Lithium Coordination in Chelating Silazanes of the
General Formula [X−Me<sub>2</sub>Si−N−SiMe<sub>2</sub>−X]<sub>2</sub>Li<sub>2</sub><sup>†</sup>
Michael Veith
Astrid Koban
Kira Fries
Patrick Spaniol
Ralf Elsässer
Andreas Rammo
Volker Huch
Ulrich Kleinsteuber
10.1021/om971142d.s002
https://acs.figshare.com/articles/journal_contribution/Lithium_Coordination_in_Chelating_Silazanes_of_the_General_Formula_X_Me_sub_2_sub_Si_N_SiMe_sub_2_sub_X_sub_2_sub_Li_sub_2_sub_sup_sup_/3781809
New derivatives of hexamethyldisilazanelithium of the general
formula [X−Me<sub>2</sub>Si−N−SiMe<sub>2</sub>−X]<sub>2</sub>Li<sub>2</sub> (X = Ph
(<b>2</b>), C<sub>4</sub>H<sub>3</sub>S (<b>3</b>),
NMe<sub>2</sub> (<b>4</b>), NEt<sub>2</sub> (<b>5</b>),
N(H)<sup>i</sup>Pr (<b>6</b>), OPh (<b>7</b>),
OSiMe<sub>3</sub> (<b>8</b>),
C<sub>4</sub>H<sub>3</sub>O (<b>9</b>)) have been synthesized and
characterized by spectroscopic means. All compounds
except <b>3</b> have been subjected to X-ray structure
determinations which reveal a common
polycyclic arrangement with a central Li<sub>2</sub>N<sub>2</sub>
four-membered ring to which four similar LiNSiY
rings are annealed along a common Li−N edge (Y can either be a carbon
atom of a π-system
(<b>2</b>, <b>3</b>), nitrogen (<b>4</b>−<b>6</b>) or
oxygen (<b>7</b>−<b>9</b>)). The common four-membered
polycyclic skeleton
Li<sub>2</sub>N<sub>2</sub>Si<sub>4</sub>Y<sub>4</sub> has a
point symmetry of approximately <i>D</i><sub>2</sub> (222) of
which only <i>C</i><sub>2</sub> (2) symmetry
is retained in the crystals of <b>4</b>, <b>5</b>,
<b>6</b>, and <b>9</b>, whereas all other derivatives have
point symmetry
<i>C</i><sub>1</sub> (1). One of the compounds crystallizes
in one enantiomeric form (<b>4</b>) in an acentric
structure. All other compounds crystallize in centrosymmetric
structures with the two
enantiomers present in the crystal. The lithium atoms in
<b>2</b>−<b>9</b> are present in a distorted
tetrahedral environment constituted by two nitrogen and two Y atoms.
From molecular
mass determinations, the compounds seem to retain their dimeric nature
in benzene, the
NMR patterns being nevertheless more simple than expected from the
crystal structures
and indicate a dynamic behavior in solution. None of these
compounds, so far, shows lithium
motion in the solid state up to room temperature, although phase
transitions seem to occur
in compound <b>8</b> at higher temperatures (<sup>13</sup>C
SPE/MAS NMR evidence). Li−N distances in
the central Li<sub>2</sub>N<sub>2</sub> ring depend on the nature of
donor groups Y: short Li−N bonds (2.024 Å)
are found for the lithium atoms coordinated by organic π-systems
together with relatively
long Li−C bonds (2.53 Å in <b>2</b>), whereas longer Li−N
bonds (2.07−2.085 Å) are encountered
for the nitrogen donors with short Li−N “donor” bonds (2.157
(<b>4</b>), 2.163 (<b>5</b>), 2.121 Å (<b>6</b>)).
If
the donor atom (Y) is oxygen, the Li−O bonds can be either shorter
than the Li−N bonds
(<b>7</b>, Li−N 2.073, Li−O 1.978 Å; <b>9</b>, Li−N
2.076, Li−O 1.977 Å) or slightly longer (<b>8</b>,
Li−N
2.021, Li−O 2.077 Å). It is remarkable that in the
trimethylsilyloxy case <b>8</b> the Li−N
distances are not equal within their standard deviations as observed in
the other cases:
two distances (average 1.96 Å) on opposite sides of the
Li<sub>2</sub>N<sub>2</sub> ring are much shorter than
the remaining two (average 2.08 Å).
1998-05-19 00:00:00
lithium atoms
Li 2 N 2 ring
SPE
trimethylsilyloxy case 8
NMR
2 Li 2