Lithium Bis(pyrazol-1-yl)borates
phenyl ring at boron, Ph* ϭ phenyl ring at a pyrazol-1-yl substitu-
ent. Elemental analyses were performed by the microanalytical
laboratory of the University of Frankfurt.
linear Ti-O-Ti cores [11]. Moreover, the fact that the biden-
tate Lewis acid 3tBu acts as an efficient chloride scavenger
is not only interesting in its own right, but explains the lith-
ium chloride contamination of the sample (in this context
it is also important to mention that we detected substantial
amounts of chloride impurity in commercial [12]crown-4
used for the crystallization of 3tBu (TXRF spectroscopy)).
As already stated above, the starting material 2 contained
not only chloride but also bromide ions. As a result,
Li([12]crown-4)Br [17] co-crystallized with the target com-
pound Li(12c4)(Et2O)[3tBuCl]. To get an estimate of the sel-
ectivity of 3tBu for chloride over bromide, we have rein-
vestigated the diffraction data of Li(12c4)(Et2O)[3tBuCl]
using the new structure model Li(12c4)(Et2O)[3tBuX] (X ϭ
Cl, Br). We assumed that ClϪ and BrϪ were sharing the
same position and the same displacement parameters and
refined the ratio of the site occupancy factors. The result
for the specimen investigated was a site occupation by ClϪ
of 99.7(5)%.
Synthesis of 2. PhLi (2.0 M, 2.80 mL, 5.60 mmol) in nBu2O was
added dropwise at Ϫ78 °C to a stirred solution of 1 (0.957 g,
2.80 mmol) in toluene (40 mL). The reaction mixture was allowed
to warm to room temperature and stirred for 10 h, whereupon a
colourless precipitate formed. After filtration, the filtrate was eva-
porated to dryness in vacuo, leaving behind a colourless solid.
Yield: 0.828 g (87 %). Crystals suitable for X-ray diffraction were
grown by recrystallization of the crude product from toluene.
11B NMR (96.3 MHz, CDCl3): 36.2 (h1/2 ϭ 270 Hz). 1H NMR (250.1 MHz,
CDCl3): 2.98, 3.02 (2 ϫ s, 2 ϫ 6H, NCH3), 7.33 (s, 4H, C6H4), 7.32Ϫ7.43
(m, 10H, Ph). 13C NMR (62.9 MHz, CDCl3): 41.6, 41.6 (2 ϫ NCH3), 127.2
(Ph-m), 127.3 (Ph-p), 132.0 (C6H4), 132.9 (Ph-o), n.o. (CB).
Synthesis of 3Ph. A solution of 2 (0.252 g, 0.74 mmol) in toluene
(30 mL) was added to a suspension of LipzPh (0.222 g, 1.48 mmol)
and HpzPh (0.213 g, 1.48 mmol) in toluene (20 mL). The mixture
was heated to reflux for 10 h and cooled to room temperature
again. All volatiles were removed in vacuo and the resulting colour-
less residue was washed with hexane (30 mL) and dried in vacuo.
Yield: 0.520 g (84 %). Anal. Calcd. for C54H42B2Li2N8 [838.42] ϫ
4 THF [72.11]: C, 74.61; H, 6.62; N, 9.94. Found: C, 74.48; H, 6.62;
N, 9.92 %. Crystals of 3Ph(THF)4 suitable for X-ray analysis were
grown by diffusion of Et2O into a saturated solution of 3Ph in THF
at room temperature.
3 Conclusion
We have synthesized sterically demanding ditopic lithium
bis(pyrazol-1-yl)borates with phenyl- (3Ph) and t-butyl sub-
stituents (3tBu) in the 3-positions of the pyrazolyl rings.
Single crystals of both compounds were grown from THF/
Et2O in the presence of Li(Cl,Br). X-ray crystallography re-
vealed profound differences in the chloride affinities of 3Ph
and 3tBu: The former compound crystallizes as THF-adduct
11B NMR (96.3 MHz, THF-d8): 2.6 (h1/2 ϭ 580 Hz). 1H NMR (300.0 MHz,
3
THF-d8): 6.37 (d, 4H, JHH ϭ 2.2 Hz, pzH-4), 6.79Ϫ6.82 (m, 4H, Ph-o),
6.97 (s, 4H, C6H4), 7.02Ϫ7.10 (m, 6H, Ph-m/p), 7.16Ϫ7.21 (m, 4H, Ph*-p),
3
7.26-7.32 (m, 8H, Ph*-o), 7.57 (d, 4H, JHH ϭ 2.2 Hz, pzH-5), 7.60Ϫ7.64
(m, 8H, Ph*-m). 13C NMR (75.5 MHz, THF-d8): 101.6 (pzC-4), 125.7 (Ph-
p), 127.2 (Ph-m), 127.4 (Ph*-p), 127.7 (Ph*-o), 128.8 (Ph*-m), 134.6, 134.7
(Ph-o, C6H4), 137.0 (Ph*-i), 139.7 (pzC-5), 154.3 (pzC-3), n.o. (CB).
3
Ph(THF)4 and can thereby be separated from the lithium
halide contaminants in analytically pure form. In contrast,
3tBu functions as inverse chelator of ClϪ and forms anions
[3tBuCl]Ϫ featuring a bent Li-Cl-Li core (the compound co-
crystallizes with Li([12]crown-4)Br). In the investigated
single crystal, the selectivity of 3tBu for ClϪ over BrϪ was
close to 100 %. We have determined the cell constants of
10 other crystals which were either identical with the ones
determined for Li(12c4)(Et2O)[3tBuCl] (6 species) or for
Li([12]crown-4)Br (4 species). However, it cannot be fully
excluded that the differences in the parameters of the
hypothetical compound Li(12c4)(Et2O)[3tBuBr] and
Li(12c4)(Et2O)[3tBuCl] are too small to allow for an unam-
biguous differentiation.
Synthesis of 3tBu. A mixture of neat LipztBu (0.229 g, 1.76 mmol)
and neat HpztBu (0.219 g, 1.76 mmol) was added to a stirred solu-
tion of 2 (0.300 g, 0.88 mmol) in toluene (20 mL) at room tempera-
ture. The resulting slurry was heated to reflux for 15 h and cooled
to room temperature again. All volatiles were removed in vacuo and
the resulting colourless residue was washed with hexane (30 mL)
and dried in vacuo. Crystals of Li(12c4)(Et2O)[3tBuCl] were grown
by slow diffusion of Et2O into a saturated solution of 1 equivalent
of 3tBu and 2 equivalents of [12]crown-4 in THF at room tempera-
ture. Anal. Calcd. for C58H84B2ClLi3N8O5 [1051.22] ϫ THF [72.11]
ϫ Li([12]crown-4)Br [263.06]: C, 60.64; H, 7.85; N, 8.08. Found:
C, 60.82; H, 7.65; N, 7.91 %.
11B NMR (80.3 MHz, THF-d8): 2.5 (h1/2 ϭ 510 Hz). 1H NMR (300.0 MHz,
3
THF-d8): 1.30 (s, 36H, C(CH3)3), 5.94 (d, 4H, JHH ϭ 2.1 Hz, pzH-4),
6.76Ϫ6.79 (m, 4H, Ph-o), 6.87 (s, 4H, C6H4), 7.07Ϫ7.12 (m, 6H, Ph-m/p),
7.39 (d, 4H, JHH ϭ 2.1 Hz, pzH-5). 13C NMR (75.5 MHz, THF-d8): 31.5
(C(CH3)3), 32.7 (C(CH3)3), 99.4 (pzC-4), 126.5 (Ph-p), 127.5 (Ph-m), 134.8,
135.3 (Ph-o, C6H4), 137.8 (pzC-5), 162.9 (pzC-3), n.o. (CB).
3
4 Experimental Section
General Considerations. All reactions and manipulations of air-sen-
sitive compounds were carried out in dry, oxygen-free argon using
˚
standard Schlenk ware. CDCl3 was passed through a 4 A molecular
X-ray Crystal Structure Determinations of 2,
3Ph(THF)4, and Li(12c4)(Et2O)[3tBuCl]
sieves column prior to use. All other solvents were freshly distilled
under argon from Na/benzophenone. Compound
1
[14],
(H,Li)pzPh, and (H,Li)pztBu were prepared following literature pro-
cedures [13, 18, 19]. NMR: Bruker AMX 250, AMX 300, AMX
400, Bruker DPX 250 spectrometers. 1H- and 13C NMR spectra
are calibrated against residual solvent signals. 11B NMR spectra
are reported relative to external BF3 ϫEt2O. All NMR spectra were
run at room temperature. Abbreviations: s ϭ singlet, d ϭ doublet,
m ϭ multiplet, n.o. ϭ signal not observed, pz ϭ pyrazol-1-yl, Ph ϭ
Data collections were performed on a Stoe-IPDS-II two-circle-dif-
fractometer with graphite-monochromated MoKα radiation. An
empirical absorption correction with the MULABS option in the
program PLATON [20] was performed for Li(12c4)(Et2O)[3tBuCl].
The structures were solved by direct methods [21] and refined with
full-matrix least-squares on F2 using the program SHELXL97 [22].
Hydrogen atoms were placed on ideal positions and refined with
Z. Anorg. Allg. Chem. 2008, 1570Ϫ1574
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1573
Morawitz, Thorsten
