Molecular Zinc Hydride Cations [ZnH]+: Synthesis, Structure, and CO2 Hydrosilylation Catalysis

Article abstract of DOI:10.1002/anie.202011480

Protonolysis of [ZnH₂]_n with the conjugated Br?nsted acid of the bidentate diamine TMEDA (N,N,N′,N′-tetramethylethane-1,2-diamine) and TEEDA (N,N,N′,N′-tetraethylethane-1,2-diamine) gave the zinc hydride cation [(L₂)ZnH]

Full text of DOI:10.1002/anie.202011480

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Accepted Article
Title: Molecular Zinc Hydride Cations [ZnH]+: Synthesis, Structure, and
CO2 Hydrosilylation Catalysis
Authors: Jun Okuda, Florian Ritter, Thomas P. Spaniol, Iskander
Douair, and Laurent Maron
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Angewandte Chemie International Edition
10.1002/anie.202011480
+
Molecular Zinc Hydride Cations [ZnH] : Synthesis,
Structure, and CO Hydrosilylation Catalysis
2
[
a]
[a]
[b]
[b]
Florian Ritter, Thomas P. Spaniol, Iskander Douair, Laurent Maron, and Jun
[
a]
Okuda *
[
a] F. Ritter, Dr. T. P. Spaniol, Prof. Dr. J. Okuda*, Institute of Inorganic Chemistry, RWTH
Aachen University, Landoltweg 1, 52056 Aachen, Germany, email: jun.okuda@ac.rwth-
aachen.de
[
b] Prof. Dr. L. Maron, Iskander Douair, CNRS, INSA, UPS, UMR 5215, LPCNO, Université
de Toulouse 135 avenue de Rangueil, 31077 Toulouse, France
Supporting information for this article is available on the WWW under
http//:www.angewandte.chemie.org or from the author
Keywords: zinc hydride • hydrosilylation • Lewis acid • chemoselective catalysis • carbon dioxide
Table of Contents
Insoluble, thermally unstable zinc dihydride [ZnH
2
]
n
can be converted into robust zinc hydride
+
cations [(L
silane. Ligand L
2
)ZnH] which catalyzes the homogeneous hydrosilylation of CO to give methoxy
2
2
can be as simple as chelating TMEDA.
1
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10.1002/anie.202011480
Abstract: Protonolysis of thermally unstable and insoluble [ZnH
2
]
n
with the conjugated
Brønsted acid of the bidentate diamine TMEDA (N,N,N′,N′-tetramethylethane-1,2-diamine) and
TEEDA (N,N,N′,N′-tetraethylethane-1,2-diamine) gave the zinc hydride cation [(L
2
)ZnH]+,
isolable either as the mononuclear THF adduct [(L
B(3,5-(CF -C )Zn)} (µ-H)
]-) or as the dimer [{(L
to [ZnH
also shown to form di- and trinuclear adducts of the elusive neutral [(L
2
)ZnH(thf)] [BArF
+
4
]- (L
2
= TMEDA; BArF
4
-
=
[
3
)
2
6
H
3
)
4
2
2
2
]2+[BArF
4
]-
2
(L
2
= TEEDA). In contrast
+
2
]
n
, the cationic zinc hydrides are thermally stable and soluble in THF. [(L
)ZnH
2
)ZnH] was
2
2
] containing both
terminal and bridging hydrido ligands. All hydride-containing cations readily inserted CO to
2
give the corresponding formate complexes. [(TMEDA)ZnH] [BArF
+
]- catalyzed the
4
hydrosilylation of CO with tertiary hydrosilanes to give stepwise formoxy silane, methyl
2
formate, and methoxy silane. The unexpected formation of methyl formate was shown to result
from the zinc-catalyzed transesterification of methoxy silane with formoxy silane, which was
eventually converted into methoxy silane as well.
Introduction
The hydrosilylation of CO
2
is catalyzed by a variety of transition metal-[1] and non-metal-
[2]
[3]
based catalysts to give formoxy silane, bis(silyl)acetal, methoxy silane, and methane. The
chemoselectivity appears to be determined by the Lewis acidity of the catalyst. A transparent
structure-selectivity relationship, however, remains unavailable, since both the nature of the
hydrosilane and the activation mode of CO
pronounced Lewis acidity, molecular zinc compounds supported by chelating ligands have
been successfully utilized as chemoselective catalysts for the hydrosilylation of CO
.[4] Most
catalysts are neutral as the result of employing mono-anionic L
X-type ligands,[4a-c, 5] whereas
2
play critical roles during catalysis. By virtue of its
2
n
cationic zinc hydrides remain relatively scarce. Assuming increased Lewis acidity due to the
positive cationic charge, Rivard et al. showed improved activity of the zinc hydride cation
precursor [(IPr)ZnH(THF)(OTf)] in ketone hydrosilylation when compared to its neutral
analogue [(IPr)ZnH(µ-H)]
2
(IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) (Figure
). The presence of Lewis acids such as boranes was found to improve the reaction rate and
to influence the degree of CO hydrosilylation, suggesting that zinc’s inherent electrophilicity
[6]
1
2
may not be sufficient on its own.[5, 7] The optimum coordination number of a highly electrophilic
zinc center can be conceived to be lower than four with the steric bulk of the ancillary ligand
as small as possible.[4, 5, 7, 12b]
2
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2+
+
N
N
Mes
THF
Mes
N
N
+
Dipp
Dipp
N
Zn
Zn
H
H
H
NMe2
H
H
Mes
N
Mes
Me
N
Zn
Zn
Zn
2
N
N
N
NMe
2
NMe2
H
N
N
Mes
NMe2
Mes
[(L
+
+
2
+
ZnH]
Zn H
ZnH]
4
[(L )
[
(L
3
)
4
)
3
4
]
Figure 1: Cationic zinc hydrides reported in the literature.[6-8]
Here we report that protonolysis of zinc dihydride[9] with the conjugated Brønsted acid of the
simple bidentate diamine TMEDA (N,N,N′,N′-tetramethylethane-1,2-diamine) and TEEDA
+
(
N,N,N′,N′-tetraethylethane-1,2-diamine), the zinc hydride cation [(L
become accessible. Using the non-nucleophilic Kobayashi anion BArF
CF -C
]-) was critical to obtain quasi tricoordinate zinc hydride cations, since other
tetraarylborates were decomposed.[10] Hydrosilylation of CO
2
)ZnH] and its derivatives
-
(BArF
-
4
4
= [B(3,5-
(
3
)
2
6
H
)
3 4
+
2
was catalyzed by [(L
2
)ZnH] to
give methoxy silane with formoxy silane and methyl formate as intermediates.
Results and Discussion
Synthesis and Structure of Zinc Hydride Cations
+
Zinc hydride cations 2a and 2b derived from [(L
2
)ZnH] were prepared by treating a suspension
9]
of [ZnH
diamine L
soluble in organic solvents such as THF or CH
0 °C in solution, and were isolated as colorless crystals in practically quantitative yields.
Notably, reacting [ZnH with neat TMEDA or TEEDA did not afford any isolable
2
] [
n
in THF at room temperature with the conjugated Brønsted acid of the bidentate
1a and 1b (L = TMEDA, TEEDA) (Scheme 1). Both complexes 2a and 2b are
Cl , show no sign of decomposition even at
2
2
2
2
7
]
2 n
compounds,[11] but rather resulted in the gradual decomposition of [ZnH
2
n
] into metallic zinc
1
and dihydrogen. The H NMR spectrum of 2a in [D
8
]THF exhibits one set of two sharp singlets
for the TMEDA ligand along with resonances of the borate anion in 1:1 ratio. Even after
prolonged drying under vacuum THF could not be removed entirely. The zinc hydride
resonance in 2a was found as a singlet at δ = 3.68 ppm in [D
8
]THF and as a broad singlet at δ
=
3.56 ppm in CD Cl indicating fluxional THF coordination in solution. Generally, the
2
2
resonance for terminal zinc hydrides is expected in the chemical shift region between 3 to 5
ppm,[6 8] whereas that for bridging zinc hydrides are usually shifted by 1 to 2 ppm toward higher
field.[12]
3
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10.1002/anie.202011480
R
F
R
N
+ [NEt3
4
H][BAr ]
N
R
R
+
2+
R
N
R
N
R
N
Et O
-NEt3
+
R
R
2
R
R
R
R
H
H
H
1
/n [ZnH2
]n
Zn
0.5
F
Zn
Zn
NCH CH NR
[
(R
2
2
2
2
)H] [BAr
a: R = Me
b: R = Et
4
]
THF, - H2
N
R
O
N
R
N
R
1
1
=
B(3,5-(CF
F
BArF
C H
3 6 3
2 4
) )
[BAr
4
]
F
4 2
[BAr ]
4
2a
2b
R = Me
R = Et
Scheme 1: Synthesis of the zinc hydride cations 2a and 2b.
1
The H NMR spectra of 2b in [D
8
]THF shows a set of signals for the TEEDA ligand in
resonances
of the ligand backbone are observed as a singlet at δ = 2.96 ppm, whereas the diastereotopic
CH signals of the ethyl groups appear as two sets of multiplets of an A BB’ spin system. The
zinc hydride resonance is observed as a singlet at δ = 3.73 ppm in [D ]THF and δ = 4.28 ppm
in CD Cl respectively, in agreement with the presence of a dissociative equilibrium.
agreement with D2h symmetry in addition to those for the borate anion. The CH
2
2
3
8
2
2
Figure 2: Cation portion in 2a (left) and 2b (right). Displacement parameters are set at 50%
probability. The borate anion and hydrogen atoms except of H1 in 2a and H1 and H1’ in 2b
are omitted for clarity.
Single crystals of 2a and 2b suitable for X-ray diffraction were obtained from CH
2
Cl /n-pentane
2
solution at −30 °C (Figure 2). The molecular structure of 2a confirms a monomeric, cationic
zinc center tetrahedrally coordinated by a terminal hydrido ligand, one TMEDA and one THF
ligand. The Zn-H bond length is 1.55(4) Å, comparable to 1.59(3) Å reported for
4
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+
F
]- (Me
TREN = tris{2-(dimethylamino)ethy}amine).[7a] The solid state
[
(Me
6
TREN)ZnH] [BAr
4
6
structure of 2b reveals a dimeric structure. Both tetrahedrally coordinated zinc centers are
bridged by two µ-hydrido ligands and separated by 2.4397(4) Å. The Zn1-H1 distance of
2
.04(3) Å and the Zn1-H1’ bond length of 2.06(4) Å are in the expected range.[12] Although the
bias toward dimerization is pronounced when the sterically more encumbered TEEDA was
used, we believe that a monomer-dimer equilibrium exists in THF solution (Scheme 1).
As coordinatively unsaturated zinc hydrides tend to aggregate to form clusters,[8, 12-13] the Lewis
+
acidic fragments [(L
2
)ZnH] 2a and 2b could be used to deaggregate polymeric [ZnH
2
n
] .
Treating a THF suspension of [ZnH
2
n
] in the presence of one equivalent of TMEDA with 2a at
room temperature resulted in a clear solution within a few minutes. Depending on the
stoichiometry, cationic zinc hydrides 3 and 4 were formed. They can also be synthesized
directly from [ZnH
2
]
n
, using the appropriate amount of 1 and L
2
(Scheme 2).
+
R
R
N
+
1/n [ZnH2 n
R
R
]
R
R
N
H
+
L2
F
Zn
Zn
[
4
BAr ]
THF
H
N
R
N
R
H
+
R
R
N
3a: R = Me
3b: R = Et
H
F
Zn
[BAr ]
4
m
N
R
O
R
2+
R
R
x
m
N
R'
R
+
1/2n [ZnH2 n
R'
2
a
]
N
H
H
Zn
:
R = Me, x = 1, m = 1
b: R = Et, x = 0, m = 2
+
0.5 L2
N
R
2
F
0.5
Zn
H
R
H
4 2
[BAr ]
R
THF
N
Zn
=
TMEDA, TEEDA
N
R
L2
R'
R'
N
R
4
4
a: R = R' = Me
R = R' = Me
b:
4c:
R = Me, R' = Et
Scheme 2: Formation of the cationic zinc hydrides 3 and 4.
Dinuclear 3a is soluble in THF and CH Cl and in contrast to 2a and 2b, sparingly soluble in
2
2
benzene or toluene. Compound 3a is stable at room temperature for at least two days but
unlike 2a and 2b decomposes at elevated temperatures. Analogously, hydride cation 3b was
1
obtained from 2b and TEEDA. The H NMR spectra in [D
8
]THF of 3a and 3b show the signals
of the diamine and the borate anion in the expected region in a 2:1 ratio. At room temperature,
no distinction is possible between the terminal and bridging hydrides: the hydride resonance
1
is found as a singlet at δ = 3.68 ppm. VT H NMR spectra of 3a in CD
2
Cl (see SI) revealed
2
splitting of the hydride resonance into two singlets in a 2:1 ratio and two sets for the TMEDA
5
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Angewandte Chemie International Edition
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+
ligand at −50 °C. A formal combination of [ZnH] and [ZnH
2
]
n
agrees well with the structure
obtained in the solid state. Single crystals of 3a suitable for X-ray diffraction were obtained
from CH Cl /n-hexane solution at −30 °C (Figure 3). In the dinuclear structure the two
)ZnH] units are bridged by a single µ-hydrido ligand. Both tetrahedral zinc centers are
separated by 2.7860(7) Å. The terminal Zn-H bond lengths of Zn1-H1 1.53(4) Å and Zn2-H3
.59(4) Å are in the same range as those determined in 2a. The bridging hydride bonds are
2
2
+
[
(L
2
1
given as Zn1-H2 1.71(4) Å and Zn2-H2 1.62(4) Å, respectively. Conventionally, two metal
[14]
centers bridged by three hydrides as in [(Me
(triphos)Rh(μ-H) Rh(triphos)]BPh
are observed in a [M ] motif. A single bridging hydride in between two zinc centers remain
5
TRENCH
2
)Lu(μ-H)
3
Lu(Me
6
TREN)][BArF
4
]
2
or
[
3
4
(triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane)[15]
2
H
3
hitherto unknown. Only few examples such as [(dippe)
2
Ni
t
2
H
3
][BPh
Pt
4
] (dippe = 1,2-
][BPh
][17] were
bis(diisopropylphosphanyl)ethane)[16] and [{( Bu)
t
P(CH
)
P( Bu)
}
H
2
2
3
2
2
2
3
4
reported to feature this bridging mode.
Figure 3: Molecular cations in 3a (left) and 4a (right). Displacement parameters are set at 50%
probability. The borate anion and hydrogen atoms except of H1, H2, H3 in 3a and H1, H2, H3,
H4 in 4a are omitted for clarity.
+
It was possible to coordinate the zinc cation [(L
neutral [(L )ZnH
2
)ZnH] toboth terminal hydrides of the putative
2
2
]. The reaction of 2a with 0.5 equivalent of [ZnH
2
] and TMEDA gave the
n
cationic trinuclear zinc hydride 4a in quantitative yield. Zinc hydride clusters 4 show good
1
solubility in THF and CH
2
Cl
2
as well as thermal stability up to 70 °C in solution. The H NMR
spectrum displays signals for the ligand and the borate anion in a 3:2 ratio. The zinc hydride
appears as a single resonance at δ = 3.62 ppm. As observed for 3, it was not possible to
distinguish between terminal and bridging hydrides at room temperature. Single crystals of 4a
suitable for X-ray diffraction were obtained from a CH
2
Cl /n-pentane solution at −30 °C (Figure
2
3). In the solid state, the C
2
-symmetrical trinuclear zinc hydride dication consists of a
6
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10.1002/anie.202011480
+
[
(L
2
)ZnH
2
] unit connected to two [(L
2
)ZnH] fragments by a µ-H bridge (L
2
= TMEDA). The
terminal Zn-H bond lengths of Zn1-H1 1.60(3) and Zn3-H4 1.59(3) Å are in the same range as
the terminal zinc hydride bond found in 2a, whereas Zn1-H2 1.75(3) and Zn3-H3 1.80(3) Å
appear to be significantly longer than Zn2-H2 1.61(3) and Zn2-H3 1.64(3) Å. In contrast to the
previously reported [Zn
3
H
4
]2+ core found in [(IMes)
with dicationic [(IMes)Zn(thf)][BPh
3
Zn
3
H
4
(thf)][(BPh
4
)]
2
, which can be
,[8][7]
] coordinated to two
regarded as an adduct of neutral [(IMes)ZnH
2
]
2
]
4 2
complex 4a can be considered as an adduct of [(TMEDA)ZnH
2
+
[
(TMEDA)ZnH] cations. The terminal zinc hydrides in 4a can be attributed to a strong repulsion
[6]
originated in both cationic zinc centers as found in [(IPr)ZnH(thf)OTf]. When 2a was reacted
with [ZnH and TEEDA, the mixed ligand trinuclear cation 4c was isolated as the sole
]
2 n
product. NMR spectroscopy as well as single crystal X-ray diffraction (see SI) revealed an
identical structural motif in both solution and solid state as compared to 4a. Notably even at
elevated temperatures (70 °C) was a ligand exchange not observed, indicating that the neutral
[
(TEEDA)ZnH
2
] in 4 originating from [ZnH
2
n
] is kinetically inert.
To gain more insight into the electronic structure and nature of the zinc hydride bonds in 2-4,
DFT and NBO calculations were performed (see SI). In 2a, the Wiberg index for the Zn1-H1
bond of 0.77 confirms a terminal hydride in the classical sense. For the optimized dimeric
structure in 2b much smaller Wiberg indices of 0.38 describe the bonding between Zn2 and
the bridging hydrides (Zn2-H1 and Zn2-H2). Donation of these bonds to Zn1 (283.9 kcal/mol
for Zn2-H1→Zn1 and 266.5 kcal/mol for Zn2-H2→Zn1) confirms that each Zn-H-Zn unit
involves a 3c-2e bond. Compound 3a which is based on a [Zn
2
H
]+ fragment contains two
3
terminal hydrides with Wiberg bond indices for Zn1-H1 and Zn2-H3 of 0.78, as expected.
Considering the bridging hydride along Zn1-H2 and Zn2-H2, indices of 0.37 are obtained. A
second order study demonstrates that the σ electron lone pair at H2 donates to the empty sp
orbitals of both zinc centers, contributing 82 kcal/mol to Zn1 and 79 kcal/mol to Zn2. This is in
line with a 3c-2e bond along the Zn1-H2-Zn2 motif. The terminal hydride bonds Zn1-H1 and
Zn3-H4 of the cation [Zn
3
H
]2+ in 4a can again be regarded as classical hydride bonds with
4
Wiberg indices of each 0.7. The bonding along the Zn2-H3-Zn3 and Zn2-H2-Zn1 motif differs
from the situation found in 3a. The Wiberg indices are given as 0.4 for the Zn2-H2 and Zn2-
H3 bond and as 0.29 between Zn1-H2 and Zn3-H3, respectively. Second order analysis
reveals a contribution of 76 kcal/mol of the σ electron lone pair at H2 to the empty sp orbital at
Zn1, just as the s electron pair at H3 donates to the empty sp orbital at Zn3. This donation from
the bridging hydrides to the empty sp orbitals of the cationic units weakens the Zn-H bonds on
Zn2. Consequently, the cationic motif should result in significant Lewis acidity and 4a can be
+
considered as an adduct of neutral [ZnH
2
] stabilized by two [ZnH] cations.
7
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Reaction of Zinc Hydride Cations with CO
2
The cationic hydrides 2-4 readily reacted with CO
2
(1 bar) in THF at room temperature to give
1
13
11
19
formate complexes 5a,b and 6a,b in quantitative yields (Scheme 3). The H, C, B and F
NMR resonances of the ligand and the anion in 5a,b and 6a,b are found in the expected
regions. The formate proton in 5a and 6a was detected as a singlet at δ = 8.23 and 8.26 ppm
1
13
in the H NMR spectrum as well as by a single resonance at 170.06 and 170.07 ppm in the C
NMR spectrum, respectively. Single crystal X-ray diffraction of 5a revealed a dimeric structure
+
2
where two [(TMEDA)Zn(O
2
CH)] units are bridged by two formate ligands in a µ-κ -O CH
2
fashion (Figure 4). The NMR spectroscopic data and structural parameters are comparable to
those of the dimeric zinc formates [(MesBDI)Zn(O
CH)]
[18]
and [(DIPPBDI)Zn(O
CH)]
.[4b]
2
2
2
2
H
2
+
R
R
R
N
R
R
O
O
O
(1bar)
N
2a
CO2
THF
,
2b
Zn
Zn
F
[
4 2
BAr ]
N
R
O
N
R
R
H
5
a: R = Me
R = Et
5b:
H
+
R
N
O
O
R
O
O
O
R
R
R
H
(1bar)
N
3a
CO2
THF
F
,
3b
Zn
Zn
[
4
BAr ]
N
R
N
R
R
O
H
R = Me
R = Et
6
a:
6b:
H
R
R
R
(1bar)
O
N
O
4a, 4b
CO2
5a, 5b
2
+
Zn
N
THF
O
O
R
H
7a: R = Me
R = Et
7b:
Scheme 3: Reaction of zinc hydrides 2-4 with carbon dioxide.
8
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Figure 4: Molecular cations in 5a (left) and 6a (right). Displacement parameters are set at 50%
probability. The borate anion and hydrogen atoms except of the formate hydrogen are omitted
for clarity.
Single crystals of 6a suitable for X-ray diffraction were obtained from a CH
2
Cl /n-pentane
2
solution at −30 °C (Figure 4). The molecular structure of 6a revealed a dimeric structure in
+
which two [(TMEDA)Zn] ions are bridged by three formato ligands. In contrast to 5a the Zn1
center is embedded in a square pyramidal coordination environment, whereas Zn2 shows a
six-coordinate zinc center with a distorted octahedral geometry. Zinc center Zn2 contains a µ-
2
κ -formate ligand. Six-coordinate zinc centers are observed when small monodentate ligands
are used or in the solid state structures of zinc-containing MOFs.[19] Reaction of 4 with CO
unexpectedly gave a mixture of doubly bridged 5 and neutral diformate 7. Compounds 7a and
b were isolated from the reaction mixture and could also be synthesized independently from
2
7
[
ZnH
2
]
n
, diamine L
2
, and CO
2
(see SI). They were characterized by NMR and IR spectroscopy.
1
Single crystal X-ray diffraction of 7a confirmed the tetrahedral molecular structure with two κ -
formate ligands. When the mixed ligand complex 4c was exposed to CO
mixture of formates 5a and 7b was formed in a 2:1 ratio. Neither 5b nor 7a was obtained,
suggesting that scrambling of the L ligands did not occur.
2
atmosphere, a
2
Hydrosilylation of CO
2
The facile reaction of the cationic zinc hydrides toward CO
in the catalytic hydrosilylation of CO . To convert CO into formoxy silane, silylacetals, and
methoxy silane by zinc-catalyzed hydrosilation, PhSiH or (EtO) SiH have predominantly been
2
prompted us to test their activity
2
2
3
3
used.[4b, 4d, 20] In addition to uncontrollable chemoselectivity, aryl and methoxy substituted
silanes undergo scrambling of silicon substituents.[21] Therefore we applied tertiary alkyl-
n
substituted silanes such as BuMe
2
SiH or Et SiH which are more likely to be inert toward this
3
9
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Angewandte Chemie International Edition
10.1002/anie.202011480
exchange.[22] The CO
hydrosilylation using these hydrosilanes was benchmarked by using 2
2
mol% of 2a or 2b in THF at 70 °C (Table 1).
Table 1: Hydrosilane and catalyst screening for the hydrosilylation of CO
.[a]
2
H
O
[
cat.]
[
Si]
+
[Si]
OCH3
+
CH
3
+ [Si]
O
[Si]
[
Si]-H + CO
2
O
O
H
O
7
0°C, THF
A
B
C
Conv.
Time
[h]
Selec. [%][b,e]
Entry[a] Hydrosilane
Cat. [mol%]
[
b]
[
%]
A/B/C
n
5
5
6
7/15/28
6/22/22
3/11/26
93/3/4
1
2
BuMe
2
2
SiH
SiH
2a (2.0)
2b (2.0)
2a (2.0)
2a (2.0)
2a (2.0)
>99
>99
>99
66
89
-
24
24
16
48
96
24
24
60
60
24
n
BuMe
3
EtMe
2
SiH
4
Et SiH
3
5[c]
6
n
n
n
BuMe
BuMe
BuMe
BuMe
BuMe
BuMe
SiH
SiH
SiH
SiH
SiH
SiH
8
0/20/0
2
2
2
-
[ZnH
[ZnH
2
2
]
]
n
n
(2.0)
(2.0)
7[d]
8
-
-
n
n
n
H][BArF
-
2
2
2
[NEt
[Zn(OTf)
BArF18 (2.0)
3
4
] (10.0)
-
1
00/0/0
9
2
] (10.0)
54
-
-
10
[
a] n(hydrosilane) = 0.18 mmol, 1 bar CO
2
, 0.5 mL of [D
determined by H NMR spectroscopy using 0.03 mM of hexamethylbenzene as internal standard. [c]
Addition of 2 mol% of BPh . [d] Addition of 1 equiv. of TMEDA. [e] Disiloxane was formed in amounts
8
]THF. [b] Conversion and selectivity
1
3
corresponding to the overall stoichiometry (see Scheme 4).
n
When 2a was used as catalyst, full conversion of BuMe
2
SiH was achieved after 24 h.
n
BuMe
BuMe
2
2
Si(O
2
CH) was identified as the major product in 57% yield along with with 15% of
) (Table1, entry 1). Unexpectedly, apart from formoxy silane and methoxy
n
Si(OCH
3
1
4
silane methyl formate was detected in 28% yield by H NMR spectroscopy (δ = 3.66 (d, JH-H
=
0
.7 Hz), 8.02 (quart., 4JH-H = 0.7 Hz) ppm).[23],[24] When 2b was used as catalyst, CO
was
2
n
hydrosilylated by BuMe
2
SiH with comparable selectivity as using 2a (Table1, entry 2).
Employing EtMe
2
SiH, full conversion of the hydrosilane was achieved after 16 h (Table 1, entry
3) with 63% of formoxy silane, 11% of methoxy silane, and 26% of methyl formate. When
sterically more demanding Et
3
SiH was used instead, the conversion rate was reduced (Table1,
n
entry 4), with chemoselectivity comparable to that when BuMe
2
SiH or EtMe
and (EtO)
can catalyze the reduction of CO
2
SiH was used.
SiH gave only
to
Hydrosilanes commonly used in CO
2
hydrosilylation such as PhSiH
3
3
poor or no conversion. Although boranes such as BPh
3
2
formoxy silane on its own,[2e] it has recently been shown that the activity of zinc hydrides can
also be improved by the addition of boranes.[7a, 20b] When a mixture containing 2a (2 mol%) and
10
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Angewandte Chemie International Edition
10.1002/anie.202011480
BPh (2 mol%) was used as catalyst, activity and chemoselectivity were reduced giving
3
formoxy silane as the major product only after 96 h (Table 1, entry 5). Methoxy silane was
detected in 20% yield, but in contrast to applying 2a as the sole catalyst, methyl formate was
formed only in trace amounts. Polymeric zinc dihydride showed no activity either with or without
added TMEDA (Table 1, entry 6, 7). Methyl formate is not formed by Brønsted or Lewis acid
catalysts, as employing [NEt
Zn(OTf) ] gave exclusively formoxy silane (Table 1, entry 9). B(3,5-(CF
decomposed under the conditions of catalysis (Table 1, entry 10).
3
H][BArF
4
] showed no conversion (Table 1, entry 8), while
[
2
3
)
2
-C
6
H
)
3 3
Table 2: Catalytic hydrosilylation of CO .
2
1
2
.) cat., [Si]-H, CO2
.) cat., n [Si]-H
O
+
+
OCH3
[Si]
O
[Si]
[
Si]-H + CO
2
[Si]
CH3
70 °C, THF
H
O
B
C
Selec. [%][b,e]
Entry[a]
1[c]
Hydrosilane
Cat.[mol%]
Conv. [%][b]
Time [h]
B/C
n
73/27
62/38
BuMe
2
SiH
SiH
SiH
SiH
2a (4.0)
2a (4.0)
2a (4.0)
2a (4.0)
>99
>99
>99
>99
16
8
2[c]
EtMe
2
3[d]
n
>99/<1
>99/<1
BuMe
2
24
14
4[d]
EtMe
2
[
a] n(hydrosilane) = 0.18 mmol, 1 bar CO
2
, 0.5 mL of [D
determined by H NMR spectroscopy using 0.06 mM of hexamethylbenzene as internal standard.
c] Addition of 1 equiv., n(hydrosilane) = 0.18 mmol and 4 mol% of catalyst in step 2. [d] Addition
8
]THF. [b] Conversion and selectivity
1
[
of 2 equiv, n(hydrosilane) = 0.36 mmol and 4 mol% of catalyst in step 2. [e] Disiloxane was
formed in amounts corresponding to the overall stoichiometry (see Scheme 4).
The chemoselective hydrosilylation of CO to give methoxy silane can be achieved in a two
2
step one pot reaction at 70 °C (Table 2). Monitoring the reaction by NMR spectroscopy showed
rapid formation of formoxy silane, whereas formation of methoxy silane and methyl formate
was slow in the presence of CO
2
. When the remaining CO was removed and catalyst as well
2
as hydrosilane were added, full consumption of the formoxy silane and increase in methyl
n
formate concentration were observed. Using BuMe
2
SiH as hydrosilane, a mixture containing
73% of methoxy silane and 27% of methyl formate was obtained after 16 h (Table 2, entry 1).
When EtMe SiH was applied instead, selectivity toward methoxy silane was slightly higher and
2
the reaction time was reduced to 8 h (Table 2, entry 2). Only trace amounts of silylacetal are
detected.[19] When the reaction mixture was treated with an excess of hydrosilane after removal
11
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Angewandte Chemie International Edition
10.1002/anie.202011480
of CO
2
and catalyst (2 equiv. of hydrosilane, 4 mol% of catalyst), full conversion into methoxy
silane and disiloxane was observed (Table 2, entry 3 and 4).
[3]
Consistent with the mechanism suggested in the literature, fast insertion of CO into the zinc
2
hydride bond and subsequent metathesis of zinc formate with hydrosilane forms formoxy
silane. The latter is subsequently reduced to the silylacetal and eventually to methoxy silane
and disiloxane. Notably, the formation of methyl formate in the initial phase of catalysis has not
been reported. Methyl formate was found when formoxy silane underwent nucleophilic
substitution with methanol using a ruthenium phosphine catalyst.[25] To gain more insight into
the chemoselectivity, we probed the individual steps by stoichiometric reactions (Scheme 4).
n
When the hydride cation 2a was treated with BuMe
2
Si(O
2
CH), an exchange reaction was
n
n
observed giving the zinc formate cation 5a and BuMe
2
SiH. The BuMe
2
SiH thus formed
immediately reacted with 5a to give methoxy silane, methyl formate, and disiloxane. In
contrast, methoxy silane remained inert to such an exchange under the same conditions. When
a mixture of isolated formoxy silane and methoxy silane was reacted in the presence of either
2a or 5a methyl formate and disiloxane were formed. In the absence of zinc catalysts formoxy
silane and methoxy silane did not react even at elevated temperatures. We therefore suggest
that methyl formate is produced by the nucleophilic attack of formoxy silane by methoxy silane,
catalyzed by the Lewis acidic zinc cation (Scheme 4c). When excess hydrosilane was present
the formation of methoxy silane and disiloxane was observed exclusively (Table 2, entry 3 and
4), because methyl formate is known to form methoxy silane by hydrosilane in the presence of
a suitable catalyst.[26] Therefore the cationic zinc hydride can act as a hydrosilylation catalyst
to convert methyl formate into methoxy silane as soon as formoxy silane is fully consumed
(
Scheme 4c). Formally methyl formate can be regarded as the equivalent of bis(silyl)acetal,[5,20]
which was only detected in trace amounts under the catalytic conditions studied. We assume
that the zinc hydride cation forms a zinc acetal [Zn]-OCH OCH . This intermediate undergoes
further attack by hydrosilane to give methoxy silane, disiloxane, and zinc hydride cation.
2
3
12
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Angewandte Chemie International Edition
10.1002/anie.202011480
Overall stoichiometry
+
CO + 3 [Si]-H
ZnH]
+ [Si]-O-[Si]
+ [Si]-O-[Si]
[
2
[Si]-OCH
3
a) Hydrosilylation of CO
2
+
[
Si]-H + CO
ZnH]
ZnH]
[
[
2
2CH
[Si]-O
+
2CH + 2 [Si]-H
Si]-O
[Si]-OCH3
[
b) Reversible reaction of formoxy silane with zinc hydride cation
+
+
2CH + [ZnH]
Zn-O CH]
2
[
Si]-O
[Si]-H + [
c) Formation of methyl formate and its reduction to give methoxy silane
Si] OCH3
[
+
[ZnH]+
O
2CH + [Si]-OCH3
O
H
+ [Si]-O-[Si]
CH3
[
Si]-O
[
Si] δ+
- [ZnH]+
H
O
O
+
ZnH]
[
H
Si]
[
O
O
+
[ZnH]+
H
H
CH3
O
+ 2 [Si]-H
2 [Si]-OCH3
H
H C
+
3
- [
ZnH]
O
H
+
[
Si]
[
Zn
]
Scheme 4: Catalytic hydrosilylation of CO
by cationic zinc hydride [ZnH]+.
2
Conclusion
In conclusion, we have demonstrated that by using simple diamines TMEDA and TEEDA as
+
supporting donor ligands, the quasi-tricoordinate cationic zinc hydrides [(L
2
)ZnH] can be
isolated as mono- or dinuclear complexes 2 and structurally characterized. Interestingly,
+
[
[
(L
(L
2
2
)ZnH] forms adducts 3 and 4 with the hypothetical neutral molecular zinc dihydride
)ZnH
2
]. Although the solid state structure of parent [ZnH
2
]
n
still remains unknown, the
, resulting in molecular zinc
+
accessibility of [(L
2
)ZnH] facilitates the de-aggregation of [ZnH
]
2 n
hydrides of improved thermal stability and solubility. In agreement with the facile CO
2
insertion
into the Zn-H bond, the hydrosilylation of CO
2
using hydosilanes to give methoxy silane are
+
]
itself.[13e] As a hitherto unknown intermediate of zinc-
catalyzed by [ZnH] but not by [ZnH
2
n
catalyzed CO hydrosilylation, methyl formate has been identified. It results from the zinc-
2
catalyzed transesterification of kinetically preferred formoxy silane by methoxy silane and is
eventually hydrosilylated to give methoxy silane as the thermodynamic product. As recently
13
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Angewandte Chemie International Edition
10.1002/anie.202011480
noted by Hazari et al. in a comprehensive study on the various effects influencing the
chemoselectivity of group 10 metal-catalyzed hydroboration of CO
2
,[27] the Lewis acidity of the
+
metal center is critical but not the sole factor. With the access of [(L
2
)ZnH] , a comparison of
various homogeneous catalyst based on 3d metals[1] can be performed and the question
addressed whether d-electron configurations affect Lewis acidity.[4l]
Notes
The authors declare no competing financial interest.
Acknowledgements
The research was funded by the Deutsche Forschungsgemeinschaft. We thank Dr. G. Fink for
NMR measurements and Profs. A. Venugopal and M. Ingleson for valuable discussions.
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Heterogeneous catalysts based on noble metals allow the formation of methyl
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2
2
16
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Angewandte Chemie International Edition
10.1002/anie.202011480
a reaction medium such as methanol. Ruthenium is used in homogeneous catalysis.
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