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

Reduction of the cyclometalation-resistant aryloxide compounds [W(OC6HPh2-2,6-R2-3,5) 2 -Cl 4 ] (1, R = Ph; 2, R = Me; 3, R = But) in the presence of a variety of phosphine ligands generates the W(II) species [W(OC 6 HR 2 Ph-η 6 -C 6 H 4 )(OC 6 HPh 2 -2,6-R 2 -3,5)(L)] (4-6) in moderate yield. Compounds 4-6 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 PMe 3 , PEt 3 , and PBu n 3 adducts 4b-d 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 O 2 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 (PMe 3 )] (X = O, NPh, Ntol; 7-10, respectively). Structural studies show a trigonal-bipyramidal geometry for 7 and 10 with equatorial O or NAr groups and an axial PMe 3 . Formation of the mixed-imido species [W(OC 6 HPh 4 -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(OC 6 -HPh4-2,3,5,6) 2 (NPh)(O)(OPMe 3 )] (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 12-14. 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 d 0 -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 (d 2 -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 1 H NMR and shown to have a first-order dependence on both [4b] and [TolN=NTol) and inverse first-order dependence on [PMe 3 ]. The kinetic data was fit to a mechanistic model involving fast exchange of azobenzene with PMe 3 followed by a rate-determining product formation. The concentration of free PMe 3 (strong inhibition) is controlled by the position of the fast coordination equilibrium of the product [W(OC 6 HPh 4 -2,3,5,6) 2 (NTol) 2 (PMe 3 )] (9). Reaction of trans-MesN=NMes (NMes = NC 6 H 3 Me 3 -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.