silanides with pendant polydonor groups, including the first
stable sodium and potassium species. Charge separation in
these novel zwitterions is most pronounced for the lithium
silanides, which is reflected in the significant up-field shift
of the central ‘‘naked’’ silicon anion of Li-4 and Li-5 in the
29Si-NMR relative to the sodium and potassium derivatives.
The naked silicon anion is available for additional metal
coordination as exemplarily shown for reactions of Li-4
with main group and transition metal fragments, leading to
hitherto unknown zwitterionic heterobimetallic silanides.
Studies regarding the synthesis of novel mono- and bimetallic
transition-metal silanides and their use as molecular catalysts
are currently underway.
The support of our work by Texas Tech University is greatly
acknowledged. David Purkiss is thanked for carrying out the
NMR experiments.
Fig. 3 Temperature dependent 23Na-NMR spectra of a 1 : 1 mixture
of 18-crown-6 and Na-4 in THF.
À15.4 ppm can be assigned to the sodium ion crown ether
complex, (18-crown-6)Na+ (eqn (6)).11,12
Notes and references
Zwitterion Na-4 (eqn (5)) behaves somewhat differently as the
sodium signal in THF only slightly shifts to higher field from
0.4 ppm to À2.5 ppm, but broadens tremendously upon adding
18-crown-6 indicating slow exchange occurring at room tempera-
ture (Fig. 3). Moreover, upon cooling progressively to À20 1C the
broad signal splits into two distinct signals, a sharp one at
3.1 ppm and a broader one at À13.5 ppm, which can be ascribed
to Na-4 and (18-crown-6)Na+ being in equilibrium. These NMR
experiments reveal podand 4À to be more stable with respect to
cation exchange than 5À. Comparison of the X-ray data of Na-4
with Na-5, which show the average M–O distances of Na-4 to be
significantly shorter than those of Na-5, supports this view.
We next examined the ability of the ‘‘naked’’ anionic silicon
to function as an additional donor for the electrophilic main
group and transition metal species (Scheme 3). We chose Li-4 as
the starting material and were pleased to see that B(C6F5)3 and
AlMe3 readily reacted to afford the first stable zwitterionic
silyl borates and aluminates 7 and 8 in good yields as crystalline
materials.13 Reaction with W(CO)6 gives with loss of one
molecule of CO the corresponding zwitterionic tungstenate
complex 9 as a stable crystalline material.14 In particular
complexes 7 and 9 are worth mentioning as they are stable on
air for weeks without any sign of decomposition. The composi-
tion of 7–9 was unambiguously confirmed by multinuclear
NMR spectroscopy and the results of elemental analysis.
In summary, we have reported a facile method for the
preparation of a series of discrete zwitterionic alkali metal
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5 For example, addition of solid NH4Cl leads to rapid protonation
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6 Similar trends are seen for the methoxy substituted silanides
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12 Similar observations were made for [Me3Si)3Si]Na(THF)n and
[Si(SiMe2OMe)3]Na(THF)2. In the 23Na-NMR the sodium atom
instantly shifts to ca. À15 ppm upon adding stoichiometric
amounts of 18-crown-6.
13 For non-zwitterionic silyl borate and aluminate species see:
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Scheme 3 Synthesis of 7–9.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4117–4119 4119