2 Homometallic Alkoxides

Homometallic Alkoxides 109
Under carefully controlled conditions, however, intermediate metal oxo-alkoxides,
MOy⊲OR⊳x 2y may be isolated. 21,617–619,645,646 (See also Chapter 5.)
A detailed mechanistic study of hydrolysis reactions was made by Bradley 617,645
who had isolated a number of metal oxo-alkoxides and assigned plausible structures for
such products. There has been a renewal of interest 21,646 in the chemistry of metal oxoalkoxides
(Chapter 5), some of which have been isolated under anhydrous conditions
also. Further, these hydrolysis reactions have assumed unprecedented technological
significance in view of their importance in the preparation of ceramic materials by the
sol–gel process (Chapter 7) which involves controlled hydrolysis of metal alkoxides
(or some other precursors).
4.3 Reactions with Alcohols, Phenols, and Silanols (as well as Organic and
Silyl Esters)
4.3.1 Alcohol Exchange Reactions
Metal alkoxides (of early transition, lanthanide, actinide, and main group metals, except
silicon) react with a variety of primary, secondary, and tertiary alcohols (R 0 OH) to set
up an equilibrium rapidly of the type shown in Eq. (2.193):
M⊲OR⊳x C yR 0 OH ⇀
↽ M⊲OR⊳x y⊲OR 0 ⊳y C yROH ⊲2.193⊳
There appear to be a number of important factors which influence the facility and
extent of substitution in alcoholysis reactions (Eq. 2.193): (i) the solubility of the reactant
and product metal alkoxides, (ii) the steric demands of the ligands and the alcohol,
(iii) the relative 1H 0 of ionization of the reactant and product alcohols, (iv) the relative
bond strengths of the alkoxides, (v) the presence of strongly bridged and more easily
replaceable terminal alkoxo groups, and (vi) the electron-donating and withdrawing
nature of the groups attached to oxygen.
Interestingly, the reaction represented by Eq. (2.193) can be pushed to yield homoleptic
alkoxides M⊲OR 0 ⊳x by fractionating out the liberated alcohol, provided it is
more volatile. In some cases a solvent like benzene which forms an azeotrope with
the liberated alcohol (EtOH or Pr i OH) not only facilitates fractionation of the alcohol
produced, but also makes it possible to carry out reactions in different stoichiometric
ratios of the reactants to obtain the desired heteroleptic alkoxides M⊲OR⊳x y⊲OR 0 ⊳y.
Another advantage of this procedure is the possibility of monitoring the progress of the
reaction by estimating the liberated isopropyl alcohol (or ethyl alcohol) by a convenient
oxidimetric method. 205,647 The alcohol-interchange reactions have, therefore, been
extensively used as synthetic strategies (Section 2.7) for the formation of a variety of
new homo- and heteroleptic metal alkoxides.
An interesting observation was reported 61 in the replaceability of only three methoxide
groups of insoluble [Zr⊲OMe⊳4]n with excess tertiary butyl alcohol resulting in the
formation of the kinetically and thermodynamically favoured derivative [(Bu t O)3Zr( -
OMe)2Zr(OBu t )3] which does not appear to react further with tertiary butyl alcohol
(Eq. 2.194)
2[Zr⊲OMe⊳4]n C 6nBu t OH ! n[⊲Bu t O⊳3Zr⊲ -OMe⊳]2
B
C 6nMeOH " ⊲2.194⊳