A heterofulvene-like germylene with a betain reactivity

Article abstract of DOI:10.1002/anie.200600237

An electronic chameleon: The planar cyclogermylene 1 can be described by the resonance structures 1A and 1B. It is accessible in 79% yield by dehydrohalogenation of the corresponding (β-diketiminato)chlorogermylene by LiN(SiMe₃)₂. Al

Full text of DOI:10.1002/anie.200600237

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Main-Group-Element Compounds
DOI: 10.1002/anie.200600237
A Heterofulvene-Like Germylene with a Betain
Reactivity
Matthias Driess,* Shenglai Yao, Markus Brym, and
Christoph van Wüllen
Dedicated to Professor Werner Kutzelnigg
Since the pioneering work on the synthesis of isolable
germylenes (R2Ge) reported by Lappert et al.,[1] remarkable
progress has been made in tuning the reactivity of the low-
coordinate Ge atom in germylenes through the presence of
bulky and/or p-donor substituents R.[2] Sterically demanding
amido or alkoxo substituents at the two-coordinate Ge atom
lead to derivatives which can serve as ligands with different
donor–acceptor abilities in coordination compounds.[3]
Among the amido-substituted germylenes, the N-heterocyclic
germylenes A,[4] B,[5] and C[6] deserve particular attention
Scheme 1. Synthesis of 2 and 3, starting from the cyclic chloro-
germylene 1.
Herein, we report the synthesis of the new N-heterocyclic
germylene 3 with an unexpected molecular structure and
reactivity. Although, the desired germylene 3 is accessible by
deprotonation of the germylene cation 2 by using strong bases
(lithium diisopropylamide or LiN(SiMe3)2), it can only be
isolated in low yield (< 10%). Fortunately, compound 3 is
simply accessible in 79% yield from the direct reaction of
chlorogermylene 1 with LiN(SiMe3)2 as a base in the molar
ratio of 1:1.
The formation of 3 from 1 is mainly due to the steric
congestion around the germanium atom which favors the
base-assisted dehydrochlorination via monodeprotonation of
a methyl group on the backbone of the b-diketiminato ligand
instead of a nucleophilic Cl/N(SiMe3)2 substitution at germa-
nium. Such a Cl substitution occurs when the analogous
chlorogermylene with less-bulky phenyl substituents at the
nitrogen atoms (R = Ph) is used, and leads solely to the
corresponding N(SiMe3)2-substituted germylene.[6b] A similar
deprotonation reaction at the backbone has been observed
for the tautomerization of a iminogermane bearing the b-
diketiminato ligand,[7b] for a cyclodiazaborane analogue[11]
and for related calcium complexes.[12] The brown-red crystals
of 3 have been fully characterized by EI-MS, combustion
analysis, NMR spectroscopy (see Supporting Information),
and X-ray diffraction analysis. The crystal structure revealed
that the six-membered GeN2C3 ring is essentially planar
(Figure 1).[13]
because they are versatile building blocks for the synthesis of
germanium compounds with remarkable electronic features.
Recent highlights include the synthesis of compounds with
À
=
resonance-stabilized X Ge Y moieties (X = F, Cl, OH; Y=
S,[7] Se,[8] NR[9]) and the formation of the N-heterocyclic
germylenium cation 2,[10] starting from the sterically encum-
bering b-diketiminato-substituted chlorogermylene
1
(Scheme 1).[6c]
The strikingly simple access to the planar GeN2C3 cation 2
from 1 and B(C6F5)3 even in the presence of water suggests
that p-electron resonance stabilization occurs.[10] The ready
formation of the GeN2C3 cation 2 prompted us to investigate
whether its p-resonance stabilization and planarity is retained
even after monodeprotonation of a ring methyl group, and
lead to the unprecedented cyclogermylene 3 which may exist
as a dipolar resonance hybrid of 3A and 3B (Scheme 1).
À
Interestingly, the Ge N bond lengths in 3, which are
almost identical, are even slightly shorter (by 3 pm) than
À
those in the germylene cation 2 whereas the endocyclic C N
À
À
bonds (N1 C2 and N2 C4) are significantly longer (by 5 pm).
À
At the same time, the endocyclic C C bond lengths of
140.2(3) (C2 C3) and 139.2(3) pm (C3 C4), and also the
exocyclic C2 C1 double bond (141.3(3) pm) indicate conju-
gated double bonds. This situation suggests a larger contri-
bution of the resonance form 3B (Scheme 1) to the electronic
ground state of 3. Additionally, the X-ray structure of 3 shows
À
À
À
[*] Prof. Dr. M. Driess, Dr. S. Yao, Dr. M. Brym, Prof. Dr. C. van Wüllen
Institute of Chemistry: Metalorganics and Inorganic Materials
Technische Universität Berlin
Strasse des 17. Juni 135, Sekr. C2, 10623 Berlin (Germany)
Fax : (+49)30-314-22168
E-mail: matthias.driess@tu-berlin.de
À
that the terminal C4 C5 bond is unusual short (145.0(3) pm),
this may result from disordering and/or packing effects which
À
À
lead to shorter C4 C5 and longer C1 C2 bonds. The presence
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
À
of alternating C C bond lengths is supported by density
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perpendicular to the planes of the six-membered rings. This
ring current affords a shielding of the ring center from the
external magnetic field, and can be calculated by quantum
chemical methods as the NICS (nucleus-independent chem-
ical shift) value.[15a–d]
Cycles with negative NICS values are qualified by
definition as aromatic. For the prototypical 6 p aromatic
molecule, benzene, we calculate an isotropic NICS value of
À10.0 ppm at the ring center (NICS(0)) which increases to
À11.3 ppm 100 pm above this location (NICS(1)) in good
agreement with results from calculations with smaller basis
sets.[15] NICS values above the ring center offer a more robust
criterion for aromaticity especially for inorganic ring systems
since in these systems the s-bond contributions are much
smaller.[15b] The optimized geometries of 2’ and 3’ reveal
different NICS(0) and NICS(1) values of À0.2 and À2.4 ppm,
respectively, for 2’ and + 3.0 and + 1.3 ppm, respectively, for
3’. This data supports the assumption that some aromatic
character can be ascribed to 2’ but this is no longer the case for
3’. Clearly, the cyclic delocalization of the p electrons in 3A’ is
disfavored due to charge separation.
Figure 1. Molecular structure of 3. The hydrogen atoms, except those
at C1, have been omitted for clarity. Selected bond lengths [pm] and
angles [8]: Ge1–N1 186.58(17), Ge1–N2 186.50(18), N1–C2 140.4(3),
N1–C6 144.4(3), C1–C2 141.3(3), C2–C3 140.2(3), C3–C4 139.2(3),
C4–C5 145.0(3), N2–C4 140.0(3), N2–C18 144.8(3); N1-Ge1-N2
95.84(8), Ge1-N1-C2 127.12(14), C2-C3-C4 129.9(2), N2-C4-C3
120.06(19), C1-C2-C3 120.2(2). Sum of bond angles at N1, N2, C2,
and C4: 3608.
The electronic situation in 3 is reminiscent of that in
fulvenes, especially heptafulvene. For heptafulvene, the
calculated contribution of the zwitterionic aromatic reso-
nance structure is rather small (from 5%[16] to 8%[17]), but this
is enough to influence the properties of the molecule (see
Supporting Information). In fact, 3 reacts like a betain, as
shown by its gentle conversion with Me3SiOTf (OTf =
OSO2CF3) at À208C, leading to the exo-CH2SiMe3 substi-
tuted (trifluoromethylsulfonato)germylene 4 in 85% yield
(Scheme 3). The composition of 4 is consistent with the results
by EI-MS, elemental analysis, and NMR spectroscopy, (see
Supporting Information). An X-ray structure analysis of 4[13]
revealed a nonplanar GeN2C3 array (see Supporting Infor-
mation) with bond lengths and angles similar to those
observed for the analogous cyclic chlorogermylene 1.[6c] The
functional (DFT) calculations on the model compound 3’ in
which the 2,6-diisopropylphenyl groups at nitrogen have been
replaced by 2,6-dimethylphenyl substituents.[14] The calcula-
À
tions revealed alternating endo- and exocyclic C C distances
À
À
À
À
(C1 C2 135 pm, C2 C3 146 pm, C3 C4 135 pm, C4 C5
151 pm) as expected for substituted buta-1,3-diene deriva-
À
tives. Remarkably, the calculations show also that the C C
bond-length equilibration is a shallow mode: only 10–
15 kJmolÀ1 are required to distort the minimum geometry
À
À
of 3’ such that both endocyclic C C bond lengths (C2 C3,
À
À
C3 C4) and the exocyclic C1 C2 bond lengths amount to
140 pm. Additionally, the calculations indicate a preference
for the mesomeric form 3B’ over 3A’ (Scheme 2). This
situation is supported by quantum chemical calculations of
the magnetic properties of 3’ (described by 3A’ and 3B’) in
comparison with those of the analogous model cation 2’
(described by the resonance forms 2A’ and 2B’; Scheme 2). A
cyclic delocalization of the 6 p electrons, as indicated by the
resonance forms 3A’ and 2A’, should lead to a ring current if
the molecules are put into an external magnetic field
Scheme 2. Resonance structures of 2’ and 3’.
Scheme 3. Synthesis of 4 and 5.
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unexpected bis(bromogermylene) 5 also results from the
unusual betain-like reactivity of 3 which reacts with 1,2-
dibromoethane in hexane at room temperature (Scheme 3).
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À
Compound 5 is the C C homocoupling product of two
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1989, 122, 433 – 436.
germylene units under concomitant addition of bromine to
each germanium atom. The yellow crystals of 5 can be isolated
in 71% yield and consist of a mixture of diastereoisomers
1
(racemic RR, SS, and meso-RS) as demonstrated by H and
13C NMR spectroscopy (see Supporting Information).
Apparently, in this reaction, 1,2-dibromoethane serves as
a chemical equivalent of elemental bromine under release of
ethene (1H NMR spectroscopic monitoring). In line with this
idea, compound 5 is also accessible by the reaction of 3 with
Br2 in the molar ratio of 1:1 but the yield is much lower (ca.
30%) and separation from side-products is difficult.
A X-ray diffraction analysis[13] shows that the crystals of 5
are the meso form (RS) with a center of symmetry and two
À
nonplanar GeN2C3 rings coupled by a C C bond (Figure 2).
The bond lengths and angles of the GeN2C3 skeleton in 5 are
À
similar to those of 1 and the C1 C1’ distance indicates the
[4]a) M. Veith, M. Grosser, Z. Naturforsch. B 1982, 37, 1375;
b) Review: M. Veith, Angew. Chem. 1987, 99, 1; Angew. Chem.
Int. Ed. Engl. 1987, 26, 1.
3
3
À
presence of an undisturbed C(sp ) C(sp ) single bond.
[5]a) A. Meller, C.-P. Gräbe, Chem. Ber. 1985, 118, 2020; b) J.
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1989, 122, 245; c) W. A. Herrmann, M. Denk, J. Behm, W.
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Chem. 1992, 104, 1489; Angew. Chem. Int. Ed. Engl. 1992, 31,
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met. Chem. 1994, 474, 73; e) A. Schäfer, W. Saak, M. Weiden-
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[6]a) A. E. Ayers, T. Klapötke, H. V. R. Dias, Inorg. Chem. 2001,
40, 1000; b) A. Akkari, J. J. Byrne, I. Saur, G. Rima, H.
Gornitzka, J. Barreau, J. Organomet. Chem. 2001, 622, 190;
c) Y. Ding, H. Hao, H. W. Roesky, M. Noltemeyer, H.-G.
Schmidt, Organometallics 2001, 20, 4806.
[7]a) L. W. Pineda, V. Jancik, H. W. Roesky, R. Herbst-Irmer,
Angew. Chem. 2004, 116, 5650 – 5652; Angew. Chem. Int. Ed.
2004, 43, 5534 – 5536; b) Y. Ding, Q. Ma, H. W. Roesky, R.
Herbst-Irmer, I. Uson, M. Noltemeyer, H.-G. Schmidt, Organo-
metallics 2002, 21, 5216 – 5220.
Figure 2. Molecular structure of 5. H atoms have been omitted for
clarity. Selected bond lengths [pm] and angles [8]: Ge1–Br1 247.16(5),
Ge1–N1 200.1(2), Ge1–N2 198.1(3), N1–C2 132.8(4), N1–C6 145.9(4),
C1–C2 152.4(4), C1–C1’ 152.5(6), C2–C3 138.2(5), C3–C4 141.0(4),
C4–C5 149.9(5), N2–C4 132.8(4), N2–C18 145.6(4); N1-Ge1-Br1
93.68(7), N2-Ge1-Br1 95.35(7), N1-Ge-N2 91.17(10), C2-N1-Ge1
122.9(2), C3-C2-N1 124.1(3), C4-C3-C2 127.6(3), N2-C4-C3 121.7(3),
N1-C2-C1 120.2(3), C1-C2-C3 115.4(3), C5-C4-C3 117.2(3), C5-C4-N2
121.0(3), C2-C1-C1’ 109.9(3). Sum of bond angles at Ge1: 280.38.
[8]a) Y. Ding, Q. Ma, H. W. Roesky, I. Uson, M. Noltemeyer, H.-G.
Schmidt, Dalton Trans. 2003, 1094 – 1098; b) I. Saur, G. Rima, H.
Gornitzka, K. Miqueu, J. Barrau, Organometallics 2003, 22,
1106 – 1109.
In summary, we have shown that the N-heterocyclic
germylene 3 is simply accessible from dehydrochlorination of
1 and has a planar GeN2C3 arrangement with a dipolar
resonance structure analogous to the fulvenes. Although the
less-polar resonance form 3B is favored in the ground state,
compound 3 shows a betain-like reactivity because of the
contribution of dipolar resonance structure 3A. Further
investigations are currently underway to use 3 as a dipolar
donor–acceptor ligand for the stabilization of s,p-sandwich
complexes of transition metals in unusual oxidation states.
[9]a) Y. Ding, Q. Ma, H. W. Roesky, R. Herbst-Irmer, I. Uson, M.
Noltemeyer, H.-G. Schmidt, Organometallics 2002, 21, 5216 –
5220; b) Y. Ding, H. Hao, H. W. Roesky, M. Noltemeyer, H.-
G. Schmidt, Organometallics 2001, 20, 4806 – 4811.
[10]M. Stender, A. D. Phillips, P. P. Power, Inorg. Chem. 2001, 40,
5314 – 5315.
[11]B. Qian, S. W. Baek, M. R. Smith III, Polyhedron 1999, 18, 2405.
[12]S. Harder, Angew. Chem. 2003, 115, 3553 – 3556; Angew. Chem.
Int. Ed. 2003, 42, 3430 – 3434.
[13] 3: monoclinic, space group P21/n, a = 12.5801(3), b = 16.2194(4),
c = 13.8696(4) Š, b = 105.3620(10)8, V= 2728.86(12) Š3, Z = 4,
1calcd = 1.191 MgmÀ3
, , 25012 collected
m(MoKa) = 1.14 mmÀ1
reflections, 8320 crystallographically independent reflections
(Rint = 0.0523), 5282 reflections with I > 2s(I), qmax = 30.538,
R(Fo) = 0.0469 (I > 2s(I)), wR(F2o) = 0.0901 (all data), 298
refined parameters. 4: monoclinic, space group P21/n, a =
11.5855(4), b = 23.3854(7), c = 13.4704(4) Š, b = 96.0450(10)8,
Received: January 19, 2006
Published online: June 7, 2006
V= 3629.3(2) Š3, Z = 4, 1calcd = 1.302 MgmÀ3
,
m(MoKa) =
Keywords: betains · C–C coupling · fulvenes · germylenes ·
.
heteroarenes
0.98 mmÀ1, 34090 collected reflections, 11087 crystallographi-
Angew. Chem. Int. Ed. 2006, 45, 4349 –4352
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4351

Communications
cally independent reflections (Rint = 0.0977), 7812 reflections
with I > 2s(I), qmax = 30.568, R(Fo) = 0.0473 (I > 2s(I)), wR(F2o) =
0.0765 (all data), 417 refined parameters. 5: monoclinic, space
group P21/n, a = 9.6029(3), b = 13.4481(5), c = 22.5907(7) Š, b =
99.7560(10)8, V= 2875.19(17) Š3, Z = 4, 1calcd = 1.315 MgmÀ3
,
m(MoKa) = 2.47 mmÀ1, 26898 collected reflections, 8821 crystal-
lographically independent reflections (Rint = 0.1391), 4961
reflections with I > 2s(I), qmax = 30.608, R(Fo) = 0.0549 (I >
2s(I)), wR(F2o) = 0.1180 (all data), 307 refined parameters.
Crystals were each mounted on a glass capillary in perfluori-
nated oil and measured in a cold N2 flow. The data of 3, 4, and 5
were collected on a Bruker-AXS SMART CCD diffractometer
at 173(2) K using MoKa radiation (l = 0.71073 Š). The structures
were solved by direct methods and were refined on F2 with the
SHELX-97[23] software package. The positions of the H atoms
were calculated and considered isotropically according to a
riding model. Absorption corrections were done using the
SADABS program.[24] CCDC-294818 (3), CCDC-294819 (4),
and CCDC-294820 (5) contain the supplementary crystallo-
graphic 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.
[14]Geometry optimizations have been performed with the Gaus-
sian03 program[18] at density functional level using the B3LYP[20]
hybrid exchange-correlation functional and polarized valence
triple zeta (TZVP)[19] basis sets. Nucleus-independent chemical
shifts (NICS)[15] have been calculated by the (Hartree-Fock)
IGLO[21] method with its integral-direct implementation.[22]
NICS values have been obtained at the ring centers (NICS(0)
value) as well as 100 pm above (NICS(1) value).
[15]a) P. von R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N. J. R.
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J. Org. Chem. 1997, 62, 2026.
[17]H. Möllerstedt, M. C. Piqueras, R. Crespo, H. Ottoson, J. Am.
Chem. Soc. 2004, 126, 13938.
[18]Gaussian03, RevisionC.02, M. J. Frisch et al. See Supporting
Information.
[19]A. Schäfer, C. Huber, R. Ahlrichs, J. Chem. Phys. 1994, 100,
5829.
[20]a) P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J.
Phys. Chem. 1994, 98, 11623; b) A. D. Becke, J. Chem. Phys.
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Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785.
[21]W. Kutzelnigg, U. Fleischer, M. Schindler in NMR Basic
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[22]U. Meier, C. van Wüllen, M. Schindler, J. Comput. Chem. 1992,
13, 551.
[23]G. M. Sheldrick, SHELX-97, Program for Crystal Structure
Determination, University of Göttingen (Germany), 1997.
[24]G. M. Sheldrick, SADABS, Program for Empirical Absorption
Correction of Area Detector Data, University of Göttingen
(Germany), 1996.
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