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-chel ating arenethiolate ligands, and a-substituted 2-pyridylmethoxides, we re reacted with W(CH2SiMe3)3Cl(=NPh) (1) and W(CH2SiMe3)2Cl-(=NPh)(OCM e3) (2), respectively. It appears that steric properties of the monoan ionic O,N- and S,N-chelating ligands are very important for inducing a n intramolecular H(alpha)-abstraction reaction; i.e., the phenoxide OC 6H4CH2NMe2-2 (a) gave rise to the formation of alkylidene precursor W( CH2SiMe3)3(=NPh)(OC6H4CH2NMe2-2) (3), in which the alkyl groups are st ill 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 CHSiMe3)(CH2SiMe3)(=NPh)(OC6H2(CH2NMe2)2-2,6-Me-4) (4'). The reaction of 1 with Na-8-quin instantaneously gave W(=CHSiMe 3)(CH2SiMe3)(=NPh)(8-quin) (5'). Also, electronic properties play an i mportant role in the formation of alkylidene complexes; i.e., the comp lexes derived from 2, containing a pi-electron-donating tert-butoxide group, are thermally more stable toward intramolecular H(alpha) abstra ction than those derived from 1. With the alpha-substituted 2-pyridylm ethoxide ligands OR1R2(2-py), in which the R substituents can be varie d easily, we were able to prepare both the alkylidene precursor and th e subsequent product of an intramolecular H(alpha)-abstraction reactio n, the tungsten(VI) alkylidene complex. The solid-state structure (X-r ay) of the trialkyltungsten(VI) precursor W(CH2SiMe3)3(=NPh)[OCPh2(2-p y)] (9) as well as the resulting alkylidene complex W(=CHSiMe3)(CH2SiM e3)(=NPh)[OCPh2(2-py)] (9') have been determined. Crystals of 9 are mo noclinic, 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 re flections 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)deg rees, 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 34 8 parameters. The two molecular structures clearly show the advantage of a d0 metal center in forming an alkylidene unit out of two alkyl gr oups. 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 s yn rotamer, whereas in the anti rotamer this will lead to considerable steric hindrance of the SiMe3 group of the alkylidene function with t he vicinal substituents in the chelating ring. The reactivity of the p resent tungsten(VI) alkylidene complexes 5', 9.', and 10' toward linea r olefins is very low. However, aldehydes react with these complexes i n Wittig-type reactions to yield predominantly the corresponding trans -olefin. Norbornene can be polymerized using these alkylidene complexe s 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.