Inorg. Chem. 2004, 43, 6300−6306
Unprecedented Oxo-Titanium Citrate Complex Precipitated from Aqueous Citrate Solutions, Exhibiting a Novel Bilayered Ti8O10 Structural Core Tim Kemmitt,*,† Najeh I. Al-Salim,† Graeme J. Gainsford,† Andrea Bubendorfer,† and Mark Waterland‡ Industrial Research Ltd., P.O. Box 31-310, Lower Hutt, New Zealand, and Institute of Fundamental Sciences, Massey UniVersity, PriVate Bag 11-222, Palmerston North, New Zealand Received February 23, 2004
Aqueous titanium citrate solutions were prepared from the reaction of citric acid with titanium 2-propoxide in a range of molar ratios. Solutions containing two or fewer citrates per titanium resulted in the slow crystallization of an insoluble titanium oxo-citrate complex. Single-crystal X-ray analysis identified the species as Ti8O10(citrate)4(H2O)12‚14H2O‚3HOPri, crystallized in the tetragonal space group I41/a, with a ) 30.775(7) Å, c ) 14.528(7) Å, V ) 13 759(8) Å3, and Z ) 8. The trianionic citrate ligands supply both carboxylate and alkoxide coordination and stabilize the structure using simultaneous chelating and bridging modes of attachment. The compound is a neutral species, exhibiting titanium in three contrasting environments. Laser Raman microscopy and 13C CPMAS solidstate NMR data were consistent with those of the X-ray crystal structure. When exposed to air, the crystals rapidly lost water and became a powder. The dehydrated powder was noncrystalline to X-rays and insoluble, but 13C NMR results demonstrated retention of the carboxylate linkages.
Introduction Strategies to temper the sensitivity of metal alkoxides to hydrolysis include modification with appropriate ligands and/ or formation of oxo-alkoxides.1,2 Carboxylate ligands have been recognized as imparting a degree of robustness to an oxo-cluster, and many examples, generally formed from the reaction of carboxylic acids with titanium alkoxides, have been reported.3-12 Titanium citrate species are technologically very important in producing high-tech materials, and the
Pechini method13 for producing barium titanate (BT) powders is a widely utilized citrate-based route, which results in a highly homogeneous and well-crystallized product. The method has shown its versatility with its adoption in several alternative systems for preparing well-characterized mixedoxide nanoparticulate powders14-18 and is now more generally referred to as the polymerized citrate route. Various chemical aspects of the route have been examined,19-21 and recently, the structure of a titanium tricitrate dianion was
* Author to whom correspondence should be addressed. E-mail:
[email protected]. † Industrial Research Ltd. ‡ Massey University. (1) Mehrotra, R. C.; Singh, A. Chem. Soc. ReV. 1996, 1-13. (2) Hubert-Pfalzgraf, L. G. Inorg. Chem. Commun. 2003, 6, 102-120. (3) Doeuff, S.; Dromzee, Y.; Taulelle, F.; Sanchez, C. Inorg. Chem. 1989, 28, 4439-4445. (4) Doeuff, S.; Dromzee, Y.; Sanchez, C. C. R. Acad. Sci. 1989, 308, 1409-1412. (5) Boyle, T. J.; Alam, T. M.; Tafoya, C. J.; Scott, B. L. Inorg. Chem. 1998, 37, 5588-5594. (6) Alam, T. M.; Boyle, T. J.; Buchheit, C. D.; Schwartz, R. W.; Ziller, J. W. Mater. Res. Soc. Symp. Proc. 1994, 346, 35-40. (7) Papiernik, R.; Hubert-Pfalzgraf, L. G.; Vaissermann, J.; Goncalves, M. C. H. B. J. Chem. Soc., Dalton Trans. 1998, 2285-2287. (8) Kickelbick, G.; Schubert, U. Eur. J. Inorg. Chem. 1998, 159-161. (9) Mijatovic, I.; Kickelbick, G.; Schubert, U. Eur. J. Inorg. Chem. 2001, 1933-1935.
(10) Pandey, A.; Gupta, V. D.; Noth, H. Eur. J. Inorg. Chem. 2002, 13511357. (11) Moraru, B.; Kickelbick, G.; Schubert, U. Eur. J. Inorg. Chem. 2001, 1295-1301. (12) Boyle, T. J.; Tyner, R. P.; Alam, T. M.; Scott, B. L.; Ziller, J. W.; Potter, B. G. J. Am. Chem. Soc. 1999, 121, 12104-12112. (13) Pechini, M. Sprague Electric Company. U.S. Patent 3,330,697, 1967. (14) Kakihana, M.; Okubo, T.; Arima, M.; Uchiyama, O.; Yashima, M.; Yoshimura, M.; Nakamura, Y. Chem. Mater. 1997, 9, 451-456. (15) Kakihana, M.; Okubo, T.; Arima, M.; Nakamura, Y.; Yashima, M.; Yoshimura, M. J. Sol-Gel Sci. Technol. 1998, 12, 95-109. (16) Duran, P.; Moure, C.; Villegas, M.; Tartaj, J.; Caballero, A. C.; Fernandez, J. F. J. Am. Ceram. Soc. 2000, 83, 1029-1032. (17) Vaqueiro, P.; Lopez-Quintela, M. A. J. Mater. Chem. 1998, 8, 161163. (18) Liu, W.; Farrington, G. C.; Chaput, F.; Dunn, B. J. Electrochem. Soc. 1996, 143, 879-884. (19) Kakihana, M.; Arima, M.; Nakamura, Y.; Yashima, M.; Yoshimura, M. Chem. Mater. 1999, 11, 438-450. (20) Tsai, J.-D.; Fang, T.-T. J. Am. Ceram. Soc. 1999, 82, 1409-1415.
6300 Inorganic Chemistry, Vol. 43, No. 20, 2004
10.1021/ic049760r CCC: $27.50
© 2004 American Chemical Society Published on Web 09/01/2004
Oxo-Titanium Citrate Complex with NoWel T8O10 Core
determined using X-ray crystallography.22 Single-crystal X-ray studies on anionic titanium peroxo citrate ions have also been reported.23,24 Our interest has been in producing titanium dioxide based photocatalysts, utilizing precursors with lower citrate contents and using water as a reaction medium.25,26 A deficit of citrate ligands encourages the formation of hydrolytic species in solution but is preferred for the formation of photocatalytic thin films.26 Citrate complexes are also of interest from a biological viewpoint as citric acid (CA) is known to be involved in metal-ion transport and metallobiological processes.27,28 We report here on the preparation of titanium citrate solutions, their characterization by Raman spectroscopy, and the preparation, isolation, and characterization of an octanuclear titanium oxocitrate complex by Raman microscopy, solid-state 13C CPMAS NMR, and X-ray crystallography. The neutral complex was isolated from aqueous solution and exhibits an entirely new titanium oxide core structure. Experimental Section General Procedures. All of the experiments and manipulations were carried out open to the atmosphere. Ti(OPri)4 (Lancaster Synthesis) and citric acid (Panreac Chimica) were used as supplied. C and H microanalyses were carried out by the Campbell Laboratory at the University of Otago. Thermogravimetric analyses were carried out on a Rheometrics STA-1500 instrument with a heating rate of 10 °C min-1. Representative Preparations. Solution A (1:2 Ti/CA). Anhydrous citric acid (19.2 g, 0.1 mol) was dissolved in 2-propanol (100 mL) and added to a rapidly stirred solution of Ti(OPri)4 (14.2 g, 0.05 mol) in 2-propanol (50 mL). A rapid reaction ensued, causing the whole solution to gel. All of the free solvent was removed on a rotary evaporator, leaving a fine white powder. Distilled water (40 mL) was added, and the mixture was heated to 70 °C to dissolve. Further reduction on the rotary evaporator removed most of the residual 2-propanol before the solution was diluted to 50 mL with water, resulting in approximately a 1.0 M solution (based on Ti), and left to stand at room temperature. Solution B (1:1 Ti/CA) was prepared similarly to solution A, using Ti(OPri)4 (28.4 g, 0.10 mol) and anhydrous citric acid (19.2 g, 0.10 mol) in anhydrous ethanol and diluting the final solution to 100 mL with distilled water to form a 1.0 M Ti solution. Solution C (2:1 Ti/CA) was prepared similarly to solution A, using Ti(OPri)4 (56.8 g, 0.20 mol) and anhydrous citric acid (19.2 g, 0.10 mol) and diluting the final solution to 200 mL with distilled water to form a 1.0 M Ti solution. Ti8O10(citrate)4(H2O)12‚14H2O‚3HOPri. Crystals appearing on the walls of the flasks from solutions A-C were removed for (21) Fang, T.-T.; Wu, M.-S.; Tsai, J.-D. J. Am. Ceram. Soc. 2002, 85, 2984-2988. (22) Zhou, Z.-H.; Deng, Y.-F.; Jiang, Y.-Q.; Wan, H.-L.; Ng, S.-W. J. Chem. Soc., Dalton Trans. 2003, 2636-2638. (23) Kakihana, M.; Tada, M.; Shiro, M.; Petrykin, V.; Osada, M.; Nakamura, Y. Inorg. Chem. 2001, 40, 891-894. (24) Dakanali, M.; Kefalas, E. T.; Raptopoulou, C. P.; Terzis, A.; Voyiatsis, G.; Kyrikou, I.; Mavromoustakos, T.; Salifoglou, A. Inorg. Chem. 2003, 42, 4632-4639. (25) Kemmitt, T.; Al-Salim, N. I.; Waterland, M.; Kennedy, V.; Markwitz, A. Curr. Appl. Phys. 2004, 4, 189-192. (26) Kemmitt, T.; Al-Salim, N. I.; Ono, H. JSR Corporation. JP Patent No. 2000334310, 2000. (27) Glusker, J. P. Acc. Chem. Res. 1980, 13, 345-352. (28) Schneider, K.; Muller, A.; Krahn, E.; Hagen, W. R.; Wassink, H.; Knuttel, K.-H. Eur. J. Biochem. 1995, 230, 666-675.
examination and X-ray crystallographic study. Preliminary X-ray data confirmed the products were all identical. The full structural analysis described below relates to the crystals obtained from solution B. Solution C had the same Ti/CA stoichiometry as the crystalline precipitate and resulted in significantly greater precipitation of the crystalline product. Yield ) 17.7 g (36.4%) based on Ti. Further precipitation of the same crystalline product continued from the mother liquor over the following 6 months. (Yields of crystals from solutions A and B were invariably much lower at
