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

Abstract

The reaction of 2,6-[ArN(SiMe3)CH2]2NC5H3 (1, BDPP(SiMe3)2, Ar = 2,6-iPr2C6H3) 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(η2-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(η2-PrC⋮CPr)(C⋮CR) (5a, R = Ph; 5b, R = Bu; 5c, R = SiMe3; 5d, R = o-Me3SiC6H4). Compound 5a reacts with phenylacetylene to give the metallacyclic derivative (BDPP)Ta[(η2-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[(η2-BuC⋮C)PrCCPrHCCBu] (6b) and (BDPP)Ta[(η2-Me3SiC⋮C)PrCCPrHCCSiMe3] (6b), respectively. Compound 5b reacts with HC⋮CPh to give (BDPP)Ta[(η2-BuC⋮C)PrCCPrHCCPh] (7b) only, establishing that the starting acetylide is retained in the final product.

Synthesis, Structure, and Reactivity of Niobium and Tantalum Alkyne Complexes

“…With regard to alkyne polymerization by a Ta alkyne complex, investigations by McConville and co-workers provide evidence for a possible mechanistic pathway. 92 As shown in Scheme 17, treating the [BDPP]Ta(RCCR) acetylide complexes 62 with excess phenylacetylene afforded the metallacyclic product 64. The authors speculated that 64 was formed by the insertion of the coordinated alkyne unit into the Ta−CCPh bond (63), followed by the coordination and insertion of a phenylacetylene unit into the newly formed Ta−"alkynevinyl" bond.…”

“…With regard to alkyne polymerization by a Ta alkyne complex, investigations by McConville and co-workers provide evidence for a possible mechanistic pathway . As shown in Scheme , treating the [BDPP]­Ta­(RCCR) acetylide complexes 62 with excess phenylacetylene afforded the metallacyclic product 64 .…”

Highlights in the Renaissance of Amidometal Chemistry

“…Alkynetantalum complexes with pyridine diamido ligands are suitable model systems to investigate the mechanism of the coordination polymerization of alkynes because successive alkyne insertion and coupling within the coordination sphere of tantalum occurs (Scheme 45). [197] Scheme 45. Sequential alkyne insertion and coupling (Ar 2,6-iPr 2 C 6 H 3 ).…”

In situ generation of uranium alkyl complexes

“…10 In order to probe whether the reactivity of 1 fc -U(CH 2 Ph) 2 is unique, 21 we decided to synthesize analogous dibenzyl complexes of a tridentate, dianionic supporting ligand, 2,6bis(2,6-diisopropylanilidomethyl)pyridine (NN py , Chart 1), because its geometry mimics that of NN fc . Complexes of Ti(IV), [22][23][24] Zr(IV), [25][26][27][28] Ta(V), 29,30 and lanthanides [31][32][33] supported by pyridine-diamide ligands are known.…”