Application of Hetero-Triphos Ligands in the Selective Ruthenium-Catalyzed Transformation of Carbon Dioxide to the Formaldehyde Oxidation State

Article abstract of DOI:10.1021/acs.organomet.9b00107

Due to the increasing demand for formaldehyde as a building block in the chemical industry as well as its emerging potential as feedstock for biofuels in the form of dimethoxymethane and the oxymethylene ethers produced therefrom, the catalytic transformation of carbon dioxide to the formaldehyde oxidation state has become a focus of interest. In this work, we present novel ruthenium complexes with hetero-triphos ligands, which show high activity in the selective transformation of carbon dioxide to dimethoxymethane. We substituted the apical carbon atom in the backbone of the triphos ligand platform with silicon or phosphorus and optimized the reaction conditions to achieve turnover numbers as high as 685 for dimethoxymethane. The catalytic systems could also be tuned to preferably yield methyl formate with turnover numbers of up to 1370, which in turn can be converted into dimethoxymethane under moderate conditions.

Full text of DOI:10.1021/acs.organomet.9b00107

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Application of Hetero-Triphos Ligands in the Selective Ruthenium-
Catalyzed Transformation of Carbon Dioxide to the Formaldehyde
Oxidation State
Max Seibicke, Max Siebert, Alexander F. Siegle, Sophie M. Gutenthaler, and Oliver Trapp*
̈
̈
̈
Department Chemie, Ludwig-Maximilians-Universitat Munchen, Butenandtstr. 5-13, 81377 Munchen, Germany
S
* Supporting Information
ABSTRACT: Due to the increasing demand for formaldehyde as a building
block in the chemical industry as well as its emerging potential as feedstock
for biofuels in the form of dimethoxymethane and the oxymethylene ethers
produced therefrom, the catalytic transformation of carbon dioxide to the
formaldehyde oxidation state has become a focus of interest. In this work, we
present novel ruthenium complexes with hetero-triphos ligands, which show
high activity in the selective transformation of carbon dioxide to dimethoxy-
methane. We substituted the apical carbon atom in the backbone of the
triphos ligand platform with silicon or phosphorus and optimized the reaction
conditions to achieve turnover numbers as high as 685 for dimethoxymethane.
The catalytic systems could also be tuned to preferably yield methyl formate
with turnover numbers of up to 1370, which in turn can be converted into
dimethoxymethane under moderate conditions.
Scheme 1. Overview of Different Ruthenium Catalysts for
the Synthesis of DMM from CO2
INTRODUCTION
■
In recent years, the utilization of carbon dioxide as renewable
feedstock has become a major focus in academia and the
chemical industry.1 Driven by the postulated and already
emerging dramatic global climate change due to the higher
concentration of atmospheric greenhouse gases, such as CO2,
the necessity for sustainable and atom-efficient processes in
industry is as high as never before.2
CO2 has been utilized as the starting material for the
synthesis of a variety of important organic molecules. A
number of these showed the potential for process develop-
ment, considering economic as well as ecologic factors.1b,3 Yet,
conversion of CO2 to the formaldehyde oxidation level remains
challenging, and only little progress was made on the road to
an industrially feasible process.3e,4
Besides the academic approaches toward formaldehyde and
its derivatives based on hydroboration,5 hydrosilylation,6 and
frustrated Lewis pairs,7 only a few attempts meeting the
industrial requirements have been published until now. In
2016, Klankermayer and Leitner showed that the catalytic
transformation of carbon dioxide to dimethoxymethane
(DMM), the dimethyl acetal of formaldehyde, is viable. By
employing a ruthenium-based triphosPh complex and Al(OTf)3
as an additive, they achieved a turnover number (TON) of 214
for DMM (Scheme 1), and methyl formate (MF) was
observed as the only side product.8 This pathway to DMM
is more atom-efficient than the common approaches, including
acid-catalyzed acetalization of formaldehyde or methanol
oxidation utilizing heterogeneous catalysts.4 Furthermore,
Schieweck and Klankermayer reported the use of modified
triphos ligands and cobalt salts for the same reaction, achieving
a TON of 157 for DMM with increased selectivity.9 Recently,
our group was able to raise TONs for DMM to 786 using a
ruthenium catalyst with an N-triphosPh ligand and approaching
the multivariate catalytic system in a stepwise optimization
process (Scheme 1). This way, maxima in activity for
temperature, partial pressure of hydrogen and carbon dioxide,
time as well as catalyst and additive loading were determined.10
These excellent results, achieved by tuning the established
ruthenium triphos system, encouraged us to investigate further
steric and electronic modifications of the triphos ligand
structure. The backbone of these tripodal ligands can be
Received: February 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00107
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Article
varied in a modular way, following established synthetic
procedures.11 We envisioned substitution of the apical atom in
the backbone with heteroatoms, such as silicon and
phosphorus (Scheme 1), to significantly alter the reactivity of
the catalytic system, as has been demonstrated in other
reports.12
distance (2a: 3.63 Å, 2b: 3.64 Å) between the apical atom and
the metal center. This trend results partly from the larger
atomic radius of silicon, which is indicated by the longer X−C
bonds of 1.88 and 1.89 Å (Table 1, entries 1−3). Furthermore,
smaller X−C−P bond angles of 110° for both compounds as
well as significantly higher X−C−P−Ru torsion angles of 35.0
and 37.1° have been found (Table 1, entries 1−3), whereas the
tetrahedral C−X−C bond angles of the apical atoms are similar
to the triphos complex. Interestingly, substitution of methyl to
phenyl on the apical silicon has no significant effect on the
complex geometry.
RESULTS AND DISCUSSION
■
We started our studies by preparing triphos ligands with silicon
and phosphorus in the backbone. To ensure comparability
with previously reported systems and to gain insights into the
effects of these structural variations, all synthesized ligands
were kept tripodal and tridentate. In addition, the investigated
complexes were prepared with the doubly charged trimethy-
lenemethane anion as the counterion.
In the reported crystal structures of N-triphos ligands, the
X−C−P bond angle is 114° and thus in between those of Si-
triphosPh and triphosPh (Table 1, entries 1−5). The distance
from nitrogen to ruthenium is the shortest at 3.46 Å, which
correlates with the largest tetrahedral C−X−C bond angle of
115° and the shortest X−C bond lengths of 1.47 and 1.46 Å
for all compared complexes (Table 1, entries 1−5). The X−
C−P−Ru dihedral angles of N-triphos ligands are smaller
(27.3 and 25.6°) than that of carbon-based triphosPh (Table 1,
entries 3−5). With this information in hand, we were curious
about the catalytic performance of the novel complexes.
We started screening the complexes 2a−c, employing the
previously optimized reaction conditions found for the N-
The synthesis of tripodal Si-triphosPh (1a,b) and P-triphosPh
(1c) ligands was achieved in a modular two-step procedure
based on publications by the groups of Tilley,13 LaBelle,14
Gramlich,15 and Alvarez.16 Deprotonation of the methyl group
́
of MePPh2, using n-BuLi and N,N,N′,N′-tetramethylethylene-
diamine (TMEDA), resulted in the formation of the
Ph2PMeLi−TMEDA complex, which was directly used in a
nucleophilic substitution. Si-triphosPh ligands (1a,b) were
obtained by reaction with phenyl- and methyltrichlorosilane
in 71−90% yield (Scheme 2A). P-triphosPh (1c) was prepared
in 66% yield using triphenyl phosphite (Scheme 2B).
triphosPh system (V = 0.5 mL, T = 90 °C, pH = 90 bar, pCO
=
2
2
20 bar, t = 18 h, ncat = 0.38 μmol, nadd = 1.56 μmol
Al(OTf)3).10 Varying the reaction temperature, we observed
maxima for TONs of DMM at 90 °C for the silicon-based
complexes 2a,b and at 100 °C for the phosphorus-based
complex 2c. Already for the first set of applied conditions, the
TON for DMM exceeded 500, using catalyst 2b. Interestingly,
with higher temperatures, the selectivity (ratio of DMM/MF)
increased for 2b and 2c but decreased for 2a (Figure 2A). In
the screening of the applied H2 pressure, only small effects on
the catalysis were observed (Figure 2B), whereas for different
CO2 pressures, the behavior of catalysts 2a−c was found to be
similar to that of the N-triphosPh system. Here, lower CO2
pressures led to enhanced selectivity, while MF formation
declined. For all three catalysts, the best performance in terms
of DMM formation was found at 20 bar CO2 pressure (Figure
2C). Next, we varied the catalyst and additive loading. As
expected, both parameters influence the catalytic system
significantly (Figure 2D,E). In all three cases, the TON for
MF shows a reciprocal correlation with the catalyst amount,
i.e., doubling the catalyst loading leads to reduction of the
TON for MF by half. With increasing catalyst loading, the
TON for DMM decreases for 2a, whereas complex 2b shows a
maximum at 0.38 μmol. For 2c, DMM formation is barely
influenced. As a consequence, the selectivity increases strongly
upon increasing the amount of catalyst. At a catalyst loading of
0.19 μmol, the TON for MF exceeds 1250 for all three
complexes (Table 2, entries 7−9) and has a maximum of 1370
for catalyst 2b, which, to the best of our knowledge, is
currently the highest reported value in this transformation
setup. Using 0.75 μmol of 2b, the selectivity improves to the
highest value of 67% (ratio of DMM/MF is 2:1). With higher
additive loading, while keeping the amount of the investigated
catalysts 2a−c constant, the TON for MF declines, whereas
the TON for DMM and the selectivity improve. By using 3.13
μmol additive, the TONs for DMM of the silicon-based
triphos complexes 2a,b were 678 and 685, respectively, thus
reaching the highest activity herein (Table 2, entries 2 and 3).
Scheme 2. Synthetic Procedure towards the Tripodal
Ligands and Their Ruthenium Complexes
The ruthenium complexes were synthesized according to a
method by Klankermayer and Leitner.17 A mixture of [Ru(2-
methylallyl)2(COD)] and the respective ligand was heated in
toluene to 110 °C (Scheme 2C) for 5 h (2a, yield: 60%), 15 h
(2b, yield: 56%), or 4.5 h (2c, yield: 16%). Crystal structures
of complexes 2a and 2b, containing the silicon-based ligands
PhSi-triphosPh (1a) and MeSi-triphosPh (1b), showed the
expected distorted octahedral geometry (Figure 1A,B).
When compared to the ruthenium complex with the carbon-
based triphosPh ligand, 2a and 2b exhibit a higher spatial
B
DOI: 10.1021/acs.organomet.9b00107
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Article
Figure 1. Molecular structure of complexes 2a (A) and 2b (B) with ellipsoids drawn at the 50% probability level (hydrogen atoms are omitted for
clarity; carbon: gray; phosphorus: orange; ruthenium: turquoise; silicon: beige).
Table 1. Selected Structural Parameters for Triphos and Hetero-Triphos Ligands in Ruthenium Complexes with tmm as the
Counterion
bc
,
space
group
torsion angle
(deg)
a
b
b
b
entry
ligand
(PhSi-triphosPh
MeSi-triphosPh
(triphosPh)18
(N-triphosPh)19
X−Ru dist. (Å) X−C bond length (Å) X−C−P bond angle (deg) C−X−C bond angle (deg)
1
2
3
4
5
)
)
P1
̅
3.63
3.64
3.56
3.46
3.46
1.88 0.00
1.89 0.00
1.55 0.00
1.47 0.00
1.46 0.00
110
110
116
114
114
0
1
0
0
1
109
110
111
115
115
0
1
0
0
1
−35.0 1.2
37.1 3.1
−30.8 2.2
27.3 0.0
−25.6 3.5
(
Pna21
P21/c
R3
10
(N-triphospTol
)
P1
̅
a
b
c
In [Ru(ligand)(tmm)] complex. Average of all three observables in the compound; the standard deviation is indicated. X−C−P−Ru torsion
angle.
Interested in the performance of the investigated catalysts
compared to the established [Ru(triphosPh)(tmm)] complex,
we conducted an additional set of experiments with the latter,
using the conditions optimized for 2a and 2b. Reaching TONs
of 658 for DMM and 343 for MF (Table 2, entry 5), the
carbon-based triphos system exhibits a similar activity in regard
of the DMM formation, within the margin of error. In contrast,
the TON for MF is notably increased for the silicon-based
complexes 2a and 2b, resulting in an overall improved catalytic
performance (Table 2, entries 2,3,7,8). P-triphos complex 2c,
however, showed considerably lower TONs for DMM, only
exceeding the triphos reference system in MF formation
(Table 2, entries 4,5,9). In general, the influence of the apex
atom on the investigated conversion is much more pronounced
in group 15, with N-triphos significantly outperforming its
phosphorus analog (Table 2, entries 1,4,6,9).
Intrigued by the high catalytic activity of catalysts 2a−c
regarding MF (Table 2, entries 7−9), we investigated its
reusability in the catalytic process. For all three catalysts, MF
was selectively converted to DMM at 90 °C and 90 bar H2 (see
the Supporting Information). These results would encourage
an industrial process for the synthesis of DMM in which the
side product MF is transformed into DMM after workup of the
catalysis products by distillation.
formation can thus potentially be attributed to electronic
effects, as silicon is known for strongly influencing the electron
density in neighboring groups.20 This is further supported by
the partly differing responses of the two silicon complexes 2a
and 2b to variation of the catalytic parameters in the screening
(Figure 2A,C,D, and Table 1, entries 1 and 2).
When exchanging nitrogen for phosphorus, the effects on
activity are much more pronounced. Taking the smallest X−
C−P−Ru torsion angles into account (Table 1, entries 4 and
5), the nitrogen-based ligands seem to fit ruthenium the best
among the compared complexes, coinciding with the highest
observed TON for DMM (Table 2, entry 1). Besides the
optimum sterical prerequisite, electronic effects become more
dominant by the proximity between the nitrogen and
ruthenium as well as the phosphine groups. With phosphorus
in the apex position, the larger atomic radius and the more
diffused X−C bonds are more likely to result in weaker fixation
in the ligand backbone, whereby the lower activity of 2c in
catalysis toward DMM can be rationalized (Table 2, entry 4).
CONCLUSIONS
■
We report the application of novel ruthenium hetero-triphos
complexes in the homogeneous catalysis of carbon dioxide to
dimethoxymethane and methyl formate with very high activity.
For this purpose, we prepared complexes bearing a silicon (2a
and 2b) or phosphorus atom (2c) in the apical position of the
ligand. Optimizing temperature, H2 and CO2 pressure as well
as catalyst and additive loading, maximum turnover numbers of
685 for DMM and 1370 for MF could be achieved using
complex 2b. These results are in the same range as the TONs
reported for the benchmark N-triphosPh system (DMM: 786;
The exchange from carbon to silicon in the ligand backbone
has improved overall activity for 2a and 2b (Table 2, entries
2,3,5,7,8). It seems that the larger atomic radius of silicon and
the accompanying longer X−C bonds are compensated by the
higher X−C−P−Ru torsion angles (Table 1, entries 1−3). We
assume that this reduces steric effects at the catalytically active
ruthenium center. The observed difference in product
C
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Figure 2. Bar plots of TONs in the catalytic investigation of the reductive transformation of CO2 to DMM and MF for the screening of reaction
parameters, such as temperature (A), H2 (B) and CO2 pressure (C) as well as catalyst (D) and additive loading (E).
MF: 1290), which represent the highest activity to date. The
catalytic system can be tuned to either form DMM (in a ratio
of 2:1) or MF (in a ratio of 4:1) as the main product. Utilizing
the same catalyst and additive, MF can be reductively
transformed into DMM under moderate conditions. Changing
the apical atom from carbon to silicon notably enhances MF
formation without affecting TONs for DMM and thus reduces
selectivity under the given conditions. The phosphorus-based
system requires higher temperatures and catalytic activity
regarding DMM is attenuated in general. Using H2 and CO2,
PhSi-triphosPh and MeSi-triphosPh can be considered as good
alternatives to N-triphosPh as ligands for the ruthenium-
catalyzed synthesis of DMM, due to their high activity and
facile preparation.
EXPERIMENTAL SECTION
■
Detailed documentation of all experiments performed and synthetic
procedures as well as spectroscopic characterization of compounds
D
DOI: 10.1021/acs.organomet.9b00107
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Article
a b c
, ,
Table 2. Overview of Catalyses with Best Results Regarding DMM and MF Using Ruthenium Triphos Catalysts.
entry
cat.
T (°C)
ncat (μmol)
nadd (μmol)
TONDMM
TONMF
d
d
1
2
3
4
5
6
7
8
9
[Ru(N-triphosPh)(tmm)]10
[Ru(PhSi-triphosPh)(tmm)] 2a
[Ru(MeSi-triphosPh)(tmm)] 2b
[Ru(P-triphosPh)(tmm)] 2c
[Ru(triphosPh)(tmm)]
[Ru(N-triphosPh)(tmm)]10
[Ru(PhSi-triphosPh)(tmm)] 2a
[Ru(MeSi-triphosPh)(tmm)] 2b
[Ru(P-triphosPh)(tmm)] 2c
90
90
90
100
90
90
90
90
100
0.38
0.38
0.38
0.38
0.38
0.19
0.19
0.19
0.19
1.56
3.13
3.13
3.13
3.13
0.78
0.78
0.78
0.78
786 62
678 21
685 65
533 17
452
1
466 37
515 51
343 49
1290 151
1350 58
1370 99
1280 76
439
6
658 54
728 102
d
d
564
6
339 27
358 18
a
b
Catalytic conditions: Additive Al(OTf)3, 0.5 mL MeOH, 90 bar H2, 20 bar CO2, 18 h. TONs were determined by 1H NMR spectroscopy using
c
d
mesitylene as the internal standard. All values are the average of two runs unless stated otherwise, and the standard deviation is indicated. This
value is the average of three runs, and the standard deviation is indicated.
used can be found in the Supporting Information. For better
readability, simplified nomenclature instead of IUPAC names is used
for ligands 1a−c.
Novel complexes 2a−c were characterized by NMR spectroscopy,
ABBREVIATIONS
COD, cyclooctadiene; DMM, dimethoxymethane; HR-MS,
high-resolution mass spectrometry; IR, infrared radiation; MF,
methyl formate; NMR, nuclear magnetic resonance; tmm,
trimethylenemethane dianion; TON, turnover number
■
high-resolution mass spectrometry (HR-MS), IR spectroscopy, and
elemental analysis. X-ray diffraction analysis from a single crystal was
obtained for compounds 2a and 2b (see the Supporting Information).
All catalyses and the corresponding analyses were performed two
times following a procedure previously reported by our group.10
The catalysis was also performed in the absence of a catalyst, a co-
catalyst, or both to demonstrate the need for the catalytic system for
the formation of DMM and MF. In these experiments, no significant
TONs for both of these compounds were obtained (see the
Supporting Information). Error bars shown in Figure 2 display the
standard deviation of the experiments (see the Supporting
Information).
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Experimental details, screening of reaction conditions,
and characterization data for all new compounds;
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(PDF)
Accession Codes
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tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
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AUTHOR INFORMATION
Corresponding Author
*E-mail: oliver.trapp@cup.uni-muenchen.de.
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ORCID
Oliver Trapp: 0000-0002-3594-5181
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.organomet.9b00107
Organometallics XXXX, XXX, XXX−XXX