Chemistry Reference
In-Depth Information
3.4.4
Alkoxides of Yttrium, Lanthanides, and Actinides
3.4.4.1 Yttrium and lanthanum alkoxides
A number of yttrium and lanthanum alkoxides have been investigated by
1
Hand
13
C spectral studies
18
,
21
by the research groups of Evans
160
,
161
,
164
,
346
,
552
and Bradley
et al
.
345
,
349 - 349c
in an attempt to provide preliminary evidence for the structures of
these derivatives. Although the spectral patterns are generally more complex, the data
are interpretable in terms of X-ray crystallographically established structures of typical
compounds of each series. For example, the
1
H NMR spectrum (in C
6
D
6
)ofY
3
(
3
-
OBu
t
)(
3
-Cl)(
2
-OBu
t
)
3
(OBu
t
)
4
(THF)
2
exhibits several broad overlapping resonances
in the
υ
1.1 - 2.1 region attributable to the
t
-butoxide ligands.
160
In THF-d
8
,these
peaks split into six sharp well-resolved signals with relative intensities of 1:2:1:2:1:1.
The
13
C f
1
Hg NMRspectrumofthecomplexinTHF-d
8
also showed six chemi-
cally nonequivalent OBu
t
groups, consistent with the solid-state structure
160
of the
derivative.
The
1
H NMR spectrum of La
3
(OBu
t
)
9
(thf)
2
in aromatic solvents shows broad
resonances characteristic of fluxional behaviour.
160
In THF-d
8
, the resonance due
to OBu
t
groups sharpens. Using the structure - shift correlations developed for
Y
3
(OBu
t
)
8
Cl(thf)
2
, the lanthanum complex contains two equivalent
3
-OBu
t
groups,
three
2
-OBu
t
groups in a ratio of 1:2, and four terminal OBu
t
groups. Even at
500 MHz, the terminal OBu
t
peaks are not fully resolved, so the number of distinct
environments is indeterminate. The
13
C NMR pattern is consistent with
1
H NMR data.
At room temperature, the
1
Hand
13
C NMR studies on tris-
tert
-alkoxides of yttrium
and lanthanum
345
reveal that the complexes are fluxional and require low temperatures
to slow down the exchange between distinguishable alkoxo ligands. Even at around
43
Ž
C
1
H NMR spectra of the trinuclear complexes [M
3
(OBu
t
)
9
(HOBu
t
)
2
](MD Y,
La) exhibit a complex pattern, each having five major peaks with intensities in the ratio
2:2:2:1:4, appearing from lower to higher fields. For interpretation of such complex
spectral data, the knowledge gained from the X-ray crystallographically characterized
lanthanum complex
345
proved invaluable. The
1
H NMR spectral data of these and some
alkoxide complexes of Y and La are listed in Table 2.20.
The
1
Hand
13
C NMR spectra
346
of [(Ph
3
CO)
2
La(
2
-OCPh
3
)
2
La(OCPh
3
)
2
] show
two types of phenyl resonances, consistent with terminal and bridging groups in the
dimer.
3.4.4.2 Thorium and uranium alkoxides
At room temperature the
1
H NMR spectrum of [Th(OBu
t
)
4
(py)
2
] reveals that the
molecule is fluxional
406
and low-temperature (75
Ž
C) spectra in toluene-d
8
failed
to freeze out a limiting structure. In benzene-d
6
the room temperature
1
HNMR
spectrum
406
of [Th
2
(OBu
t
)
8
(HOBu
t
)] shows only one broad signal due to OBu
t
groups
at
υ
1.54 and a broad peak at
υ
3.22 for alcoholic OH protons in the intensity ratio
80:1, consistent with these assignments.
In an interesting
1
H NMR experiment the reaction of Th
2
(OBu
t
)
8
(HOBu
t
) with
pyridine has been studied
406
which supports the stepwise changes as illustrated in
scheme 2.4:
