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

Abstract

Hydrogenolysis of tantalum methyl complex [Ta(L)Me3] (1, L = [η5:σ-Me2C(C5H4)(C2B10H10)]) in the presence of terminal or internal alkynes R1CCR2 gave the corresponding π-alkyne complexes [TaL(η2-R1CCR2)Me] (2a–2g) in 71–93% yields. Their reactions with various kinds of unsaturated molecules were investigated. Complex 2b reacted with trimethylsilyl acetylene to give an η3-butadienyl complex 3. Complexes 2a–2c reacted with XylNC to give aza-η3-butadienyl complexes 5a–5c via the η2-iminoacyl η2-alkyne intermediates 4a–4c. Treatment of 2f with CO in the presence of 1,2-bis(dimethylphosphino)ethane (dmpe) afforded a tantallafuran complex 6. The reaction of 2a with PhNCO produced an oxo N-alkenyl η2-iminoacyl complex 7, whereas 2b reacted with PhNCO to generate a metallacyclic complex 8. The structures of these complexes have been confirmed by single-crystal X-ray analyses.

Metallaaromaticity involving a d0 early transition metal centre: synthesis, structure, and aromaticity of tantallapyridinazirine complexes

“…During our study on organotantalum alkyne complexes bearing a multidentate bulky [Me 2 C(C 5 H 4 )(C 2 B 10 H 10 )] 2− ligand, we observed sequential insertion-C–C coupling reactions with unsaturated molecules on a Ta( v ) metal centre, facilely yielding a variety of d 0 -tantalum metallacycles. 12 Based on these results, we speculate that the steric bulkiness of the σ-carboranyl ligand can effectively protect the electrophilic metal centre during multi-step insertion reactions, facilitating the formation of large metallacycles. 13 To our delight, the multiple insertions of isocyanides into alkyl tantalum–alkyne complexes [η 5 :σ-Me 2 C(C 5 H 4 )(C 2 B 10 H 10 )]TaMe(η 2 -RC CH) delivered an unprecedented class of tantallapyridinazirine complexes, of which the tantallapyridinium ring manifests aromatic characters.…”

“…14 The organotantalum alkynes complexes [η 5 :σ-Me 2 C(C 5 H 4 )(C 2 B 10 H 10 )]TaMe(η 2 -RCCH) [R = SiMe 3 ( 2 ), t Bu ( 3 ), p Tol ( 4 )] were freshly prepared via the hydrogenolysis of 1 in the presence of the corresponding alkynes. 12 Treatment of in situ formed 2 with two equiv. of 2,6-dimethylphenyl isocyanide (XylNC) in toluene at room temperature afforded complex 5 in 75% isolated yield.…”

“…The coordination is followed by the insertion of the coordinated XylNC to the Ta–Me bond to give η 2 -iminoacyl η 2 -alkyne complex E . 12 These two steps are very facile and exergonic by 28.9 kcal mol −1 . The C–C bond formation contributes significantly to the stability of E .…”

“…These exocyclic C– H resonances were downfield shifted compared with the open-chain (6.24 ppm) or cyclic (5.92–6.25 ppm) olefinic C H s for our recently reported tantalum complexes ( Scheme 2a ). 12 Such a deshielded exocyclic proton is indicative of an aromatic environment. 5 The 13 C{ 1 H} NMR spectrum of 5 displayed the signals of C α (3), C β (4), C γ (5), and C δ (6) at 222.4, 92.7, 127.9, and 141.7 ppm, respectively.…”

“…Coordination of the first molecule of XylNC to 2 is the initial event of the reaction and is slightly exergonic with a small barrier, as expected. The coordination is followed by the insertion of the coordinated XylNC to the Ta–Me bond to give η 2 -iminoacyl η 2 -alkyne complex E. 12 These two steps are very facile and exergonic by 28.9 kcal mol −1 . The C–C bond formation contributes significantly to the stability of E. Since E is formally a 16 e − species, further coordination of the second equiv.…”