Full Paper
doi.org/10.1002/chem.202102233
Chemistry—A European Journal
denes can be classified into two types, i) two-coordinate
germyliumylidenes, which have both electrophilic and nucleo-
philic centers (Scheme 1b, Type A), ii) three- coordinate
germyliumylidenes where the nucleophilic character is more
pronounced due to the occupancy of the two p-orbitals
(Scheme 1b, Type B).[4]
Among the type B classification, bis-NHC stabilized germy-
liumylidenes are comparatively Lewis basic and relatively more
stable than type A germyliumylidenes, due to the occupied p-
orbitals. Thus, on one hand they can be used as a robust Lewis
base catalyst whilst on the other hand they can be used as a
Lewis acid catalyst, due to the ability of the GeÀ CNHC σ* orbitals
to accept electrons, which provides an additional cooperative
site for potential catalytic application. Despite these unique
electronic features, the catalytic application of germyliumyli-
denes is in its infancy. Recent examples from the groups of
Rivard and Nagendran have shown the role of type A and B
germyliumylidenes, respectively, in the hydroboration of
carbonyls.[6] However, the catalytic application of bis-NHC-
stabilized germyliumylidenes is still unknown.[5]
points towards the dependence on the cationic germanium
center during the catalysis. With the above points considered,
°
use of 5 mol% of 1 with PhSiH3 in CD3CN at 60 C provides the
optimal reaction conditions for this study.
Whilst the catalytic activity of 1 is lower in comparison to
our previously reported germa-acylium ion catalyst (TOF: I=
13.2 hÀ 1 vs. 1=7.9 hÀ 1 for PhSiH3 at 60 C),[7] germyliumylidene 1
°
has the added advantage of being a more stable catalyst as
well as requiring fewer synthetic steps. Using the optimized
conditions mentioned above, the longevity of catalyst 1 was
examined in which additional PhSiH3 and 1 bar of CO2 were
added to the J-Young NMR tube at the end of the cycle. This
process could be repeated four times before a small drop in
turnover was observed (TOF: Run 1=8 hÀ 1 vs. Run 4=6 hÀ 1). In
contrast, the previously reported germa-acylium ion catalyst
decomposed after the third cycle.
A series of stoichiometric reactions were undertaken to
probe the mechanism. No reaction was observed with 1 and
CO2 in the absence of silane, even after prolonged heating at
°
60 C. Additionally, no reaction was observed with varying
Very recently, we have reported hydrosilylation of CO2 with
a germa-acylium ion (I, [MesTerGe(O)(NHC)2]Cl) (MesTer=2,6-
(2,4,6-Me3C6H2)2C6H3; NHC=IMe4 =1,3,4,5-tetramethylimidazol-
2-ylidene) which proceeds through the active germylene
species (II, MesTerGe(OSiHPh2)(NHC)).[7] Importantly, in our case
the Lewis basicity of the Ge(II) center facilitates the hydride
transfer from silane to CO2 via the formation of a hyper-
coordinate silane intermediate. Moreover, it has been shown
equivalents of hydrosilane under the optimal catalytic con-
ditions. This suggests a cooperative silane/CO2 mechanism, and
therefore, the mechanism was investigated computationally
(Figure S50). In a similar fashion to the previous case,[7] the SiÀ H
bond in PhSiH3 is activated by the germanium lone pair in 1M+
and the hydride transfer from the hypercoordinate silane to the
free CO2 occurs in a concerted process via the transition state
TS-1 (Figure 1). IRC calculations confirm the direct formation of
the transition state from the separated species (1M+ +PhSiH3 +
CO2) following a three-component mechanism. Additionally,
participation of the germanium lone pair in “concerted SN2@Si-
acceptor” mechanism[9] rather than the classical activation of
hydrosilanes is also clearly evident from the substantially longer
GeÀ Si distance (2.787 Å) in TS-1. This step needs to surmount
an energy barrier of 28.6 kcalmolÀ 1 to provide the resulting
intermediate (INT-1). The Lewis basicity of NHC-stabilized
germyliumylidene towards PhSiH3 cannot be explained by
drastically high energy separation (11.0 eV) between the
germanium lone pair orbital in free 1M+ and SiÀ H σ* orbital in
free PhSiH3 (Figure S51a). However, when the silane approaches
the Ge center, significant stabilization of the SiÀ H σ* orbital in
the PhSiH3 fragment of TS-1 results in favorable interaction
(8.3 eV) with the germanium lone pair in the 1M+ fragment,
thus enhancing the reactivity of germyliumylidene and high-
lighting the Lewis basic nature of 1M+ (Figure S51b). In the
alternative pathway, oxidative addition of silane across the Ge
center in 1M+ with concomitant liberation of IMe4 demands
slightly lesser energy barrier of 28.2 kcalmolÀ 1 (Figure S52), as
suggested by favorable interaction (8.2 eV) between the SiÀ H σ
and GeÀ CIMe4 σ* orbitals in TS-7 (Figure S53b). However, CO2
insertion into the Ge(IV) hydride species (INT-5) in the
subsequent step demands drastically high intrinsic activation
barrier of 44.9 kcalmolÀ 1 and can be safely discarded. Moreover,
1M+ does not coordinate with either CO2 or silane. Thus 1[Cl]
assisted hydrosilylation of CO2 is proposed to occur through
three-component pathway rather than a two component
mechanism. In agreement with experimental findings, the
that the Lewis acidity and Lewis basicity are both important for
[2d,e,h,8]
the catalytic transformation of CO2
This encouraged us to
examine the recently reported bis NHC stabilized germyliumyli-
dene range of catalytic
[
MesTerGe(NHC)2]Cl (1) towards
a
reductive functionalization reactions, with a particular emphasis
on C=O reduction i.e., CO2 and carbonyls.
Results and Discussion
Following a similar protocol to the previous NHC-stabilized
germa-acylium catalysis,[7] compound 1 was found to transform
CO2 into the corresponding hydrosilylated products in both a
stoichiometric and catalytic manner in the presence of phenyl-
1
silane (PhSiH3). The H NMR spectrum revealed the complete
°
consumption of PhSiH3 within 2.5 h at 60 C, with the formation
of silylformate, bis(silyl)acetal and silylated methanol observed
(see Supporting Information, Figure S1). As expected, use of
more sterically protected silanes required increased reaction
times (PhSiH3 2.5 h vs. Ph2SiH2 3.5 h), and in the case of Ph3SiH,
higher temperatures and prolonged reaction times are required
°
(28 h at 80 C). Furthermore, solvent optimization studies found
increased rates of reaction in polar solvents (e.g., CD3CN)
compared to non-polar solvents (e.g., C6D6). This, however, can
also be attributed to the low solubility of the catalyst in non-
polar solvents. To consider the influence of the counter ion in
catalysis, the reaction was performed using the anion-ex-
changed 1[BArF], [BArF={(3,5-(CF3)2C6H5)4B], catalyst in CD3CN,
no significant change in the rate of reaction was found. This
Chem. Eur. J. 2021, 27, 1–8
2
© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
��
These are not the final page numbers!