THE TUNGSTEN-TUNGSTEN TRIPLE BOND .18. BRIDGING AND TERMINAL ALLYL LIGANDS IN COMPLEXES OF THE FORMULA W2(R)2(NME2)4, WHERE R=C3H5 AND C4H7

The reaction between RMgCl (two equivalents) and 1,2-W2Cl2(NMe2)4 in hydrocarbon solvents affords the compounds W2R2(NMe2)4, where R = allyl and 1- and 2-methyl-allyl. In the solid state the molecular structure of W2(C3H5)2(NMe2)4 has C2 symmetry with bridging allyl ligands and terminal W-NMe2 ligands. The W-W distance 2.480(1) angstrom and the C-C distances, 1.47(l) angstrom, imply an extensive mixing of the allyl pi-MOs with the W=W pi-MOs, and this is supported by an MO calculation on the molecule W2-(C3H5)2(NH2)4 employing the method of Fenske and Hall. The most notable interaction is the ability of the (W=W)6+ centre to donate to the allyl pi*-MO (pi3). This interaction is largely responsible for the long W-W distance, as well as the long C-C distances, in the allyl ligand. The structure of the 2-methyl-allyl derivative W2(C4H7)2(NMe2)4 in the solid state reveals a gauche-W2C2N4 core with W-W = 2.286(1) angstrom and W-C = 2.18(1) angstrom, typical of W=W and W-C triple and single bonds, respectively. In solution (toluene-d8) H-1 and C-13 NMR spectra over a temperature range - 80-degrees-C to + 60-degrees-C indicate that both anti- and gauche-W2C2N4 rotamers are present for the 2-methyl-allyl derivative. In addition, there is a facile fluxional process that equilibrates both ends of the 2-methyl-allyl ligand on the NMR time-scale. This process leads to a coalescence at 100-degrees-C and is believed to take place via an eta3-bound intermediate. The 1-methyl-allyl derivative also binds in an eta1 fashion in solution and temperature-dependent rotations about the W-N, W-C and C=C bonds are frozen out at low temperatures. The spectra of the allyl compound W2(C3H5)2(NMe2)4 revealed the presence of two isomers in solution-one of which can be readily reconciled with the presence of the bridging isomer found in the solid state while the other is proposed to be W2(eta3-C3H5)2(NMe2)4. The compound W2R2(NMe2)4 where R = 2,4-dimethyl-pentadiene was similarly prepared and displayed dynamic NMR behaviour explainable in terms of facile eta1 = eta3 interconversions.