Communication pubs.acs.org/IC
Small Titanium Oxo Clusters: Primary Structures of Titanium(IV) in Water Guanyun Zhang, Jie Hou, 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 250199, P. R. China S Supporting Information *
Scheme 1. Strategy for the Synthesis of Compounds 1−3
ABSTRACT: For sol−gel synthesis of titanium oxide, the titanium(IV) precursors are dissolved in water to form clear solutions. However, the solution status of titanium(IV) remains unclear. Herein three new and rare types of titanium oxo clusters are isolated from aqueous solutions of TiOSO4 and TiCl4 without using organic ligands. Our results indicate that titanium(IV) is readily hydrolyzed into oxo oligomers even in highly acidic solutions. The present clusters provide precise structural information for future characterization of the solution species and structural evolution of titanium(IV) in water and, meanwhile, are new molecular materials for photocatalysis.
To a TiCl4 (1.0 M) solution was added CsCl (1.0 M), followed by slow evaporation for around 1 week, to give 1 in 71% yield (based on titanium). Compound 1 crystallizes in relatively large greenish block crystals, in the form of Cs 2 [Ti 3 (μ 2 O)3(OH2)4Cl8]·4CsCl (1), belonging to the orthorhombic crystal system (space group Fdd2). 1 is the mixed cesium salt of Cl− and [Ti3(μ2-O)3(OH2)4Cl8]2− (1a). In 1a, there are no intercluster bridging ligands. As shown in Figure 1, in the Ti3O3
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n the sol−gel synthesis of metal oxide nanomaterials, the water-soluble precursors quickly hydrolyze and condense into liquid sol.1 The lack of an in-depth understanding of solutionphase species is a great challenge in this synthesis science.2 For centuries, aqueous solutions of TiCl4 and TiOSO4 have been the most important precursors for the sol−gel synthesis of TiO2 nanomaterials;3 however, little is known about the speciation or the initial stage of these precursors’ hydrolysis and condensation in water. Titanium oxo clusters are the trapped “snapshots” of intermediate structures in the sol−gel growth of TiO2 nanocrystals.4 Their isolation and structural determination will be the first step toward the solution-phase speciation.4g,5 Recently, solution-based in situ structural characterization techniques like solution X-ray scattering2,6 have provided new opportunities for the structural determination of solution species, but this also needs the combination with the precise structural information on known aqueous-isolated compounds. Moreover, the titanium oxo clusters are nano building blocks for the preparation of organic−inorganic hybrid materials7 and molecular materials, which may find applications in solar energy harvesting.5 However, to date, no titanium(IV) oxo oligomer has been isolated from TiCl4 or TiOSO4 aqueous solution except [Ti8O12(OH2)24]8+, whose structure was solved nearly 30 years ago.8 We now report the isolation, characterization, and electronic properties of three small titanium oxo clusters. Notably, TiCl4 and TiOSO4 readily dissolve in water, forming highly acidic clear solutions. With the added salts, which can decrease the solubility of the solution species, the titanium oxo clusters crystallized at room temperature upon solvent evaporation (Scheme 1). © XXXX American Chemical Society
Figure 1. Ball-and-stick structures of 1a, 2a, and 3a. Color code: blue, Ti; red, O; yellow, S; green, Cl. H-atoms are omitted for clarity.
ring, the Ti−μ2-O bond lengths are 1.81−1.82 Å, the Ti···Ti distances are in the range of 3.45−3.47 Å, and the Ti−μ2-O−Ti bond angles are ca. 144° (Table S2). 1a is capped with four terminal O atoms with Ti−Ot distances of 2.14−2.15 Å and doped with eight Cl atoms with Ti−Cl distances of 2.34−2.49 Å. The bond-valence-sum calculations (Table S3) confirm that μ2O atoms are oxo ligands, while the terminal O atoms are aqua ligands. To a clear solution containing 20% TiOSO4 was added 2.0 M CsCl, which yielded a white precipitate. The precipitate was dissolved in concentrated HCl (3.0 M) with additional CsCl (0.5 M). Slow evaporation of the solvent under ambient conditions gave light-green crystals of compound 2 (yield ≈ 30% based on titanium). X-ray diffraction (XRD) indicates that it belongs to the monoclinic crystal system (space group C2). 2 is also a mixed salt containing a unique tetratitanium oxoanion, [Ti4(μ2O)4(OH2)6 Cl2(SO4)4]2− (2a; Figure 1), separated by Cs+ and Received: January 18, 2016
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DOI: 10.1021/acs.inorgchem.6b00129 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry Cl− ions. The four Ti atoms in 2a are connected with μ2-O in a Ti4O4 ring, with Ti−μ2-O bond lengths of 1.78−1.91 Å, Ti···Ti distances of 3.43−3.53 Å, and Ti−μ2-O−Ti angles of ca. 144°. The four SO4 serve as bidentate bridging ligands, with Ti− O(SO3) in the range of 1.89−1.99 Å. 2a is capped with six terminal aqua ligands and doped with two Cl ligands. Compound 3, in the form [Ti4O4(OH2)8(SO4)4]·2TEAC· H2SO4·10H2O, was obtained as colorless block crystals by slow evaporation of a mixture of TiCl4 (1.0 M), H2SO4 (3.0 M), and tetraethylammonium chloride (TEAC; 1.0 M). The neutral [Ti4O4(OH2)8(SO4)4] cluster {[Ti4O4(OH2)8(SO4)4] (3a); Figure 1} follows the structure of 2a and has aqua ligands in the place of Cl. Viewed together, the titanium oxo clusters in compounds 1−3 serve as intercalation layers for, and meanwhile are separated by, their cocrystallized salts (Figure 2). This suggests that the added
one-pot reaction indicates that they are formed under the same conditions. Although both exhibit the same condensation degree, apparently 2 is more hydrolyzed than 1. When the structures of 3 and 2 are compared, 3 can be regarded as the hydrolysis product of 2. The present clusters 1a, 2a, and 3a and the previously reported [Ti8O12(OH2)24]8+ are the first oligomers isolated from an aqueous solution of TiOSO4 or TiCl4,8 different from the titanium oxo alkoxides synthesized by the solvothermal treatment of titanium(IV) precursors in anhydrous solutions.5,7,10 Notably, TiCl4 and TiOSO4 are the most widely used inorganic salts of titanium(IV) for the sol−gel synthesis of TiO 2 nanomaterials. It is generally accepted that the sol−gel methods to prepare TiO2 undergo two steps: hydrolysis resulting in the formation of a metal hydroxide and condensation giving the Ti− O−Ti bonds.3a,11 However, the structures of 1a, 2a, and 3a demonstrate or suggest that (1) titanium(IV) becomes oligomers in aqueous solutions even under very acidic conditions, (2) condensation occurs simultaneously with hydrolysis, (3) thoroughly hydrolyzed low-nuclearity oligomers can exist, and (4) condensation gives oxo clusters (due to the highly acidic oxo ligands) instead of hydroxo-bridged clusters different from zirconium(IV) and hafnium(IV) even though all are IVB elements.6b,12 The different types of bridging O atoms [the bridging oxo in the present clusters and the bridging hydroxo in zirconium(IV)/hafnium(IV) clusters] may be relevant to the smaller size and thereby the larger charge density of titanium(IV). The total charges of the titanium oxo clusters are related to their degree of hydrolysis and condensation; i.e., because of the Cl ligands, the charge of 1a is 2−, while 2a is more negatively charged than 3a. Zirconium(IV)/hafnium(IV) clusters are usually polycationic apparently because of their low-charged bridging hydroxo and terminal aqua ligands.2,12a,13 Further, the clusters 1a, 2a, and 3a can be viewed as snapshots of intermediate structures in the sol−gel growth of TiO2 nanocrystals.4g,5 In fact, when a solution of TiCl4 (1.0 M) was aged at room temperature (20−25 °C) for over 1 month, a white precipitate insoluble in water was slowly produced. XRD analysis of the precipitate revealed it to be rutile TiO2 (Figure S5). The structural parameters of compounds 1−3 are summarized in Table S2. Prior to this work, this information for water-isolated titanium oxo clusters was scarce. By contrast, the extensive use of solution-based in situ structural characterization, especially highenergy X-ray scattering, has greatly enriched the knowledge of solution-phase speciation of transition-metal ions recently.2,6 For example, Soderholm and co-workers used a synchrotron X-ray beam to get the scattering profiles of zirconium(IV) aqueous solutions, and in the pair distance distribution function, the fitted peaks were able to be assigned by comparison with the structural information derived from single-crystal crystallography.2,12a,13 Therefore, we believe the present precise structural information on the primary hydrolysis products of titanium(IV) in water will be a prerequisite for understanding the hydrolysis and condensation of titanium(IV) precursors, which remained relatively unexplored before, and ultimately serve as a preliminary step in studying the sol−gel synthesis of TiO2.3a,d,e,14 Compounds 1−3 were further characterized by Fourier transform infrared, Raman, and electron-spin resonance (ESR) spectra. The frequencies in the range of 700−900 cm−1 are tentatively assigned to Ti−O IR vibrational modes, while those in the range of 450−550 cm−1 are assigned to bending modes of Ti−O bonds (Figures S6 and S7). Compounds 1−3 are unstable upon attempted dissolution in acidic water and are insoluble in
Figure 2. Cluster-packing diagram of 1a (A), 2a (B), and 3a (C and C′). The Ti atoms are located at the center of the blue octahedra, while the SO4 ligands are drawn as yellow tetrahedra. Cs and Cl atoms are drawn in gray and green, respectively. In panels C and C′, TEA+, Cl−, and SO42− in the matrix are omitted because of the use of “squeeze” in solving the structure (Supporting Information). However, they should fill the void between the {001} planes, which are comprised of squarepacked cluster 3a with Cl atoms at the square centers.
salts are important for crystallizing the titanium oxo clusters. It is apparent that, with these added salts, the solubility of the solution species decreased and the production of crystals from these solutions occurred easier. In fact, without the added salts, evaporation of TiCl4 or TiOSO4 solutions to a minimum volume under the same conditions for syntheses of 1−3 gave highly soluble viscous materials containing many tiny crystals whose IR spectra suggest that they are mixtures. 1a has been the smallest titanium oxo cluster so far and has a high condensation degree (γ, defined as the number of μ-O atoms divided by the number of Ti atoms; γ = 1 for 1a).7 For comparison, the smallest titanium oxo cluster isolated by the solvothermal approach in anhydrous isopropyl alcohol, Ti3O(OiPr)10 (iPr = isopropyl),9 has a much smaller condensation ratio of 0.33. Meanwhile, the three Ti atoms in Ti3O(OiPr)10 are connected with a μ3-O atom on top of the {Ti3} triangle. By contrast, no μ3-O atom is observed in the available simple titanium oxo clusters without organic chelates. Compounds 1 and 2 can be crystallized simultaneously from the same solution by a protocol slightly different from that for the synthesis of 2. For this, the precipitate by mixing 20% TiOSO4 and 2.0 M CsCl was dissolved in 1.5 M HCl and 0.5 M CsCl and allowed to evaporate under ambient conditions for around 1 month. Compounds 1 and 2 crystallized separately and were distinguished by their appearance. The formation of 1 and 2 in a B
DOI: 10.1021/acs.inorgchem.6b00129 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Province (Grant ZR2014BQ003). C.-H.T. acknowledges Shandong University for a startup fund (Grant 104.205.2.5).
organic solvents; hence, their solution chemistry has not been studied. ESR measurements of the solid samples showed no evidence of titanium(III) in compounds 1−3 at room temperature, indicating that all Ti atoms are in their highest oxidation states. The electronic band structures were determined by diffuse-reflectance UV−vis spectroscopy (Figure 3).4d,e,g,h,15 The
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(1) (a) Feinle, A.; Elsaesser, M. S.; Hüsing, N. Chem. Soc. Rev. 2016, DOI: 10.1039/C5CS00710K. (b) Dunn, B.; Zink, J. I. Acc. Chem. Res. 2007, 40, 729−729. (c) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33−72. (d) Brinker, C. J.; Scherer, G. W. Sol−Gel Science: The Physics and Chemistry of Sol−Gel Processing; Academic Press, Inc.: New York, 1990. (2) Hu, Y.; Knope, K. E.; Skanthakumar, S.; Kanatzidis, M. G.; Mitchell, J. F.; Soderholm, L. J. Am. Chem. Soc. 2013, 135, 14240− 14248. (3) (a) Cargnello, M.; Gordon, T. R.; Murray, C. B. Chem. Rev. 2014, 114, 9319−9345. (b) Wang, X.; Li, Z.; Shi, J.; Yu, Y. Chem. Rev. 2014, 114, 9346−9384. (c) Sang, L.; Zhao, Y.; Burda, C. Chem. Rev. 2014, 114, 9283−9318. (d) Fattakhova-Rohlfing, D.; Zaleska, A.; Bein, T. Chem. Rev. 2014, 114, 9487−9558. (e) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891−2959. (4) (a) Lv, Y.; Cheng, J.; Steiner, A.; Gan, L.; Wright, D. S. Angew. Chem. 2014, 126, 1965−1969. (b) Liu, Z.; Lei, J.; Frasconi, M.; Li, X.; Cao, D.; Zhu, Z.; Schneebeli, S. T.; Schatz, G. C.; Stoddart, J. F. Angew. Chem., Int. Ed. 2014, 53, 9193−9197. (c) Negre, C. F. A.; Young, K. J.; Oviedo, M. B.; Allen, L. J.; Sánchez, C. G.; Jarzembska, K. N.; Benedict, J. B.; Crabtree, R. H.; Coppens, P.; Brudvig, G. W.; Batista, V. S. J. Am. Chem. Soc. 2014, 136, 16420. (d) Lv, Y.; Cheng, J.; Matthews, P. D.; Holgado, J. P.; Willkomm, J.; Leskes, M.; Steiner, A.; Fenske, D.; King, T. C.; Wood, P. T.; Gan, L.; Lambert, R. M.; Wright, D. S. Dalton Trans. 2014, 43, 8679−8689. (e) Matthews, P. D.; King, T. C.; Wright, D. S. Chem. Commun. 2014, 50, 12815−23. (f) Piszczek, P.; Radtke, A.; Muziol, T.; Richert, M.; Chojnacki, J. Dalton Trans. 2012, 41, 8261− 8269. (g) Benedict, J. B.; Freindorf, R.; Trzop, E.; Cogswell, J.; Coppens, P. J. Am. Chem. Soc. 2010, 132, 13669−13671. (h) Benedict, J. B.; Coppens, P. J. Am. Chem. Soc. 2010, 132, 2938−2944. (5) Coppens, P.; Chen, Y.; Trzop, E. Chem. Rev. 2014, 114, 9645− 9661. (6) (a) Sadeghi, O.; Zakharov, L. N.; Nyman, M. Science 2015, 347, 1359. (b) Ruther, R. E.; Baker, B. M.; Son, J.; Casey, W. H.; Nyman, M. Inorg. Chem. 2014, 53, 4234−4242. (c) Gossard, A.; Toquer, G.; Grandjean, S.; Grandjean, A. J. Sol-Gel Sci. Technol. 2014, 71, 571−579. (d) Antonio, M. R.; Nyman, M.; Anderson, T. M. Angew. Chem., Int. Ed. 2009, 48, 6136−6140. (7) Rozes, L.; Sanchez, C. Chem. Soc. Rev. 2011, 40, 1006−1030. (8) Reichmann, M. G.; Hollander, F. J.; Bell, A. T. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1987, 43, 1681−1683. (9) Day, V. W.; Eberspacher, T. A.; Chen, Y.; Hao, J.; Klemperer, W. G. Inorg. Chim. Acta 1995, 229, 391−405. (10) Matthews, P. D.; King, T. C.; Wright, D. S. Chem. Commun. 2014, 50, 12815−12823. (11) Lu, Z.; Lindner, E.; Mayer, H. A. Chem. Rev. 2002, 102, 3543− 3578. (12) (a) Kalaji, A.; Soderholm, L. Inorg. Chem. 2014, 53, 11252− 11260. (b) Fitzgerald, M.; Pappas, I.; Zheng, C.; Xie, Z.; Huang, X.; Tao, S.; Pan, L. Dalton Trans. 2009, 6289. (c) Pan, L.; Heddy, R.; Li, J.; Zheng, C.; Huang, X.; Tang, X.; Kilpatrick, L. Inorg. Chem. 2008, 47, 5537−5539. (d) Singhal, A.; Toth, L. M.; Lin, J. S.; Affholter, K. J. Am. Chem. Soc. 1996, 118, 11529−11534. (e) Norén, B.; et al. Acta Chem. Scand. 1973, 27, 1369−1384. (f) Mak, T. C. W. Can. J. Chem. 1968, 46, 3491−3500. (13) Kalaji, A.; Skanthakumar, S.; Kanatzidis, M. G.; Mitchell, J. F.; Soderholm, L. Inorg. Chem. 2014, 53, 6321−6328. (14) Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W. Chem. Rev. 2014, 114, 9919−9986. (15) (a) Chen, Y.; Trzop, E.; Makal, A.; Chen, Y.; Coppens, P. Dalton Trans. 2014, 43, 3839−3841. (b) Chen, Y.; Sokolow, J.; Trzop, E.; Chen, Y.; Coppens, P. J. Chin. Chem. Soc. 2013, 60, 887−890. (c) Chen, Y.; Sokolow, J. D.; Trzop, E.; Coppens, P. Dalton Trans. 2013, 42, 15285− 15287. (d) Chen, Y.; Trzop, E.; Sokolow, J. D.; Coppens, P. Chem. - Eur. J. 2013, 19, 16651−16655.
Figure 3. Solid-state diffuse-reflectance UV−vis spectra of 1, 2, and 3.
HOMO−LUMO transitions (Eg) obtained by extrapolation of the linear portions of the Kubelka−Munk function are 2.88 eV for 1, 2.76 eV for 2, and 2.97 eV for 3. All of these values are redshifted (to the visible region of the solar spectrum) with respect to both pure-phase TiO2 (e.g., for anatase TiO2, Eg = 3.2 eV) and large undoped polyoxotitanates like Ti17O24(OiPr)20 (HOMO− LUMO gap ≈ 4.25 eV4h). It is worth noting that Cl modification of 3 (giving 2) causes the absorption edge to red shift, likely because of Cl-to-Ti charge-transfer transitions. These clusters might find applications in photocatalysis. In conclusion, we report the syntheses and characterization of three new titanium oxo clusters isolated from aqueous solutions. The very low-nuclearity structures herein are the primary hydrolysis products of the most widely used titanium(IV) precursors for the sol−gel synthesis of TiO2 in water. The present study enriches the precise structural information for further elucidation of the solution speciation and structural evolution of titanium(IV) in water and meanwhile provides new molecular materials that might be useful in photocatalysis.
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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.6b00129. X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) Experimental details, data of crystallography, and spectra (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Y.W. gratefully acknowledges financial support by the National Natural Science Foundation of China (Grants 21473104 and 21401117) and by the Natural Science Foundation of Shandong C
DOI: 10.1021/acs.inorgchem.6b00129 Inorg. Chem. XXXX, XXX, XXX−XXX
