Successive Insertion of Alkynes into a Tantalum−Alkynyl Bond: Implications for the Coordination Polymerization of Alkynes

The reaction of 2,6-[ArN(SiMe 3 )CH 2 ] 2 NC 5 H 3 ( 1, BDPP(SiMe 3 ) 2, Ar = 2,6- i Pr 2 C 6 H 3 ) with TaCl 5 yields the complex mer -(BDPP)TaCl 3 ( 2 ) and 2 equiv of ClSiMe 3 . The reduction of compound 2 with excess Na/Hg in the presence of alkynes yields the pseudo 5-coordinate Ta(III) derivatives (BDPP)Ta(η 2 -RC⋮CR‘)Cl ( 3a, R = R‘ = Pr; 3b, R = R‘ = Et; 3c, R = R‘ = Ph; 3d, R = Ph, R‘ = H). An X-ray study of 3a revealed a distorted square pyramidal geometry with the chloride occupying the apical position. Compound 3a reacts with LiC⋮CR to give the acetylide octyne derivatives (BDPP)Ta(η 2 -PrC⋮CPr)(C⋮CR) ( 5a, R = Ph; 5b, R = Bu; 5c, R = SiMe 3; 5d, R = o -Me 3 SiC 6 H 4 ). Compound 5a reacts with phenylacetylene to give the metallacyclic derivative (BDPP)Ta[(η 2 -PhC⋮C)PrC CPrHC CPh] ( 6a ). An X-ray study of 6a again revealed a square pyramidal geometry with the alkenyl carbon occupying the apical position. Similarly, compounds 5b, c react with HC⋮CBu and HC⋮CSiMe 3 to give the metallacycles (BDPP)Ta[(η 2 -BuC⋮C)PrC CPrHC CBu] ( 6b ) and (BDPP)Ta[(η 2 -Me 3 SiC⋮C)PrC CPrHC CSiMe 3 ] ( 6b ), respectively. Compound 5b reacts with HC⋮CPh to give (BDPP)Ta[(η 2 -BuC⋮C)PrC CPrHC CPh] ( 7b ) only, establishing that the starting acetylide is retained in the final product.