Facile synthesis of a rare chlorosilylene-BH3 adduct

Article abstract of DOI:10.1002/ejic.201001188

The first stable chlorosilylene-BH₃ adduct was synthesized in the reaction of N-heterocyclic carbene (NHC) stabilized dichlorosilylene either with equivalent amounts of LiBH₄ or with equivalent amounts of a BH₃ ·THF soluti

Full text of DOI:10.1002/ejic.201001188

SHORT COMMUNICATION
DOI: 10.1002/ejic.201001188
Facile Synthesis of a Rare Chlorosilylene–BH3 Adduct
[a]
[a]
[a]
[a]
ˇ
ˇ
Ramachandran Azhakar, Gasper Tavcar, Herbert W. Roesky,* Jakob Hey, and
Dietmar Stalke*[a]
Dedication to Professor Hansgeorg Schnöckel on the occasion of his 70th birthday
Keywords: Silylenes / Boranes / Lewis acid adducts
The first stable chlorosilylene–BH3 adduct was synthesized
in the reaction of N-heterocyclic carbene (NHC) stabilized
dichlorosilylene either with equivalent amounts of LiBH4 or
with equivalent amounts of a BH3·THF solution. The chloro-
silylene–BH3 adduct was characterized by single-crystal X-
ray structural analysis, NMR and IR spectroscopy, EI mass
spectrometry, and elemental analysis.
tivity of LSiCl2 with the Lewis acid B(C6F5)3[10] and investi-
gated the reactivity with transition metal complexes.[11] In-
spired by exploring the chemistry further towards reducing
agents, we treated LSiCl2 with the strong reducing agent
LiBH4. In the present report, we describe LSiCl2 that forms
LSiCl2·BH3 either with equivalent amounts of LiBH4 or
with equivalent amouts of a BH3·THF solution, which is
the first stable chlorosilylene–BH3 adduct.
Introduction
Silylenes are considered as silicon analogues of carbenes,
existing as a neutral bivalent silicon atom[1] with two non-
bonding electrons as the HOMO, which possesses nucleo-
philic character, and an empty p-orbital as the LUMO,
which exhibits electrophilic character. Having both the nu-
cleophilic and electrophilic reactive sites at the silicon atom,
silylenes are considered to have an ambiphilic character and
behave as Lewis acids as well as Lewis bases.[2] Due to the
availability of both nucleophilic and electrophilic reaction
center, the chemistry of silylenes is of intense interests to
scientists.[1–5] After the report of the first stable N-heterocy-
clic silylene (NHSi) by West et al.,[6] the chemistry of stable
N-heterocyclic silylenes (NHSis) has attracted much atten-
tion, whereas the NHCs (N-heterocyclic carbenes) have led
to burgeoning research activities and to many applications
in chemistry.[7,8] Recently, we reported on the high-yield
synthesis of NHC-stabilized dichlorosilylene LSiCl2 [L =
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] by em-
ploying a new synthetic procedure comprising the reductive
elimination of HCl from trichlorosilane in the presence of
NHC under mild reaction conditions.[9a] Furthermore, we
synthesized PhC(NtBu)2SiCl in high yield using lithium
bis(trimethylsilyl)amide as dehydrochlorinating agent.[9b]
These routes are completely different from the other con-
ventional synthetic procedures reported so far by using
strong reducing agents, such as potassium or potassium
graphite, for the synthesis of silylene or other compounds
with low-valent silicon. We have also reported on the reac-
Results and Discussion
The reaction of equimolar amounts of LSiCl2 (LЈ) with
LiBH4 afforded the colorless crystalline solid LSiCl2·BH3
(LЈ·BH3) in quantitative yield with good solubility in THF.
(Scheme 1). We surmised that there should be an elimi-
nation of LiH. It is interesting to mention that stable car-
bene reacts with LiMH4 (M = Al, Ga, In) to form carbene–
MH3 complexes.[12] Further, catalytic amounts of SnCl4,[13]
[14]
[15]
TiCl4
and CoCl2
are known to react with LiBH4 or
NaBH4 to give stoichiometric amounts of BH3. The same
adduct LЈ·BH3 was also synthesized by direct reaction of
[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
Scheme 1. Synthesis of LЈ·BH3.
Eur. J. Inorg. Chem. 2011, 475–477
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
475

R. Azhakar, G. Tavcˇar, H. W. Roesky, J. Hey, D. Stalke
SHORT COMMUNICATION
equivalent amounts of LЈ with BH3·THF solution. More- is smaller when compared with that of LЈ·B(C6F5)3
over, LЈ·BH3 is stable in solution as well as in the solid [132.3(2)°]. The average Si–Cl distance in LЈ·BH3 is
state at room temperature under an inert gas. It has been 2.0839(7) Å [Si–Clav of LЈ 2.1664(17) Å]. LЈ·BH3 exhibits
characterized by elemental analysis, spectroscopic methods an Si–C bond length of 1.937(2) Å, which is shorter when
and single-crystal X-ray structural studies.
compared to that of LЈ [Si–C 1.985(4) Å].
The coordination of the boron atom to the silicon atom
can be readily noticed in the 29Si NMR spectrum. The ad-
duct resonates at δ = 30.72 ppm. Moreover, it shows a quar-
tet resonance, which indicates the coupling of the silicon
nucleus with the quadrupolar 11B (I = 3/2) nucleus. The
coupling constant JSiB is 60 Hz. The 11B NMR spectrum of
LЈ·BH3 exhibits a resonance at δ = –38.75 ppm and displays
a quartet resonance with an intensity of 1:3:3:1 indicating
the coupling of the hydrogen atoms to the boron atom in
Conclusions
In this study, we report on the first stable chlorosilylene–
BH3 adduct from the reaction of N-heterocyclic carbene
stabilized dichlorosilylene either with equivalent amounts
of LiBH4 or with equivalent amounts of a BH3·THF solu-
tion.
1
the BH3 unit (JBH = 95 Hz). The H NMR spectrum of
LЈ·BH3 exhibits a quartet resonance in the range δ = –1.02
to +0.40 ppm for the B–H environment with an intensity of
1:1:1:1, which indicates the coupling of the hydrogen nuclei
with the quadrupolar 11B nucleus (JHB = 94 Hz). In the IR
spectrum we were able to observe the band for the νB–H
vibrations at 2356 and 2320 cm–1. Compound LЈ·BH3
shows the molecular ion peak in the EI mass spectrum at
Experimental Section
General: All manipulations were carried out under dinitrogen by
using standard Schlenk techniques and in a dinitrogen-filled glove
box. Solvents were purified by a solvent purification system
MBRAUN MB SPS-800. All chemicals were purchased from Ald-
rich and used without further purification. LЈ was prepared as re-
ported in the literature.[9] 1H, 13C, 11B and 29Si NMR spectra were
m/z = 500.1. In addition to that the structure was confirmed recorded with a Bruker Avance DPX 200 or a Bruker Avance DRX
500 spectrometer by using [D8]THF as solvent. Chemical shifts δ
are given relative to SiMe4. The EI mass spectrum was obtained
by using a Finnigan MAT 8230 instrument. The IR spectrum was
recorded with a Bio-Rad Digilab FTS7 spectrometer in the range
of 4000–350 cm–1 as nujol mulls. Elemental analysis was performed
by the Institut für Anorganische Chemie, Universität Göttingen.
The melting point was measured in a sealed glass tube with a Büchi
B-540 melting point apparatus.
by single-crystal X-ray structural analysis.
The molecular structure of LЈ·BH3 is shown in Figure 1.
The adduct crystallizes in the monoclinic space group P21/
n. Both the silicon and boron atoms are four-coordinate
and display a distorted tetrahedral geometry. The silicon
coordination environment is derived from one carbon atom
of the carbene, two chlorine atoms and one boron atom,
whereas the boron atom features three hydrogen atoms and
one silicon atom. The silicon–boron bond length in LЈ·BH3
is 1.965(2) Å, which is shorter than that in LЈ·B(C6F5)3
[2.106(6) Å], and the angle ЄC–Si–B in LЈ·BH3 [119.61(9)°]
Synthesis of LЈ·BH3
Method A: THF (60 mL) was added to a 100 mL Schlenk flask
containing LЈ (2.31 g, 4.73 mmol) and LiBH4 (0.11 g, 5.05 mmol),
and the reaction mixture was stirred at room temperature over-
night. The solution was filtered, and the solvent was removed in
vacuo. Finally, the compound was washed with toluene (20 mL),
and the solvents were evaporated to dryness to obtain a pure color-
less crystalline solid (1.95 g, 81.93%).
Method B: To a solution of LЈ (2.00 g, 4.10 mmol) in THF (60 mL)
a BH3·THF solution (1.0 m, 4.2 mL, 4.20 mmol) was added at
–78 °C, and then the flask was allowed to come to room tempera-
ture and the mixture stirred overnight. Then the solution was fil-
tered, and the solvent was removed in vacuo. Finally, the com-
pound was washed with toluene (20 mL), and the solvents were
evaporated to dryness to obtain a pure colorless crystalline solid
(1.55 g, 75.60%).
M.p. 210 °C (dec.). C27H39BCl2N2Si (501): calcd. C 64.67, H 7.84,
1
N 5.59; found C 64.42, H 7.84, N 5.46. H NMR (200 MHz, [D8]-
THF, 25 °C): δ = –1.02 to 0.40 (q, JHB = 94 Hz, 3 H, BH3), 1.15
[d, J = 6.84 Hz, 12 H, CH(CH3)2], 1.38 [d, J = 6.72 Hz, 12 H,
CH(CH3)2], 2.59 [m, 4 H, CH(CH3)2], 7.34 (m, 4 H, ArH), 7.51
(m,
2 H, ArH), 7.89 (s, 2
H, CH) ppm. 13C{1H} NMR
Figure 1. Molecular structure of LЈ·BH3. Anisotropic displacement
parameters are depicted at the 50% probability level. Hydrogen
atoms are omitted for clarity except for the BH3 moiety. Selected
bond lengths [Å] and angles [°]: B(1)–Si(1) 1.965(2), C(1)–Si(1)
1.937(2), Si(1)–Cl(1) 2.0787(7), Si(1)–Cl(2) 2.0892(8); C(1)–Si(1)–
B(1) 119.61(9), Cl(1)–Si(1)–Cl(2) 101.91(3), C(1)–Si(1)–Cl(1)
101.95(6), C(1)–Si(1)–Cl(2) 99.44(6), B(1)–Si(1)–Cl(1) 116.34(8),
B(1)–Si(1)–Cl(2) 114.76(8).
(125.75 MHz, [D8]THF, 25 °C): δ = 22.87, 26.06 [CH(CH3)2], 30.08
[CH(CH3)2], 124.83, 125.43, 128.35, 131.95, 134.32, 146.44, 156.48
(NCH and C6H3) ppm. 11B{1H} NMR(160.46 MHz, [D8]THF,
25 °C): δ = –38.75 (q, JBH = 95 Hz, 1 B, BH3) ppm. 29Si{1H} NMR
(99.36 MHz, [D8]THF, 25 °C): δ = 30.72 (q, JSiB = 60 Hz) ppm.
EI-MS: m/z: 500.1 [M+]. FT-IR (Nujol): ν = 2356 (w), 2320 (shoul-
˜
der) [ν(B–H)] cm–1.
476
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Eur. J. Inorg. Chem. 2011, 475–477

Facile Synthesis of a Rare Chlorosilylene–BH3 Adduct
Maciejewski, J. Organomet. Chem. 2003, 686, 321–331; m) Y.
Xiong, S. Yao, M. Driess, Inorg. Chem. 2009, 48, 7522–7524.
[5] a) M. Besora, F. Maseras, A. Lledós, O. Eisenstein, Inorg.
Chem. 2002, 41, 7105–7112; b) A. Shinohara, J. McBee, T. D.
Tilley, Inorg. Chem. 2009, 48, 8081–8083; c) M.-D. Su, S.-Y.
Chu, Inorg. Chem. 1999, 38, 4819–4823; d) Y. Xiong, S. Yao,
M. Driess, J. Am. Chem. Soc. 2009, 131, 7562–7563; e) A.
Meltzer, S. Inoue, C. Präsang, M. Driess, J. Am. Chem. Soc.
2010, 132, 3038–3046; f) Y. Xiong, S. Yao, R. Müller, M.
Kaupp, M. Driess, J. Am. Chem. Soc. 2010, 132, 6912–6913.
[6] M. Denk, R. Lennon, R. Hayashi, R. West, A. V. Belyakov,
H. P. Verne, A. Haaland, M. Wagner, N. Metzler, J. Am. Chem.
Soc. 1994, 116, 2691–2692.
[7] a) S. P. Nolan, in N-Heterocyclic Carbenes in Synthesis, Wiley-
VCH, Weinheim, Germany, 2006; b) F. Glorius, in N-Heterocy-
clic Carbenes in Transition Metal Catalysis, Springer-Verlag,
Berlin, 2007; c) J. Chun, I. G. Jung, H. J. Kim, M. Park, M. S.
Lah, S. U. Son, Inorg. Chem. 2009, 48, 6353–6355; d) P.-C.
Chiang, M. Rommel, J. W. Bode, J. Am. Chem. Soc. 2009, 131,
8714–8718; e) V. J. Catalano, A. L. Moore, J. Shearer, J. Kim,
Inorg. Chem. 2009, 48, 11362–11375; f) N. D. Harrold, R. Wa-
terman, G. L. Hillhouse, T. R. Cundari, J. Am. Chem. Soc.
2009, 131, 12872–12873; g) A. Sinha, S. M. W. Rahaman, M.
Sarkar, B. Saha, P. Daw, J. K. Bera, Inorg. Chem. 2009, 48,
11114–11122; h) A. G. Tennyson, E. L. Rosen, M. S. Collins,
V. M. Lynch, C. W. Bielawski, Inorg. Chem. 2009, 48, 6924–
6933; i) Y. Lee, B. Li, A. H. Hoveyda, J. Am. Chem. Soc. 2009,
131, 11625–11633; j) P. M. Zimmerman, A. Paul, C. B. Mus-
grave, Inorg. Chem. 2009, 48, 5418–5433; k) S. Groysman,
R. H. Holm, Inorg. Chem. 2009, 48, 621–627.
Crystallography: Colorless crystals suitable for single-crystal X-ray
analysis were obtained by keeping LЈ·BH3 in THF solution at
–32 °C for 3 d. A single crystal was mounted at low temperature in
inert oil under argon by applying the X-Temp2 device.[16] The data
of LЈ·BH3 was collected with a Bruker D8 three-circle dif-
fractometer equipped with a SMART APEX II CCD detector and
an INCOATEC Mo microsource with INCOATEC Quazar mirror
optics.[17] The data were integrated with SAINT,[18] and an empiri-
cal absorption correction with SADABS[19] was applied. The struc-
ture was solved by direct methods (SHELXS-97) and refined
against all data by full-matrix least-squares methods on F2
(SHELXL-97).[20] All non-hydrogen atoms were refined with aniso-
tropic displacement parameters. The hydrogen atoms were refined
isotropically in calculated positions by using a riding model with
their Uiso values constrained to 1.5ϫUeq of their pivot atoms for
terminal sp3 carbon atoms and 1.2ϫUeq for all other carbon
atoms. CCDC-791729 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Acknowledgments
The authors would like to thank the Deutsche Forschungsgemein-
schaft for supporting this work. R. A. and G. T. are thankful to
the Alexander von Humboldt Stiftung for research fellowships.
D. S. and J. H. are grateful to the State of Niedersachsen for provid-
ing the “Catalysis of Sustainable Synthesis” (CaSuS) PhD program.
[8] a) H. Clavier, S. P. Nolan, Annu. Rep. Prog. Chem. Sect. B:
Org. Chem. 2007, 103, 193–222; b) L. Jafarpour, S. P. Nolan,
Adv. Organomet. Chem. 2000, 46, 181–222; c) T. Weskamp,
V. P. W. Böhm, W. A. Herrmann, J. Organomet. Chem. 2000,
600, 12–22.
[9] a) R. S. Ghadwal, H. W. Roesky, S. Merkel, J. Henn, D. Stalke,
Angew. Chem. 2009, 121, 5793–5796; Angew. Chem. Int. Ed.
2009, 48, 5683–5686; b) S. S. Sen, H. W. Roesky, D. Stern, J.
Henn, D. Stalke, J. Am. Chem. Soc. 2010, 132, 1123–1126.
[10] R. S. Ghadwal, H. W. Roesky, S. Merkel, D. Stalke, Chem. Eur.
J. 2010, 16, 85–88.
[11] a) J. Li, S. Merkel, J. Henn, K. Meindl, A. Döring, H. W. Roes-
ky, R. S. Ghadwal, D. Stalke, Inorg. Chem. 2010, 49, 775–777;
b) G. Tavcˇar, S. S. Sen, R. Azhakar, A. Thorn, H. W. Roesky,
Inorg. Chem. 2010, 49, 10199–10202.
[12] M. D. Francis, D. E. Hibbs, M. B. Hursthouse, C. Jones, N. A.
Smithies, J. Chem. Soc., Dalton Trans. 1998, 3249–3254.
[13] S. Kano, Y. Tanaka, S. Shibuya, J. Chem. Soc., Chem. Com-
mun. 1979, 796–796.
[14] a) H. S. Lee, K. Isagawa, H. Toyoda, Y. Otsuji, Chem. Lett.
1984, 673–676; b) S. Kano, Y. Tanaka, S. Hibino, J. Chem.
Soc., Chem. Commun. 1980, 414–415; c) K. Burgess, W. A.
van der Donk, Organometallics 1994, 13, 3616–3620.
[15] N. Satyanarayana, M. Periyasamy, Tetrahedron Lett. 1984, 25,
2501–2504.
[1]
a) M. Haaf, T. A. Schmedake, R. West, Acc. Chem. Res. 2000,
33, 704–714; b) P. P. Gasper, R. West, in The Chemistry of Or-
ganic Silicon Compounds (Eds.: Z. Rappoport, Y. Apeloig),
John Wiley and Sons, Chichester, 1998, vol. 2, part 3, pp. 2463–
2567; c) B. Gehrhus, M. F. Lappert, J. Organomet. Chem. 2001,
617–618, 209–223.
a) R.-E. Li, J.-H. Sheu, M.-D. Su, Inorg. Chem. 2007, 46, 9245–
9253; b) P. V. Bharatam, R. Moudgil, D. Kaur, Inorg. Chem.
2003, 42, 4743–4749; c) P. V. Bharatam, R. Moudgil, D. Kaur,
Organometallics 2002, 21, 3683–3690.
[2]
[3] T. A. Schmedake, M. Haaf, B. J. Paradise, A. J. Millevolte, D.
Powell, R. West, J. Organomet. Chem. 2001, 636, 17–25.
[4] a) M. Haaf, R. Hayashi, R. West, J. Chem. Soc., Chem. Com-
mun. 1994, 33–34; b) R. West, M. Denk, Pure Appl. Chem.
1996, 68, 785–788; c) J. D. Feldman, G. P. Mitchell, J.-O. Nolte,
T. D. Tilley, J. Am. Chem. Soc. 1998, 120, 11184–11185; d) B.
Gehrhus, P. B. Hitchcock, M. F. Lappert, H. Maciejewski, Or-
ganometallics 1998, 17, 5599–5601; e) W. Yang, H. Fu, H.
Wang, M. Chen, Y. Ding, H. W. Roesky, A. Jana, Inorg. Chem.
2009, 48, 2058–2060; f) S. H. A. Petri, D. Eikenberg, B. Neu-
mann, H.-G. Stammler, P. Jutzi, Organometallics 1999, 18,
2615–2618; g) T. A. Schmedake, M. Haaf, B. J. Paradise, D.
Powell, R. West, Organometallics 2000, 19, 3263–3265; h) J. M.
Dysard, T. D. Tilley, Organometallics 2000, 19, 4726–4732; i)
S. B. Clendenning, B. Gehrhus, P. B. Hitchcock, D. F. Moser,
J. F. Nixon, R. West, J. Chem. Soc., Dalton Trans. 2002, 484–
490; j) D. Amoroso, M. Haaf, G. P. A. Yap, R. West, D. E.
Fogg, Organometallics 2002, 21, 534–540; k) A. Meltzer, C.
Präsang, M. Driess, J. Am. Chem. Soc. 2009, 131, 7232–7233;
l) A. G. Avent, B. Gehrhus, P. B. Hitchcock, M. F. Lappert, H.
[16] a) D. Stalke, Chem. Soc. Rev. 1998, 27, 171–178; b) T. Kottke,
D. Stalke, J. Appl. Crystallogr. 1993, 26, 615–619.
[17] T. Schulz, K. Meindl, D. Leusser, D. Stern, J. Graf, C.
Michaelsen, M. Ruf, G. M. Sheldrick, D. Stalke, J. Appl. Crys-
tallogr. 2009, 42, 885–891.
[18] SAINT, Bruker AXS Inc., Madison, Wisconsin, USA, 2000.
[19] G. M. Sheldrick, SADABS, University of Göttingen, Germany,
2000.
[20] G. M. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112–122.
Received: November 10, 2010
Published Online: December 22, 2010
Eur. J. Inorg. Chem. 2011, 475–477
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