A Post-Functionalizable Iso-Polyoxotitanate Cage Cluster - Inorganic

Article pubs.acs.org/IC

A Post-Functionalizable Iso-Polyoxotitanate Cage Cluster Jie Hou, Junyi Hu, Qing Sun, Guanyun Zhang, Chen-Ho Tung, and Yifeng Wang* Key Lab of Colloid and Interface Science of the Education Ministry, Department of Chemistry and Chemical Engineering, Shandong University, Ji’Nan 250100, P. R. China S Supporting Information *

ABSTRACT: During solvothermal alcoholysis of a mixture of TiI4 and Ti(OiPr)4, a {I@Ti22} cage cluster encapsulating an OH and iodide guests is crystallized. The {I@Ti22} host−guest cluster surface is postfunctionalizable with catecholate and carboxylate ligands. The synthetic details, structural characterization, spectroscopic properties of the obtained cages clusters are provided. The present study provides candidates for modeling ligand exchange and electron-hole transfer at the titanate nanoparticle surface, and meanwhile offers new opportunities for understanding the TiO2 nanocrystalline formation in solvothermal processes.

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INTRODUCTION Due to their earth abundance and wide applications, especially their potential as photoanodes in solar energy conversion, titanium oxides have been extensively synthesized and studied. Recently, titanium oxide nanoclusters (polyoxotitanates)1the molecular form of titanatehave also been attracting increasing interest, for their roles as prenucleation clusters and/or intermediates of TiO2 nanocrystal formation,2 their precise structures as models for simulating the interfacial structures and photophysical processes in dye-sensitized solar cells,1a and as nanobuilding blocks for the bottom-up assembly of organic−inorganic hybrid materials.1c Despite a great number of polyoxotitanates having been reported to date,1 very rare among them are cage structures which enclose anions (only two and with hybrid metal-oxo shells).1a,3 Such cage clusters are of broad interest in the field of synthetic chemistry and host−guest chemistry, not only for fundamental research but also for their potential applications like MRI.3−5 Herein, we provide the synthesis, structure, spectroscopy, and postfunctionalization study of a novel {I@Ti22} host−guest iso-polyoxotitanate cluster encapsulating an OH and an iodide guests, isolated as an intermediate for synthesis of anatase TiO2 using the solvothermal strategy. Nonaqueous solvothermal alcoholysis of titanium halides (TiX4) has been long and extensively applied for synthesis of TiO2-tailored nanomaterials, which provides good control over nanoparticle size, morphology, and phase composition.6 It has been established that the alcoholysis of Ti4+ shown below to produce TiO2 involves reactions 1 and 2, in which the μ-O in the oxide originates from the alcohol:7 TiX4 + 4R‐OH → Ti(OR)4 + 4HX, X = halide

(1)

Ti(OR)4 + TiX4 → 2TiO2 + 4RX

(2)

© XXXX American Chemical Society

While the detailed mechanism/steps relevant to condensation of the TiX4 precursors and the nucleation of TiO2 nanocrystals remain obscure,2a we anticipate that a few clusters are involved as intermediates in this process and are interested in isolation of them to obtain molecular titanium oxides. For this purpose, we “snapshot” the solvothermal alcoholysis reaction solution at different stages simply by cooling it, and the polyoxotitanate cluster compounds may crystallize.

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RESULTS AND DISCUSSION In this work, by the solvothermal alcoholysis using TiI4 and Ti(OiPr)4 as precursors (eq 2) and our “snapshot” strategy, three polyoxotitanates of increasing condensation degrees (γ; defined as number of μ-O divided by number of Ti-atoms;1a,c for TiO2, γ = 2) were stepwise isolated with reaction time as the major intermediary products (Scheme 1). Scheme 1. Relationship of 1−5 and TiO2a

a

The percentage values are the highest isolated yields based on Ti. H2DTBC = 3,5-di-tert-butylcatechol.

Received: April 19, 2016

A

DOI: 10.1021/acs.inorgchem.6b00982 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

longer time (e.g., 2 weeks) at 100 °C or at higher temperatures for the same period (e.g., 3 days at 150 °C) began to yield anatase TiO2 as the sole precipitate (γ = 2; Figure S5). Comparing the TixOy frameworks of 2a and 3a with TiO2, it can be seen that they all exhibit a common (Ti-μ3-O)n ladderlike building block (Figure 2). It appears that the length

Remarkable is the isolation of compound 3, in the form [I@ Ti22(μ2-O)7(μ3-O)24(μ3-OH)(μ1-OiPr)22(μ2-OiPr)](I3) (denoted as 3aI3), which contains a cationic nanocluster 3a (Figure 1). Compound 3a exhibits an iodide and an OH−

Figure 1. (Left) Wireframe and (right) combined ball-and-stick and polyhedral views of 3a. Color scheme: blue, Ti; green, the Oin atom; red, bridging-O; pink, alkoxide-O; gray, C; deep purple, I. For clarity, all the H atoms and the isopropyl groups of the right panel are selectively omitted. The overall dimension of 3a, specified by the O(iPr)···O(iPr) distance as indicated in the right panel, is 13.1 Å; its diameter is 12.4 Å along the body diameter.

Figure 2. TixOy framework of (A) 2a, (B) 3a, and (C) anatase TiO2. They all feature a zigzag (Ti-μ3-O)n ladderlike building block, as highlighted in panel C.

of the (Ti-μ3-O)n ladder (i.e., value of n) is relevant to the condensation degree, i.e., n = 4 for 2a, n = 8 for 3a, and n = ∞ for anatase TiO2. Hence clusters 1a, 2a, and 3a are considered as intermediates of anatase TiO2. Because no more clusters which have larger condensation degree (1.45 < γ < 2) were obtained, and 3a coexists with the anatase precipitate, we consider 3a to be the largest structure that can be isolated in this system. Structurally, cluster 3a is the second example of the structure {anion@M22O32} (M = transition metal ion). The first member 6− V of this family, [(ClO4)@HVIV (6a), was 8 V14(μ2-O)32(Ot)22] 10 reported 20 years ago. The dimension of the shell of 3a is close to that of 6a (see Figure S6 for comparison). On the other hand, ignoring the OH, cluster 3a may be viewed as analogous to the recently reported [Br@Ti15O24(CoBr)6(OiPr)18]+ (7a)3 and [I@Ti15O22(MnI)2(OiPr)17] (8a),1a which are also cage clusters encapsulating halide anions. However, different from 7a and 8a, which have heterometal ions in the cage shells, 3a has been the first iso-polyoxotitanate host−guest cluster. Importantly, we now show that the shell of cluster 3a is postfunctionalizable. To date, among all the known polyoxotitanates, the OEt groups at certain positions of Ti16O16(OEt)32 are known to be labile and exchanged by OnPr without disruption of the Ti-oxo framework,11 and Ti17O24(OiPr)20 is the only one that has been postfunctionalized with catecholates and carboxylates.12 The catecholate- and carboxylate-functionalized polyoxotitanates are important because they provide molecular models of the dye/TiO2 interface for understanding dye binding modes and electron transfer processes in dye sensitized solar cells.1a Ligand exchange is essential in functionalization of the polyoxotitanate framework, because direct synthesis of functionalized polyoxotitanates is not predictable and usually gives low nuclearity clusters. Compound 3 was reacted with H2DTBC or pivalic acid in isopropanol, giving 4 and 5 (see Supporting Information), respectively, upon solvent evaporation at room temperature in a glovebox. The yield is acceptable, i.e., 41% of 4 and 49% of 5 based on 3. Cluster 4a (Figure 3), in the form [I@Ti22(μ2-O)6(μ3O)24(μ2-OH)2(μ1-OiPr)20(DTBC)2]+, was derived by exchange

enclosed in the ca. 7.5 Å body diameter cavity formed by the 22 Ti, seven μ2-O, 24 near-planar μ3-O, and one μ2-OiPr. The electron spin resonance (ESR; Figure S1) and X-ray photoelectron spectroscopy (XPS; Figure S2) indicate that all of the Ti atoms in 3a are in the +4 oxidation state. Nineteen of the 22 Ti atoms in the shell are five-coordinated and have chemically similar square-based pyramidal geometries by bonding to four equatorial oxo-atoms of the shell and an axial isopropoxide group. The other three Ti atoms are six-coordinated and are bonded to the same sp3 μ3-O (labeled as Oin; green in Figure 1), resulting in an OTi3 trigonal pyramid inside the cage shell. Protonation of the Oin is evidenced by charge balance, bond valence calculation (Table S3), and the Fourier transform infrared spectrum (FTIR; Figure S3), which shows a sharp O− H stretch mode at ca. 3370 cm−1, and 1H NMR of 4a (provided later). Oin has the nearest distance among all the O atoms to the encapsulated iodide anion, i.e., 3.51 Å, slightly smaller than the sum of the van der Waals radii of I and O (3.55 Å). Hence, the location of the iodide anion is greatly constrained by the OH guest and the highly positively charged Ti22O3224+ shell. For the solvothermal synthesis reaction, [Ti3(OiPr)11](I3) (1; Figure S4; γ = 0) was first isolated in the early stage before noticeable condensation occurred. Cluster [Ti3(OiPr)11]+ (1a) has the same structure as the cation of the previously reported [Ti3(OiPr)11][FeCl4] salt.8 Prolonged reaction time to 2−3 days followed by standing at room temperature for 1 day yielded red block crystals of [Ti12O14(OiPr)18](I3)2 (2; γ = 1.17; Figure S4), while reaction for 5 days gave dark-red block crystals of 3 (γ = 1.45), as the major products. Cluster [Ti12O14(OiPr)18]2+ (2a) is new and contains an α-Keggin-type Ti12O16 titanate framework which is the same as that in [Ti12O16(OiPr)16].9 The two μ2-O2− bridges in the latter are replaced with two [μ2-OiPr]− to give 2a, leaving 2a with a net charge of +2. The highest isolated yields of 1, 2, and 3 are 5%, 10%, and 80%, respectively. The relative low yield of 1 may be relevant to its high solubility and/or the low stability (e.g., equilibrium with monomers) in isopropanol. It was also noticed that there is no clear boundary timing for the separation of the cluster compounds, i.e., 1 and 2 and 2 and 3 could be both found in the same reaction solution. Reaction for an even B

DOI: 10.1021/acs.inorgchem.6b00982 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (Left) Wireframe and (right) combined ball-and-stick and polyhedral visualization of 4a. Color scheme is the same as described in Figure 1. The protons of the two green Oin atoms are omitted. In the polyhedral view, the isopropyl groups are selectively omitted. The inset shows the binding mode of catecholate−O to Ti.

of three OiPr of 3a with two equivalents of DTBC (eq 3 and an illustration shown in Figure S7). 3a + 2H 2DTBC → 4a + 3HOi Pr

(3)

During the postfunctionalization, the Oin of 3a disappears while two μ2-O on the inner wall (green in Figure 3) of the cage shell of 4a were generated. On the basis of charge balance, bond valence calculation (Table S3), FTIR, and the 1H NMR spectrum (shown immediately below), the two Oin of 4a are both protonated. Based on a geometrical analysis, one of the two Oin in 4a is derived from the Oin of 3a, while the other Oin should be converted from a μ2-O in the shell of 3a with the associative incoming of a DTBC (Figure S7). Ignoring the DTBC ligands, cluster 4a belongs to the C2v point group. The DTBC ligands bind to the titanium-oxo framework adopting the bridging bidentate mode (inset in Figure 3). This is the first observation of the bridging bidentate mode of catecholate on nuclearity >10 polyoxotitanate clusters; by contrast, only chelate monodentate mode has been observed before.12a With the two DTBC ligands, the symmetry of 4a decreases to C2. ESR data (Figure S1) indicate that 4 has a weak signal of Ti(III) in the dark which slightly increases upon visible light irradiation, attributed to the charge transfer from π orbitals of the DTBC benzene ring to the Ti 3d orbitals.7a,13 The ability of postfunctionalization (and recrystallization) of 3a, in addition with the IR data (Figure S8), firmly implies its solution stability at room temperature, such as in isopropanol, acetonitrile, CHCl3, and acetone, in which compound 3 is readily soluble. Compound 3 is soluble but slowly decomposes into a milky sol in toluene and in benzene. Meanwhile, 3 is not sensitive to moisture, as its IR maintains under ambient conditions for 2 days, i.e., ca. 20 °C and ca. 30% relative humidity. The 1H NMR spectrum of 3 in CDCl3 (Figure 4A) shows two sets of chemical shift in the ratio of nearly 24:138. The peaks in the range of 4.6−5.0 ppm are assigned to H atoms of the secondary-C while those in the range of 1.28−1.52 ppm are assigned to the methyl-H. The proton of the Oin of 3a is not assigned and is possibly overlapped with the signals of isopropoxide-H. In the 1H NMR spectrum of 4 (Figure 4B), besides the peaks of methyl-H (0.9−1.6 ppm, ca. 156 H) and the − CH groups (4.3−5.3 ppm, ca. 20 H), another three peaks at 7.34, 6.88, and 5.85 ppm are observed. The relative intensity of the latter peaks is near 1:1:1 and is ca. 1/10 of the peak area of the −CH group. Therefore, the 7.34 and 6.88 ppm peaks are assigned to benzene-H, and the 5.85 ppm peak is assigned to the protons (of Oin) inside the Ti22O32 cage of cluster 4a. The two protons

Figure 4. 1H NMR (A) of 3 and (B) of 4 in CDCl3. 13C NMR spectra are provided as Figures S9 and S10. The arrowed peaks are assigned to isopropanol impurity and are not included in the peak area calculations.

inside the cage of 4a confirm the protonation of the Oin of 3a (recall eq 3 and Figure S7) as well. In agreement with its solution stability, in the 13C NMR spectrum of 4a, the six carbon atoms of the benzene ring and the 10 secondary-C of the isopropoxide are well separated (Figure S10). The clear elucidation of the NMR spectra is beneficial for studying the ligand-exchange processes (e.g., kinetics and isotherm) in future. Compounds 3 and 4 were further characterized by UV−vis spectroscopy (Figures S11−S14). The indirect HOMO− LUMO transition (Eg) values1b,2b,12c,14 are 3.51 eV for 3 and 3.50 eV for 4, respectively. For comparison, the large, undoped polyoxotitanate Ti17O24(OiPr)20 has an indirect HOMO− LUMO gap of 3.40 eV.12c Hence, the iodide atoms in the cages do not modify the electronic structures of 3a and 4a.

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CONCLUDING REMARKS The host−guest cage structure, the functionalizable shell, and the solution stability will make 3a (as well as 4a and 5a) a good model for host−guest chemistry, supramolecular assembly, and photoinduced electron/hole transfer. Meanwhile, the present work provides new opportunities for better understanding TiO2 nanocrystalline formation and the lability of the ligands on the surface of a titanium oxide.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00982. X-ray crystallographic data in CIF format of compound 1 (CIF) C

DOI: 10.1021/acs.inorgchem.6b00982 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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X-ray crystallographic data in CIF format of compound 2 (CIF) X-ray crystallographic data in CIF format of compound 3 (CIF) X-ray crystallographic data in CIF format of compound 4 (CIF) X-ray crystallographic data in CIF format of compound 5 (CIF) Detailed synthesis, characterization, and more discussion (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-531-88363632. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (21473104&21401117), Natural Science Foundation of Shandong Province (ZR2014BQ003), and Shandong University (104.205.2.5) for the financial supports.

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DOI: 10.1021/acs.inorgchem.6b00982 Inorg. Chem. XXXX, XXX, XXX−XXX