New derivatives of hexamethyldisilazanelithium of the general
formula [X−Me2Si−N−SiMe2−X]2Li2 (X = Ph
(2), C4H3S (3),
NMe2 (4), NEt2 (5),
N(H)iPr (6), OPh (7),
OSiMe3 (8),
C4H3O (9)) have been synthesized and
characterized by spectroscopic means. All compounds
except 3 have been subjected to X-ray structure
determinations which reveal a common
polycyclic arrangement with a central Li2N2
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
(2, 3), nitrogen (4−6) or
oxygen (7−9)). The common four-membered
polycyclic skeleton
Li2N2Si4Y4 has a
point symmetry of approximately D
2 (222) of
which only C
2 (2) symmetry
is retained in the crystals of 4, 5,
6, and 9, whereas all other derivatives have
point symmetry
C
1 (1). One of the compounds crystallizes
in one enantiomeric form (4) in an acentric
structure. All other compounds crystallize in centrosymmetric
structures with the two
enantiomers present in the crystal. The lithium atoms in
2−9 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 8 at higher temperatures (13C
SPE/MAS NMR evidence). Li−N distances in
the central Li2N2 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 2), whereas longer Li−N
bonds (2.07−2.085 Å) are encountered
for the nitrogen donors with short Li−N “donor” bonds (2.157
(4), 2.163 (5), 2.121 Å (6)).
If
the donor atom (Y) is oxygen, the Li−O bonds can be either shorter
than the Li−N bonds
(7, Li−N 2.073, Li−O 1.978 Å; 9, Li−N
2.076, Li−O 1.977 Å) or slightly longer (8,
Li−N
2.021, Li−O 2.077 Å). It is remarkable that in the
trimethylsilyloxy case 8 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
Li2N2 ring are much shorter than
the remaining two (average 2.08 Å).
