2 Homometallic Alkoxides

478 Alkoxo and Aryloxo Derivatives of Metals
In the case of the group 5 metals niobium and tantalum the reduction potentials
for the mixed alkyl, aryloxides was shown to correlate with absorptions in the electronic
spectra. 203 In particular a band(s) assignable to alkyl to metal charge transfer
was blue-shifted for compounds that are more difficult to reduce. It was argued that
both reduction and the ligand-to-metal charge transfer (LMCT) process are
populating a lowest unoccupied molecular orbital (LUMO), that is sensitive to the
-donor aryloxides. In the case of identical derivatives of niobium and tantalum the
second row d-block metal compound was more readily reduced (LMCT band redshifted).
Furthermore, the alkoxide [Nb(OPr i )2(CH2SiMe3)3] was found to have a more
negative (inaccessible) reduction potential compared to the aryloxide [Nb(OC6H3Me2-
2,6)2(CH2SiMe3⊳3]. 203 The much higher energy for the alkyl to metal charge transfer
band also indicates that the alkoxide ligand is a stronger -donor than the aryloxide
ligand.
5 ARYLOXIDE LIGAND REACTIVITY
5.1 Cyclometallation of Aryloxide Ligands
The aryloxide ligand is able to undergo cyclometallation at various metal centres via
a number of mechanistic pathways. The reactivity can involve activation of the ortho-
CH bond of the phenoxy nucleus itself, as well as aliphatic, benzylic, or aromatic
CH bonds of attached substituents. The products of these reactions are typically stable
four-, five- or six-membered oxa-metallacycles. 27,28
The cyclometallation of simple phenoxides can be achieved at mono- and
polymetallic centres via oxidative addition pathways. The cluster carbonyl [Os3(CO)12]
initially reacts with phenol to produce the hydrido, phenoxide [Os3⊲ 2-H)( 2-
OPh)(CO)11] which on subsequent thermolysis yields the nonacarbonyl whose
structural parameters are consistent with a 1,3-cyclohexadienone bonding description
(Eq. 6.59). 204
[Os3(H)(OPh)(CO)10]
∆, −CO O
(OC) 3Os
H
Os
Os(CO) 3
H
(CO) 3
(6.59)
There is also an extensive series of Ru6 “raft” like clusters that contain orthometallated
phenoxides bound with not only the oxygen and ortho-metallated carbon
bridging two metal centres but also the phenoxy ring -bound to another metal
centre. 205–207
The addition of phenol to the tungsten complex [W( 2 -Me2P-CH2)(PMe3)4]
produces the compound [W(O- 1 -C6H4)(H)2(PMe3)4] which contains a four-membered
metallacycle ring. 208 The substituted substrates 2,6-dimethylphenol and 2,4,6trimethylphenol
react similarly to yield five-membered metallacycles. The substrates,
HOC6H4R-2 (R D Et, Pr i , Bu t , Ph) undergo metallation at the phenoxide nucleus