.
Angewandte
Communications
[
19]
is assigned to CH CN (CH CN in [D ]THF: s, 1.95 ppm).
the molecular system may be explained by the reaction of
acrylonitrile with two adjacent surface dimers rather than
a single dimer on the surface. The group of Bent showed that
the formation of an interdimer cyclic ketenimine was
thermodynamically more favored compared to a single intra-
3
3
8
Both azadigermetines 7 and 9 show similar behavior upon
thermolysis in the presence of DMB.
Reversible cycloadditions are unknown in digermene
chemistry, and indeed, are very rare in the chemistry of the
heavier ditetrelenes and ditetrelynes. The chemistry observed
here is comparable to the reversible addition of ethylene to
[8]
dimer adduct, although the difference was not appreciable.
Considering the strain associated with the incorporation of
a ketenimine into a small ring, the formation of a seven-
membered cyclic ketenimine (1b) seems more reasonable
than a six-membered ketenimine (1a). Isolation of stable
[
1]
a distannyne. The reversible insertion of an isonitrile to
a disilene and the reversible addition of a germylene to an
[9a]
[9c]
alkene are notable related examples. The reversible com-
plexation of unsaturated molecules, such as alkenes, is a key
feature in the reactivity of transition metals which has led to
the postulation that unsaturated main group compounds,
noted primarily for their challenging syntheses and novel
bonding, now have the potential of being developed into
catalysts which offer several key advantages (low cost and
toxicity) over their transition metal counterparts. In a similar
vein, Bent has postulated that the Ge(100)-2 1 surface may
also serve as a catalyst because the buckled surface dimers
exhibit donor and acceptor properties characteristic of
transition-metal-based catalysts and the adsorbate–surface
[
22]
seven-membered cyclic allenes containing a SiÀSi
or a
bond provides support for the formation of
[6c]
GeÀGe
analogous cyclic ketenimines on the surface. In addition,
seven-membered cyclic organic ketenimines exhibit a signifi-
cant degree of bending (~ 155 88 ) which manifests in a lowering
[
23,24]
of the ketenimine stretching frequency
as was observed in
[2]
[7]
the surface cyclic ketenimine 1 although the incorporation
of the GeÀGe bond within the cyclic ketenimine is expected
to reduce some of the strain. Accordingly, the ketenimine
stretching vibration in 1b is not as low as in the all-carbon
À1
derivative (1954 versus 1924 cm , respectively). The surface
[20]
bonds are of an intermediate strength.
results suggest that azadigermetine 9 may be the kinetic
product and that the isomeric cyano-substituted digermetane
may be the thermodynamic product produced under equilib-
rium conditions. Given that digermene 4 is known to undergo
conversion to Ge Mes (with decomposition) in solution after
The reversible addition of nitriles to digermene 4 is
completely consistent with what was observed on the Ge(100)
reconstructed surface for acetonitrile, albeit, the binding of
the nitrile to the molecular digermene appears to be stronger
than the surface digermene perhaps due to substituent effects
or the different geometries at the unsaturated germaniums
3
6
[
12]
long periods of time,
we have not investigated this
possibility; however, using a digermene which is stable in
solution for prolonged periods, this hypothesis could be
tested.
[3]
[12]
(
cis- versus trans-bent).
The lack of conversion of the
azadigermetines to their enamine tautomers, as observed in
[10]
the analogous disilene system, is likely a consequence of
the reversibility of the cycloaddition. Perhaps, in the diger-
mene case, the relative amount of enamine did not increase
due to competitive dissociation of the nitrile. Interestingly, we
previously investigated the addition of acrylonitrile to diger-
mene 4 at elevated temperatures and, under these conditions,
no acrylonitrile adduct was isolated. We now understand
that an adduct was likely not observed due to the reversible
binding of the nitrile to the digermene.
Finally, it is interesting to compare the reactivity of
digermene 4 with nitriles to that of a stable digermyne
(Ar’GeGeAr’, Ar’ = C H -2,6-(C H -2,6-iPr )). Digermyne
6
3
6
3
2
Ar’GeGeAr’ reacted with benzonitrile to give the 2:1
adduct 13 and with tetracyanoethylene to give the unusual,
complex cage species 14; simple 1:1 cycloaddition adducts
[21]
[16]
were not obtained (Figure 2). Although the mechanisms for
In contrast, the structures of adducts formed upon
addition of acrylonitrile to the germanium surface dimers in
comparison to the molecular digermene are different. The
addition of acrylonitrile to the Ge(100)-2 1 surface at room
temperature results in the formation of cyclic ketenimines
[
7]
(
1a,b) and a cyano-substituted digermetane (2), whereas we
observe the formation of an azadigermetine (9); no evidence
for ketenimine formation or cycloaddition through the C=C
bond of acrylonitrile was obtained. Analysis of 9 by IR
spectroscopy revealed an absorption at 1556 cm which was
À1
[16]
Figure 2. Nitrile adducts of a digermyne.
assigned to the C=N stretching vibration; no such absorption
was observed by MIR-IR spectroscopy of the surface adducts.
DFT calculations show that both the cyclic ketenimines 1 and
digermetane 2 are formed from an initial donor adduct and
the formation of 13 and 14 are unknown, the authors
postulated that the formation of these products likely begins
with a single electron transfer and involves radicals revealing
the diradical character of the digermyne. This is in contrast to
the heterolytic mechanism proposed by us for the reaction of
the digermenes with nitriles. A complete understanding of the
differences in reactivity between these multiply bonded
germanium compounds requires further investigations.
[8]
that 1 is the kinetic and 2 is the thermodynamic product. The
calculations also predict that cycloaddition through the CꢀN
bond of acrylonitrile would not be observed on the Ge
surface, despite having a relatively low kinetic barrier, due to
the weak binding energy of the adduct, and hence, a short
[
7,8]
surface lifetime.
The lack of formation of a ketenimine in
6
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Angew. Chem. Int. Ed. 2015, 54, 6600 –6603