Chemistry Reference
In-Depth Information
Table 2.11
Volatilities and molecular complexities of cerium and thorium alkoxides
Ce(OR)
4
Th(OR)
4
R
B.p. (
Ž
C/mm)
Molecular complexity
B.p. (
Ž
C/mm)
Molecular complexity
CHMe
2
160 - 170/0.5
3.1
200 - 210/0.1
3.8
CMe
3
140 - 150/0.1
2.5
160/0.1
3.4
CEt
3
154/0.05
1.1
148/0.05
1.0
CMe
2
Et
240/0.1
2.4
208/0.3
2.8
CMeEt
2
140/0.06
-
148/0.1
-
CMeEtPr
n
150/0.05
1.0
153/0.1
1.7
CMeEtPr
i
-
-
139/0.05
1.0
In contrast to the monomeric nature of tertiary alkoxides of titanium, zirconium, and
hafnium, the corresponding cerium and thorium lower tertiary alkoxides exhibit asso-
ciation, which decreases with increasing chain length of the groups attached to the
tertiary carbon atom and finally Th(OCMeEtPr
i
)
4
286
and Ce(OCMeEtPr
n
)
4
141
show
monomeric behaviour. On the basis of the above observations, the order of volatility of
some quadrivalent metal alkoxides may be assigned: Si(OR)
4
>
Ge(OR)
4
>
Ti(OR)
4
>
Hf(OR)
4
>
Zr(OR)
4
>
Ce(OR)
4
>
Th(OR)
4
. However, for monomeric tertiary alkox-
ides, the order of volatility is Hf
OR
t
4
>
Zr
OR
t
4
>
Ti
OR
t
4
.
3.2.8
Alkoxides of Group 5 Metals
Vanadium trimethoxide and triethoxide are nonvolatile solids,
218
whereas almost all
known vanadium tetra-alkoxides are volatile derivatives with the exception of the
tetramethoxide which decomposes under similar conditions.
333
Vanadium tetraethoxide
and tetra-
n
-propoxide sublime under 0.5 mm pressure at 100- 110
Ž
C and 140 - 150
Ž
C
bath temperatures, respectively, whereas tetra-
n
-butoxide, tetra isobutoxide, tetra-
isopropoxide and tetra-
tert
-butoxide distil at 150 - 160
Ž
C/0.5, 70 - 80
Ž
C/0.1 and
60 - 70
Ž
C/0.1 mm pressures, respectively. The heats of formation of liquid and gaseous
vanadium tetra-
tert
-butoxide, determined by reaction calorimetry,
307
were found to be
334 š 0
.
8and328 kcal mol
1
, respectively. Using the relation for the calculation
of
tetra-
tert
-butoxide)
467
average
bond
dissociation
energy
(used
for
chromium
(Section 3.2.9), Bradley
et al
.
307
found the average value of
D
for V(OBu
t
)
4
to be
87.5 kcal.
Ebullioscopic molecular weight determinations show that vanadium tetramethoxide
is trimeric. The tetraethoxide is dimeric, whereas the tetra-
n
-propoxide,
n
-butoxide
and
n
-amyloxide show average degrees of association of 1.38, 1.31, and 1.27, respec-
tively.
333
All secondary and tertiary alkoxides of vanadium are monomeric except the
isopropoxide which shows slight association (1.17 complexity) in boiling benzene.
The pentavalent niobium and tantalum alkoxides are reasonably volatile and can
be distilled unchanged in the pressure range 0.05 - 10 mm,
e.g.
pentamethoxides of
niobium
468
and tantalum
469
have been distilled at 153
Ž
C/0.1 or 200
Ž
C/5.5 and 130
Ž
C/0.2
or 189
Ž
C/10.0 mm pressures, respectively. The data in Table 2.12 indicate that the boiling
points are dependent on chain length; furthermore, the methoxides and ethoxides of
tantalum are more volatile than the niobium analogues whereas for higher
n
-alkoxides,
the reverse is true.
279
,
280
,
468
,
469
