Chemistry Reference
In-Depth Information
states,
e.g.
[W(OC
6
H
3
Me
2
-2,6)Cl
5
],
432
[W(OC
6
H
3
Pr
i
2
-2,6)
4
],
433
[W(OC
6
H
3
Ph-
6
-
C
6
H
5
)(H)(PMePh
2
)
2
],
434
and [W(OC
6
H
3
Ph-
6
-C
6
H
5
)(OAr)(dppm)].
29
,
435
The reduction of the tetrachloride [W(OC
6
H
3
Ph
2
-2,6)
2
Cl
4
] in the presence of
phosphine ligands leads to either bis-cyclometallated compounds,
e.g.
[W(OC
6
H
3
Ph-
1
-C
6
H
4
)
2
(PMePh
2
)
2
] or else to produce the deep-green W(
II
) species [W(OC
6
H
3
Ph-
6
-C
6
H
5
)(OAr)(PMePh
2
)].
436
,
437
The latter compound can be thermally converted to
the former with the elimination of H
2
. The adducts [W(OC
6
H
3
Ph-
1
-C
6
H
4
)
2
L
2
](L
2
D a
variety of pyridines and bipyridines) are paramagnetic in solution. The temperature
dependence of the NMR spectra has been used to determine the singlet- triplet energy
gap for the d
2
-W(
IV
) metal centre and a pyridine ligand
-acidity scale developed.
438
The use of metallation-resistant 2,3,5,6-tetraphenylphenoxide ligands gives the corre-
sponding, thermally stable
6
-arene derivatives in higher yield and with a variety of
different phosphine ligands.
439
Structural studies of these
-arene compounds show
the strongly metal-bound arene ring to be reduced, leading to structural parameters
consistent with a bonding picture (Scheme 6.9). These compounds can carry out the
four-electron reduction of a variety of small molecules including the four-electron
cleavage of the N
D
N double bond in azobenzenes (Scheme 6.9).
440
Mechanistic studies
show that the process occurs at a single metal centre, and the pathway has been analysed
theoretically.
441
With suitable ketones and aldehydes, products derived by insertion of
the carbonyl function into a W-C(metallanorbornadiene) bond are observed.
442
Ph
Ph
Ph
Ph
Ph
O
W
Ph
O
W
OAr
OAr
PMe
3
PMe
3
O=O
tolN=Ntol
PhN=Ntol
PhN=O
ArO
ArO
ArO
O
O
Ntol
NPh
NPh
ArO
W
ArO
W
ArO
W
ArO
W
O
Ntol
Ntol
OAr
Me
3
PO
PMe
3
PMe
3
PMe
3
Scheme 6.9
The aryloxide ligand has played an important part in the development of the olefin/
alkyne metathesis chemistry associated with molybdenum and tungsten. Many early
catalyst systems consisted of Mo/W aryloxide compounds “activated” in the presence of
suitable substrates.
443
Examples include the metathesis of olefins by [W
OAr
x
Cl
6
x
]
precursors treated with [EtAlCl
2
] and related reagents.
444 - 447
In one study of the meta-
thesis of 2-pentene catalysed by [W
OAr
4
Cl
2
] it was shown that electron-withdrawing
groups in the 4-position of the phenoxide increase the activity considerably. A linear
free-energy relationship was observed over a limited range of substituents (Cl, Br,
H, Me, OMe). The presence of methyl substituents in the 2- and 6-positions on the
