Striking reactivity of ylide-like germylene toward terminal alkynes: [4+2] cycloaddition versus C-H bond activation

Article abstract of DOI:10.1039/b811952j

The isolable ylide-like N-heterocyclic germylene LGe: (2) {L = CH[(C=CH₂)CMe][N(aryl)]₂, aryl = 2,6-ⁱPr ₂C₆H₃} shows an unprecedented dual reactivity toward terminal alkynes: its reaction with acetylene leads via [4+2] cycloaddition to the novel intramolecular donor stabilised germylene 3, while conversion of phenylacetylene furnishes the analogous cycloadduct 4 along with a C-H bond activation product, the novel N-donor stabilised alkynyl germylene 5. The Royal Society of Chemistry.

Full text of DOI:10.1039/b811952j

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Striking reactivity of ylide-like germylene toward terminal alkynes:
[4+2] cycloaddition versus C–H bond activationw
¨
Shenglai Yao,a Christoph van Wullenb and Matthias Driess*a
Received (in Cambridge, UK) 14th July 2008, Accepted 5th August 2008
First published as an Advance Article on the web 18th September 2008
DOI: 10.1039/b811952j
The isolable ylide-like N-heterocyclic germylene LGe: (2) {L =
CH[(CQCH2)CMe][N(aryl)]2, aryl = 2,6-iPr2C6H3} shows an
unprecedented dual reactivity toward terminal alkynes: its reac-
tion with acetylene leads via [4+2] cycloaddition to the novel
intramolecular donor stabilised germylene 3, while conversion of
phenylacetylene furnishes the analogous cycloadduct 4 along
with a C–H bond activation product, the novel N-donor stabi-
lised alkynyl germylene 5.
Scheme 1 The mesomeric forms of zwitterionic 1 and 2.
In recent decades, silylenes and germylenes have been the focus
of considerable research due to their carbene-like properties.1–3
They are highly reactive and represent indispensable building
blocks for the synthesis of unique low-coordinated silicon or
germanium compounds with remarkable electronic features.
Since the seminal work by Lappert and co-workers on isolable
divalent Ge, Sn, and Pb carbene-like compounds,3a even the
isolation of silylenes and new types of germylenes stable at room
Scheme 2 The dichotomic reactivity of 1 toward HCRCH.
Exposure of a brown-red solution of 2 in hexane to HCRCH
at room temperature leads to a yellow solution, from which
yellow crystals of the air-sensitive cycloadduct 3 can be isolated
in 81% yield (Scheme 3). The composition and constitution of
compound 3 are proven by elemental analysis, 1H and 13C NMR
spectroscopy (see ESIw). Solid 3 melts at 155 1C and undergoes
slow decomposition without cycloreversion. Its structure has been
established by single-crystal X-ray diffraction analysis (Fig. 1).
Compound 3 represents a new type of intramolecular donor
stablised N-heterocyclic germylene. The structural analysis
revealed that 3 consists of a bicyclo[2,2,2]octane-like core with
a C5N2Ge skeleton and Ge(II) and the g-C atom as bridgehead
atoms (Fig. 1). Thus the entire b-diketiminate ligand has under-
gone modification and the Ge and the g-C atom are now in a
non-planar GeN2C3 six-membered ring. Accordingly, the Ge(II)
centre adopts a pyramidal coordination with the sum of bond
angles of 267.41, similar to the situation in related Ge(II) b-
diketiminate complexes.3,4 The Ge1–N2 distance of 206.1(3) pm
in 3 is significantly longer than that of the Ge1–N1 bond
[198.5(3) pm], implying a stronger donor character for N2.
The latter is consistent with pronounced shortening of the
N2–C4 distance [130.6(4) pm] compared to that of the N1–C2
bond [135.1(4) pm]. The C30–C31 distance of the 1,2-ethylene-
diyl bridge [132.1(5) pm] is in the common range of localised
CQC distances. Moreover, the 1H NMR spectrum of 3 reveals
temperature has been achieved by judicious choice of sterically
demanding substituents on the Si(II) and Ge(II) atoms, which
provide a kinetic and thermodynamic stabilisation with p-donor
groups bonded to the divalent silicon or germanium atom.
Taking advantage of the intramolecular stabilisation of low-
valent silicon and germanium, respectively, by N-donor substi-
tution, a number of isolable N-heterocyclic divalent germanium
and silicon complexes have been synthesised.3–5a–c Very recently,
we developed a facile route to a novel type of donor-supported
zwitterionic NHC-homologues,5d the N-heterocyclic silylene
LSi: 15a and its germanium homologue LGe: 2 (Scheme 1).6
Owing to their unique ylide-like character, silylene 1 and
germylene 2 show remarkably distinct reactivities toward both
electrophiles and nucleophiles in comparison to the behaviour
of other N-heterocyclic silylenes and germylenes, respec-
tively.5–8 Interestingly, the 1,4-dipolar nature of 1 facilitates
a marked preference for C–H insertion versus autocatalytic
[2+1] cycloaddition of the CRC bond of terminal alkynes at
divalent silicon (Scheme 2).8 Herein, we wish to report the
remarkably different reactivity of germylene 2 toward terminal
alkynes, affording the [4+2] cycloaddition products 3 and 4 as
well as the 1,4-insertion (C–H bond activation) product 5,
respectively.
a 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
b Fachbereich fur Chemie, Technische Univesitat Kaiserslautern,
¨
¨
Erwin-Schrodinger-Strasse, D-67663 Kaiserslautern, Germany
¨
w Electronic supplementary information (ESI) available: Synthesis,
characterisation and crystallographic data of 3, 4, and 5, and DFT
calculations of model systems. CCDC reference numbers
695015–695017. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b811952j
Scheme 3 The reaction of 2 with HCRCH and HCRCPh.
Chem. Commun., 2008, 5393–5395 | 5393
ꢀc
This journal is The Royal Society of Chemistry 2008

View Article Online
Fig. 1 The molecular structure of 3. Thermal ellipsoids are drawn at
50% probability level. Hydrogen atoms, except those at C1, C30, and
C31, are omitted for clarity. Selected bond lengths (pm) and angles (1):
Ge1–N1 198.5(3), Ge1–C30 199.1(4), Ge1–N2 206.1(3), N1–C2 135.1(4),
C1–C2 137.3(4), N2–C4 130.6(4), C2–C3 151.1(5), C3–C4 151.3(5),
C3–C31 157.8(5), C30–C31 132.1(5), N1–Ge1–C30 90.0(1), N1–Ge1–N2
89.7(1), C30–Ge1–N2 87.7(1), C2–C3–C4 113.6(3), C2–C3–C31 108.3(3),
C4–C3–C31 107.4(3). The sum of angles at Ge 267.4.
Fig. 2 The molecular structure of 4. Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms, except those at C1 and C30, are
omitted for clarity. Selected bond lengths (pm) and angles (1): Ge1–N1
195.9(1), Ge1–C30 201.6(2), Ge1–N2 210.0(1), N1–C2 137.8(2), C1–C2
134.4(2), N2–C4 129.0(2), C2–C3 152.4(2), C3–C4 151.8(2), C3–C31
154.6(2), C30-C31 132.6(2), C31–C32 148.7(2), N1–Ge1–C30 91.05(6),
N1–Ge1–N2 89.09(5), C30–Ge1–N2 84.82(6), C4–C3–C2 113.6(1),
C4–C3–C31 107.9(1), C2–C3–C31 109.2(1). The sum of angles at Ge 265.0.
4
a doublet of doublets (3JHH = 6.7 Hz, JHH = 1.3 Hz) at
identical to the respective value observed in Mamx-
Ge(CRCPh) (Mamx = methylamino-methyl-m-xylyl).10
The question arises, how are these compounds formed in the
reaction of 2 with alkynes? Although the mechanism is still
unknown, we reasoned that the [4+2] cycloadditon is pro-
moted by the electrophilic character of the two-coordinate
germanium(II) centre and the nucleophilic character of the
endocyclic g-C atom in the GeN2C3 ring in 2. Related
cycloaddition reactions of unsaturated organic substrates to
b-diketiminate ligands have been previously observed for
transition metal b-diketiminate cations [e.g. Ru(II)11 and
Fe(II)12] and for a Al(III) b-diketiminate cation,13 respectively.
The reactivity of 2 toward alkynes is in marked contrast to
that of the silylene homologue 1 which leads exclusively to
Si(IV) addition products {[2+1] cycloaddition of CRC and
Si(II) vs. 1,1-insertion of the C–H bond at Si(II)}.8 This can be
explained by the pronounced inert-electron pair effect of Ge(II)
vs. Si(II) and the lower basicity of the lone-pair electrons at
Ge(II), respectively. In fact, the formation of 3 and 4 is
supported by density functional theory (DFT) calculations
using the model system 2A, in which the 2,6-iPr2C6H3 groups
at nitrogen have been replaced by phenyl (see ESIw). The
calculations revealed that, in contrast to the reactivity of the
silicon-homologue of 2A, the [4+2] cycloaddition product 3A
is the thermodynamic product (ꢁ126.4 kJ molꢁ1) with a
remarkably low barrier of only 33.5 kJ molꢁ1 for the reaction
of 2A with HCRCH (Scheme 4). Interestingly, C–H bond
activation of HCRCH with 2A leads to the second stable
product of the system (ꢁ98.7 kJ molꢁ1), the 1,4 adduct 5A,
with a higher activation energy of 87.7 kJ molꢁ1. The latter
barrier is relatively high and explains the absence of such a
species in the case of the reaction of 2 with HCRCH.
Apparently, the situation is drastically changed by applying
PhCRCH which possesses a more acidic C–H bond and leads
4.25 ppm for the g-H atom in accordance with the aliphatic
character of the g-C bridgehead atom. In contrast, two doublets of
=
3
doublets at much lower field [6.96 (3JHH = 10.1 Hz, JHH
4
6.7 Hz) and 7.60 ppm (3JHH = 10.1 Hz, JHH = 1.3 Hz)] have
1
been observed for the H nuclei of the 1,2-ethylenediyl group.
Similar low-field shifts of the hydrogen atoms in Ge-vinyl com-
pounds have been previously observed for Me3GeCHQCH2.9
Interestingly, conversion of 2 with phenylacetylene in n-
hexane leads to the analogous [2+4] cycloadduct 4 along with
the alkynyl germylene 5 (Scheme 3). The products can be
separated by fractional crystallisation and isolated in the form
of yellow crystals in 67% (4) and 16% yield (5), respectively.
They have been characterised by elemental analysis, mass
spectrometry, 1H and 13C NMR spectroscopy (see ESIw).
The 1H NMR spectrum of 5 shows the presence of two
chemically equivalent excocyclic b-methyl groups. In other
words, the former exocyclic methylene group in 2 has been
converted to a b-methyl group through 1,4-addition of the
phenylacetylene (C–H bond activation) to the germylene. This
and the molecular structure of 4 has been confirmed by X-ray
diffraction analysis (Fig. 2 and 3). As expected, the charac-
teristic metric features of 4 are similar to those of 3 despite the
terminal phenyl group bonded to one of the C atoms of the
1,2-ethylenediyl bridge. Apparently, the cycloaddition of phenyl-
acetylene occurs regioselectively owing to steric hindrance
induced by the large aryl groups at nitrogen. In line with that,
germylene 2 does not react with diphenylacetylene even at
elevated temperatures. Compound 5 is the second example of
an isolable alkynyl germylene characterised structurally.10 It
possesses a pyramidally coordinated Ge(II) atom and reveals
an almost planar GeN2C3 ring similar to the situation in
related b-diketiminate complexes of divalent germanium.3,4
The C16RC17 distance of 121.9(5) pm in 5 is in the typical
range for CRC distances in other alkyne derivatives and
ꢀc
This journal is The Royal Society of Chemistry 2008
5394 | Chem. Commun., 2008, 5393–5395

View Article Online
product in an autocatalytic process and can be isolated in high
yields (Scheme 2).8 It should be mentioned here that isolable
germacycloprop-2-enes have been generated employing transient
germylenes and acetylene.14
In summary, the unprecedented reactivity of the ylide-like
germylene 2 toward terminal alkynes is reported. Reaction of
the zwitterionic germylene 2 with HCRCR leads to the
unexpected [4+2] cycloadducts 3 (R = H) and 4 (R = Ph)
but also to the second isolated alkynyl germylene 5 (R = Ph).
The novel compounds represent remarkably strong nucleo-
philic Ge(II) ligands in metal coordination chemistry for
catalysis. Respective investigations are currently underway.
Financial support from the Deutsche Forschungsge-
meinschaft (Cluster of Excellence ‘‘Unifying Concepts in
Catalysis’’, EXC 314/1) is gratefully acknowledged.
Notes and references
1 For reviews on silylenes, see: (a) N. J. Hill and R. West, J. Organomet.
Chem., 2004, 689, 4165; (b) M. Kira, J. Organomet. Chem., 2004, 689,
4475; (c) B. Gehrhus and M. F. Lappert, J. Organomet. Chem., 2001,
617–618, 209; (d) P. P. Gaspar and R. West, in The Chemistry of
Organic Silicon Compounds, ed. Z. Rappoport and Y. Apeloig, Wiley
and Sons, Chichester, 1998, vol. 2, Part 3, pp. 2463–2567.
Fig. 3 Molecular structure of 5. Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms are omitted for clarity. Selected bond
lengths (pm) and angles (1): Ge1–C16 197.6(4), Ge1–N1 198.5(2), N1–C2
134.5(3), C2–C3 139.0(3), C16–C17 121.9(5), C16–Ge1–N1 93.35(9),
N10–Ge1–N1 91.3(1), C2–C3–C20 127.9(3), C17–C16–Ge1 171.0(3),
C16–C17–C18 176.6(4). The sum of angles at Ge is 278. Symmetry
transformations used to generate equivalent atoms (0): x,ꢁy + 3/2, z.
2 For reviews on germylenes, see: (a) J. Barrau, J. Escudie
´
and J.
Satge, Chem. Rev., 1990, 90, 283; (b) M. Weidenbruch, Eur. J.
´
Inorg. Chem., 1999, 373; (c) K. W. Klinkhammer, Recent advances
in structural chemistry of organic germanium, tin and lead com-
pounds, in The Chemistry of Organic Germanium, Tin and Lead
Compounds, ed. Z. Rappoport, Wiley, New York, 2002, vol. 2, ch.
to the corresponding compound 5 at least as a minor product.
As in the silicon-homologue of 2A, formation of the 1,1-
adduct 6A by direct insertion of Ge(II) into the C–H bond
faces a very high activation barrier (4200 kJ molꢁ1). In
contrast to the silicon case, 6A is thermodynamically dis-
favoured compared to its tautomer 5A (Scheme 4).
4, pp. 284–332; (d) O. Kuhl, Coord. Chem. Rev., 2004, 248, 411; (e)
¨
N. Tokitoh and R. Okazaki, Coord. Chem. Rev., 2000, 210, 251; (f)
M. Weidenbruch, J. Organomet. Chem., 2002, 646, 39; (g) J.
Barrau and G. Rima, Coord. Chem. Rev., 1998, 178, 593.
3 (a) D. H. Harris, M. F. Lappert, J. B. Pedley and G. J. Sharp,
J. Chem. Soc., Dalton Trans., 1976, 945; (b) S. Nagendran and H. W.
Roesky, Organometallics, 2008, 27, 457 and references cited therein.
4 (a) L. W. Pineda, V. Jancik, H. W. Roesky, D. Neculai and A. M.
Neculai, Angew. Chem., Int. Ed., 2004, 43, 1419; (b) L. W. Pineda,
V. Jancik, K. Starke, R. B. Oswald and H. W. Roesky, Angew.
Chem., Int. Ed., 2006, 45, 2602; (c) I. Saur, S. G. Alonso and J.
Barrau, Appl. Organomet. Chem., 2005, 19, 414.
Striking differences in comparison with the Si(II) homologue
result also for the [2+1] cycloaddition product of 2A with
HCRCH, that is, the corresponding germacycloprop-3-ene,
which is thermodynamically disfavoured by 53.5 kJ molꢁ1. In
contrast, the silicon-homologues react readily to the correspond-
ing N-heterocyclic silacycloprop-3-enes which result as a kinetic
5 (a) M. Driess, S. Yao, M. Brym, C. van Wullen and D. Lentz, J.
¨
Am. Chem. Soc., 2006, 128, 9628; (b) M. Driess, S. Yao, M. Brym
and C. van Wullen, Angew. Chem., Int. Ed., 2006, 45, 6730; (c) S.
¨
¨
Yao, M. Brym, C. van Wullen and M. Driess, Angew. Chem., Int.
Ed., 2007, 46, 4659; (d) For zwitterionic NHC’s see: A. J. Arduengo
III, T. P. Bannenberg, D. Tapu and W. J. Marshall, Tetrahedron
Lett., 2005, 46, 6847, and cited references therein.
6 M. Driess, S. Yao, M. Brym and C. van Wullen, Angew. Chem.,
Int. Ed., 2006, 45, 4349.
¨
7 Y. Xiong, S. Yao, M. Brym and M. Driess, Angew. Chem., Int. Ed.,
2007, 46, 4511.
8 S. Yao, C. van Wullen, X.-Y. Sun and M. Driess, Angew. Chem.,
¨
Int. Ed., 2008, 47, 3250.
9 B. Machiniiec, H. Lawicka, M. Majchrzak, M. Kubicki and I.
Kownacki, Chem.–Eur. J., 2006, 12, 244.
10 P. Jutzi, S. Keitemeyer, B. Neumann and H.-G. Stammler, Orga-
nometallics, 1999, 18, 4778.
11 A. D. Phillips, G. Laurenczy, R. Scopelliti and P. J. Dyson,
Organometallics, 2007, 26, 1120.
12 E. A. Greory, R. J. Lachicotte and P. L. Holland, Organometallics,
2005, 24, 1803.
13 C. E. Radzewich, M. P. Coles and R. F. Jordan, J. Am. Chem.
Soc., 1998, 120, 9384.
14 (a) M. P. Egorov, S. P. Kolesnikov, Yu. T. Struchkov, M. Yu. Antipin,
S. V. Sereda and O. M. Nefedov, J. Organomet. Chem., 1985, 290, C27;
(b) N. Tokitoh, K. Kishikawa, T. Matsumoto and R. Okazaki, Chem.
Lett., 1995, 827; (c) N. Fukaya, M. Ichinohe, Y. Kabe and A.
Sekiguchi, Organometallics, 2001, 20, 3364; (d) F. Meiners, W. Saak
and M. Weidenbruch, Z. Anorg. Allg. Chem., 2002, 628, 2821.
Scheme 4 An energy profile for the reaction of 2A with HCRCH,
including DFT-calculated structures of the model compounds 3A, 5A,
6A and their corresponding transition states TS-3A, TS-5A and TS-
6A, respectively.
ꢀc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 5393–5395 | 5395