Chemistry Reference
In-Depth Information
[Ti
3
3
-O
-OPr
i
4
fMe
2
C
O
CHC
O
CH
2
C
O
Me
2
g]
66
in which the tridentate ligand
formed by condensation of acetone spans the three Ti atoms.
2.2
Formation of Oxo-alkoxides by Nonhydrolytic Reactions of Alkoxides
Metal oxo-alkoxides unintentionally obtained during attempts to prepare metal alkox-
ides have generally resulted from the presence of adventitious water leading to hydroly-
sis, but it appears that the formation of some oxo-alkoxides is due to the instability of
the sought after alkoxide.
An early example was the failure to obtain penta-tertiary alkoxides of niobium
owing to the preferential formation of the oxo compounds Nb
2
O
OR
8
and NbO
OR
3
in contrast to tantalum which formed thermally stable Ta
OBu
t
5
. This behaviour was
explained in terms of the greater tendency of niobium to form the metal-oxo double
bond.
13
In the reactions of W
NMe
2
6
with ROH the hexa-alkoxides W
OR
6
(R D Me,
Et, Pr
i
, allyl) were formed but not the tertiary butoxide which gave WO
OBu
t
4
instead.
14
The tendency of MoO
OR
4
compounds to form MoO
2
OR
2
by ether elim-
ination has also been noted (Eq. 5.4).
15
L
x
M
OR
2
! L
x
M
D
O C R
2
O
5
.
4
The formation of multiple metal - oxo bonds by penta or hexa valent transition metals
(d
0
) is not surprising but the more recent discovery that scandium, yttrium, and
the tervalent lanthanides form oxo-isopropoxides rather than triisopropoxides was
unexpected.
16
Considerable efforts have been devoted to ensuring that the formation of
these pentanuclear oxo-alkoxides [M
5
5
-O
3
-OPr
i
4
-OPr
i
4
OPr
i
5
] was not due
to hydrolysis, and there is evidence that elimination of ether is involved. An alternative
mode of decomposition would be alkene elimination with concomitant formation of
alcohol (Eq. 5.5).
L
x
MfOCH
CH
3
2
g
2
! L
x
M
D
O C CH
2
D
CHMe C Me
2
CHOH
5
.
5
This process would be expected to be facilitated by tertiary alkoxo groups but it is
significant that the tris-tertiary alkoxides of these metals are readily prepared and
are thermally stable. It is also noteworthy that whereas scandium forms pentanuclear
oxo-alkoxides Sc
5
O
OR
13
with ethoxo and isopropoxo ligands, the trifluorethoxo and
hexafluoroisopropoxo ligands form stable tris-alkoxides.
17
The thermal desolvation of
[Ce
2
OPr
i
8
Pr
i
OH
2
] gave rise to the oxo-species [Ce
4
O
OPr
i
14
] by elimination of
diisopropyl ether.
67
Heterometallic oxo-alkoxides may also be formed by conversion from heterometallic
alkoxides. Thus the insoluble [Pb
OPr
i
2
]
x
reacted with Ti
OPr
i
4
to form [Pb
4
Ti
4
O
3
OPr
i
18
]
and [Pb
2
Ti
2
4
-O
3
-OPr
i
2
-OPr
i
4
OPr
i
4
] presumably by ether elimination.
18
Similarly, from reactions of barium alkoxides and titanium alkoxides the heterometallic
oxo-alkoxides f[Ba
4
Ti
4
O
4
OPr
i
16
Pr
i
OH
4
][Ba
4
Ti
4
O
4
OPr
i
10
Pr
i
OH
3
]g
19
and
[Ba
2
Ti
4
O
OMe
18
MeOH
7
]
20
were obtained as a result of ether elimination, although
similar complexes are readily prepared when the barium alkoxide solution is aerobically
oxidized.
21
