2 Homometallic Alkoxides

20 Alkoxo and Aryloxo Derivatives of Metals
Specific intermediate products according to Eq. (2.18) may be isolated by controlling
the conditions (solvent, stoichiometry, or temperature). For example, the equimolar
reaction of TiCl4 with Pr i OH in dichloromethane at room temperature has been
shown 108 to yield the dimeric complex [TiCl3⊲HOPr i ⊳⊲ -Cl⊳]2. The above reaction
in 2–3 molar ratios gives the dimeric complexes [TiCl2⊲OPr i ⊳⊲HOPr i ⊳⊲ -Cl⊳]2 and
[TiCl2⊲OPr i ⊳⊲HOPr i ⊳⊲ -OPr i ⊳]2 as outlined in Scheme 2.1 on the basis of X-ray
structures of the products.
Cl
Pr i O
Pr i
Cl
Ti
O
4HOPri 2TiCl4 +
H
Pri O
Pri O
H
O
Ti
Cl
Pr i
OPr i
Cl
− 2HCl
− 2HCl
Scheme 2.1
Cl
Pr i O
Pr i
Pr i
Cl
Ti
O
H
Cl
O
H
H
Cl
Cl
H
O
Ti
Cl
O
Ti
Cl
+ 2HOPr i
Interestingly, the reaction of metallic uranium in isopropyl alcohol in the presence of
stoichiometric amounts of iodine 109 has been shown to afford a mixed iodide–isopropoxide
of uranium(IV), UI2⊲OPr i ⊳2⊲Pr i OH⊳2 (Eq. 2.24):
U C 2I2 C 4Pr i OH Pri OH
! UI2⊲OPr i ⊳2⊲Pr i OH⊳2 C 2HI ⊲2.24⊳
This type of reaction appears to have considerable promise for the preparation of other
polyvalent metal–iodide–isopropoxide complexes.
Out of the p-block elements, anhydrous chlorides of electronegative elements
boron, 110–112 silicon, 2,113–117 and phosphorus 118,119 react vigorously with alcohols to
yield homoleptic alkoxo derivatives [M(OR) x ] (Eq. 2.25). Although no detailed studies
have been made, AlCl3 120,121 and NbCl5 122,123 undergo only partial substitution, while
GeCl4 124,125 does not appear to react at all with alcohols.
Pr i
OPr i
Cl
Pr i
OPr i
MClx C xROH ! M(OR) x C nHCl " ⊲2.25⊳
⊲M D B, x D 3; Si, x D 4; P,⊲As⊳ x D 3⊳
The reactions indicated above occur with primary and secondary alcohols only, and
have been studied mainly with ethyl and isopropyl alcohols. With a tertiary alcohol
(Bu t OH), silicon tetrachloride yields almost quantitatively Si(OH) 4 and Bu t Cl. 126 This
Cl