From NHC→germylenes to stable NHC→germanone complexes

Article abstract of DOI:10.1039/b914060c

The unique isolable N-heterocyclic carbene (NHC)→germanone complexes 3a and 3b were synthesised and structurally characterised, starting from the corresponding NHC-germylene precursors 2a and 2b through straightforward oxygenation with N₂O.

Full text of DOI:10.1039/b914060c

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From NHC-germylenes to stable NHC-germanone complexesw
Shenglai Yao, Yun Xiong and Matthias Driess*
Received (in Cambridge, UK) 14th July 2009, Accepted 21st August 2009
First published as an Advance Article on the web 8th September 2009
DOI: 10.1039/b914060c
The unique isolable N-heterocyclic carbene (NHC)-germanone
complexes 3a and 3b were synthesised and structurally
characterised, starting from the corresponding NHC–germylene
precursors 2a and 2b through straightforward oxygenation
with N2O.
nucleophilicity of the low-valent centres.10 Thus, the promis-
ing donor ability of NHC ligands prompted us to synthesise
carbene–germylene adducts which, in turn, appear as suitable
precursors for the preparation of isolable NHC–germanone
adducts. We report here the facile synthesis and structural
characterisation of the first NHC-stabilised germanones 3a
and 3b, which result from gentle oxygenation of the novel
NHC–germylene precursors 2a and 2b (Scheme 2).
Heavier group 14 element-chalcogen multiply bonded species,
metallanechalcogenones (R2MQX: M = Si, Ge, Sn; X = O,
S, Se, Te), are congeners of ubiquitous ketones which can serve
as building blocks in contemporary organometallic chemistry.1
Since Corriu et al. described the first stable diarylsilanethiones
and diarylsilaneselones supported by an intramolecular
nitrogen donor in 1989,2 several examples of metallanechalco-
genones have been synthesised.1–3 Among these, germanium
compounds with double bonds to chalcogens (I and II, X = S,
Se, Te, Scheme 1) were more explored than those of other
heavier group 14 elements.1,3 Actually, the first evidence
for germanones, that is, germanium homologues of ketones
containing a GeQO double bond, was reported in 1971.4 The
lack of isolable heavier group 14 element-oxygen doubly
bonded species is probably due to the extremely high polarity
of the intrinsically betaine-like group 14-oxygen p-bond
(R2MQO 2 R2M+–Oꢀ) and hence their tendency to
polymerise. In spite of remarkable achievement in the
synthesis of germanones as reactive intermediates,1,5 they
could not as yet be characterised structurally.
Treatment of the N-heterocyclic germylene 111 with an
equimolar amount of 1,3,4,5-tetramethylimidazol-2-ylidene
and 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene,12 respectively,
in toluene at ꢀ30 1C, leads to an immediate colour change
from brown red to yellow and quantitative formation of the
corresponding germylene–NHC complexes 2a and 2b as
1
indicated by H NMR spectroscopy (Scheme 2). Subsequent
work-up of the reaction solution afforded the crystalline
products 2a and 2b in 85% and 86% yield, respectively. The
compositions of 2a and 2b were proven by elemental analysis,
and multinuclear NMR spectroscopy (see ESIw). Their
molecular structures were confirmed by single-crystal X-ray
diffraction analyses (Fig. 1).13
Compound 2a crystallises in the monoclinic space group
ꢀ
P21/n, while crystals of 2b are triclinic (space group P1) with
one molecule toluene in the asymmetric unit. In both
structures, the pyramidal coordination geometry of the
germanium atom comprises two nitrogen atoms from the
C3N2 backbone and the carbon(II) atom of the NHC ligand.
The sum of bond angles around the GeII centre is found to be
288.01 (2a) and 293.51 (2b), respectively, which implies that the
vertex of the germanium atom is occupied by a lone pair of
electrons with pronounced 4s-character. The six-membered
C3N2Ge rings in both cases remain almost planar with
significantly longer Ge–N distances, ranging from 1.912(2) to
1.943(2) A, in comparison to those in 1 (1.865 (2) and 1.866(2) A).11
Additionally, the five-membered C3N2 ring of the NHC
moieties is planar. As expected, the dative Ge–C30 (carbene)
Recently, we have been interested in synthesising isolable
compounds with a double bond between group 14 and 16
elements by taking advantage of thermodynamic and/or
kinetic stabilisation. Accordingly, we succeeded in the pre-
paration and structural characterisation of a remarkable
robust silaformamide–borane complex with a donor–acceptor-
supported silicon-oxygen double bond,6 starting from a
ylide-like silylene.7 In addition, we described the isolation of
a silanoic silylester III (Scheme 1)8 containing a resonance
stabilised SiQO p-bond by oxygenation of a siloxysilylene.6
Furthermore, we reported the isolation of the NHC
(N-heterocyclic carbene)-supported silanone IV (Scheme 1)
by facile oxygenation of a NHC–silylene adduct.9 Remark-
ably, the donor ability of NHC ligands not only provides a
facile method for the stabilisation of low-coordinate main
group atoms including germylenes but also increases the
Technische Universitat Berlin, Institute of Chemistry: Metalorganics
¨
and Inorganic Materials, Sekr. C2, Strasse des 17. Juni 135,
D-10623 Berlin, Germany. E-mail: matthias.driess@tu-berlin.de;
Fax: +49 (0)30-314-29732; Tel: +49 (0)30-314-29731
w Electronic supplementary information (ESI) available: Synthesis,
characterisation and crystallographic data of 2a, 2b, 3a, and 3b.
CCDC 731869–731872. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b914060c
Scheme 1 Germanechalcogenones I, donor-supported germanechalco-
genones II, intramolecular-N-donor-stabilised silanoic silylester III,
and donor-supported silanone IV.
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directly from the reaction mixture (Scheme 2). The NHC–
germanone adducts 3a and 3b are marginally soluble in
hydrocarbons and diethyl ether but well soluble in dichloro-
methane. They were fully characterised by elemental analysis,
EI mass spectrometry, IR, 1H, and 13C NMR spectroscopy as
well as single-crystal X-ray diffraction analyses (Fig. 2, see also
ESIw).13
3a crystallises in the monoclinic space group P21/c with one
molecule of diethyl ether and one molecule of dichloro-
methane in the asymmetric unit. In contrast, crystals of 3b
were solvent-free with two independent molecules in the
asymmetric unit. One of the molecular structures of 3b is
depicted. In spite of the different N-organo groups in the NHC
ligands, the structures of 3a and 3b reveal similar geometry,
which is reminiscent of the situation observed for the silicon
analogue IV. The germanium centres adopt a strongly distorted
tetrahedral coordination, featuring a terminal oxygen atom.
In addition, the six-membered C3N2Ge rings in these structures
are significantly puckered, while the five-membered NHC ring
remains planar. Remarkably, the Ge–O distances (1.672(3) A
for 3a, 1.664(2) and 1.670(2) A for 3b) are distinctly shorter
(around 7%) than a typical Ge–O single bond in germoxanes
(1.75 to 1.85 A)15a as well as the germanium oxygen
distance observed in the GeQO moiety of (EDTA)GeQO
(1.77 (1) A, EDTA = ethylenediaminetetraacetate) previously
Scheme 2 Synthesis of 2a and 2b and 3a and 3b from germylene 1.
Fig. 1 Molecular structure of 2a and 2b. Thermal ellipsoids are
drawn at 50% probability level. H atoms are omitted for clarity.
Selected distances (A) and angles (1) for 2a: Ge1–N1 1.927(3), Ge1–N2
1.929(3), Ge1–C30 2.149(3), N1–C2 1.385(4), N2–C4 1.407(4),
N3–C30 1.345(4), N3–C32 1.386(5), N4–C30 1.355(4), N4–C31
1.383(4), C2–C3 1.430(5), C3–C4 1.359(5), N1–Ge1–N2 95.1(1),
N1–Ge1–C30 95.5(1), N2–Ge1–C30 97.4(1), N3–C30–N4 104.3(3).
Selected distances (A) and angles (1) for 2b: Ge1–N2 1.912(2),
Ge1–N1 1.943(2), Ge1–C30 2.192(3), N1–C2 1.387(3), N2–C4
1.399(3), N3–C30 1.357(3), N3–C31 1.398(3), N4–C30 1.353(3),
N4–C32 1.391(3), C2–C3 1.459(4), C3–C4 1.350(4), N2–Ge1–N1
95.35(9),
N2–Ge1–C30
103.29(9),
N1–Ge1–C30
94.85(9),
Fig. 2 Molecular structure of 3a and 3b. Thermal ellipsoids are
drawn at 50% probability level. H atoms are omitted for clarity.
Selected distances (A) and angles (1) for 3a: Ge1–O1 1.672(3), Ge1–N2
1.854(4), Ge1–N1 1.859(3), Ge1–C30 2.020(4), N1–C2 1.401(6),
N2–C4 1.395(6), N3–C30 1.347(5), N4–C30 1.346(5), C2–C3
1.417(7), C3–C4 1.380(7), O1–Ge1–N2 116.1(2), O1–Ge1–N1
117.3(2), N2–Ge1–N1 100.5(2), O1–Ge1–C30 109.7(2), N2–Ge1–C30
104.7(2), N1–Ge1–C30 107.4(2), N4–C30–N3 104.7(3). Selected
distances (A) and angles (1) for 3b (only one of the two independent
molecules in the asymmetric unit is shown): mol. 1: Ge1–O1 1.670(2),
Ge1–N2 1.852(2), Ge1–N1 1.871(2), Ge1–C30 2.040(2), N1–C2
1.397(3), N2–C4 1.409(3), O1–Ge1–N2 115.25(8), O1–Ge1–N1
N4–C30–N3 105.4(2).
bonds are with 2.147(3) A (2a) and 2.192(3) A (2b),
respectively, significantly longer than a common Ge–C single
bond in related organogermanes (1.96 to 2.04 A).14
The compounds 2a and 2b are well soluble in hydrocarbons,
ethereal solvents and remain unchanged both in the solid state
and in solution in an oxygen- and moisture-free atmosphere.
The apparently larger sensitivity of the latter toward
oxygenating reagents in comparison to that of 1 is due to
the higher nucleophilicity of the GeII atom by the dative
NHC - Ge: interaction. In contrast to 1, which is resistant
towards N2O at ambient temperature and 1 atm pressure,
exposure of yellow solutions of 2a and 2b in toluene to N2O
at room temperature leads to the formation of the
NHC–germanone adducts 3a and 3b, which precipitate in
the form of colourless crystals in high yields (3a 77%; 3b 84%,)
118.35(8), N2–Ge1–N1 100.47(9), O1–Ge1–C30
108.48(9),
N2–Ge1–C30 111.08(9), N1–Ge1–C30 102.36(9), N3–C30–N4
106.1(2). Mol. 2: Ge1–O1 1.664(2), Ge1–N2 1.853(2), Ge1–N1
1.866(2), Ge1–C30 2.053(3), N1–C2 1.400(3), N2–C4 1.415(3),
O1–Ge1–N2 113.88(9), O1–Ge1–N1 118.83(9), N2–Ge1–N1
100.54(9),
O1–Ge1–C30
109.0(1),
N2–Ge1–C30
110.3(1),
N1–Ge1–C30 103.5(1), N3–C30–N4 105.9(2).
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Chem. Commun., 2009, 6466–6468 | 6467

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Scheme 3 Resonance structures 3, 30 and 300 (Ar = 2,6-iPr2C6H3,
R = Me, iPr).
characterised by Seifullina and co-workers,15b suggesting
significant GeQO double bond character in the compounds.
Interestingly, although the coordination number on the
germanium centre in 3a and 3b increases from three in 2a
and 2b to four, the Ge–N distances, ranging from 1.852(2) to
1.871(2) A, are shorter than those found in 2a and 2b (1.912(2)
to 1.943(2) A). The same is true for the Ge–C(carbene)
distances (2.020(4) A for 3a, 2.040(2) and 2.053(3) A for 3b),
which are shorter than those of the corresponding values in 2a
(2.149(3) A) and 2b (2.192(3) A). Apparently, the shortening
of the Ge–N and Ge–C distances in 3a and 3b is indicative of
the importance of betaine-like resonance forms 30 and 300 for
the stabilisation of the GeQO double bond (Scheme 3).
Furthermore, due to the coordination of the NHC to the
germanium atom in 3a and 3b, the hypothetical N2GeQO
moieties lose planarity, affording a strongly distorted tetra-
hedral coordination geometry around germanium. Notably, a
similar situation has been observed for intramolecular donor-
stabilised germanethiones3 and silanones, respectively.6,8,9
In conclusion, we have reported the facile synthesis of the
novel NHC–germylene adducts 2a and 2b with highly nucleo-
philic GeII atoms. The latter adducts react readily with N2O at
ambient temperature to give the isolable germanone–NHC
complexes 3a and 3b.
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¨
8 S. Yao, Y. Xiong, M. Brym and M. Driess, J. Am. Chem. Soc.,
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9 Y. Xiong, S. Yao and M. Driess, J. Am. Chem. Soc., 2009, 131,
7562.
10 For examples, see: (a) Y. Wang, Y. Xie, Y. Wei, R. B. King,
H. F. Schaefer III, P. von R. Schleyer and G. H. Robinson,
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P. J. Ragogna and K. M. Baines, J. Am. Chem. Soc., 2007, 129,
15138; (c) P. A. Rupar, M. C. Jennings, P. J. Ragogna and
K. M. Baines, Organometallics, 2007, 26, 4109; (d) P. A. Rupar,
V. N. Staroverov, P. J. Ragogna and K. M. Baines, J. Am. Chem.
Soc., 2007, 129, 15138.
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12 N. Kuhn and T. Kratz, Synthesis, 1993, 561.
¨
13 Crystal data for 2a: C36H52GeN4, M = 613.41, monoclinic, space
group P21/n, Z = 4, a = 12.5971(6), b = 16.7102(6), c =
17.2809(6) A, b = 105.778(4)1. V = 3500.6(2) A3, T = 150 K,
15 170 reflections collected, 6140 independent (Rint = 0.0656)
which were used in all calculations. The final wR(F2) was 0.0871
(all data). Crystal data for 2b: C47H68GeN4, M = 761.64, triclinic,
Financial support from the Cluster of Excellence ‘‘Unifying
Concepts in Catalysis’’ (EXC 314/1) funded by the Deutsche
Forschungsgemeinschaft and administered by the Technische
ꢀ
space group P1, Z = 2, a = 11.1525(4), b = 14.2463(5), c =
Universitat Berlin is gratefully acknowledged. NHC =
¨
N-heterocyclic carbenes.
14.3356(6) A, a = 74.43&(3)1, b = 80.643(3)1, g = 87.108(3)1.
V = 2164.9(2) A3, T = 150 K, 16 315 reflections collected, 7585
independent (Rint = 0.0464) which were used in all calculations.
The final wR(F2) was 0.0760 (all data). Crystal data for 3a:
C41H64Cl2GeN4O2, M = 788.45, monoclinic, space group P21/c,
Z = 4, a = 11.6635(3), b = 24.5936(7), c = 15.6283(3) A, b =
102.959(2)1. V = 4368.8(2) A3, T = 150 K, 18 717 reflections
collected, 7646 independent (Rint = 0.0321) which were used in all
calculations. The final wR(F2) was 0.1693 (all data). Crystal data
for 3b: C40H60GeN4O, M = 685.51, monoclinic, space group
P21/c, Z = 8, a = 21.2684(5), b = 16.9410(4), c = 21.6339(5) A,
b = 94.333(2)1. V = 7772.6(3) A3, T = 150 K, 71 416 reflections
collected, 13 672 independent (Rint = 0.0693) which were used in
all calculations. The final wR(F2) was 0.0757 (all data). Crystals of
2a, 2b, 3a, and 3b were covered with a perfluorinated oil prior to
mounting on the goniometer in a cold N2 flow and measured on an
Oxford Diffraction Xcalibur S Sapphire at 150 K (Mo-Ka radiation,
l = 0.71073 A).
Notes and references
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(f) N. Tokitoh and R. Okazaki, Adv. Organomet. Chem., 2001, 47,
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Chalcogen Chemistry, ed. F. A. Devillanova, The Royal Society of
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2 P. Arya, J. Boyer, F. Carre0, R. Corriu, G. Lanneau, J. Lapasset,
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3 For selected recent examples of stable germanium–chalcogen
´
doubly bonded compounds, see: (a) Y. Ding, Q. Ma, I. Uson,
H. W. Roesky, M. Noltemeyer and H.-G. Schmidt, J. Am. Chem.
14 P. Jutzi, S. Keitemeyer, B. Neumann and H.-G. Stammler,
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15 (a) K. M. Baines and W. G. Stibbs, Coord. Chem. Rev., 1995, 145,
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This journal is The Royal Society of Chemistry 2009
6468 | Chem. Commun., 2009, 6466–6468