Tantalum(V) ethoxide

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Tantalum(V) ethoxide
Other names

  • Tantalum ethylate
  • Tantalum(V) ethylate
  • Pentaethyl tantalate
  • Tantalum pentaethoxide
  • Pentaethoxytantalum(V)
  • Tantalum(5+) pentaethanolate

Identifiers
CAS number 6074-84-6
PubChem 2733710
ChemSpider 21169490
EC-number 228-010-2
Jmol-3D images {{#if:[Ta+5].[O-]CC.[O-]CC.[O-]CC.[O-]CC.[O-]CC|Image 1
Properties
Molecular formula C10H25O5Ta
Molar mass 406.25 g mol-1
Appearance Colorless liquid
Density 1.566 g/cm3 (at 25°C)
Melting point 21 °C; 70 °F; 294 K
Boiling point 145 °C (at 0.0133 kPa)
Solubility in water miscible
Solubility Organic solvents
Refractive index (nD) 1.488 [1]
Hazards[2]
MSDS External MSDS
R-phrases R10, R34
S-phrases S16, S26, S36/37/39, S45
NFPA 704
1
2
2
W
Flash point 31 °C; 87 °F; 304 K
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Tantalum(V) ethoxide is an metalorganic compound with formula Ta2(OC2H5)10, often abbreviated as Ta2(OEt)10. It is a colorless solid that dissolves in some organic solvents but hydrolyzes readily.[3] In solution, it aggregates to form bioctahedral[4] dimers[5] with composition [(EtO)4Ta(μ-OEt)]2. Several approaches for preparing tantalum(V) ethoxide are known but salt metathesis from tantalum(V) chloride is generally the most successful synthetic approach.[6] It is mainly used for the manufacture of tantalum(V) oxide thin-film materials by approaches including chemical vapor deposition,[7] atomic layer deposition,[8] and sol-gel processing.[9] These materials have semiconductor,[8] electrochromic,[10] and optical[7] applications.

Structure

Metal alkoxides rarely adopt monomeric structures and tantalum(V) ethoxide is no exception. Early studies established that tantalum alkoxides aggregate in solution as dimers[5] with octahedral six-coordinate tantalum metal centres.[11] Subsequent crystallographic analysis established that the methoxide and isopropoxides of niobium adopt bioctahedral structures.[4][12] From a geometric perspective, the ten ethoxide ligand oxygen atoms of the Ta2(OEt)10 molecule in solution define a pair of octahedra sharing a common edge with the two tantalum atoms located at their centres.[4] From a bonding perspective, each tantalum centre is surrounded octahedrally by four monodentate and two bridging ethoxide ligands. The oxygen atoms of the bridging ethoxides are each bonded to both tantalum centres, and these two ligands are cis to one another within the coordination sphere. The formula [(EtO)4Ta(μ-OEt)]2 more comprehensively represents this dimeric structure, though the simplified formula is commonly used for most purposes.

Preparation

Tantalum pentachloride, Ta2Cl10, provides a convenient starting point for preparation of tantalum(V) ethoxide. Direct reaction with ethanol is possible and is accompanied by the production of hydrogen chloride, a strong acid in solution, according to the equation (Et, C2H5, refers to the ethyl group):

10 EtOH + Ta2Cl10 → Ta2(OEt)10 + 10 HCl

Unfortunately, acidic conditions favour the generation of mixed chloride-ethoxide species TaClx(OC2H5)5−x, 0 < x < 5, decreasing the yield and complicating the purification of the tantalum(V) ethoxide product.[6] For this reason, ammonia is usually added to trap liberated HCl and maintain basic reaction conditions.[6]

10 EtOH + Ta2Cl10 + 10 NH3 → Ta2(OEt)10 + 10 NH4Cl

Basic conditions also increase the concentration of ethoxide ions, favouring the reaction as they are markedly more powerful nucleophiles for the substitution of chloride ligands than is the parent alcohol. For this reason, direct salt metathesis using an alkali metal alkoxide is often the most successful synthetic approach:[6]

10 NaOEt + Ta2Cl10 → Ta2(OEt)10 + 10 NaCl

The same compound can be prepared electrochemically.[13][4] The two half-equations and the overall equation[13] for this reaction are:

cathode: 2 EtOH + 2 e− → 2 EtO− + H2
anode: Ta → "Ta5+" + 5 e−
overall: 2 Ta + 10 EtOH → 2 "Ta5+" + 10 EtO− + 5 H2 → Ta2(OEt)10 + 5 H2

Commercial production of tantalum(V) ethoxide using this electrochemical approach has been successfully employed in Russia.[13] The compound can also be prepared by direct reaction of tantalum metal with ethanol, in which case the overall equation is the same as that shown above for the electrochemical approach.[6]

Reactions

The most important reaction of tantalum alkoxides is their hydrolysis to produce films and gels of tantalum oxides. Although these reactions are complex, the formation of a tantalum(V) oxide film by hydrolysis[3] can be described by this simplified equation:

Ta2(OC2H5)10 + 5 H2O → Ta2O5 + 10 C2H5OH

Tantalum(V) ethoxide optical coatings can be produced by low pressure chemical vapour deposition.[7] At pressures as low as 1.33 mPa and temperatures of 700 °C, a silica film of the desired depth is first deposited by the decomposition of tetraethoxysilane, Si(OEt)4, or di-t-butyoxydiacetoxysilane, Si(OC(CH3)3)2(OOCCH3)2, then tantalum(V) ethoxide is introduced.[7] As in the case of niobium(V) ethoxide, the ethoxide precursor thermally decomposes to produce the oxide layer with the associated release of diethyl ether:

Ta2(OEt)10 → Ta2O5 + 5 Et–O–Et

Pyrolysis also produces a tantalum(V) oxide film by chemical vapor deposition in which case the tantalum(V) ethoxide is completely oxidised, producing carbon dioxide and water vapor:[14]

Ta2(OC2H5)10 + 30 O2 → Ta2O5 + 20 CO2 + 25 H2O

Amorphous tantalum(V) oxide films can also be prepared by atomic layer deposition or by a pulsed chemical vapour deposition technique in which tantalum(V) ethoxide and tantalum(V) chloride are applied alternately.[8] At temperatures approaching 450 °C the films produced have refractive indices and permittivity properties similar to those produced from convential approaches.[8] The preparation of these films occurs with the loss of chloroethane:[8]

Ta2(OC2H5)10 + Ta2Cl10 → 2 Ta2O5 + 10 C2H5Cl

Sol-gel processing also produces thin films of tantalum(V) oxide[9] using a similar chemical approach. Sol-gel routes using tantalum(V) ethoxide to generate layered perovskite materials have also been developed.[15]

Applications

Tantalum(V) oxide films have a variety of applications including as optical films with refractive indices as high as 2.039[16] and as a thin-film dielectric material in dynamic random access memory and semiconductor field-effect transistors.[8] The approach chosen for preparation of these materials is determined by the desired properties. Direct hydrolysis is appropriate when the presence of residual water or the use of high temperatures for drying is acceptable. Micropatterns can be produced by site-selective deposition using the hydrolysis approach by forming a self-assembled monolayer followed by high temperature annealling.[17] Chemical vapour deposition allows control of the thickness of the film on a nanometre scale, which is essential for some applications. Direct pyrolysis is convenient for optical applications,[7] where transparent materials with low light loss due to absorption is important,[16] and has also been used to prepare nitride read-only memory.[14] Electrochromism is the property of some materials to change color when charge is applied,[18] and is the means by which so-called smart glass operates. Films produced by tantalum(V) ethoxide hydrolysis has been used to prepare amorphous tantalum(V) oxide films suitable for electrochromic applications.[10]

Mixed-metal thin-films have also been prepared from this compound. For example, lithium tantalate, LiTaO3, films are desirable for their non-linear optical properties and have been prepared by first reacting tantalum(V) ethoxide with lithium dipivaloylmethanate, LiCH(COC(CH3)3)2, to prepare a precursor suitable for metalorganic vapour phase epitaxy (a form of chemical vapor deposition).[19] Films of strontium tantalate, Sr(TaO3)2, have also been prepared using atomic layer deposition approaches and their properties investigated.[20]

A research application of tantalum(V) ethoxide is for the synthesis of novel compounds with interesting chemical or geometric properties. In a reaction related to hydrolysis, treatment with carboxylic acids gives oxo-alkoxide-carboxylates, e.g., Ta4O4(OEt)8(OOCCH3)4.[6] The Ta4O4 core of such compounds have a cubic geometry, like the carbon atom core of the hydrocarbon cubane. Each oxo ligand bridges between three tantalum centres, which are each also associated with two alkoxide and one carboxylate ligands.

References

  1. ↑ "Tantalum Ethoxide and Niobium Ethoxide". Materian Advanced Chemicals. Retrieved 19 October 2012. 
  2. ↑ "Tantalum (V) ethoxide 99.98% trace metals basis". Sigma Aldrich. Retrieved 18 October 2012. 
  3. ↑ 3.0 3.1 Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0487-3. 
  4. ↑ 4.0 4.1 4.2 4.3 Turova, N. Y.; Korolev, A. V.; Tchebukov, D. E.; Belokon, A. I.; Yanovsky, A. I.; Struchkov, Y. T. (1996). "Tantalum(V) Alkoxides: Electrochemical Synthesis, Mass-Spectral Investigation and Oxoalkoxocomplexes". Polyhedron 15 (21): 3869–3880. doi:10.1016/0277-5387(96)00092-7. 
  5. ↑ 5.0 5.1 Bradley, D. C.; Holloway, C. E. (1968). "Nuclear Magnetic Resonance Studies on Niobium and Tantalum Penta-alkoxides". J. Chem. Soc. A: 219–223. doi:10.1039/J19680000219. 
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 Schubert, U. (2003). "Sol–Gel Processing of Metal Compounds". Comprehensive Coordination Chemistry II 7: 629–656. doi:10.1016/B0-08-043748-6/06213-7. 
  7. ↑ 7.0 7.1 7.2 7.3 7.4 Baumeister, P. W. (2004). Optical Coating Technology. SPIE Press. p. 7. ISBN 9780819453136. 
  8. ↑ 8.0 8.1 8.2 8.3 8.4 8.5 Kukli, K.; Ritala, M.; Leskelä, M. (2000). "Atomic Layer Deposition and Chemical Vapor Deposition of Tantalum Oxide by Successive and Simultaneous Pulsing of Tantalum Ethoxide and Tantalum Chloride". Chem. Mater. 12 (7): 1914–1920. doi:10.1021/cm001017j. 
  9. ↑ 9.0 9.1 Winter, S.; Velten, D.; Aubertin, F.; Hoffmann, B.; Heidenau, F.; Ziegler, G. (2008). "Chemical Surface Modifications". In Breme, J.; Kirkpatrick, C. J.; Thull, R. Metallic Biomaterial Interfaces. John Wiley & Sons. p. 51. ISBN 9783527318605. 
  10. ↑ 10.0 10.1 Tepehan, F. Z.; Ghodsi, F. E.; Ozer, N.; Tepehan, G. G. (1999). "Optical Properties Of Sol-Gel Dip-Coated Ta2O5 Films For Electrochromic Applications". Sol. Eng. Mat. Sol. Cells 59 (3): 265–275. doi:10.1016/S0927-0248(99)00041-0. 
  11. ↑ Bradley, D. C.; Holloway, H. (1961). "Metal Oxide Alkoxide Polymers: Part II. The Hydrolysis Of Tantalum Pentaethoxide". Can. J. Chem. 39 (9): 1818–1826. doi:10.1139/v61-239. 
  12. ↑ Mehrotra, R. C.; Singh, A. (1997). "Recent Trends in Metal Alkoxide Chemistry". Progress in Inorganic Chemistry 46: 239ff. doi:10.1002/9780470166475.ch4. 
  13. ↑ 13.0 13.1 13.2 Bradley, D. C.; Mehrotra, R. C.; Rothwell, I. P.; Singh, A. (2001). Alkoxo and Aryloxo Derivatives of Metals (2nd ed.). Academic Press. p. 18. ISBN 9780121241407. 
  14. ↑ 14.0 14.1 US patent, Chang, K. K. & Chen, C.-H., "Method For Fabricating A Nitride Read-Only-Memory (NROM)", issued 2002-10-08, assigned to Macronix International Co. Ltd. 
  15. ↑ Nalwa, H. S. (2001). Handbook of Advanced Electronic and Photonic Materials and Devices: Chalcogenide Glasses and Sol-Gel Materials. Academic Press. p. 208. ISBN 9780125137553. 
  16. ↑ 16.0 16.1 Oubaha, M.; Elmaghrum, S.; Copperwhite, R.; Corcoran, B.; McDonagh, C.; Gorin, A. (2012). "Optical Properties of High Refractive Index Thin Films Processed at Low-Temperature". Opt. Mater. 34 (8): 1366–1370. doi:10.1016/j.optmat.2012.02.023. 
  17. ↑ Masuda, Y.; Wakamatsu, S.; Koumoto, K. (2004). "Site-Selective Deposition And Micropatterning Of Tantalum Oxide Thin Films Using A Monolayer". J. Eur. Ceram. Soc. 24 (2): 301–307. doi:10.1016/S0955-2219(03)00230-9. 
  18. ↑ Mortimer, R. J. (2011). "Electrochromic Materials". Ann. Rev. Mater. Res. 41: 241–268. doi:10.1146/annurev-matsci-062910-100344. 
  19. ↑ Wernberg, A. A.; Braunstein, G.; Paz-Pujalt, G.; Gysling, H. J.; Blanton, T. N. (1993). "Solid-Phase Epitaxial-Growth Of Lithium Tantalate Thin-Films Deposited By Spray-Metalorganic Chemical-Vapor-Deposition". Appl. Phys. Lett. 63 (3): 331–333. doi:10.1063/1.110061. 
  20. ↑ Lee, W. J.; You, I. K.; Ryu, S. O.; Yu, B. G.; Cho, K. I.; Yoon, S. G.; Lee, C. S. (2001). "SrTa2O6 Thin Films Deposited By Plasma-Enhanced Atomic Layer Deposition". Jpn. J. Appl. Phys. 40 (12): 6941–6944. doi:10.1143/JJAP.40.6941. 
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