2 Homometallic Alkoxides

Table 2.10 Vapour pressure of Ti, Zr, and Hf tert-butoxides
Ti(OBu t )4 Zr(OBu t )4 Hf(OBu t )4
Homometallic Alkoxides 65
1Hv 1Hv 1Hv
T Ž C P (mm) (kcal mol 1 ) P (mm) (kcal mol 1 ) P (mm) (kcal mol 1 ) PZr/Ti PHf/Zr
27 0.049 15.8 0.066 15.9 0.069 16.3 1.34 1.05
47 0.25 15.2 0.34 15.3 0.36 15.5 1.34 1.08
67 1.00 14.7 1.35 14.8 1.48 14.7 1.35 1.09
87 3.26 14.1 4.42 14.2 4.80 13.9 1.36 1.09
107 9.04 13.5 12.30 13.7 13.05 13.2 1.36 1.06
127 21.41 13.0 29.77 13.1 30.49 12.4 1.39 1.02
heated at 340 Ž C under 760 mm pressure for about 3 hours; the isopropoxide on the other
hand decomposed under these conditions to give volatile products, propylene, isopropyl
alcohol, and a solid residue containing 65.8% Zr, which is in the range required for
Zr2O3(OPr i )2-ZrO2. Compared to these derivatives, the tert-butoxide and tert-amyloxide
are thermally unstable. The former is stable for about 70 h at 250 Ž C/760 mm under special
conditions whereas the latter is stable indefinitely at 220 Ž C/760 mm, for about 30 h at
250 Ž C/175 mm, and under special conditions for about 6 h at 320 Ž C/760 mm pressure.
These alkoxides also, on thermal decomposition, yielded alkene, tert-alcohols and solid
residues having Zr contents that are also within the range required for Zr2O3(OR t )2-ZrO2.
The following order of the thermal stability of these alkoxides may thus be assigned
in which the ethoxide appears to be thermally more stable than the more volatile
secondary and tertiary alkoxides: Zr(OEt) 4 × Zr⊲OPr i ⊳4 > Zr⊲OBu t ⊳4 ¾ Zr⊲OAm t ⊳4.
Furthermore, the kinetics of thermal decomposition of zirconium tetra-tert-amyloxide 465
studied in a clean glass apparatus in the temperature range 208–247 Ž C indicated that
the tertiary amyl alcohol formed during decomposition eliminates water; a molecule of
water produced from one molecule of alcohol would produce another two molecules
of alcohol by hydrolysis of zirconium tert-amyloxide and hence a chain reaction would
be set up. Since the dehydration of the tertiary alcohol is a rapid first order reaction,
it may be the rate controlling step in the thermal decomposition of tertiary alkoxides.
In comparison with titanium, zirconium, and hafnium alkoxides, the thorium and
cerium tetra alkoxides are less volatile; their methoxides and ethoxides are insoluble
solids, which could not be sublimed even at higher temperatures in vacuo. 143,466 The
long chain normal alkoxides are, however, soluble derivatives but these also could not
be volatilized in vacuo except for cerium tetrapentyloxide which could be sublimed
at 260 Ž C/0.05 mm. 143 The soluble isopropoxide and tertiary alkoxides of cerium and
thorium could also be volatilized (Table 2.11) under reduced pressure. 141,286,287 The
normal alkoxides of cerium, e.g. n-propoxide and n-pentyloxide show molecular complexity
143 in the range of 4.2–4.3 in refluxing benzene and this was reduced to a considerable
extent in boiling toluene where the observed values fall in the range 3.4–3.44.
The analogous thorium alkoxides show higher associations; thus the n-butoxide and
n-pentyloxide show associations of the order of 6.44 and 6.20, respectively, in boiling
benzene. This appears to indicate that increasing the chain length does not have an
appreciable effect on the degree of polymerization of normal alkoxides of cerium and
thorium.
The isopropoxides of cerium and thorium also show higher association (3.1 and
3.8, respectively, in boiling benzene) than titanium, zirconium, and hafnium alkoxides.