Alkyne Exchange Reactions of Silylalkyne Complexes of Tantalum:  Mechanistic Investigation and Its Application in the Preparation of New Tantalum Complexes Having Functional Alkynes (PhC⋮CR (R = COOMe, CONMe2))

Silylalkyne complexes of tantalum with the general formula TaCl3(R1C⋮CR2)L2 (1, R1 = R2 = SiMe3, L2 = DME; 2, R1 = SiMe3, R2 = Me, L2 = DME; 6, R1 = R2 = SiMe3, L = py; 7, R1 = SiMe3, R2 = Me, L = py) reacted with an internal alkyne to give corresponding alkyne complexes via an alkyne exchange reaction. These silylalkyne complexes were newly prepared and structurally characterized. Kinetic measurements revealed that the rates of the exchange reactions of the DME complexes 1 and 2 were first-order dependent on the concentration of the complex and the reaction proceeded via dissociative pathway. The exchange reaction rates of the bis(pyridine) complexes 6 and 7 were slower. In contrast to the DME complexes, the exchange reaction of 6 proceeded via an associative interchange pathway, and the exchange mechanism for 7 was an associative pathway. During the exchange, two pyridine ligands coordinated strongly to the tantalum center and the intermediate is proposed to be a bis(alkyne) complex. New tantalum complexes having functional alkynes (PhC⋮CR (R = COOMe, CONMe2)) were prepared from the silylalkyne complexes via the alkyne exchange reaction.