TUNGSTEN(VI) PHENYLIMIDO ALKYLIDENE COMPLEXES CONTAINING A MONOANIONIC O,N-CHELATING LIGAND AND THEIR ISOLATED PRECURSOR COMPLEXES - X-RAY STRUCTURES OF W(CH2SIME3)3(=NPH)[OCPH2(2-PY)] AND W(=CHSIME3)(CH2SIME3)(=NPH)[OCPH2(2-PY)]

The lithium salts of selected phenoxides, with one or two potentially chelating o-CH2NMe2 substituents, 8-quinolinolate (8-quin), ortho-chelating arenethiolate ligands, and a-substituted 2-pyridylmethoxides, were reacted with W(CH2SiMe3)3Cl(=NPh) (1) and W(CH2SiMe3)2Cl-(=NPh)(OCMe3) (2), respectively. It appears that steric properties of the monoanionic O,N- and S,N-chelating ligands are very important for inducing an intramolecular H(alpha)-abstraction reaction; i.e., the phenoxide OC6H4CH2NMe2-2 (a) gave rise to the formation of alkylidene precursor W(CH2SiMe3)3(=NPh)(OC6H4CH2NMe2-2) (3), in which the alkyl groups are still intact, whereas the ''pincer phenolate'' OC6H2(CH2NMe2)2-2,6-Me-4 (b), with two o-CH2NMe2 substituents, directly led to the formation of alkylidene complex W(=CHSiMe3)(CH2SiMe3)(=NPh)(OC6H2(CH2NMe2)2-2,6-Me-4) (4'). The reaction of 1 with Na-8-quin instantaneously gave W(=CHSiMe3)(CH2SiMe3)(=NPh)(8-quin) (5'). Also, electronic properties play an important role in the formation of alkylidene complexes; i.e., the complexes derived from 2, containing a pi-electron-donating tert-butoxide group, are thermally more stable toward intramolecular H(alpha) abstraction than those derived from 1. With the alpha-substituted 2-pyridylmethoxide ligands OR1R2(2-py), in which the R substituents can be varied easily, we were able to prepare both the alkylidene precursor and the subsequent product of an intramolecular H(alpha)-abstraction reaction, the tungsten(VI) alkylidene complex. The solid-state structure (X-ray) of the trialkyltungsten(VI) precursor W(CH2SiMe3)3(=NPh)[OCPh2(2-py)] (9) as well as the resulting alkylidene complex W(=CHSiMe3)(CH2SiMe3)(=NPh)[OCPh2(2-py)] (9') have been determined. Crystals of 9 are monoclinic, space group P2(1)/c, with unit-cell dimensions a = 10.974(1) angstrom, b = 18.531(3) angstrom, and c = 20.307(2) angstrom, beta = 106.47(1)degrees, Z = 4, final R = 0.063, and R(w) = 0.053 for 2644 reflections with I > 2.5sigma(I) and 415 parameters. Crystals of 9' are triclinic, space group P1BAR, with unit-cell dimensions a = 9.4359(5) angstrom, b = 10-8887(7) angstrom, and c 16.611(1) alpha = 81.60(1)degrees, beta = 88.37(1)degrees, gamma = 79.19(1)degrees, Z=2, final R=0.042, and R(w) = 0.026 for 5879 reflections with I > 2.5sigma(I) and 348 parameters. The two molecular structures clearly show the advantage of a d0 metal center in forming an alkylidene unit out of two alkyl groups. The chemical shift of H-6, the hydrogen atom bonded at the ortho position with respect to the pyridyl nitrogen, as a probe shows that the pyridyl group in alkylidene complexes 9' and W(=CHSiMe3)(CH2SiMe3)(=NPh)[OCH(CMe3)(2-py)] (10') can coordinate intramolecularly in the syn rotamer, whereas in the anti rotamer this will lead to considerable steric hindrance of the SiMe3 group of the alkylidene function with the vicinal substituents in the chelating ring. The reactivity of the present tungsten(VI) alkylidene complexes 5', 9.', and 10' toward linear olefins is very low. However, aldehydes react with these complexes in Wittig-type reactions to yield predominantly the corresponding trans-olefin. Norbornene can be polymerized using these alkylidene complexes as a catalyst in a ring-opening metathesis reaction at 70-degrees-C to give polymeric cyclopentenes with greater-than-or-equal-to 90% cis vinylene bonds.