The tungsten-tungsten triple bond18. Bridging and terminal allyl ligands in complexes of the formula W 2(R) 2(NMe 2) 4, where R  C 3H 5 and C 4H 7

Abstract

The reaction between RMgCl (two equivalents) and 1,2-W 2Cl 2(NMe 2) 4 in hydrocarbon solvents affords the compounds W 2R 2(NMe 2) 4, where R = allyl and 1− and 2-methyl-allyl. In the solid state the molecular structure of W 2(C 3H 5) 2(NMe 2) 4 has C 2 symmetry with bridging allyl ligands and terminal WNMe 2 ligands. The WW distance 2.480(1) Å and the CC distances, 1.47(1) Å, imply an extensive mixing of the allyl π-MOs with the WW π-MOs, and this is supported by an MO calculation on the molecule W 2(C 3H 5) 2(NH 2) 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 π *-MO (π 3). 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 W 2(C 4H 7) 2(NMe 2) 4 in the solid state reveals a gauche-W 2C 2N 4 core with WW = 2.286(1) Å and WC = 2.18(1) Å, typical of WW and WC triple and single bonds, respectively. In solution (toluene-d 8) 1H and 13C NMR spectra over a temperature range −80°C to +60°C indicate that both anti- and gauche- W 2C 2N 4 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°C and is believed to take place via an η 3-bound intermediate. The 1-methyl-allyl derivative also binds in an η 1 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 W 2(C 3H 5) 2(NMe 2) 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 W 2(η 3-C 3H 5) 2(NMe 2) 4. The compound W 2R 2(NMe 2) 4 where R = 2,4-dimethyl- pentadiene was similarly prepared and displayed dynamic NMR behaviour explainable in terms of facile η 1 = η 3 interconversions.