Lithium complex of an abnormal carbene

Article abstract of DOI:10.1002/ejic.201100574

A lithium complex of an abnormal carbene with the composition [H ₃BC{{N(2,6-iPr₂C₆H₃)} ₂CHCLi(THF)₃}] was synthesized by the lithiation reaction of the N-heterocyclic carbene borane adduct

Full text of DOI:10.1002/ejic.201100574

SHORT COMMUNICATION
DOI: 10.1002/ejic.201100574
Lithium Complex of an Abnormal Carbene
[a]
[a]
[a]
[a]
ˇ
ˇ
Anukul Jana, Ramachandran Azhakar, Gasper Tavcar, Herbert W. Roesky,*
Ina Objartel,[a] and Dietmar Stalke*[a]
Dedicated to the memory of Professor Hans Georg von Schnering
Keywords: Carbenes / Boranes / Lithium
A lithium complex of an abnormal carbene with the composi-
tion [H3BC{{N(2,6-iPr2C6H3)}2CHCLi(THF)3}] was synthe-
sized by the lithiation reaction of the N-heterocyclic carbene
borane adduct {IPr·BH3, IPr = [C{[N(2,6-iPr2C6H3)]CH}2] (1)}
with nBuLi. The lithiated species was proven by NMR spec-
troscopy and unambiguously established by single-crystal X-
ray structural analysis.
applications in synthetic chemistry (ionic, organometallic,
and radical reactions).[9] Braunschweig et al. incorporated
the NHC·BH3 adduct into the coordination sphere of tran-
sition metals to afford η1-end-on-coordinated borane com-
plexes.[10] Very recently, we reported a facile route to abnor-
mal NHCs via NHC base-stabilized dichlorosilylenes.[11] In-
spired by the above results, we wanted to synthesize the
lithium complex of an abnormal carbene by treating
IPr·BH3 (2) with nBuLi to lithiate the fourth position of
the five-membered imidazole ring. The reaction proceeds as
expected, and herein we report on the lithiated species of
an IPr·BH3 adduct, with the composition [H3BC{[N(2,6-
iPr2C6H3)]2CHCLi(THF)3}] (3). The formation of 3 was
proven by NMR spectroscopy and unambiguously estab-
lished by single-crystal X-ray structural analysis.
Introduction
In the last two decades, the chemistry of N-heterocyclic
carbenes (NHCs) has developed tremendously after the
milestone report of a thermodynamically as well as a kinet-
ically stable carbene in the absence of moisture and oxygen
by Arduengo et al. in 1991.[1] This discovery initiated fur-
ther studies devoted to the synthesis of new NHCs and to
their use as ligands in transition-metal complexes.[2,3] Many
highly reactive compounds with low-valent elements that
are stabilized with NHCs have been documented.[4,5a] We
also reported the NHC-stabilized dichlorosilylene and
studied its reactivity.[5] Depending on the availability of the
lone pair of electrons on the five-membered imidazole ring,
carbenes are classified as normal carbenes (A in Figure 1)
and abnormal carbenes (B in Figure 1). Abnormal N-het-
erocyclic carbenes (aNHCs) are kinetically less stable than
their normal isomers and possess strong electron-donating
properties. Recently, Bertrand et al. reported on stable
aNHC,[6] and Robinson et al. described the lithium complex
of a dicarbene with lithium coordination at the second and
fourth position of the five-membered imidazole ring.[7]
Furthermore, they synthesized the Lewis acid adduct by co-
ordinating two molecules of BH3 each at the second and
fourth position of the five-membered imidazole ring.[8] The
NHC·BR3 (R = H, aryl, alkyl) adduct finds wide-spread
Figure 1. Schematic representation of normal carbene (A) and ab-
normal carbene (B).
Results and Discussion
The reaction of equimolar amounts of IPr·BH3 (2) with
nBuLi afforded the lithiated species of IPr·BH3 with the
composition [H3BC{{N(2,6-iPr2C6H3)}2CHCLi(THF)3}]
(3) in quantitative yield (Scheme 1). Compound 3 is a color-
less crystalline solid with good solubility in THF and has
been characterized by NMR spectroscopy and single-crys-
tal X-ray structural studies.
[a] Institut für Anorganische Chemie der Universität Göttingen,
Tammannstrasse 4, 37077 Göttingen, Germany
Fax: +49-551-393-373
E-mail: hroesky@gwdg.de
dstalke@chemie.uni-goettingen.de
3686
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 3686–3689

Lithium Complex of an Abnormal Carbene
dral geometry. The coordination environment consists of
three oxygen atoms from the THF solvent molecules and
one imidazole carbon atom of the carbene moiety. The Li–
Oav and Li(1)–C(3) bond lengths are 1.983 and 2.122(3) Å,
respectively, which is quite comparable to the bond lengths
reported in the literature.[12] The boron atom is also four
coordinate[13] and has a distorted tetrahedral geometry that
comprises three hydrogen atoms and one carbon atom. The
carbon–boron bond length in 3 is 1.6041(19) Å.
Scheme 1. Synthesis of 3.
The formation of the lithiated species 3 was readily no- Conclusions
ticed in the 1H NMR spectrum. Compound 3 shows an
In this study, we report on the synthesis of a lithium
upfield resonance for the imidazole ring hydrogen atom at
δ = 6.18 ppm in [D8]THF relative to that for 2 (δ =
7.26 ppm).[7] Moreover, the CH(CH3)2 protons in 3 display
three resonances [δ = 1.05 (6 H), 1.08 (6 H), and 1.16 ppm
(12 H)], whereas 2 exhibits two resonances [δ = 1.17 (12 H)
complex of an abnormal carbene. The functionalization of
the carbon atom of the imidazole ring at the fourth position
with lithium is an interesting concept. This allows further
modifications of the electronic and steric properties through
a transmetalation process, which is currently under investi-
gation.
1
and 1.26 ppm (12 H)] in the related H NMR spectra. Fur-
ther, the CH(CH3)2 proton in 3 resonates at δ = 2.73 (2 H)
and 2.84 ppm (2 H), while that of 2 resonates at δ =
2.61 ppm (4 H). The 7Li NMR spectrum shows a single
resonance at δ = 0.64 ppm. The 11B NMR spectrum of 3
exhibits a resonance at δ = –35.59 ppm and displays a quar-
tet resonance with an intensity of 1:3:3:1, which indicates
the coupling of the hydrogen atoms to the boron atom in
Experimental Section
General: All manipulations were carried out in an inert atmosphere
of dinitrogen by using standard Schlenk techniques and in a dini-
the BH3 unit (JBH = 85 Hz). In addition, the structure of trogen-filled glove box. Solvents were purified by the MBRAUN
solvent purification system MB SPS-800. All chemicals were pur-
chased from Aldrich and used without further purification. Com-
pounds 1 and 2 were prepared as reported in the literature.[14] 1H,
7Li, and 11B NMR spectra were recorded with a Bruker Avance
DPX 200, a Bruker Avance DRX 300, or a Bruker Avance DRX
500 spectrometer, by using [D8]THF as solvent. Chemical shifts δ
are given relative to SiMe4. EI-MS spectra were obtained by using
a Finnigan MAT 8230 instrument. Melting points were measured
in a sealed glass tube on a Büchi B-540 melting point apparatus.
compound 3 was unambiguously established by low-tem-
perature single-crystal X-ray structure analysis.
The molecular structure of 3 is displayed in Figure 2.
¯
Compound 3 crystallizes in the triclinic space group P1. In
3, the lithium is four coordinate and has a distorted tetrahe-
Synthesis of 3: To a solution of 2 (0.50 g, 4.10 mmol) in THF
(60 mL) was added an nBuLi solution (1.0 m, 4.2 mL, 4.20 mmol)
at –78 °C. The mixture was then allowed to come to room tempera-
ture and stirred for 12 h. Finally, the solution was filtered, and the
solvent was removed in vacuo and evaporated to dryness to obtain
the pure colorless crystalline solid of 3 (0.72 g, 93.22%). Mp: 112–
1
115 °C (dec). H NMR (200 MHz, [D8]THF, 25 °C): δ = 1.05 [d, J
= 6.92 Hz, 6 H, CH(CH3)2], 1.08 [d, J = 6.9 Hz, 6 H, CH(CH3)2],
1.16 [d, J = 6.9 Hz, 12 H, CH(CH3)2], 2.73 [m, 2 H, CH(CH3)2],
2.84 [m, 2 H, CH(CH3)2], 6.18 (s, 1 H, CH), 7.02–7.19 (m, 6 H,
ArH) ppm. 13C{1H} NMR (125.29 MHz, [D8]THF, 25 °C): δ =
22.86 [CH(CH3)2], 23.07 [CH(CH3)2], 24.22 [CH(CH3)2], 24.38
[CH(CH3)2], 24.86 [CH(CH3)2], 25.03 [CH(CH3)2], 28.06
[CH(CH3)2], 28.15 [CH(CH3)2], 122.10, 123.57, 127.21, 127.50,
129.54, 135.21, 138.08, 143.86, 145.61, 145.92 (NCH, NCLi, aro-
matic) ppm. 11B{1H} NMR (96.29 MHz, [D8]THF, 25 °C): δ =
Figure 2. Molecular structure of 3. Anisotropic displacement pa-
rameters are depicted at the 50% probability level. Hydrogen atoms
are omitted for clarity, with the exception of those for the BH3
moiety and H(2). Selected bond lengths [Å] and angles [°]: Li(1)–
C(3) 2.123(3), Li(1)–O(1) 1.993(2), Li(1)–O(2) 1.986(2), Li(1)–O(3)
1.969(2), B(1)–C(1) 1.6041(19); O(3)–Li(1)–O(2) 99.43(11), O(3)–
Li(1)–O(1) 107.19(12), O(2)–Li(1)–O(1) 105.44(12), O(3)–Li(1)–
C(3) 126.95(13), O(2)–Li(1)–C(3) 108.56(11), O(1)–Li(1)–C(3)
107.36(11).
7
–35.59 (q, JBH = 85 Hz, BH3) ppm. Li{1H} NMR (116.64 MHz,
[D8]THF, 25 °C): δ = 0.64 ppm.
Crystallography: Colorless crystals suitable for single-crystal X-ray
analysis were obtained by storing 3 in THF solution at –32 °C for
3 d. A shock-cooled crystal was selected and mounted under nitro-
gen atmosphere by using the device X-TEMP2.[15] The data set was
collected on an INCOATEC microfocus source with a mirror optics
Eur. J. Inorg. Chem. 2011, 3686–3689
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3687

A. Jana, R. Azhakar, G. Tavcˇar, H. W. Roesky, I. Objartel, D. Stalke
SHORT COMMUNICATION
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Crystal Data for 3: C39H62BLiN2O3, Mr
=
624.66,
0.2ϫ0.1ϫ0.05 mm, a = 10.369(2), b = 10.965(2), c = 18.731(3) Å,
α = 73.74(2)°, β = 89.37(3)°, γ = 69.21(2)°, V = 1902.0(6) Å3, Z =
2, ρcalcd. = 1.091 Mg/m3, μ (Mo-Kα) = 0.067 mm–1, 2θmax = 50.7°,
40602 reflections measured, 6982 independent (Rint = 0.029), R1 =
0.0387 [IϾ2σ(I)], wR2 = 0.0960 (all data), residual density peaks:
0.242 to –0.219 eÅ–3. CCDC-826662 contains the supplementary
crystallographic data for 3. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments
[4]
[5]
The authors thank the Deutsche Forschungsgemeinschaft for sup-
porting this work. R. A. and G. T. are thankful to the Alexander
von Humboldt Stiftung for research fellowships. D. S. kindly ac-
knowledges funding from the Deutsche Forschungsgemeinschaft
(DFG) Priority Programme 1178, the DNRF funded Centre of
Materials Crystallography and the doctoral programme Catalysis
for Sustainable Synthesis, provided by the Land Niedersachsen.
Support of the Chemetall, Frankfurt and Langelsheim and the
Volkswagenstiftung is appreciated.
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Received: June 6, 2011
Published Online: July 28, 2011
Eur. J. Inorg. Chem. 2011, 3686–3689
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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