Tantalum isopropoxide

Alkaline-earth alkoxides are insoluble and involatile in contrast to the double alkoxides formed by dissolving the alkaline earth metal in a solution of niobium or tantalum isopropoxide in propan-2-ol. With the exception of the magnesium derivative which decomposes on heating, they can be sublimed in vacuo, thermal stability diminishing in the order Ba > Sr > Ca. The calcium and strontium derivatives are monomeric in refluxing benzene, the others are insoluble. Attempts to elucidate structures by n.m.r. spectroscopy were foiled by the rapid exchange of geminal dimethyl protons (see also p. 66). 

The NMR spectrum of niobium pentaisopropoxide at room temperature indicates a broad resonance and an average donblet which at low temperatnres was resolved into peaks of the dimer (at S 1.25 and 4.90) and monomer (at S 1.19 and 4.68). Although the monomer/dimer equilibrium for niobium pentaisopropoxide also existed in benzene solution, the value of the equilibrium constant at a given temperatnre was about 100 times greater for niobium than for tantalum isopropoxide, thereby demonstrating the greater degree of dissociation of niobium isopropoxide into monomeric species. 

The metal salts of alcohols are called alkoxides, M(OR), where M is a metal and R is an alkyl group. Typical examples in the synthesis of ferroelectric thin films include titanium isopropoxide [Ti(OPr )4 where Pr represents isopropyl], tantalum ethoxide [Ta(OEt)5 where Et represents ethyl], and zirconium butoxide butanol [Zr(OBu )4 n-BuOH where Bu represents -butyl], an alkoxide/alcohol compound. The structure of this compound, shown in Figure 27.3a, illustrates its dimeric nature, that is, the fact that each molecule contains two metal centers. 

The phenoxides of niobium and tantalum have been obtained from their isopropoxides by a similar reaction carried out in cyclohexane (equation 25). 

In attempts to prepare tantalum pentaisopropoxide and penta-tm-butoxide by alcoholysis of tantalum pentamethoxide with isopropyl alcohol or tert-butyl alcohol, respectively, Bradley etal could isolate only monomethoxide tetra-isopropoxide or tetra-tert-butoxide ... 

The NMR spectra of the isopropoxides of niobium and tantalum were found to be both temperature (Fig. 2.5) and concentration (Fig. 2.6) dependent, consistent with the equilibrium M2(OPF) 2M(OPF)5. For example the pentaisopropoxide of tantalum in cyclohexane solution gave three septets in the range 8 4.7-5.0. The intensity of the high-field signals increased on raising the temperature at the expense of the... 

Compared to the tetra-alkoxoaluminates, the corresponding hexa-alkoxoniobate and -tantalate derivatives of most of the metals are less stable. Of these two, the niobate products tend to disproportionate more readily this is reflected in the conductivity curves obtained in the titrations of niobium and tantalum pentaisopropoxide with alkali (Na, K) isopropoxides in isopropyl alcohol (see Fig. 3.2). 

The hexanuclear tantalum heterometallic oxo-isopropoxide [Ta4Zn2( i3-0)2(/[i-0)2(M-OIV)6(OPr )8l2] has a centrosymmetric structure comprising two triangular Ta2Zn(/u,3-0)(/Li0Pr )3l units linked by two nearly linear Ta(/Li-0)Ta bridges. Tantalum has distorted octahedral and zinc tetrahedral coordination. ... 

Fu D., Suzuki K., Kato K. Local piezoelectric response in bismuth-based ferroelectric thin films investigated by scanning force microscopy. Jpn. J. Appl. Phys. 2002 41 L1103-L1105 GovU S., Kapoor P.N., Mehrotra R.C. Double isopropoxides ofniobium and tantalum with alkaline-earth metals. J. Inorg. Nucl. Chem. 1976 38 172-172 Hirano S., Kato K. Synthesis ofLiNbOs by hydrolysis of metal aUcoxides. Advanced Ceramics Materials 1987 2 142-145... 

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