2 Homometallic Alkoxides

384 Alkoxo and Aryloxo Derivatives of Metals
M⊲OR⊳y C H2O ! M⊲OH⊳⊲OR⊳y 1 C ROH ⊲5.2⊳
M⊲OH⊳⊲OR⊳y 1 C M⊲OR⊳y ! M2O⊲OR⊳2y 2 C ROH ⊲5.3⊳
Studies on the hydrolysis of titanium tetra-alkoxides revealed that the reactions represented
by both Eqs (5.2) and (5.3) are extremely rapid, and no hydroxo compounds
were isolated. 5 Similar results were found with the hydrolysis of the alkoxides of
zirconium, 6 tantalum(V), 7 tin(IV), 8 and uranium(V). 8 In these ebulliometric studies of the
hydrolysis of metal alkoxides in their progenitive alcohols it was found that the degrees
of polymerization of metal oxo-alkoxides formed were surprisingly low for a given
degree of hydrolysis, and structures were proposed based on certain models. However,
just as titanium tetraethoxide, which was essentially trimeric in solution, proved to
be a tetramer in the crystalline state so the oxo-ethoxide [Ti6O4⊲OEt⊳16] proposed in
solution was shown to be [Ti7O4⊲OEt⊳20] by X-ray crystallography, thus emphasizing
the difficulty of assigning structures to species in solution. Klemperer and co-workers
have used 17 O enriched water to great effect in studying the hydrolysis of titanium
alkoxides. 9 They were able to assign chemical shifts to the oxo-ligand in different
environments (e.g. -O, 650–850; 3-O, 450–650; 4-O, 250–450 ppm) by reference
to the known structures [Ti7O4⊲OEt⊳20], [Ti8O6⊲OCH2Ph⊳20], and [Ti10O8⊲OEt⊳24].
The 17 O NMR spectrum of partially hydrolysed Ti⊲OEt⊳4 showed the presence of
[Ti7O4⊲OEt⊳20] and[Ti8O6⊲OEt⊳20] but not [Ti10O8⊲OEt⊳24]. Using magic angle spinning
(MAS) 17 O NMR they demonstrated the presence of mainly 3-O with some 4-O
in the xerogels formed by hydrolysis of Ti⊲OEt⊳4 and the formation of first anatase
(υ17O , 561 ppm) and then rutile (υ17O , 591 ppm) (TiO2) on firing the xerogel above
600 Ž C. Although [Ti3⊲ 3-O⊳⊲ 3-OPr i ⊳⊲ -OPr i ⊳3⊲OPr i ⊳6] was insufficiently stable to be
isolated it was identified by 17 O NMR in the initial stages of hydrolysis of Ti⊲OPr i ⊳4 in
isopropanol with the undecanuclear species [Ti11O13⊲OPr i ⊳18] being formed as the more
stable species. 10 The latter compound and the dodecanuclear species [Ti12O16⊲OPr i ⊳16],
which occurs in two isomeric forms, were isolated and characterized structurally. 11
The importance of steric factors was demonstrated by the fact that the mixed ligand
trinuclear compound [Ti3⊲ 3-O⊳⊲ 3-OMe⊳⊲ -OPr i ⊳3⊲OPr i ⊳6], which was identified in
solution by 17 O NMR, was sufficiently stable to be crystallized from solution and
its X-ray structure was determined. 10 The value of 17 Oand 13 C NMR in identifying
complex polyoxo-alkoxides in solution was illustrated by studies on the hydrolysis of
Ti⊲OBu t ⊳4 in Bu t OH. The hydrolysis, which was relatively slow at room temperature,
gave initially a species proposed to be [Ti3O⊲OBu t ⊳10] but prolonged heating at 100 Ž C
gave rise to the giant molecule [Ti18O27⊲OH⊳⊲OBu t ⊳17] whose 17 O NMR spectrum was
in accord with the requirements of the X-ray crystal structure. 12 By using 17 OMAS
NMR it was shown that the sol–gel produced directly by hydrolysis of Ti⊲OBu t ⊳4
was significantly different in constitution from the product obtained by hydrolysis of
[Ti18O27⊲OH⊳⊲OBu t ⊳17], a result which clearly has some bearing on the mechanism
of the sol–gel process. The structures of the known titanium oxo-alkoxides and those
of zirconium, niobium, tantalum, tin, and aluminium will be discussed in Section 5.
As an alternative to adding water to a metal alkoxide it has been shown that acetone
will undergo aldol-type condensation in the presence of a metal alkoxide leading to
the formation of an oxo-alkoxide. For example the zinc alkoxide [Zn⊲OCEt3⊳2] was
converted to a gel 65 whilst titanium tetraisopropoxide gave a trinuclear species