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Synthetic and Mechanistic Studies of the Four-Electron Reduction of Dioxygen, NN, and NO Double Bonds by Tungsten(II) Aryloxide Compounds

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journal contribution
posted on 02.02.2004, 00:00 authored by Margaret R. Lentz, Jonathan S. Vilardo, Mark A. Lockwood, Phillip E. Fanwick, Ian P. Rothwell
Reduction of the cyclometalation-resistant aryloxide compounds [W(OC6HPh2-2,6-R2-3,5)2Cl4] (1, R = Ph; 2, R = Me; 3, R = But) in the presence of a variety of phosphine ligands generates the W(II) species [W(OC6HR2Ph-η6-C6H4)(OC6HPh2-2,6-R2-3,5)(L)] (46) in moderate yield. Compounds 46 contain a three-legged piano-stool geometry with one of the aryloxide ligands chelated to the tungsten via an ortho phenyl ring. Solid-state structural parameters for the PMe3, PEt3, and PBun3 adducts 4bd indicate a shortening of the W−C(ipso) and W−C(para) distances consistent with a tungstanorbornadiene resonance contribution. Solution studies of 4b show that exchange on the NMR time scale of ortho and meta protons occurs at elevated temperatures via phosphine dissociation and not π-arene dissociation. Reaction of 4b with O2 or the trans-diazines PhNNPh, TolNNTol, and PhNNTol (Tol = 4-methylphenyl) led to the corresponding bis(oxo) and bis(arylimido) derivatives [W(OC6HPh4-2,3,5,6)2(X)2(PMe3)] (X = O, NPh, Ntol; 710, respectively). Structural studies show a trigonal-bipyramidal geometry for 7 and 10 with equatorial O or NAr groups and an axial PMe3. Formation of the mixed-imido species [W(OC6HPh4-2,3,5,6)2(NPh)(NTol)(PMe3)] (10) did not involve generation of any 8 or 9. Cleavage of nitrosobenzene by 4b led to [W(OC6HPh4-2,3,5,6)2(NPh)(O)(OPMe3)] (11) containing terminal oxo and phenylimido groups and a phosphine oxide donor ligand. Compound 4b was also found to ring-open the substrates pyridazine, benzo[c]cinnoline, and phthalazine to produce the three new seven-membered diaza metallacycles 1214. Structural studies show 12 and 14 to contain planar rings, whereas the two backbone phenyl rings in 13 are twisted by 33° with respect to each other. Structural parameters are more consistent with a formulation as a d0-W(VI) metal center with a 2,7-diazatungstahepta-1,3,5,7-tetraene ring and two tungsten−imido bonds for 12 and 14, although there is some evidence for a 2,7-diazatungstahepta-2,4,6-triene (d2-W(IV) metal center) resonance contribution for 14. A kinetic study of the reaction of 4b with trans-4,4‘-dimethylazobenzene (TolNNTol) to produce 9 was carried out. The rate of disappearance of 4b was monitored using 1H NMR and shown to have a first-order dependence on both [4b] and [TolNNTol] and inverse first-order dependence on [PMe3]. The kinetic data was fit to a mechanistic model involving fast exchange of azobenzene with PMe3 followed by a rate-determining product formation. The concentration of free PMe3 (strong inhibition) is controlled by the position of the fast coordination equilibrium of the product [W(OC6HPh4-2,3,5,6)2(NTol)2(PMe3)] (9). Reaction of trans-MesNNMes (NMes = NC6H3Me3-2,4,6) with 4b was slow even at 100 °C. However, photolysis to produce cis-MesNNMes led to rapid room-temperature formation of [W(OC6HPh4-2,3,5,6)2(NMes)2] (15). The many orders of magnitude rate acceleration for cis-diazines over their trans counterparts combined with all the other mechanistic work makes a compelling argument that these cleavage reactions (four-electron reductions) are taking place at a single metal center.