Tantalum η²-Vinyl Complexes Bearing an [η⁵:σ-Me₂C(C₅H₄)(C₂B₁₀H₁₀)]^2– Ligand: Syntheses and Structural Characterizations

Abstract

Transition-metal η2-vinyl complexes, also known as 1-metallacyclopropenes, constitute an important subset of metal alkenyl complexes, which are limited to groups 6–8 metals only. We report in this paper a strategy for the synthesis of Ta η2-vinyl complexes. Hydrogenolysis of tantalum alkyl complex LTaMe3 (1, L = [η5:σ-Me2C­(C5H4)­(C2B10H10)]) in the presence of dmpe ([Me2PCH2]2) followed by addition of p-tolylacetylene gave an η2-vinyl complex, LTa­(η2-H2CCC6H4CH3)­(dmpe) (2). Replacement of p-tolylacetylene with t-butyl acetylene resulted in the formation of an alkynyl η2-vinyl complex LTa­(η2-HCC t Bu)­(Me2PCH2CH2PMe2)­(CC t Bu) (3). Using trimethylsilyl acetylene as the substrate gave both LTa­(η2-H2CCSiMe3)­(dmpe) (4) and LTa­(η2-HCCSiMe3)­(Me2PCH2CH2PMe2)­(CCSiMe3) (5). A mechanistic study revealed that the Ta η2-vinyl complexes 2–5 were formed through a tantalum trihydride intermediate, which was trapped by CO to give LTaH­(CO)­(dmpe) (6).

Synthesis of Organotantalum Alkyne Complexes [{η5:σ-Me2C(C5H4)(C2B10H10)}TaMe(η2-R1C≡CR2)] and Their Reactions with Unsaturated Molecules

“…The Ta atom is η 5 -bound to the Cp ring, σ-bound to the cage carbon, and η 2 -bound to the alkyne and the iminoacyl ligand in a four-legged piano-stool geometry. The C­(cage)–Ta distances in 4a and 4b are in agreement with the reported values of C­(cage)–Ta distances. , The C–C­(alkyne) distances in 4a (1.303(5) Å) and 4b (1.32(3) Å) compare with those in the parent alkyne complexes 2a (1.321(5) Å) and 2b (1.317(4) Å).…”

“…X-ray analyses reveal that 5 have metallaheterocyclic structures with an azabutadienyl moiety, arising from alkyne–iminoacyl coupling. The C­(cage)–Ta distances in 5a – 5c are close to the reported values of C­(cage)–Ta distances. , The typical structure of 5a is displayed in Figure , in which the Ta is η 5 -bound to the Cp ring, σ-bound to the cage carbon, and incorporated into an azabutadienyl unit. The azabutadienyl fragment in 5a – 5c is almost coplanar with the dihedral angle of C3–C4–C5–N1 = 1.8–4.5°.…”

“…The C(cage)−Ta distances in 4a and 4b are in agreement with the reported values of C(cage)−Ta distances. 12,13 The C−C(alkyne) distances in 4a (1.303(5) Å) and 4b (1.32(3) Å) compare with those in the parent alkyne complexes 2a (1.321(5) Å) and 2b (1.317(4) Å).…”

“…11 Very recently, we found that [Ta{η 5 :σ-Me 2 C(C 5 H 4 )-(C 2 B 10 H 10 )}Me 3 ] (1) was easily transformed to [Ta{η 5 :σ-Me 2 C(C 5 H 4 )(C 2 B 10 H 10 )}(dmpe)H 3 ] (dmpe = 1,2-bis-(dimethylphosphino)ethane) under 1 atm of H 2 and 1 equiv of dmpe at ambient temperature. 12 We wondered if such tantalum polyhydrides could serve as starting materials of Ta III for the preparation of Ta alkynes. In fact, treatment of complex 1 with H 2 (1 atm) and alkynes gave the corresponding Ta alkynes, [Ta{η 5 :σ-Me 2 C(C 5 H 4 )(C 2 B 10 H 10 )}(Me)(alkyne)].…”

“…The C(cage)−Ta distances in 5a−5c are close to the reported values of C(cage)−Ta distances. 12,13 The typical structure of 5a is displayed in Figure 6, in which the Ta is η 5 -bound to the Cp ring, σ-bound to the (II). 21 As listed in Table 2, the selected structural data of 5a− 5c over the Ta−azabutadienyl moiety are similar to those found in I 28 and II.…”