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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. In solution, it aggregates to form bioctahedral dimers 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. It is mainly used for the manufacture of tantalum(V) oxide thin-film materials by approaches including chemical vapor deposition, atomic layer deposition, and sol-gel processing. These materials have semiconductor, electrochromic, and optical applications.
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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 with octahedral six-coordinate tantalum metal centres. Subsequent crystallographic analysis established that the methoxide and isopropoxides of niobium adopt bioctahedral structures. 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. 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 HClUnfortunately, 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. For this reason, ammonia is usually added to trap liberated HCl and maintain basic reaction conditions.
10 EtOH + Ta2Cl10 + 10 NH3 → Ta2(OEt)10 + 10 NH4ClBasic 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:
10 NaOEt + Ta2Cl10 → Ta2(OEt)10 + 10 NaClThe same compound can be prepared electrochemically. The two half-equations and the overall equation 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 H2Commercial production of tantalum(V) ethoxide using this electrochemical approach has been successfully employed in Russia. 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.
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 can be described by this simplified equation:
Ta2(OC2H5)10 + 5 H2O → Ta2O5 + 10 C2H5OHTantalum(V) ethoxide optical coatings can be produced by low pressure chemical vapour deposition. 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. 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–EtPyrolysis 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:
Ta2(OC2H5)10 + 30 O2 → Ta2O5 + 20 CO2 + 25 H2OAmorphous 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. At temperatures approaching 450 °C the films produced have refractive indices and permittivity properties similar to those produced from conventional approaches. The preparation of these films occurs with the loss of chloroethane:
Ta2(OC2H5)10 + Ta2Cl10 → 2 Ta2O5 + 10 C2H5ClSol-gel processing also produces thin films of tantalum(V) oxide using a similar chemical approach. Sol-gel routes using tantalum(V) ethoxide to generate layered perovskite materials have also been developed.
Applications
Tantalum(V) oxide films have a variety of applications including as optical films with refractive indices as high as 2.039 and as a thin-film dielectric material in dynamic random access memory and semiconductor field-effect transistors. 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. 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, where transparent materials with low light loss due to absorption is important, and has also been used to prepare nitride read-only memory. Electrochromism is the property of some materials to change color when charge is applied, 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.
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). Films of strontium tantalate, Sr(TaO3)2, have also been prepared using atomic layer deposition approaches and their properties investigated.
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. The Ta4O4 core of such compounds form a cubane-type cluster. Each oxo ligand bridges between three tantalum centres, which are each also associated with two alkoxide and one carboxylate ligands.