Chemistry Reference
In-Depth Information
Table 5.1
(
continued
)
M-O
1
MOM
cm
1
cm
1
Compound
Reference
[Os
2
O
2
O
2
OCMe
2
CH
2
NBu
t
2
]
960
660
78
[U
3
O
4
OBu
t
10
]
931, 898
-
64
[UO
2
OBu
t
2
Ph
3
PO
2
]
861 (822)
-
44
1
Values in parentheses are
M-
18
O
frequencies (cm
1
).
be explored using
1
Hand
13
C, the oxo ligands using
17
O, and the metal using a
range of metal nuclei (
27
Al,
119
Sn,
207
Pb,
89
Y,
47
,
49
Ti,
51
V,
93
Nb, and
183
W). Fluxional
behaviour has been studied using variable-temperature (VT) studies and in many cases
the limiting low-temperature spectrum is in accordance with the known X-ray crystal
structure. Metals with
I
D
2
nuclei give useful spectra but the quadrupolar nuclei give
broad lines with obvious disadvantages. Although the
17
O nucleus is quadrupolar the
chemical shift
8
and line width are very sensitive to the oxo environment. Klemperer and
co-workers
9-12
have demonstrated the value of
17
O NMR in characterizing titanium
oxo-alkoxides. Some data on
17
O chemical shifts are given in Table 5.2 relating specif-
ically to the oxo ligand. Unambiguous spectra were obtained by carrying out hydrolysis
with
17
O-enriched water. Thus the Mo
D
O terminal oxo ligands in MoO
2
OR
2
and
MoO
OR
4
gave shifts in the 970 - 862 ppm range
34
and the bridging oxo-ligands (
,
3
,
4
) were identified in a number of titanium oxo-alkoxides.
9-12
The chemical shifts
for
4
-and
5
-O in several aluminium oxo-alkoxides have also been determined.
79
Heterometal oxo-alkoxides have also been studied using
17
O NMR. In the cases of the
niobotungstate anions in
Bu
4
N
3
[Nb
2
W
4
O
18
OMe
]and
Bu
4
N
3
[Nb
2
W
4
O
18
OBu
t
]
it was possible to identify to which metal atoms the different oxo ligands were
bonded.
80
Similar data have been reported for several more hexanuclear complexes
NBu
4
x
[
MeO
MW
5
O
18
]
x
(M D Ti, Zr;
x
D 3. M D Nb, Ta;
x
D 2).
81
In these cases
the
6
-
17
O resonances were seen at 56 to 71 ppm (see Table 5.2). In the lead
oxo-alkoxides [Pb
4
4
-O
OBu
t
6
]and[Pb
6
3
-O
4
OBu
t
4
] and the zirconium lead
heterometal oxo-alkoxide [Pb
3
Zr
4
-O
OBu
t
8
] the relatively sharp
17
O resonances
show the expected coupling to the
207
Pb (22%) isotope.
22
The
17
O resonances for
3
-O(Ba, Ti) and
5
-O
Ba
2
Ti
3
were in accordance with the X-ray crystal structure
of the hetero metal oxo-alkoxide [Ba
4
Ti
13
5
-O
6
3
-O
12
OC
2
H
4
OMe
24
].
82
4.3
Mass Spectra
Mass spectral studies on metal oxo-alkoxides have sometimes given valuable analytical
data. As with metal alkoxides the parent molecular ion P D [M
x
O
y
OR
z
]
C
is usually
of very low intensity and the strongest high mass fragment ion is often P Me, P R,
or P OR.
The appearance of P Me or P R is then a confirmation that the oxo-alkoxide has
retained the integrity of its M
x
O
y
core in spite of the disruptive effects of 70 eV elec-
trons. For example, although [Sc
5
O
OEt
13
] gave a parent molecular ion
17
the penta-
nuclear isopropoxo species [M
5
5
-O
OPr
i
13
] did not and were thus characterized
as (P OPr
i
)(MD Sc,Y)or(P 3OPr
i
)(MD Yb)
16a
. Indium oxo-isopropoxide was
