A cyclic germadicarbene ( germylone ) from germyliumylidene

Article abstract of DOI:10.1021/ja402477w

By employing the chelate dicarbene 1, the new chlorogermyliumylidene complex 2 could be synthesized and isolated in 95% yield. Dechlorination of 2 with sodium naphthalenide furnishes the unique cyclic germadicarbene 3 which could be isolated in 45% yield. Compound 3 is the first isolable Ge(0) complex with a single germanium atom stabilized by a dicarbene. Its molecular structure is in accordance with DFT calculations which underline the peculiar electronic structure of 3 with two lone pairs of electrons at the Ge atom.

Full text of DOI:10.1021/ja402477w

Communication
pubs.acs.org/JACS
A Cyclic Germadicarbene (“Germylone”) from Germyliumylidene
Yun Xiong, Shenglai Yao, Gengwen Tan, Shigeyoshi Inoue, and Matthias Driess*
Department of Chemistry: Metalorganics and Inorganic Materials, Technische Universitat Berlin, Strasse des 17. Juni 135, Sekr. C2,
̈
D-10623 Berlin, Germany
S
* Supporting Information
calculations reported by the research groups of Frenking3a−g
and Apeloig.3h Accordingly, the isolated and structurally
ABSTRACT: By employing the chelate dicarbene 1, the
new chlorogermyliumylidene complex 2 could be synthe-
sized and isolated in 95% yield. Dechlorination of 2 with
characterized bent allenes reported by the groups of Bertrand4a
and Regitz,4b as well as the heavier Group 14 analogues
sodium naphthalenide furnishes the unique cyclic
germadicarbene 3 which could be isolated in 45% yield.
Compound 3 is the first isolable Ge(0) complex with a
single germanium atom stabilized by a dicarbene. Its
molecular structure is in accordance with DFT calculations
which underline the peculiar electronic structure of 3 with
two lone pairs of electrons at the Ge atom.
(trisilaallene, trigermaallene, and 1,3-digermasilaallene) pub-
lished by the groups of Kira5 and Wiberg (tristannaallene),6
should rather be interpreted as ylidones (Scheme 1, B), since
these compounds have bent structures with nonlinear E−E−E
bond angles (E = C − Sn) ranging from 122 to 156°, and the
four substituents at the terminal E atoms deviate from
orthogonal arrangement as in genuine allenes.3−6
In order to synthesize isolable ylidones, carbenes have
proven to serve as suitable donors, although even phosphanes7
and heavier carbene analogues have been employed successfully
as mentioned above. Very recently, Roesky and Frenking et al.
reported the first silylone (L:)2Si D (Scheme 1),8a bearing the
cyclic alkyl amino carbene (cAAC) L:. Compound D results
from dechlorination of the precursor C with KC8 (Scheme 1).8b
The latter investigations revealed that, in contrast to the
remarkable biradical electronic structure of precursor C, the
siladicarbene D is a silylone instead of a Si(II) silylene.
However, D has a closed-shell singlet ground state with a non-
negligible contribution from the singly excited state owing to
the low electronic excitation energy (small HOMO−LUMO
gap). In other words, D has a biradicaloid character. The two
carbene ligands L: in D coordinate to the Si atom in a much
more acute C−Si−C bond angle of 117° compared with those
observed for B (122−156°). The slightly shorter Si−C bond
distance of 1.841(2) Å in D than the calculated value for
(NHC)2Si (1.869 Å)3e−g exhibits the better π acceptor
properties of the cAAC ligand than NHC,8a which is reflected
by its frontier orbitals: its HOMO is a π-type orbital which has
the largest extension at Si but exhibits significant Si−C π
bonding. NBO analysis of the siladicarbene D gives one σ lone-
pair orbital and a three-center C−Si−C π orbital with 40% at Si
and 30% at each C atom as shown in Scheme 1.8a
ecently, the use of N-heterocyclic carbenes (NHCs) as
Rligands to stabilize Group 14 elements in the zero
oxidation state has attracted significant research interest. For
example, through dechlorination of the NHC-stabilized
compounds NHC→SiCl4 and NHC→GeCl2 (NHC = 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene) the groups of
Robinson1 and Jones2 were able to isolate the fascinating
Si(0) and Ge(0) species L:→SiSi←:L and L:→GeGe←:L
(L = NHC), respectively. In the latter compounds each Si(0)
and Ge(0) atom features one lone pair of electrons. Another
striking new class of compounds with Group 14 atoms
represents the so-called ylidones L:→E←:L (Scheme 1, A),
(L: = donor ligand, E = C − Pb),3 in which the E atom retains
its valence electrons as two lone pairs and the bonding situation
between the two donor ligands L: and the single atom E can be
best described in terms of donor−acceptor interactions; they
are expected to possess unusual reactivity owing to the peculiar
electronic structures in accordance with results from theoretical
Scheme 1. Dicoordinate E(0) Complexes
Taking into account that chelate ligands with strong push−
pull effect4a,9 will lead automatically to acute bending angles, we
planned to synthesize ylidones of type E (Scheme 2). Recently,
we described the coordination properties of the chelate ligand
1,8-bis(tributylphosphazenyl)naphthalene toward Si(II) and
Ge(II) which furnishes the potential chlorosilyliumylidene
precursors F and its Ge analogue G, respectively (Scheme 2).10
However, their reduction reactions with KC8 did not lead to the
desired ylidone H but to reduction of the chelate ligand. This
can be explained by DFT calculations which revealed that the
Received: March 9, 2013
Published: March 21, 2013
© 2013 American Chemical Society
5004
dx.doi.org/10.1021/ja402477w | J. Am. Chem. Soc. 2013, 135, 5004−5007

Journal of the American Chemical Society
Communication
Scheme 2. Attempts To Prepare Ylidones Featuring Chelate
Ligands
Figure 1. Molecular structure of the chlorogermyliumylidene cation in
2. Thermal ellipsoids are drawn at the 50% probability level. H atoms,
counteranion (Cl−), and one acetonitrile lattice solvent molecule are
omitted for clarity. Selected interatomic distances (Å) and angles (°):
Ge1−Cl1 2.310(1), Ge1−C1 2.058(3), Ge1−C2 2.057(3), C1−N1
1.349(4), C1−N2 1.341(4), C2−N3 1.347(4), C2−N4 1.385(5), C1−
Ge1−C2 85.2(1), C1−Ge1−Cl1 91.8(1), C2−Ge1−Cl1 90.5(1),
Ge1−C1−N1 129.1(2), Ge1−C1−N2 126.0(2), N1−C1−N2
104.8(3), Ge1−C2−N3 126.2(2), Ge1−C2−N4 128.8(2), N3−C2−
N4 104.9(3).
LUMOs of F and G are mainly represented by the π system of
the naphthalene moiety.
Based on the formation of F and G and inspired by the
successful preparation of D we explored the reactivity of the
bis-N-heterocyclic carbene (bNHC) 1 (Scheme 3) toward
Scheme 3. Generation of the Chlorogermyliumylidene
Chloride 2, Starting from bNHC 1, and Its Dechlorination to
the Cyclic Germadicarbene (“Germylone”) 3
conformation with N2, N3, C1, and C2 being nearly coplanar,
and Ge1 and C3 displaced away from that plane in the same
direction as the Ge−Cl bonding vector is pointing. The
dihedral angle between the plane defined by C3, N2, N3 and
the plane defined by N2, N3, C2, C1 is 38.7°, whereas the
dihedral angle between the plane defined by Ge1, C1, C2 and
the plane defined by N2, N3, C2, C1 is 18.4°. The Ge1−Cl1
bond in 2 is oriented nearly perpendicular to both Ge−C
bonds, with Cl1−Ge1−C1 and Cl1−Ge1−C2 bond angles of
90.(1)° and 91.8(1)°, respectively, which are smaller than those
in G (average 95.8°). The sum of angles around the carbene
carbon atoms in 2 is 359.90° and 359.88°, respectively,
indicating a nearly ideal trigonal-planar geometry. The Ge1−
Cl1 distance of 2.310(1) Å in 2 is longer than those in G
(2.278(1) Å), in the NHC-GeCl2 adduct (average 2.277 Å13),
and in the related four-coordinate Ge(II) species 1,2,3,4-
tetrakis(di-tert-butylmethylsilyl)-3,4-dichlorotetragermetene
(2.248(1) Å14) and [GeCl(pmdta)][GeCl3] (2.266(2) Å15). In
contrast, the Ge1−C1 (2.058(3) Å) and Ge1−C2 (2.057(3) Å)
bond lengths in 2 are significantly shorter than that in NHC-
GeCl2 (2.112(2) Å),13 indicating a stronger interaction of the
chelate carbene carbon atoms toward the Ge atom in 2 than
that in NHC-GeCl2.
In order to gain more insight into the electronic features,
DFT calculations (B3LYP/6-31G(d) level of theory) have been
performed for the chlorogermyliumylidene cation in 2.Geom-
etry optimization of the latter yielded a structure similar to that
observed experimentally. Its HOMO and LUMO are depicted
in Figure 2. The HOMO is essentially a σ lone-pair orbital at
the Ge(II) center. Strikingly, the LUMO consists mainly of a π-
type orbital at Ge, which is in marked contrast to the situation
of compounds F and G. In addition, a strong interaction
between the chelate carbene carbons and the Ge center in the
germyliumylidene in 2 is supported by the Wiberg bond index
values of the Ge−C bonds (0.608 and 0.611). We expected that
these features may facilitate the two-electron reduction of the
Ge(II) atom of the chlorogermyliumylidene in 2 to give the
desired germylone 3.
GeCl2·dioxane, aiming at synthesizing the first germadicarbene
(“germylone”) E through dehalogenation (Scheme 2). To our
best knowledge, the coordination ability of 1 has merely been
investigated toward a few transition metals such as chromium11
and iron,12 but its utilization to stabilize low-valent p-block
main-group elements has not yet been reported.
Similar to the formation of G, treatment of an equimolar
amount of bNHC 1 with GeCl2·dioxane in THF at room
temperature furnished the desired Ge(II) salt [bNHC-
GeCl]+Cl− 2 as an off-white powder in 95% yield (Scheme
3). Compound 2 is the first germyliumylidene cation supported
by a chelating bis-carbene. In contrast to G, compound 2 is
insoluble in THF but soluble in acetonitrile, exhibiting its
stronger ionicity. The high-resolution ESI-mass spectrum of 2
dissolved in acetonitrile gives in the positive mode a peak at m/
z = 577.21484 (calcd 577.21478) corresponding to the
[bNHC-GeCl]+ cation. In comparison to 1, the donor 13C
carbene nuclei in 2 show an upfield shift (δ = 166 ppm for 2 vs
220 ppm for 1) in the 13C NMR spectrum. Single crystals of 2
suitable for X-ray diffraction analysis were grown in actonitrile
solutions at −20 °C. Similar to G, the Ge(II) atom in 2 is three-
fold coordinated by two carbon atoms from the bis-carbene
ligand 1 and one chlorine atom (Figure 1); the chloride
counterion is located far away from the Ge center (6.53 Å).
The six-membered C3N2Ge ring is puckered in a boat
In fact, the dechlorination of 2 with sodium naphthalenide
leads to the cyclic germadicarbene 3 (Scheme 3). When a cold
5005
dx.doi.org/10.1021/ja402477w | J. Am. Chem. Soc. 2013, 135, 5004−5007

Journal of the American Chemical Society
Communication
the plane defined by C3, N2, N3 and the plane defined by N2,
N3, C1, C2 (46° in 3 vs 38.7° in 2), and a smaller dihedral
angle between the plane defined by Ge1, C1, C2 and the plane
defined by C1, C2, N2, N3 (13° in 3 vs 18.4° in 2). In addition,
the sum of angles around each carbene carbon atom in 3
(359.95° and 359.59°), just as in 2, indicates an almost ideal
trigonal-planar coordination geometry. The Ge−C bond
distances of 1.967(2) and 1.962(2) Å in 3 are significantly
shorter by about 0.10 Å than those in the precursor 2 (2.058(3)
and 2.057(3) Å), and distinctly shorter by about 0.15 Å than
that in NHC-GeCl2 (2.112(2) Å).12 The short Ge−C bond
Figure 2. HOMO (left) and LUMO (right) of the chlorogermylium-
ylidene cation in 2. Hydrogen atoms are omitted for clarity.
2
distances in 3 are comparable to the Ge−Csp single bond in
solution of 2 molar equiv of sodium naphthalenide in THF
(−30 °C) was added to the suspension of 2 in THF (−30 °C),
the reaction mixture turned immediately dark red. Compound
3 could be isolated from toluene solutions of the reaction
mixtures as dark red crystals in 45% yield. The dark red crystals
of 3 dissolved in toluene showed UV−vis absorption bands at λ
= 286, 420, and 564 nm which are comparable with those found
for the bent trigermaallene (trigermaylidone, λ = 280, 380, 435,
496, and 630 nm)5b and D (390 and 584 nm).8a Interestingly,
the chemical shift of the carbene carbons in 3, compared to that
of the precursor 2 (δ = 166 ppm), appears downfield (δ = 196
ppm) in the 13C NMR spectrum, which is relatively close to
that observed for the silylone D (δ = 210.9 ppm).8a
Compound 3 is extremely air and moisture sensitive. It
exhibits also slight temperature sensitivity. Solutions of 3 in
THF are stable for 2 days in a sealed NMR tube at room
temperature and then show traces of “free” bNHC 1 (1H
NMR). Compound 3 crystallizes at −20 °C in toluene
solutions in the monoclinic space group P21/c (Figure 3). X-
Ph3GeC(F)CF2 (1.969(6) Å)16 but slightly longer than the
3
Ge−Csp single bonds in 9,10-dihydro-10-germa-9-sila-
anthracene (1.903−1.913 Å)17 and 2,3,5,6-tetrakis(trimethyl-
germyl)-1,4-benzoquinones (1.941(6)−1.965(7) Å).18
In order to shed light on the electronic nature of compound
3, DFT calculations [B3LYP/6-31G(d)] were performed. The
optimized structure of 3 in the singlet ground state is in good
agreement with the metric data observed by X-ray analysis.
Furthermore, it was found through DFT calculation that the
structure with the first triplet state is by 33.2 kcal mol−1 less
stable than the singlet ground state. The HOMO of 3 consists
of the π-type orbital at the Ge center, including Ge−C π
bonding (Figure 4, left). This π interaction (back-bonding)
Figure 4. HOMO (left) and HOMO−1 (right) of cyclic germaylidone
3. Hydrogen atoms are omitted for clarity.
may explain the relatively short Ge−C bond lengths in 3. On
the other hand, the HOMO−1 represents a σ lone-pair orbital
at Ge (Figure 4, right). In addition, the proton affinities (PA) of
compound 3 were calculated to certificate the germylone
character. The first and second values PA(1) PA(2) were
estimated to be 279.6 and 175.0 kcal mol−1, respectively. It
follows from the relatively large value for PA(2) that compound
3 clearly possesses germylone character. These trends can also
be observed in the acyclic silylone D,8a and similar structural
and electronic features of related species (carbone, silylone,
germylone, etc.) have been theoretically predicted by Frenking
and co-workers.3
In summary, we report here for the first time the bis-carbene-
stabilized chlorogermyliumylidene cation 2 which could be
successfully dehalogenated by sodium naphthalenide to give the
unique isolable cyclic germadicarbene (germylone) 3. The
latter contains a single Ge atom in the formal oxidation state
zero which is coordinated by the bis-carbene chelate ligand 1.
Figure 3. Molecular structure of 3. Thermal ellipsoids are drawn at the
50% probability level. H atoms and toluene lattice solvent molecules
are omitted for clarity. Selected interatomic distances (Å) and angles
(°): Ge1−C1 1.965(3), Ge1−C2 1.961(3), C1−N1 1.382(4), C1−N2
1.378(4), C2−N3 1.377(4), C2−N4 1.375(4), C1−Ge1−C2 86.5(1),
Ge1−C1−N2 128.6(2), Ge1−C1−N1 128.1(2), N1−C1−C2
103.3(1), Ge1−C2−N3 127.7(2), Ge1−C2−N4 129.0(2), N3−C2−
N4 102.9(2).
ray diffraction analysis reveals that the Ge atom in 3 is two-
coordinate with a C−Ge−C angle of 86.6(1)°. This angle is
only slightly smaller than the corresponding value for the
precursor 2 (85.2(1)°), but much more acute than the Ge−
Ge−Ge bond angle of the trigermaylidone of type B (Scheme
1) reported by Kira and co-workers (122.6°).5b In fact,
compound 3 represents the most bent allene or ylidone
hitherto structurally characterized (117−156°).4−6,8 Similar to
2, the six-membered C3N2Ge ring in 3 is also puckered in a
boat conformation, but exhibits a larger dihedral angle between
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details for the synthesis, and spectroscopic and
crystallographic (CIF) data for 2 and 3. This material is
available free of charge via the Internet at http://pubs.acs.org.
5006
dx.doi.org/10.1021/ja402477w | J. Am. Chem. Soc. 2013, 135, 5004−5007

Journal of the American Chemical Society
Communication
(17) McCarthy, W. Z.; Corey, J. Y.; Corey, E. R. Organometallics
1984, 3, 255.
(18) Tsutsui, S.; Sakamoto, K.; Yoshida, H.; Kunai, A. Organo-
AUTHOR INFORMATION
Corresponding Author
matthias.driess@tu-berlin.de
■
metallics 2004, 23, 1554.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Deutsche Forschungsgemeinschaft (DR
14/2) for financial support. S.I. thanks the Alexander von
Humboldt foundation (Sofja Kovalevskaja Program) for
financial support.
REFERENCES
■
(1) Wang, Y.; Xie, Y.; Wie, P.; King, R. B.; Schaefer, H. F.; Schleyer,
P. v. R.; Robinson, G. H. Science 2008, 321, 1069.
(2) Sidiropoulos, A.; Jones, C.; Stasch, A.; Klein, S.; Preking, G.
Angew. Chem., Int. Ed. 2009, 48, 9701.
(3) (a) Tonner, R.; Frenking, G. Angew. Chem., Int. Ed. 2007, 46,
8695. (b) Tonner, R.; Frenking, G. Chem.Eur. J. 2008, 14, 3260.
(c) Tonner, R.; Frenking, G. Chem.Eur. J. 2008, 14, 3273.
(d) Tonner, R.; Frenking, G. Pure Appl. Chem. 2009, 81, 597.
(e) Takagi, N.; Shimizu, T.; Frenking, G. Chem.Eur. J. 2009, 15,
3448. (f) Takagi, N.; Shimizu, T.; Frenking, G. Chem.Eur. J. 2009,
15, 8593. (g) Takagi, N.; Tonner, R.; Frenking, G. Chem.Eur. J.
2012, 18, 1772. (h) Kosa, M.; Karni, M.; Apeloig, Y. J. Chem. Theory
Comput. 2006, 2, 956.
(4) (a) Dyker, C. A.; Lavallo, V.; Donnadieu, B.; Bertrand, G. Angew.
Chem., Int. Ed. 2008, 47, 3206. (b) Hofmann, M. A.; Bergstrasser, U.;
Reiss, G. J.; Nyulaszi, L.; Regitz, M. Angew. Chem., Int. Ed. 2000, 39,
1261. (c) Furstner, A.; Alcarazo, M.; Goddard, R.; Lehmann, C. W.
̈
Angew. Chem., Int. Ed. 2008, 47, 3210. (d) Alcarazo, M.; Lehmann, C.
W.; Anoop, A.; Thiel, W.; Furstner, A. Nat. Chem. 2009, 1, 295.
̈
(5) (a) Ishida, S.; Iwamoto, T.; Kabuto, C.; Kira, M. Nature 2003,
421, 725. (b) Kira, M. Chem. Commun. 2010, 46, 2893. (c) Iwamoto,
T.; Masuda, H.; Kabuto, C.; Kira, M. Organometallics 2005, 24, 197.
(d) Kira, M.; Iwamoto, T.; Ishida, S.; Masuda, H.; Abe, T.; Kabuto, C.
J. Am. Chem. Soc. 2009, 131, 17135. (e) Tanaka, H.; Inoue, S.;
Ichinohe, M.; Driess, M.; Sekiguchi, A. Organometallics 2011, 30, 3475.
(6) Wiberg, N.; Lerner, H.-W.; Vasisht, S.-K.; Wagner, S.;
Karaghiosoff, K.; Noth, H.; Ponikwar, W. Eur. J. Inorg. Chem. 1999,
̈
1211.
(7) Ramirez, F.; Desai, N. B.; Hansen, B.; McKelvie, N. J. Am. Chem.
Soc. 1961, 83, 3539.
(8) (a) Mondal, K. C.; Roesky, H. W.; Schwarzer, M. C.; Frenking,
G.; Niepotter, B.; Wolf, H.; Herbst-Irmer, R.; Stalke, D. Angew. Chem.,
̈
Int. Ed. 2013, 52, 2963. (b) Mondal, K. C.; Roesky, H. W.; Schwarzer,
M. C.; Frenking, G.; Tkach, I.; Wolf, H.; Kratzert, D.; Herbst-Irmer,
R.; Diepotter, B.; Stalke, D. Angew. Chem., Int. Ed. 2013, 52, 1801.
̈
(9) (a) Johnson, R. P. Chem. Rev. 1989, 89, 1111. (b) Christl, M. In
Modern Allene Chemistry; Krause, N., Hashmi, A. S. K., Eds.; Wiley-
VCH: Weinheim, 2004; pp 243−357.
(10) (a) Xiong, Y.; Yao, S.; Inoue, S.; Irran, E.; Driess, M. Angew.
Chem., Int. Ed. 2012, 51, 10074. (b) Xiong, Y.; Yao, S.; Inoue, S.;
Berkefeld, A.; Driess, D. Chem. Commun. 2012, 48, 12198.
(11) Kreisel, K. A.; Yap, G. P. A.; Theopold, K. H. Organometallics
2006, 25, 4670.
(12) (a) Zlatogorsky, S.; Muryn, C. A.; Tuna, F.; Evans, D. J.;
Ingleson, M. J. Organometallics 2011, 30, 4974. (b) Zlatogorsky, S.;
Ingleson, M. J. Dalton Trans. 2012, 41, 2685.
(13) Thimer, K. C.; Al-Rafia, S. M. I; Ferguson, M. J.; McDonald, R.;
Rivard, E. Chem. Commun. 2009, 7119.
(14) Singh, A. P; Roesky, H. W.; Carl, E.; Stalke, D.; Demers, J.-P.;
Lange, A. J. Am. Chem. Soc. 2012, 134, 4998.
(15) Cheng, F.; Dyke, J. D.; Ferrante, F.; Hector, A. L.; Levason, W.;
Reid, G.; Webster, M.; Zhang, W. Dalton Trans. 2010, 39, 847.
(16) Brisdon, A. K.; Crossley, I. R.; Pritchard, R. G.; Warren, J. E.
Inorg. Chem. 2002, 41, 4748.
5007
dx.doi.org/10.1021/ja402477w | J. Am. Chem. Soc. 2013, 135, 5004−5007