Cleavage of a N-H bond of ammonia at room temperature by a germylene

Article abstract of DOI:10.1021/ic801964u

The reaction of LGeCI [1; L = CH{(CMe)(2,6-ⁱPr₂C ₆H₃N)}₂] with 1,3-di-tert-butylimidazol-2- ylidene results in the formation of the germylene L′Ge [2; L′ = CH{(C=CH₂)(CMe)(2,6-ⁱP

Full text of DOI:10.1021/ic801964u

Inorg. Chem. 2009, 48, 798-800
Cleavage of a N-H Bond of Ammonia at Room Temperature by a
Germylene
Anukul Jana, Ina Objartel, Herbert W. Roesky,* and Dietmar Stalke
Institut fu¨r Anorganische Chemie der UniVersita¨t Go¨ttingen, Tammannstrasse 4,
37077 Go¨ttingen, Germany
Received October 15, 2008
The reaction of LGeCl [1; L ) CH{(CMe)(2,6-iPr2C6H3N)}2] with
1,3-di-tert-butylimidazol-2-ylidene results in the formation of the
germylene L′Ge [2; L′ ) CH{(CdCH2)(CMe)(2,6-iPr2C6H3N)2}]. 2
reacts with ammonia under N-H cleavage to give LGeNH2 (3).
This type of reaction can also be used to activate primary amines.
3 is characterized by microanalysis, multinuclear NMR spectros-
copy, and X-ray structural analysis. The single-crystal X-ray
structural analysis indicates 3 to be a monomer, and the germanium
atom shows a trigonal-pyramidal environment with a stereochemi-
cally active lone pair.
Scheme 1. Preparation of 2
expected that the reaction of LGeCl with an excess of
ammonia leads to the product LGeNH2. However, in a broad
temperature range (-40 to +25 °C), the starting material
does not react with ammonia. From our previous experience
for the preparation of LGeOH3 and LAl(NH2)28 using LGeCl
and LAlCl2, respectively, we employed N-heterocyclic
carbenes (NHCs) as HCl acceptors. Herein we follow the
route for the preparation of LGeNH2 from LGeCl, NH3, and
NHC.
A toluene solution of 1 equiv of LGeCl and 1 equiv of
1,3-di-tert-butylimidazol-2-ylidene9 under stirring immedi-
ately changed the color from yellow to brown-red after a
few minutes, and a white precipitate is formed10a (Scheme
1). There was no reaction when a solution of 1 equiv of
LGeCl and 1 equiv of 1,3-bis(2,4,6-trimethylphenyl)imida-
zol-2-ylidene was used.
In recent years, we have been interested in the synthesis
of low-valent germanium compounds and reported on the
preparation and structural characterization of LGeCl,1 LGeF,2
LGeOH,3 and LGeH4 [L ) CH{(CMe)(2,6-iPr2C6H3N)}2].
These small reactive molecules are ideal precursors for the
preparation of heterometallic germylenes, which are active
catalysts in olefin polymerization reactions.5 One important
missing species in this series is LGeNH2. To date, there is
only one example known of a stable germanium(II) amide
featuring the parent NH2 group.6 A conventional route for
the preparation of GeNH2-containing compounds is the
ammonolysis of the corresponding halide.7 Therefore, it was
The insoluble white precipitate was identified as 1,3-di-
tert-butylimidazolium chloride and the soluble part as the
germylene 2, which was confirmed by NMR spectroscopy
and compared to a previously reported sample.11 We
postulate that the reaction proceeds via cleavage of a C-H
methyl bond and formation of the C-H carbonium ion
(Scheme 1). This type of C-H cleavage reaction was
reported for nucleophilic carbenes.12 The proof for C-H
*
To whom correspondence should be addressed. E-mail:
hroesky@gwdg.de.
(1) Ding, Y.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-G. Organo-
metallics 2001, 20, 1190–1194.
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Organometallics 2001, 20, 4806–4811.
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Angew. Chem. 2004, 116, 1443-1445; Angew. Chem., Int. Ed. 2004,
43, 1419-1421.
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Chem. 2005, 44, 3537–3540. (b) Yang, Y.; Roesky, H. W.; Jones,
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798 Inorganic Chemistry, Vol. 48, No. 3, 2009
10.1021/ic801964u CCC: $40.75  2009 American Chemical Society
Published on Web 12/31/2008

COMMUNICATION
bond cleavage at the backbone methyl group of 1 by 1,3-
di-tert-butylimidazol-2-ylidene is shown by the formation
of 2 and the elimination of HCl under isolation of 1,3-di-
tert-butylimidazolium chloride. The previously reported
germylene 2 was formed by the reaction of 1 with LiN-
(SiMe3)2. Similar deprotonation reactions at the backbone
have been observed for the borane-stabilized germylene
hydride,2 the tautomerization of a iminogermane bearing the
ꢀ-diketiminato ligand,13 a cyclodiazaborane analogue,14 and
related calcium complexes.15 This is so far a unique example
where the deprotonation reaction at the backbone has been
demonstrated using 1,3-di-tert-butylimidazol-2-ylidene.
Activation of the ammonia molecule has attracted con-
siderable interest over the years. Although ammonia usually
forms simple Lewis acid-base adducts with transition-metal
complexes, there is also some N-H activation of ammonia
via transition-metal complexes known.16,17 In contrast to
these studies, the corresponding chemistry of stable main-
group molecular compounds for activation of ammonia is
virtually nonexistent. In 2007, the first example of ammonia
splitting was reported by Bertrand and co-workers using
Figure 1. Anisotropic displacement parameters, depicted at the 50%
probability level of 3. Hydrogen atoms, except those vital at the nitrogen
atom N3, are omitted for clarity. Selected bond lengths [Å] and angles [deg]:
Ge1-N3 1.845(2), Ge1-N1 2.030(2), Ge1-N2 2.030(2); N1-Ge1-N2
88.60(8), N3-Ge1-N1 95.52(10), N3-Ge1-N2 95.55(10).
Scheme 2. Preparation of 3
stable carbenes.18 Very recently, Power and co-workers
reported the activation of ammonia by the heavier group 14
element carbene analogue SnAr2 [Ar ) C6H3-2,6(C6H3-2,6-
iPr2)2],19 and Stephan and co-workers reported the activation
of amine by a frustrated Lewis pair.20
(10) All manipulations were carried out under anaerobic and anhydrous
conditions with Schlenk techniques. (a) L′Ge (2): 1,3-di-tert-butyl-
imidazol-2-ylidene (0.360 g, 2.0 mmol) and LGeCl (1.050 g, 2.0 mmol)
were dissolved in toluene (30 mL) at room temperature. The reaction
mixture was stirred, and the color of the solution changed from yellow
to brown-red. A white precipitate was formed. The reaction mixture
was stirred for another 1 h, then the white precipitate was separated
by filtration, and the remaining red solution was evaporated. The
residue was dissolved in n-hexane. Storage of the solution at -30 °C
for 1 day yielded brown-red crystals suitable for a single-crystal X-ray
diffraction experiment. Yield: 0.630 g (65%). 1H NMR (500 MHz,
C6D6): δ ) 7.04-7.21 (m, 6H, Ar-H), 5.43 (s, 1H, γ-CH), 3.91 (s,
1H, CH2), 3.69 (sept, 2H, CH(CH3)2), 3.44 (sept, 2H, CH(CH3)2), 3.26
(s, 1H, CH2), 1.41 (s, 3H, CH3), 1.35 (d, 6H, CH(CH3)2), 1.29 (d, 6H,
CH(CH3)2), 1.20 (d, 6H, CH(CH3)2), 1.15 (d, 6H, CH(CH3)2) ppm.
(b) LGeNH2 (3): Dry ammonia gas was added to a red solution of 2
(0.490 g, 1 mmol) in toluene (20 mL) at room temperature. The
reaction mixture became orange. For a further 5 min, the ammonia
gas was bubbled through the solution. After that, the solvent was
removed in vacuum and the residue was extracted with n-hexane. The
solution was reduced to half of the volume. Storage of the solution at
-30 °C for 1 day yielded yellow crystals suitable for a single-crystal
Herein we show the N-H cleavage of ammonia by
employing L′Ge [2; L′
) CH{(CdCH2)(CMe)(2,6-
iPr2C6H3N)}2], which generates at mild conditions a terminal
GeNH2 group (Scheme 2).10 The addition of an excess of
b
dry ammonia gas to a red solution of 2 in toluene leads to a
rapid change of the color to orange. The solution was
evaporated and extracted with n-hexane. Concentration of
the solution yielded yellow crystals of LGeNH2 (3). X-ray
crystal structure analysis afforded a monomeric structure,
as illustrated in Figure 1.21 Surprisingly, 3 is monomeric in
1
X-ray diffraction experiment. Yield: 0.455 g (95%). Mp: 162 °C. H
(18) Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand,
G. Science 2007, 316, 439–441.
NMR (500 MHz, C6D6): δ ) 7.12-7.18 (m, 6H, Ar-H), 4.79 (s, 1H,
γ-CH), 3.54 (sept, 2H, CH(CH3)2), 3.47 (sept, 2H, CH(CH3)2), 1.60
(s, 6H, CH3), 1.35 (d, 6H, CH(CH3)2), 1.28 (d, 6H, CH(CH3)2), 1.24
(d, 6H, CH(CH3)2), 1.14 (d, 6H, CH(CH3)2) ppm. 13C{1H} NMR
(125.77 MHz, C6D6): δ ) 163.20 (CN), 145.95, 143.77, 142.74,
125.81, 124.66, 124.18 (Ar-C), 95.55 (γ-C), 29.19 (CH(CH3)2), 28.60
(CH(CH3)2), 28.25 (CH(CH3)2), 26.96 (CH(CH3)2), 24.76 (CH(CH3)2),
24.67 (CH(CH3)2), 23.40 (CH3) ppm. IR (Nujol, KBr): ν ) 3431 vw,
3333 vw (N-H) cm -1. EI-MS (70 eV): m/z (%): 507 (100) [M+].
Anal. Calcd for C29H43GeN3 (507.27): C, 68.79; H, 8.56; N, 8.30.
Found: C, 69.25; H, 8.82; N, 7.66.
(19) Peng, Y.; Ellis, B. D.; Wang, X.; Power, P. P. J. Am. Chem. Soc.
2008, 130, 12268–12269.
(20) Chase, P. A.; Stephan, D. W. Angew. Chem. 2008, 120, 7543-7547;
Angew. Chem., Int. Ed. 2008, 47, 7433-7437.
(21) Single-crystal structure analysis of 3: The data set was collected on a
Bruker TXS-Mo rotating anode equipped with INCOATEC Helios
mirror optics (Mo KR, λ ) 71.073 pm). The crystal was mounted in
a shock-cooled oil drop at the tip of a fibre.22 The integration was
performed with SAINT V7.46A, which was followed by an empirical
absorption correction with SADABS-2008/1. The structure was solved
by direct methods and refined with SHELXL against F2.23 The positions
of the hydrogen atoms H3A and H3B at the amine nitrogen atom N3
were taken from the difference map and refined with distance restraints.
C29H43GeN3, M ) 506.25 g/mol, crystal size 0.25 × 0.03 × 0.03 mm,
monoclinic, C2/c, a ) 2490.2(5) pm, b ) 1516.7(3) pm, c ) 1467.2(3)
pm, ꢀ ) 95.261(3)°, V ) 5.5180(18) nm; Z ) 8, Fcalcd ) 1.219 Mg/
m3, µ ) 1.131 mm-1, T ) 100(2) K, 2θmax ) 50.76°, 25 829 reflections
measured, of which 5069 were independent, Rint ) 0.0526, R1 )
0.0385 [I > 2σ(I)], wR2 ) 0.1048 (all data), +0.609/-0.799 e/Å3
residual densities. Complete crystallographic data are deposited at the
Cambridge Crystallographic Data Centre, where it can be downloaded
free of charge from www.ccdc.cam.ac.uk/data_request/cif at CCDC
705305.
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118, 4455-4458; Angew. Chem., Int. Ed. 2006, 45, 4349-4352.
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R. HelV. Chim. Acta 1999, 82, 2348–2364.
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Schmidt, H.-G. Organometallics 2002, 21, 5216–5220.
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2414.
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Ed. 2003, 42, 3430-3434.
(16) Casalnuovo, A. L.; Calabrese, J. C.; Milstein, D. Inorg. Chem. 1987,
26, 971–973.
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1082.
(22) (a) Stalke, D. Chem. Soc. ReV. 1998, 27, 171–178. (b) Kottke, T.;
Stalke, D. J. Appl. Crystallogr. 1993, 26, 615–619.
Inorganic Chemistry, Vol. 48, No. 3, 2009 799

COMMUNICATION
the solid state, and what is even more striking is that the
NH2 group is not involved in any kind of hydrogen bonding,
as shown by X-ray structural analysis and IR spectroscopy.
The IR spectrum displays two weak sharp bands at 3431
and 3333 cm-1 that are attributed to the two N-H stretching
modes of the NH2 group. These wavenumbers are close to
those at 3380 and 3308 cm-1 observed for (Ar*GeNH2)2 [Ar*
) C6H3-2,6-(C6H3-2,6-iPr2)].6
In summary, we have shown that 1,3-di-tert-butylimidazol-
2-ylidene cleaves the C-H bond of a methyl group of 1 and
leads to the formation of the carbene analogue germylene 2.
Compound 2 reacts quantitatively with ammonia at room
temperature under N-H bond cleavage and oxidative addi-
tion to 3, demonstrating an example of sustainable chemistry.
1,3-Di-tert-butylimidazolium chloride can be converted to
the starting material using a strong base. Preliminary studies
of 2 with 2,6-iPr2C6H3NH2 show the complete conversion
of 2 and afford LGeN(H)-2,6-iPr2C6H3. Other N-H and P-H
cleavage reactions according to this route should be possible.
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft, Bruker AXS, Delft, IN-
COATEC, Geesthacht, and the Go¨ttinger Akademie der
Wissenschaften.
Supporting Information Available: Crystallographic data for
3 (CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
(23) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112–120.
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