Successive insertion of alkynes into a tantalum-alkynyl bond: Implications for the coordination polymerization of alkynes

The reaction of 2,6-[ArN(SiMe3)CH2](2)NC5H3 (1, BDPP(SiMe3)(2), Ar = 2,6(i)Pr(2)C(6)H(3)) with TaCl5 yields the complex mer-(BDPP)TaCl3 (2) and 2 equiv of ClSiMe3. The reduction of compound 2 with excess Na/Hg in the presence of alkynes yields the pseudo 5-coordinate Ta(III) derivatives (BDPP)Ta(eta(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(eta(2)-PrC=CPr)(C=CR) (5a, R = Ph; 5b, R = Bu; 5c, R = SiMe3; 5d, R = omicron-Me3SiC6H4). Compound 5a reacts with phenylacetylene to give the metallacyclic derivative (BDPP)Ta[(eta(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=CSiMe3 to give the metallacycles (BDPP)Ta[(eta(2)-BuC=C)PrC=CPrHC=CBu] (6b) and (BDPP)Ta[(eta(2)Me(3)SiC=C)PrC=CPrHC=C SiMe3] (6b), respectively. Compound 5b reacts with HC=CPh to give (BDPP)Ta[(eta(2)-BuC=C)PrC=CPrHC=CPh] (7b) only, establishing that the starting acetylide is retained in the final product.