Angewandte
Chemie
DOI: 10.1002/anie.201407751
Ge–Ge Double Bonds Very Important Paper
II
A Multiply Functionalized Base-Coordinated Ge Compound and Its
Reversible Dimerization to the Digermene**
Anukul Jana, Volker Huch, Henry S. Rzepa, and David Scheschkewitz*
Dedicated to Professor Kaspar Hegetschweiler on the occasion of his 60th birthday
Abstract: Stable compounds with a Ge=Ge bond are usually
elements under mild conditions are therefore urgently
needed. Encouraging proofs-of-principle reactions include
the equilibrium between Lappertꢀs stannylene I and its
prepared under relatively harsh reaction conditions that are
incompatible with many functional groups. In particular,
unsaturated functionalities are not tolerated owing to their
facile reaction with low-coordinate germanium compounds.
We now report the synthesis of an imino-functionalized
germanium(II) species, stabilized by coordination of an N-
heterocyclic carbene (NHC), by reaction of an isonitrile with
a heavier NHC-coordinated vinylidene. Removal of the NHC
by a Lewis acid results in dimerization to the corresponding
digermene with a Ge=Ge bond. The reversibility of this process
[4]
dimer, the reversible reaction of distannynes of type II
[
5]
with ethylene, and the reversible formation of the arsasilene
[6]
III (Scheme 1). In his remarkable 2010 Review, Power
is demonstrated by addition of two equivalents of NHC to the
isolated digermene.
Scheme 1. Selected examples of low-valent Group 14 compounds
formed by reversible reactions (II: Ar=terphenyl-type substituent, III:
R=2,6-iPr C H ).
S
table multiply bonded species play a pivotal role in the
2
6
3
[1]
chemistry of heavier main-group elements. The unique
properties of heavier element double bonds (inherently small
HOMO–LUMO gap, conformational flexibility and high
reactivity) render the idea of their incorporation into
conjugated systems an exciting prospect. Prompted by the
successful preparation of P=P- and P=C-containing conju-
identified reversibility as one of the conceptual similarities
[
7]
between main-group and transition metals. In the case of
germanium, there are numerous reports on germylene–
[
8]
digermene equilibria in solution, but in general a lack of
residual functionality prevents the further exploitation of this
behavior for the creation of extended systems. Compounds
with Ge=Ge bonds are usually obtained from either nucle-
[2]
gated polymers, intense efforts towards analogous Group 14
systems followed, but are to date restricted to model
[
3]
compounds with two heavier element double bonds. These
difficulties mainly arise from the low selectivity of suitable
preparative procedures in the presence of multiple reactive
sites. Synthetic methods to selectively (and possibly rever-
sibly) generate multiple bonds between heavier Group 14
ophilic substitution reactions, photolysis of germanes, or
[
9]
reductive dehalogenation of 1,1-dichlorogermanes.
As a consequence, digermenes with residual functionality
in the periphery of the double bond are rare: a 1,2-dibromo-
[
8f]
digermene was reported by Tokitoh et al. and a lithium
digermenide, albeit not fully characterized, has been de-
[
*] Dr. A. Jana, Dr. V. Huch, Prof. Dr. D. Scheschkewitz
Krupp-Lehrstuhl fꢀr Allgemeine und Anorganische Chemie
Universitꢁt des Saarlandes
[10]
scribed by the groups of Masamune and later Weidenbruch.
We now disclose the synthesis of an cyclic N-heterocyclic
carbene(NHC)-coordinated germanium(II) compound with
no less than two different residual functionalities: an exocy-
clic imino group and a chloro substituent. Decomplexation of
66125 Saarbrꢀcken (Germany)
E-mail: scheschkewitz@mx.uni-saarland.de
Homepage: http://www.uni-saarland.de/fak8/scheschkewitz/
index.html
the NHC by triphenylborane (BPh ) results in dimerization to
3
Prof. Dr. H. S. Rzepa
Department of Chemistry, Imperial College London
London SW7 2AZ (UK)
the corresponding digermene, a process which is reversed by
adding anew two equivalents of the NHC.
The concept of the stabilization of low-valent main-group
species by the coordination of strong bases has allowed the
isolation of various compounds with otherwise inaccessible
Dr. A. Jana
New Address: Tata Institute of Fundamental Research, Centre for
Interdisciplinary Sciences
[11]
structural motifs. In particular in the case of germanium,
2
1, Brundavan Colony, Narsingi, Hyderabad-500075 (India)
**] Funding by the the Deutsche Forschungsgemeinschaft (DFG)
SCHE906/5-1), Alfried Krupp von Bohlen und Halbach-Founda-
numerous low-valent species have been obtained employing
[
[
11d,12]
this strategy.
We recently reported the synthesis of
(
[13]
[14]
silagermenylidenes 1 and 2 (Scheme 2) and established
the formal [2+2] cycloaddition of 1 with phenylacetylene as
tion, and COST Action CM1302 (Smart Inorganic Polymers) is
gratefully acknowledged.
well as the donor ability of 2 towards the Fe(CO) fragment.
Subsequently, we disclosed that an NHC exchange in 2
4
Angew. Chem. Int. Ed. 2015, 54, 289 –292
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
289