reported in the literature.30,32 The colors of complexes PTC-71, PTC-72, and PTC-74 change from colorless to purple-blue, while complex PTC-73 exhibits a less strong color change from colorless to light yellow (Figure 4). The EPR measurements at 110 K were carried out to study the electronic states of the Ti core in each PTC complex after photochromic transformation (Figures S16−S19). The EPR spectra exhibit the characterized signals of paramagnetic Ti(III) centers in a distorted rhombic ligand field with calculated parameters of 1.947 for PTC-71, 1.994 for PTC-72, and 1.946 for PTC-74. The EPR spectrum of PTC-73 shows the signals of paramagnetic super oxide diatomic O2•− ion resonances with a g value of 2.007.

Figure 3. Polyhedral and ball-and-stick representations of complexes PTC-73 (a, c) and PTC-74 (b, d). Polyhedral color code: red, TiO5; green, TiO6.

73 are located separately in the inside or outside of the rectangle. Also, the length of the rectangle in complex PTC-73 (1.03 nm) is almost the same as its width (1.01 nm). In contrast, the difference between length (1.24 nm) and width (1.10 nm) of the rectangle in complex PTC-74 is more noticeable due to the difference in arrangement of adjacent {Ti3} planes and the geometrical configuration of the coordinating ligands used. Comparing to that of PTC-71 and PTC-72, the packing distance of the adjacent clusters in PTC-

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CONCLUSIONS In summary, four dicarboxylates bridged titanium-oxo clusters with {Ti3(μ3-O)} building units are successfully synthesized through facile one-step solvothermal reactions. By tuning the configuration of the applied dicarboxylate ligands, the geoD

DOI: 10.1021/acs.inorgchem.8b00586 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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metrical structures of obtained PTCs can be categorized into three types, including a linear dimeric [Ti3O]2L2 complex (PTC-71), triangular [Ti3O]3L3 structure (PTC-72), and unusual rectangular [Ti3O]4L4 architectures (PTC-73 and PTC-74). The optical band gaps of the obtained PTCs also highly depend on the types of coordinating dicarboxylate ligands, and all four PTCs complexes exhibit interesting photochromic behavior upon UV−vis light irradiation in the presence of alcohol. This work not only provides some insights in understanding the influence of employed coordinating ligands on the structural diversity of PTCs but also reveals the promising potential applications of obtained PTCs in driving photochemical reactions.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00586. EPR spectra, TGA data, powder X-ray diffraction patterns, FT-IR spectra, solid-state UV−vis absorption spectra (PDF) Accession Codes

CCDC 1810862−1810865 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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Figure 4. The color change of PTC-71 to PTC-74 under UV−vis light irradiation (light source: xenon lamp).

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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 NSFC (21473202 and 21673238), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000), and the Natural Science Foundation of Fujian Province (2017J06009). E

DOI: 10.1021/acs.inorgchem.8b00586 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.8b00586 Inorg. Chem. XXXX, XXX, XXX−XXX