Chemistry Reference
In-Depth Information
OH, NO
2
) (Eq. 5.29):
56
2H
3
V
10
O
3
28
C 6RC
CH
2
OH
3
! 3V
6
O
13
f
OCH
2
3
CRg
2
C 12H
2
O C V
2
O
5
5
.
29
The methoxo polyvanadate [V
6
O
12
OMe
7
]
has been prepared by a similar method.
57
Using organic-soluble tetrabutyl ammonium salts of [Mo
2
O
7
]
2
and [Mo
8
O
26
]
4
the mixed alkoxo polyoxomolybdate species [Mo
8
O
20
OMe
4
f
OCH
2
3
CMeg
2
]
2
58
and
[Mo
3
O
6
OMe
f
OCH
2
3
CMeg
2
]
59
were obtained. The interesting heteropolyalkoxo
metallate anion [V
2
Mo
2
O
8
OMe
2
f
OCH
2
3
C
CH
2
OH
g
2
]
2
was prepared by the reac-
tion of [V
2
O
2
Cl
2
f
OCH
2
2
C
CH
2
OH
2
g
2
] with [Mo
2
O
7
]
2
.
60
The oxo vanadatrane
[OV
OC
2
H
4
3
N] was prepared by treating ammonium metavanadate with triethanol-
amine.
61
3
CHEMICAL REACTIVITY OF OXO-ALKOXIDES
The further hydrolysis of metal oxo-alkoxides to produce metal oxides is of consider-
able importance in the sol - gel process.
Klemperer and co-workers
9-12
have shown that with increasing oxo content tita-
nium oxo-alkoxides become increasingly resistant to hydrolysis in alcoholic solutions,
although gel formation occurs readily by hydrolysis in nonalcoholic solvents.
11
Alcohol exchange of metal oxo-alkoxides has also been studied. Thus the molyb-
denum oxo-alkoxides [MoO
2
OR
2
](RD Pr
i
,CH
2
Bu
t
) were obtained from the tertiary
butoxo compound (Eq. 5.30).
34
MoO
2
OBu
t
2
C 2ROH ! MoO
2
OR
2
C 2Bu
t
OH
5
.
30
Of considerable interest is the selective alkoxo exchange shown by the more hydrolysed
titanium oxo-alkoxides. The two isomeric forms of [Ti
12
O
16
OPr
i
16
] both reacted with
ethanol (Eq. 5.31) to form isomers of [Ti
12
O
16
OEt
6
OPr
i
10
] by exchange of terminal
isopropoxo groups bonded to the 6 five-coordinated Ti atoms in each molecule.
11
Ti
12
O
16
OPr
i
16
C 6EtOH ! Ti
12
O
16
OEt
6
OPr
i
10
C 6Pr
i
OH
5
.
31
Similarly [Ti
11
O
13
OPr
i
18
] was converted to [Ti
11
O
13
OEt
5
OPr
i
13
]
11
and the giant
oxo-
tert
-butoxide [Ti
18
O
28
H
OBu
t
17
] also exchanged the 5 terminal Bu
t
O groups
bonded to five-coordinated titanium with tertiary amyl alcohol.
12
These reactions
demonstrated the robust nature of the metal - oxo core structure in these compounds.
Alcohol exchange of the pentanuclear oxo-alkoxides [M
5
O
OPr
i
13
](MD Gd,
Pr) led to reorganization of the M
5
O
14
framework and formation of hexanuclear
[Gd
6
4
-O
OC
2
H
4
OMe
16
]
62
and octanuclear [Pr
8
4
-O
4
OC
2
H
4
OMe
16
Me
3
PO
2
]
63
species. Some surprising results were obtained in alcohol exchange reactions involving
the sparingly soluble [UO
2
OMe
2
.
MeOH
].
43
With isopropanol a disproportion-
ation occurred with formation of insoluble [U
2
O
5
OPr
i
2
Pr
i
OH
2
] and soluble
[UO
OPr
i
4
Pr
i
OH
]. The latter compound disproportionated on heating
in vacuo
producing the volatile hexa-alkoxide U
OPr
i
6
. Similar results were obtained using
tert
-
butanol and the hexa-alkoxide U
OBu
t
6
was remarkably air stable. Other work
64
showed
that in the reaction of UO
2
Cl
2
with KOBu
t
in THF the soluble trinuclear complex
