Chemistry Reference
In-Depth Information
stoichiometric reaction involved the formation of alkene and water (Eq. 7.1).
Zr
OR
4
! ZrO
2
C 4alkene C 2H
2
O
7
.
1
Several years later Mazdiyasni and Lynch
6
reported the deposition of ZrO
2
,HfO
2
,
and yttrium-stabilized ZrO
2
as a continuous process using chemical vapour deposi-
tion of the metal alkoxide vapour in an inert gas atmosphere. The oxides of yttrium,
dysprosium, and ytterbium were similarly deposited from the metal isopropoxides at
200 - 300
Ž
C under nitrogen.
7
The mechanism of the decomposition of metal alkoxides
to metal oxides has been the subject of several studies. Recent research by Nix
et al
.
8
has shown that Ti
OPr
i
4
begins depositing TiO
2
(on a TiO
2
surface) at low pressure
at
ca.
177
Ž
C with the formation of a mixture of acetone, isopropanol, and propene.
At higher temperatures (
>
490 K) the organic product is propene. The first-order rates
of decomposition of adsorbed intermediate at various temperatures gave an activation
energy of
ca.
85 kJ mol
1
.
It needs to be borne in mind that metal alkoxides do not invariably give rise
to pure metal oxides by the MOCVD process. For example it has been shown by
Chisholm
et al
.
9
that whereas Al
2
OBu
t
6
,Mo
2
OBu
t
6
,andW
2
OBu
t
6
do deposit
the oxides
-Al
2
O
3
,MoO
2
,andWO
2
respectively, the cyclohexyloxides of Mo and W
behave differently. At
ca.
210
Ž
CMo
2
OC
6
H
11
-
c
6
eliminates a mixture of cyclohex-
anol, cyclohexanone, cyclohexene, and cyclohexane forming a material of composition
“Mo
2
C
4
O
4
” which is stable up to 550
Ž
C but at higher temperatures (660 - 706
Ž
C) this
is converted by loss of CO and CO
2
into molybdenum carbide
-Mo
2
C. The decom-
position of W
2
OC
6
H
11
-
c
6
follows a similar course at 200 - 250
Ž
C giving “W
2
C
4
O
4
”
but at higher temperature (800
Ž
C) this loses CO forming W metal.
In order to be useful as a precursor for MOCVD a metal alkoxide needs to have
a reasonable vapour pressure within a temperature range of room temperature to
ca.
100
Ž
C, and this requires mononuclear compounds. The oligomeric nature of the lower
alkoxides [M
OR
x
]
n
(R D Me, Et, etc.) can be overcome by using bulky alkoxides
such as the
tert
-butoxide when the metals are in a high valency state (
x
D 4
,
5or
6), and monomeric species (
n
D 1) are thus obtained. However, the screening effect
of
tert
-alkoxy groups is not capable of producing monomeric alkoxides of the alkali
metals, alkaline earths, or the trivalent metals although some trivalent alkoxides give
dimeric species M
2
OR
6
which are sufficiently volatile for low-pressure MOCVD.
Two strategies have been developed for enhancing the volatility of metal alkox-
ides. One approach uses functionalized alkoxides,
e.g.
OCH
2
CH
2
X(XD OMe, OBu,
NEt
2
) which may act as chelating ligands thereby preventing oligomerization.
10-12
For example the copper(
II
) complex Cu
OC
2
H
4
NEt
2
2
sublimes at 60
Ž
C
in vacuo
.
12
The other approach is to replace CH
3
groups in the
tert
-alkoxide by CF
3
groups. The
electron-attracting effect of the CF
3
groups weakens the donor ability of the oxygen,
thus weakening the alkoxide bridges in the oligomer and raising the volatility. The
weaker intermolecular forces between CF
3
groups also enhances volatility. The metal
becomes more electrophilic when bonded to fluoro alkoxide groups and has a greater
tendency to coordinate with additional neutral ligands.
This is well illustrated by the tri-alkoxides of yttrium and the lanthanides where
the
[M
3
OBu
t
9
Bu
t
OH
2
]
tert
-butoxides
are
trinuclear
species
having
moderate
volatility.
13
With
the
hexafluoro-
tert
-butoxides
mononuclear
complexes
such
as
[MfOCMe
CF
3
2
g
3
thf
3
]and[MfOCMe
CF
3
2
g
3
diglyme
] were obtained.
14
These
