2 Homometallic Alkoxides

636 Alkoxo and Aryloxo Derivatives of Metals
[Al(OC6H2But 2-2,6-Me-4)2Me] 561 and [CpAl(OC6H2But 2-2,6-Me-4)2], 562 have been
characterized. In the case of mono(aryloxides) a ubiquitous stoichiometry is [(R)2M( 2-
OAr)2M(R)2] (MDAl, Ga, In, Tl) although the metal coordination number varies
from four with simple aryloxides to five for chelating ligands (Tables 6.48–6.51). In
the presence of donor ligands, tetrahedral adducts of aluminium aryloxides are very
common, e.g. [Al(OC6H3Pr i 2 -2,6)3(py)], 563 [Al(OC6H2Bu t 2
-2,6-Me-4)2(H)(OEt2)], 557
[Al(OC6H2Bu t 2 -2,6-Me-4)Me2(NH3)], 564 and [Al(OC6H2Bu t 3 -2,4,6)Cl2(OEt2)]. 565 With
smaller ligands, five-coordinate species are possible, e.g. trigonal bipyramidal
[Al(OC6H3Pr i 2 -2,6)2(H)(THF)2]. 566 Particularly important given their intermediacy in
a variety of organic reactions (see above) are adducts formed with ketones and related
carbonyl compounds. 567
6.2.15 Group 14 Metal Aryloxides
The “germylene” and “stannylene” aryloxides [M(OC6H2Bu t 2 -2,6-Me-4)2] (MD Ge,
Sn) can be obtained by treatment of [MfN⊲SiMe3⊳2g2] with phenol. 568 An
intermediate [Sn(OC6H2Bu t 2 -2,6-Me-4)fN⊲SiMe3⊳2g] has been isolated and structurally
characterized. 569 All of these molecules are V-shaped with O–M–O angles of less than
100 Ž . They will act as two-electron donors to metal fragments, e.g. to [Fe(CO)4]. 570
Addition of N3C(O)OAr to [GefN⊲SiMe3⊳2g2] was found to lead to phenoxides such
as [Ge⊲OPh⊳⊲CNO⊳fN⊲SiMe3⊳⊲C6H2Me3⊳g2]. 571 Tin bis(aryloxides) have also been
reported to be produced by addition of phenols to [(C5H4Me)2Sn] 572 and [Sn(acac)2]. 573
Aryloxides of Ge(IV) andSn(IV) can be obtained by reacting the tetrahalides with
LiOAr or reacting [M(NMe2)4] with phenols. 105 The bonding in these derivatives
(Section 4.2) as well as the sometimes facile activation of arene CH bonds at Sn(IV)
metal centres (Section 5.1) has been discussed above. “Hypervalent” anions such as
[Me3Sn(OC6H3Me2-2,6)2] have been characterized and their bonding analysed. 574
Although [SnR4] compounds do not react with phenols under normal conditions, the
“hypervalently activated” [Me2N(CH2)3SnPh3] will undergo stepwise elimination of
benzene and formation of corresponding mono and bis(phenoxides) with phenol. 575
6.2.16 Group 15 Metal Aryloxides
The synthesis of antimony(III) 576,577 and bismuth(III) aryloxides can be achieved by
reacting the trichlorides with either phenols or group 1 metal aryloxides or by treating
trialkyls with phenolic reagents, typically containing electron-withdrawing substituents.
In one case using [NaOC6H2(CF3)3-2,4,6] the reaction failed owing to C–F bond activation
by bismuth. 578 The homoleptic [Bi(OC6H3Me2-2,6)3] 579 is obtained via the chloride
and is a distorted pyramidal monomer (Table 6.56). Dimeric intermediates such as
[Bi2( -OC6H3Me2-2,6)2Cl4(THF)2] have been isolated. 580 The pentafluorophenoxide
(obtained from [BiPh3]) is dimeric, with the electrophilic metal centre coordinating
molecules such as toluene and THF. 581,582 Further reaction with NaOC6F5 leads to polymeric
mixed-metal aryloxides. 583 The compound [BiEt3] reacts slowly with HOPh and
HOC6F5 to form a mono-aryloxide, which is polymeric in the solid state. 584 Aryloxides
of antimony(V) and bismuth(V) can be obtained from [MPh5] substrates (Tables 6.55
and 6.56). Alternatively the dihalides [X2BiPh3] (XD Cl, Br) can be substituted with
NaOAr reagents. 585 Isolated species such as [Bi(OC6F5)(Br)Ph3] (which undergoes