ELECTROPHILE-PROMOTED CARBYNE CO COUPLING AT A TANTALUM CENTER

Siloxycarbyne complexes of the type [Ta(=COSiR3) (CO) (dmpe)2], R3 = ( t)BuPh2 (1b) or Ph3 (1c), have been prepared by routes analogous to th ose previously described for [Ta(=COSi-(i)Pr3)(CO)(dmpe)2] (1a). Compo unds 1a-c react with R'3SiCl to afford the acetylene complexes [Ta(R3S iOC=COSiR'3)(dmpe)2Cl] (R3 = (i)Pr3, R'3 = Me3 (2a); R3 = (t)BuPh2, R' 3 = Me3 (2b); R3 = Ph3, R'3 = Me3 (2c); R3 = (t)BuPh2, R'3 = Et3 (2d)) . The mechanism of C-C bond formation in these systems has been elucid ated by stopped-flow kinetic studies of the reaction of 1a with excess Me3SiCl in THF to form 2a. Addition of (n-pentyl)4NCl increased the r ate of reaction between 1a and excess Me3SiCl, but produced [Ta(Me3SiO C=COSiMe3)(dmpe)2Cl] rather than 2a. A pathway for chloride-induced ex change of silyl groups between the starting material and excess Me3SiC l has been elucidated. Addition of (n-butyl)4NBPh4 also strongly accel erated the observed reaction rates, but in this case the product forme d was 2a. The observed rate increases were attributed to an increase i n the ionic strength of the solution. At a constant salt concentration of 23.9 mM (n-butyl)4NBPh4, the reaction was first order in both 1a a nd Me3SiCl, with a second order rate constant of 1.71 +/- 0.04 M-1 s-1 at 22 +/- 1-degrees-C. These results are consistent with a mechanism involving electrophilic attack by the silyl reagent on the CO ligand o f 1 as the rate-determining step of the reaction. Addition of AlEt3 to lb allowed the isolation of [Ta(=COSi(t)BuPh2)(COAlEt3)(dmpe)2] (3b), a model for the initial species formed upon silylation of the CO liga nd in 1b. Comparison of the spectroscopic and structural properties of 3b with those of 1b revealed that C-C bond formation does not occur u pon addition of this particular Lewis acid. The structure of 3b reveal ed several important features, the most significant of which is a decr ease in the C-Ta-C angle to 73.4(4)degrees from the value of 89.1(3)de grees in 1b. The corresponding angle in the reductively coupled produc t, [Ta(Et3SiOC=COSi(t)BuPh2)(dmpe)2Cl] (2d), the structure of which is also reported, is 36.5(2)degrees.