2 Homometallic Alkoxides

62 Alkoxo and Aryloxo Derivatives of Metals
Table 2.7
alkoxides
Boiling points and molecular complexities of titanium and zirconium tetra
B.p. ( ŽC/mm) Molecular complexity
R Ti(OR)4Zr(OR)4 Ti(OR)4 Zr(OR)4
C2H5 103/0.1 190/0.1 2.4 3.6
CH3CH2CH2CH2 142/0.1 243/0.1 – 3.4
CH3(CH2)7 214/0.1 – 1.4 3.4
(CH3)2CH 49/0.1 160/0.1 1.4 3.0
(C2H5)2CH 112/0.1 181/0.1 1.0 1.0
(C3Hn 7 )2CH
(CH3)3C
CH3(CH2)4
(CH3)2CH(CH2)2
(CH3C2H5)CHCH2
(CH3)3CCH2
(C2H5)2CH
(CH3C3H
156/0.1
93.8/5.0
175/0.8
148/0.1
154/0.5
105/0.05
112/0.05
163/0.1
89/5.0
256/0.01
247/0.01
238/0.01
188/0.2
178/0.05
1.0
1.0
1.4
1.2
1.1
1.3
1.0
1.0
1.0
3.2
3.3
3.7
2.4
2.0
n 7 )CH
(CH3C3H
135/1.0 175/0.05 1.0 2.0
i 7 )CH
(CH3)2C2H5C
131/0.5
98/0.1
156/0.01
95/0.1
1.0
1.0
2.0
1.0
undergo molecular dissociation at the concentrations at which the tetrabutoxide is
almost dissociated. 456 Similarly dimeric dichloride dibutoxide as well as monomeric
trichloride monobutoxide also do not undergo any dissociation over a wide concentration
range. The above results appear to indicate that the presence of the more
electronegative chlorine atom increases the positive charge and acceptor properties
of the central titanium atom, and consequently the strength of the alkoxide bridges
increases which prevents the molecules from depolymerizing.
Barraclough et al. 457 measured the molecular complexities of titanium alkoxides
in dioxane solvent and observed that the complexities are concentration dependent
except with monomeric tert-butoxide which shows concentration-independent molecular
weight. For the normal alkoxides, as the concentration is increased, the molecular
weight increases and there was no indication of a limiting value of molecular weight
up to a concentration of 0.5 M.
Titanium isopropoxide shows an average association of 1.4, whereas its tertiary and
higher secondary alkoxides are essentially monomeric in refluxing benzene. 113,273,274,432
Titanium tetrakis(hexafluoroisopropoxide) also shows an average molecular association
of 1.5 in boiling benzene. 458
On the basis of the boiling points measured for various normal alkoxides at different
pressures, Cullinane et al. 459 observed that the results do not conform to the relation,
log p D a b/T (p is the pressure in mm at which boiling point T was observed and
a and b are constants). However, the latent heats and entropies of vaporization and the
Trouton constants indicate that titanium tetraethoxide exhibits anomalous behaviour,
i.e. it shows a distinctly higher Trouton constant (42.5) than those observed for npropoxide,
n-butoxide, n-amyloxide, and n-hexyloxide (29.1, 35.6, 39.4, and 40.5,
respectively).
Bradley et al. 306,307 measured the heat of formation of liquid trimeric titanium ethoxide
and the value was found to be 1H Ž
f Ti⊲OC2H5⊳4⊲liq⊳ D 349 š 1.4kcalmol 1 , from
which the standard heat of formation of monomeric gaseous titanium ethoxide was