Niobium isopropoxides

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. 

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

Similarly, the reaction of niobium pentaethoxide with isopropyl alcohol yielded monoethoxide tetra-isopropoxide ... 

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). 

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