Chemistry Reference
In-Depth Information
the corresponding dialkoxides [Be(OR)
2
]
n
can be partially due to steric factors, but the
replacement of one of the electron-withdrawing alkoxo groups by an electron-releasing
alkyl group would tend to make the bond much less polar in character.
Similar to beryllium
n
-dialkoxides, zinc dialkoxides are also insoluble and nonvola-
tile compounds.
222
,
447
The alkylzinc alkoxides are, however, less polymeric and exhibit
higher volatility. For example, the cryoscopic molecular weight determination in ben-
zene indicates that methylzinc methoxide and
tert
-butoxide as well as ethylzinc
tert
-
butoxide are tetrameric
450
with sublimation temperatures of 60, 95, and 105
Ž
C, respec-
tively under 0.0001 mm pressure.
390
Similar behaviour has also been observed in the case of magnesium and other alka-
line earth metal alkoxides. The normal dialkoxides of magnesium, calcium, strontium,
and barium are insoluble and nonvolatile
39 - 41
whereas the alkylmagnesium alkox-
ides are soluble in common organic solvents and some of these (especially methyl
and ethyl derivatives) can be volatilized
in vacuo
,
386 - 391
,
451
usually accompanied by
decomposition.
The properties
22
,
39 - 41
of alkaline earth metal alkoxides appear to be dominated
by their ionic character and preference for attaining metal coordination numbers ½6,
leading to rather large associated species.
3.2.5 Alkoxides of Scandium, Yttrium, and Lanthanides
The alkoxide derivatives of lanthanide elements (which are larger in size and more
electropositive) are less soluble than other trivalent (say group 13) alkoxides, prob-
ably owing to their ionic nature and higher molecular aggregation
via
alkoxo bridging.
Although methoxides and ethoxides of scandium, yttrium, and lanthanides are insoluble
and nonvolatile,
6
,
215
the isopropoxides of yttrium,
54
lanthanum,
150
neodymium,
150
,
153
samarium,
155
gadolinium,
149
,
157
dysprosium,
54
holmium,
158
erbium,
157
and ytter-
bium
54
,
103
,
157
could be sublimed under reduced (0.1 mm) pressure in the temperature
range 180 - 280
Ž
C. The sublimation temperature appears to be influenced by the size of
the lanthanide metal; the smaller the size of the metal atom the greater the volatility.
However, the authenticity of these tris-isopropoxides has been questioned (see Ch. 5,
p. 385).
158a
,
b
Brown and Mazdiyasni
55
carried out thermogravimetric analyses of yttrium, dyspro-
sium, and ytterbium isopropoxides and observed a 30% weight loss up to 200 - 250
Ž
C
with the formation of an intermediate hydroxide which was converted to the oxide in
the temperature range 750 - 850
Ž
C.
The molecular complexity of lanthanide isopropoxides in solution has been reported
to be higher than four,
155
,
158
although these were reported earlier to be dimeric.
54
On the basis of mass spectral data, Mazdiyasni
et al
.
54
reported tetrameric nature
for neodymium, erbium, and lutetium isopropoxides. The tetrameric structure of
[Ln(OCH
2
Bu
t
)
3
]
4
(Ln D La, Nd) has recently been confirmed by X-ray crystallo-
graphy,
452a
whereas the t-butoxide is trinuclear [La
3
(OBu
t
)
9
(Bu
t
OH)
2
].
452b
3.2.6 Alkoxides of Actinides
Bradley
et al
.
290
purified uranium tetra-alkoxides by sublimation under reduced pres-
sure and also tried to correlate the volatility of uranium tetra-alkoxides with those of
other tetravalent metal alkoxides as shown in Table 2.6 (which includes for comparison
relevant data for tetravalent titanium and zirconium derivatives).
