Chemistry Reference
In-Depth Information
Volatility and degree of polymerization (
n
) of metal amyloxides
273
,
294
Al(OR)
3
Table 2.5
Ti(OR)
4
Zr(OR)
4
B.p. (
Ž
C/mm)
B.p. (
Ž
C/mm)
B.p. (
Ž
C/mm)
Rin[M(OR)
x
]
n
n
n
n
CH
3
(CH
2
)
4
255/1.0
4.0
175/0.8
1.4
256/0.01
(CH
3
)
2
CH(CH
2
)
2
195/0.1
4.0
148/0.1
1.2
247/0.1
3.3
(CH
3
)(C
2
H
5
)CHCH
2
200/0.6
4.1
154/0.5
1.1
238/0.1
3.7
(CH
3
)
3
CCH
2
180/0.8
2.07
105/0.05
1.3
188/0.2
2.4
(C
2
H
5
)
2
CH
165/1.0
2.08
112/0.05
1.0
178/0.5
2.0
(CH
3
)(C
3
H
7
)CH
162/0.5
2.06
135/1.0
1.0
175/0.05
2.0
(CH
3
)(C
3
H
i
7
)CH
162/0.6
1.98
131/0.5
1.0
156/0.01
2.0
(CH
3
)
2
(C
2
H
5
)C
154/0.5
1.97
98/0.1
1.0
95/0.1
1.0
In order to assess the relative effectiveness (Section 3.1) of the two synergetic factors,
i.e.
inductive effect and steric bulk of the alkyl groups, a detailed study was carried
out in 1952
273
on the molecular complexities of all the 8 isomeric tertiary amyloxides
of titanium and zirconium (Table 2.5). Amongst these, the neopentyloxides are of
special significance as the neopentyl alcohol, in spite of being a primary alcohol, had
been known to cause steric effects comparable to those of secondary alcohols. The
closer resemblance of the neopentoxides of both the metals to their secondary rather
than primary amyloxides tended to indicate the greater influence of steric factors in
determining the molecular complexity and volatility of these derivatives.
The corresponding data on isomeric amyloxides of aluminium are also included in
Table 2.5, indicating the importance of steric factors in their cases also. In fact, detailed
investigations were also carried out almost simultaneously
293
,
294
on aluminium alkox-
ides as even aluminium tertiary amyloxide exhibits a dimeric behaviour and volatility
similar to secondary amyloxides; this has been ascribed to the electron-deficient nature
of aluminium, leading to a stronger bridged structure of the type, (Am
t
O)
2
Al(
-
OAm
t
)
2
Al(OAm
t
)
2
. The strength of this type of bridged structure is also reflected by its
presence in the vapour state. An interesting 'ageing' tendency was observed in freshly
distilled trimeric fAl
OPr
i
3
g
3
which slowly changes into a stable tetrameric crystalline
form, to which Bradley assigned the interesting structure [Alf
-OPr
i
2
Al
OPr
i
2
g
3
],
in which the central aluminium atom is hexa-coordinate whereas the three surrounding
aluminium atoms are tetra-coordinate.
577
These early studies by Bradley
et al
.
113
,
273
,
274
coupled with those of Mehrotra
293
,
294
laid the foundation of a clearer understanding for the first time of the molecular
complexity and volatility of metal alkoxide derivatives, and led to extensive inves-
tigations over several decades on the alkoxides of metals throughout the periodic
table, which confirmed the above conclusions; these are described groupwise in Sec-
tions 3.2.3 - 3.2.14, followed by an account of different physico-chemical techniques
employed for structural elucidation of metal alkoxides in general.
3.2.2
Special Characteristics of Metal Methoxides
Apart from the monomeric methoxides of the metalloids (B, Si, Ge, As, and Te) which
are very volatile and soluble in organic solvents, most of the metal methoxides (except
niobium and tantalum pentamethoxides and uranium hexamethoxide) are comparatively
nonvolatile and insoluble in common organic solvents. Attempts have been made to
