Tantalum complexes activation reactions

TaCl593 reacted with metallic sodium in neat trimethylphosphine to give the phosphinocarbene tantalum complexes 100 and 101, respectively. These reactions are the first examples of double activation of coordinated trimethylphosphine via oxidative cleavage of a substituent methyl C-H bond. A similar process was also observed in the reduction of tantalum pentabrom-ide with magnesium turnings in the presence of dimethylphenylphosphine.94... 

Tantalum(III) complexes, 655-667 activation reactions, 660 alkoxo, 664 carboxylatotrihalo, 664 dialkylphosphido, 665 3-diketonatotribalo, 664 dithiocarbamato, 664 halo... 

Tantalum is severely attacked at ambient temperatures and up to about 100°C in aqueous atmospheric environments in the presence of fluorine and hydrofluoric acids. Flourine, hydrofluoric acid and fluoride salt solutions represent typical aggressive environments in which tantalum corrodes at ambient temperatures. Under exposure to these environments the protective TajOj oxide film is attacked and the metal is transformed from a passive to an active state. The corrosion mechanism of tantalum in these environments is mainly based on dissolution reactions to give fluoro complexes. The composition depends markedly on the conditions. The existence of oxidizing agents such as sulphur trioxide or peroxides in aqueous fluoride environments enhance the corrosion rate of tantalum owing to rapid formation of oxofluoro complexes. 

Triazacyclohexane also gives rise to very active catalysts with the use of chromium [13] as do ligands of the type RS(CH2)2NH(CH2)2SR [14], The latter coordinate in a meridional fashion, while the former can only coordinate in a facial fashion. Recently examples using cyclopentadienyl titanium complexes [15] and tantalum have been reported [16], The diversity of the chromium systems and the new metal systems show that very likely more catalysts will be discovered that are useful for this reaction, including 1-octene producing catalysts (1-octene is in high demand as a comonomer for ethene polymerisation for certain grades of polyethylene). 

Attempts to prepare Ta(NEt2)5 by metathetic exchange reactions were found to lead to a complex in which activation of a )3-CH bond of an NEt2 ligand occurs with the elimination of diethylamine (equation 92), 244 345 Presumably steric pressure at the tantalum center promotes the cyclometallation reaction to generate the iminomethyl group. 

Reactivity characteristic of alkylidene complexes of tantalum is that the a-carbon is susceptible to electrophilic attack, in contrast to the electron-deficient a-carbon of Fischer-type carbene complexes of group 6 transition metals [62]. Based on this unique property of the alkylidene metal-carbon double bond, a range of new types of reactions has been developed. The discovery of the alkylidene complexes of tantalum was a key to understanding the mechanism of olefin metathesis, and they continue to play important roles in C—H bond activation, alkyne polymerization, and ring-opening metathesis polymerization. 

The kinetics of polymerization are remarkably similar to those for the reaction initiated by the analogous titanium complexes 25 and 26 first-order in initiator, zero-order in monomer for the major part of the reaction, and activation energy of 105 kJ mol Propagation via tantalacyclobutane and tantalum carbene complexes is clearly implied. 

Alkoxide ligands play an important spectator role in the chemistry of metal-carbon multiple bonds. Schrock and coworkers have shown that niobium and tantalum alkylidene complexes are active toward the alkene metathesis reaction. One of the terminating steps involves a j8-hydrogen abstraction from either the intermediate metallacycle or the alkylidene ligand. In each case the -hydrogen elimination is followed by reductive elimination. The net effect is a [1,2] H-atom shift, as shown in equations (73) and (74), and a breakdown in the catalytic cycle. Replacing Cl by OR ligands suppresses these side reactions and improves the efficiency of the alkylidene catalysts. ... 

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