Niobium penta-alkoxides

A new series of niobium derivatives (62) of the IV-phenylurethanes has been obtained586 from the insertion reaction between niobium penta-alkoxides and phenyl isocyanate. Most of the products are yellow liquids, one is white and the rest are brown or black solids or semi-solids. They exhibit v(C=0) in the range 1690— 1730 cm-1. Insertion of up to three molecules of phenyl isocyanate occurs readily at... 

Thomas prepared tantalum and niobium penta-alkoxides by the method of Eq. (2.101). For example, the alcoholysis of tris-(dialkylamido) monoalkylimidotan-talum yielded pentaalkoxides very conveniently ... 

The fragmentation pattern for tantalum penta-alkoxides Ta(OR)5 (R = Me, Ft, Pr, CH2CH20Me), in their mass spectra is similar to that for alkoxides of zirconium and hafnium, niobium, molybdenum, and tungsten. For example, at first the loss of OR groups occurs, followed by elimination of the molecules of unsaturated hydrocarbons (usually, with the same number of carbon atoms as R). However, the decomposition of Ta(OMe)s differs from that of other homologues by the loss of HCHO molecules and H atoms. Elimination of ethers, R2O, which is accompanied by the formation of the metal oxoalkoxide ions of the above mentioned alkoxides is typical. In the absence of X-ray crystallographic data, considerable effort had been directed earlier to throw light on the molecular complexities of aluminium trialkoxides in different (solid, liquid, or vapour) states. 

Bradley D.C., Holloway R.C. Nuclear magnetic resonance on niobium and tantalum penta-alkoxides. J. Chem. Soc. 1968 A 219-223... 

By contrast, the alkali alkoxide route appears to be inapplicable for the synthesis of zirconium tetra-alkoxides or niobium (tantalum) penta-aUcoxides, as these tend to form heterobimetallic alkoxides with alkali metal alkoxides (Chapter 3, Section 3.2.1.1), which volatilize out during final purification, whereas alkali titanium alkoxides, even if formed, dissociate readily to give volatile titanium alkoxides. 

An early example was the failure to obtain penta-tertiary alkoxides of niobium owing to the preferential formation of the oxo compounds Nb20(0R)g and NbO(OR)3 in contrast to tantalum which formed thermally stable Ta(OBu )5. This behaviour was explained in terms of the greater tendency of niobium to form the metal-oxo double bond. In the reactions of W(NMe2)e with ROH the hexa-alkoxides W(OR>6 (R = Me, Et, Pr , allyl) were formed but not the tertiary butoxide which gave WO(OBu )4 instead. " The tendency of MoO(OR)4 compounds to form Mo02(OR>2 by ether elimination has also been noted (Eq. 5.4). ... 

Niobium-Based Catalysts Other soluble alkoxides such as those of titanium (IV) [27, 35, 36] and Group 5 metals [21, 28, 37] are active catalysts in the direct carboxylation of alcohols. The reaction mechanism has been elucidated with penta-alkoxo species of Group 5 elements [21, 28, 37] combining DFT and experimental studies. The overall reaction mechanism is depicted in Scheme 6.2. 

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