29Si–1H IMPACT HMBC: a suitable tool for analyzing silylated derivatives
standard variable–temperature unit (BVT 3000). All 2D spectra were
obtained with 24–64 t1 increments and zero filled to 1024 points
in the F1 dimension. 4096 points have been used zero filled to
8192 points in the F2 dimension. Fast 2D spectra (4094 Â 24)
have been processed with 64 forward predicted points. More-
over, a p/2 shifted sine weighting was applied in both dimen-
sions prior to Fourier transformation and all spectra are pre-
sented in magnitude-mode. Other experimental details can be
found in the figure captions.
2/ Synthesis of products
Synthesis of pentamethyl(phenyl)disilane (1)[9]
To a solution of chlorotrimethylsilane (2.5 mL, 19.5 mmol, 1.6 eq.)
in THF (10 mL) maintained under argon, a solution of freshly
prepared dimethylphenylsilyllithium (12.3 mmol) in THF (30 mL)
was added dropwise. After stirring for 1.5 h at room temperature,
the reaction mixture was quenched with water (10 mL), extracted
with Et2O, and then washed with brine. After evaporation of
solvents and distillation under reduced pressure (110 ꢀC/10À2 mbar),
compound 1 was isolated as a colorless oil (2.0 g, 9.6 mmol, 78%).
Dimethylphenylsilyllithium is freshly prepared by treatment of
a solution of chloro(dimethyl)phenylsilane in THF with lithium
under ultrasonic waves at room temperature.
Figure 6. A/ Compound 2 dissolved in toluene-d8 at 298 K. Experimental
data points are obtained from integration of 1D spectral intensities over
1
the H region. Three different HMBC experiments have been used (●) the
IMPACT HMBC [see Fig. 1(A)] with a optimized excitation pulse, (▲) with a
n
90ꢀ flip angle, and (■) with a standard HMBC [see Fig. 1(B)]. Δ= 1/2 JSi–Η
[10a,b]
has been optimized for getting the best sensitivity and 71 ms has been
found. The DIPSI-2 period has been fixed to 40 ms, 5 kHz rf field (50ms pulse)
as a compromised between losses and polarization gains.[4] B/ 1H-29Si 1D IM-
PACT (for a = 90ꢀ in green, dotted lines, and a = 140ꢀ in red, dashed lines)
and standard HMBC in blue, solid lines, for the silylated rare-earth complex
2 in toluene - d8. The same experimental time has been used for the three
HMBC experiments to compare Signal to Noise ratios.
Synthesis of complex [Li(THF)4][Y(CH2SiMe3)4] (2)
In an argon-filled glove-box, to a suspension of YCl3 (0.387 g,
1.98 mmol) in dry THF (10 mL), a solution of LiCH2SiMe3 (0.745 g,
7.91 mmol) in THF (3mL) at room temperature was added. The
reaction mixture was stirred for 40 min at room temperature and
THF was evaporated in vacuo. The resulting solid was dissolved in
dry toluene; the mixture was centrifugated and evaporated in
vacuo to give a slightly pale yellow solid (0.536 g, 72%).
time. For an excitation angle of 90ꢀ the experiment gave smaller
gains: from 1.0 to 1.4 (see Fig. 6 B).
Acknowledgements
Conclusions
J.F warmly thanks his father for an unfailing daily help. He also
gratefully acknowledges J-P Baltaze as well as A. Dos Santos for
their precious assistance on spectrometers. J. H. and E. S. thank
ANR agency for the funding support (Grant 07 367).
The Si–H IMPACT HMBC is a powerful experiment to record quickly
and with a higher sensitivity than the classical HMBC experiment
correlation between 1H and 29Si. This comes from the reduction of
the apparent relaxation time but we have demonstrated that the
use of an optimized a excitation angle provide a higher increase of
the sensitivity. This experiment has allowed gaining an average
factor of 2 for a disilane and a tetrasilylated derivative. Recycling
times smaller than 100ms permit to record 2D spectra in only
30–40s with an increasing sensitivity factor from 2.3 to 3.0. This
class of experiment could be applied for the detection of signals
of low concentrated samples and/or for low abundant nuclei. Such
fast experiment could be used to follow kinetics in a range of less
than a minute. The first result obtained on an yttrium complex is
very encouraging in the field of organometallic chemistry and
would promise to follow fast kinetic events occurring close to the
metallic center.
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