2 Homometallic Alkoxides

Table 2.6 Volatilities of tetravalent metal alkoxides
Homometallic Alkoxides 61
Radius ( ˚A) Sublimation distillation temperature ( Ž C/mm)
M(OR)4 Atomic Ionic (M 4C ) M(OMe)4 M(OEt)4 M(OPr n )4 M(OPr i )4
Ti 1.32 0.64 170/0.1 103/0.1 124/0.1 49/0.1
Zr 1.45 0.87 280/10 5 180/0.1 208/0.1 160/0.1
U 1.42 1.05 >300/10 4 220/0.01 240/0.1 160/0.01
Ce 1.65 1.02 >200/in vac >200/in vac >200/in vac 160–170/0.05
Th 1.65 1.10 >300/0.05 >300/0.05 — 200–210/0.05–0.1
Uranium pentaethoxide 379 has been found to be thermally stable up to 170 Ž C in vacuo
and it has been distilled unchanged at 123 Ž C/0.001, 145 Ž C/0.01, and 160 Ž C/0.5 mm
pressures, but it decomposes when heated in the range 180–200 Ž C in vacuo. Uranium
pentamethoxide, penta-n-propoxide, and pentaisopropoxide have also been sublimed
or distilled respectively 289 at 190–210 Ž C/0.1, 162–164 Ž C/0.001, and 160 Ž C/0.1 mm
pressure. The purification of the uranium hexa-alkoxides has also been achieved by
distillation in vacuo; thus uranium hexamethoxide, hexaethoxide, hexa-n-propoxide
have been distilled respectively 380 at 87 Ž C/0.1, 93 Ž C/0.18, 105–107 Ž C/0.001 mm pressures.
Jones et al. 453 have claimed that uranyl diethoxide, UO2(OEt)2 retained three
moles of ethanol even on heating at a higher temperature of 200 Ž C/0.004 mm pressure,
but Bradley et al. 216 observed that uranyl dimethoxide and diethoxide lost their attendant
alcohol molecules on being heated to 100 Ž C/0.05 and 80 Ž C/0.05 mm pressures,
respectively.
3.2.7 Alkoxides of Group 4 Metals
Titanium and zirconium tetra alkoxides are all volatile derivatives including the methoxides
275 which sublime at 180 Ž C and 280 Ž C, respectively under 0.1 mm pressure.
Table 2.7 shows the boiling points and molecular complexities of titanium and zirconium
alkoxides, which decrease with increasing branching of the alkoxo groups. Furthermore,
the boiling points and complexities (including some data already given
in Tables 2.4 and 2.5) are higher for zirconium alkoxides than for their titanium
analogues. 113,274,326,432 The molecular complexity of titanium methoxide has been
reported to be tetrameric. 270 Caughlan et al. 454 measured the molecular weight of
the tetraethoxide cryoscopically in benzene and found it to be concentration dependent
with equilibrium between monomeric and trimeric species in the concentration range
0–0.3 M. Bradleyet al. 432,455 on the other hand found the complexities 2.4 and 2.8
ebullioscopically and cryoscopically in benzene, respectively, and these values were
found to be concentration independent in the concentration range 2–100 ð 10 3 M.
Higher normal alkoxides of titanium are trimeric in the concentration range 0–0.03 M;
the isopropoxide shows an average association of 1.4 whereas tertiary alkoxides are
essentially monomeric in refluxing benzene. 113,273,274,432
Martin and Winter 456 also observed three-fold molecular complexity for titanium
tetra-n-butoxide and the molecular weight was shown to be concentration dependent.
The replacement of butoxy groups by the more electronegative chlorine atoms
appears to affect the molecular complexity of tetra-n-butoxide considerably. Thus
it has been observed that the trimeric titanium monochloride tributoxide does not