2 Homometallic Alkoxides

4.3.2 Phenol–Alcohol Exchange Reactions
Homometallic Alkoxides 111
The characteristic ability of metal alkoxides to undergo facile exchange reactions with
hydroxylic reagents has been extensively utilized for preparing homo- and heteroleptic
phenoxides of a large number of elements 652 by the general reaction (Eq. 2.198):
M⊲OR⊳x C yPhOH ! M⊲OR⊳x y⊲OPh⊳y C yROH " ⊲2.198⊳
⊲M D Y, 155 Pr, Nd, 153 Sm, 155 Gd, and Er; 157 y D 1, 2, 3; R D Pr i ; x D 3. M D U; 653
y D 1, 2, 3, 4; R D Et; x D 5. M D Ti, 654 Zr; 655 y D 4; R D Bu t , x D 4. M D Ti; 276
y D 1, 2, 3, 4; R D Et, Pr i ; x D 4. M D VO; 656 y D 3; R D Et; x D 3. M D Al; 657
y D 1, 2, 3; R D Bu t ; x D 3. M D Si. 658,659 Ge; 660 y D 1, 2, 3, 4; R D Et; x D 4⊳
This method tends to work well with simple phenol or phenols substituted with
electron-withdrawing groups (F, Cl, NO2), owing to the increased acidity of the phenols
and their lower volatility relative to alcohols.
In some cases, a control of stoichiometry allows the isolation of mixed alkoxidephenoxide
derivatives. 153,155,157,276,652 In the case of uranium(V) ethoxide, only partial
substitution can be achieved even by reacting with an excess of phenol. 653
Reaction of Mo2⊲OPr i ⊳6 with excess 2,6-dimethylphenol yields only partially substituted
product 661 Mo2(OPr i )2(OC6H3Me2-2,6)4 for steric reasons.
The important role of steric congestion around the central atom in determining the
composition of the final product has also been demonstrated 662–664 during the reactions
of other metal alkoxides [Ti⊲OPr i ⊳4, Zr⊲OPr i ⊳4.Pr i OH, M⊲OPr i ⊳5 with M D Nb or Ta,
Al⊲OPr i ⊳3,Sb(OPr i )3) with sterically hindered phenols (HOC6H3Me2-2,6; HOC6H3Pr i 2 -
2,6; HOC6H3Bu t 2 -2,6].
It is noteworthy that calix[n]arenes, which are a family of macrocyclic reagents
with n D 4, 5, 6, 7, or 8 phenolic groups in the ring (2-XII) can also find the metal
ions via the deprotonated phenolate ligand. 665,666 For example, treating two equivalents
of Ti(OR)4 (R D Pr i ,Bu t ) or Zr(OPr i )4 with p-tert-butylcalix[8]arene (LH8 when
n D 8) in the presence of a base such as alkali metal (M 0 D Li, Na, K) alkoxides or
amines ⊲R 0 NH2⊳ has led to the synthesis 665 of derivatives of the type [M 0 or R 0 NH3] C
[(MOR)2(p-tert-butylcalix[8]arene)] (M D Ti, Zr). Chisholm et al. 667 have shown
that the reaction of Mo2⊲OBu t ⊳6, with monomethylated calix[4]arene ligand ⊲H3L 0 ⊳
in benzene at room temperature affords an interesting derivative Mo2[ 4 -L 0 ⊳2.
Bu t
CH2
OH
(2-XII)
4.3.3 Silanol–Alcohol Exchange Reactions
n
(n = 4, 5, 6, 7, 8)
Like alcohols, silanols have been found to react with metal alkoxides to form the
corresponding homo- and heteroleptic metal siloxides (Eq. 2.199).
M⊲OR⊳x C yR 0 3 SiOH ⇀
↽ M⊲OR⊳x y⊲OSiR 0 3 ⊳y C yROH " ⊲2.199⊳