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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- 作者：Margaret R. Lentz ; Jonathan S. Vilardo ; Mark A. Lockwood ; Phillip E. Fanwick ; and Ian P. Rothwell
- 刊名：Organometallics
- 出版年：2004
- 出版时间：February 2, 2004
- 年：2004
- 卷：23
- 期：3
- 页码：329 - 343
- 全文大小：457K
- 年卷期：v.23,no.3(February 2, 2004)
- ISSN：1520-6041
文摘
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 ligandsgenerates the W(II) species [W(OC6HR2Ph-
6-C6H4)(OC6HPh2-2,6-R2-3,5)(L)] (4-6) in moderate yield. Compounds 4-6 contain a three-legged piano-stool geometry with one of thearyloxide ligands chelated to the tungsten via an ortho phenyl ring. Solid-state structuralparameters for the PMe3, PEt3, and PBun3 adducts 4b-d indicate a shortening of theW-C(ipso) and W-C(para) distances consistent with a tungstanorbornadiene resonancecontribution. Solution studies of 4b show that exchange on the NMR time scale of orthoand meta protons occurs at elevated temperatures via phosphine dissociation and not 
-arenedissociation. 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; 7-10, respectively). Structural studiesshow a trigonal-bipyramidal geometry for 7 and 10 with equatorial O or NAr groups and anaxial 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 anda phosphine oxide donor ligand. Compound 4b was also found to ring-open the substratespyridazine, benzo[c]cinnoline, and phthalazine to produce the three new seven-membereddiaza 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 d0-W(VI) metal centerwith a 2,7-diazatungstahepta-1,3,5,7-tetraene ring and two tungsten-imido bonds for 12and 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 onboth [4b] and [TolN=NTol] and inverse first-order dependence on [PMe3]. The kinetic datawas fit to a mechanistic model involving fast exchange of azobenzene with PMe3 followed bya rate-determining product formation. The concentration of free PMe3 (strong inhibition) iscontrolled 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) with4b was slow even at 100 C. However, photolysis to produce cis-MesN=NMes led to rapidroom-temperature formation of [W(OC6HPh4-2,3,5,6)2(NMes)2] (15). The many orders ofmagnitude rate acceleration for cis-diazines over their trans counterparts combined withall the other mechanistic work makes a compelling argument that these cleavage reactions(four-electron reductions) are taking place at a single metal center.
6-C6H4)(OC6HPh2-2,6-R2-3,5)(L)] (4-6) in moderate yield. Compounds 4-6 contain a three-legged piano-stool geometry with one of thearyloxide ligands chelated to the tungsten via an ortho phenyl ring. Solid-state structuralparameters for the PMe3, PEt3, and PBun3 adducts 4b-d indicate a shortening of theW-C(ipso) and W-C(para) distances consistent with a tungstanorbornadiene resonancecontribution. Solution studies of 4b show that exchange on the NMR time scale of orthoand meta protons occurs at elevated temperatures via phosphine dissociation and not -arenedissociation. 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; 7-10, respectively). Structural studiesshow a trigonal-bipyramidal geometry for 7 and 10 with equatorial O or NAr groups and anaxial 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 anda phosphine oxide donor ligand. Compound 4b was also found to ring-open the substratespyridazine, benzo[c]cinnoline, and phthalazine to produce the three new seven-membereddiaza 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 d0-W(VI) metal centerwith a 2,7-diazatungstahepta-1,3,5,7-tetraene ring and two tungsten-imido bonds for 12and 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 onboth [4b] and [TolN=NTol] and inverse first-order dependence on [PMe3]. The kinetic datawas fit to a mechanistic model involving fast exchange of azobenzene with PMe3 followed bya rate-determining product formation. The concentration of free PMe3 (strong inhibition) iscontrolled 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) with4b was slow even at 100 C. However, photolysis to produce cis-MesN=NMes led to rapidroom-temperature formation of [W(OC6HPh4-2,3,5,6)2(NMes)2] (15). The many orders ofmagnitude rate acceleration for cis-diazines over their trans counterparts combined withall the other mechanistic work makes a compelling argument that these cleavage reactions(four-electron reductions) are taking place at a single metal center.