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J Chem Crystallogr (2007) 37:825–829
in monodentate ethers and mostly dimeric in bidentate
DME [10].
In all cases, the isolated crystals were quickly suspended
in mineral oil at ambient temperature, a suitable crystal was
selected, cut to an appropriate size and mounted with the
help of mineral oil on a 50 lm MicroMesh MiTeGen
Micromount.
Experimental Section
Materials
Crystallographic Study
2-Methyl-1-phenyl-1-propanone
(isobutyrophenone,
Aldrich) was purified by vacuum distillation and stored
under nitrogen. Lithium bis(trimethylsilyl)amide (Li-
HMDS, Aldrich, 1.0 M solution in hexanes) was used
without further purification. TMEDA (Aldrich) was dis-
tilled from calcium hydride and stored under nitrogen.
Hexane (Fisher) was degassed with nitrogen and stored
Diffraction data were collected using a Bruker AXS
SMART APEX CCD diffractometer at 100(2) K using
monochromatic Mo Ka radiation with the omega scan
technique. The unit cells were determined using SMART,
[11] and SAINT+ [12], and the data were corrected for
absorption using SADABS [13] The structure was solved
by direct methods and refined by full matrix least squares
against F2 with all reflections using Shelxtl [14]. Refine-
ment of an extinction coefficient was found to be
insignificant. All non-hydrogen atoms were refined aniso-
tropically. The TMEDA ligand in the dimer is disordered
over two positions with an occupancy ratio of 0.839(3) to
0.161(3). All disordered atoms were restrained with a rigid
bond restraint, i.e. the components of the anisotropic dis-
placement parameters for 1,2 and 1,3 distances in the
direction of the bonds were restrained to be equal within a
standard deviation of 0.01. The anisotropic displacement
parameters of the minor component were further restrained
to be isotropic within a standard deviation of 0.01. Methyl
carbon atom C10c in the monoclinic hexamer is located in
very close proximity to Li atom Li1 and, to take potentially
unusual C–H bond lengths due to LiꢀꢀꢀH interactions into
account, its hydrogen atoms were located in the difference
density Fourier map and their positions were fully refined.
All other hydrogen atoms were placed in calculated posi-
tions, and all hydrogen atoms (including those on C10c)
were isotropically refined with a displacement parameter
1.5 (methyl) or 1.2 times (aromatic) that of the adjacent
carbon atom. Crystal data and experimental details for the
three complexes are listed in Table 1.
˚
over 4 A molecular sieves. Benzene-d6 (Cambridge Iso-
tope Laboratories) was distilled from sodium metal and
benzophenone and was stored under nitrogen.
Preparation of the Lithioisobutyrophenone Crystals
The triclinic LiIBP crystals (CCDC-653236) were seren-
dipitously prepared during an NMR kinetics experiment
where 1 mL of a 1.0 M LiHMDS/hexanes solution
(0.41 mmol) was placed in an NMR tube and the solvent
removed under vacuum. Benzene-d6 (0.82 mL) was added
to the tube and over 50 min, isobutyrophenone (47.4 mg,
0.32 mmol) was added in five portions as the solution was
kept at 5 ꢁC. The solution was allowed to remain in the
NMR for 24 h at room temperature at which point crystals
were observed to have grown. The monoclinic LiIBP
hexamer complex (CCDC-653235) was prepared by plac-
ing 1 mL of a 1.0 M LiHMDS/hexanes solution in a
1.5 mL gas chromatography vial and the solvent was
removed under vacuum; this resulted in 68.2 mg
(0.41 mmol) of solid LiHMDS. About 1 mL of benzene-d6
was added to dissolve LiHMDS. Isobutyrophenone
(108.6 lL, 0.74 mmol) was added to the vial and it was
sealed under nitrogen. The solution was stirred and a solid
formed after several minutes. The solution was reheated to
dissolve the solid and was then allowed to slowly cool to
room temperature over the course of several hours, forming
crystals suitable for X-ray analysis.
Results and Discussion
Unsolvated LiIBP Hexamers
The dimeric LiIBP–TMEDA complex (CCDC-653234)
was prepared by placing LiHMDS (132.1 mg, 0.79 mmol),
1.5 mL of hexanes, isobutyrophenone (92.7 lL,
0.63 mmol) and TMEDA (93.3 lL, 1.0 mmol) in a sealed
vial under nitrogen. A solid appeared after several minutes.
The vial was heated until the solid had dissolved and
allowed to slowly cool back to room temperature over the
course of several hours, yielding crystals suitable for X-ray
analysis.
Thermal ellipsoid plots of the two LiIBP hexamers are
shown in Figs. 1 and 2. While these two complexes are
crystallographically distinct and exhibit a different pack-
ing, their overall molecular structures are similar. Fig. 3
presents the two hexamer structures (without hydrogen
atoms) with their Li6O6 cores overlaid. The structures of
the two hexamers differ in the conformations of their en-
olate moieties. Both exhibit typical organolithium hexamer
123
Nichols, Michael A.
