Efficient CO2 Insertion and Reduction Catalyzed by a Terminal Zinc Hydride Complex

Article abstract of DOI:10.1002/anie.201801800

The terminal zinc hydride complex [Tntm]ZnH (2; Tntm=tris(6-tert-butyl-3-thiopyridazinyl)methanide) is an efficient hydrosilylation catalyst of CO₂ at room temperature without the need of Lewis acidic additives. The inherent electrophilicity of

Full text of DOI:10.1002/anie.201801800

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Efficient CO2 Insertion and Reduction Catalyzed by a Terminal
Zinc Hydride Complex
Authors: Michael Tüchler, Lisa Gärtner, Susanne Fischer, Adrian
Daniel Boese, Ferdinand Belaj, and Nadia C. Mösch-Zanetti
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201801800
Angew. Chem. 10.1002/ange.201801800
Link to VoR: http://dx.doi.org/10.1002/anie.201801800
http://dx.doi.org/10.1002/ange.201801800

10.1002/anie.201801800
Angewandte Chemie International Edition
COMMUNICATION
Efficient CO2 Insertion and Reduction Catalyzed by a Terminal
Zinc Hydride Complex
Michael Tüchler,a Lisa Gärtner, a Susanne Fischer,a,b A. Daniel Boese,b Ferdinand Belaj, a Nadia C.
Mösch-Zanetti a*
Abstract: The terminal zinc hydride complex [Tntm]ZnH (2) (Tntm =
tris(6-tert-butyl-3-thiopyridazinyl)methanide) was found to be an
efficient hydrosilylation catalyst of CO2 for the first time at room
temperature without the need of Lewis acidic additives. The inherent
electrophilicity of the system leads to selective formation of the
monosilylated product (MeO)3SiO2CH (at room temperature with a
TOF of 22.2 h-1 and at 45°C with a TOF of 66.7 h-1). In absence of
silanes the intermediate formate complex [Tntm]Zn(O2CH) (3) is
quantitatively formed within
5 min. All complexes are fully
characterized via H and 13C NMR spectroscopy and single crystal
X-ray diffraction analyses. Density functional theory (DFT)
calculations reveal a high positive charge on zinc and the increased
preference of the ligand to adopt a κ3-coordination mode.
1
The use of carbon dioxide as substrate for catalytic
transformations has received considerable attention not only due
to environmental aspects, but also because it represents a
cheap and potentially useful C1 feedstock.[1] Reduction to the
formic acid level is of particular interest, because it can also be
used as fuel.[2] Furthermore, due to its excellent potential as H2
carrier, the demand of formic acid is expected to increase in the
near future.[3]
The catalytic reduction of CO2 with silanes is well known in
literature, mainly using ruthenium complexes with a combination
of high CO2 pressure (20 – 80 bar) and moderate to high
temperatures (45-100°C),[4] but also Cu,[5] Ir,[6] and Pd[7]
complexes were found to catalyze this reaction under mild
conditions. In terms of price, abundance and toxicity of the metal
center, the use of zinc based systems is of significant interest.
Thus, it is surprising that only very few examples of zinc
catalyzed hydrosilylation of CO2 have been reported in literature:
two cationic, N-heterocyclic carbene (NHC) stabilized zinc
complexes, a dicationic NHC-stabilized zinc hydride cluster, as
well as the tris(thiopyridyl)methane based [Tptm]ZnH and the
tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methane based
[TismPriBenz]ZnH complexes were shown to catalytically react
with CO2 in presence of silanes (Figure 1).[8–12]
Figure 1. Reported zinc based catalysts for the hydrosilylation of CO2.[8–12]
However, these systems require either high temperatures and
high pressures or only show low to moderate turnover numbers
and turnover frequencies. Only the recently introduced
[TismPriBenz
]
based system was found to catalyze the
hydrosilylation of CO2 under ambient conditions, albeit with the
use of
a borane additive and with only low turnover
frequencies.[11]
Herein we show that the recently introduced [Tntm] system[13]
allows the isolation of the terminal zinc hydride complex
[Tntm]ZnH. Compared to the similar [Tptm]ZnH complex, the
more electron deficient thiopyridazine heterocycles were
expected to render the metal center more Lewis acidic and thus
more reactive (Figure 1). Indeed, we found [Tntm]ZnH to
catalytically reduce CO2 with trimethoxysilane at room
temperature and under mild conditions. This represents the first
zinc based system, capable of catalytically reducing CO2 at
room temperature without the need of any additive and is also
the most active system up to date. The intermediate formate
complex [Tntm]Zn(O2CH) was isolated and fully characterized.
[Tntm]ZnH (2) was prepared by reaction of the respective zinc
trimethylsilanolate complex [Tntm]Zn(OSiMe3) (1) with a slight
excess of (MeO)3SiH in THF in 85% yield, similar to a previously
described method.[14]
[a]
[b]
M. Tüchler, L. Gärtner, S. Fischer, Prof. F. Belaj, Prof. N.C. Mösch-
Zanetti
Institute of Chemistry, Inorganic Chemistry
University of Graz
Schubertstrasse 1, 8010 Graz, Austria
E-mail: nadia.moesch@uni-graz.at
S. Fischer, Prof. A.D. Boese
Institute of Chemistry, Physical and Theoretical Chemistry
University of Graz
Heinrichstrasse 28, 8010 Graz, Austria
In solid state the hydrido compound is moderately stable at
ambient atmosphere whereas in solution it is sensitive towards
moisture and decomposes to yet unidentified products. However,
Supporting information for this article is given via a link at the end of
the document
This article is protected by copyright. All rights reserved.

10.1002/anie.201801800
Angewandte Chemie International Edition
COMMUNICATION
1
in methylene chloride, also in absence of moisture, 2 is only
stable below -30°C, while at room temperature it decomposes in
the course of 4 – 6 h, predominantly forming [κ4-C,N,S,S-
Tntm]Zn(Pn) (4, Pn = 6-tert-butyl-3-thiopyridazine, see Figures
S8-S10, S19, SI). Thus, NMR spectroscopy in CD2Cl2 is possible
if recorded immediately after sample preparation, revealing the
resonance for the hydride at 4.35 ppm in CD2Cl2 and 5.75 ppm
in C6D6, which is typical for zinc hydride complexes, ranging
from 3.99 ppm in neutral to 5.60 ppm in monoanionic
complexes.[14,15,16,17] Furthermore, also in the AT-IR spectrum the
Zn-H stretching vibration is found at 1769 cm-1 as a sharp signal.
The formation of the hydride complex was unambiguously
confirmed via single crystal X-ray diffraction analysis which
reveals a distorted tetrahedral compound with the ligand
exhibiting the κ3-C,N,N coordination mode (Figure 2). The
hydrogen atom was identified by a difference Fourier map
leading to a Zn1-H1 bond length of 1.72(3) Å. In comparison to
the few structurally characterized, mononuclear zinc hydride
complexes, this represents a rather long Zn-H distance, with the
majority being in the range of 1.384 Å - 1.600 Å.[18],[17] Only a
diaminophenolate based (1.75(3) Å)[16] and a cationic, NHC-
stabilized zinc hydride complex (1.92(3) Å)[17] were found
exhibiting longer Zn-H distances.
Subsequent H NMR measurements revealed full conversion of
the silane to the respective silyl formate within 4.5 h at room
temperature. This is evidenced by resonances at 7.67 ppm and
3.44 ppm in the 1H NMR spectrum (1:9 ratio) and 158.5 ppm and
51.2 ppm in the 13C NMR spectrum. Furthermore, in analogy to
(EtO)3SiO2CH, after addition of HCl the formed (MeO)3SiO2CH
rapidly reacts to methyl formate and methanol in approximately
80% yield, determined by H NMR spectroscopy.[8,19] It is also
1
noteworthy that during the course of catalysis the solution
remains homogenous and without any change of color indicating
no decomposition of 2 and the subsequently formed zinc
formate complex 3 (vide infra).
Increasing the reaction temperature to 45°C leads to
a
significantly faster reaction, achieving 99% conversion after 2 h
(TOF 49.5 h-1). The catalytically active zinc hydride complex 2 is
even more efficient at 45°C, revealing 95% conversion after 1
hour (TOF 95 h-1) and full conversion after 1.5 h (TOF 66.7 h-1),
as displayed in Table 1.
Table 1. Reported zinc catalysts for the hydrosilylation of CO2
catalyst
CO2
[bar]
T
[°C]
Silane
TOF
[h-1]
ref
this work
this work
this work
this work
[11]
[Tntm]ZnH
1
45
45
rt
(MeO)3SiH
(MeO)3SiH
(MeO)3SiH
(MeO)3SiH
PhSiH3
66.7
49.5
23.7
22.2
0.8
[Tntm]Zn(OSiMe3)
[Tntm]Zn(O2CH)
[Tntm]Zn(OSiMe3)
[TismPriBenz]ZnH [a]
[TismPriBenz]ZnH [b]
[(IDipp)ZnMe]+
1
1
1
rt
1
rt
[11]
1
rt
Ph3SiH
0.1
[12]
1.5
1.5
6.8
1.5
40
90
90
100
90
60
Et3SiH
5.0
[12]
[(IDipp)Zn(C6F5)]+
[Tptm]ZnH
Et3SiH
32.4
2.9
[8]
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
[9]
[(aIDipp)(nIDipp)ZnMe]+
[(IMes)3Zn3H4(THF)]2+
0.23
1.25
[10]
[a] additional 5 mol% of B(C6F5)3 (2 equiv per Zn) [b] additional 10 mol% of
B(C6F5)3 (5 equiv per Zn)
Figure 2. Molecular structure of [Tntm]ZnH (2). Hydrogen atoms, except for
the Zn-H are omitted for clarity. Selected bond lengths and angles: C1-Zn1:
2.115(4) Å, Zn1-H1: 1.72(3) Å, C1-Zn1-H1: 135.6(11)°.
The catalytic efficiency of our system is significantly higher
compared to the few previously described zinc based systems.
They require moderate to high CO2 pressure and higher
temperatures, while the [Tntm]-based system is highly efficient
at low pressure and moderate to ambient temperatures (Table 1).
The very recently reported [TismPriBenz]ZnH complex represents
the only other example capable of reducing CO2 at room
temperature, albeit it requires the addition of a borane. However,
even in presence of the activator, the observed turnover
frequencies are low (Table 1). Thus, the here presented [Tntm]
based complexes represent the fastest zinc based catalysts for
the hydrosilylation of CO2 to date.
The zinc hydride complex 2, as well as the precursor complex 1
and the intermediately formed zinc formate complex 3 (vide
infra) were found to be excellent catalysts for the hydrosilylation
of CO2, as displayed in Scheme 1. Thus,
1 mol% of
[Tntm]Zn(OSiMe3) (1), together with trimethoxysilane and
mesitylene as internal standard were suspended in C6D6 under 1
atm of CO2.
In order to evaluate the mechanism, we also performed the
reaction in absence of silane. While 1 remains unreactive under
CO2 atmosphere, the zinc hydride complex 2 reacts quickly to
the respective formate complex [Tntm]Zn(O2CH) (3), which can
be isolated in 87% yield as a light yellow solid (Scheme 2).
Scheme 1. Catalytic hydrosilylation of CO2 using [Tntm]Zn(OSiMe3) (1),
[Tntm]ZnH (2) and [Tntm]Zn(O2CH) (3) under mild conditions.
This article is protected by copyright. All rights reserved.

10.1002/anie.201801800
Angewandte Chemie International Edition
COMMUNICATION
The successful insertion of CO2 is apparent in the 1H NMR
spectrum in CD2Cl2, where the resonance for the zinc hydride is
absent with the concomitant presence of a broad singlet at 8.47
ppm for the formate proton. An additional signal at 168.42 ppm
in the 13C NMR spectrum, as well as an IR band at 1621 cm-1
also indicates the successful formation of complex 3. These
signals are in the same region as Parkin’s [Tptm]Zn(O2CH)[14] or
Na(O2CH).[20]
the C=O band of 3 at 1621 cm-1 during the reaction (Figure 4,
Figure S17, SI), it is apparent that for about 120 s no change of
transmittance occurs. This might correspond to the time for CO2
to exchange with N2 in the flask. After that, the transmittance for
the C=O band decreases significantly over a period of about 100
s, indicating the formation of the zinc formate complex 3.
Similarly, but not as distinctive, also the transmittance for the Zn-
H band increases, revealing the consumption of [Tntm]ZnH.
After approximately 250 s overall, the transmittance of both, the
Zn-H and C=O bands, remain virtually identical, only showing
the drift of the device (due to evaporation of methylene chloride
under CO2 flow). Also, after 500 s, the formate complex starts to
precipitate due to its high concentration.
Scheme 2. Preparation of [Tntm]Zn(O2CH) (3) via CO2 insertion into the Zn-H
bond.
Additionally, the molecular structure of 3 was confirmed by
single crystal X-ray diffraction analysis (Figure 3). It reveals the
ligand to be coordinated to the zinc center in a κ4-C,N,N,N
atrane – motif and the formate O2 oxygen being involved in a
hydrogen bridging bond to a CH2Cl2 molecule (O2-H 2.174(6) Å).
However, according to DFT calculations in the absence of
CH2Cl2, the formate was found to coordinate bidentate leading to
a distorted octahedral geometry as displayed in Scheme 2.
Furthermore, the κ4 coordination of the ligand in 3 was found to
be more stable by 8.58 kJ/mol compared to the respective κ3
coordination (Table S4, SI).
Figure 4. In-situ IR spectroscopy: transmittance of the Zn-H and the C=O
bands plotted against time.
We attribute this fast reaction with CO2 to the high electrophilicity
of the zinc center which is supported by calculations of the NBO
charge. For comparison we also calculated the NBO charges of
the zinc systems listed in Figure 1 and found all of them ranging
from 1.32 to 1.57 (Table S5, SI). Thus, it is apparent that a high
charge is crucial for the activation of CO2 which is in accordance
to previous findings showing (pre)coordination to precede
hydride transfer in mononuclear zinc hydride complexes.[21]
Not surprisingly, due to the similar NBO charges of the [Tntm]
and [Tptm] systems, their calculated activation energies for the
CO2 insertion are almost identical (11.6 kcal/mol for [Tptm] vs.
12.7 kcal/mol for [Tntm], Figure S23, SI) arguing against this
step being rate determining. However, a significantly higher
energy was found for our zinc formate complex (-15.5 kcal/mol
for [Tntm] vs. -19.2 kcal/mol for [Tptm], Figure S23, SI). The
energetically higher [Tntm]Zn(O2CH) allows for faster
subsequent reactions explaining the higher catalytic efficiency.
Furthermore, the high flexibility of the [Tntm] ligand, evidenced
by the low energy differences of the κ3 and κ4 coordination
modes (Table S4, SI), leads to a dynamic equilibrium between
the two modes as demonstrated by VT NMR spectroscopy of 1
and 2. Calculations reveal a pronounced tendency for κ3
coordination which is additionally beneficial for the fast catalytic
transformation of CO2, as a vacant coordination site for strong
substrate pre-coordination is available.
Figure 3. Molecular structure of [κ4-C,N,N,N-Tntm]Zn(O2CH)·CH2Cl2 (3).
Hydrogen atoms except for the formate proton and those, bond to methylene
chloride are omitted for clarity. Selected bond lengths and angles: C1-Zn1:
2.144(7) Å, Zn1-O1: 1.944(5) Å, C1-Zn1-O1 175.9(2)°.
The fast reaction of [Tntm]ZnH with CO2 at -40°C led us to
further evaluate the reactivity of the zinc hydride complex. Thus,
we performed an in-situ IR study of the CO2 insertion reaction
displayed in Scheme 2. For this purpose, a concentrated
solution of 2 in methylene chloride (0.11M) was prepared, and
the flask was purged with CO2. IR spectra were measured
continuously every 25 seconds over a period of 30 min.
Comparing the progress of the Zn-H band of 2 at 1765 cm-1 and
In conclusion, we present here a highly active zinc based
catalytic system capable of CO2 reduction with trimethoxysilane
at mild conditions (1 atm, room temperature). The metal is
coordinated by a tris(thiopyridazinyl)methanide ligand which
provides an electron-poor environment enabling fast reaction
This article is protected by copyright. All rights reserved.

10.1002/anie.201801800
Angewandte Chemie International Edition
COMMUNICATION
[21] a) J. Gajewy, J. Gawronski, M. Kwit, Eur. J. Org. Chem. 2013, 307–
318; b) D. L. DuBois, D. E. Berning, Appl. Organomet. Chem. 2000, 14,
860–862; c) M. M. Deshmukh, S. Sakaki, Inorg. Chem. 2014, 53, 8485–
8493.
with nucleophilic substrates. The combination of the flexibility of
the ligand regarding denticity, as well as the formation of an
electrophilic zinc center is key to success in the here presented
highly demanding reaction. Further use of the thiopyridazine
methanide ligand is currently investigated.
Acknowledgements
We thank Fabian Wiedemaier for useful discussions. Financial
support from NAWI Graz is gratefully acknowledged.
Keywords: Carbon dioxide fixation • Homogeneous catalysis •
Hydrides • Hydrosilylation • Zinc
[1]
[2]
a) Q. Liu, L. Wu, R. Jackstell, M. Beller, Nat. Commun. 2015, 6, 5933;
b) S. Bontemps, Coord. Chem. Rev. 2016, 308, 117–130; c) M. Aresta,
Carbon Dioxide as Chemical Feedstock, Wiley-VCH Verlag GmbH &
Co. KGaA, Weinheim, Germany, 2010.
a) M. G. Mura, L. D. Luca, G. Giacomelli, A. Porcheddu, Adv. Synth.
Catal. 2012, 354, 3180–3186; b) A. Otto, T. Grube, S. Schiebahn, D.
Stolten, Energy Environ. Sci. 2015, 8, 3283–3297; c) L. Plasseraud,
ChemSusChem 2010, 3, 631–632.
[3]
[4]
a) A. K. Singh, S. Singh, A. Kumar, Catal. Sci. Technol. 2016, 6, 12–40;
b) D. Mellmann, P. Sponholz, H. Junge, M. Beller, Chem. Soc. Rev.
2016, 45, 3954–3988.
a) P. Deglmann, E. Ember, P. Hofmann, S. Pitter, O. Walter, Chem. Eur.
J. 2007, 13, 2864–2879; b) A. Jansen, H. Görls, S. Pitter,
Organometallics 2000, 19, 135–138; c) A. Jansen, S. Pitter, J. Mol.
Catal. A: Chem. 2004, 217, 41–45; d) G. Süss-Fink, J. Reiner, J.
Organomet. Chem. 1981, 221, C36-C38; e) H. Koinuma, F. Kawakami,
H. Kato, H. Hirai, J. Chem. Soc., Chem. Commun. 1981, 213–214; f) P.
G. Jessop, Top. Catal. 1998, 5, 95–103.
[5]
[6]
a) L. Zhang, J. Cheng, Z. Hou, Chem. Commun. 2013, 49, 4782–4784;
b) K. Nakamae, M. Tanaka, B. Kure, T. Nakajima, Y. Ura, T. Tanase,
Chem. Eur. J. 2017, 23, 9457–9461.
a) R. Lalrempuia, M. Iglesias, V. Polo, P. J. Sanz Miguel, F. J.
Fernández-Alvarez, J. J. Pérez-Torrente, L. A. Oro, Angew. Chem. Int.
Ed. 2012, 51, 12824–12827; b) E. A. Jaseer, M. N. Akhtar, M. Osman,
A. Al-Shammari, H. B. Oladipo, K. Garcés, F. J. Fernández-Alvarez, S.
Al-Khattaf, L. A. Oro, Catal. Sci. Technol. 2015, 5, 274–279.
J. Takaya, N. Iwasawa, J. Am. Chem. Soc. 2017, 139, 6074–6077.
W. Sattler, G. Parkin, J. Am. Chem. Soc. 2012, 134, 17462–17465.
D. Specklin, C. Fliedel, C. Gourlaouen, J.-C. Bruyere, T. Avilés, C.
Boudon, L. Ruhlmann, S. Dagorne, Chem. Eur. J. 2017, 23, 5509–5519.
[7]
[8]
[9]
[10] A. Rit, A. Zanardi, T. P. Spaniol, L. Maron, J. Okuda, Angew. Chem. Int.
Ed. 2014, 53, 13273–13277.
[11] M. Rauch, G. Parkin, J. Am. Chem. Soc. 2017, 139, 18162–18165.
[12] D. Specklin, F. Hild, C. Fliedel, C. Gourlaouen, L. F. Veiros, S. Dagorne,
Chem. Eur. J. 2017, 23, 15908–15912.
[13] M. Tüchler, S. Holler, E. Huber, S. Fischer, A. D. Boese, F. Belaj, N. C.
Mösch-Zanetti, Organometallics 2017, 36, 3790–3798.
[14] W. Sattler, G. Parkin, J. Am. Chem. Soc. 2011, 133, 9708–9711.
[15] a) J. Spielmann, D. Piesik, B. Wittkamp, G. Jansen, S. Harder, Chem.
Commun. 2009, 3455–3456; b) R. Han, I. B. Gorrell, A. G. Looney, G.
Parkin, J. Chem. Soc., Chem. Commun. 1991, 717–719.
[16] N. J. Brown, J. E. Harris, X. Yin, I. Silverwood, A. J. P. White, S. G.
Kazarian, K. Hellgardt, M. S. P. Shaffer, C. K. Williams,
Organometallics 2014, 33, 1112–1119.
[17] P. A. Lummis, M. R. Momeni, M. W. Lui, R. McDonald, M. J. Ferguson,
M. Miskolzie, A. Brown, E. Rivard, Angew. Chem. Int. Ed. 2014, 53,
9347–9351.
[18] M. Rombach, H. Brombacher, H. Vahrenkamp, Eur. J. Inorg. Chem.
2002, 153–159.
[19] M.-A. Courtemanche, M.-A. Légaré, L. Maron, F.-G. Fontaine, J. Am.
Chem. Soc. 2013, 135, 9326–9329.
[20] R. Newman, J. Phys. Chem. 1952, 20, 1663–1664.
This article is protected by copyright. All rights reserved.

10.1002/anie.201801800
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
CO2 Inside the Hydride: The terminal
zinc hydride complex [Tntm]ZnH,
based on the electron deficient tris(6-
tert-butyl-3-thiopyridazinyl)methanide
ligand was found being an excellent
catalyst for the hydrosilylation of CO2
at room temperature, forming
Michael Tüchler, Lisa Gärtner, Susanne
Fischer, A. Daniel Boese, Ferdinand
Belaj, Nadia C. Mösch-Zanetti *
Page No. – Page No.
Efficient CO2 Insertion and Reduction
Catalyzed by a Terminal Zinc Hydride
Complex
exclusively the respective silyl
formate. We attribute this fast reaction
to the electron deficient properties of
the [Tntm] ligand in combination with
its flexible coordination.
This article is protected by copyright. All rights reserved.