Tantalum complexes transfer reactions

The reactivity of a remarkable electronically unsaturated tantalum methyli-dene complex, [p-MeCgH4C(NSiMe3)2]2Ta( = CH2)CH3, has been investigated. Electrophilic addition and olefination reactions of the Ta = CH2 functionality were reported. The alkylidene complex participates in group-transfer reactions not observed in sterically similar but electronically saturated analogs. Reactions with substrates containing unsaturated C-X (X = C, N, O) bonds yield [Ta] = X compounds and vinylated organic products. Scheme 117 shows the reaction with pyridine N-oxide, which leads to formation of a tantalum 0x0 complex. ... 

Chemical vapor deposition processes are complex. Chemical thermodynamics, mass transfer, reaction kinetics and crystal growth all play important roles. Equilibrium thermodynamic analysis is the first step in understanding any CVD process. Thermodynamic calculations are useful in predicting limiting deposition rates and condensed phases in the systems which can deposit under the limiting equilibrium state. These calculations are made for CVD of titanium - - and tantalum diborides, but in dynamic CVD systems equilibrium is rarely achieved and kinetic factors often govern the deposition rate behavior. 

The cationic tantalum dihydride Cp2(CO)Ta(H)2]+ reacts at room temperature with acetone to generate the alcohol complex [Cp2(C0)Ta(H01Pr)]+, which was isolated and characterized [45]. The mechanism appears to involve protonation of the ketone by the dihydride, followed by hydride transfer from the neutral hydride. The OH of the coordinated alcohol in the cationic tantalum alcohol complex can be deprotonated to produce the tantalum alkoxide complex [Cp2(C0)Ta(01Pr)]. Attempts to make the reaction catalytic by carrying out the reaction under H2 at 60 °C were unsuccessful. The strong bond between oxygen and an early transition metal such as Ta appears to preclude catalytic reactivity in this example. 

Moise and coworkers had shown that a cationic dihydride of tantalum, Cp2(CO)Ta(H)2, reacts with acetone to give the alcohol complex Cp2(CO)Ta(HO Pr) [45]. This reaction was proposed to occur by proton transfer from the dihydride, followed by hydride transfer to the protonated ketone. Unfortunately, attempts to make this reaction catalytic under H2 were not successful. Early transition metals like Ta form strong bonds to oxygen, with no alcohol being observed at 60 °C in attempted catalytic reactions. 

Formation of this complex may occur via an initial formation of a low-valent tantalum intermediate which oxidatively adds one terminal P-Cph bond to the metal centre resulting in transfer of the phenyl ligand to the metal and formation of complex 42. The P NMR spectrum of the reaction mixture showed formation of a mixture of cyclooligophosphanes, the phosphinidene-bridged complex [ Ta( -C5Me5)Cl( U-PPh) 2] as well as numerous low-intensity multiplets which could not... 

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