Dinuclear coordination of a novel four-electron reduced μ-η¹,η³-allene ligand with β-agostic C-H⋯Ta interaction: Direct addition of allene to (C₅Me ₅)₂Ta₂

The organoditantalum complexes (η-C₅Me₅) ₂Ta₂(μ-X)₄ (X = Cl, Br) react with 1-5 equiv. of allene under mild conditions to afford the fluxional allene adducts (C₅Me₅)₂Ta₂(μ-η¹, η³-C₃H₄)(μ-X)X₃ as shown by spectroscopic and spectrometric data. Low-temperature-limit proton NMR data at -95°C for (C₅Me₅)₂Ta₂(μ- η¹,η³-C₃H₄)(μ-Cl)Cl ₃ show a large chemical shift range for the inequivalent allene hydrogens, with a high-field value for one hydrogen at δ -0.36, and inequivalent Cp* groups. The low-temperature-limit ¹³C NMR resonances for the allene ligand are found at δ 204.4 (central C), δ 70.3 (CH₂, ¹J_CH = 154 Hz), and the substantially upfield value of δ 42.9. The latter resonance has remarkably different coupling constants (¹J_CH = 173, 154 Hz). The allene coordinates in a novel alkylidene-diyl fashion in the solid-state, as shown by X-ray diffractometry on (C₅Me₅) ₂Ta₂(μ-η¹,η³-C ₃H₄)(μ-Cl)Cl₃ at 200 K. One tantalum is doubly-bonded to the central allenic carbon (Ta1C22 distance, 2.011(7) Å) and interacts with an allene hydrogen in a probable β-agostic manner (Ta1-H21A distance of 2.25(10) Å). The second tantalum is coordinated to the allene in a hybrid of η³-allylic and sigma bonding, with Ta2-C distances to the allene methylenes of 2.206(8) and 2.276(9) Å and a Ta2-C22 bond distance to the central allenic carbon of 2.253(7) Å. The allene hydrogens are oriented above and below the C-C-C plane, consistent with appreciable σ-bonding between the Ta(2) and the allene methylenes. The bent allene ligand has a C-C-C angle of 106.0(6)°and can be viewed as reduced by four electrons, with concomitant oxidation of the two d² tantalum centers of the organoditantalum(III) reactant. The long Ta⋯Ta nonbonded distance of 3.3052(8) Å is consistent with two d⁰ tantalum centers in an organoditantalum(V) product.