Dinuclear coordination of a novel four-electron reduced μ-η 1,η 3-allene ligand with β-agostic C–H⋯Ta interaction: Direct addition of allene to (C 5Me 5) 2Ta 2(μ-X) 4 (X = Cl, Br), and molecular structure of (C 5Me 5) 2Ta 2(μ-η 1,η 3-C 3H 4)(μ-Cl)Cl 3

Abstract

The organoditantalum complexes (η-C 5Me 5) 2Ta 2(μ-X) 4 (X = Cl, Br) react with 1–5 equiv. of allene under mild conditions to afford the fluxional allene adducts (C 5Me 5) 2Ta 2(μ-η 1,η 3-C 3H 4)(μ-X)X 3 as shown by spectroscopic and spectrometric data. Low-temperature-limit proton NMR data at −95 °C for (C 5Me 5) 2Ta 2(μ-η 1,η 3-C 3H 4)(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 (CH 2, 1 J CH = 154 Hz), and the substantially upfield value of δ 42.9. The latter resonance has remarkably different coupling constants ( 1 J CH = 173, 154 Hz). The allene coordinates in a novel alkylidene-diyl fashion in the solid-state, as shown by X-ray diffractometry on (C 5Me 5) 2Ta 2(μ-η 1,η 3-C 3H 4)(μ-Cl)Cl 3 at 200 K. One tantalum is doubly-bonded to the central allenic carbon (Ta1 C22 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 η 3-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 d 2 tantalum centers of the organoditantalum(III) reactant. The long Ta⋯Ta nonbonded distance of 3.3052(8) Å is consistent with two d 0 tantalum centers in an organoditantalum(V) product.