2 Homometallic Alkoxides

82 Alkoxo and Aryloxo Derivatives of Metals
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 C6D6) ofY3( 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-d8, 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-d8 also showed six chemically
nonequivalent OBu t groups, consistent with the solid-state structure 160 of the
derivative.
The 1 H NMR spectrum of La3(OBu t )9(thf)2 in aromatic solvents shows broad
resonances characteristic of fluxional behaviour. 160 In THF-d8, the resonance due
to OBu t groups sharpens. Using the structure–shift correlations developed for
Y3(OBu t )8Cl(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 [M3(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 [(Ph3CO)2La( 2-OCPh3)2La(OCPh3)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-d8 failed
to freeze out a limiting structure. In benzene-d6 the room temperature 1 H NMR
spectrum 406 of [Th2(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 Th2(OBu t )8(HOBu t ) with
pyridine has been studied 406 which supports the stepwise changes as illustrated in
scheme 2.4: